Plasmonic Optical Fiber-Grating Immunosensing: A Review
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paper_title: Fiber optic SPR biosensing of DNA hybridization and DNA-protein interactions.

paper_content:

In this paper we present a fiber optic surface plasmon resonance (SPR) sensor as a reusable, cost-effective and label free biosensor for measuring DNA hybridization and DNA-protein interactions. This is the first paper that combines the concept of a fiber-based SPR system with DNA aptamer bioreceptors. The fibers were sputtered with a 50nm gold layer which was then covered with a protein repulsive self-assembled monolayer of mixed polyethylene glycol (PEG). Streptavidin was attached to the PEG's carboxyl groups to serve as a versatile binding element for biotinylated ssDNA. The ssDNA coated SPR fibers were first evaluated as a nucleic acid biosensor through a DNA-DNA hybridization assay for a random 37-mer ssDNA. This single stranded DNA showed a 15 nucleotides overlap with the receptor ssDNA on the SPR fiber. A linear calibration curve was observed in 0.5-5 microM range. A negative control test did not reveal any significant non-specific binding, and the biosensor was easily regenerated. In a second assay the fiber optic SPR biosensor was functionalized with ssDNA aptamers against human immunoglobulin E. Limits of detection (2nM) and quantification (6nM) in the low nanomolar range were observed. The presented biosensor was not only useful for DNA and protein quantification purposes, but also to reveal the binding kinetics occurring at the sensor surface. The dissociation constant between aptamer and hIgE was equal to 30.9+/-2.9nM. The observed kinetics fully comply with most data from the literature and were also confirmed by own control measurements.
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paper_title: Fibre-optic evanescent field absorption sensor based on a U-shaped probe

paper_content:

A fibre-optic evanescent field absorption sensor based on a U-shaped sensing probe is described. The influences of fibre core diameter, bending radius of the probe and the refractive index of the fluid on the sensitivity of the sensor are evaluated experimentally. The results are compared with the theoretical results obtained using geometrical optics and based on two-dimensional treatment. A good qualitative agreement is found between them.
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paper_title: Surface Plasmon Resonance-Based Fiber Optic Sensors: Principle, Probe Designs, and Some Applications

paper_content:

Surface plasmon resonance technique in collaboration with optical fiber technology has brought tremendous advancements in sensing of various physical, chemical, and biochemical parameters. In this review article, we present the principle of SPR technique for sensing and various designs of the fiber optic SPR probe reported for the enhancement of the sensitivity of the sensor. In addition, we present few examples of the surface plasmon resonance- (SPR-) based fiber optic sensors. The present review may provide researchers valuable information regarding fiber optic SPR sensors and encourage them to take this area for further research and development.
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paper_title: Local Excitation, Scattering, and Interference of Surface Plasmons

paper_content:

The optical probe of a scanning near-field optical microscope is shown to act as a point source of surface plasmon (SP) polaritons on gold and silver films. Plasmon excitation manifests itself by emission of light in the direction of the SP resonance angle, originating from an area with the shape of a dipole radiation pattern whose extension is given by the SP decay length. Interaction with selected, individual surface inhomogeneities gives rise to characteristic modifications of the emitted radiation, which provide detailed information about SP scattering, reflection, and interference phenomena.
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paper_title: Bending effects in optical fibers

paper_content:

Mode coupling at bends in optical fibers supporting one or only a few guided modes is analyzed by considering the local normal modes for the corresponding straight waveguide. Matrix elements giving the strength of coupling between guided modes at a corner bend, and for coupling between guided modes and radiation modes, are calculated as a function of guiding strength for this "geometrical" effect. The correction to these matrix elements due to the longitudinal strain in a bent fiber is also determined. The increase in propagation constant for the fundamental mode of a fiber wrapped in a coil of constant radius is calculated from information on the coupling strengths and mode propagation constants. The phase shift and attenuation of the fundamental mode caused by a spatially periodic microbending of the fiber axis are also considered. Finally, potential applications of these effects in fiber-optic devices such as mode converters, phase shifters, switches, and sensors are discussed.
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paper_title: A fiber-optic chemical sensor based on surface plasmon resonance

paper_content:

Abstract A fiber-optic chemical sensor is presented which utilizes surface plasmon resonance excitation. The sensing element of the fiber has been fabricated by removing a section of the fiber cladding and symmetrically depositing a thin layer of highly reflecting metal onto the fiber core. A white-light source is used to introduce a range of wavelengths into the fiber optic. Changes in the sensed parameters (e.g., bulk refractive index, film thinkness and film refractive index) are determined by measuring the transmitted spectral-intensity distribution. Experimental results of the sensitivity and the dynamic range in the measurement of the refractive indices of aqueous solutions are in agreement with the theoretical model of the sensor.
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paper_title: Surface plasmon subwavelength optics

paper_content:

Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal's surface, the properties of surface plasmons—in particular their interaction with light—can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.
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paper_title: Fiber-Optic Sensors Based on Surface Plasmon Resonance: A Comprehensive Review

paper_content:

Since the introduction of optical fiber technology in the field of sensor based on the technique of surface plasmon resonance (SPR), fiber-optic SPR sensors have witnessed a lot of advancements. This paper reports on the past, present, and future scope of fiber-optic SPR sensors in the field of sensing of different chemical, physical, and biochemical parameters. A detailed mechanism of the SPR technique for sensing purposes has been discussed. Different new techniques and models in this area that have been introduced are discussed in quite a detail. We have tried to put the different advancements in the order of their chronological evolution. The content of the review article may be of great importance for the research community who are to take the field of fiber-optic SPR sensors as its research endeavors.
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paper_title: Surface plasmon resonance sensors: review

paper_content:

Abstract Since the first application of the surface plasmon resonance (SPR) phenomenon for sensing almost two decades ago, this method has made great strides both in terms of instrumentation development and applications. SPR sensor technology has been commercialized and SPR biosensors have become a central tool for characterizing and quantifying biomolecular interactions. This paper attempts to review the major developments in SPR technology. Main application areas are outlined and examples of applications of SPR sensor technology are presented. Future prospects of SPR sensor technology are discussed.
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paper_title: Fiber optic SPR biosensing of DNA hybridization and DNA-protein interactions.

paper_content:

In this paper we present a fiber optic surface plasmon resonance (SPR) sensor as a reusable, cost-effective and label free biosensor for measuring DNA hybridization and DNA-protein interactions. This is the first paper that combines the concept of a fiber-based SPR system with DNA aptamer bioreceptors. The fibers were sputtered with a 50nm gold layer which was then covered with a protein repulsive self-assembled monolayer of mixed polyethylene glycol (PEG). Streptavidin was attached to the PEG's carboxyl groups to serve as a versatile binding element for biotinylated ssDNA. The ssDNA coated SPR fibers were first evaluated as a nucleic acid biosensor through a DNA-DNA hybridization assay for a random 37-mer ssDNA. This single stranded DNA showed a 15 nucleotides overlap with the receptor ssDNA on the SPR fiber. A linear calibration curve was observed in 0.5-5 microM range. A negative control test did not reveal any significant non-specific binding, and the biosensor was easily regenerated. In a second assay the fiber optic SPR biosensor was functionalized with ssDNA aptamers against human immunoglobulin E. Limits of detection (2nM) and quantification (6nM) in the low nanomolar range were observed. The presented biosensor was not only useful for DNA and protein quantification purposes, but also to reveal the binding kinetics occurring at the sensor surface. The dissociation constant between aptamer and hIgE was equal to 30.9+/-2.9nM. The observed kinetics fully comply with most data from the literature and were also confirmed by own control measurements.
---
paper_title: Fibre-optic evanescent field absorption sensor based on a U-shaped probe

paper_content:

A fibre-optic evanescent field absorption sensor based on a U-shaped sensing probe is described. The influences of fibre core diameter, bending radius of the probe and the refractive index of the fluid on the sensitivity of the sensor are evaluated experimentally. The results are compared with the theoretical results obtained using geometrical optics and based on two-dimensional treatment. A good qualitative agreement is found between them.
---
paper_title: Surface Plasmon Resonance-Based Fiber Optic Sensors: Principle, Probe Designs, and Some Applications

paper_content:

Surface plasmon resonance technique in collaboration with optical fiber technology has brought tremendous advancements in sensing of various physical, chemical, and biochemical parameters. In this review article, we present the principle of SPR technique for sensing and various designs of the fiber optic SPR probe reported for the enhancement of the sensitivity of the sensor. In addition, we present few examples of the surface plasmon resonance- (SPR-) based fiber optic sensors. The present review may provide researchers valuable information regarding fiber optic SPR sensors and encourage them to take this area for further research and development.
---
paper_title: Bending effects in optical fibers

paper_content:

Mode coupling at bends in optical fibers supporting one or only a few guided modes is analyzed by considering the local normal modes for the corresponding straight waveguide. Matrix elements giving the strength of coupling between guided modes at a corner bend, and for coupling between guided modes and radiation modes, are calculated as a function of guiding strength for this "geometrical" effect. The correction to these matrix elements due to the longitudinal strain in a bent fiber is also determined. The increase in propagation constant for the fundamental mode of a fiber wrapped in a coil of constant radius is calculated from information on the coupling strengths and mode propagation constants. The phase shift and attenuation of the fundamental mode caused by a spatially periodic microbending of the fiber axis are also considered. Finally, potential applications of these effects in fiber-optic devices such as mode converters, phase shifters, switches, and sensors are discussed.
---
paper_title: A fiber-optic chemical sensor based on surface plasmon resonance

paper_content:

Abstract A fiber-optic chemical sensor is presented which utilizes surface plasmon resonance excitation. The sensing element of the fiber has been fabricated by removing a section of the fiber cladding and symmetrically depositing a thin layer of highly reflecting metal onto the fiber core. A white-light source is used to introduce a range of wavelengths into the fiber optic. Changes in the sensed parameters (e.g., bulk refractive index, film thinkness and film refractive index) are determined by measuring the transmitted spectral-intensity distribution. Experimental results of the sensitivity and the dynamic range in the measurement of the refractive indices of aqueous solutions are in agreement with the theoretical model of the sensor.
---
paper_title: Fiber-Optic Sensors Based on Surface Plasmon Resonance: A Comprehensive Review

paper_content:

Since the introduction of optical fiber technology in the field of sensor based on the technique of surface plasmon resonance (SPR), fiber-optic SPR sensors have witnessed a lot of advancements. This paper reports on the past, present, and future scope of fiber-optic SPR sensors in the field of sensing of different chemical, physical, and biochemical parameters. A detailed mechanism of the SPR technique for sensing purposes has been discussed. Different new techniques and models in this area that have been introduced are discussed in quite a detail. We have tried to put the different advancements in the order of their chronological evolution. The content of the review article may be of great importance for the research community who are to take the field of fiber-optic SPR sensors as its research endeavors.
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paper_title: Design and characteristics of refractive index sensor based on thinned and microstructure fiber Bragg grating.

paper_content:

A refractive index sensor based on the thinned and microstructure fiber Bragg grating (ThMs-FBG) was proposed and realized as a chemical sensing. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated and the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length, and position. The sandwich-solution etching method was utilized to realize the microstructure of the ThMs-FBG, and the photographs of the microstructure were obtained. Experimental results demonstrated that the reflectance spectrum, phase shift down-peak wavelength, and reflected optical intensity of the ThMs-FBG all depended on the surrounding refractive index. However, only the down-peak wavelength of the ThMs-FBG changed with the surrounding temperature. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800 and 14 mum, respectively, and the position of the microstructure of the ThMs-FBG is in the middle of grating region, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/refractive index unit with the wide range of 1.33-1.457 and a high resolution of 1.2 x 10(-3). The temperature sensitivity was 0.0103 nm/ degrees C, which was approximately equal to that of common FBG.
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paper_title: Point-by-point written fiber-Bragg gratings and their application in complex grating designs

paper_content:

The point-by-point technique of fabricating fibre-Bragg gratings using an ultrafast laser enables complete control of the position of each index modification that comprises the grating. By tailoring the local phase, amplitude and spacing of the grating’s refractive index modulations it is possible to create gratings with complex transmission and reflection spectra. We report a series of grating structures that were realized by exploiting these flexibilities. Such structures include gratings with controlled bandwidth, and amplitude- and phase-modulated sampled (or superstructured) gratings. A model based on coupled-mode theory provides important insights into the manufacture of such gratings. Our approach offers a quick and easy method of producing complex, non-uniform grating structures in both fibres and other mono-mode waveguiding structures.
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paper_title: Fiber grating spectra

paper_content:

In this paper, we describe the spectral characteristics that can be achieved in fiber reflection (Bragg) and transmission gratings. Both principles for understanding and tools for designing fiber gratings are emphasized. Examples are given to illustrate the wide variety of optical properties that are possible in fiber gratings. The types of gratings considered include uniform, apodized, chirped, discrete phase-shifted, and superstructure gratings; short-period and long-period gratings; symmetric and tilted gratings; and cladding-mode and radiation-mode coupling gratings.
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paper_title: Hydrogen sensor based on side-polished fiber Bragg gratings coated with thin palladium film

paper_content:

A new type of hydrogen sensor based on a side-polished fiber Bragg grating (FBG) coated with thin palladium film was demonstrated experimentally. The used FBG with the reflectivity of 90% is fabricated in a hydrogen-loaded single-mode fiber (SMF-28) by using the phase mask writing technique of a KrF excimer laser. The experimental results show that proposed sensor can be applied for hydrogen concentration measurements.
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paper_title: Formation of Bragg gratings in optical fibers by a transverse holographic method.

paper_content:

Bragg gratings have been produced in germanosilicate optical fibers by exposing the core, through the side of the cladding, to a coherent UV two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244 nm. Fractional index perturbations of approximately 3 x 10(-5) have been written in a 4.4-mm length of the core with a 5-min exposure. The Bragg filters formed by this new technique had reflectivities of 50-55% and spectral widths, at half-maximum, of 42 GHz.
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paper_title: Thinned fiber Bragg gratings as high sensitivity refractive index sensor

paper_content:

In this work, the numerical and experimental analysis on the use of thinned fiber Bragg gratings as refractive index sensors have been carried out. Wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization for an almost full etched cladding sensor is presented demonstrating good agreement with numerical results and resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for outer refractive index around 1.45 and 1.333, respectively.
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paper_title: Theoretical and experimental study on etched fiber Bragg grating cladding mode resonances for ambient refractive index sensing

paper_content:

The theoretical model of etched fiber Bragg grating (FBG) backward cladding-mode resonances for ambient refractive index sensing is presented. The dependent behaviors of the mode resonances have been analyzed in the etching process and the ambient refractive index changed. The analysis is based on the classical coupling-mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Experimental data match the theoretical model wonderfully. This model not only describes the relationship between the FBG backward cladding-mode resonances and the ambient index but also is valuable for the design of a flexible highly sensitive ambient index sensor.
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paper_title: Etched fiber Bragg grating for refractive index distribution measurement

paper_content:

Abstract We present a refractive index (RI) distribution sensor based on an etched fiber Bragg grating (FBG). The FBG is etched by hydrofluoric acid solution until the residual diameter of its core is about 5.73 μm, and then it is installed in a micro-tube where the RI of the liquid medium decreases linearly along its axis. The reflection spectrum of the etched FBG changes from a single resonant peak into many micro-resonant peaks under the inhomogeneous liquid medium. The difference of the wavelengths between the maximum micro-resonant peak and the minimum one changes linearly with the RI gradient, and the average change of the wavelengths of them changes linearly with the average change of the RI as well. Experimental results show that the sensitivity of RI gradient and the average sensitivity of RI for this etched FBG sensor is about 336.2 nm mm/RIU and 49.44 nm/RIU, respectively, when the surrounding RI ranges from 1.330 to 1.3586. Associated with the sensitivity of the RI gradient and the average sensitivity of the RI, the linear RI distribution of inhomogeneous liquid medium surrounding the etched FBG can be obtained.
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paper_title: Optical low-coherence reflectometry (OLCR) characterization of efficient Bragg gratings in optical fiber

paper_content:

Keywords: [OFD] Note: Ouellette, FConference on Photosensitivity and Self-Organization in Optical Fibers and WaveguidesAUG 17-18, 1993QUEBEC, CANADA0-8194-1303-8 Reference LOA-ARTICLE-1993-003 Record created on 2009-01-20, modified on 2017-05-10
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paper_title: Demonstration of etched cladding fiber Bragg grating-based sensors with hydrogel coating

paper_content:

A novel hydrogel-coated single-mode fiber Bragg grating (FBG) sensor is investigated. The sensor uses a swellable polymer material, hydrogel, as the active sensing component. The sensing mechanism is based on the stress that is induced in the chemically sensitive swellable hydrogel coating. The stress shifts the Bragg wavelength of the FBG. By reducing the cladding diameter of FBG through etching with hydrofluoric acid, the tension tuning force of FBG can be lowered. Therefore, the stress induced in hydrogel can shift the wavelength of FBG more and the sensitivity of the sensor is improved. When the grating fiber is etched 37.5 μm in diameter, the sensitivity of sensor is approximately 10-fold larger than that without fiber etching.
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paper_title: Visible wavelength fiber Bragg grating arrays for high speed biomedical spectral sensing

paper_content:

Spectral data for each pixel in a confocal spatial scan are acquired by mapping spectral slices into the time domain with an array of visible fiber Bragg gratings. Multispectral images of biomedical tissue can be generated in real time.
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paper_title: D-shaped fiber grating refractive index sensor induced by an ultrashort pulse laser

paper_content:

The fabrication of fiber Bragg gratings was here demonstrated using ultrashort pulse laser point-by-point inscription. This is a very convenient means of creating fiber Bragg gratings with different grating periods and works by changing the translation speed of the fiber. The laser energy was first optimized in order to improve the spectral properties of the fiber gratings. Then, fiber Bragg gratings were formed into D-shaped fibers for use as refractive index sensors. A nonlinear relationship was observed between the Bragg wavelength and liquid refractive index, and a sensitivity of ∼30 nm/RIU was observed at 1.450. This shows that D-shaped fiber Bragg gratings might be used to develop promising biochemical sensors.
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paper_title: A Temperature-Insensitive Cladding-Etched Fiber Bragg Grating Using a Liquid Mixture with a Negative Thermo-Optic Coefficient

paper_content:

To compensate for the temperature dependency of a standard FBG, a cladding-etched FBG immersed with a liquid mixture having a negative thermo-optic coefficient is presented, and its characteristics are investigated. The Bragg wavelength of the cladding-etched FBG is shifted counter to the direction of the Bragg wavelength shift of a conventional FBG according to the mixing ratio of glycerin to water; thus, the temperature-dependent Bragg wavelength shift was almost compensated by using a liquid mixture of water (50%) and glycerin (50%) having the negative thermo-optic coefficient of −5 × 10−4 °C−1.
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paper_title: Point-by-point fabrication of micro-Bragg gratings in photosensitive fibre using single excimer pulse refractive index modification techniques

paper_content:

Optical fibre Bragg reflectors have been fabricated using a single pulse of high power 249 nm excimer laser light to photoinduce point-by-point each individual index element forming the grating. Bragg reflectors with a length of 360 μm and reflectivity of 70% have been made.
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paper_title: Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing

paper_content:

Photosensitivity in optical fibres properties of Fibre Bragg gratings inscribing Bragg gratings in optical fibres Fibre Bragg grating theory applications of Bragg gratings in communications Fibre Bragg grating sensors impact of Fibre Bragg gratings.
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paper_title: Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask

paper_content:

A photolithographic method is described for fabricating refractive index Bragg gratings in photosensitive optical fiber by using a special phase mask grating made of silica glass. A KrF excimer laser beam (249 nm) at normal incidence is modulated spatially by the phase mask grating. The diffracted light, which forms a periodic, high‐contrast intensity pattern with half the phase mask grating pitch, photoimprints a refractive index modulation into the core of photosensitive fiber placed behind, in proximity, and parallel, to the mask; the phase mask grating striations are oriented normal to the fiber axis. This method of fabricating in‐fiber Bragg gratings is flexible, simple to use, results in reduced mechanical sensitivity of the grating writing apparatus and is functional even with low spatial and temporal coherence laser sources.
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paper_title: A fibre Bragg grating refractometer

paper_content:

An opto-chemical in-fibre Bragg grating (FBG) sensor for refractive index measurement in liquids has been developed using fibre side-polishing technology. At a polished site where the fibre cladding has partly been removed, a FBG is exposed to a liquid analyte via evanescent field interaction of the guided fibre mode. The Bragg wavelength of the FBG is obtained in terms of its dependence on the refractive index of the analyte. Modal and wavelength dependences have been investigated both theoretically and experimentally in order to optimize the structure of the sensor. Using working wavelengths far above the cut-off wavelength results in an enhancement of the sensitivity of the sensor. Measurements with different mode configurations lead to the separation of cross sensitivities. Besides this, a second FBG located in the unpolished part can be used to compensate for temperature effects. Application examples for monitoring fuels of varying quality as well as salt concentrations under deep borehole conditions are presented.
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paper_title: Demonstration of an etched cladding fiber Bragg grating filter with reduced tuning force requirement

paper_content:

A novel tunable fiber Bragg grating (FBG) filter with a chemically etched cladding has been demonstrated. By reducing the cladding diameter of a FBG through etching with HF, the tension tuning force can be lowered by over an order of magnitude. Wavelength tuning up to a failure point of 21.2 nm has been demonstrated along with repeated cycling over a 9 nm tuning range. Observations of increased coupling to resonant cladding modes in the etched FBG's are also noted.
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paper_title: Fiber Bragg grating technology fundamentals and overview

paper_content:

The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.
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paper_title: Side-polished fiber Bragg grating refractive index sensor with TbFeCo magnetoptic thin film

paper_content:

Fiber Bragg grating (FBG) is side-polished to enable interaction with sensitive materials around FBG fiber core. Using TbFeCo magneto-optic thin film deposited onto FBG fiber core as transducer, a FBG refractive index senor for magnetic field/current characterization is first proposed and demonstrated in this paper. Magnetic field sensing experiments show 19 pm of wavelength shift at a magnetic field intensity of 50 mT, the average linearity of magnetic field response is 0.9877.
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paper_title: Recent research progress of optical fiber sensors based on D-shaped structure

paper_content:

Abstract The review summarizes recent studies on D-shaped optical fibers and their recent applications in optical sensors. The configurations and working principles of D-shaped optical fibers are introduced. For each optical fiber sensor, the structure principles and measuring methods are all discussed in detail, with their optimal characteristics and performances being compared. Results from various studies show that it is possible to realize a high-sensitivity optical fiber sensor with a simple structure, good mechanical properties, and strong anti-interference ability. This may be due to the excellent structural design of the D-shaped optical fiber. Finally, key issues and new challenges on the D-shaped optical fiber are discussed.
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paper_title: Fiber Bragg Gratings in the Visible Spectral Range With Ultraviolet Femtosecond Laser Inscription

paper_content:

In this letter, we investigate the inscription of fiber Bragg gratings in the visible spectral range using deep ultraviolet femtosecond laser exposure and two-beam interferometry. The properties of first-order reflection gratings and third-order gratings for use in the visible wavelength range are compared. Stronger gratings have been achieved for first-order reflecting Bragg gratings compared with third-order gratings. We demonstrate a fiber Bragg grating with a grating period of 226 nm and a filtering efficiency of more than 30 dB.
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paper_title: Side-polished fiber Bragg grating hydrogen sensor with WO 3 -Pd composite film as sensing materials

paper_content:

WO3-Pd composite films were deposited on the side-face of side-polished fiber Bragg grating as sensing elements by magnetron sputtering process. XRD result indicates that the WO3-Pd composite films are mainly amorphous. Compared to standard FBG coated with same hydrogen sensitive film, side-polished FBG significantly increase the sensor’s sensitivity. When hydrogen concentrations are 4% and 8% in volume percentage, maximum wavelength shifts of side-polished FBG are 25 and 55 pm respectively. The experimental results show the sensor’s hydrogen response is reversible, and side-polished FBG hydrogen sensor has great potential in hydrogen’s measurement.
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paper_title: Wide Range Refractive Index Measurement Using a Multi-Angle Tilted Fiber Bragg Grating

paper_content:

The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU.
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paper_title: Demodulation technique for weakly tilted fiber Bragg grating refractometer

paper_content:

In this letter, a demodulation technique is presented in order to measure the surrounding refractive index in the range 1-1.45 by means of a weakly tilted fiber Bragg grating. This technique is based on the global monitoring of the cladding modes in the transmitted spectrum and on the computation of two statistical parameters. We report a resolution of 210/sup -4/ in terms of the refractive index as well as a temperature-insensitive behavior.
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paper_title: Tilted fiber phase gratings

paper_content:

A detailed theoretical treatment is presented of bound-mode to bound-mode Bragg reflection and bound-mode to radiation-mode coupling loss in a tilted optical-fiber phase grating. Numerical predictions of the effects of grating tilt on the spectral characteristics of such a grating are calculated. These predictions are compared with experimentally measured spectra of strong gratings written by ultraviolet irradiation of deuterium-sensitized fiber with grating tilt angles ranging from 0° to 15°. Good agreement is obtained between the theoretical predictions and the experimental results.
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paper_title: Optical fiber refractometer using narrowband cladding-mode resonance shifts.

paper_content:

Short-period fiber Bragg gratings with weakly tilted grating planes generate multiple strong resonances in transmission. Our experimental results show that the wavelength separation between selected resonances allows the measurement of the refractive index of the medium surrounding the fiber for values between 1.25 and 1.44 with an accuracy approaching 1x10(-4). The sensor element is 10 mm long and made from standard single-mode telecommunication grade optical fiber by ultraviolet light irradiation through a phase mask.
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paper_title: Ultrasensitive plasmonic sensing in air using optical fibre spectral combs

paper_content:

Fibre sensors are key to many minimally-invasive detection techniques but, owing to an index mismatch, they are often limited to aqueous environments. Here, Caucheteur et al. develop a high-resolution fibre gas sensor with a tilted in-fibre grating that allows coupling to higher-order plasmon modes.
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paper_title: Wide Range Refractive Index Measurement Using a Multi-Angle Tilted Fiber Bragg Grating

paper_content:

The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU.
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paper_title: In-fibre directional transverse loading sensor based on excessively tilted fibre Bragg gratings

paper_content:

We report a distinctive polarization mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with a tilted angle exceeding 45°. The ex-45° TFBGs exhibit pronounced polarization mode splitting resulted from the birefringence induced by the grating structure asymmetry. We have fabricated TFBGs with a tilted structure at 81° and studied their properties under transverse load applied to their equivalent fast and slow axes. The results show that the light coupling to the orthogonally polarized modes of the 81°-TFBGs changes only when the load is applied to their slow axis, giving a prominent directional loading response. For the view of real applications, we further investigated the possibility of interrogating such a TFBG-based load sensor using low-cost and compact-size single wavelength source and power detector. The experimental results clearly show that the 81°-TFBGs plus the proposed power-measurement interrogation scheme may be developed to an optical fibre vector sensor system capable of not just measuring the magnitude but also recognizing the direction of the applied transverse load. Using such an 81°-TFBG based load sensor, a load change as small as 1.6 × 10-2 g may be detected by employing a standard photodiode detector.
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paper_title: Ultrasensitive plasmonic sensing in air using optical fibre spectral combs

paper_content:

Fibre sensors are key to many minimally-invasive detection techniques but, owing to an index mismatch, they are often limited to aqueous environments. Here, Caucheteur et al. develop a high-resolution fibre gas sensor with a tilted in-fibre grating that allows coupling to higher-order plasmon modes.
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paper_title: Graphene-induced unique polarization tuning properties of excessively tilted fiber grating

paper_content:

By exploiting the polarization-sensitive coupling effect of graphene with the optical mode, we investigate the polarization modulation properties of a hybrid waveguide of graphene-integrated excessively tilted fiber grating (Ex-TFG). The theoretical analysis and experimental results demonstrate that the real and imaginary parts of complex refractive index of few-layer graphene exhibit different effects on transverse electric (TE) and transverse magnetic (TM) cladding modes of the Ex-TFG, enabling stronger absorption in the TE mode and more wavelength shift in the TM mode. Furthermore, the surrounding refractive index can modulate the complex optical constant of graphene and then the polarization properties of the hybrid waveguide, such as resonant wavelength and peak intensity. Therefore, the unique polarization tuning property induced by the integration of the graphene layer with Ex-TFG may endow potential applications in all-in-one fiber modulators, fiber lasers, and biochemical sensors.
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paper_title: Optic sensors of high refractive-index responsivity and low thermal cross sensitivity that use fiber Bragg gratings of >80° tilted structures

paper_content:

For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80 degrees tilted structures have been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80 degrees tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340 nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs.
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paper_title: Wide Range Refractive Index Measurement Using a Multi-Angle Tilted Fiber Bragg Grating

paper_content:

The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU.
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paper_title: Negative axial strain sensitivity in gold-coated eccentric fiber Bragg gratings

paper_content:

New dual temperature and strain sensor has been designed using eccentric second-order fiber Bragg gratings produced in standard single-mode optical fiber by point-by-point direct writing technique with tight focusing of 800 nm femtosecond laser pulses. With thin gold coating at the grating location, we experimentally show that such gratings exhibit a transmitted amplitude spectrum composed by the Bragg and cladding modes resonances that extend in a wide spectral range exceeding one octave. An overlapping of the first order and second order spectrum is then observed. High-order cladding modes belonging to the first order Bragg resonance coupling are close to the second order Bragg resonance, they show a negative axial strain sensitivity (-0.55 pm/με) compared to the Bragg resonance (1.20 pm/με) and the same temperature sensitivity (10.6 pm/°C). With this well conditioned system, temperature and strain can be determined independently with high sensitivity, in a wavelength range limited to a few nanometers.
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paper_title: Cladding mode coupling in highly localized fiber Bragg gratings: modal properties and transmission spectra

paper_content:

The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs con- siderably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications.
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paper_title: Surface plasmon resonance in eccentric femtosecond-laser-induced fiber Bragg gratings

paper_content:

Highly localized refractive index modulations are photo-written in the core of pure silica fiber using point-by-point focused UV femtosecond pulses. These specific gratings exhibit a comb-like transmitted amplitude spectrum, with polarization-dependent narrowband cladding mode resonances. In this work, eccentric gratings are surrounded by a gold sheath, allowing the excitation of surface plasmon polaritons (SPP) for radially-polarized light modes. The spectral response is studied as a function of the surrounding refractive index and a maximum sensitivity of 50 nm/RIU (refractive index unit) is reported for a well-defined cladding-mode resonance among the spectral comb. This novel kind of plasmonic fiber grating sensor offers rapidity of production, design flexibility, and high temperature stability.
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paper_title: Cladding mode coupling in highly localized fiber Bragg gratings II: complete vectorial analysis

paper_content:

Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. The transverse localization of the resonant grating causes strong coupling to cladding modes of high azimuthal and radial order. In this paper, we show how the reflected cladding modes can be fully analyzed, taking their vectorial nature, orientation and degeneracies into account. The observed modes’ polarization and intensity distributions are directly tied to the dispersive properties and show abrupt transitions in nature, strongly correlated with changes in the coupling strengths.
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paper_title: Off-axis ultraviolet-written fiber Bragg gratings for directional bending measurements.

paper_content:

Off-axis fiber Bragg gratings are inscribed by ultraviolet irradiation limited to expose only a portion of the fiber core cross section. The coupling to cladding modes is significantly increased, and the amplitude of the cladding mode resonances becomes sensitive to bending in magnitude and direction. Sensitivities ranging from +1.17 dB/m(-1) to -1.25 dB/m(-1) were obtained for bending in different directions relative to the offset direction of the grating, for curvatures from 0 to 1.1 m(-1), a range ideal for the shape sensing of large structures. The bending sensor response is also shown to be independent of temperature and the surrounding refractive index.
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paper_title: Ultrasensitive plasmonic sensing in air using optical fibre spectral combs

paper_content:

Fibre sensors are key to many minimally-invasive detection techniques but, owing to an index mismatch, they are often limited to aqueous environments. Here, Caucheteur et al. develop a high-resolution fibre gas sensor with a tilted in-fibre grating that allows coupling to higher-order plasmon modes.
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paper_title: Long-period fiber gratings as band-rejection filters

paper_content:

We present a new class of long-period fiber gratings that can be used as in-fiber, low-loss, band-rejection filters. Photoinduced periodic structures written in the core of standard communication-grade fibers couple light from the fundamental guided mode to forward propagating cladding modes and act as spectrally selective loss elements with insertion losses act as backreflections <-80 dB, polarization-mode-dispersions <0.01 ps and polarization-dependent-losses <0.02 dB.
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paper_title: Characterization of Long-period Grating Refractive Index Sensors and Their Applications

paper_content:

The influence of grating length and bend radius of long-period gratings (LPGs) on refractive index sensing was examined. Sensitivity to refractive indexes smaller than that of silica could be enhanced by bending LPGs. Bent LPGs lost sensitivity to refractive indexes higher than that of silica, whereas a 20-mm-long LPG arranged in a straight line had considerable sensitivity. These experimental results demonstrated that the sensitivity characteristics of LPGs to refractive index could be controlled by grating length and bend radius.
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paper_title: Long period fibre gratings for structural bend sensing

paper_content:

The authors examine the changes in wavelength and attenuation of long period fibre gratings subjected to bends with curvatures from 0 to 4.4 m/sup -1/. The wavelength change with curvature is nonlinear, with a projected minimum detectable curvature change of 2/spl times/10/sup -3/ m/sup -1/. The magnitude of the bend-induced wavelength shift depends on the rotation of the cylindrical fibre relative to the bending plane.
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paper_title: Measurements of refractive index sensitivity using long-period grating refractometer

paper_content:

Abstract We report the development and demonstration of a long-period grating refractometer. The principle of operation is based on the using of a long-period grating that is structurally induced by a CO 2 laser, and where the resonance wavelengths are shifted as the refractive index of medium surrounding the cladding of the long-period grating. The different concentrations for three types of solutions (ethylene glycol, salt, and sugar) were experimentally measured, and results show that, as a refractometer, this fiber-based device not only can differentiate chemicals based on their refractive index, but it can also become a concentration indicator of a particular chemical solution, and applied in the oil and petroleum industry.
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paper_title: Optical Fiber-Excited Surface Plasmon Resonance Spectroscopy of Single and Ensemble Gold Nanorods

paper_content:

Gold nanorods are deposited on the core surface of an etched optical fiber, and their surface plasmon resonance is excited by the evanescent wave from the optical fiber. The excitation of the surface plasmon produces strong light scattering from individual nanorods, allowing for single-particle scattering-based imaging and spectroscopy. We systematically characterize the dependence of the scattering spectra of individual nanorods on the refractive index of the surrounding medium. As the refractive index is increased, the scattering spectra red shift steadily. The refractive index sensitivity and sensing figure of merit, which is the index sensitivity divided by the scattering peak line width, are found to reach 200 nm/RIU (RIU = refractive index unit) and 3.8, respectively. We further investigate the ensemble response of the plasmon resonance of Au nanorods to varying dielectric environments by measuring the intensity and spectrum of the light transmitted through the fiber. The ensemble index sensitivity ...
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paper_title: Tuning the Resonance of the Excessively Tilted LPFG-Assisted Surface Plasmon Polaritons: Optimum Design Rules for Ultrasensitive Refractometric Sensor

paper_content:

An excessively tilted LPFG-assisted surface plasmon polaritons sensor (Ex-TLPFG assisted SPP sensor) with ultrahigh sensitivity is proposed and numerically investigated using the finite-element-method-based full-vector complex coupled mode theory. We show that the SPP mode is transited (or excited) gradually from both the p-polarized TM $_{0,j}$ and EH $_{v,j}$ ( $v\geq 1$ ) modes, and hence, the proposed SPP sensor can be tuned to achieve strong resonance of either the degenerate TM $_{0,j}$ and EH $_{2,j}$ modes or EH $_{1,j}$ mode to optimize the sensitivity for analyte refractive index sensing. The results confirm that a transition point corresponding to the phase matching curve of the SPP mode is obtained, which can be used to predict the optimized grating period. By this approach, ultrasensitive SPP refractometric sensor can be obtained and the sensitivity can be further improved through a simple method: reducing the fiber cladding combined with an optimized grating period. We demonstrate that a giant sensitivity as high as $10100\,\text{nm}/\text{RIU}$ is achieved for the degenerate TM $_{0,32}$ and EH $_{2,32}$ modes (or $7400\,\text{nm/RIU}$ for the EH $_{1,32}$ mode). These appealing characteristics make the proposed Ex-TLPFG-assisted SPP sensor ideal for biochemical analyte sensing applications.
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paper_title: Lab on Fiber Technology for biological sensing applications

paper_content:

This review presents an overview of “Lab on Fiber” technologies and devices with special focus on the design and development of advanced fiber optic nanoprobes for biological applications. Depending on the specific location where functional materials at micro and nanoscale are integrated, “Lab on Fiber Technology” is classified into three main paradigms: Lab on Tip (where functional materials are integrated onto the optical fiber tip), Lab around Fiber (where functional materials are integrated on the outer surface of optical fibers), and Lab in Fiber (where functional materials are integrated within the holey structure of specialty optical fibers). ::: ::: This work reviews the strategies, the main achievements and related devices developed in the “Lab on Fiber” roadmap, discussing perspectives and challenges that lie ahead, with special focus on biological sensing applications.
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paper_title: Wavelength-based localized surface plasmon resonance optical fiber biosensor

paper_content:

Abstract Two types of localized surface plasmon resonance (LSPR)-based optical fiber biosensors using gold nanospheres (GNSs) and gold nanorods (GNRs) have been developed and their performance characteristics evaluated and cross-compared successfully in this work. Based on the results obtained from the optimization of each of these types of biosensor and reported by the authors elsewhere, GNSs with a diameter of 60 nm and GNRs with an aspect ratio of 4.1 were specifically chosen in this work for the fabrication of two representative sensor probes, with an aim to create a highly sensitive and wavelength-based LSPR sensor to overcome the limitations arising from other intensity-based sensors. In order to develop effective LSPR biosensors, both GNSs and GRNs respectively were immobilized on an unclad surface of an optical fiber, prior to the functionalization with human IgG in order to create a device for the detection of anti-human IgG, at different concentrations. The experimental results obtained from tests carried out show that the sensitivities of GNSs and GNRs-based LSPR sensors to refractive index variation are 914 and 601 nm/RIU respectively; however as biosensors they have demonstrated the same detection limit of 1.6 nM for the detection of anti-human IgG.
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paper_title: Colloidal gold-modified optical fiber for chemical and biochemical sensing.

paper_content:

A novel class of fiber-optic evanescent-wave sensor was constructed on the basis of modification of the unclad portion of an optical fiber with self-assembled gold colloids. The optical properties and, hence, the attenuated total reflection spectrum of self-assembled gold colloids on the optical fiber changes with different refractive index of the environment near the colloidal gold surface. With sucrose solutions of increasing refractive index, the sensor response decreases linearly. The colloidal gold surface was also functionalized with glycine, succinic acid, or biotin to enhance the selectivity of the sensor. Results show that the sensor response decreases linearly with increasing concentration of each analyte. When the colloidal gold surface was functionalized with biotin, the detection limit of the sensor for streptavidin was 9.8 x 10(-11) M. Using this approach, we demonstrate proof-of-concept of a class of refractive index sensor that is sensitive to the refractive index of the environment near the colloidal gold surface and, hence, is suitable for label-free detection of molecular or biomolecular binding at the surface of gold colloids.
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paper_title: Universal scaling of the figure of merit of plasmonic sensors

paper_content:

We demonstrate an improvement by more than 1 order of magnitude of the figure of merit (FoM) of plasmonic nanoparticle sensors by means of the diffractive coupling of localized surface plasmon resonances. The coupling in arrays of nanoparticles leads to Fano resonances with narrow line widths known as surface lattice resonances, which are very suitable for the sensitive detection of small changes in the refractive index of the surroundings. We focus on the sensitivity to the bulk refractive index and find that the sensor FoM scales solely with the frequency difference between the surface lattice resonance and the diffracted order grazing to the surface of the array. This result, which can be extended to other systems with coupled resonances, enables the design of plasmonic sensors with a high FoM over broad spectral ranges with unprecedented accuracy.
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paper_title: Towards a Uniform Metrological Assessment of Grating-Based Optical Fiber Sensors: From Refractometers to Biosensors

paper_content:

A metrological assessment of grating-based optical fiber sensors is proposed with the aim of providing an objective evaluation of the performance of this sensor category. Attention was focused on the most common parameters, used to describe the performance of both optical refractometers and biosensors, which encompassed sensitivity, with a distinction between volume or bulk sensitivity and surface sensitivity, resolution, response time, limit of detection, specificity (or selectivity), reusability (or regenerability) and some other parameters of generic interest, such as measurement uncertainty, accuracy, precision, stability, drift, repeatability and reproducibility. Clearly, the concepts discussed here can also be applied to any resonance-based sensor, thus providing the basis for an easier and direct performance comparison of a great number of sensors published in the literature up to now. In addition, common mistakes present in the literature made for the evaluation of sensor performance are highlighted, and lastly a uniform performance assessment is discussed and provided. Finally, some design strategies will be proposed to develop a grating-based optical fiber sensing scheme with improved performance.
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paper_title: An enhanced LSPR fiber-optic nanoprobe for ultrasensitive detection of protein biomarkers.

paper_content:

A miniaturized, localized surface plasmon resonance (LSPR)-coupled fiber-optic (FO) nanoprobe is reported as a biosensor that is capable of label-free, sensitive detection of a cancer protein biomarker, free prostate specific antigen (f-PSA). The biosensor is based on the LSPR at the reusable dielectric-metallic hybrid interface with a robust, gold nano-disk array at the fiber end facet that is directly fabricated using EBL and metal lift-off process. The f-PSA has been detected with a mouse anti-human PSA monoclonal antibody (mAb) as a specific receptor linked with a self-assembled monolayer at the LSPR-FO facet surfaces. Experimental investigation and data analysis found near field refractive index (RI) sensitivity at ~226 nm/RIU with current LSPR-FO nanoprobe, and demonstrated the lowest limit of detection (LOD) at 100 fg/mL (~3 fM) of f-PSA in PBS solutions. The control experimentation using 5mg/mL bovine serum albumin in PBS and nonspecific surface test shows the excellent specificity and selectivity in the detection of f-PSA in PBS. These results present important progress towards a miniaturized, multifunctional fiber-optic technology that integrates informational communication and sensing function for developing a high performance, label-free, point-of-care (POC) device.
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paper_title: Annealing of gold nanostructures sputtered on glass substrate

paper_content:

The effects of annealing at 300 °C on gold nanostructures sputtered onto glass substrate were studied using XRD, SAXSees, the Van der Pauw method and ellipsometry. As-sputtered and annealed samples exhibit a different dependence of the gold lattice parameter on the sputtering time. With increasing sputtering time the average thickness of the layer and the size of gold crystallites increased. Another rapid enlargement of the crystallites is observed after annealing. The volume resistivity decreases rapidly with the increasing sputtering time for both, as-deposited and annealed structures. With increasing sputtering time initially discontinuous gold coverage changes gradually in a continuous one. Electrically continuous gold coverage on the as-sputtered and annealed samples exhibits the same concentration of free charge carriers and Hall mobility. Optical constants of as-deposited and annealed gold films determined by ellipsometry support resistivity measurements and clearly manifest the presence of plasmons in discontinuous films.
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paper_title: Bioinspired fabrication of optical fiber SPR sensors for immunoassays using polydopamine-accelerated electroless plating

paper_content:

This study presents a facile, rapid and effective method for the fabrication of optical fiber surface plasmon resonance (SPR) sensors via polydopamine (PDA)-accelerated electroless plating (ELP). The bioinspired PDA coating formed through the facile self-polymerization of dopamine (DA) was utilized as a versatile material for optic-fiber functionalization. Gold seeds were then rapidly and firmly adsorbed onto the PDA functional layer by amino and imino intermediates generated during the polymerization, and a gold film sensor was fabricated after metal deposition. The fabrication time of the sensor was decreased by 6–12 times, and the fabricated sensor exhibited higher sensitivity, better reproducibility and adhesion stability compared with those fabricated by the traditional ELP. Some key experimental parameters, including DA polymerization temperature, DA polymerization time, and plating time, were investigated in detail. The optimized sample exhibited high sensitivity ranging from 1391 nm per RIU to 5346 nm per RIU in the refractive index range of 1.328 to 1.386. Scanning electron microscopy images indicated that the sensor surface consisted of gold nanoparticles with a uniform particle size and an orderly arrangement, and the film thickness was approximately 60 nm. Another PDA layer was formed on the gold film for facile immobilization of antibodies. The sensor exhibited effective antibody immobilization ability and high sensitivity for human IgG detection over a wide range of concentrations from 0.5 to 40 μg mL−1, which indicate the potential applications of the fabricated sensor in immunoassays.
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paper_title: Thermal annealing of gold coated fiber optic surfaces for improved plasmonic biosensing

paper_content:

Abstract The morphological properties of thin gold (Au) films sputtered onto fiber optic (FO) substrates play an essential role in the overall performance of the sensing devices relying on surface plasmon resonance (SPR) effects. In this work, the influence of thermal treatments on the structural changes of the Au layer coated on the FO-SPR sensors, and consequently on their plasmonic biosensing performance, was systematically investigated. First, the sensors were exposed to different annealing temperatures for different durations and their sensitivity was evaluated by refractometric measurements in sucrose dilutions. The attained results suggested the optimal annealing conditions that were further validated using a split-plot experimental design statistical model. Room-temperature scanning tunneling microscopy (STM) imaging of the FO substrates revealed changes in the granular surface texture of the thermally treated Au films that could be linked to the observed increase in sensitivity of the treated sensors. The FO sensors, annealed under optimal conditions, were finally tested as label-free aptamer-based biosensors for the detection of Ara h 1 peanut allergen. Remarkably, the results demonstrated a superior biosensing performance of the thermally treated FO sensors, as the limit of detection (LOD) was improved with up to two orders of magnitude compared to a similar non-annealed FO-SPR sensor. This significant increase in sensitivity represents a major step forward in the facile and cost-effective preparation of FO-SPR sensors capable of label-free detection of various biomolecular targets.
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paper_title: Ion beam-induced enhanced adhesion of gold films deposited on glass

paper_content:

Abstract The metallisation of glass for decorative and/or functional purposes is now a well-established technique. The most popular methods are electroplating or sputter deposition. In order to obtain suitable adhesion, substrate pretreatment is a substantial part of a vapour-deposited coating that generally cannot be dispensed with. The pretreatment costs can reach the same order of magnitude as those associated with the actual coating process, or can even exceed them. In this work we studied the adhesion of Au thin films on glass. The substrates were pretreated by an ion-beam-mixing step, consisting of the deposition of Au/C bilayers or C/Au/C multilayers followed by Xe + implantation. After such preparation, the specimens were further coated (using a sputtering machine) with 150-nm-thick Au or Au-alloy films. Adhesion properties of the films were examined using a scratch tester in conjunction with scanning electron microscopy. It was observed that, without the ion-beam-mixing pretreatment, the coatings were poorly adherent. Strong adhesion enhancement was observed in the pretreated samples. The key mechanism envisaged to explain this is related to the formation of mixed SiC–Au phases at the interface region. Moreover, the mechanical properties of the pure and alloyed Au films were quantified by nano-indentation, and hardness results are in good agreement with a simple rigid-sphere model of substitutional hardening.
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paper_title: Silanization of solid surfaces via mercaptopropylsilatrane: a new approach of constructing gold colloid monolayers

paper_content:

Mercaptopropylsilatrane (MPS) was investigated as a novel self-assembled film on silica surfaces and also as a novel adhesive layer for the construction of a gold colloid monolayer on silica surfaces. We compare the preparation procedure and film quality of the MPS films to those of (3-mercaptopropyl)trimethoxysilane (MPTMS), which is commonly used for anchoring of gold nanoparticles on silica surfaces. The films were characterized by Fourier transform infrared spectroscopy, contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and Ellman's reagent to determine surface mercaptan concentration. The process in preparing the MPS films involves more environmentally friendly aqueous or polar organic solvents, takes significantly shorter time (<30 min), and results in more uniform and reproducible films due to its insensitivity to moisture. The MPS films also have higher mercaptan surface density than that of the MPTMS films, resulting in higher saturation coverage of the gold colloid monolayers on the MPS-coated substrates. The higher ambient stability of the MPS films as compared to the MPTMS films is important for applications where sufficient durability of the self-assembled films under ambient conditions is required. Thus, mercaptosilatrane may have the potential to replace mercaptosilane for surface modification and as an adhesive layer for the construction of a noble metal colloid monolayer on oxide surfaces.
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paper_title: Oxides and nitrides as alternative plasmonic materials in the optical range [Invited]

paper_content:

As alternatives to conventional metals, new plasmonic materials offer many advantages in the rapidly growing fields of plasmonics and metamaterials. These advantages include low intrinsic loss, semiconductor-based design, compatibility with standard nanofabrication processes, tunability, and others. Transparent conducting oxides such as Al:ZnO, Ga:ZnO and indium-tin-oxide (ITO) enable many high-performance metamaterial devices operating in the near-IR. Transition-metal nitrides such as TiN or ZrN can be substitutes for conventional metals in the visible frequencies. In this paper we provide the details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications.
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paper_title: Improved detection limits of protein optical fiber biosensors coated with gold nanoparticles.

paper_content:

The study presented herein investigates a novel arrangement of fiber-optic biosensors based on a tilted fiber Bragg grating (TFBG) coated with noble metal nanoparticles, either gold nanocages (AuNC) or gold nanospheres (AuNS). The biosensors constructed for this study demonstrated increased specificity and lowered detection limits for the target protein than a reference sensor without gold nanoparticles. The sensing film was fabricated by a series of thin-film and monolayer depositions to attach the gold nanoparticles to the surface of the TFBG using only covalent bonds. Though the gold nanoparticle integration had not yet been optimized for the most efficient coverage with minimum number of nanoparticles, binding AuNS and AuNC to the TFBG biosensor decreased the minimum detected target concentrations from 90 nM for the reference sensor, to 11 pM and 8 pM respectively. This improvement of minimum detection is the result of a reduced non-specific absorption onto the gold nanoparticles (by functionalization of the external surface of the gold nanoparticles), and of an optical field enhancement due to coupling between the photonic modes of the optical fiber and the localized surface plasmon resonances (LSPR) of the gold nanoparticles. This coupling also increased the sensitivity of the TFBG biosensor to changes in its local environment. The dissociation constant (Kd) of the target protein was also characterized with our sensing platform and found to be in good agreement with that of previous studies.
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paper_title: High resolution fiber optic surface plasmon resonance sensors with single-sided gold coatings

paper_content:

The surface plasmon resonance (SPR) performance of gold coated tilted fiber Bragg gratings (TFBG) at near infrared wavelengths is evaluated as a function of the angle between the tilt plane orientation and the direction of single- and double-sided, nominally 50 nm-thick gold metal depositions. Scanning electron microscope images show that the coating are highly non-uniform around the fiber circumference, varying between near zero and 50 nm. In spite of these variations, the experimental results show that the spectral signature of the TFBG-SPR sensors is similar to that of simulations based on perfectly uniform coatings, provided that the depositions are suitably oriented along the tilt plane direction. Furthermore, it is shown that even a (properly oriented) single-sided coating (over only half of the fiber circumference) is sufficient to provide a theoretically perfect SPR response with a bandwidth under 5 nm, and 90% attenuation. Finally, using a pair of adjacent TFBG resonances within the SPR response envelope, a power detection scheme is used to demonstrate a limit of detection of 3 × 10−6 refractive index units.
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paper_title: Identification and Quantification of Celery Allergens Using Fiber Optic Surface Plasmon Resonance PCR

paper_content:

Abstract: Accurate identification and quantification of allergens is key in healthcare, biotechnology and food quality and safety. Celery (Apium graveolens) is one of the most important elicitors of food allergic reactions in Europe. Currently, the golden standards to identify, quantify and discriminate celery in a biological sample are immunoassays and two-step molecular detection assays in which quantitative PCR (qPCR) is followed by a high-resolution melting analysis (HRM). In order to provide a DNA-based, rapid and simple detection method suitable for one-step quantification, a fiber optic PCR melting assay (FO-PCR-MA) was developed to determine different concentrations of celery DNA (1 pM–0.1 fM). The presented method is based on the hybridization and melting of DNA-coated gold nanoparticles to the FO sensor surface in the presence of the target gene (mannitol dehydrogenase, Mtd). The concept was not only able to reveal the presence of celery DNA, but also allowed for the cycle-to-cycle quantification of the target sequence through melting analysis. Furthermore, the developed bioassay was benchmarked against qPCR followed by HRM, showing excellent agreement (R2 = 0.96). In conclusion, this innovative and sensitive diagnostic test could further improve food quality control and thus have a large impact on allergen induced healthcare problems.
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paper_title: Enhanced Biosensor Platforms for Detecting the Atherosclerotic Biomarker VCAM1 Based on Bioconjugation with Uniformly Oriented VCAM1-Targeting Nanobodies

paper_content:

Surface bioconjugation of biomolecules has gained enormous attention for developing advanced biomaterials including biosensors. While conventional immobilization (by physisorption or covalent couplings using the functional groups of the endogenous amino acids) usually results in surfaces with low activity, reproducibility and reusability, the application of methods that allow for a covalent and uniformly oriented coupling can circumvent these limitations. In this study, the nanobody targeting Vascular Cell Adhesion Molecule-1 (NbVCAM1), an atherosclerotic biomarker, is engineered with a C-terminal alkyne function via Expressed Protein Ligation (EPL). Conjugation of this nanobody to azidified silicon wafers and Biacore™ C1 sensor chips is achieved via Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry to detect VCAM1 binding via ellipsometry and surface plasmon resonance (SPR), respectively. The resulting surfaces, covered with uniformly oriented nanobodies, clearly show an increased antigen binding affinity, sensitivity, detection limit, quantitation limit and reusability as compared to surfaces prepared by random conjugation. These findings demonstrate the added value of a combined EPL and CuAAC approach as it results in strong control over the surface orientation of the nanobodies and an improved detecting power of their targets-a must for the development of advanced miniaturized, multi-biomarker biosensor platforms.
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paper_title: Self-assembled Monolayers for Biosensors

paper_content:

The use of self-assembled monolayers (SAMs) in various fields of ::: research is rapidly growing. In particular, many biomedical fields apply ::: SAMs as an interface-layer between a metal surface and a solution or ::: vapour. This review summarises methods for the formation of SAMs upon the ::: most commonly used materials and techniques used for monolayer ::: characterisation. Emphasis will lie on uniform, mixed and functionalised ::: monolayers applied for immobilisation of biological components including ::: (oligo-)nucleotides, proteins, antibodies and receptors as well as ::: polymers. The application of SAMs in today’s research, together with ::: some applications will be discussed.
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paper_title: Optical biosensors for food quality and safety assurance—a review

paper_content:

Food quality and safety is a scientific discipline describing handling, preparation and storage of food in ways that prevent food borne illness. Food serves as a growth medium for microorganisms that can be pathogenic or cause food spoilage. Therefore, it is imperative to have stringent laws and standards for the preparation, packaging and transportation of food. The conventional methods for detection of food contamination based on culturing, colony counting, chromatography and immunoassay are tedious and time consuming while biosensors have overcome some of these disadvantages. There is growing interest in biosensors due to high specificity, convenience and quick response. Optical biosensors show greater potential for the detection of pathogens, pesticide and drug residues, hygiene monitoring, heavy metals and other toxic substances in the food to check whether it is safe for consumption or not. This review focuses on optical biosensors, the recent developments in the associated instrumentation with emphasis on fiber optic and surface plasmon resonance (SPR) based biosensors for detecting a range of analytes in food samples, the major advantages and challenges associated with optical biosensors. It also briefly covers the different methods employed for the immobilization of bio-molecules used in developing biosensors.
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paper_title: Free-Energy-Driven Lock/Open Assembly-Based Optical DNA Sensor for Cancer-Related microRNA Detection with a Shortened Time-to-Result

paper_content:

Quantification of cancer biomarker microRNAs (miRs) by exquisitely designed biosensors with a short time-to-result is of great clinical significance. With immobilized capture probes (CPs) and fluorescent-labeled signal probes (SPs), surface-involved sandwich-type (SST) biosensors serve as powerful tools for rapid, highly sensitive, and selective detection of miR in complex matrices as opposed to the conventional techniques. One key challenge for such SST biosensors is the existence of false-negative signals when the amount of miRs exceeds SPs in solution phase for a surface with a limited number of CP. To meet this challenge, a dynamic lock/open DNA assembly was designed to rationally program the pathway for miR/SP hybrids. Based on secondary structure analysis and free-energy assessment, a “locker” strand that partially hybridizes with target miR by two separated short arms was designed to stabilize target miR, preventing possible false-negative signals. The strategy was demonstrated on a fiber-based flu...
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paper_title: Facile screening of potential xenoestrogens by an estrogen receptor-based reusable optical biosensor.

paper_content:

The apparent increase in hormone-induced cancers and disorders of the reproductive tract has led to a growing demand for new technologies capable of screening xenoestrogens. We reported an estrogen receptor (ER)-based reusable fiber biosensor for facile screening estrogenic compounds in environment. The bioassay is based on the competition of xenoestrogens with 17β-estradiol (E2) for binding to the recombinant receptor of human estrogen receptor α (hERα) protein, leaving E2 free to bind to fluorophore-labeled anti-E2 monoclonal antibody. Unbound anti-E2 antibody then binds to the immobilized E2-protein conjugate on the fiber surface, and is detected by fluorescence emission induced by evanescent field. As expected, the stronger estrogenic activity of xenoestrogen would result in the weaker fluorescent signal. Three estrogen-agonist compounds, diethylstilbestrol (DES), 4-n-nonylphenol (NP) and 4-n-octylphenol (OP), were chosen as a paradigm for validation of this assay. The rank order of estrogenic potency determined by this biosensor was DES>OP>NP, which were consistent with the published results in numerous studies. Moreover, the E2-protein conjugate modified optical fiber was robust enough for over 300 sensing cycles with the signal recoveries ranging from 90% to 100%. In conclusion, the biosensor is reusable, reliable, portable and amenable to on-line operation, providing a facile, efficient and economical alternative to screen potential xenoestrogens in environment.
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paper_title: Non-antibody protein-based biosensors

paper_content:

Biosensors that depend on a physical or chemical measurement can be adversely affected by non-specific interactions. For example, a biosensor designed to measure specifically the levels of a rare analyte can give false positive results if there is even a small amount of interaction with a highly abundant but irrelevant molecule. To overcome this limitation, the biosensor community has frequently turned to antibody molecules as recognition elements because they are renowned for their exquisite specificity. Unfortunately antibodies can often fail when immobilised on inorganic surfaces, and alternative biological recognition elements are needed. This article reviews the available non-antibody-binding proteins that have been successfully used in electrical and micro-mechanical biosensor platforms.
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paper_title: Plasmonic Fiber Optic Refractometric Sensors: From Conventional Architectures to Recent Design Trends

paper_content:

Surface Plasmon Resonance (SPR) fiber sensor research has grown since the first demonstration over 20 year ago into a rich and diverse field with a wide range of optical fiber architectures, plasmonic coatings, and excitation and interrogation methods. Yet, the large diversity of SPR fiber sensor designs has made it difficult to understand the advantages of each approach. Here, we review SPR fiber sensor architectures, covering the latest developments from optical fiber geometries to plasmonic coatings. By developing a systematic approach to fiber-based SPR designs, we identify and discuss future research opportunities based on a performance comparison of the different approaches for sensing applications.
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paper_title: Small biomolecule immunosensing with plasmonic optical fiber grating sensor.

paper_content:

This study reports on the development of a surface plasmon resonance (SPR) optical fiber biosensor based on tilted fiber Bragg grating technology for direct detection of small biomarkers of interest for lung cancer diagnosis. Since SPR principle relies on the refractive index modifications to sensitively detect mass changes at the gold coated surface, we have proposed here a comparative study in relation to the target size. Two cytokeratin 7 (CK7) samples with a molecular weight ranging from 78 kDa to 2.6 kDa, respectively CK7 full protein and CK7 peptide, have been used for label-free monitoring. This work has first consisted in the elaboration and the characterization of a robust and reproducible bioreceptor, based on antibody/antigen cross-linking. Immobilized antibodies were then utilized as binding agents to investigate the sensitivity of the biosensor towards the two CK7 antigens. Results have highlighted a very good sensitivity of the biosensor response for both samples diluted in phosphate buffer with a higher limit of detection for the larger CK7 full protein. The most groundbreaking nature of this study relies on the detection of small biomolecule CK7 peptides in buffer and in the presence of complex media such as serum, achieving a limit of detection of 0.4 nM.
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paper_title: Polarized spectral combs probe optical fiber surface plasmons.

paper_content:

The high-order cladding modes of conventional single mode fiber come in semi-degenerate pairs corresponding to mostly radially or mostly azimuthally polarized light. Using tilted fiber Bragg gratings to excite these mode families separately, we show how plasmonic coupling to a thin gold coating on the surface of the fiber modifies the effective indices of the modes differently according to polarization and to mode order. In particular, we show the existence of a single “apolarized” grating resonance, with equal effective index for all input polarization states. This special resonance provides direct evidence of the excitation of a surface plasmon on the metal surface but also an absolute wavelength reference that allows for the precise localization of the most sensitive resonances in refractometric and biochemical sensing applications. Two plasmon interrogation methods are proposed, based on wavelength and amplitude measurements. Finally, we use a biotin-streptavidin biomolecular recognition experiment to demonstrate that differential spectral transmission measurements of a fine comb of cladding mode resonances in the vicinity of the apolarized resonance provide the most accurate method to extract information from plasmon-assisted Tilted fiber Bragg gratings, down to pM concentrations and at least 10−5 refractive index changes.
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paper_title: Surface plasmon resonance sensor interrogation with a double-clad fiber coupler and cladding modes excited by a tilted fiber Bragg grating.

paper_content:

We present a novel optical fiber surface plasmon resonance (SPR) sensor scheme using reflected guided cladding modes captured by a double-clad fiber coupler and excited in a gold-coated fiber with a tilted Bragg grating. This new interrogation approach, based on the reflection spectrum, provides an improvement in the operating range of the device over previous techniques. The device allows detection of SPR in the reflected guided cladding modes and also in the transmitted spectrum, allowing comparison with standard techniques. The sensor has a large operating range from 1.335 to 1.432 RIU, and a sensitivity of 510.5 nm/RIU. The device shows strong dependence on the polarization state of the guided core mode which can be used to turn the SPR on or off.
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paper_title: Tilted Fiber Bragg Grating Sensor with Graphene Oxide Coating for Humidity Sensing

paper_content:

In this study, we propose a tilted fiber Bragg grating (TFBG) humidity sensor fabricated using the phase mask method to produce a TFBG that was then etched with five different diameters of 20, 35, 50, 55 and 60 μm, after which piezoelectric inkjet technology was used to coat the grating with graphene oxide. According to the experimental results, the diameter of 20 μm yielded the best sensitivity. In addition, the experimental results showed that the wavelength sensitivity was −0.01 nm/%RH and the linearity was 0.996. Furthermore, the measurement results showed that when the relative humidity was increased, the refractive index of the sensor was decreased, meaning that the TFBG cladding mode spectrum wavelength was shifted. Therefore, the proposed graphene oxide film TFBG humidity sensor has good potential to be an effective relative humidity monitor.
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paper_title: Miniaturized Long-Period Fiber Grating Assisted Surface Plasmon Resonance Sensor

paper_content:

This paper presents the design and fabrication of a fiber-optic refractive index (RI) sensor. The novel concept employs a long-period fiber grating (LPG) to achieve surface plasmon resonance (SPR) of a single cladding mode at the gold-coated tip of a single-mode fiber. The sensor combines a high level of sensitivity, a miniaturized sensing area and simple intensity-based interrogation, and is intended for biosensing using portable point-of-care devices or highly integrated lab-on-chip systems.
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paper_title: Label-Free Detection of Cancer Biomarkers Using an In-Line Taper Fiber-Optic Interferometer and a Fiber Bragg Grating

paper_content:

A compact and label-free optical fiber sensor based on a taper interferometer cascaded with a fiber Bragg grating (FBG) is proposed and experimentally demonstrated for detection of a breast cancer biomarker (HER2). The tapered fiber-optic interferometer is extremely sensitive to the ambient refractive index (RI). In addition, being insensitive to the RI variation, the FBG can be applied as a temperature thermometer due to its independent response to the temperature. Surface functionalization to the sensor is carried out to achieve specific targeting of the unlabeled biomarkers. The result shows that the proposed sensor presents a low limit-of-detection (LOD) of 2 ng/mL, enabling its potentials of application in early diagnosis on the breast cancer.
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paper_title: Cladding mode coupling in highly localized fiber Bragg gratings: modal properties and transmission spectra

paper_content:

The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs con- siderably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications.
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paper_title: A Novel Low-Power-Consumption All-Fiber-Optic Anemometer with Simple System Design

paper_content:

A compact and low-power consuming fiber-optic anemometer based on single-walled carbon nanotubes (SWCNTs) coated tilted fiber Bragg grating (TFBG) is presented. TFBG as a near infrared in-fiber sensing element is able to excite a number of cladding modes and radiation modes in the fiber and effectively couple light in the core to interact with the fiber surrounding mediums. It is an ideal in-fiber device used in a fiber hot-wire anemometer (HWA) as both coupling and sensing elements to simplify the sensing head structure. The fabricated TFBG was immobilized with an SWCNT film on the fiber surface. SWCNTs, a kind of innovative nanomaterial, were utilized as light-heat conversion medium instead of traditional metallic materials, due to its excellent infrared light absorption ability and competitive thermal conductivity. When the SWCNT film strongly absorbs the light in the fiber, the sensor head can be heated and form a "hot wire". As the sensor is put into wind field, the wind will take away the heat on the sensor resulting in a temperature variation that is then accurately measured by the TFBG. Benefited from the high coupling and absorption efficiency, the heating and sensing light source was shared with only one broadband light source (BBS) without any extra pumping laser complicating the system. This not only significantly reduces power consumption, but also simplifies the whole sensing system with lower cost. In experiments, the key parameters of the sensor, such as the film thickness and the inherent angle of the TFBG, were fully investigated. It was demonstrated that, under a very low BBS input power of 9.87 mW, a 0.100 nm wavelength response can still be detected as the wind speed changed from 0 to 2 m/s. In addition, the sensitivity was found to be -0.0346 nm/(m/s) under the wind speed of 1 m/s. The proposed simple and low-power-consumption wind speed sensing system exhibits promising potential for future long-term remote monitoring and on-chip sensing in practical applications.
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paper_title: Analysis of the Characteristics of PVA-Coated LPG-Based Sensors to Coating Thickness and Changes in the External Refractive Index

paper_content:

Recent research has shown that the transmission spectrum of a long period grating (LPG) written into an optical fiber is sensitive to the thickness and the refractive index (RI) of a thin layer deposited on it, a concept which forms the basis of a number of optical fiber sensors. The research reported herein is focused, in particular, on sensors of this type to create a design with an optimized thickness of the deposited layers of polyvinyl alcohol (PVA) on LPGs forming the basis of a highly sensitive probe. In creating such sensors, the dip-coating technique is used to minimize deleterious effects in the attenuation bands resulting from the inhomogeneity of the coated surface. In this paper, LPGs with different coating thicknesses are uniformly coated with thin layers of PVA and submerged in oils of known RI over the range from 1.30 to 1.70, to create effective RI probes. It is observed that when the coating thickness reaches a particular value that enables a substantial redistribution of the optical power within the overlay, a maximum sensitivity of the sensor can be achieved, even when the overlay has a RI higher than that of the cladding. The experimental results obtained through the characterization of the devices developed are shown to be in good agreement with the results of a theoretical model.
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paper_title: KLT-Based Interrogation Technique for FBG Multiplexed Sensor Tracking

paper_content:

The Karhunen–Loeve transform (KLT) is used to retrieve the wavelength information of several fiber Bragg gratings (FBGs) that are acting as a multiplexed sensor. The modulated light of a broadband source is launched to the FBG cascade in order to capture the electrical frequency response of the system. Thanks to dispersive media, the wavelengths of the FBGs are mapped in radio-frequency delays. Wavelength changes are determined by the amplitude change of the samples in the impulse response, a change which is followed by the eigenvalue calculated by the KLT routine. The use of the KLT routine reduces by three orders of magnitude the amount of points needed to have a subdegree resolution in temperature sensing, while keeping the accuracy almost intact.
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paper_title: Fiber grating assisted surface plasmon resonance for biochemical and electrochemical sensing

paper_content:

Surface Plasmon resonance (SPR) optical fiber sensors can be used as a cost-effective and relatively simple-toimplement alternative to well established bulky prism configurations for in-situ high sensitivity biochemical and electrochemical measurements. The miniaturized size and remote operation ability offer them a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo. Grating-assisted and polarization control are two key properties of fiber-optic SPR sensors to achieve unprecedented sensitivities and limits of detection. The biosensor configuration presented here utilizes a nano-scale metal-coated tilted fiber Bragg grating (TFBG) imprinted in a commercial single mode fiber core with no structural modifications. Such sensor provides an additional resonant mechanism of high-density narrow cladding mode spectral combs that overlap with the broader absorption of the surface Plasmon for high accuracy interrogation. In this talk, we briefly review the principle, characterization and implementation of plasmonic TFBG sensors, followed by our recent developments of the “surface” and “localized” affinity studies of the biomolecules for real life problems, the electrochemical actives of electroactive biofilms for clean energy resources, and ultra-highly sensitive gas detection.
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paper_title: A novel immunosensor based on excessively tilted fiber grating coated with gold nanospheres improves the detection limit of Newcastle disease virus.

paper_content:

Abstract A novel immunosensor for detecting Newcastle disease virus (NDV) was developed using excessively tilted fiber grating (Ex-TFG) coated with gold nanospheres (AuNs). AuNs were coated on the Ex-TFG surface via Au–S bonds using 3-mercaptopropyltrimethoxysilane (MPTMS), and the activated staphylococcal protein A (SPA) was linked to AuNs by covalent bonds via cysteamine. AuNs greatly enhanced the impact of the analyte on the fiber cladding mode through the local surface Plasmon resonance (LSPR) effect, thus improving the detection limit and sensitivity of the immunosensor. Meanwhile, SPA enhanced the bioactivity of anti-NDV monoclonal antibodies (MAbs), thus promoting the effectiveness of specific binding events on the fiber surface. Immunoassays were performed by monitoring the resonance wavelength shift of the proposed sensor under NDV samples containing different particle amounts. Specificity was assessed, and clinical tests for NDV were performed by contrast experiments. Experimental results showed that the detection limit for NDV was about 5~10 times improved compared to that of reference Ex-TFG without AuN treatment. Moreover, the novel biosensor was reusable and could potentially be applied in clinic.
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paper_title: Graphene-Based Long-Period Fiber Grating Surface Plasmon Resonance Sensor for High-Sensitivity Gas Sensing

paper_content:

A graphene-based long-period fiber grating (LPFG) surface plasmon resonance (SPR) sensor is proposed. A monolayer of graphene is coated onto the Ag film surface of the LPFG SPR sensor, which increases the intensity of the evanescent field on the surface of the fiber and thereby enhances the interaction between the SPR wave and molecules. Such features significantly improve the sensitivity of the sensor. The experimental results demonstrate that the sensitivity of the graphene-based LPFG SPR sensor can reach 0.344 nm%-1 for methane, which is improved 2.96 and 1.31 times with respect to the traditional LPFG sensor and Ag-coated LPFG SPR sensor, respectively. Meanwhile, the graphene-based LPFG SPR sensor exhibits excellent response characteristics and repeatability. Such a SPR sensing scheme offers a promising platform to achieve high sensitivity for gas-sensing applications.
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paper_title: Optical hydrogen sensor based on etched fiber Bragg grating sputtered with Pd/Ag composite film

paper_content:

Abstract A novel fiber optical fiber hydrogen sensor based on etched fiber Bragg grating coated with Pd/Ag composite film is proposed in this paper. Pd/Ag composite films were deposited on the side-face of etched fiber Bragg grating (FBG) as sensing elements by magnetron sputtering process. The atomic ratio of the two metals in Pd/Ag composite film is controlled at Pd:Ag = 76:24. Compared to standard FBG coated with same hydrogen sensitive film, etched FBG can significantly increase the sensor’s sensitivity. When hydrogen concentrations are 4% in volume percentage, the wavelength shifts of FBG-125 μm, FBG-38 μm and FBG-20.6 μm are 8, 23 and 40 pm respectively. The experimental results show the sensor’s hydrogen response is reversible, and the hydrogen sensor has great potential in hydrogen’s measurement.
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paper_title: Cancer biomarker sensing using packaged plasmonic optical fiber gratings: Towards in vivo diagnosis.

paper_content:

This work presents the development of an innovative plasmonic optical fiber (OF) immunosensor for the detection of cytokeratin 17 (CK17), a biomarker of interest for lung cancer diagnosis. The development of this sensing platform is such that it can be assessed in non-liquid environments, demonstrating that a surface plasmon resonance (SPR) can be excited in this case. For this purpose, detections have been first carried out on CK17 encapsulated in gel matrix in the aim of mimicking tissue samples. Gold-coated OF immunosensors were embedded in a specifically designed packaging providing enough stiffness to penetrate into soft matters. Resulting reflected spectra have revealed, for the first time, the presence of a stable SPR signal recorded in soft matters. Experiments conducted to detect CK17 trapped in a porous polyacrylamide gel matrix have highlighted the specific and selective biosensor response towards the target protein. Finally, the packaged OF immunosensor has been validated by a preliminary test on human lung biopsy, which has confirmed the ex-vivo CK17 detection. Consequently, this work represents an important milestone towards the detection of biomarkers in tissues, which is still a clinical challenge for minimally-invasive in vivo medical diagnosis.
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paper_title: Optical bio-sensing devices based on etched fiber Bragg gratings coated with carbon nanotubes and graphene oxide along with a specific dendrimer

paper_content:

We demonstrate that etched fiber Bragg gratings (eFBGs) coated with single walled carbon nanotubes (SWNTs) and graphene oxide (GO) are highly sensitive and accurate biochemical sensors. Here, for detecting protein concanavalin A (Con A), mannose-functionalized poly(propyl ether imine) (PETIM) dendrimers (DMs) have been attached to the SWNTs (or GO) coated on the surface modified eFBG. The dendrimers act as multivalent ligands, having specificity to detect lectin Con A. The specificity of the sensor is shown by a much weaker response (factor of similar to 2500 for the SWNT and similar to 2000 for the GO coated eFBG) to detect non specific lectin peanut agglutinin. DM molecules functionalized GO coated eFBG sensors showed excellent specificity to Con A even in the presence of excess amount of an interfering protein bovine serum albumin. The shift in the Bragg wavelength (Delta lambda(B)) with respect to the lambda(B) values of SWNT (or GO)-DM coated eFBG for various concentrations of lectin follows Langmuir type adsorption isotherm, giving an affinity constant of similar to 4 x 10(7) M-1 for SWNTs coated eFBG and similar to 3 x 10(8) M-1 for the GO coated eFBG. (C) 2014 Elsevier B.V. All rights reserved.
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paper_title: [INVITED] Tilted fiber grating mechanical and biochemical sensors ☆

paper_content:

The tilted fiber Bragg grating (TFBG) is a new kind of fiber-optic sensor that possesses all the advantages of well-established Bragg grating technology in addition to being able to excite cladding modes resonantly. This device opens up a multitude of opportunities for single-point sensing in hard-to-reach spaces with very controllable cross-sensitivities, absolute and relative measurements of various parameters, and an extreme sensitivity to materials external to the fiber without requiring the fiber to be etched or tapered. Over the past five years, our research group has been developing multimodal fiber-optic sensors based on TFBG in various shapes and forms, always keeping the device itself simple to fabricate and compatible with low-cost manufacturing. This paper presents a brief review of the principle, fabrication, characterization, and implementation of TFBGs, followed by our progress in TFBG sensors for mechanical and biochemical applications, including one-dimensional TFBG vibroscopes, accelerometers and micro-displacement sensors; two-dimensional TFBG vector vibroscopes and vector rotation sensors; reflective TFBG refractometers with in-fiber and fiber-to-fiber configurations; polarimetric and plasmonic TFBG biochemical sensors for in-situ detection of cell, protein and glucose.
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paper_title: Plasmonic nanoshell functionalized etched fiber Bragg gratings for highly sensitive refractive index measurements

paper_content:

A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 μm, and therefore offers the possibility of a localized refractive index measurement.
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paper_title: Sensitivity-enhanced FBG demodulation system with multi-sideband filtering method

paper_content:

Abstract A multi-sideband filtering demodulation scheme with enhanced high sensitivity was proposed to demodulate weak fiber Bragg grating signals, and a novel data processing method was employed to reduce the influence of the background noise. The relationship of the sensing sensitivity, number of sidebands, and their slope coefficients were researched theoretically, numerically, and experimentally. The results showed that the sensing sensitivity was in proportion to the number of the sidebands and their slope coefficients. The power sensitivities for single and double CWDMs demodulating FBG in the experimental system were 6.960 dB/nm and 15.187 dB/nm, respectively, which were in agreement with the theoretical results. In addition, the data processing method we used to obtain the optical power was experimentally proven to be better than the method of directly measuring the optical power because it reduces the background noise.
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paper_title: Power-referenced refractometer with tilted fiber Bragg grating cascaded by chirped grating

paper_content:

Abstract A power-referenced refractometer operating in reflection mode is proposed and experimentally demonstrated based on a tilted-fiber Bragg grating (TFBG) cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The optical signal reflected by the CFBG passes twice through the TFBG that enhances sensitivity of the refractometer. In addition, the optical signal is propagating all the way in the fiber core so that the extra insertion loss is low. Refractive index measurement with sensitivity up to 597.2 μW/R.I.U. is achieved within the range from 1.333 to ~1.42. The maximum detectable refractive index is ~1.45.
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paper_title: Plasmonic Fiber-Optic Refractometers Based on a High Q-Factor Amplitude Interrogation

paper_content:

A nano-scale gold-coated tilted fiber Bragg grating sensor with clear surface plasmon resonance is proposed and experimentally demonstrated. The transmission spectrum of the sensor provides a fine comb of narrowband resonances that overlap with the broader absorption of the surface plasmon and thus provides a unique tool to measure small shifts of the plasmon and identify the changes of the surrounding refractive index (SRI) with high accuracy. Meanwhile, our proposed sensor provides an amplitude interrogation method for SRI measurement, which is much simpler and cost-effective than the traditional wavelength monitoring. Experimental results obtained in sucrose solutions with different refractive indices ranging from 1.3330 to 1.3410 show that the sensitivities of our proposed sensor are 450 nm/RIU and 2040 dB/RIU by using the wavelength and amplitude interrogation methods, respectively. Furthermore, the Q-factor of amplitude interrogation signal has been increased from $\sim 70$ to $\sim 6000$ by using the narrowband cladding resonance instead of broader SPR absorption.
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paper_title: Narrowband interrogation of plasmonic optical fiber biosensors based on spectral combs

paper_content:

Abstract Gold-coated tilted fiber Bragg gratings can probe surface Plasmon polaritons with high resolution and sensitivity. In this work, we report two configurations to interrogate such plasmonic biosensors, with the aim of providing more efficient alternatives to the widespread spectrometer-based techniques. To this aim, the interrogation is based on measuring the optical power evolution of the cladding modes with respect to surrounding refractive index changes instead of computing their wavelength shift. Both setups are composed of a broadband source and a photodiode and enable a narrowband interrogation around the cladding mode that excites the surface Plasmon resonance. The first configuration makes use of a uniform fiber Bragg grating to filter the broadband response of the source in a way that the final interrogation is based on an intensity modulation measured in transmission. The second setup uses a uniform fiber grating too, but located beyond the sensor and acting as a selective optical mirror, so the interrogation is carried out in reflection. Both configurations are compared, showing interesting differential features. The first one exhibits a very high sensitivity while the second one has an almost temperature-insensitive behavior. Hence, the choice of the most appropriate method will be driven by the requirements of the target application.
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paper_title: Multiplexing of Surface Plasmon Resonance Sensing Devices on Etched Single-Mode Fiber

paper_content:

It is proposed the multiplexing of optical fiber-based surface plasmon resonance (SPR) sensors deployed in a ladder topology, addressed in wavelength by combining each sensor with specific fiber Bragg gratings (FBGs) and considering intensity interrogation. In each branch of the fiber layout, the FBGs are located after the sensor and the peak optical power reflected by the FBGs is a function of the relative spectral position between the SPR sensor and the FBG resonances, with the former dependent on the refractive index of the surrounding medium. The concept is tested for the multiplexing of two SPR sensors fabricated in an etched region of a single-mode fiber showing intrinsic refractive index sensitivity up to 5000 nm/RIU, which translates into a sensitivity of ∼829 dB/RIU from the interrogation approach considered. The obtained refractive index resolution is in the order of 10−4 RIU, and the crosstalk level between the sensors was found negligible.
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paper_title: Fibre optic surface plasmon resonance sensor system designed for smartphones

paper_content:

A fibre optic surface plasmon resonance (SPR) sensor system for smartphones is reported, for the first time. The sensor was fabricated by using an easy-to-implement silver coating technique and by polishing both ends of a 400 µm optical fibre to obtain 45° end-faces. For excitation and interrogation of the SPR sensor system the flash-light and camera at the back side of the smartphone were employed, respectively. Consequently, no external electrical components are required for the operation of the sensor system developed. In a first application example a refractive index sensor was realised. The performance of the SPR sensor system was demonstrated by using different volume concentrations of glycerol solution. A sensitivity of 5.96·10−4 refractive index units (RIU)/pixel was obtained for a refractive index (RI) range from 1.33 to 1.36. In future implementations the reported sensor system could be integrated in a cover of a smartphone or used as a low-cost, portable point-of-care diagnostic platform. Consequently it offers the potential of monitoring a large variety of environmental or point-of-care parameters in combination with smartphones.
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paper_title: Surface Plasmon Resonances in Oriented Silver Nanowire Coatings on Optical Fibers

paper_content:

Silver nanowires 1–3 μm in length and diameters of 0.04–0.05 μm were synthesized by a polyol process and deposited on a single mode optical fiber with the Langmuir–Blodgett technique. For nanowire surface coverage of ∼40% and partial orientation of their long axis obtained by controlling the deposition parameters, the optical properties of the nanowire coating become identical to those of a uniform metal coating obtained by sputtering or evaporation. Excitation of the nanowires by the polarized evanescent field of fiber cladding modes at near-infrared wavelengths near 1.5 μm results in surface plasmon-like resonances in the transmission spectrum of the optical fiber. The polarization-dependent loss (PDL) spectrum of the tilted fiber Bragg grating used to excite the cladding modes shows a pronounced characteristic dip indicative of a plasmon resonance for radially polarized light waves and complete shielding of light for azimuthally polarized light. The PDL dip shifts at a rate of 650 nm/(refractive index ...
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paper_title: Analysis of a plasmonic based optical fiber optrode with phase interrogation

paper_content:

Optical fiber optrodes are attractive sensing devices due to their ability to perform point measurement in remote locations. Mostly, they are oriented to biochemical sensing, quite often supported by fluorescent and spectroscopic techniques, but with the refractometric approach considered as well when the objective is of high measurement performance, particularly when the focus is on enhancing the measurand resolution. In this work, we address this subject, proposing and analyzing the characteristics of a fiber optic optrode relying on plasmonic interaction. A linearly tapered optical fiber tip is covered by a double overlay: the inner one–a silver thin film and over it–a dielectric layer, with this combination allowing to achieve, at a specific wavelength range, surface plasmonic resonance (SPR) interaction sensitive to the refractive index of the surrounding medium. Typically, the interrogation of the SPR sensing structures is performed, considering spectroscopic techniques, but in principle, a far better performance can be obtained, considering the reading of the phase of the light at a specific wavelength located within the spectral plasmonic resonance. This is the approach which is studied here in the context of the proposed optical fiber optrode configuration. The analysis performed shows the combination of a silver inner layer with a dielectric titanium oxide layer with tuned thicknesses enables sensitive phase reading and allows the operation of the fiber optic optrode sensor in the third telecommunication wavelength window.
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paper_title: KLT-Based Interrogation Technique for FBG Multiplexed Sensor Tracking

paper_content:

The Karhunen–Loeve transform (KLT) is used to retrieve the wavelength information of several fiber Bragg gratings (FBGs) that are acting as a multiplexed sensor. The modulated light of a broadband source is launched to the FBG cascade in order to capture the electrical frequency response of the system. Thanks to dispersive media, the wavelengths of the FBGs are mapped in radio-frequency delays. Wavelength changes are determined by the amplitude change of the samples in the impulse response, a change which is followed by the eigenvalue calculated by the KLT routine. The use of the KLT routine reduces by three orders of magnitude the amount of points needed to have a subdegree resolution in temperature sensing, while keeping the accuracy almost intact.
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paper_title: Self-optimized metal coatings for fiber plasmonics by electroless deposition

paper_content:

We present a novel method to prepare optimized metal coatings for infrared Surface Plasmon Resonance (SPR) sensors by electroless plating. We show that Tilted Fiber Bragg grating sensors can be used to monitor in real-time the growth of gold nano-films up to 70 nm in thickness and to stop the deposition of the gold at a thickness that maximizes the SPR (near 55 nm for sensors operating in the near infrared at wavelengths around 1550 nm). The deposited films are highly uniform around the fiber circumference and in spite of some nanoscale roughness (RMS surface roughness of 5.17 nm) the underlying gratings show high quality SPR responses in water.
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paper_title: Theoretical and experimental study of differential group delay and polarization dependent loss of Bragg gratings written in birefringent fiber

paper_content:

In this paper, we completely study the wavelength dependency of differential group delay (DGD) and polarization dependent loss (PDL) for Bragg gratings written in birefringent fibers. Based on the coupled mode theory, we present analytical expressions for the evolution with wavelength of the transmission coefficient, the DGD and the PDL. The wavelength dependencies of these evolutions on the birefringence are then discussed. Experimental results are finally presented for an apodized FBG written in a bow-tie fiber. A very good agreement between theory and experience is reported, confirming the validity of the theoretical analysis.
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paper_title: Polarization-Assisted Fiber Bragg Grating Sensors: Tutorial and Review

paper_content:

Fiber Bragg gratings (FBGs) are inherently sensitive to temperature, axial strain, and pressure, which can be easily measured by a shift of the Bragg wavelength in their reflected/transmitted power spectrum. FBG sensors acquire many more additional sensing modalities and applications when the polarization of the interrogating light is controlled. For the polarization to have an effect, the cylindrical symmetry of the fiber must be broken, either by the structure of the fiber itself, by that of the FBG, or by the perturbation to be measured. Polarization control allows for sensing parameters that are spatially oriented, such as lateral force, bending or twist, and also for measurements of the properties of anisotropic media. Most importantly, polarization control enables high quality all-fiber surface plasmon resonance (SPR) FBG sensors and localized SPR-assisted sensing. This tutorial will cover the theory of polarized measurements in fiber gratings, their experimental implementation, and review a selection of the most important applications.
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paper_title: High resolution interrogation of tilted fiber grating SPR sensors from polarization properties measurement.

paper_content:

The generation of surface plasmon resonances (SPRs) in gold-coated weakly tilted fiber Bragg gratings (TFBGs) strongly depends on the state of polarization of the core guided light. Recently, it was demonstrated that rotating the linear state of polarization of the guided light by 90° with respect to the grating tilt allows to turn the SPR on and off. In this work, we measure the Jones matrix associated to the TFBG transmission properties in order to be able to analyze different polarization-related parameters (i.e. dependency on wavelength of polarization dependent loss and first Stokes parameter). As they contain the information about the SPR, they can be used as a robust and accurate demodulation technique for refractometry purposes. Unlike other methods reported so far, a tight control of the input state of polarization is not required. The maximum error on refractive index measurement has been determined to be ~1 10(-5) refractive index unit (RIU), 5 times better than intensity-based measurements on the same sensors.
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paper_title: Cancer biomarker sensing using packaged plasmonic optical fiber gratings: Towards in vivo diagnosis.

paper_content:

This work presents the development of an innovative plasmonic optical fiber (OF) immunosensor for the detection of cytokeratin 17 (CK17), a biomarker of interest for lung cancer diagnosis. The development of this sensing platform is such that it can be assessed in non-liquid environments, demonstrating that a surface plasmon resonance (SPR) can be excited in this case. For this purpose, detections have been first carried out on CK17 encapsulated in gel matrix in the aim of mimicking tissue samples. Gold-coated OF immunosensors were embedded in a specifically designed packaging providing enough stiffness to penetrate into soft matters. Resulting reflected spectra have revealed, for the first time, the presence of a stable SPR signal recorded in soft matters. Experiments conducted to detect CK17 trapped in a porous polyacrylamide gel matrix have highlighted the specific and selective biosensor response towards the target protein. Finally, the packaged OF immunosensor has been validated by a preliminary test on human lung biopsy, which has confirmed the ex-vivo CK17 detection. Consequently, this work represents an important milestone towards the detection of biomarkers in tissues, which is still a clinical challenge for minimally-invasive in vivo medical diagnosis.
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paper_title: Plasmonic nanoshell functionalized etched fiber Bragg gratings for highly sensitive refractive index measurements

paper_content:

A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 μm, and therefore offers the possibility of a localized refractive index measurement.
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paper_title: Evanescent wave long-period fiber bragg grating as an immobilized antibody biosensor

paper_content:

An immunosensor using a long-period grating (LPG) was used for sensitive detection of antibody−antigen reactions. Goat anti-human IgG (antibody) was immobilized on the surface of the LPG, and detection of specific antibody−antigen binding was investigated. This sensor operates using total internal reflection where an evanescent field interacts with bound antibody immobilized over the grating region. The reaction between antibody and antigen altered the LPG transmission spectrum and was monitored in real time as a change in refractive index, thereby eliminating the need for labeling antigen molecules. Human IgG binding was observed to be concentration dependent over a range of 2−100 μg mL-1, and equilibrium bound antigen levels could be attained in ∼5 min using an initial rate determination. Binding specificity was confirmed using human interleukin-2 and bovine serum albumin as controls, and nonspecific adsorption of proteins did not significantly interfere with detection of binding. Antigen detection in a...
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paper_title: Detecting hybridization of DNA by highly sensitive evanescent field etched core fiber Bragg grating sensors

paper_content:

Highly sensitive fiber Bragg grating sensors were developed by etching away the cladding and part of the core of the fiber and detecting the change of Bragg wavelength due to the change of index of the surrounding medium. A sensitivity of 1394 nm/riu was achieved when the diameter of the grating core was 3.4 /spl mu/m and the index of the surrounding medium was close to the index of the core of the fiber. Assuming a detectable spectral resolution of 0.01 nm realized in the experiment, the sensor achieves a minimum detectable index resolution of 7.2/spl times/10/sup -6/. Higher sensitivity at lower surrounding index was achieved by using higher order modes excited in the Bragg grating region. The use of the fiber Bragg grating sensor was further investigated to detect hybridization of DNA. Single stranded DNA oligonucleotide probes of 20 bases were immobilized on the surface of the fiber grating using relatively common glutarahyldehyde chemistry. Hybridization of complimentary target single strand DNA oligonucleotide was monitored in situ and successfully detected. The demonstrated fiber Bragg grating sensors provide an elegant method to monitor biological changes in an in situ manner, and provide temporal information in a single experiment.
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paper_title: Giant sensitivity of long period gratings in transition mode near the dispersion turning point: an integrated design approach

paper_content:

We report an original design approach based on the modal dispersion curves for the development of long period gratings in transition mode near the dispersion turning point exhibiting ultrahigh refractive index sensitivity. The theoretical model predicting a giant sensitivity of 9900 nm per refractive index unit in a watery environment was experimentally validated with a result of approximately 9100 nm per refractive index unit around an ambient index of 1.3469. This result places thin film coated LPGs as an alternative to other fiber-based technologies for high-performance chemical and biological sensing applications.
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paper_title: Sensitive optical biosensors for unlabeled targets: A review

paper_content:

This article reviews the recent progress in optical biosensors that use the label-free detection protocol, in which biomolecules are unlabeled or unmodified, and are detected in their natural forms. In particular, it will focus on the optical biosensors that utilize the refractive index change as the sensing transduction signal. Various optical label-free biosensing platforms will be introduced, including, but not limited to, surface plasmon resonance, interferometers, waveguides, fiber gratings, ring resonators, and photonic crystals. Emphasis will be given to the description of optical structures and their respective sensing mechanisms. Examples of detecting various types of biomolecules will be presented. Wherever possible, the sensing performance of each optical structure will be evaluated and compared in terms of sensitivity and detection limit.
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paper_title: Manufacturing and Spectral Features of Different Types of Long Period Fiber Gratings: Phase-Shifted, Turn-Around Point, Internally Tilted, and Pseudo-Random

paper_content:

The manufacturing and spectral features of different types of long period fiber gratings (LPFGs), ranging from phase-shifted, turn-around point, and internally tilted gratings, to pseudo-random gratings, are described and discussed in detail. LPFGs were manufactured on boron-germanium co-doped photosensitive optical fibers with the point-by-point technique using an excimer KrF laser operating at 248 nm. The developed experimental setup to manufacture high-quality LPFGs was designed to totally customize any type of gratings with the possibility of setting different parameters, such as the grating period (or pitch), the number of grating planes, the number of laser shots for each plane, etc. Some important spectral features of the LPFGs’ spectra were taken into account. This allows realizing homemade devices useful in several fiber-based applications, such as optical filtering, coupling systems, random lasers, physical and chemical sensing, and biosensing.
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paper_title: Long period fiber grating nano-optrode for cancer biomarker detection

paper_content:

Abstract We report an innovative fiber optic nano-optrode based on Long Period Gratings (LPGs) working in reflection mode for the detection of human Thyroglobulin (TG), a protein marker of differentiated thyroid cancer. The reflection-type LPG (RT-LPG) biosensor, coated with a single layer of atactic polystyrene (aPS) onto which a specific, high affinity anti-Tg antibody was adsorbed, allowed the label-free detection of Tg in the needle washouts of fine-needle aspiration biopsies, at concentrations useful for pre- and post-operative assessment of the biomarker levels. Analyte recognition and capture were confirmed with a parallel on fiber ELISA-like assay using, in pilot tests, the biotinylated protein and HRP-labeled streptavidin for its detection. Dose-dependent experiments showed that the detection is linearly dependent on concentration within the range between 0 and 4 ng/mL, while antibody saturation occurs for higher protein levels. The system is characterized by a very high sensitivity and specificity allowing the ex-vivo detection of sub ng/ml concentrations of human Tg from needle washouts of fine-needle aspiration biopsies of thyroid nodule from different patients.
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paper_title: Small biomolecule immunosensing with plasmonic optical fiber grating sensor.

paper_content:

This study reports on the development of a surface plasmon resonance (SPR) optical fiber biosensor based on tilted fiber Bragg grating technology for direct detection of small biomarkers of interest for lung cancer diagnosis. Since SPR principle relies on the refractive index modifications to sensitively detect mass changes at the gold coated surface, we have proposed here a comparative study in relation to the target size. Two cytokeratin 7 (CK7) samples with a molecular weight ranging from 78 kDa to 2.6 kDa, respectively CK7 full protein and CK7 peptide, have been used for label-free monitoring. This work has first consisted in the elaboration and the characterization of a robust and reproducible bioreceptor, based on antibody/antigen cross-linking. Immobilized antibodies were then utilized as binding agents to investigate the sensitivity of the biosensor towards the two CK7 antigens. Results have highlighted a very good sensitivity of the biosensor response for both samples diluted in phosphate buffer with a higher limit of detection for the larger CK7 full protein. The most groundbreaking nature of this study relies on the detection of small biomolecule CK7 peptides in buffer and in the presence of complex media such as serum, achieving a limit of detection of 0.4 nM.
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paper_title: Cancer biomarker sensing using packaged plasmonic optical fiber gratings: Towards in vivo diagnosis.

paper_content:

This work presents the development of an innovative plasmonic optical fiber (OF) immunosensor for the detection of cytokeratin 17 (CK17), a biomarker of interest for lung cancer diagnosis. The development of this sensing platform is such that it can be assessed in non-liquid environments, demonstrating that a surface plasmon resonance (SPR) can be excited in this case. For this purpose, detections have been first carried out on CK17 encapsulated in gel matrix in the aim of mimicking tissue samples. Gold-coated OF immunosensors were embedded in a specifically designed packaging providing enough stiffness to penetrate into soft matters. Resulting reflected spectra have revealed, for the first time, the presence of a stable SPR signal recorded in soft matters. Experiments conducted to detect CK17 trapped in a porous polyacrylamide gel matrix have highlighted the specific and selective biosensor response towards the target protein. Finally, the packaged OF immunosensor has been validated by a preliminary test on human lung biopsy, which has confirmed the ex-vivo CK17 detection. Consequently, this work represents an important milestone towards the detection of biomarkers in tissues, which is still a clinical challenge for minimally-invasive in vivo medical diagnosis.
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paper_title: Small biomolecule immunosensing with plasmonic optical fiber grating sensor.

paper_content:

This study reports on the development of a surface plasmon resonance (SPR) optical fiber biosensor based on tilted fiber Bragg grating technology for direct detection of small biomarkers of interest for lung cancer diagnosis. Since SPR principle relies on the refractive index modifications to sensitively detect mass changes at the gold coated surface, we have proposed here a comparative study in relation to the target size. Two cytokeratin 7 (CK7) samples with a molecular weight ranging from 78 kDa to 2.6 kDa, respectively CK7 full protein and CK7 peptide, have been used for label-free monitoring. This work has first consisted in the elaboration and the characterization of a robust and reproducible bioreceptor, based on antibody/antigen cross-linking. Immobilized antibodies were then utilized as binding agents to investigate the sensitivity of the biosensor towards the two CK7 antigens. Results have highlighted a very good sensitivity of the biosensor response for both samples diluted in phosphate buffer with a higher limit of detection for the larger CK7 full protein. The most groundbreaking nature of this study relies on the detection of small biomolecule CK7 peptides in buffer and in the presence of complex media such as serum, achieving a limit of detection of 0.4 nM.
---
paper_title: Cancer biomarker sensing using packaged plasmonic optical fiber gratings: Towards in vivo diagnosis.

paper_content:

This work presents the development of an innovative plasmonic optical fiber (OF) immunosensor for the detection of cytokeratin 17 (CK17), a biomarker of interest for lung cancer diagnosis. The development of this sensing platform is such that it can be assessed in non-liquid environments, demonstrating that a surface plasmon resonance (SPR) can be excited in this case. For this purpose, detections have been first carried out on CK17 encapsulated in gel matrix in the aim of mimicking tissue samples. Gold-coated OF immunosensors were embedded in a specifically designed packaging providing enough stiffness to penetrate into soft matters. Resulting reflected spectra have revealed, for the first time, the presence of a stable SPR signal recorded in soft matters. Experiments conducted to detect CK17 trapped in a porous polyacrylamide gel matrix have highlighted the specific and selective biosensor response towards the target protein. Finally, the packaged OF immunosensor has been validated by a preliminary test on human lung biopsy, which has confirmed the ex-vivo CK17 detection. Consequently, this work represents an important milestone towards the detection of biomarkers in tissues, which is still a clinical challenge for minimally-invasive in vivo medical diagnosis.
---
paper_title: Small biomolecule immunosensing with plasmonic optical fiber grating sensor.

paper_content:

This study reports on the development of a surface plasmon resonance (SPR) optical fiber biosensor based on tilted fiber Bragg grating technology for direct detection of small biomarkers of interest for lung cancer diagnosis. Since SPR principle relies on the refractive index modifications to sensitively detect mass changes at the gold coated surface, we have proposed here a comparative study in relation to the target size. Two cytokeratin 7 (CK7) samples with a molecular weight ranging from 78 kDa to 2.6 kDa, respectively CK7 full protein and CK7 peptide, have been used for label-free monitoring. This work has first consisted in the elaboration and the characterization of a robust and reproducible bioreceptor, based on antibody/antigen cross-linking. Immobilized antibodies were then utilized as binding agents to investigate the sensitivity of the biosensor towards the two CK7 antigens. Results have highlighted a very good sensitivity of the biosensor response for both samples diluted in phosphate buffer with a higher limit of detection for the larger CK7 full protein. The most groundbreaking nature of this study relies on the detection of small biomolecule CK7 peptides in buffer and in the presence of complex media such as serum, achieving a limit of detection of 0.4 nM.
---
paper_title: Cancer biomarker sensing using packaged plasmonic optical fiber gratings: Towards in vivo diagnosis.

paper_content:

This work presents the development of an innovative plasmonic optical fiber (OF) immunosensor for the detection of cytokeratin 17 (CK17), a biomarker of interest for lung cancer diagnosis. The development of this sensing platform is such that it can be assessed in non-liquid environments, demonstrating that a surface plasmon resonance (SPR) can be excited in this case. For this purpose, detections have been first carried out on CK17 encapsulated in gel matrix in the aim of mimicking tissue samples. Gold-coated OF immunosensors were embedded in a specifically designed packaging providing enough stiffness to penetrate into soft matters. Resulting reflected spectra have revealed, for the first time, the presence of a stable SPR signal recorded in soft matters. Experiments conducted to detect CK17 trapped in a porous polyacrylamide gel matrix have highlighted the specific and selective biosensor response towards the target protein. Finally, the packaged OF immunosensor has been validated by a preliminary test on human lung biopsy, which has confirmed the ex-vivo CK17 detection. Consequently, this work represents an important milestone towards the detection of biomarkers in tissues, which is still a clinical challenge for minimally-invasive in vivo medical diagnosis.
---
paper_title: Plasmonic nanoshell functionalized etched fiber Bragg gratings for highly sensitive refractive index measurements

paper_content:

A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 μm, and therefore offers the possibility of a localized refractive index measurement.
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paper_title: Fiber Bragg grating assisted surface plasmon resonance sensor with graphene oxide sensing layer

paper_content:

Abstract A single mode fiber Bragg grating (FBG) is used to generate Surface Plasmon Resonance (SPR). The uniform gratings of the FBG are used to scatter light from the fiber optic core into the cladding thus enabling the interaction between the light and a thin gold film in order to generate SPR. Applying this technique, the cladding around the FBG is left intact, making this sensor very robust and easy to handle. A thin film of graphene oxide (GO) is deposited over a 45 nm gold film to enhance the sensitivity of the SPR sensor. The gold coated sensor demonstrated high sensitivity of approximately 200 nm/RIU when tested with different concentrations of ethanol in an aqueous medium. A 2.5 times improvement in sensitivity is observed with the GO enhancement compared to the gold coated sensor.
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paper_title: Graphene-controlled fiber Bragg grating and enabled optical bistability.

paper_content:

We report a graphene-assisted all-optical control of a fiber Bragg grating (FBG), which enables in-fiber optical bistability and switching. With an optical pump, a micro-FBG wrapped by graphene evolves into chirped and phase-shifted FBGs, whose characteristic wavelengths and bandwidths could be controlled by the pump power. Optical bistability and multistability are achieved in the controlled FBG based on a shifted Bragg reflection or Fabry–Perot-type resonance, which allow the implementation of optical switching with an extinction ratio exceeding 20 dB and a response time in tens of milliseconds.
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paper_title: Femtosecond Laser Inscribed Bragg Gratings in Low Loss CYTOP Polymer Optical Fiber

paper_content:

We report on the first inscription of fiber Bragg gratings (FBGs) in cyclic transparent optical polymer (CYTOP)-perfluorinated polymer optical fibers (POFs). We have used a direct write method with a femtosecond laser operating in the visible. The FBGs have a typical reflectivity of 70%, a bandwidth of 0.25 nm, a 3-mm length, and an index change of $\sim 10^{-4}$ . The FBGs operate in the $C$ -band, where CYTOP offers key advantages over polymethyl methacrylate optical fibers, displaying significantly lower optical loss in the important near-infrared (NIR) optical communications window. In addition, we note that CYTOP has a far lower affinity for water absorption and a core-mode refractive index that coincides with the aqueous index regime. These properties offer several unique opportunities for POF sensing at NIR wavelengths, such as compatibility with existing optical networks, the potential for POF sensor multiplexing and suitability for biosensing. We demonstrate compatibility with a commercial Bragg grating demodulator.
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paper_title: Challenges in the fabrication of fibre Bragg gratings in silica and polymer microstructured optical fibres

paper_content:

This paper reviews the state-of-the-art of grating fabrication in silica and polymer microstructured optical fibres. It focuses on the difficulties and challenges encountered during photo-inscription of such gratings and more specifically on the effect of the air hole lattice microstructure in the cladding of the fibre on the transverse coupling of the coherent writing light to the core region of the fibre. Experimental and computational quantities introduced thus far to assess the influence of the photonic crystal lattice on grating writing efficiency are reviewed as well, together with techniques that have been proposed to mitigate this influence. Finally, early proposals to adapt the microstructure in view of possibly enhancing multi-photon grating fabrication efficiency are discussed.
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paper_title: Graphene-coated microfiber Bragg grating for high-sensitivity gas sensing.

paper_content:

A graphene coated microfiber Bragg grating (GMFBG) for gas sensing is reported in this Letter. Taking advantage of the surface field enhancement and gas absorption of a GMFBG, we demonstrate an ultrasensitive approach to detect the concentration of chemical gas. The obtained sensitivities are 0.2 and 0.5 ppm for NH3 and xylene gas, respectively, which are tens of times higher than that of a GMFBG without graphene for tiny gas concentration change detection. Experimental results indicate that the GMFBG-based NH3 gas sensor has fast response due to its highly compact structure. Such a miniature fiber-optic element may find applications in high sensitivity gas sensing and trace analysis.
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paper_title: Surface plasmon excitation at near-infrared wavelengths in polymer optical fibers.

paper_content:

We report the first excitation of surface plasmon waves at near-infrared telecommunication wavelengths using polymer optical fibers (POFs) made of poly(methyl methacrylate) (PMMA). For this, weakly tilted fiber-Bragg gratings (TFBGs) have been photo-inscribed in the core of step-index POFs and the fiber coated with a thin gold layer. Surface plasmon resonance is excited with radially polarized modes and is spectrally observed as a singular extinction of some cladding-mode resonances in the transmitted amplitude spectrum of gold-coated TFBGs. The refractometric sensitivity can reach ∼550 nm/RIU (refractive index unit) with a figure of merit of more than 2000 and intrinsic temperature self-compensation. This kind of sensor is particularly relevant to in situ operation.
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paper_title: Plasmonic structure: fiber grating formed by gold nanorods on a tapered fiber.

paper_content:

The authors demonstrated the fabrication of a fiber Bragg grating-like plasmonic nanostructure on the surface of a tapered optical fiber using gold nanorods (GNRs). A multimode optical fiber with core and cladding diameters of 105 and 125 μm, respectively, was used to make a tapered fiber using a dynamic etching process. The tip diameter was ∼100 nm. Light from a laser was coupled to the untapered end of the fiber, which produced a strong evanescent field around the tapered section of the fiber. The gradient force due to the evanescent field trapped the GNRs on the surface of the tapered fiber. The authors explored possible causes of the GNR distribution. The plasmonic structure will be a good candidate for sensing based on surface enhanced Raman scattering.
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paper_title: Fiber-optic anemometer based on single-walled carbon nanotube coated tilted fiber Bragg grating

paper_content:

In this work, a novel and simple optical fiber hot-wire anemometer based on single-walled carbon nanotubes (SWCNTs) coated tilted fiber Bragg grating (TFBG) is proposed and demonstrated. For the hot-wire wind speed sensor design, TFBG is an ideal in-fiber sensing structure due to its unique features. It is utilized as both light coupling and temperature sensing element without using any geometry-modified or uncommon fiber, which simplifies the sensor structure. To further enhance the thermal conversion capability, SWCNTs are coated on the surface of the TFBG instead of traditional metallic materials, which have excellent thermal characteristics. When a laser light is pumped into the sensor, the pump light propagating in the core will be easily coupled into cladding of the fiber via the TFBG and strongly absorbed by the SWCNTs thin film. This absorption acts like a hot-wire raising the local temperature of the fiber, which is accurately detected by the TFBG resonance shift. In the experiments, the sensor’s performances were investigated and controlled by adjusting the inherent angle of the TFBG, the thickness of SWCNTs film, and the input power of the pump laser. It was demonstrated that the developed anemometer exhibited significant light absorption efficiency up to 93%, and the maximum temperature of the local area on the fiber was heated up to 146.1°C under the relatively low pump power of 97.76 mW. The sensitivity of −0.3667 nm/(m/s) at wind speed of 1.0 m/s was measured with the selected 12° TFBG and 1.6 μm film.
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paper_title: Oxides and nitrides as alternative plasmonic materials in the optical range [Invited]

paper_content:

As alternatives to conventional metals, new plasmonic materials offer many advantages in the rapidly growing fields of plasmonics and metamaterials. These advantages include low intrinsic loss, semiconductor-based design, compatibility with standard nanofabrication processes, tunability, and others. Transparent conducting oxides such as Al:ZnO, Ga:ZnO and indium-tin-oxide (ITO) enable many high-performance metamaterial devices operating in the near-IR. Transition-metal nitrides such as TiN or ZrN can be substitutes for conventional metals in the visible frequencies. In this paper we provide the details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications.
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paper_title: Small biomolecule immunosensing with plasmonic optical fiber grating sensor.

paper_content:

This study reports on the development of a surface plasmon resonance (SPR) optical fiber biosensor based on tilted fiber Bragg grating technology for direct detection of small biomarkers of interest for lung cancer diagnosis. Since SPR principle relies on the refractive index modifications to sensitively detect mass changes at the gold coated surface, we have proposed here a comparative study in relation to the target size. Two cytokeratin 7 (CK7) samples with a molecular weight ranging from 78 kDa to 2.6 kDa, respectively CK7 full protein and CK7 peptide, have been used for label-free monitoring. This work has first consisted in the elaboration and the characterization of a robust and reproducible bioreceptor, based on antibody/antigen cross-linking. Immobilized antibodies were then utilized as binding agents to investigate the sensitivity of the biosensor towards the two CK7 antigens. Results have highlighted a very good sensitivity of the biosensor response for both samples diluted in phosphate buffer with a higher limit of detection for the larger CK7 full protein. The most groundbreaking nature of this study relies on the detection of small biomolecule CK7 peptides in buffer and in the presence of complex media such as serum, achieving a limit of detection of 0.4 nM.
---
paper_title: Optical bio-sensing devices based on etched fiber Bragg gratings coated with carbon nanotubes and graphene oxide along with a specific dendrimer

paper_content:

We demonstrate that etched fiber Bragg gratings (eFBGs) coated with single walled carbon nanotubes (SWNTs) and graphene oxide (GO) are highly sensitive and accurate biochemical sensors. Here, for detecting protein concanavalin A (Con A), mannose-functionalized poly(propyl ether imine) (PETIM) dendrimers (DMs) have been attached to the SWNTs (or GO) coated on the surface modified eFBG. The dendrimers act as multivalent ligands, having specificity to detect lectin Con A. The specificity of the sensor is shown by a much weaker response (factor of similar to 2500 for the SWNT and similar to 2000 for the GO coated eFBG) to detect non specific lectin peanut agglutinin. DM molecules functionalized GO coated eFBG sensors showed excellent specificity to Con A even in the presence of excess amount of an interfering protein bovine serum albumin. The shift in the Bragg wavelength (Delta lambda(B)) with respect to the lambda(B) values of SWNT (or GO)-DM coated eFBG for various concentrations of lectin follows Langmuir type adsorption isotherm, giving an affinity constant of similar to 4 x 10(7) M-1 for SWNTs coated eFBG and similar to 3 x 10(8) M-1 for the GO coated eFBG. (C) 2014 Elsevier B.V. All rights reserved.
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paper_title: Plasmonic nanoshell functionalized etched fiber Bragg gratings for highly sensitive refractive index measurements

paper_content:

A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 μm, and therefore offers the possibility of a localized refractive index measurement.
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Title: Plasmonic Optical Fiber-Grating Immunosensing: A Review

Section 1: Introduction
Description 1: Introduce the basic concepts of biosensors, label-free optical biosensors, and their applications. Explain the mechanism of surface plasmon resonance (SPR) and emphasize the advantages of optical fiber-based SPR sensors over traditional configurations.

Section 2: Review of Grating Configurations Used for SPR Excitation
Description 2: Review the different grating configurations used for SPR excitation in optical fibers, including their operating principles and practical benefits.

Section 3: Unclad Uniform Fiber Bragg Gratings
Description 3: Describe unclad uniform fiber Bragg gratings (FBGs), their structure, production techniques, sensitivity to temperature and axial strain, and their application in SPR excitation.

Section 4: Tilted-Fiber Bragg Gratings
Description 4: Discuss tilted-fiber Bragg gratings (TFBGs), their unique features, light coupling mechanisms, and their application for refractometry through surrounding index changes.

Section 5: Excessively Tilted Fiber Gratings
Description 5: Explain excessively tilted fiber gratings (ETFGs), their behavior, production techniques, and their effectiveness in SPR-based sensing.

Section 6: Eccentric Fiber Bragg Gratings
Description 6: Cover eccentric fiber Bragg gratings (EFBGs), focusing on their structure, production methods, and sensitivity to surrounding refractive index changes, comparing them to TFBGs.

Section 7: Long-Period Fiber Gratings
Description 7: Detail long-period fiber gratings (LPFGs), their role as forward-going cladding mode couplers, and their sensitivity to surrounding refractive index changes.

Section 8: Additional Considerations
Description 8: Discuss other important considerations in the development of SPR optical fiber sensors, including metal film coatings, nanoparticles, and the figure of merit (FOM) in sensor performance.

Section 9: Interactions with Metals and Surface Biochemical Functionalization
Description 9: Elaborate on the methods of metal layer deposition on fiber gratings and the subsequent surface functionalization for biosensing applications.

Section 10: Interrogation of Plasmonic Fiber-Grating (Bio)chemical Sensors
Description 10: Evaluate different techniques for interrogating plasmonic fiber-grating chemical and biochemical sensors, including spectral and intensity interrogation methods.

Section 11: Spectrometer-Based Interrogation
Description 11: Explain the principles and setups for spectrometer-based interrogation of plasmonic FBG sensors, and techniques for enhancing measurement resolution.

Section 12: Intensity or Optical Power-Based Interrogation
Description 12: Discuss intensity or optical power-based interrogation techniques and their practical implementation for plasmonic FBG sensors.

Section 13: Other Interrogation Techniques
Description 13: Explore alternative interrogation techniques using polarization analysis and phase measurement, highlighting their advantages in specific applications.

Section 14: Protein and Cell Detection and Quantification
Description 14: Summarize experimental demonstrations of fiber gratings used for protein and cell detection, focusing on their performance and practical implementations.

Section 15: Overview of Plasmonic Fiber-Grating Immunosensors
Description 15: Present a general survey of recent literature on plasmonic fiber-grating immunosensors, including their detection limits and performance indicators.

Section 16: Detection of Cancer Biomarkers
Description 16: Outline the steps involved in developing plasmonic fiber-grating immunosensors for cancer biomarker detection, emphasizing practical implementation in clinical diagnostics.

Section 17: Conclusions
Description 17: Conclude the review by summarizing the current state of fiber grating-based SPR immunosensors, their potential clinical applications, and future research directions.