Inkjet-Printed Wireless Chemiresistive Sensors—A Review
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paper_title: Chemical sensors for portable, handheld field instruments

paper_content:

A review of three commonly used classes of chemical sensor technologies as applicable to implementation in portable, handheld field instruments is presented. Solid-state gas and chemical sensors have long been heralded as the solution to a wide variety of portable chemical sensing system applications. However, advances in optical sensing technology have reduced the size of supporting infrastructure to be competitive with their solid-state counterparts. Optical, solid-state, and hybrid arrays of sensors have application for portable instruments, but issues of insufficient selectivity and sensitivity continue to hamper the widespread introduction of these miniaturized sensors for solving chemical sensing problems in environments outside the laboratory. In this article, we evaluate three of the major classes of compact chemical sensors for portable applications: (solid-state) chemiresistors, (solid-state) CHEMFETs, and (optical) surface plasmon resonance sensors (SPR). These sensors are evaluated and reviewed, according to the current state of research, in terms of their ability to operate at low-power, small-size, and relatively low-cost in environments, with numerous interferents and variable ambient conditions.
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paper_title: One-step inkjet printing of conductive silver tracks on polymer substrates

paper_content:

A one-step process to fabricate conductive features on flexible polymer substrates by inkjet printing an organometallic silver ink directly onto a substrate that is heated to 130 °C is presented. This process led to the immediate sintering of the printed features. The samples were left for 5 min at elevated temperature, which resulted in conductive silver features with a resistivity of eight times the bulk silver value. The combination of this ink and the simultaneous printing/sintering process opens up routes for the direct fabrication of conductive features on common polymer substrates that could be applied, for example, in roll-to-roll production of flexible microelectronic systems.
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paper_title: Printed Motes for IoT Wireless Networks: State of the Art, Challenges, and Outlooks

paper_content:

Although wireless sensor networks (WSNs) have been an active field of research for many years, the modules incorporated by WSN nodes have been mainly manufactured utilizing conventional fabrication techniques that are mostly subtractive, requiring significant amounts of materials and increased chemical waste. The new era of the Internet of Things (IoT) will see the fabrication of numerous small form factor devices for wireless sensing for a plurality of applications, including security, health, and environmental monitoring. The large volume of these devices will require new directions in terms of manufacturing cost and energy efficiency, which will be achieved with redesigned, energy-aware modules. This paper presents the state of the art of printed passives, sensors, energy harvesting modules, actives, and communication front ends, and summarizes the challenges of implementing modules that feature low power consumptions without compromising the low fabrication cost. The plethora of the modules presented herein will facilitate the implementation of low cost, additively manufactured, energy-aware IoT nodes that can be fabricated in large volumes with green processes.
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paper_title: Inkjet printing of organic electronics – comparison of deposition techniques and state-of-the-art developments

paper_content:

Inkjet printing represents a solution dispensing technique that is characterized by its non-contact, material-efficient and reproducible processing. This critical review discusses the use of inkjet printing for organic electronics with a focus on the applicability as well as the drying behavior. The nascent inkjet printing technique is compared to commonly used solution deposition methods, like spin-coating and doctor blading. Basic drying principles of inkjet printed features are understood and fundamental correlations between processing properties and film characteristics can be drawn. It is, however, a long way to gain a full understanding of the complete drying process, since the process conditions as well as the ink properties correlate in a complex relation with the final device properties. Nevertheless, inkjet printing has the potential to evolve as one of the most promising film preparation techniques in the future and has already been applied successfully in combinatorial screening workflows and for the preparation of organic solar cell devices.
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paper_title: Inkjet-printed electrochemical sensors

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Inkjet printing is a digital and non-contact printing technique that is beginning to be used in the manufacture of electronic devices, including electrochemical sensors. This article covers the main works featuring inkjet printing for the production of electrochemical sensors, and focuses especially on the works reported since 2015. The main factor limiting the application of inkjet printing in the manufacture of advanced devices stems mainly from the strict rheological conditions that the ink needs to meet. This is the reason why inkjet is best used in combination with other manufacturing techniques. Given their high degree of maturity and accessibility, home/office printers will likely lead a future wave in the manufacture of electronic devices, particularly at the development and prototyping stages.
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paper_title: Inkjet-printed silver tracks : low temperature curing and thermal stability investigation

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In this contribution the curing behavior and conductivity development of several commercially available silver inks is discussed. In addition, the preparation and characterization of a silver particle ink that shows a curing temperature as low as 80 °C is described. Good to excellent conductivity values of 5 to 56% of bulk silver have been reached by using a very small amount of organic additives without any strong adsorbing groups such as amines, amides or mercapto groups. This low curing temperature opens new routes to produce conductive features on polymeric foils that have a low Tg, like PET. Furthermore, the temperature stability of silver tracks, prepared by inkjet printing different colloidal silver suspensions, was investigated. Hereto, the resistance was on-line measured during heating of the silver tracks, from room temperature to 650 °C.
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paper_title: Properties and Application Perspective of Hybrid Titania-Silica Patterns Fabricated by Inkjet Printing

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A hybrid titania-silica cold-setting sol has been developed that can be deposited onto a wide variety of surfaces without the need for high-temperature fixing and that is suitable for material printing deposition. Thin hybrid titania-silica coatings were patterned onto glass and PET substrates by inkjet printing. Well-defined hybrid titania-silica patterns, with thicknesses ranging from 40 to 400 nm, were fabricated by overprinting 1 to 10 layers. Excellent mechanical, optical, and photocatalytic properties were observed, making the reported material well suited for the fabrication of transparent self-cleaning coatings both on mineral and organic substrates. The printed patterns exhibit photoelectrochemical activity that can be further improved by thermal or photonic curing. A concept of fully printed interdigitated photoelectrochemical cells on flexible PET substrates utilizing the reported hybrid photocatalyst is disclosed as well.
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paper_title: Chemical Sensors : An Introduction for Scientists and Engineers

paper_content:

Fundamentals.- Semiconductor Structures as Chemical Sensors.- Mass-Sensitive Sensors.- Conductivity Sensors and Capacitive Sensors.- Thermometric and Calorimetric Sensors.- Electrochemical Sensors.- Optical Sensors.- Chemical Sensors as Detectors and Indicators.- Sensor Arrays and Micro Total Analysis Systems.
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paper_title: Wireless chemical sensors and biosensors: A review

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Abstract Parallel advances in chemical sensing and wireless communication technologies have sparked the development of wireless chemical sensors (WCSs). These hybrid devices enable wireless determination, collection and distribution of (bio)chemical analytical information in a way that is significantly impacting the Sensor Internet of Things with applications in healthcare, defence, sport, the environment, and agriculture. Challenges and examples for each of the major chemical sensor and major radio technologies related to different application areas are reviewed, including the latest trends emerging from wearable sensors. The review focuses on radio-based WCSs, and finds that ubiquitous wireless technologies such as Bluetooth, ZigBee, radio-frequency identification (RFID) and near-field communication (NFC) are helping make analytical (bio)chemical sensing appropriate and realistic for mass market adoption, in particular for two major classes of chemical sensor – electrochemical and optical. The review provides an in-depth analysis of academic WCS research publications over the ten year period 2007–2017.
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paper_title: Combinatorial approaches toward patterning nanocrystals

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A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals is presented. The process is based on lithographic patterning of organic monolayers that contain a photolabile protection group and are covalently bound to SiO2 surfaces. The process results in spatially and chemically distinct interaction sites on a single substrate. Nanocrystal assembly occurs with a high selectivity on just one type of site. We report on the production of binary, tertiary, and quatemary patterns of nanocrystals. We highlight and discuss the differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, we investigate the assembled structures using photoluminescence and absorption spectroscopy.
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paper_title: Inkjet printed (bio)chemical sensing devices

paper_content:

Inkjet printing has evolved from an office printing application to become an important tool in industrial mass fabrication. In parallel, this technology is increasingly used in research laboratories around the world for the fabrication of entire (bio)chemical sensing devices or single functional elements of such devices. Regularly stated characteristics of inkjet printing making it attractive to replace an alternative material deposition method are low cost, simplicity, high resolution, speed, reproducibility, flexibility, non-contact, and low amount of waste generated. With this review, we give an overview over areas of (bio)chemical sensing device development profiting from inkjet printing applications. A variety of printable functional sensor elements are introduced by examples, and the advantages and challenges of the inkjet method are pointed out. It is demonstrated that inkjet printing is already a routine tool for the fabrication of some (bio)chemical sensing devices, but also that novel applications are being continuously developed. Finally, some inherent limitations of the method and challenges for the further exploitation of this technology are pointed out.
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paper_title: Printed Motes for IoT Wireless Networks: State of the Art, Challenges, and Outlooks

paper_content:

Although wireless sensor networks (WSNs) have been an active field of research for many years, the modules incorporated by WSN nodes have been mainly manufactured utilizing conventional fabrication techniques that are mostly subtractive, requiring significant amounts of materials and increased chemical waste. The new era of the Internet of Things (IoT) will see the fabrication of numerous small form factor devices for wireless sensing for a plurality of applications, including security, health, and environmental monitoring. The large volume of these devices will require new directions in terms of manufacturing cost and energy efficiency, which will be achieved with redesigned, energy-aware modules. This paper presents the state of the art of printed passives, sensors, energy harvesting modules, actives, and communication front ends, and summarizes the challenges of implementing modules that feature low power consumptions without compromising the low fabrication cost. The plethora of the modules presented herein will facilitate the implementation of low cost, additively manufactured, energy-aware IoT nodes that can be fabricated in large volumes with green processes.
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paper_title: Wireless chemical sensors and biosensors: A review

paper_content:

Abstract Parallel advances in chemical sensing and wireless communication technologies have sparked the development of wireless chemical sensors (WCSs). These hybrid devices enable wireless determination, collection and distribution of (bio)chemical analytical information in a way that is significantly impacting the Sensor Internet of Things with applications in healthcare, defence, sport, the environment, and agriculture. Challenges and examples for each of the major chemical sensor and major radio technologies related to different application areas are reviewed, including the latest trends emerging from wearable sensors. The review focuses on radio-based WCSs, and finds that ubiquitous wireless technologies such as Bluetooth, ZigBee, radio-frequency identification (RFID) and near-field communication (NFC) are helping make analytical (bio)chemical sensing appropriate and realistic for mass market adoption, in particular for two major classes of chemical sensor – electrochemical and optical. The review provides an in-depth analysis of academic WCS research publications over the ten year period 2007–2017.
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paper_title: Materials and transducers toward selective wireless gas sensing.

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Wireless sensors are devices in which sensing electronic transducers are spatially and galvanically separated from their associated readout/display components. The main benefits of wireless sensors, as compared to traditional tethered sensors, include the non-obtrusive nature of their installations, higher nodal densities, and lower installation costs without the need for extensive wiring.1–3 These attractive features of wireless sensors facilitate their development toward measurements of a wide range of physical, chemical, and biological parameters of interest. Examples of available wireless sensors include devices for sensing of pH, pressure, and temperature in medical, pharmaceutical, animal health, livestock condition, automotive, and other applications.4–7 ::: ::: Some implementations of wireless gas sensors can be already found in monitoring of analyte gases (e.g. carbon dioxide, water vapor, oxygen, combustibles) in relatively interference-free industrial and indoor environments.8,9 However, unobtrusive wireless gas sensors are urgently needed for many more diverse applications ranging from wearable sensors at the workplace, urban environment, and battlefield, to monitoring of containers with toxic industrial chemicals while in transit, to medical monitoring of hospitalized and in-house patients, to detection of food freshness in individual packages, and to distributed networked sensors over large areas (also known as wireless sensor networks, WSNs). Unfortunately, in these and numerous other practical applications, the available wireless gas sensors fall short of meeting emerging measurement needs in complex environments. In particular, existing wireless gas sensors cannot perform highly selective gas detection in the presence of high levels of interferences and cannot quantitate several components in gas mixtures. ::: ::: 1.1. Diversity Of Monitoring Needs Of Volatiles ::: The monitoring of numerous gases of environmental, industrial, and homeland security concern is needed over the broad range of their regulated exposure concentrations. Figure 1 illustrates the relationships between several regulated exposure levels spanning several orders of magnitude of gas concentrations. Typical examples of concentrations of regulated exposure are presented in Table 110–14 for three groups of toxic volatiles such as volatile organic compounds (VOCs), toxic industrial chemicals (TICs), and chemical warfare agents (CWAs). These examples demonstrate the need for gas sensing capabilities with broad measurement dynamic ranges to cover 2 – 4 orders of magnitude in gas concentrations. ::: ::: ::: ::: Figure 1 ::: ::: Examples of regulated vapor-exposure limits established by different organizations: ::: ::: GPL: General Population Limit, established by USACHPPM – U.S. Army Center for Health Promotion and Preventative Medicine; ::: ::: ::: PEL: Permissible Exposure Limit, established ::: ::: ::: ... ::: ::: ::: ::: ::: ::: Table 1 ::: ::: Examples of regulated concentration levels (in ppm by volume) from three representative classes of toxic gases: VOCs, TICs, and CWAs.10–14 ::: ::: ::: ::: Additional needs for detection of volatiles originate from medical diagnostics, food safety, process monitoring, and other areas.15–17 In those applications, the types and levels of detected volatiles can provide the needed information for further control actions.
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paper_title: Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, and Resolution

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Inkjet printing is viewed as a versatile manufacturing tool for applications in materials fabrication in addition to its traditional role in graphics output and marking. The unifying feature in all these applications is the dispensing and precise positioning of very small volumes of fluid (1–100 picoliters) on a substrate before transformation to a solid. The application of inkjet printing to the fabrication of structures for structural or functional materials applications requires an understanding as to how the physical processes that operate during inkjet printing interact with the properties of the fluid precursors used. Here we review the current state of understanding of the mechanisms of drop formation and how this defines the fluid properties that are required for a given liquid to be printable. The interactions between individual drops and the substrate as well as between adjacent drops are important in defining the resolution and accuracy of printed objects. Pattern resolution is limited by the extent to which a liquid drop spreads on a substrate and how spreading changes with the overlap of adjacent drops to form continuous features. There are clearly defined upper and lower bounds to the width of a printed continuous line, which can be defined in terms of materials and process variables. Finer-resolution features can be achieved through appropriate patterning and structuring of the substrate prior to printing, which is essential if polymeric semiconducting devices are to be fabricated. Low advancing and receding contact angles promote printed line stability but are also more prone to solute segregation or “coffee staining” on drying.
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paper_title: Architecture for the efficient manufacturing by printing of heated, planar, resistive transducers on polymeric foil for gas sensing

paper_content:

Abstract This paper presents a new coplanar architecture to implement the electrodes and the heating element of printed gas sensors. The design includes two electrodes and three contacts. One of the electrodes works as heating element and, simultaneously, drains the sensing current. The electrical modelling of the transducers and its electronic implementation are reported. As this new approach is related to the electrical topology of the system, a reduction in the number of conductive elements will be obtained for any geometry, deposition method, and gas sensing layer. Compared with other coplanar topologies, this approach simplifies the transducers processing to a single printing step, avoiding the use of an inter-dielectric layer between heater and electrodes, and reducing the amount of electrically conductive ink necessary to pattern heater and electrodes of the transducers. This cost-effective architecture and process was applied to the fabrication of heated transducers for metal-oxide gas sensors. The two electrodes were made by inkjet printing of gold on polyimide foil. For the validation of the concept, a Pt loaded WO 3 sensing layer was grown on top of these transducers printed with the proposed topology. When tested against H 2 , diluted in air, their response was as good as the one obtained employing the classical topology. This simple architecture has strong potential for the realization of fully printed resistive gas sensors and can be implemented as well in clean room processed transducers.
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paper_title: All inkjet-printed chemical gas sensors based on CNT/polymer nanocomposites: Comparison between double printed layers and blended single layer

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Gas sensors based on carbon nanotube (CNT)/ polymer nanocomposite were fabricated by a fully inkjet-printing method. The CNT/polymer layers acting as sensing layer were prepared by 2 methods; (i) deposition of separated layer of CNT and polymer to form a composite-like structure at the interface and (ii) deposition of blend CNT and polymer to obtain completely composite layer. Difference of sensor responses was demonstrated. We have found that the sensor with a compositelike layer provides a good response to volatile organic compounds (VOCs) as compared to the sensor obtained from printing CNT/polymer composite. The results have also demonstrated that the capability of the CNT/polymer composite sensor allows detecting VOCs in broad range of concentration.
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paper_title: Monitoring the quality of raw poultry by detecting hydrogen sulfide with printed sensors

paper_content:

Abstract Food quality and safety are controlled by legislation and monitored both by food industry players and regional and national authorities in a food supply chain. The need for more precise estimation of shelf life and faster results from food pathogen tests has resulted in the development of novel food quality sensors. Intelligent food packages are the concept toward traceability and real time monitoring of food. Herein we present the usage of printed and low-cost copper acetate-based sensors for monitoring the quality of raw broiler meat. The sensor operates by detecting hydrogen sulfide (H 2 S) as an end product of the microbial metabolism. The sensors responded to H 2 S at +6 °C accumulated in modified atmosphere, packaged broiler meat. A concept of the sensor platform, in which the sensor is combined with a printed planar coil antenna and printed capacitor to construct a wirelessly readable printed resonance circuit, is also presented. The sensor is suitable for large-scale production, which could make it inexpensive enough to be integrated in a low-cost retail food package.
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paper_title: A Wearable Hydration Sensor with Conformal Nanowire Electrodes

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A wearable skin hydration sensor in the form of a capacitor is demonstrated based on skin impedance measurement. The capacitor consists of two interdigitated or parallel electrodes that are made of silver nanowires (AgNWs) in a polydimethylsiloxane (PDMS) matrix. The flexible and stretchable nature of the AgNW/PDMS electrode allows conformal contact to the skin. The hydration sensor is insensitive to the external humidity change and is calibrated against a commercial skin hydration system on an artificial skin over a wide hydration range. The hydration sensor is packaged into a flexible wristband, together with a network analyzer chip, a button cell battery, and an ultralow power microprocessor with Bluetooth. In addition, a chest patch consisting of a strain sensor, three electrocardiography electrodes, and a skin hydration sensor is developed for multimodal sensing. The wearable wristband and chest patch may be used for low-cost, wireless, and continuous monitoring of skin hydration and other health parameters.
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paper_title: Direct measurement of ammonia in simulated human breath using an inkjet-printed polyaniline nanoparticle sensor

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Abstract A sensor fabricated from the inkjet-printed deposition of polyaniline nanoparticles onto a screen-printed silver interdigitated electrode was developed for the detection of ammonia in simulated human breath samples. Impedance analysis showed that exposure to ammonia gas could be measured at 962 Hz at which changes in resistance dominate due to the deprotonation of the polymer film. Sensors required minimal calibration and demonstrated excellent intra-electrode baseline drift (≤1.67%). Gases typically present in breath did not interfere with the sensor. Temperature and humidity were shown to have characteristic impedimetric and temporal effects on the sensor that could be distinguished from the response to ammonia. While impedance responses to ammonia could be detected from a single simulated breath, quantification was improved after the cumulative measurement of multiple breaths. The measurement of ammonia after 16 simulated breaths was linear in the range of 40–2175 ppbv (27–1514 μg m −3 ) ( r 2 = 0.9963) with a theoretical limit of detection of 6.2 ppbv (4.1 μg m −3 ) (SN −1 = 3).
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paper_title: Electrical Performance Characterization of an Inkjet-Printed Flexible Circuit in a Mobile Application

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Consumer electronic devices tend to become ever smaller and lighter, and one way to reduce their size and weight is to use flexible circuit boards in, e.g., mobile phones, cameras, and laptop computers. Usually, flexible circuits are used in a device body to reduce device size and weight, to place a circuit on a nonflat surface, or to add a movable property in the device structure. Flexible circuits can be wrapped around the device body in a stationary application or to provide a hinge or a joint between moving parts in a dynamic application. There are many reasons for seeking new manufacturing technologies; to produce flexible circuits with cost-efficient materials, environmental values, and fewer manufacturing steps being some of the preferred ones. Inkjet printing is an emerging technology for manufacturing flexible electronics and the number of materials that can be deposited by inkjet is constantly expanding. Fabricating printed circuit boards by inkjet technology differs from the common manufacturing methods in some ways. Because inkjet technology is an additive process, minimal amounts of material is wasted during fabrication, and as no material is taken away, all deposited material goes to create a circuit. In large-scale manufacturing, this could have a positive environmental impact. In addition, digital control of the printing process enables easy and fast modification of circuit board layout files, which is beneficial for prototyping and small-volume manufacturing. This paper focuses on the applicability of inkjet technology for manufacturing flexible circuits. As a demonstrator device, we use a flexible circuit working as a user interface module in a mobile phone, and we report on its design and fabrication and the testing and measurement of its electrical performance. Comparison of electromagnetic compatibility/electrostatic discharge and antenna tests of the original flexible circuit and the inkjet-printed prototype shows that the electrical performance of the inkjet-printed circuit is similar or close to that of the original product.
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paper_title: A printed capacitive--resistive double sensor for toluene and moisture sensing

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This paper presents a flexible printed sensor to detect two different magnitudes: toluene content and relative humidity. This device has been manufactured by using two printing techniques: inkjet printing to define the electrodes and screen printing to deposit the sensitive material. The combination in the same area of two different sensors enables the monitoring of two different gaseous chemicals without interference between them. On one hand, moisture content is associated with changes in the electrical permittivity of the chosen substrate (capacitive effect). On the other hand, toluene concentration is measured through variations in volume of the developed composite (resistive effect). Changes to relative humidity and toluene content have been measured in both capacitive and resistive parts as well as their responses to temperature variations.
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paper_title: Chemical Gas Sensor Based on a Flexible Capacitive Microwave Transducer Associated with a Sensitive Carbon Composite Polymer Film

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This communication presents results on the feasibility of an inkjet printed chemical gas sensor suitable for a real time multi-sensing platform. The prototype targets volatile organic compounds (VOCs) such as ethanol vapor to monitor environmental pollution. The designed and ink-jet printed microwave sensor is presented. Preliminary results have shown the influence of ethanol vapor on the electrical properties of the sensor at microwave frequency range. The sensor’s sensitivity to ethanol vapor has been estimated to −2.48 kHz/ppm. The final aim of this work is to develop a low cost sensor for Internet of Things (IoT) applications.
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paper_title: A novel graphene-based inkjet-printed WISP-enabled wireless gas sensor

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In this paper we demonstrate the design and development of a low-cost, self-powered, wireless sensor solution based on the WISP platform and utilizing thin films produced from environmentally friendly, water-based, inkjet printed graphene oxide (GO) ink. The sensor demonstrates good response to ammonia gas (NH3), yielding a 6% normalized resistance change within 15 minutes after exposure to a concentration of 500 ppm. In addition, excellent recovery time is achieved using the graphene thin films, with over 30% of material recovery observed within 5 minutes without exposure to high temperature or any UV treatments. In addition to reporting the first ever integration of inkjet-printed water soluble GO inks into low cost RF electronics fabricated on flexible substrates, we also bring gas sensing capabilities to RFID tags relying on purely wireless digital transmission of the sensed information. The introduction of mass producible, stable, environmentally friendly, inkjet printable GO on organic paper/Kapton substrates lays the foundation for the development of a wide range of new low-cost, high performance graphene-based devices, such as inkjet-printed diodes, capacitors and transistors.
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paper_title: Inkjet Printing of Silver Nanowires for Stretchable Heaters

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Inkjet printing is a promising technique for large-scale printed flexible and stretchable electronics. However, inkjet printing of silver nanowires (AgNWs) still presents many challenges. In this study, inkjet printing of high-concentration AgNW ink on flexible substrates is demonstrated, and liquid polydimethylsiloxane (PDMS) is then spin-coated on top of the printed AgNWs patterns to form stretchable conductors. We analyzed the relationship between the surface microstructure and electrical property of the stretchable AgNW conductors during various stretching/releasing cycles. Three consecutive stages of the resistance change, including conditioning, equilibrium, and rising stages, can be observed as a result of the morphology change. We also have demonstrated that the inkjet-printed stretchable AgNW conductor can be used as a stretchable heater. All of these characteristics indicate the excellent potential of inkjet printing of AgNWs for developing large-area flexible and stretchable electronics.
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paper_title: Inkjet-printed gold nanoparticle chemiresistors: Influence of film morphology and ionic strength on the detection of organics dissolved in aqueous solution

paper_content:

Abstract The influence of film morphology on the performance of inkjet-printed gold nanoparticle chemiresistors has been investigated. Nanoparticles deposited from a single-solvent system resulted in a “coffee ring”-like structure with most of the materials deposited at the edge. It was shown that the uniformity of the film could be improved if the nanoparticles were deposited from a mixture of solvents comprising N -methyl-2-pyrrolidone and water. Electrical conductivity measurements showed that both “coffee ring” and “flat” films were qualitatively similar suggesting that the films have similar nanoscale structures. To form the functional chemiresistor device, the 4-(dimethylamino)pyridine coating on the nanoparticle was exchanged with 1-hexanethiol to provide a hydrophobic sensing layer. The performance of 1-hexanethiol coated gold nanoparticle chemiresistors to small organic molecules, toluene, dichloromethane and ethanol dissolved in 1 M KCl in regard to changes in impedance and response times was unaffected by the film morphology. For larger hydrocarbons such as octane, the rate of uptake of the analyte into the film was significantly faster when the flatter nanoparticle film was used as opposed to the “coffee ring” film which has a thicker edge. Furthermore, the presence of potassium and chloride ions in the solution media does not significantly affect the impedance of the nanoparticle film at 1 Hz (
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paper_title: Inkjet Printing of Interdigitated Capacitive Chemical Sensors with Reduced Size by the Introduction of a Dielectric Interlayer

paper_content:

A new fabrication strategy is proposed to reduce the size of sensors based on inkjet-printed interdigitated electrodes (IDE). The process is compatible with fabrication at low temperature and can be applied to flexible organic foils. The new fabrication concept consists of the deposition of a thin dielectric layer of parylene-C onto the first electrodes comb prior to the printing of the second one. In this way, the combs are not longer on the same plane, preventing undesired short-circuits and solving the typical problems of definition and yield associated to inkjet printing of IDE devices with small pitch. The proposed strategy results especially relevant for capacitive-based devices, where short-circuits render the device non-functional. Moreover, it permits enhancing their capacitance per surface area. The validation of the exposed process for sensors application has been carried out by functionalizing the IDE structure with a humidity sensing layer, and characterizing the resulting sensor against changes in relative humidity (R.H.). (C) 2012 Elsevier Ltd .... Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.
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paper_title: Enhanced-performance wireless conformal "smart skins" utilizing inkjet-printed carbon-nanostructures

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This paper introduces for the first time the integration of a UHF radio frequency identification (RFID) antenna with reduced graphene oxide (rGO), developed using direct-write techniques and utilizing an RFID chip for chemical gas detection. The module is realized by inkjet printing on a lowcost paper-based substrate, and the RFID tag is designed for the North America UHF RFID band. The electrical impedance of the rGO thin film changes in the presence of very small quantities of certain toxic gases, resulting in a variation of the backscattered power level which is easily detected by the RFID reader to realize reliable wireless toxic gas sensing. The inkjet printed RFID tag demonstrated a change in backscattered power of 9.18% upon exposure of 40 ppm NO2 for 5 minutes.
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paper_title: Study of the correlation between sensing performance and surface morphology of inkjet-printed aqueous graphene-based chemiresistors for NO2 detection

paper_content:

The extremely high sensitivity to the external environment and the high specific surface area, as well as the absence of bulk phenomena that could interfere with the response signal, make graphene highly attractive for the applications in the field of sensing. Among the various methods for producing graphene over large areas, liquid phase exfoliation (LPE) appears to be very promising, especially if combined with inkjet printing (IJP), which offers several advantages, including the selective and controlled deposition of small ink volumes and the versatility of the exploitable inks and substrates. Herein we present a feasibility study of chemiresistive gas sensors inkjet-printed onto paper substrates, in which a LPE graphene suspension dispersed in a water/isopropanol (H2O/IPA) mixture is used as sensing ink. The device performances, in terms of relative conductance variations, upon exposure to NO2 at standard ambient temperature and pressure, are analysed. In addition, we examine the effect of the substrate morphology and, more specifically, of the ink/substrate interaction on the device performances, by comparing the response of different chemiresistors fabricated by dispensing the same suspension also onto Al2O3 and Si/SiO2 substrates and carrying out a supportive atomic force microscopy analysis. The results prove the possibility to produce sensor devices by means of a wholly environmentally friendly, low-cost process that meets the requests coming from the increasing field of paper-based electronics and paving the way towards a flexible, green-by-design mass production.
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paper_title: Inkjet-printed gas sensors: metal decorated WO3 nanoparticles and their gas sensing properties

paper_content:

Inkjet deposition is an attractive technology to localize nanomaterials in an area-selective manner on virtually any kind of surfaces. Great advantages of the method are effective usage of materials, low processing temperatures and few required manufacturing steps, thus enabling rapid prototyping and bulk production with reasonably low cost. A number of different electrical devices such as light emitting diodes, transistors and solar cells have already been demonstrated, reflecting the versatility of inkjet printing. In this paper, we collect the contemporary results on inkjet deposited gas sensors and show examples of such gas sensing devices based on surface modified WO3 nanoparticles for efficient discrimination of various gaseous analytes from sub-ppm up to nearly 0.1% concentration levels in air.
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paper_title: Inkjet-printed line morphologies and temperature control of the coffee ring effect.

paper_content:

We have studied inkjet-printed drops of a conductive polymer. We show how varying drop spacing and temperature lead to several different printed line morphologies and offer a simple geometric explanation for these various forms. Also, by controlling the evaporation profile of drying drops and lines, we demonstrate control of the coffee ring effect by which solute is transferred to the rim. Under appropriate conditions, we are able to enhance or eliminate the coffee ring effect in our drying features.
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paper_title: Influence of the doping method on the sensitivity of Pt-doped screen-printed SnO2 sensors

paper_content:

In this work, we study the influence of the introduction method of Pt atoms on the sensitivity to traces of ethanol of Pt-doped SnO2 sensors. The tin oxide films were obtained by a screen-printing process. Two different methods were employed to introduce Pt atoms on SnO2 films. In the first one, the Pt atoms were added to the screen-printed tin oxide layer by using RF magnetron sputtering and a subsequent thermal treatment. The second method consisted of mixing SnO2 and Pt pastes before the screen-printing process. The different active layers (including un-doped tin oxide) were carefully examined relative to their sensitivity to ethanol at different working temperatures. Sensors prepared by the second method showed sensitivity to ethanol four times higher than one of the sensors prepared by the first method and 12 times higher than un-doped sensors. XPS and scanning electron microscopy (SEM) measurements showed that this behaviour could be associated with the spatial distribution of the doping elements within the tin oxide film. While in Pt-sputtered sensors most of the Pt atoms were found at the surface of the active layer, for the sensors made by mixing Pt and SnO2 pastes, a homogeneous distribution of the Pt atoms was observed. These sensors show high sensitivity and fast response time to ethanol vapours, with a detection limit in the ppb range.
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paper_title: Gold nanoparticle chemiresistor sensors: direct sensing of organics in aqueous electrolyte solution.

paper_content:

A novel chemiresistor sensor for detection of organic analytes in high-conductivity aqueous electrolyte solution is reported. The chemiresistor sensor is based on thin films of gold nanoparticles capped with a 1-hexanethiol monolayer that is inkjet printed onto a microelectrode. In order for a change in nanoparticle film resistance to be measured, the electronic conduction must preferentially occur through the nanoparticle film rather than through the high-conductivity electrolyte solution. This was achieved by miniaturizing the chemiresistor device such that the double layer capacitance of the electrodes in contact with the electrolyte solution gives rise to a significantly larger impedance compared to the nanoparticle film resistance. This system was shown to be sensitive to simple organics dissolved in an aqueous electrolyte solution. The organic analytes, dissolved in the aqueous solution, partition into the hydrophobic nanoparticle film causing the nanoparticle film to swell, resulting in an increase in the low-frequency impedance of the sensor. An increase in the impedance, at 1 Hz, of the gold nanoparticle chemiresistor on exposure to toluene, dichloromethane, and ethanol dissolved in 1 M KCl solution was demonstrated with detection limits of 0.1, 10, and 3000 ppm, respectively. Titration curves over 3 orders of magnitude could be obtained for analytes such as toluene.
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paper_title: Fully inkjet-printed parallel-plate capacitive gas sensors on flexible substrate

paper_content:

Small fully inkjet-printed gas sensors based on capacitive parallel-plate (PP) structures have been realized on flexible plastic foil and characterized. A gas sensing layer was inkjet-printed between inkjet-printed bottom and top silver electrodes. Compared with comb electrode (CE) geometries, PP structures drastically reduce the developing complexity of gas sensors on polymeric foil, avoiding the substrate parasitic signal. Furthermore, the use of porous inkjet-printed metal makes the patterning of complex grids on the top electrode unnecessary, since such porosity permits the analyte to flow into the sensing layer. This low demanding patterning resolution facilitates the miniaturization of the inkjet-printed sensors, introducing significant improvements in their sensing performances, such as sensitivity or response time. The printed sensing devices were characterized against pulses of relative humidity (R.H.) and their performances were analyzed.
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paper_title: A Fully Inkjet-Printed Wireless and Chipless Sensor for CO2 and Temperature Detection

paper_content:

A study on a low-cost wireless fully inkjet-printed chipless sensor on a flexible laminate with three different inks is presented. It is based on two split ring resonator 90° oriented between each other to allow for independent responses on two polarizations. A deposit of a polymer/single walled carbon nanotube composite ink is used to allow for the detection of CO ::: 2 ::: as well as temperature. In this paper, it is shown that the inkjet printing of a polymer-based coating on top of the sensing/reactive deposit can significantly reduce the sensitivity to CO ::: 2 ::: , whereas the temperature sensitivity stays at same. Simulations and experimental results verify the repeatability of this topology.
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paper_title: A Zigbee-based wireless wearable electronic nose using flexible printed sensor array

paper_content:

A wearable electronic nose (e-nose) has been developed by integrating a low cost chemical sensor array with a wireless communication for applications in healthcare. Its sensing unit was fabricated by a fully inkjet-printing technique, comprising eight different sensor elements manufactured by varying printing patterns and sensing materials. These sensors have shown response to a wide variety of complex odors. A wearable e-nose prototype using Zigbee wireless technology was designed as a compact armband for monitoring the axillary odor released from human body. Preliminary results based on principal component analysis (PCA) could classify different odors released from the human body upon various activities.
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paper_title: Low-cost and flexible printed graphene–PEDOT:PSS gas sensor for ammonia detection

paper_content:

Abstract This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene–poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH 3 ) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV–visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of π–π interactions between graphene and PEDOT:PSS. The ink-jet printed graphene–PEDOT:PSS gas sensor exhibits high response and high selectivity to NH 3 in a low concentration range of 25–1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH 3 molecules via π electrons network. The NH 3 -sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted.
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paper_title: Fabrication of interdigitated electrodes by inkjet printing technology for apllication in ammonia sensing

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In this paper interdigitated electrodes for gas sensors were fabricated by inkjet printing technology. Silver electrodes were inkjet printed on Si/SiO2 substrates instead of traditional photolithography method. The inkjet printing parameters to obtain desired dimensions, thickness of the electrodes and distance between the interdigitated electrodes were optimized in this study. The fabricated interdigitated silver electrodes were tested for application in ammonia gas sensors. Conductive polyaniline (PANI) layer was coated on the silver interdigitated electrodes by drop-coating. Ammonia detection of the PANI-coated chips was characterized with a gas measurement system in which humidity and ammonia concentrations were well-controlled. The electrical conductivity of the PANI films coated on the electrodes was measured when the PANI films were exposed to nitrogen and ammonia. The conductivity of the PANI films decreased significantly due to the deprotonation process of PANI upon ammonia expodure. The recovery time was about 15 min by heating up the polymer chip at 60 °C. The results showed that the silver electrodes fabricated by inkjet printing technique could be used as a sensor platform for ammonia detection.
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paper_title: Inkjet printing of chemiresistive sensors based on polymer and carbon nanotube networks

paper_content:

We report about the manufacturing of sensor devices for detection of volatile organic compounds (VOCs) in the air. The sensor comprises three fully inkjet-printed layers of (i) silver interdigitated electrodes (SIDE), (ii) multi-walled carbon nanotubes (MWCNTs) and (iii) poly (styrene-co-maleic acid) partial isobutyl/methyl mixed ester (PSE). We have found that inkjet printing of MWCNT layers and a PSE layer on top yields an interfacial layer leading to the formation of a MWCNTs/PSE composite-like structure. The printed layers act as chemiresistive vapor sensors due to the change of electrical resistance in presence of selected VOCs. The sensor responses to various VOCs were studied by varying the concentration of the chemicals in the air between 50 and 500 ppm. The sensing mechanism can be understood as a fractional volume change of the inkjet-printed polymer that occurs during exposure to VOCs, resulting in a structural alteration of the MWCNT-polymer network. We have found that the sensor response can be defined as a function of the VOC's concentration. To investigate the morphologies of the inkjet-printed thin films, profilometry and scanning electron microscopy (SEM) were employed.
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paper_title: Formation and Stability of Lines Produced by Inkjet Printing

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To produce stable lines with parallel sides through inkjet printing, individual drops are deposited on a surface so that they coalesce; this initial liquid line (or bead) must remain stable until it forms a solid. The stable line width is shown to be bounded by two limits, with the lower bound (minimum line width) determined by the maximum drop spacing for stable coalescence and the upper bound determined by the minimum drop spacing below which a bulging instability occurs. The maximum stable track width is also a function of the velocity at which an inkjet printhead traverses the substrate. These bounds are presented in dimensionless form and are shown to agree well with experiment. To enable easier determination of the stability of an arbitrary ink/substrate combination, both the upper and lower bounds are presented in graphical forms to define a region of bead stability in an appropriate parameter space.
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paper_title: Printed electronics: the challenges involved in printing devices, interconnects, and contacts based on inorganic materials

paper_content:

Printed electronics represent an emerging area of research that promises large markets due to the ability to bypass traditional expensive and inflexible silicon-based electronics to fabricate a variety of devices on flexible substrates using high-throughput printing approaches. This article presents a summary of work to date in the field of printed electronics and the materials chemistry involved. In particular, the focus is upon the use of metal- and metal oxide-containing inks in the preparation of contacts and interconnects. The review discusses the challenges associated with processing these types of inks and ways to successfully obtain the desired features.
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paper_title: Materials and transducers toward selective wireless gas sensing.

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Wireless sensors are devices in which sensing electronic transducers are spatially and galvanically separated from their associated readout/display components. The main benefits of wireless sensors, as compared to traditional tethered sensors, include the non-obtrusive nature of their installations, higher nodal densities, and lower installation costs without the need for extensive wiring.1–3 These attractive features of wireless sensors facilitate their development toward measurements of a wide range of physical, chemical, and biological parameters of interest. Examples of available wireless sensors include devices for sensing of pH, pressure, and temperature in medical, pharmaceutical, animal health, livestock condition, automotive, and other applications.4–7 ::: ::: Some implementations of wireless gas sensors can be already found in monitoring of analyte gases (e.g. carbon dioxide, water vapor, oxygen, combustibles) in relatively interference-free industrial and indoor environments.8,9 However, unobtrusive wireless gas sensors are urgently needed for many more diverse applications ranging from wearable sensors at the workplace, urban environment, and battlefield, to monitoring of containers with toxic industrial chemicals while in transit, to medical monitoring of hospitalized and in-house patients, to detection of food freshness in individual packages, and to distributed networked sensors over large areas (also known as wireless sensor networks, WSNs). Unfortunately, in these and numerous other practical applications, the available wireless gas sensors fall short of meeting emerging measurement needs in complex environments. In particular, existing wireless gas sensors cannot perform highly selective gas detection in the presence of high levels of interferences and cannot quantitate several components in gas mixtures. ::: ::: 1.1. Diversity Of Monitoring Needs Of Volatiles ::: The monitoring of numerous gases of environmental, industrial, and homeland security concern is needed over the broad range of their regulated exposure concentrations. Figure 1 illustrates the relationships between several regulated exposure levels spanning several orders of magnitude of gas concentrations. Typical examples of concentrations of regulated exposure are presented in Table 110–14 for three groups of toxic volatiles such as volatile organic compounds (VOCs), toxic industrial chemicals (TICs), and chemical warfare agents (CWAs). These examples demonstrate the need for gas sensing capabilities with broad measurement dynamic ranges to cover 2 – 4 orders of magnitude in gas concentrations. ::: ::: ::: ::: Figure 1 ::: ::: Examples of regulated vapor-exposure limits established by different organizations: ::: ::: GPL: General Population Limit, established by USACHPPM – U.S. Army Center for Health Promotion and Preventative Medicine; ::: ::: ::: PEL: Permissible Exposure Limit, established ::: ::: ::: ... ::: ::: ::: ::: ::: ::: Table 1 ::: ::: Examples of regulated concentration levels (in ppm by volume) from three representative classes of toxic gases: VOCs, TICs, and CWAs.10–14 ::: ::: ::: ::: Additional needs for detection of volatiles originate from medical diagnostics, food safety, process monitoring, and other areas.15–17 In those applications, the types and levels of detected volatiles can provide the needed information for further control actions.
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paper_title: Ammonia gas sensors ink-jet printed on textile substrates

paper_content:

This work describes the fabrication of ammonia gas sensors on textile substrates by using the reactive inkjet printing for the deposition the polyaniline gas sensing layers. These layers were printed over vacuum deposited gold electrodes designed for the purposes of sensor conductivity analysis. The sensing properties such as sensing linearity, response and recovery times were studied in the range of ammonia gas concentrations from 15 to 100 ppm. The fabricated sensors showed a relevant increase in relative resistance from 0.30 to 14.4 times after exposing to ammonia gas at the concentrations of 15–100 ppm for 120 sec.
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paper_title: Inkjet-printed, flexible, high performance, carbon nanomaterial based sensors for ammonia and DMMP gas detection

paper_content:

In this paper, we report the most sensitive inkjet printed reduced graphene oxide based ammonia sensor as well as the first inkjet printed carbon nanotube based dimethyl methylphosphonate (DMMP) vapor sensor to date. The processes utilized to fabricate the sensors, including substrate surface modification, inclusion of porosity enhancing nanospheres and chemical functionalization, are outlined. The testing set-up and protocols are described and finally the measurement results are shown with displayed sensitivities of 8.5% for 30 ppm of ammonia and 5% for 2.5 ppm of DMMP. These results may set the foundation for fully inkjet printed, conformal, ‘zero-power’ and low cost ubiquitous wireless multi-gas motes.
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paper_title: Fabrication of Polyaniline-Based Gas Sensors Using Piezoelectric Inkjet and Screen Printing for the Detection of Hydrogen Sulfide

paper_content:

This work describes a fully printable polyaniline-copper (II) chloride sensor for the detection of hydrogen sulfide gas. The sensing device is composed of screen printed silver interdigitated electrode (IDE) on a flexible PET substrate with inkjet printed layers of polyaniline and copper (II) chloride. The sensor is employed as a chemiresistor with changes in measured current being correlated with concentration. On exposure to hydrogen sulfide, 2.5 ppmv (parts per million by volume) is clearly detectable with a linear relationship between measured current and concentration over the 10-100 ppmv region. The detection mechanism is discussed with respect to the hydrogen sulfide response, the choice of electrode materials in addition to UV-vis and surface enhanced Raman spectroscopy (SERS) characterization.
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paper_title: Spreading and drying impact on printed pattern accuracy due to phase separation of a colloidal ink

paper_content:

Drop-on-demand inkjet printing relies on the dispensing and precise positioning of very small amount of liquid on substrate before its transformation to a solid. Consequently, many steps are involved during inkjet printing process that must be understood to fabricate accurate patterns. In this work, multicomponent colloidal mixture spreading and drying is investigated. A systematic study has been performed to explain the impact of the jetting frequency, the drop spacing, and the substrate exposure to UV ozone on a colloidal silver ink behavior. It is demonstrated that phase separation between stabilizing agent and colloids can occur during the spreading step. This phenomenon is described and appears to be related to the colloidal nature of the ink. It is shown that it can have a negative impact on pattern accuracy and several routes are given to overcome these issues.
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paper_title: Towards fully printed capacitive gas sensors on flexible PET substrates based on Ag interdigitated transducers with increased stability

paper_content:

Interdigitated capacitive transducers have been inkjet printed onto flexible substrates and optimized for gas sensing applications. Their characteristics have been improved by tuning the annealing/sintering conditions and making use of additional passivation procedures, such as Ag electroplating with Ni or Parylene-C coating of the whole device surface. The as-prepared transducers printed with Ag ink could be utilized in capacitive gas sensing structures up to 70% relative humidity only. Higher humidity levels irreversibly altered the nominal capacitance and conductance of the devices, limiting their practical application. This drawback could be eliminated through the upgrading routes specified above and stable operation over wide temperature ranges has been achieved. From the acquired data a quite accurate description of the sorption processes, involving the printed conducting layers and the supporting material, has been inferred. Furthermore the parasitic sensitivity to test vapours from the class of Volatile Organic Compounds in the concentration range of Threshold Limit Value - Time-Weighted Average as possible interfering analytes has been estimated. Test gas sensors obtained from the optimized transducers by additional coating with gas sensing films poly(ether urethane) successfully passed the laboratory evaluations and seem to be appropriate for use in real devices. (C) 2012 Elsevier B. V. All rights reserved.
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paper_title: A Novel Wearable Electronic Nose for Healthcare Based on Flexible Printed Chemical Sensor Array

paper_content:

A novel wearable electronic nose for armpit odor analysis is proposed by using a low-cost chemical sensor array integrated in a ZigBee wireless communication system. We report the development of a carbon nanotubes (CNTs)/polymer sensor array based on inkjet printing technology. With this technique both composite-like layer and actual composite film of CNTs/polymer were prepared as sensing layers for the chemical sensor array. The sensor array can response to a variety of complex odors and is installed in a prototype of wearable e-nose for monitoring the axillary odor released from human body. The wearable e-nose allows the classification of different armpit odors and the amount of the volatiles released as a function of level of skin hygiene upon different activities.
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paper_title: Inkjet printing of polymer functionalized CNT gas sensor with enhanced sensing properties

paper_content:

Abstract We report on fully inkjet printed carbon nanotubes gas sensors on flexible substrates with improved ethanol sensing characteristics. The sensors were fabricated via two simple steps: printing the electrodes and the carbon nanotubes based - active layer. Carbon nanotubes were functionalized with carboxylic acid (COOH) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). The performance analysis shows a significant enhancement in the sensitivity of the sensor with polymer wrapped nanotubes in comparison with the oxidized nanotubes, with enhancement factors greater than 2.5. Moreover, a remarkable improvement in the sensor response and recovery times after polymer functionalization is noted which bodes well for fully printable environmental sensors on flexible plastic circuits.
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paper_title: Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres

paper_content:

Abstract Carbon nanofibres decorated with metal nanoparticles were deposited over kapton, a polyimide flexible substrate, onto which array interdigitated electrodes in one side and a common heater in the backside were printed using inkjet printing technique. The control over metal type (as-grown, Au, Pd) and the decoration percentage gave us the possibility to improve the sensors response, and also enhance the selectivity by taking advantage of the different interaction behaviors of tested target gas molecules with different hybrid materials that occur at room temperature.
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paper_title: The performance of inkjet-printed copper acetate based Hydrogen sulfide gas sensor on a flexible plastic substrate - varying ink composition and print density

paper_content:

Abstract Low-cost and robust hydrogen sulfide (H2S) gas sensors can be utilized in different industrial applications. Earlier we have demonstrated an inexpensive wirelessly readable copper acetate based H2S gas sensor which was successfully employed for monitoring the quality of raw poultry. In this study we have thoroughly investigated and optimized the performance of inkjet-printed copper acetate based H2S gas sensor on flexible plastic substrate at room temperature. The effect of ink composition, print density, number of print nozzles and temperature of the substrate on sensor performance was tested. The long term stability of these sensors after exposure to H2S was studied extensively and was optimized as a function of the print density of copper acetate. The conversion of copper acetate to copper sulfide upon reaction with H2S was established by X-ray photoelectron spectroscopy. We believe that the optimized sensor developed in this study with respect to stability, repeatability and material consumption will pave the way for the commercial use of these sensors e.g. in food quality monitoring and environmental applications.
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paper_title: Inkjet printed (bio)chemical sensing devices

paper_content:

Inkjet printing has evolved from an office printing application to become an important tool in industrial mass fabrication. In parallel, this technology is increasingly used in research laboratories around the world for the fabrication of entire (bio)chemical sensing devices or single functional elements of such devices. Regularly stated characteristics of inkjet printing making it attractive to replace an alternative material deposition method are low cost, simplicity, high resolution, speed, reproducibility, flexibility, non-contact, and low amount of waste generated. With this review, we give an overview over areas of (bio)chemical sensing device development profiting from inkjet printing applications. A variety of printable functional sensor elements are introduced by examples, and the advantages and challenges of the inkjet method are pointed out. It is demonstrated that inkjet printing is already a routine tool for the fabrication of some (bio)chemical sensing devices, but also that novel applications are being continuously developed. Finally, some inherent limitations of the method and challenges for the further exploitation of this technology are pointed out.
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paper_title: Carbon-Nanotube Loaded Antenna-Based Ammonia Gas Sensor

paper_content:

Carbon nanotubes (CNTs) have been researched extensively for gas-sensing applications due to their unique electrical, chemical, and structural properties. Single-walled carbon nanotubes (SWNTs) have been predominantly used due to their superior electrical conductivity and higher sensitivity relative to multiwalled CNTs. This paper presents the design and characterization of a novel planar sensor fabricated on paper substrate to detect small concentrations of ammonia gas, using the shift in resonance frequency of a patch antenna as the discriminator. We have investigated three main design issues in depth. First, functionalization of the SWNTs with a polymer is studied in order to enhance the gas detection sensitivity. Second, a thin film of the functionalized SWNT is characterized to create a surface impedance model for the explanation and prediction of the resonance shift due to different gas concentrations. Finally, as a proof of concept, functionalized SWNTs are integrated into a patch antenna design and the return loss is measured in a closed-system environment to show high sensitivity for low concentrations of ammonia gas. The proposed antenna-based wireless gas sensor can be utilized in several applications, given its small form factor, light weight, and little to no power requirements.
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paper_title: Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets

paper_content:

Individual graphene oxide sheets subjected to chemical reduction were electrically characterized as a function of temperature and external electric fields. The fully reduced monolayers exhibited conductivities ranging between 0.05 and 2 S/cm and field effect mobilities of 2−200 cm2/Vs at room temperature. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest that charge transport occurs via variable range hopping between intact graphene islands with sizes on the order of several nanometers. Furthermore, the comparative study of multilayered sheets revealed that the conductivity of the undermost layer is reduced by a factor of more than 2 as a consequence of the interaction with the Si/SiO2 substrate.
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paper_title: Self-Assembled Gold Nanoparticle/Alkanedithiol Films: Preparation, Electron Microscopy, XPS-Analysis, Charge Transport and Vapor-Sensing Properties

paper_content:

Gold nanoparticle/alkanedithiol films were prepared via layer-by-layer self-assembly. For the assembly process, dodecylamine-stabilized Au nanoparticles with an average size of 4 nm and alkanedithiols with different alkylene chain lengths (C6, C9, C12, C16) were used. The thickness of the films was determined by AFM and ranged between 26 and 34 nm. FE-SEM and TEM images indicate that the particle size within the film materials was similar to that of the dodecylamine-stabilized particles used for film preparation. The composition of the films was analyzed by XPS. The absence of the nitrogen signal indicated that the dodecylamine ligands were quantitatively exchanged by alkanedithiol molecules during film assembly. Two sulfur signals were observed, which could be assigned to sulfur bound to gold (S−Au) and to free thiol groups (S−H). As indicated by the relative signal intensities, about 60% of the alkanedithiol molecules were bound with both ends to the nanoparticles, whereas 40% were bound with only one t...
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paper_title: Architecture for the efficient manufacturing by printing of heated, planar, resistive transducers on polymeric foil for gas sensing

paper_content:

Abstract This paper presents a new coplanar architecture to implement the electrodes and the heating element of printed gas sensors. The design includes two electrodes and three contacts. One of the electrodes works as heating element and, simultaneously, drains the sensing current. The electrical modelling of the transducers and its electronic implementation are reported. As this new approach is related to the electrical topology of the system, a reduction in the number of conductive elements will be obtained for any geometry, deposition method, and gas sensing layer. Compared with other coplanar topologies, this approach simplifies the transducers processing to a single printing step, avoiding the use of an inter-dielectric layer between heater and electrodes, and reducing the amount of electrically conductive ink necessary to pattern heater and electrodes of the transducers. This cost-effective architecture and process was applied to the fabrication of heated transducers for metal-oxide gas sensors. The two electrodes were made by inkjet printing of gold on polyimide foil. For the validation of the concept, a Pt loaded WO 3 sensing layer was grown on top of these transducers printed with the proposed topology. When tested against H 2 , diluted in air, their response was as good as the one obtained employing the classical topology. This simple architecture has strong potential for the realization of fully printed resistive gas sensors and can be implemented as well in clean room processed transducers.
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paper_title: All inkjet-printed chemical gas sensors based on CNT/polymer nanocomposites: Comparison between double printed layers and blended single layer

paper_content:

Gas sensors based on carbon nanotube (CNT)/ polymer nanocomposite were fabricated by a fully inkjet-printing method. The CNT/polymer layers acting as sensing layer were prepared by 2 methods; (i) deposition of separated layer of CNT and polymer to form a composite-like structure at the interface and (ii) deposition of blend CNT and polymer to obtain completely composite layer. Difference of sensor responses was demonstrated. We have found that the sensor with a compositelike layer provides a good response to volatile organic compounds (VOCs) as compared to the sensor obtained from printing CNT/polymer composite. The results have also demonstrated that the capability of the CNT/polymer composite sensor allows detecting VOCs in broad range of concentration.
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paper_title: Direct measurement of ammonia in simulated human breath using an inkjet-printed polyaniline nanoparticle sensor

paper_content:

Abstract A sensor fabricated from the inkjet-printed deposition of polyaniline nanoparticles onto a screen-printed silver interdigitated electrode was developed for the detection of ammonia in simulated human breath samples. Impedance analysis showed that exposure to ammonia gas could be measured at 962 Hz at which changes in resistance dominate due to the deprotonation of the polymer film. Sensors required minimal calibration and demonstrated excellent intra-electrode baseline drift (≤1.67%). Gases typically present in breath did not interfere with the sensor. Temperature and humidity were shown to have characteristic impedimetric and temporal effects on the sensor that could be distinguished from the response to ammonia. While impedance responses to ammonia could be detected from a single simulated breath, quantification was improved after the cumulative measurement of multiple breaths. The measurement of ammonia after 16 simulated breaths was linear in the range of 40–2175 ppbv (27–1514 μg m −3 ) ( r 2 = 0.9963) with a theoretical limit of detection of 6.2 ppbv (4.1 μg m −3 ) (SN −1 = 3).
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paper_title: A novel highly-sensitive antenna-based “smart skin” gas sensor utilizing carbon nanotubes and inkjet printing

paper_content:

Carbon nanotubes (CNTs) have attractive features for implementation in wireless sensor nodes due to their small size, light weight and low power requirements, and their ability to be functionalized with conductive polymers for the electronic detection of a range of chemical and biological agents. In this paper, we present a single-walled CNT sensor for gas detection that is printed on paper substrate and integrated with a co-planar RF antenna for potential application as a light-weight wireless sensor node. The CNT thin-film loads the antenna and changes its resonant frequency upon exposure to the gas, the resonant frequency shift then being used as a discriminator for the gas detection in trace quantities. Measurements of a CNT-based sensor when exposed to low levels of ammonia reveal a resonance frequency shift of 300 MHz for a patch antenna centered around 6 GHz. To the authors' knowledge, this is the highest frequency shift reported in literature.
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paper_title: A printed capacitive--resistive double sensor for toluene and moisture sensing

paper_content:

This paper presents a flexible printed sensor to detect two different magnitudes: toluene content and relative humidity. This device has been manufactured by using two printing techniques: inkjet printing to define the electrodes and screen printing to deposit the sensitive material. The combination in the same area of two different sensors enables the monitoring of two different gaseous chemicals without interference between them. On one hand, moisture content is associated with changes in the electrical permittivity of the chosen substrate (capacitive effect). On the other hand, toluene concentration is measured through variations in volume of the developed composite (resistive effect). Changes to relative humidity and toluene content have been measured in both capacitive and resistive parts as well as their responses to temperature variations.
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paper_title: Chemiresistor coatings from Pt- and Au-nanoparticle/nonanedithiol films: sensitivity to gases and solvent vapors

paper_content:

Abstract Chemiresistor coatings are fabricated via layer-by-layer self-assembly using 1,9-nonanedithiol (NT) and dodecylamine-stabilized Pt- or Au-nanoparticles. The film thickness, as determined by atomic force microscopy (AFM), is 66±2 and 31±1 nm, respectively. These film materials show linear current–voltage characteristics and conductivities in the range of ∼10 −3 and ∼10 −4 Ω −1 cm −1 at room temperature. The sensitivity of the films is investigated by dosing them with the gases ammonia (NH 3 ), carbon monoxide (CO) and vapors of water and toluene in the concentration range from 300 ppb to 5000 ppm. To all analytes the films respond with an increase of their resistance. Although the total signal intensities are rather weak, these sensors show a very high signal-to-noise ratio, which enables a detection limit for NH 3 below 100 ppb. The response dynamics and the curvature of the response isotherms indicate that NH 3 and CO bind significantly stronger to the film material than water or toluene. This result is interpreted with the assumption that NH 3 and CO bind to vacant sites on the metal nanoparticle cores, whereas water and toluene interact preferably with the NT linker molecules.
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paper_title: A novel graphene-based inkjet-printed WISP-enabled wireless gas sensor

paper_content:

In this paper we demonstrate the design and development of a low-cost, self-powered, wireless sensor solution based on the WISP platform and utilizing thin films produced from environmentally friendly, water-based, inkjet printed graphene oxide (GO) ink. The sensor demonstrates good response to ammonia gas (NH3), yielding a 6% normalized resistance change within 15 minutes after exposure to a concentration of 500 ppm. In addition, excellent recovery time is achieved using the graphene thin films, with over 30% of material recovery observed within 5 minutes without exposure to high temperature or any UV treatments. In addition to reporting the first ever integration of inkjet-printed water soluble GO inks into low cost RF electronics fabricated on flexible substrates, we also bring gas sensing capabilities to RFID tags relying on purely wireless digital transmission of the sensed information. The introduction of mass producible, stable, environmentally friendly, inkjet printable GO on organic paper/Kapton substrates lays the foundation for the development of a wide range of new low-cost, high performance graphene-based devices, such as inkjet-printed diodes, capacitors and transistors.
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paper_title: Inkjet-printed gold nanoparticle chemiresistors: Influence of film morphology and ionic strength on the detection of organics dissolved in aqueous solution

paper_content:

Abstract The influence of film morphology on the performance of inkjet-printed gold nanoparticle chemiresistors has been investigated. Nanoparticles deposited from a single-solvent system resulted in a “coffee ring”-like structure with most of the materials deposited at the edge. It was shown that the uniformity of the film could be improved if the nanoparticles were deposited from a mixture of solvents comprising N -methyl-2-pyrrolidone and water. Electrical conductivity measurements showed that both “coffee ring” and “flat” films were qualitatively similar suggesting that the films have similar nanoscale structures. To form the functional chemiresistor device, the 4-(dimethylamino)pyridine coating on the nanoparticle was exchanged with 1-hexanethiol to provide a hydrophobic sensing layer. The performance of 1-hexanethiol coated gold nanoparticle chemiresistors to small organic molecules, toluene, dichloromethane and ethanol dissolved in 1 M KCl in regard to changes in impedance and response times was unaffected by the film morphology. For larger hydrocarbons such as octane, the rate of uptake of the analyte into the film was significantly faster when the flatter nanoparticle film was used as opposed to the “coffee ring” film which has a thicker edge. Furthermore, the presence of potassium and chloride ions in the solution media does not significantly affect the impedance of the nanoparticle film at 1 Hz (
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paper_title: Inkjet Printing of Interdigitated Capacitive Chemical Sensors with Reduced Size by the Introduction of a Dielectric Interlayer

paper_content:

A new fabrication strategy is proposed to reduce the size of sensors based on inkjet-printed interdigitated electrodes (IDE). The process is compatible with fabrication at low temperature and can be applied to flexible organic foils. The new fabrication concept consists of the deposition of a thin dielectric layer of parylene-C onto the first electrodes comb prior to the printing of the second one. In this way, the combs are not longer on the same plane, preventing undesired short-circuits and solving the typical problems of definition and yield associated to inkjet printing of IDE devices with small pitch. The proposed strategy results especially relevant for capacitive-based devices, where short-circuits render the device non-functional. Moreover, it permits enhancing their capacitance per surface area. The validation of the exposed process for sensors application has been carried out by functionalizing the IDE structure with a humidity sensing layer, and characterizing the resulting sensor against changes in relative humidity (R.H.). (C) 2012 Elsevier Ltd .... Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.
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paper_title: Inkjet Printed Graphene-Based Chemiresistive Sensors to NO2

paper_content:

In this work, the possibility of manufacturing chemiresistive gas sensing devices by inkjet printing different LPE (Liquid Phase Exfoliation) graphene suspensions, formulated in standard organic solvents or aqueous mixtures, on rigid and flexible substrates has been studied. The sensing film has been obtained by printing a different number of graphene layers, depending on the specific ink/substrate system. The device performances have been investigated upon exposure to different concentrations of NO2 at ambient pressure and temperature, addressing the device-to-device variation as function of the number of printed layers and the base conductance.
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paper_title: Enhanced-performance wireless conformal "smart skins" utilizing inkjet-printed carbon-nanostructures

paper_content:

This paper introduces for the first time the integration of a UHF radio frequency identification (RFID) antenna with reduced graphene oxide (rGO), developed using direct-write techniques and utilizing an RFID chip for chemical gas detection. The module is realized by inkjet printing on a lowcost paper-based substrate, and the RFID tag is designed for the North America UHF RFID band. The electrical impedance of the rGO thin film changes in the presence of very small quantities of certain toxic gases, resulting in a variation of the backscattered power level which is easily detected by the RFID reader to realize reliable wireless toxic gas sensing. The inkjet printed RFID tag demonstrated a change in backscattered power of 9.18% upon exposure of 40 ppm NO2 for 5 minutes.
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paper_title: Sensor Technologies: Healthcare, Wellness and Environmental Applications

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Sensor Technologies: Healthcare, Wellness and Environmental Applications explores the key aspects of sensor technologies, covering wired, wireless, and discrete sensors for the specific application domains of healthcare, wellness and environmental sensing. It discusses the social, regulatory, and design considerations specific to these domains. The book provides an application-based approach using real-world examples to illustrate the application of sensor technologies in a practical and experiential manner. The book guides the reader from the formulation of the research question, through the design and validation process, to the deployment and management phase of sensor applications. The processes and examples used in the book are primarily based on research carried out by Intel or joint academic research programs. Sensor Technologies: Healthcare, Wellness and Environmental Applications provides an extensive overview of sensing technologies and their applications in healthcare, wellness, and environmental monitoring. From sensor hardware to system applications and case studies, this book gives readers an in-depth understanding of the technologies and how they can be applied. I would highly recommend it to students or researchers who are interested in wireless sensing technologies and the associated applications. Dr. Benny Lo Lecturer, The Hamlyn Centre, Imperial College of London This timely addition to the literature on sensors covers the broad complexity of sensing, sensor types, and the vast range of existing and emerging applications in a very clearly written and accessible manner. It is particularly good at capturing the exciting possibilities that will occur as sensor networks merge with cloud-based big data analytics to provide a host of new applications that will impact directly on the individual in ways we cannot fully predict at present. It really brings this home through the use of carefully chosen case studies that bring the overwhelming concept of 'big data' down to the personal level of individual life and health. Dermot Diamond Director, National Centre for Sensor Research, Principal Investigator, CLARITY Centre for Sensor Web Technologies, Dublin City University "Sensor Technologies: Healthcare, Wellness and Environmental Applications takes the reader on an end-to-end journey of sensor technologies, covering the fundamentals from an engineering perspective, introducing how the data gleaned can be both processed and visualized, in addition to offering exemplar case studies in a number of application domains. It is a must-read for those studying any undergraduate course that involves sensor technologies. It also provides a thorough foundation for those involved in the research and development of applied sensor systems. I highly recommend it to any engineer who wishes to broaden their knowledge in this area!" Chris Nugent Professor of Biomedical Engineering, University of Ulster What youll learnThe relevant sensing approaches and the hardware and software components required to capture and interpret sensor data. The importance of regulations governing medical devices. A design methodology for developing and deploying successful home- and community-based technologies, supported by relevant case studies. Health, wellness, and environmental sensing applications and how they work. The challenges and future directions of sensing in these domains. Who this book is for Sensor Technologies: Healthcare, Wellness and Environmental Applicationsis targeted at clinical and technical researchers, engineers, and students who want to understand the current state of the art in sensor applications in these domains. The reader gains a full awareness of the key technical and non-technical challenges that must be addressed in the development of successful end-to-end sensor applications. Real-world examples help give the reader practical insights into the successful development, deployment, and management of sensor applications. The reader will also develop an understanding of the personal, social, and ethical impact of sensor applications, now and in the future.
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paper_title: Interdigitated electrode fabricated by integration of ink-jet printing with electroless plating and its application in gas sensor

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Abstract A method for the fabrication of PEDOT–PSS film modified interdigitated electrode by integrating inkjet printing with electroless plating technique is reported. Palladium nanoparticles stabilized in water by styrene-N-isopropylacrylamide oligomer (St-co-NIPAAm) are inkjet printed onto a flexible PET surface to create the catalytic sites, onto which nickel is subsequently deposited by an electroless plating method to obtain the desired interdigitated electrode pattern. As a first step the influence of the electroless plating conditions (plating temperature, plating time and stabilizer concentration) to prepare the interdigitated electrode are investigated. Then, a galvanic displacement reaction is utilized to deposit a monolayer of gold coating on the nickel film surface to increase the electrical conductivity. Finally a layer of PEDOT–PSS thin film is inkjet-printed on top of the gold interdigitated electrode and used as the sensor to detect CO2. The device is exposed to CO2 gas at concentration of 100 ppm at room temperature (300 K) and the obtained results are presented. Based on our proposed mechanism for the CO2 sensing, the PEDOT–PSS modified interdigitated electrode has good potential as a novel sensor for polar gases and vapors.
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paper_title: Vapor sensitivity of networked gold nanoparticle chemiresistors : Importance of flexibility and resistivity of the interlinkage

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In this study we investigated the response behavior of chemiresistors made from differently interlinked networks of gold nanoparticles. Our results show that the degree of flexibility and conductivity of the interlinkage between the nanoparticles has a profound impact on the response characteristics of such sensors. 1,12-Dodecanedithiol was used as a hydrophobic, flexible linker compound. For comparison, [4]-staffane-3,3‘ ‘‘-dithiol provided a rigid, rodlike linkage, and 4,4‘-terphenyldithiol was used as a rigid linker with enhanced conductivity due to its delocalized aromatic moieties. As determined by AFM, all three sensor coatings had similar thicknesses (∼30 nm), but the degree of interlinkage, as measured by XPS, was significantly higher in the case of the flexible network. All three materials showed linear current−voltage characteristics. The vapor sensitivity was tested by dosing the sensors with toluene, 1-propanol, 4-methyl-2-pentanone, and water in the concentration range 100−5000 ppm at 0% rela...
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paper_title: Influence of the doping method on the sensitivity of Pt-doped screen-printed SnO2 sensors

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In this work, we study the influence of the introduction method of Pt atoms on the sensitivity to traces of ethanol of Pt-doped SnO2 sensors. The tin oxide films were obtained by a screen-printing process. Two different methods were employed to introduce Pt atoms on SnO2 films. In the first one, the Pt atoms were added to the screen-printed tin oxide layer by using RF magnetron sputtering and a subsequent thermal treatment. The second method consisted of mixing SnO2 and Pt pastes before the screen-printing process. The different active layers (including un-doped tin oxide) were carefully examined relative to their sensitivity to ethanol at different working temperatures. Sensors prepared by the second method showed sensitivity to ethanol four times higher than one of the sensors prepared by the first method and 12 times higher than un-doped sensors. XPS and scanning electron microscopy (SEM) measurements showed that this behaviour could be associated with the spatial distribution of the doping elements within the tin oxide film. While in Pt-sputtered sensors most of the Pt atoms were found at the surface of the active layer, for the sensors made by mixing Pt and SnO2 pastes, a homogeneous distribution of the Pt atoms was observed. These sensors show high sensitivity and fast response time to ethanol vapours, with a detection limit in the ppb range.
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paper_title: Gold nanoparticle chemiresistor sensors: direct sensing of organics in aqueous electrolyte solution.

paper_content:

A novel chemiresistor sensor for detection of organic analytes in high-conductivity aqueous electrolyte solution is reported. The chemiresistor sensor is based on thin films of gold nanoparticles capped with a 1-hexanethiol monolayer that is inkjet printed onto a microelectrode. In order for a change in nanoparticle film resistance to be measured, the electronic conduction must preferentially occur through the nanoparticle film rather than through the high-conductivity electrolyte solution. This was achieved by miniaturizing the chemiresistor device such that the double layer capacitance of the electrodes in contact with the electrolyte solution gives rise to a significantly larger impedance compared to the nanoparticle film resistance. This system was shown to be sensitive to simple organics dissolved in an aqueous electrolyte solution. The organic analytes, dissolved in the aqueous solution, partition into the hydrophobic nanoparticle film causing the nanoparticle film to swell, resulting in an increase in the low-frequency impedance of the sensor. An increase in the impedance, at 1 Hz, of the gold nanoparticle chemiresistor on exposure to toluene, dichloromethane, and ethanol dissolved in 1 M KCl solution was demonstrated with detection limits of 0.1, 10, and 3000 ppm, respectively. Titration curves over 3 orders of magnitude could be obtained for analytes such as toluene.
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paper_title: A Fully Inkjet-Printed Wireless and Chipless Sensor for CO2 and Temperature Detection

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A study on a low-cost wireless fully inkjet-printed chipless sensor on a flexible laminate with three different inks is presented. It is based on two split ring resonator 90° oriented between each other to allow for independent responses on two polarizations. A deposit of a polymer/single walled carbon nanotube composite ink is used to allow for the detection of CO ::: 2 ::: as well as temperature. In this paper, it is shown that the inkjet printing of a polymer-based coating on top of the sensing/reactive deposit can significantly reduce the sensitivity to CO ::: 2 ::: , whereas the temperature sensitivity stays at same. Simulations and experimental results verify the repeatability of this topology.
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paper_title: Multifunctional nanoparticles: Analytical prospects

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Abstract Multifunctional nanoparticles are among the most exciting nanomaterials with promising applications in analytical chemistry. These applications include (bio)sensing, (bio)assays, catalysis and separations. Although most of these applications are based on the magnetic, optical and electrochemical properties of multifunctional nanoparticles, other aspects such as the synergistic effect of the functional groups and the amplification effect associated with the nanoscale dimension have also been observed. Considering not only the nature of the raw material but also the shape, there is a huge variety of nanoparticles. In this review only magnetic, quantum dots, gold nanoparticles, carbon and inorganic nanotubes as well as silica, titania and gadolinium oxide nanoparticles are addressed. This review presents a narrative summary on the use of multifuncional nanoparticles for analytical applications, along with a discussion on some critical challenges existing in the field and possible solutions that have been or are being developed to overcome these challenges.
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paper_title: A Zigbee-based wireless wearable electronic nose using flexible printed sensor array

paper_content:

A wearable electronic nose (e-nose) has been developed by integrating a low cost chemical sensor array with a wireless communication for applications in healthcare. Its sensing unit was fabricated by a fully inkjet-printing technique, comprising eight different sensor elements manufactured by varying printing patterns and sensing materials. These sensors have shown response to a wide variety of complex odors. A wearable e-nose prototype using Zigbee wireless technology was designed as a compact armband for monitoring the axillary odor released from human body. Preliminary results based on principal component analysis (PCA) could classify different odors released from the human body upon various activities.
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paper_title: Low-cost and flexible printed graphene–PEDOT:PSS gas sensor for ammonia detection

paper_content:

Abstract This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene–poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH 3 ) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV–visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of π–π interactions between graphene and PEDOT:PSS. The ink-jet printed graphene–PEDOT:PSS gas sensor exhibits high response and high selectivity to NH 3 in a low concentration range of 25–1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH 3 molecules via π electrons network. The NH 3 -sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted.
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paper_title: SELECTIVITY IMPROVEMENT OF SNO2 FILMS BY SUPERFICIAL METALLIC FILMS

paper_content:

The purpose of this paper is to demonstrate the possibility of modifying the sensitivity of tin dioxide (SnO 2 ) films by depositing metallic catalysts on the surface of SnO 2 layer. The aim is to reduce the effect of ethanol which is considered as an interfering gas in many domestic or industrial applications. Two catalysts have been so studied (platinum and palladium), deposited on two types of SnO 2 layers which present different textures. Both catalysts reduce the sensitivities to ethanol, carbon monoxide and methane. But the decrease of the alcohol sensitivity is more significant and consequently, the relative sensitivities to CO and CH 4 are increased. This effect is more important with the SnO 2 layers which have a high porosity. An other application where tin dioxide sensors can be used is the automotive application. In this case, nitrogen oxides are considered as interfering gases for the carbon monoxide detection. Several catalysts are studied in order to decrease the influence of nitrogen oxides and especially the conductance decrease under NO 2 . One of the catalysts which can be used to reduce the nitrogen oxide effect is rhodium. The results obtained with such rhodium films deposited on SnO 2 sensors allow us to consider future applications in the automotive field.
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paper_title: Influence on the gas sensor performances of the metal chemical states introduced by impregnation of calcinated SnO2 sol–gel nanocrystals

paper_content:

Abstract The effects of the introduction of Pt and Pd by impregnation in sol–gel fabricated SnO 2 nanoparticles after calcination are reported in this paper. The differences in base resistance and sensitivity of sensors prepared using these powders are presented and explained — taking into account the chemical states of the metal additives and the generated surface states in the band gap of the SnO 2 .
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paper_title: Gold Nanoparticle/Organic Networks as Chemiresistor Coatings: The Effect of Film Morphology on Vapor Sensitivity

paper_content:

Networked films comprising gold nanoparticles (4 nm core diameter) and dodecanedithiol were deposited via layer-by-layer self-assembly. The film thickness was controlled by the number of deposition cycles and ranged from submonolayer to multilayer coverage with up to 60 nm thickness. FE-SEM and XPS revealed island growth during the first four to five deposition cycles. At room temperature, films based on islands showed slightly nonlinear current−voltage curves, whereas thicker films gave Ohmic behavior. Between 100 and 300 K, the temperature dependence of conductance was consistent with an Arrhenius model for activated charge transport. The chemical sensitivity of the films was characterized by dosing them with vapors of toluene, 4-methyl-2-pentanone, 1-propanol, and water while monitoring their relative differential resistances. Thin films responded with a decrease in the resistance, whereas thicker films responded with a resistance increase. The results indicate that swelling along the film normal may b...
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paper_title: Fabrication of interdigitated electrodes by inkjet printing technology for apllication in ammonia sensing

paper_content:

In this paper interdigitated electrodes for gas sensors were fabricated by inkjet printing technology. Silver electrodes were inkjet printed on Si/SiO2 substrates instead of traditional photolithography method. The inkjet printing parameters to obtain desired dimensions, thickness of the electrodes and distance between the interdigitated electrodes were optimized in this study. The fabricated interdigitated silver electrodes were tested for application in ammonia gas sensors. Conductive polyaniline (PANI) layer was coated on the silver interdigitated electrodes by drop-coating. Ammonia detection of the PANI-coated chips was characterized with a gas measurement system in which humidity and ammonia concentrations were well-controlled. The electrical conductivity of the PANI films coated on the electrodes was measured when the PANI films were exposed to nitrogen and ammonia. The conductivity of the PANI films decreased significantly due to the deprotonation process of PANI upon ammonia expodure. The recovery time was about 15 min by heating up the polymer chip at 60 °C. The results showed that the silver electrodes fabricated by inkjet printing technique could be used as a sensor platform for ammonia detection.
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paper_title: Electroanalysis moves towards paper-based printed electronics: carbon black nanomodified inkjet-printed sensor for ascorbic acid detection as a case study

paper_content:

Abstract Herein we demonstrated, for the first time, the possibility to use the paper employed in printed electronics (i.e. p_e:smart) as substrate to develop a paper-based sensor. To improve the electrochemical performances of the inkjet-printed sensor, a dispersion based on carbon black nanoparticles was used to modify the working electrode, allowing for a highly performant nanomodified electrochemical sensor platform. This disposable sensor was characterized both electrochemically and morphologically, and it has been successively challenged towards a model analyte namely ascorbic acid. It has been evidenced that the presence of carbon black as nanomodifier decreased the overpotential for ascorbic acid oxidation (from 0.47 V to 0.28 V) with respect to the unmodified sensor and boosted the sensitivity (ca. 3-times). The applicability of this printed electrochemical sensor was demonstrated for the detection of ascorbic acid in a dietary supplement, quantifying 999 ± 130 mg with respect to the 1000 mg reported on the label.
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paper_title: Inkjet printing of chemiresistive sensors based on polymer and carbon nanotube networks

paper_content:

We report about the manufacturing of sensor devices for detection of volatile organic compounds (VOCs) in the air. The sensor comprises three fully inkjet-printed layers of (i) silver interdigitated electrodes (SIDE), (ii) multi-walled carbon nanotubes (MWCNTs) and (iii) poly (styrene-co-maleic acid) partial isobutyl/methyl mixed ester (PSE). We have found that inkjet printing of MWCNT layers and a PSE layer on top yields an interfacial layer leading to the formation of a MWCNTs/PSE composite-like structure. The printed layers act as chemiresistive vapor sensors due to the change of electrical resistance in presence of selected VOCs. The sensor responses to various VOCs were studied by varying the concentration of the chemicals in the air between 50 and 500 ppm. The sensing mechanism can be understood as a fractional volume change of the inkjet-printed polymer that occurs during exposure to VOCs, resulting in a structural alteration of the MWCNT-polymer network. We have found that the sensor response can be defined as a function of the VOC's concentration. To investigate the morphologies of the inkjet-printed thin films, profilometry and scanning electron microscopy (SEM) were employed.
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paper_title: Inkjet-printed, flexible, high performance, carbon nanomaterial based sensors for ammonia and DMMP gas detection

paper_content:

In this paper, we report the most sensitive inkjet printed reduced graphene oxide based ammonia sensor as well as the first inkjet printed carbon nanotube based dimethyl methylphosphonate (DMMP) vapor sensor to date. The processes utilized to fabricate the sensors, including substrate surface modification, inclusion of porosity enhancing nanospheres and chemical functionalization, are outlined. The testing set-up and protocols are described and finally the measurement results are shown with displayed sensitivities of 8.5% for 30 ppm of ammonia and 5% for 2.5 ppm of DMMP. These results may set the foundation for fully inkjet printed, conformal, ‘zero-power’ and low cost ubiquitous wireless multi-gas motes.
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paper_title: Fabrication of Polyaniline-Based Gas Sensors Using Piezoelectric Inkjet and Screen Printing for the Detection of Hydrogen Sulfide

paper_content:

This work describes a fully printable polyaniline-copper (II) chloride sensor for the detection of hydrogen sulfide gas. The sensing device is composed of screen printed silver interdigitated electrode (IDE) on a flexible PET substrate with inkjet printed layers of polyaniline and copper (II) chloride. The sensor is employed as a chemiresistor with changes in measured current being correlated with concentration. On exposure to hydrogen sulfide, 2.5 ppmv (parts per million by volume) is clearly detectable with a linear relationship between measured current and concentration over the 10-100 ppmv region. The detection mechanism is discussed with respect to the hydrogen sulfide response, the choice of electrode materials in addition to UV-vis and surface enhanced Raman spectroscopy (SERS) characterization.
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paper_title: Inkjet printing of conductive materials: a review

paper_content:

Purpose – The purpose of this paper is to present an exhaustive review of research studies and activities in the inkjet printing of conductive materials.Design/methodology/approach – This paper gives a detailed literature survey of research carried out in inkjet printing of conductive materials.Findings – This article explains the inkjet printing process and the various types of conductive inks. It then examines the various factors that affect the quality of inkjet printed interconnects such as printing parameters, materials and substrate treatments. Methods of characterising both the inkjet printing process and the electrical properties of printed conductive materials are also presented. Finally relevant applications of this technology are described.Originality/value – Inkjet printing is currently one of the cheapest direct write techniques for manufacturing. The use of this technique in electronic manufacturing, where interconnects and other conductive features are required is an area of increasing rele...
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paper_title: Towards fully printed capacitive gas sensors on flexible PET substrates based on Ag interdigitated transducers with increased stability

paper_content:

Interdigitated capacitive transducers have been inkjet printed onto flexible substrates and optimized for gas sensing applications. Their characteristics have been improved by tuning the annealing/sintering conditions and making use of additional passivation procedures, such as Ag electroplating with Ni or Parylene-C coating of the whole device surface. The as-prepared transducers printed with Ag ink could be utilized in capacitive gas sensing structures up to 70% relative humidity only. Higher humidity levels irreversibly altered the nominal capacitance and conductance of the devices, limiting their practical application. This drawback could be eliminated through the upgrading routes specified above and stable operation over wide temperature ranges has been achieved. From the acquired data a quite accurate description of the sorption processes, involving the printed conducting layers and the supporting material, has been inferred. Furthermore the parasitic sensitivity to test vapours from the class of Volatile Organic Compounds in the concentration range of Threshold Limit Value - Time-Weighted Average as possible interfering analytes has been estimated. Test gas sensors obtained from the optimized transducers by additional coating with gas sensing films poly(ether urethane) successfully passed the laboratory evaluations and seem to be appropriate for use in real devices. (C) 2012 Elsevier B. V. All rights reserved.
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paper_title: A Novel Wearable Electronic Nose for Healthcare Based on Flexible Printed Chemical Sensor Array

paper_content:

A novel wearable electronic nose for armpit odor analysis is proposed by using a low-cost chemical sensor array integrated in a ZigBee wireless communication system. We report the development of a carbon nanotubes (CNTs)/polymer sensor array based on inkjet printing technology. With this technique both composite-like layer and actual composite film of CNTs/polymer were prepared as sensing layers for the chemical sensor array. The sensor array can response to a variety of complex odors and is installed in a prototype of wearable e-nose for monitoring the axillary odor released from human body. The wearable e-nose allows the classification of different armpit odors and the amount of the volatiles released as a function of level of skin hygiene upon different activities.
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paper_title: Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres

paper_content:

Abstract Carbon nanofibres decorated with metal nanoparticles were deposited over kapton, a polyimide flexible substrate, onto which array interdigitated electrodes in one side and a common heater in the backside were printed using inkjet printing technique. The control over metal type (as-grown, Au, Pd) and the decoration percentage gave us the possibility to improve the sensors response, and also enhance the selectivity by taking advantage of the different interaction behaviors of tested target gas molecules with different hybrid materials that occur at room temperature.
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paper_title: Gold Nanoparticles in Chemical and Biological Sensing

paper_content:

Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13 ::: ::: In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28 ::: ::: Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37 ::: ::: ::: ::: Figure 1 ::: ::: Physical properties of AuNPs and schematic illustration of an AuNP-based detection system. ::: ::: ::: ::: In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.
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paper_title: A review of sensor-based methods for monitoring hydrogen sulfide

paper_content:

We review sensor-based methods commonly employed for monitoring hydrogen sulfide (H2S), and recent developments in H2S-sensing instrumentation. We evaluate the basic quality-assurance parameters of different sensor types for quantifying H2S in terms of major operational criteria (e.g., response time, limit of detection, common operating range of concentrations, and stability). We also describe the applicability of these sensor-based methods with respect to practicality in various environmental settings. Finally, we highlight the limitations and the future prospects of these sensor-based methods. a 2011 Elsevier Ltd. All rights reserved.
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paper_title: Inkjet printed (bio)chemical sensing devices

paper_content:

Inkjet printing has evolved from an office printing application to become an important tool in industrial mass fabrication. In parallel, this technology is increasingly used in research laboratories around the world for the fabrication of entire (bio)chemical sensing devices or single functional elements of such devices. Regularly stated characteristics of inkjet printing making it attractive to replace an alternative material deposition method are low cost, simplicity, high resolution, speed, reproducibility, flexibility, non-contact, and low amount of waste generated. With this review, we give an overview over areas of (bio)chemical sensing device development profiting from inkjet printing applications. A variety of printable functional sensor elements are introduced by examples, and the advantages and challenges of the inkjet method are pointed out. It is demonstrated that inkjet printing is already a routine tool for the fabrication of some (bio)chemical sensing devices, but also that novel applications are being continuously developed. Finally, some inherent limitations of the method and challenges for the further exploitation of this technology are pointed out.
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paper_title: All inkjet-printed chemical gas sensors based on CNT/polymer nanocomposites: Comparison between double printed layers and blended single layer

paper_content:

Gas sensors based on carbon nanotube (CNT)/ polymer nanocomposite were fabricated by a fully inkjet-printing method. The CNT/polymer layers acting as sensing layer were prepared by 2 methods; (i) deposition of separated layer of CNT and polymer to form a composite-like structure at the interface and (ii) deposition of blend CNT and polymer to obtain completely composite layer. Difference of sensor responses was demonstrated. We have found that the sensor with a compositelike layer provides a good response to volatile organic compounds (VOCs) as compared to the sensor obtained from printing CNT/polymer composite. The results have also demonstrated that the capability of the CNT/polymer composite sensor allows detecting VOCs in broad range of concentration.
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paper_title: A printed capacitive--resistive double sensor for toluene and moisture sensing

paper_content:

This paper presents a flexible printed sensor to detect two different magnitudes: toluene content and relative humidity. This device has been manufactured by using two printing techniques: inkjet printing to define the electrodes and screen printing to deposit the sensitive material. The combination in the same area of two different sensors enables the monitoring of two different gaseous chemicals without interference between them. On one hand, moisture content is associated with changes in the electrical permittivity of the chosen substrate (capacitive effect). On the other hand, toluene concentration is measured through variations in volume of the developed composite (resistive effect). Changes to relative humidity and toluene content have been measured in both capacitive and resistive parts as well as their responses to temperature variations.
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paper_title: Inkjet Printing of Interdigitated Capacitive Chemical Sensors with Reduced Size by the Introduction of a Dielectric Interlayer

paper_content:

A new fabrication strategy is proposed to reduce the size of sensors based on inkjet-printed interdigitated electrodes (IDE). The process is compatible with fabrication at low temperature and can be applied to flexible organic foils. The new fabrication concept consists of the deposition of a thin dielectric layer of parylene-C onto the first electrodes comb prior to the printing of the second one. In this way, the combs are not longer on the same plane, preventing undesired short-circuits and solving the typical problems of definition and yield associated to inkjet printing of IDE devices with small pitch. The proposed strategy results especially relevant for capacitive-based devices, where short-circuits render the device non-functional. Moreover, it permits enhancing their capacitance per surface area. The validation of the exposed process for sensors application has been carried out by functionalizing the IDE structure with a humidity sensing layer, and characterizing the resulting sensor against changes in relative humidity (R.H.). (C) 2012 Elsevier Ltd .... Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.
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paper_title: Low-cost and flexible printed graphene–PEDOT:PSS gas sensor for ammonia detection

paper_content:

Abstract This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene–poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH 3 ) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV–visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of π–π interactions between graphene and PEDOT:PSS. The ink-jet printed graphene–PEDOT:PSS gas sensor exhibits high response and high selectivity to NH 3 in a low concentration range of 25–1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH 3 molecules via π electrons network. The NH 3 -sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted.
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paper_title: Fabrication of interdigitated electrodes by inkjet printing technology for apllication in ammonia sensing

paper_content:

In this paper interdigitated electrodes for gas sensors were fabricated by inkjet printing technology. Silver electrodes were inkjet printed on Si/SiO2 substrates instead of traditional photolithography method. The inkjet printing parameters to obtain desired dimensions, thickness of the electrodes and distance between the interdigitated electrodes were optimized in this study. The fabricated interdigitated silver electrodes were tested for application in ammonia gas sensors. Conductive polyaniline (PANI) layer was coated on the silver interdigitated electrodes by drop-coating. Ammonia detection of the PANI-coated chips was characterized with a gas measurement system in which humidity and ammonia concentrations were well-controlled. The electrical conductivity of the PANI films coated on the electrodes was measured when the PANI films were exposed to nitrogen and ammonia. The conductivity of the PANI films decreased significantly due to the deprotonation process of PANI upon ammonia expodure. The recovery time was about 15 min by heating up the polymer chip at 60 °C. The results showed that the silver electrodes fabricated by inkjet printing technique could be used as a sensor platform for ammonia detection.
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paper_title: Electroanalysis moves towards paper-based printed electronics: carbon black nanomodified inkjet-printed sensor for ascorbic acid detection as a case study

paper_content:

Abstract Herein we demonstrated, for the first time, the possibility to use the paper employed in printed electronics (i.e. p_e:smart) as substrate to develop a paper-based sensor. To improve the electrochemical performances of the inkjet-printed sensor, a dispersion based on carbon black nanoparticles was used to modify the working electrode, allowing for a highly performant nanomodified electrochemical sensor platform. This disposable sensor was characterized both electrochemically and morphologically, and it has been successively challenged towards a model analyte namely ascorbic acid. It has been evidenced that the presence of carbon black as nanomodifier decreased the overpotential for ascorbic acid oxidation (from 0.47 V to 0.28 V) with respect to the unmodified sensor and boosted the sensitivity (ca. 3-times). The applicability of this printed electrochemical sensor was demonstrated for the detection of ascorbic acid in a dietary supplement, quantifying 999 ± 130 mg with respect to the 1000 mg reported on the label.
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paper_title: Inkjet printing of chemiresistive sensors based on polymer and carbon nanotube networks

paper_content:

We report about the manufacturing of sensor devices for detection of volatile organic compounds (VOCs) in the air. The sensor comprises three fully inkjet-printed layers of (i) silver interdigitated electrodes (SIDE), (ii) multi-walled carbon nanotubes (MWCNTs) and (iii) poly (styrene-co-maleic acid) partial isobutyl/methyl mixed ester (PSE). We have found that inkjet printing of MWCNT layers and a PSE layer on top yields an interfacial layer leading to the formation of a MWCNTs/PSE composite-like structure. The printed layers act as chemiresistive vapor sensors due to the change of electrical resistance in presence of selected VOCs. The sensor responses to various VOCs were studied by varying the concentration of the chemicals in the air between 50 and 500 ppm. The sensing mechanism can be understood as a fractional volume change of the inkjet-printed polymer that occurs during exposure to VOCs, resulting in a structural alteration of the MWCNT-polymer network. We have found that the sensor response can be defined as a function of the VOC's concentration. To investigate the morphologies of the inkjet-printed thin films, profilometry and scanning electron microscopy (SEM) were employed.
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paper_title: Inkjet-printed, flexible, high performance, carbon nanomaterial based sensors for ammonia and DMMP gas detection

paper_content:

In this paper, we report the most sensitive inkjet printed reduced graphene oxide based ammonia sensor as well as the first inkjet printed carbon nanotube based dimethyl methylphosphonate (DMMP) vapor sensor to date. The processes utilized to fabricate the sensors, including substrate surface modification, inclusion of porosity enhancing nanospheres and chemical functionalization, are outlined. The testing set-up and protocols are described and finally the measurement results are shown with displayed sensitivities of 8.5% for 30 ppm of ammonia and 5% for 2.5 ppm of DMMP. These results may set the foundation for fully inkjet printed, conformal, ‘zero-power’ and low cost ubiquitous wireless multi-gas motes.
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paper_title: Fabrication of Polyaniline-Based Gas Sensors Using Piezoelectric Inkjet and Screen Printing for the Detection of Hydrogen Sulfide

paper_content:

This work describes a fully printable polyaniline-copper (II) chloride sensor for the detection of hydrogen sulfide gas. The sensing device is composed of screen printed silver interdigitated electrode (IDE) on a flexible PET substrate with inkjet printed layers of polyaniline and copper (II) chloride. The sensor is employed as a chemiresistor with changes in measured current being correlated with concentration. On exposure to hydrogen sulfide, 2.5 ppmv (parts per million by volume) is clearly detectable with a linear relationship between measured current and concentration over the 10-100 ppmv region. The detection mechanism is discussed with respect to the hydrogen sulfide response, the choice of electrode materials in addition to UV-vis and surface enhanced Raman spectroscopy (SERS) characterization.
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paper_title: Towards fully printed capacitive gas sensors on flexible PET substrates based on Ag interdigitated transducers with increased stability

paper_content:

Interdigitated capacitive transducers have been inkjet printed onto flexible substrates and optimized for gas sensing applications. Their characteristics have been improved by tuning the annealing/sintering conditions and making use of additional passivation procedures, such as Ag electroplating with Ni or Parylene-C coating of the whole device surface. The as-prepared transducers printed with Ag ink could be utilized in capacitive gas sensing structures up to 70% relative humidity only. Higher humidity levels irreversibly altered the nominal capacitance and conductance of the devices, limiting their practical application. This drawback could be eliminated through the upgrading routes specified above and stable operation over wide temperature ranges has been achieved. From the acquired data a quite accurate description of the sorption processes, involving the printed conducting layers and the supporting material, has been inferred. Furthermore the parasitic sensitivity to test vapours from the class of Volatile Organic Compounds in the concentration range of Threshold Limit Value - Time-Weighted Average as possible interfering analytes has been estimated. Test gas sensors obtained from the optimized transducers by additional coating with gas sensing films poly(ether urethane) successfully passed the laboratory evaluations and seem to be appropriate for use in real devices. (C) 2012 Elsevier B. V. All rights reserved.
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paper_title: Inkjet printing of polymer functionalized CNT gas sensor with enhanced sensing properties

paper_content:

Abstract We report on fully inkjet printed carbon nanotubes gas sensors on flexible substrates with improved ethanol sensing characteristics. The sensors were fabricated via two simple steps: printing the electrodes and the carbon nanotubes based - active layer. Carbon nanotubes were functionalized with carboxylic acid (COOH) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). The performance analysis shows a significant enhancement in the sensitivity of the sensor with polymer wrapped nanotubes in comparison with the oxidized nanotubes, with enhancement factors greater than 2.5. Moreover, a remarkable improvement in the sensor response and recovery times after polymer functionalization is noted which bodes well for fully printable environmental sensors on flexible plastic circuits.
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paper_title: Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres

paper_content:

Abstract Carbon nanofibres decorated with metal nanoparticles were deposited over kapton, a polyimide flexible substrate, onto which array interdigitated electrodes in one side and a common heater in the backside were printed using inkjet printing technique. The control over metal type (as-grown, Au, Pd) and the decoration percentage gave us the possibility to improve the sensors response, and also enhance the selectivity by taking advantage of the different interaction behaviors of tested target gas molecules with different hybrid materials that occur at room temperature.
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paper_title: Carbon-Nanotube Loaded Antenna-Based Ammonia Gas Sensor

paper_content:

Carbon nanotubes (CNTs) have been researched extensively for gas-sensing applications due to their unique electrical, chemical, and structural properties. Single-walled carbon nanotubes (SWNTs) have been predominantly used due to their superior electrical conductivity and higher sensitivity relative to multiwalled CNTs. This paper presents the design and characterization of a novel planar sensor fabricated on paper substrate to detect small concentrations of ammonia gas, using the shift in resonance frequency of a patch antenna as the discriminator. We have investigated three main design issues in depth. First, functionalization of the SWNTs with a polymer is studied in order to enhance the gas detection sensitivity. Second, a thin film of the functionalized SWNT is characterized to create a surface impedance model for the explanation and prediction of the resonance shift due to different gas concentrations. Finally, as a proof of concept, functionalized SWNTs are integrated into a patch antenna design and the return loss is measured in a closed-system environment to show high sensitivity for low concentrations of ammonia gas. The proposed antenna-based wireless gas sensor can be utilized in several applications, given its small form factor, light weight, and little to no power requirements.
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paper_title: All inkjet-printed chemical gas sensors based on CNT/polymer nanocomposites: Comparison between double printed layers and blended single layer

paper_content:

Gas sensors based on carbon nanotube (CNT)/ polymer nanocomposite were fabricated by a fully inkjet-printing method. The CNT/polymer layers acting as sensing layer were prepared by 2 methods; (i) deposition of separated layer of CNT and polymer to form a composite-like structure at the interface and (ii) deposition of blend CNT and polymer to obtain completely composite layer. Difference of sensor responses was demonstrated. We have found that the sensor with a compositelike layer provides a good response to volatile organic compounds (VOCs) as compared to the sensor obtained from printing CNT/polymer composite. The results have also demonstrated that the capability of the CNT/polymer composite sensor allows detecting VOCs in broad range of concentration.
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paper_title: A novel highly-sensitive antenna-based “smart skin” gas sensor utilizing carbon nanotubes and inkjet printing

paper_content:

Carbon nanotubes (CNTs) have attractive features for implementation in wireless sensor nodes due to their small size, light weight and low power requirements, and their ability to be functionalized with conductive polymers for the electronic detection of a range of chemical and biological agents. In this paper, we present a single-walled CNT sensor for gas detection that is printed on paper substrate and integrated with a co-planar RF antenna for potential application as a light-weight wireless sensor node. The CNT thin-film loads the antenna and changes its resonant frequency upon exposure to the gas, the resonant frequency shift then being used as a discriminator for the gas detection in trace quantities. Measurements of a CNT-based sensor when exposed to low levels of ammonia reveal a resonance frequency shift of 300 MHz for a patch antenna centered around 6 GHz. To the authors' knowledge, this is the highest frequency shift reported in literature.
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paper_title: A novel graphene-based inkjet-printed WISP-enabled wireless gas sensor

paper_content:

In this paper we demonstrate the design and development of a low-cost, self-powered, wireless sensor solution based on the WISP platform and utilizing thin films produced from environmentally friendly, water-based, inkjet printed graphene oxide (GO) ink. The sensor demonstrates good response to ammonia gas (NH3), yielding a 6% normalized resistance change within 15 minutes after exposure to a concentration of 500 ppm. In addition, excellent recovery time is achieved using the graphene thin films, with over 30% of material recovery observed within 5 minutes without exposure to high temperature or any UV treatments. In addition to reporting the first ever integration of inkjet-printed water soluble GO inks into low cost RF electronics fabricated on flexible substrates, we also bring gas sensing capabilities to RFID tags relying on purely wireless digital transmission of the sensed information. The introduction of mass producible, stable, environmentally friendly, inkjet printable GO on organic paper/Kapton substrates lays the foundation for the development of a wide range of new low-cost, high performance graphene-based devices, such as inkjet-printed diodes, capacitors and transistors.
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paper_title: Enhanced-performance wireless conformal "smart skins" utilizing inkjet-printed carbon-nanostructures

paper_content:

This paper introduces for the first time the integration of a UHF radio frequency identification (RFID) antenna with reduced graphene oxide (rGO), developed using direct-write techniques and utilizing an RFID chip for chemical gas detection. The module is realized by inkjet printing on a lowcost paper-based substrate, and the RFID tag is designed for the North America UHF RFID band. The electrical impedance of the rGO thin film changes in the presence of very small quantities of certain toxic gases, resulting in a variation of the backscattered power level which is easily detected by the RFID reader to realize reliable wireless toxic gas sensing. The inkjet printed RFID tag demonstrated a change in backscattered power of 9.18% upon exposure of 40 ppm NO2 for 5 minutes.
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paper_title: A Zigbee-based wireless wearable electronic nose using flexible printed sensor array

paper_content:

A wearable electronic nose (e-nose) has been developed by integrating a low cost chemical sensor array with a wireless communication for applications in healthcare. Its sensing unit was fabricated by a fully inkjet-printing technique, comprising eight different sensor elements manufactured by varying printing patterns and sensing materials. These sensors have shown response to a wide variety of complex odors. A wearable e-nose prototype using Zigbee wireless technology was designed as a compact armband for monitoring the axillary odor released from human body. Preliminary results based on principal component analysis (PCA) could classify different odors released from the human body upon various activities.
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paper_title: RFID-enabled ultrasensitive wireless sensors utilizing inkjet-printed antennas and carbon nanotubes for gas detection applications

paper_content:

This paper presents, for the first time, the integration of conformal paper-based RFID's with a Single Walled Carbon Nanotube (SW-CNT) composite for the development of a chipless RFID-enabled wireless sensor node for toxic gas detection and breathing-gas-content estimation. The electrical performance of the inkjet-printed SWCNT-based ultrasensitive sensor is reported up to 1GHz. The whole module is realized by inkjet-printing on a low-cost “green” paper-based substrate designed to operate in the European UHF RFID band. The electrical conductivity of the SWCNT film changes in the presence of ultra-small quantities of gases like ammonia and nitrogen dioxide, resulting in the variation of the backscattered power level which can be easily detected by the RFID reader to realize reliable early-warning toxic gas detection or breathing monitoring with potentially profound effects on ubiquitous low-cost “green” quality-of-life applications
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paper_title: Battery-free RFID-enabled wireless sensors

paper_content:

This paper introduces the realization of batter-free RFID-enable wireless sensors by integrating conformal RFID antennas with inkjet-printed carbon nanotubes (CNT) composites in a chipless RFID fashion for gas detection. The whole module is realized by inkjet printing on a low-cost paper-based substrate and the RFID tag is designed for the European UHF RFID band. The electrical conductivity of the CNT film changes in the presence of very small quantities of gases like ammonia, methanol, ethanol, acetone and nitrogen oxide (NO x ), resulting in the variation of the backscattered power level which can be easily detected by the RFID reader to realize reliable wireless toxic gas sensing. The electrical performance characterization of the inkjet-printed CNT film is also reported in the UHF band.
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paper_title: Inkjet printing of chemiresistive sensors based on polymer and carbon nanotube networks

paper_content:

We report about the manufacturing of sensor devices for detection of volatile organic compounds (VOCs) in the air. The sensor comprises three fully inkjet-printed layers of (i) silver interdigitated electrodes (SIDE), (ii) multi-walled carbon nanotubes (MWCNTs) and (iii) poly (styrene-co-maleic acid) partial isobutyl/methyl mixed ester (PSE). We have found that inkjet printing of MWCNT layers and a PSE layer on top yields an interfacial layer leading to the formation of a MWCNTs/PSE composite-like structure. The printed layers act as chemiresistive vapor sensors due to the change of electrical resistance in presence of selected VOCs. The sensor responses to various VOCs were studied by varying the concentration of the chemicals in the air between 50 and 500 ppm. The sensing mechanism can be understood as a fractional volume change of the inkjet-printed polymer that occurs during exposure to VOCs, resulting in a structural alteration of the MWCNT-polymer network. We have found that the sensor response can be defined as a function of the VOC's concentration. To investigate the morphologies of the inkjet-printed thin films, profilometry and scanning electron microscopy (SEM) were employed.
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paper_title: A Novel Conformal RFID-Enabled Module Utilizing Inkjet-Printed Antennas and Carbon Nanotubes for Gas-Detection Applications

paper_content:

This letter introduces for the first time the integration of a conformal radio frequency identification (RFID) antenna with a single-walled carbon nanotube (SWCNT) composite in a chipless RFID node for toxic gas detection. The electrical performance characterization of the inkjet-printed SWCNT film is also reported for the first time up to 1 GHz. The whole module is realized by inkjet printing on a low-cost paper-based substrate, and the RFID tag is designed for the European UHF RFID band. The electrical conductivity of the SWCNT film changes in the presence of very small quantities of toxic gases like ammonia and nitrogen oxide, resulting in the variation of the backscattered power level, which can be easily detected by the RFID reader to realize reliable wireless toxic gas sensing.
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paper_title: 3D‐Printed Disposable Wireless Sensors with Integrated Microelectronics for Large Area Environmental Monitoring

paper_content:

Large area environmental monitoring can play a crucial role in dealing with crisis situations. However, it is challenging as implementing a fixed sensor network infrastructure over large remote area is economically unfeasible. This work proposes disposable, compact, dispersible 3D-printed wireless sensor nodes with integrated microelectronics which can be dispersed in the environment and work in conjunction with few fixed nodes for large area monitoring applications. As a proof of concept, the wireless sensing of temperature, humidity, and H2S levels are shown which are important for two critical environmental conditions namely forest fires and industrial leaks. These inkjet-printed sensors and an antenna are realized on the walls of a 3D-printed cubic package which encloses the microelectronics developed on a 3D-printed circuit board. Hence, 3D printing and inkjet printing are uniquely combined in order to realize a low-cost, fully integrated wireless sensor node.
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paper_title: A Novel Wearable Electronic Nose for Healthcare Based on Flexible Printed Chemical Sensor Array

paper_content:

A novel wearable electronic nose for armpit odor analysis is proposed by using a low-cost chemical sensor array integrated in a ZigBee wireless communication system. We report the development of a carbon nanotubes (CNTs)/polymer sensor array based on inkjet printing technology. With this technique both composite-like layer and actual composite film of CNTs/polymer were prepared as sensing layers for the chemical sensor array. The sensor array can response to a variety of complex odors and is installed in a prototype of wearable e-nose for monitoring the axillary odor released from human body. The wearable e-nose allows the classification of different armpit odors and the amount of the volatiles released as a function of level of skin hygiene upon different activities.
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Title: Inkjet-Printed Wireless Chemiresistive Sensors—A Review

Section 1: Introduction
Description 1: Provide an overview of the chemical sensors market, focusing on the growing demand for advanced monitoring technologies and the potential of inkjet printing as a manufacturing method for chemical sensors.

Section 2: Structure and Transduction Mechanism
Description 2: Discuss the typical structure of chemiresistive sensors, including the role of interdigitated electrodes (IDEs) and the interaction between the sensing layer and analytes.

Section 3: Inkjet-Printed Sensors
Description 3: Examine the benefits and challenges of inkjet printing technology for manufacturing sensor components and complete devices.

Section 4: Substrate and Electrode Materials
Description 4: Review different types of substrates and electrode materials used in inkjet-printed chemiresistive sensors, highlighting their cost-effectiveness and flexibility.

Section 5: Partly Inkjet-Printed Chemiresistors
Description 5: Discuss examples of chemiresistive sensors where only part of the sensor (either electrode or sensing layer) is produced using inkjet printing, and the benefits and limitations of these approaches.

Section 6: Fully Inkjet-Printed Chemiresistors
Description 6: Highlight research on fully inkjet-printed chemiresistive sensors, focusing on the materials and methods used to achieve complete inkjet-printed devices.

Section 7: Integration of Inkjet-Printed Chemiresistors into Wireless Applications
Description 7: Explore the integration of inkjet-printed chemiresistive sensors into wireless systems, including RFID and IoT applications, and review the performance of these sensor systems.

Section 8: Conclusions and Outlook
Description 8: Summarize the main findings of the review, emphasize the potential and challenges of inkjet-printed chemiresistive sensors, and provide an outlook on future research directions in this field.