Formaldehyde Gas Sensors: A Review
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paper_title: A facile and highly sensitive colorimetric sensor for the detection of formaldehyde based on electro-spinning/netting nano-fiber/nets

paper_content:

Abstract A novel strategy for highly sensitive colorimetric detection of gaseous formaldehyde was developed based on Methyl Yellow-impregnated electro-spinning/netting (ESN) nylon 6 nano-fiber/nets (NFN). The sensor presented a significant reflectance intensity decreasing band at 550 nm which induce the visual color changes from yellow to red after exposure to formaldehyde and allowed for the detection of formaldehyde with a low detection limit of 50 ppb. Upon exposure to a series of volatile organic compounds (VOCs), only formaldehyde could induce a yellow-to-red color change observable by the naked eye, which clearly exhibited that Methyl Yellow-impregnated nylon 6 NFN membranes could act as highly selective and sensitive strips to detect formaldehyde with minor interference from other VOCs. Additionally, the colorimetric sensor showed good reproducibility under cyclic sensing experiments. Furthermore, the colorimetric responses were visualized quantitative by using a color-differentiation map prepared form converted RGB (red, green and blue) values. As-prepared Methyl Yellow-impregnated nylon 6 NFN sensor strips successfully combined with the visualized detection of color map and fascinating structure of NFN membranes, which indicated promising composite materials as a simple and economical alternative to replace the traditional formaldehyde sensors and facilitated the design and development of other label-free colorimetric sensors toward various analytes based on the NFN template.
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paper_title: Effect of formaldehyde on the expression of adhesion molecules in nasal microvascular endothelial cells: the role of formaldehyde in the pathogenesis of sick building syndrome

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BACKGROUND ::: Formaldehyde is associated with sick building syndrome (SBS), a set of diffuse and irritative symptoms predominantly involving the eyes and the respiratory tract. However, its pathophysiological mechanism in SBS has not yet been clarified. ::: ::: ::: OBJECTIVE ::: In this study we investigated the effect of formaldehyde on the expression of adhesion molecules on human mucosal microvascular endothelial cells (HMMECs). Furthermore, we investigated the effect of formaldehyde on adhesiveness of HMMECs to eosinophils. ::: ::: ::: MATERIALS AND METHODS ::: HMMECs were incubated with various concentrations of formaldehyde (1 ng/mL-1 microg/mL) for 24 h, and the expressions of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule (VCAM-1) on HMMECs were evaluated by flow cytometry. The change in the expression of ICAM-1 and VCAM-1 mRNA was then evaluated by reverse transcriptase polymerase chain reaction. To understand the role of formaldehyde in eosinophilic inflammation of the nasal mucosa, we examined the effects of formaldehyde on the adhesiveness between HMMECs and eosinophils by eosinophil adhesion assay. ::: ::: ::: RESULTS ::: Formaldehyde increased the surface expressions of ICAM-1 and VCAM-1 on HMMECs. Formaldehyde also induced ICAM-1 and VCAM-1 mRNA. In addition, the adhesiveness between HMMECs and eosinophils was also increased by formaldehyde. ::: ::: ::: CONCLUSION ::: These in vitro studies suggest that formaldehyde may play a role as the irritant of the nasal mucosa by increasing the expressions of adhesion molecules on HMMECs and by enhancing the adhesiveness between HMMECs and eosinophils.
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paper_title: Development of highly selective and stable potentiometric sensors for formaldehyde determination.

paper_content:

Two types of biosensors selective to formaldehyde have been developed on the basis of pH-sensitive field effect transistor as a transducer. Highly or partially purified alcohol oxidase (AOX) and the permeabilised cells of methylotrophic yeast Hansenula polymorpha (as a source of AOX) have been used as sensitive elements. The response time in steady-state measurement mode is in the range of 10-60 s for the enzyme-based sensors and 60-120 s for the cell-based sensor. When measured in kinetic mode the response time of all biosensors developed was less than 5 s. The linear dynamic range of the sensor output signals corresponds to 5-200 mM formaldehyde for highly and partially purified alcohol oxidase, and 5-50 mM formaldehyde for the cells. The operational stability of the biosensors is not less than 7 h, and the relative standard deviation of intra-sensor response is approximately 2 and 5% for the enzyme- and cell-based sensors, respectively. When stored at 4 degrees C, the enzyme and cell sensor responses have been found stable for more than 60 and 30 days, respectively. Both types of biosensors demonstrate a high selectivity to formaldehyde with no potentiometric response to primary alcohols, including methanol, or glycerol and glucose. The possible reasons of such unexpected high selectivity of AOX-based FET-sensors to formaldehyde are discussed. The influence of the biomembrane composition and the effect of different buffers on the sensor response to formaldehyde are also discussed.
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paper_title: On-line detection of atmospheric formaldehyde by a conductometric biosensor.

paper_content:

Atmospheric formaldehyde (CH(2)O) was detected under continuous flow conditions by an on-line system comprising of a wet scrubber for a continuous transfer of the pollutant to an aqueous solution, a micro-reactor containing immobilized formaldehyde dehydrogenase (FDH) and a conductometric transducer. By this system atmospheric formaldehyde concentrations in the range 0.05-2 ppm were detected with a sensitivity of 20 microS/ppm. In this concentration range the immobilized enzyme oxidized all the sampled formaldehyde molecules to formic acid, avoiding cumbersome calibration procedures. The operational stability of the biosensor was at least 3 months, working continuously 10 h/day at room temperature.
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paper_title: Evaluation of formaldehyde emission from different types of wood-based panels and flooring materials using different standard test methods

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In this study, formaldehyde emission (FE) and content (FC) from different types of wood-based panels mainly; particleboard (PB), medium and high density fiberboard (MDF and HDF) and plywood (PLW) and flooring materials [HDF laminate, solid wood, solid bamboo and polyvinyl chloride (PVC)] were measured using different test methods namely; European small-scale chamber (EN 717-1), gas analysis (EN 717-2), the American small-scale chamber (ASTM D 6007-02), and the perforator (EN 120) methods. FE was affected with high significance by board type and thickness of PB (P < 0.0001), but there was no significant effect from the moisture content (MC %) of PB (P ¼ 0.94) and PLW (P ¼ 0.195). The corrected FC values measured EN 120 were declined when the MC % decreased. Furthermore, the liberation of formaldehyde was enhanced by the process of painting when 200 g/m 2 oil-based paint was applied for MDF and HDF. There was a strong positive correlation among the four test methods (R 2 values ranged between 0.88 and 0.94) concerning the formaldehyde values from PB-16 mm, and approximately the same indication of formaldehyde values, as well as similar behavior, were seen for each method. Moreover, the results indicate surprisingly that there was a good correlation between EN 120 and ASTM D 6007-02, with R 2 values of 0.93. The measurements of FE from flooring panels were ranged between 0.003 and 0.125 mg/m 3 and the PVC flooring with UV-curable layer only had emissions ranged between 0.003 and 0.008 mg/m 3 as measured by EN 717-1. Values of the emittable formaldehyde concentrations from most of the products investigated in the present study were below the limits that are mandatory in the Czech Republic.
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paper_title: On-line collection/concentration of trace amounts of formaldehyde in air with chromatomembrane cell and its sensitive determination by flow injection technique coupled with spectrophotometric and fluorometric detection.

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A simple, rapid and highly sensitive method for the determination of trace amounts of formaldehyde in air by using flow injection analysis (FIA) system coupled with a three-hole chromatomembrane cell (CMC) was investigated by using a spectrophotometer and a fluorometer. The CMC was applied to on-line collection/concentration of trace amounts of formaldehyde in air into water as an absorbing solution; formaldehyde in the air was found to be quantitatively transferred into the absorbing solution in CMC. The solution, containing absorbed formaldehyde, was introduced into the carrier stream of the FIA system. The amount of formaldehyde in an absorbing solution was measured spectrophotometrically and fluorometrically after the reaction with a mixed reagent of acetylacetone and ammonium acetate at pH 5.6-5.8. The amount of formaldehyde in the absorbing solution, measured by the proposed system, could be converted to the concentration of formaldehyde in the air sample. A calibration graph prepared by a series of standard formaldehyde aqueous solutions was adopted. The formaldehyde in indoor air, determined as exampled by the proposed spectrophotometric FIA, was found to be 5.14+/-0.08ppbv for 20ml of the air sample at the air flow rate of 6mlmin(-1), and the relative standard deviation (R.S.D.) was 1.56%. The limit of detections (LODs) of HCHO in an absorbing solution was 2x10(-8)M (0.6ppb) and 8x10(-9)M (0.2ppb), respectively, by the spectrophotometric and the fluorometric FIA, and the LODs of HCHO in air sample of 40ml were 0.05 and 0.03ppbv, respectively. The interferences from foreign species were examined; tolerable concentrations of other aldehydes were more than 50-fold of formaldehyde (1x10(-6)M).
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paper_title: Performance of a biological degradation method for indoor formaldehyde removal

paper_content:

Abstract Formaldehyde is a common gaseous pollutant affecting the indoor air quality and the health of occupants in buildings. Various technologies have been tried, such as adsorption, photocatalytic oxidization and botanical filtrations. However, the removal of indoor formaldehyde is still a challenging problem due to the low rate, byproduct formation and low efficiency of the above mentioned methods. Biological degradation technology with high removal efficiency has been successfully applied in industries for effluents and waste gas treatments. However, the potential biohazard risk from possible microorganism leakages of the biological system is the biggest concern of indoor air biofiltration systems. This study investigated the performance of a biological degradation system for formaldehyde removal at typical indoor condition. A microbial immobilization method was applied in the biofiltration system to reduce the potential biohazard risk. Bacterial strain of Pseudomonas putida was embedded in calcium alginate gel, which was then formed to a novel packing material called microbial granules. To investigate the operating performance of the biofilter in practical indoor environment, the concentrations of experimental inlet formaldehyde was controlled between 0.222 and 1.339 mg m−3, while the corresponding removal efficiency of the biofilter system varied from 68.6% to 93.5%. The pressure drop was maintained at 120 Pa with air flow rate increased up to 1.5 L min−1. The effect of inlet concentration, empty bed residence time, and nutrient feeding rate on the performance of the system was investigated as well. No microbial leakages were detected at air downstream during the experimental study, while the high microbial activity of bacteria entrapped in the packing material was observed.
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paper_title: Advances in SAW Gas Sensors Based on the Condensate-Adsorption Effect

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A surface-acoustic-wave (SAW) gas sensor with a low detection limit and fast response for volatile organic compounds (VOCs) based on the condensate-adsorption effect detection is developed. In this sensor a gas chromatography (GC) column acts as the separator element and a dual-resonator oscillator acts as the detector element. Regarding the surface effective permittivity method, the response mechanism analysis, which relates the condensate-adsorption effect, is performed, leading to the sensor performance prediction prior to fabrication. New designs of SAW resonators, which act as feedback of the oscillator, are devised in order to decrease the insertion loss and to achieve single-mode control, resulting in superior frequency stability of the oscillator. Based on the new phase modulation approach, excellent short-term frequency stability (±3 Hz/s) is achieved with the SAW oscillator by using the 500 MHz dual-port resonator as feedback element. In a sensor experiment investigating formaldehyde detection, the implemented SAW gas sensor exhibits an excellent threshold detection limit as low as 0.38 pg.
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paper_title: Potentiometric detection of formaldehyde in air by an aldehyde dehydrogenase FET

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Abstract This paper reports on a system for sampling atmospheric formaldehyde by dissolution in an aqueous solution followed by the monitoring of the aldehyde using an ion-sensitive field-effect transistor (ISFET) in conjunction with an enzyme specific for this pollutant. Formaldehyde dehydrogenase from Pseudomonas putida was chosen on the basis of its characteristics to be coupled to the ISFET transducer. This enzyme, using oxidized nicotinamide adenine dinucleoside (NAD) as cofactor, catalyses the oxidation of a molecule of formaldehyde with the parallel production of two protons, which are sensed by the ISFET. The working conditions were chosen to obtain a linear response of the sensor up to 200 μM formaldehyde. On the basis of the enrichment obtained by the sampling system, the detection limit of 10 μM formaldehyde in aqueous solution, achieved by the ISFET biosensor, corresponds to an atmospheric concentration of the formaldehyde in the ppb range.
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paper_title: Evaluation of three test methods in determination of formaldehyde emission from particleboard bonded with different mole ratio in the urea–formaldehyde resin

paper_content:

Abstract The use of urea–formaldehyde (UF) resins with lower contents of free formaldehyde in the board industry has led to products with very low emissions of formaldehyde. In this study, a detailed account is given of how UF resins with different mole ratios of formaldehyde to urea within the range of 0.97–1.27 influence the formaldehyde emission. In order to evaluate simpler laboratory methods for determining the formaldehyde emission from particleboard, the desiccator (JIS A 5908, 2003) and the EN 120 (European Standard, 1991) have been compared with the 1 m3 chamber method SS 270236 (Swedish Standard of determining formaldehyde emission with chamber test, 1988). Good relationships were obtained between the methods studied with correlation coefficients of >0.9. The value of formaldehyde emission decreased linearly with the mole ratio of formaldehyde to urea down to 1.05 where the effect smoothed out. The effect on the formaldehyde emission of temperature and storage time for tested boards also was studied. The heat treatment lowered the perforator value with resins that had mole ratios equal to or higher than 1.15, at the two lowest mole ratios 0.97 and 1.01, there was a tendency for the heat treatment to increase the perforator value. After the boards were stored for 6 months at 23 °C and 50% RH, the perforator values were unchanged or negligibly changed with the lower mole ratios and there was a more pronounced change with a higher mole ratio of 1.27.
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paper_title: Formaldehyde Detection in Air by Laser-Induced Fluorescence

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A method is described for the instantaneous concentration measurement of formaldehyde (H2CO) in air by laser-induced fluorescence of the H2CO (A1A″-X˜1A1) electronic transition. Concentrations down to about 13 ng/litre (about 10 ppb) can be determined in about 100 seconds without any sample collection, water extraction, or further chemical treatment. The influence of the presence of water vapor and methanol vapor in the air on the accuracy of the method has been investigated. It appeared that if the humidity of the ambient air is not taken into account, the detection limit is about 0.15 ppm H2CO in air; the presence of methanol has no influence on the accuracy.
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paper_title: Chemical sensors: definitions and classification

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A chemical sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal. The chemical information, mentioned above, may originate from a chemical reaction of the analyte or from a physical property of the system investigated.
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paper_title: A biosensor for monitoring formaldehyde using a new lipophilic tetrathiafulvalene-tetracyanoquinodimethane salt and a polyurethane membrane

paper_content:

Abstract A format for a disposable screen-printed biosensor was investigated for monitoring formaldehyde. The screen-printed sensor format comprised a working electrode (WE) modified with platinised carbon, a new lipophilic tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) salt as mediator, a plasticised polyurethane membrane (TECOFLEX SG80, PU) with anionic sites and the enzyme (formaldehyde dehydrogenase FOH) and co-factor (β-NAD+). The new lipophilic TTF-TCNQ salt gave a smooth, robust electrode surface rather than microcrystalline deposits of the salt observed with TTF-TCNQ salts. Entrapment of the enzyme and co-factor behind a plastcised PU membrane with anionic sites enhanced the sensitivity and stability of the sensor. A linear response between 10−3 and 10−4 mol dm−3 was observed. To demonstrate the feasibility of the use of the screen-printed sensor to monitoring formaldehyde in ambient air, preliminary experiments were carried out in the vicinity of a local medium density fibreboard factory and its performance compared to that of a biosensor made from the same platinised carbon and enzyme system, but without the lipophilic TTF-TCNQ and membrane, using a commercial ‘Rank’ cell. Samples were collected using a Molspin ‘Super-sniffer’.
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paper_title: Formaldehyde and VOC emissions at different manufacturing stages of wood-based panels

paper_content:

Abstract Formaldehyde and VOC emissions from wood-based panels are recognized as one of the major causes of poor indoor air quality. These emissions may be strongly influenced by the raw materials and manufacturing techniques of the panels. However, there is very little published information regarding this issue. In this study we determine and compare formaldehyde and VOC emissions, including their species and content, at different manufacturing stages of wood-based panels such as urea-formaldehyde resin, wood chip, wood fiber after resin application, medium density fiberboard, and phenol-formaldehyde resin. We found that: (1) in total, 34 individual VOCs were identified, but none of them were in all the specimens, indicating considerable VOC species changes during the manufacturing process; (2) the formaldehyde in wood-based panels came primarily from urea-formaldehyde resin, and there existed a linear relationship between the formaldehyde content in resins and formaldehyde specific emission rate from wood-base panels; (3) VOCs in wood-based panels mainly came from the wood chips; (4) the drying and hot-pressing techniques were helpful in reducing the formaldehyde and VOC emissions from wood-based panels. Some suggestions are presented for the control of formaldehyde and VOC emissions from wood-based panels during the manufacturing process.
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paper_title: Development of a novel hand-held formaldehyde gas sensor for the rapid detection of sick building syndrome

paper_content:

Abstract The majority of conventional sensors for monitoring formaldehyde (HCHO) in air have several drawbacks, in terms of sensitivity, selectivity, and sampling time. In this report, a novel hand-held HCHO gas sensor for the sick building syndrome (SBS) prevention and control is developed with a significantly short sampling time. A well-described reagent, 4-amino hydrazine-5-mercapto-1,2,4-triazole (AHMT) was utilized in the development of the novel gas sensor. AHMT of 100 μl and potassium hydroxide solution were dropped on a circular filter by using a specially developed hanging drop kit. When the filter was exposed to HCHO gas at the rate of 400 ml/min, AHMT reagent on the filter reacted with HCHO to produce a change in its color. The color change was recorded by measuring the intensity of reflected light using a photodiode, which was illuminated by an LED at 540 nm. The response was defined as % of the voltage drop. The filter color intensity increased with sampling time in a concentration range of 0.04–1 ppm HCHO, and also the filter gave no response in the absence of HCHO. The limit-of-detection (LOD) of 0.04 ppm HCHO within a sampling time of 3 min was achieved. Thus, this method could be used to detect HCHO at the WHO standards (0.08 ppm) within 3 min. Moreover, our device did not respond to other SBS causing reagents, such as other aldehydes and volatile organic compounds. In conclusion, we have developed a rapid, selective and simple HCHO gas sensor for SBS, which is also a suitable device for on-field operations.
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paper_title: Determination of formaldehyde in air by gas chromatography

paper_content:

Abstract Formaldehyde in air was analyzed by gas chromatography (GC) using instruments equipped with thermal conductivity, flame-ionization and photoionization detectors. A method of making formaldehyde standards in air, to calibrate the GC system, was tested. The rate of polymerization increase in air, with increase in relative humidity, was also determined. The application of the method to the determination of formaldehyde desorbing from urea-formaldehyde foam and particle board is discussed.
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paper_title: Microfabricated Formaldehyde Gas Sensors

paper_content:

Formaldehyde is a volatile organic compound that is widely used in textiles, paper, wood composites, and household materials. Formaldehyde will continuously outgas from manufactured wood products such as furniture, with adverse health effects resulting from prolonged low-level exposure. New, microfabricated sensors for formaldehyde have been developed to meet the need for portable, low-power gas detection. This paper reviews recent work including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, and nanowire-based sensors. This paper also investigates the promise of polymer-based sensors for low-temperature, low-power operation.
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paper_title: Emission behavior of formaldehyde and TVOC from engineered flooring in under heating and air circulation systems

paper_content:

Abstract Formaldehyde and volatile organic compounds (VOCs) from the adhesive, flooring, and flooring with adhesive were measured using a desiccator, a 20-L chamber and a field and laboratory emission cell (FLEC). Flooring with an adhesive is similar to that used in construction was applied to a floor heating system and an air circulation system, and the surface temperature of the flooring was set to 20 °C, 26 °C and 32 °C. The rate of formaldehyde emission from the flooring was the highest at 32 °C using a desiccator and decreased with time. The formaldehyde and aldehyde emissions from the samples using a 20-L chamber and FLEC showed a similar tendency. The VOCs emission trends with the 20-L chamber and FLEC were similar. The rate of formaldehyde and TVOC emission determined using FLEC was higher than that determined using the 20-L chamber method. The flooring emitted primarily benzene, toluene, ethylbenzene, styrene, xylene, as well as some unknown VOCs. There was a strong correlation between formaldehyde and TVOC emission for the 20-L chamber and FLEC. Samples using a floor heating system showed higher formaldehyde emission than those using an air circulation system. The level of TVOC emission was higher from the samples using an air circulation system than those using the floor heating system.
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paper_title: A simplified apparatus for ambient formaldehyde detection via GC-pHID

paper_content:

Abstract An apparatus has been developed for the direct determination of ambient, atmospheric formaldehyde using gas chromatography. A loop sample of 1–6 ml is transferred onto the column and the formaldehyde is focused with a cold trap. Following separation, formaldehyde is detected with an argon doped, pulsed discharge helium ionization detector. Absolute values are obtained via calibration with a permeation source. A detection limit of 42 pptv is calculated. The relatively high duty cycle (∼11 h−1) allows measurements to be taken on timescales that are short compared to the atmospheric lifetime of formaldehyde. The reliability of the apparatus has been investigated via a 4-week field campaign at an urban site in Leeds, UK. Diurnal profiles are observed with evidence for both primary emissions from traffic and secondary, photochemical production. A decrease in formaldehyde concentrations was observed during rain events.
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paper_title: New chemiluminescent derivatizing agent for the analysis of aldehydes and ketones by high-performance liquid chromatography with peroxyoxalate chemiluminescence

paper_content:

Abstract Three methods of reductive amination have been investigated for the derivatization of aldehydes and ketones with a new chemiluminescent agent, 3-aminofluoranthene. Use of a borane-pyridine complex was found to give the highest product yield. Analytes were separated by high-performance liquid chromatography (HPLC) and detected by peroxyoxalate chemiluminescence. Detection limits were in the fmol range, chemiluminescence detectability being thirty times better than fluorescence.
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paper_title: Workplace air sampling and polarographic determination of formaldehyde.

paper_content:

A polarographic method has been developed for the determination of formaldehyde in workplace air. Samples are collected in midget fritted glass bubblers containing an aqueous solution of 10% methanol. The collected formaldehyde in the absorbent solution is derivatized with hydrazine reagent to form a formaldehyde hydrazone compound. An aliquot of the resulting solution is analyzed by differential pulse polarography at a dropping mercury electrode. The sample generation system, impinger collection, polarographic analysis, and precision and accuracy data are described. The method was validated over the range of 5.8 to 17.7 mg/cu m, which corresponds to 0.5 to 2 times the (peak) OSHA-PEL for a 30-liter sample at a flow rate of 1 Lpm. The average recovery was 103%. The pooled coefficient of variation or relative standard deviation was 0.08.
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paper_title: Sensory eye irritation in humans exposed to mixtures of volatile organic compounds.

paper_content:

Eight subjects participated in a controlled eyes-only exposure study of human sensory irritation in ocular mucosal tissue. The authors investigated dose-response properties and the additive effects of three mixtures of volatile organic compounds. The dose-response relationships for these mixtures showed increases in response intensity as concentration increased. Replication of exposure did not result in significantly different dose- response relationships. Moreover, the result implied that components of the three mixtures interacted additively to produce ocular irritation, a result referred to as simple agonism. Finally, the authors addressed the comparability of two methods to measure sensory irritation intensity (visual analogue scale and a comparative scale). The results indicated that the two rating methods produced highly comparable results.
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paper_title: On-line detection of atmospheric formaldehyde by a conductometric biosensor.

paper_content:

Atmospheric formaldehyde (CH(2)O) was detected under continuous flow conditions by an on-line system comprising of a wet scrubber for a continuous transfer of the pollutant to an aqueous solution, a micro-reactor containing immobilized formaldehyde dehydrogenase (FDH) and a conductometric transducer. By this system atmospheric formaldehyde concentrations in the range 0.05-2 ppm were detected with a sensitivity of 20 microS/ppm. In this concentration range the immobilized enzyme oxidized all the sampled formaldehyde molecules to formic acid, avoiding cumbersome calibration procedures. The operational stability of the biosensor was at least 3 months, working continuously 10 h/day at room temperature.
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paper_title: Portable sick house syndrome gas monitoring system based on novel colorimetric reagents for the highly selective and sensitive detection of formaldehyde.

paper_content:

Formaldehyde (HCHO) emitted from the furniture and the walls in the rooms injures the eyes, nose, and respiratory organs and causes allergies, which is called sick house syndrome. We designed and synthesized novel colorimetric HCHO-sensing molecules (KD-XA01 and KD-XA02) which possess an enaminone structure and developed a hand-held instrument to monitor indoor HCHO gas with the use of KD-XA01. These sensing molecules produced speedy color changes from colorless to yellow under mild conditions, which was caused by the fact that the enaminone structure in the reagent reacts with HCHO to give a lutidine derivative. This reaction took place not only in the solution phase but also in the solid phase (surface of the cellulose paper). To take advantage of this phenomena, a handy and rapid monitoring system has been developed for detecting indoor HCHO gas using a highly sensitive and selective detection tablet constructed from the porous cellulose paper that contains silica gel as an adsorbent, KD-XA01, and phosphoric acid under optimum conditions. This instrument detected the surface color change of the tablet from white to yellow, which was monitored as a function of the intensity of the reflected light illuminated by an LED (475 nm). The response was proportional to the HCHO concentration at a constant sampling time and flow rate; 0.05 ppm HCHO, which is under the standard value set by the World Health Organization, was able to be detected in 5 min. The detection limit was 0.005 ppm. This monitoring system was not interfered by carbonyl compounds such as acetaldehyde and acetone, alcohols, hydrocarbons, and typical gases such as carbon monoxide, carbon dioxide, nitrogen dioxide, etc., which contributes to the measurement of correct HCHO concentrations. It was possible to monitor the HCHO gas in the room of a new apartment and school using this instrument; the response values were in good agreement with those obtained by the standard DNPH method. This highly sensitive, selective, and handy HCHO gas monitor is widely applicable and convenient for users who are not specialists in this field.
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paper_title: Real-time detection of formaldehyde by a sensor

paper_content:

A hand-held formaldehyde (HCHO) gas detector has been developed. This fluorescence-based system uses a standard microscope glass slide both as a transducer and a sensor. The detection of gaseous formaldehyde is performed using the well-known Sawicki reaction with Fluoral-P, a reagent present in the nanoporous thin film which allows a selective reaction with formaldehyde. The nanoporous film of hybrid organic-inorganic polymer, prepared via the sol-gel method, is coated on the microscope slide by spin-coating, enabling reproducible coatings of various shapes and thicknesses. Experiments were carried out in the laboratory to calibrate the instrument, with the objective to perform measurements indoor and outdoor. The real-time monitoring allows to detect formaldehyde pollution peaks.
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paper_title: Development of a novel hand-held formaldehyde gas sensor for the rapid detection of sick building syndrome

paper_content:

Abstract The majority of conventional sensors for monitoring formaldehyde (HCHO) in air have several drawbacks, in terms of sensitivity, selectivity, and sampling time. In this report, a novel hand-held HCHO gas sensor for the sick building syndrome (SBS) prevention and control is developed with a significantly short sampling time. A well-described reagent, 4-amino hydrazine-5-mercapto-1,2,4-triazole (AHMT) was utilized in the development of the novel gas sensor. AHMT of 100 μl and potassium hydroxide solution were dropped on a circular filter by using a specially developed hanging drop kit. When the filter was exposed to HCHO gas at the rate of 400 ml/min, AHMT reagent on the filter reacted with HCHO to produce a change in its color. The color change was recorded by measuring the intensity of reflected light using a photodiode, which was illuminated by an LED at 540 nm. The response was defined as % of the voltage drop. The filter color intensity increased with sampling time in a concentration range of 0.04–1 ppm HCHO, and also the filter gave no response in the absence of HCHO. The limit-of-detection (LOD) of 0.04 ppm HCHO within a sampling time of 3 min was achieved. Thus, this method could be used to detect HCHO at the WHO standards (0.08 ppm) within 3 min. Moreover, our device did not respond to other SBS causing reagents, such as other aldehydes and volatile organic compounds. In conclusion, we have developed a rapid, selective and simple HCHO gas sensor for SBS, which is also a suitable device for on-field operations.
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paper_title: Real-time detection of formaldehyde by a sensor

paper_content:

A hand-held formaldehyde (HCHO) gas detector has been developed. This fluorescence-based system uses a standard microscope glass slide both as a transducer and a sensor. The detection of gaseous formaldehyde is performed using the well-known Sawicki reaction with Fluoral-P, a reagent present in the nanoporous thin film which allows a selective reaction with formaldehyde. The nanoporous film of hybrid organic-inorganic polymer, prepared via the sol-gel method, is coated on the microscope slide by spin-coating, enabling reproducible coatings of various shapes and thicknesses. Experiments were carried out in the laboratory to calibrate the instrument, with the objective to perform measurements indoor and outdoor. The real-time monitoring allows to detect formaldehyde pollution peaks.
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paper_title: Microfabricated Formaldehyde Gas Sensors

paper_content:

Formaldehyde is a volatile organic compound that is widely used in textiles, paper, wood composites, and household materials. Formaldehyde will continuously outgas from manufactured wood products such as furniture, with adverse health effects resulting from prolonged low-level exposure. New, microfabricated sensors for formaldehyde have been developed to meet the need for portable, low-power gas detection. This paper reviews recent work including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, and nanowire-based sensors. This paper also investigates the promise of polymer-based sensors for low-temperature, low-power operation.
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paper_title: Detection of formaldehyde vapor using mercaptophenol-coated piezoresistive cantilevers

paper_content:

Piezoresistive cantilevers coated with mercaptophenol self-assembled monolayers were used to detect formaldehyde vapor at room temperature. The three isomers of mercaptophenol, namely 2-, 3-, and 4-mercaptophenol, were tested as probe molecules. The 3-mercaptophenol-coated cantilever showed the largest deflection due to high reactivity between formaldehyde and 3-mercaptophenol. When the 3-mercaptophenol-coated cantilever was exposed to interferents such as benzene, toluene, p-xylene, water, or ethanol, the deflection direction of the cantilever was the opposite of that induced by the chemical reaction with formaldehyde, indicating that 3-mercaptophenol is very specific to formaldehyde. The detection limit of the 3-mercaptophenol-coated cantilever was determined to be better than 0.01 ppm.
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paper_title: A facile and highly sensitive colorimetric sensor for the detection of formaldehyde based on electro-spinning/netting nano-fiber/nets

paper_content:

Abstract A novel strategy for highly sensitive colorimetric detection of gaseous formaldehyde was developed based on Methyl Yellow-impregnated electro-spinning/netting (ESN) nylon 6 nano-fiber/nets (NFN). The sensor presented a significant reflectance intensity decreasing band at 550 nm which induce the visual color changes from yellow to red after exposure to formaldehyde and allowed for the detection of formaldehyde with a low detection limit of 50 ppb. Upon exposure to a series of volatile organic compounds (VOCs), only formaldehyde could induce a yellow-to-red color change observable by the naked eye, which clearly exhibited that Methyl Yellow-impregnated nylon 6 NFN membranes could act as highly selective and sensitive strips to detect formaldehyde with minor interference from other VOCs. Additionally, the colorimetric sensor showed good reproducibility under cyclic sensing experiments. Furthermore, the colorimetric responses were visualized quantitative by using a color-differentiation map prepared form converted RGB (red, green and blue) values. As-prepared Methyl Yellow-impregnated nylon 6 NFN sensor strips successfully combined with the visualized detection of color map and fascinating structure of NFN membranes, which indicated promising composite materials as a simple and economical alternative to replace the traditional formaldehyde sensors and facilitated the design and development of other label-free colorimetric sensors toward various analytes based on the NFN template.
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paper_title: Determination of ultra-trace formaldehyde in air using ammonium sulfate as derivatization reagent and capillary electrophoresis coupled with on-line electrochemiluminescence detection.

paper_content:

The reaction between formaldehyde and ammonium ion to produce hexamethylenetetramine is well known. The reaction conditions are very easily controlled in situ and the experiment operation is very simple. However, such derivatization reaction for trace formaldehyde determination using capillary electrophoresis (CE) electrochemiluminescence (ECL) has not been reported before. In this study, the application of ammoniun sulfate as derivatization reagent to in-situ determination of formaldehyde in air was reported. Based on ECL enhancement of tris(2,2'-bipyridyl)ruthenium(II) with hexamethylenetetramine, a novel approach for the determination of ultra-trace formaldehyde in air using CE coupled with on-line ECL of tris(2,2'-bipyridyl)ruthenium(II) has been developed. The parameters affecting separation and detection such as detection potential, concentration and pH of phosphate buffer, and electrokinetic voltage, were investigated. Under the optimal conditions, the linear concentration range of formaldehyde in air was from 0.48 μg/m(3) to 96 mg/m(3) (linear range covering 5 orders of magnitude). The limit of detection (3σ) was 0.15 μg/m(3). The relative standard deviations of peak height and migration time for six consecutive injection of 1 ng/mL formaldehyde derivative were 0.9% and 0.8%, respectively. The recoveries of formaldehyde in air were between 99.3% and 101%.
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paper_title: Chemical sensors: definitions and classification

paper_content:

A chemical sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal. The chemical information, mentioned above, may originate from a chemical reaction of the analyte or from a physical property of the system investigated.
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paper_title: Ultraviolet-assisted gas sensing: A potential formaldehyde detection approach at room temperature based on zinc oxide nanorods

paper_content:

Zinc oxide nanorods were synthesized using a simple hydrothermal method. Formaldehyde-sensing characteristics were investigated without and with ultraviolet (UV) light irradiation. The gas response of nanorods to 110 ppm formaldehyde with UV light irradiation was about 120 times higher than that without UV light irradiation. The detection limit was as low as 1.8 ppm at room temperature. This high gas response under the UV light is attributed to photocatalytic oxidation. Furthermore, the significant improvements of the selectivity and recovery time were obtained after irradiating the nanorods with UV light. Our results demonstrated that applying UV light irradiation on the zinc oxide nanorods is an effective approach to achieve higher response and excellent selectivity to formaldehyde at room temperature.
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paper_title: Direct detection of formaldehyde in air by a novel NAD+- and glutathione-independent formaldehyde dehydrogenase-based biosensor.

paper_content:

An amperometric enzyme-based sensor-system for the direct detection of formaldehyde in air is under investigation. The biosensor is based on a native bacterial NAD(+)- and glutathione-independent formaldehyde dehydrogenase as biorecognition element. The enzyme was isolated from Hyphomicrobium zavarzinii strain ZV 580, grown on methylamine hydrochloride in a fed-batch process. The sensor depends on the enzymatic conversion of the analyte to formic acid. Released electrons are detected in an amperometric measurement at 0.2V vs. Ag/AgCl reference electrode by means of a redox-mediator. To optimize the sensing device, Ca(2+) and pyrroloquinoline quinone (PQQ) were added to the buffer solution as reconstitutional substances. At this stage, the sensor shows linear response in the tested ppm-range with a sensitivity of 0.39 microA/ppm. The signal is highly reproducible with respect to sensitivity and base line signal. Reproducibility of sensitivity is more than 90% within the same bacterial batch and even when enzyme of different bacterial batches is used.
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paper_title: Solubility of gaseous formaldehyde in liquid water and generation of trace standard gaseous formaldehyde

paper_content:

Stable concentrations of gaseous formaldehyde (atm) may be generated at the 10/sup -9/-atm level and up by equilibration of air with an aqueous formaldehyde (M) solution through a microporous membrane tube. The solubility behavior of formaldehyde at 293.2 K obeys the equation (HCHO(aq)) = 16650(HCHO(g))/sup 1.0798/ for HCHO(g) ranging from 3.3 x 10/sup -9/ to 1.3 x 10/sup -6/ atm or HCHO(aq) ranging from 1.0 x 10/sup -5/ to 6.9 x 10/sup -3/ M. When the aqueous concentration is expressed in mole fraction units, the predictions of the corresponding equation are in good agreement with the vapor pressure data obtained by Blair and Ledbury at aqueous concentrations as high as X/sub HCHO/ = 0.254 (13.4 M). At low HCHO concentrations, the temperature dependence between 278.2 and 298.2 K is given by (HCHO(aq)) = 10/sup ((4538/T)-11.34)/ (HCHO(g))/sup ((252.2/T) + 0.2088) with an uncertainty of less than 10%.
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paper_title: Study on a micro-gas sensor with SnO2-NiO sensitive film for indoor formaldehyde detection

paper_content:

Abstract A micro-gas sensor is fabricated based on a micro-hotplate (MHP) with micromachining technology. The SnO 2 –NiO nanometer polycrystalline composite synthesized by a chemical co-precipitation method is coated onto a micro-hotplate as a sensitive film. The response and recovery time of the micro-gas sensor decrease rapidly with increasing the formaldehyde (HCHO) concentration at relatively low concentrations. The micro-gas sensor shows good stability, low power consumption of 180 mW at the MHP operation temperature of 300 °C, and high selectivity in the presence of interference gases such as alcohol, toluene, α-pinene and acetone. The micro-gas sensor can detect 0.06 ppm HCHO, which is the indoor exposure limit for HCHO referring to Chinese environmental protection agency (EPA).
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paper_title: Formaldehyde sensor based on Ni-doped tetrapod-shaped ZnO nanopowder induced by external magnetic field

paper_content:

Abstract The sensors based on Ni-doped ZnO nanopowder with tetrapod-shape (T-ZnO) were fabricated by screen-printing technique with external magnetic field in different direction. The morphologies and crystal structures of the thick film were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), respectively. Gas-sensing property of sensors responded to 100 ppm formaldehyde was also detected. The results show that the direction of magnetic field has crucial effect on the sensor sensitivity. The sensors based on 5 wt% Ni-doped T-ZnO induced by magnetic field in parallel direction to the thick film surface, has the optimization sensitivity, the shortest response and recovery time, which are 10.6, 16 and 15 s, respectively. The magnetic-field induction model and the gas-sensing mechanism of the Ni-doped T-ZnO are proposed.
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paper_title: High sensitive and selective formaldehyde sensors based on nanoparticle-assembled ZnO micro-octahedrons synthesized by homogeneous precipitation method

paper_content:

Abstract Nanoparticle-assembled ZnO micro-octahedrons were synthesized by a facile homogeneous precipitation method. The ZnO micro-octahedrons are hexagonal wurtzite with high crystallinity. Abundant structure defects were confirmed on ZnO surface by photoluminescence. Gas sensors based on the ZnO micro-octahedrons exhibited high response, selectivity and stability to 1–1000 ppm formaldehyde at 400 °C. Especially, even 1 ppm formaldehyde could be detected with high response ( S = 22.7). It is of interest to point out that formaldehyde could be easily distinguished from ethanol or acetaldehyde with a selectivity of about 3. The high formaldehyde response is mainly attributed to the synergistic effect of high contents of electron donor defects (Zn i and V O ) and highly active oxygen species (O 2− ) on the ZnO surface.
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paper_title: Enhanced Sensing Characteristics in MEMS-based Formaldehyde Gas Sensor

paper_content:

This paper presents a novel micro fabrication for formaldehyde gas sensors to enhance sensitivity and detection resolution capabilities. Therefore, two different types of fabrication sequences of gas sensors were considered, different positions of micro heaters and sensing layers to compare the effects of different areas of the sensing layers contact with the surrounding gas. The MEMS-based formaldehyde gas sensor consists of a quartz substrate, a thin-film NiO/Al2O3 sensing layer, an integrated Pt micro-hotplate, and Pt inter-digitized electrodes (IDEs) to measure the resistance variation of sensing layers caused by formaldehyde oxidation at the oxide surface. This abstract offers comparisons of the characteristics of sensors in different areas of the sensing layers contacting the surrounding gas as well as those to decrease the thickness of the sensing layer and deposits of the sensing layer using co-sputtering technology with NiO/Al2O3 to improve the sensitivity limits of the sensors. The experimental data indicated that increasing the area of the sensing layer that contacts with the surrounding gas and decreasing the thickness of the sensing layer in the sputtering process and then co-sputtered NiO/Al2O3 sensing layers, significantly enhanced the sensing characteristics of the developed formaldehyde sensor.
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paper_title: Studies of influence of structural properties and thickness of NiO thin films on formaldehyde detection

paper_content:

Abstract The aim of the present work is to test the performance of nickel oxide (NiO) thin films to low concentrations of formaldehyde (HCHO). NiO thin films were deposited on alumina substrates by RF reactive magnetron sputtering in a mixed atmosphere of argon and oxygen. A Pt heating resistor was deposited on the reverse side for an accurate control of the operating temperature. Samples were annealed in synthetic air for 4 h at 700 °C in order to stabilise their microstructure. Two different thicknesses (150 and 300 nm) were deposited in order to study the influence of this parameter on both the microstructure and sensor response. Both XRD analysis and FEG-SEM images show a smaller grain size for the 150 nm-thick samples. The best operating temperature was established at 340 and 300 °C for 150 and 300 nm-thick samples respectively. A higher sensitivity was obtained for the samples of lower thickness for a set of HCHO concentrations ranging from 5 to 20 ppm. Moreover, the repeatability of the experiments was tested for the most sensitive samples.
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paper_title: CdO activated Sn-doped ZnO for highly sensitive, selective and stable formaldehyde sensor

paper_content:

Sn-, Ni-, Fe- and Al-doped ZnO and pure ZnO are prepared by coprecipitation method, and characterized by scanning electron microscope (SEM), energy diffraction spectra (EDS) and X-ray diffraction (XRD). Their formaldehyde gas sensing properties are evaluated and the results show that 2.2 mol% Sn dopant can increase the response of ZnO by more than 2 folds, while other dopants increase little response or even decrease response. Further, CdO is used to activate ZnO based formaldehyde sensing material. It is demonstrated that 10 mol% CdO activated 2.2 mol% Sn-doped ZnO has the highest formaldehyde gas response, with a linear sensitivity of similar to 10/ppm at lowered work temperature of 200 degrees C than 400 degrees C of pure ZnO, and high selectivity over toluene, CO and NH(3), as well as good stability tested in 1 month. (C) 2011 Published by Elsevier B.V.
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paper_title: Shuttle-like ZnO nano/microrods: Facile synthesis, optical characterization and high formaldehyde sensing properties

paper_content:

Abstract Shuttle-like ZnO nano/microrods were successfully synthesized via a low temperature (80 °C), “green” (without any organic solvent or surfactant) and simple hydrothermal process in the solution of zinc chloride and ammonia water. X-ray diffraction and Raman spectroscopy indicated that the ZnO nano/microrods are a well-crystallized hexagonal wurtzite structure. Yet photoluminescence analysis showed that abundant intrinsic defects (52.97% electron donor defects and 45.49% electron acceptor defects) exist on the surface of ZnO crystals. Gas sensors based on the shuttle-like ZnO nano/microrods exhibited high sensitivity, rapid response–recovery and good selectivity to formaldehyde in the range of 10–1000 ppm at an optimum operating temperature of 400 °C. Through applying linear fitting to the plot of sensitivity versus formaldehyde concentration in logarithmic forms, the chemisorbed oxygen species on the ZnO surface were found to be O 2− (highly active among O 2 , O 2 − and O − species). Notably, formaldehyde can be easily distinguished from acetaldehyde with a selectivity of about 3. The high formaldehyde sensitivity is mainly attributed to the synergistic effect of abundant electron donor defects (52.97%) and highly active oxidants (surface adsorbed O 2− species) co-existed on the surfaces of ZnO.
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paper_title: NiO thin-film formaldehyde gas sensor

paper_content:

Abstract The suitability of both pure and Li-doped NiO as a thin-film resistive gas sensor for formaldehyde has been investigated. Pure NiO had a linear formaldehyde sensitivity of 0.825 mV ppm −1 while that for 0.5 at.% Li-doped NiO was 0.488 mV ppm −1 at 600°C. These gas-sensing materials also showed similar sensitivity for methanol and acetone as well as a reduced sensitivity for toluene and ethanol. Chloroform was a poison for these gas-sensing materials. Due to resistive noise, the detection limit for formaldehyde was found to be ≈40 ppm.
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paper_title: Ultraviolet-assisted gas sensing: A potential formaldehyde detection approach at room temperature based on zinc oxide nanorods

paper_content:

Zinc oxide nanorods were synthesized using a simple hydrothermal method. Formaldehyde-sensing characteristics were investigated without and with ultraviolet (UV) light irradiation. The gas response of nanorods to 110 ppm formaldehyde with UV light irradiation was about 120 times higher than that without UV light irradiation. The detection limit was as low as 1.8 ppm at room temperature. This high gas response under the UV light is attributed to photocatalytic oxidation. Furthermore, the significant improvements of the selectivity and recovery time were obtained after irradiating the nanorods with UV light. Our results demonstrated that applying UV light irradiation on the zinc oxide nanorods is an effective approach to achieve higher response and excellent selectivity to formaldehyde at room temperature.
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paper_title: Visible-light activate mesoporous WO3 sensors with enhanced formaldehyde-sensing property at room temperature

paper_content:

Abstract The formaldehyde (HCHO) sensing properties of mesoporous WO 3 sensors were investigated via being activated by the visible light at room temperature. It is found that the response of the mesoporous WO 3 sensors to HCHO (100 ppm) reached to 1.3 × 10 −7 (Ωs) −1 under white light (100 W/m 2 ) and 9.5 × 10 −8 (Ωs) −1 under blue light (100 W/m 2 ), respectively, which are 4 times and 9 times as much as the response of the commercial WO 3 , mainly due to the high surface area and the existence of the mesopores. Moreover, the response of the WO 3 sensors is linearly increased with the increase of the light intensity and the formaldehyde concentration, and high humidity significantly decreases the gas sensing properties of the mesoporous WO 3 under visible light. Also, compared to the sensors made by commercial WO 3 material, the mesoporous WO 3 sensors exhibit high sensitivity to acetone and toluene. Enhancement of the gas sensing properties of the mesoporous WO 3 is attributed to the higher surface interaction rate among photoelectron, photoinduced oxygen ion and target gas.
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paper_title: Investigation on formaldehyde gas sensor with ZnO thick film prepared through microwave heating method

paper_content:

Abstract ZnO nanopowders were prepared through microwave heating method. ZnO thick film sensors were fabricated by using ZnO nanopowders as sensing materials. The phase composition and morphology of the material particles were characterized by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The gas-sensing properties of the sensors based on ZnO nano-materials were investigated. It was found that the sensor based on ZnO nano-materials (low power, 10× 10 min) exhibited very high responses to benzene and toluene when operating at 440 and 370 °C, respectively; but the sensor based on ZnO (low power, 10× 10 min) showed very low responses to benzene and toluene when operating at 205–215 °C. The sensor based on ZnO (low power, 10× 10 min) showed high response and good selectivity to dilute formaldehyde when operating at 210 °C; especially, the response to 0.001 ppm HCHO attained 7.4 when operating at 210 °C.
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paper_title: Multi-wall carbon nanotube gas sensors modified with amino-group to detect low concentration of formaldehyde

paper_content:

Abstract Gas sensors with multi-wall carbon nanotubes (MWCNTs) modified with amino-groups on interdigitated electrodes (IDE) were fabricated to detect low concentration of formaldehyde at room temperature. Effects of content of amino groups on sensing responses against various interfering circumstances and low concentration of formaldehyde were investigated. The sensor behaved high relative resistance changes to formaldehyde and lower response to interfering gases such as acetone, carbon dioxide, ammonia, toluene and ethanol. When the concentration of formaldehyde was 20 ppb, the relative resistance changes of the sensor modified with 18.19% amino-group reached 1.73%. The sensor displayed high chemical selectivity, fast response and good reproducibility to low concentration of formaldehyde, which was attributed to the properties of MWNTs and the interaction between the surface of MWCNTs and amino-group.
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paper_title: Fabrication and formaldehyde gas-sensing property of ZnO–MnO2 coplanar gas sensor arrays

paper_content:

Abstract Nanostructured flat-type coplanar gas sensor arrays of ZnO with different MnO 2 additive concentration were fabricated by a combination of screen-printing technology and solution growth process. The morphologies and crystal structures were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The results showed that ZnO nanostructures were induced on the surface of gas sensor arrays through the solution growth process. The nanostructures were composed of nanowalls and nanosheets with the thickness of about 50–200 nm and length of about 1–2 μm. The resistance–temperature characteristic and the response to formaldehyde revealed that the solution growth process could effectively control the morphologies, change the aspect ratio of ZnO nanostructures and significantly improve the response of gas sensor arrays. In addition, the response to formaldehyde of ZnO sensors with 0.5 wt% and 1.0 wt% MnO 2 additive was higher than that of pure ZnO sensor.
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paper_title: A self-heating gas sensor with integrated NiO thin-film for formaldehyde detection

paper_content:

Abstract This study develops a MEMS-based formaldehyde gas sensor based on a suspended silicon nitride microstructure with an integrated micro Pt heater, a thin-film NiO sensing layer and Pt interdigitated electrodes (IDEs) to measure the resistance changes of the NiO layer in the presence of formaldehyde. A specific orientation of the NiO layer is observed as the substrate temperature in the sputtering process is increased. The increase in substrate temperature assists in the formation of a NiO layer with the correct stoichiometric ratio (1:1). When formaldehyde is present in the atmosphere, oxidation occurs near the heated NiO sensing layer. This oxidization causes a change in the electrical conductivity of the NiO film, and hence changes the measured resistance between the interdigitated electrodes. The formaldehyde concentration is then determined from the change in the measured resistance. The application of a voltage to the Pt heaters causes the temperature of the micro-hotplate to increase, which in turn enhances the sensitivity of the sensor. The current experimental results show that the sub-micrometer grain sizes of the sputtered oxide thin film yield a high degree of sensitivity (0.33 Ω ppm −1 ), a low hysteresis value (0.7 ppm), a detection capability of less than 0.8 ppm, a quick response time (13.2 s), a quick recovery time (40.0 s) and a high selectivity over a wide range of formaldehyde concentrations in the presence of interfering species, such as acetone, ethanol and methanol. The novel micro formaldehyde gas sensor developed in this study is ideal for applications aimed at preventing and controlling sick building syndrome (SBS).
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Title: Formaldehyde Gas Sensors: A Review

Section 1: Origin and Measurement Technology of Formaldehyde in Air
Description 1: Describe the origins of formaldehyde, its risks, current exposure limits, and the evolution of measurement technologies for its detection, including spectrophotometry and gas chromatography.

Section 2: Receptor-Based Formaldehyde Sensors
Description 2: Provide an overview of receptor-based sensors that transform chemical information into measurable energy, including spectrometric, piezoresistive, and colorimetric types.

Section 3: Spectrometric Type
Description 3: Detail the operation, principles, and examples of spectrometric formaldehyde sensors, including those based on color change detection and fluorescence emission.

Section 4: Piezoresistive Type
Description 4: Explain the mechanisms and examples of piezoresistive formaldehyde sensors, focusing on how resistance changes with mechanical strain and chemical exposure.

Section 5: Colorimetric Type
Description 5: Discuss the design and operation of colorimetric sensors, including changes in reflectance intensity and their responses to formaldehyde concentrations.

Section 6: Transducer-Based Formaldehyde Sensors
Description 6: Outline the design and functioning of transducer-based formaldehyde sensors, including amperometric and conductimetric types, and their application in real-time measurement.

Section 7: Amperometric Type
Description 7: Present the principles and examples of amperometric sensors for formaldehyde detection, including the role of electric current changes in response to gas presence.

Section 8: Conductimetric Type
Description 8: Describe the mechanisms and examples of conductimetric sensors, focusing on conductivity changes due to adsorption of formaldehyde gas and the use of metal-oxide semiconductors.

Section 9: Conclusions
Description 9: Summarize the advances in sensor technology, compare the discussed sensors, and highlight the applicability and potential for various formaldehyde gas sensors in low-cost, high-performance applications.