Preparation, characterization and gas-sensing properties of mesoporous SnO₂ powders have been described. SnO₂ powders prepared from sodium stannate by utilizing a self-assembly of n-cetylpyridinium chloride were characterized to have hexagonally ordered mesoporous structure (d₁₀₀≈4.4 nm). Treatment of the as-prepared powders with phosphoric acid was effective for depressing the growth of SnO₂ crystallites and then maintaining the ordered mesoporous structure (d₁₀₀≈3.1 nm, mean pore diameter≈1.5-2.5 nm) up to elevated temperatures, while the pore volume decreased. H₂ sensitivity of mesoporous SnO₂ was largely dependent on its specific surface area, the sensitivity being highest (R_g/R_a≈337 to 1000 ppm H₂ at 400°C) with the highest specific surface area (374 m² g^-1 after calcination at 600°C). Mesoporous SnO₂ powder with larger mesopores (mean pore diameter: ca. 5.2 nm, surface area: 198 m² g^-1 after calcination at 600°C), which was prepared by utilizing a block copolymer with a large molecular weight (ca. 5800) as a surfactant, showed higher H₂ sensitivity (R_g/R_a≈77 to 1000 ppm H₂ at 400°C) than that of n-cetylpyridinium chloride-derived SnO₂ with the same surface area but smaller mesopores. However, electrical resistance levels in air of the monolith sensors based on the mesoporous materials were close to the limit for practical measurement. Then, the mesoporous SnO₂ was utilized as a surface coating layer of conventional SnO₂ powder, which was prepared by pyrolysis of tin oxalate. The coating was effective for improving the gas-sensing properties, while maintaining the sensor resistance in air at a practical level. For example, NO_x sensitivity of conventional SnO₂ markedly increased with the coating, while H₂ sensitivity of the coated sensor was lower than that of mesoporous SnO₂.

A fabricated hydrogen sensor diode of Pt - thin TiO_x layer - SiC structure was found to exhibit a large voltage response up to 3 volts. Diodes with and without the TiO_x layer were compared in terms of current-voltage characteristics. The difference in voltage obtained in an H₂ atmosphere and that obtained in an O₂ atmosphere at a constant forward current was studied. A simple model consisting of a resistance and Schottky diode connected in series was assumed. The voltage change results from changes in both barrier height and series resistance. The change in series resistance is due to a property of TiO_x under exposure to hydrogen.

Resistor-type sensor devices using a samaria-doped ceria electrolyte with different working electrodes were fabricated for detecting carbon monoxide (CO) in reformed gases between 250 and 425°C. The electrochemical oxidation of H₂ over a Pd working electrode was the most sensitive to a reversible poisoning by 50-4000 ppm CO in the presence of 50% hydrogen, 10% carbon dioxide, and 6% water vapor, which provided an increase in the electrode-reaction resistance at the electrode-electrolyte interface. The resulting sensor signal was strongly dependent on the CO concentration, where linear relationships between the signal and the logarithm of the CO concentration were obtained, especially below 1000 ppm with 90% response and 90% recovery times of about 20 seconds. An improvement in the CO-sensing properties of the present sensor device above 300°C was also carried out by the addition of Au to the Pd working electrode.

A simple gas sensor system applicable for monitoring various environmentally deteriorating gases was fabricated by the combination of a newly developed cerium ion conducting solid electrolyte, (Ce_xZr_1-x)_4/(4-x)Nb(PO₄)₃, and yttria stabilized zirconia (YSZ). By applying the oxycarbonate or the potassium nitrite mixed gadolinium oxide based solid solution as an auxiliary electrode, the sensor can detect CO₂ gas or NO gas, respectively, with demonstrating a rapid, continuous, and reproducible response, which obeys the theoretical Nernst relationship. The present sensor fabrication proposes the approaches to pursue for detecting various gases only by exchanging the auxiliary electrode suitable for the target gas sensing.

The mixed-potential type NO_x sensor attached with the oxidation-catalyst electrode was fabricated by laminating and sintering YSZ green-sheets, on which an NO_x sensing electrode consisting of metal oxide (Cr₂O₃), an O₂ sensing electrode, an NO_x conversion electrode, Pt heaters, and a pair of thermocouples were printed. The sensor was capable of detecting low concentrations of total NO_x. The use of the oxidation-catalyst electrode was found to decrease largely the interference of reducing gases (C₃H₈, CO) to the NO_x sensitivity. The results obtained here indicate that the present sensor has great possibility of being utilized as an on-board NO_x sensor.

An electrode coated with a bilayer consisting of a polyion complex membrane (PIC) layer on a hemin/ferrocene layer was prepared and applied to measuring H₂O₂. Response to H₂O₂ on the electrode was measured at −0.2 V vs. Ag/AgCl; lower detection limit was 1 μM, linear range, up to 50 μM. Current by interferants was completely prevented. In contrast, an electrode with a monolayer consisting of PIC containing hemin and ferrocene showed a discernible interfering current (current ratio by l-ascorbate to H₂O₂ was ~35%). Thus the bilayer modified electrode has been proved to be useful as a H₂O₂-sensor even if the electrochemical interferants co-exist in the samples.

Organophosphorus pesticides (OPPs), diazinon (0.01-0.05 ppm) and ethylthiometon (0.01-0.2 ppm), were determined by monitoring the activity change of acetylcholinesterase (AChE), based on the chemisorption/desorption-process of thiocholine on a silver electrode as follows. Thiocholine, which was produced through the AChE-catalyzed reaction, was chemisorbed on the silver electrode surface, and then was cathodically desorbed. The electrical charge passed through the desorption process was proportional to the number of adsorbed thiocholine molecules, hence to the AChE activity. Thus the inhibition of the AChE activity by OPPs could be monitored from the decrease in the electrical charge for the desorption process.

An amperometric ADP-sensing electrode was prepared by immobilizing pyruvate kinase and pyruvate oxidase on an electrode coated with an oxygen-permeable, poly (dimethylsiloxane) layer. The concentration of ADP (2 μM-0.4 mM) could be determined from the decrease in the cathodic current at −0.4 V vs. Ag/AgCl. The bienzyme electrode was applied to the determination of cratine kinase (CK); the enzyme activity (4-1,000 U l^-1) was measured from the rate of current decrease, which was caused by the production of ADP through CK-catalyzed reaction.

Piezoelectric devices coated with amino-functional poly(styrene-co-chloromethyl styrene) derivatives were fabricated to develop a simple system that monitors NO₂ gas in air. The introduction of NO₂ gas irreversibly decreased the oscillation frequencies of all of the devices tested. The sensitivity estimated from the hourly change in the frequency demonstrated a ppm level of NO₂ detection. It was also confirmed that other gases, such as O₂, CO₂, and SO₂, functioning as an interference gas, showed little response and good selectivity toward NO₂. The effect of humidity was also not serious.

Amperometric hydrocarbon sensor based on the oxygen pumping current was studied for monitoring hydrocarbon in exhaust gas. It was found that the oxygen pumping current through the sensor using La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ electrolyte at 1 V application monotonically increased with C₃H₆ concentration, but hardly depends on oxygen concentration when Pt and SnO₂ were used for the active and the inactive electrode for oxidation, respectively. The oxygen pumping current of the developed sensor under 1 V application is hardly changed by coexisting CO and H₂O. Consequently, the developed amperometric hydrocarbon sensor can detect hydrocarbon with high selectivity.

A hetero-layered ZnO/SnO₂ thin film was prepared by the sol-gel method to improve the sensing properties for the CO gas. The CO sensitivity and selectivity of the hetero-layered ZnO/SnO₂ thin film were both drastically improved than that of the single-layered Pd-SnO₂ thin film at a high operating temperature range from 300°C to 500°C. It was considered that the columnar ZnO layer deposited onto the SnO₂ layer was very effective in decreasing the influence of the interference gases, such as NO_x.

The determination of heavy metal ions based on quenching of the rare earth luminescence was investigated. A few heavy metal ions efficiently quenched the luminescence of Ce³⁺ in polymerized cellulose films which were obtained by Na-CMC (sodium carboxymethyl cellulose) solution and CeCl₃ solution or the mixed solution of CeCl₃ and a heavy metal chloride. Also, the decay time of Ce³⁺ ions(τ) in Ce³⁺-Cu ⁺, Ce³⁺-Cu²⁺, Ce³⁺-Zn²⁺ and Ce³⁺-Pb²⁺ system was shorter than that of only Ce³⁺ system (τ₀). The linear relationship between the concentration of Cu²⁺, Zn²⁺ or Pb²⁺ ions and τ₀/τ reflected the dynamic quenching, as predicted by the Stern-Volmer equation.

A sensor-chip-module integrated portable surface-plasmon-resonance (SPR) immunosensor was employed for monitoring the indirect competitive immunoreaction of the monoclonal antibody against 2-hydroxybiphenyl between the analyte (2-hydroxybiphenyl, abbreviated as HBP hereafter) in test solution and the HBP-protein conjugate immobilized on sensor chip. This sensor was found to detect HBP rather sensitively in the concentration range of 0.1-1000 ppb. The sensor chip could be reusable several times by removing the antibody anchored to the sensor chip on treatment with a pepsin solution (pH 2.0) for a few minutes.

A novel microfabricated electrochemical hydrogen cyanide gas sensor was developed. This sensor consisted of a gas permeable membrane, an electrolyte solution, a working electrode, a counter electrode, a sensor body and other components. Several microparts were produced using microfabrication techniques; a microporous silicon membrane by DeepRIE, a quartz glass sensor body by dicing, and Ag/Pt/Ti thin membranes as electrodes by ECR sputtering. The size of the sensor was 5×5×3 mm, which was 1/467 when the volume compared with commercially available sensors. This sensor showed good performance for hydrogen cyanide gas detection with respect to linearity and rapid-ness of response.

A microbial chip for bioassay was fabricated and its performance was characterized by scanning electrochemical microscopy (SECM). The microbial chip was fabricated by filling the micropores on a glass substrate with collagen gel embedding Escherichia coli (E.coli) cells. For measuring the activity of E.coli cells, the SECM measurement using ferricyanide as an electron mediator showed that the respiration activity of the immobilized E.coli cells increased with the concentration of glucose in the solution. The results suggested that the microbial chip can be used as a chemical sensor for nutritive species.

The immobilization of two enzymes on a pair of Au microband electrodes was performed by using electrochemical desorption of self-assembled monolayer (SAM) of alkanethiol. Both of the Au electrodes were coated with the SAM of n-octadecanethiol (ODT-SAM) at first. The ODT-SAM on one of the Au electrodes was electrochemically removed by applying reductive potential. The resulting naked Au surface was re-coated with the SAM of 2-aminoethanethiol (AET). These treatments resulted in a couple of Au electrodes coated with the ODT-SAM and AET-SAM, respectively. Horseradish peroxidase (HRP) was selectively immobilized on the AET-SAM by using crosslinking agent, glutaraldehyde. On the other hand, diaphorase (Dp) was immobilized on the surface of ODT-SAM by hydrophobic interaction. The imaging of the resulting substrate with scanning electrochemical microscope (SECM) demonstrated the enzymatic activities of HRP and Dp at the AET- and ODT- treated Au electrodes, respectively.

We recently isolated a novel fructosyl-amine oxidase (FAOD) from the soil bacterium Arthrobacter sp. FV 1-1, cloned the gene encoding FAOD and succeeded in the recombinant production. This bacterial FAOD showed high specificity towards fructosyl valine (FV), a proteolytic product of hemoglobin A1c (HbA1c), but was almost completely inactive towards ε-fructosyl-lysine (ε-FK), a proteolytic product of glycated albumin. By utilizing recombinant FAOD expressed in E. coli, an FAOD sensor based on hydrogen peroxide detection was constructed. The sensor showed FV specificity, with a minimum detectable concentration of 1 μM. The sensitivity was 7.1 mAM^-1cm^-2, which is 17 times higher than that of the previously reported FAOD-based enzyme sensors. Because the bacterial FAOD-based sensor could detect FV concentration and discriminate FV in the presence of ε-FK, it has the great potential for use in the measurement of HbA1c in whole blood samples.

A carbon fiber microelectrode (CFM) was devised and applied to the dielectrophoretic selection of a viable single-cell of rice and BY-2. When 10 MHz electric field was applied, only a viable single-cell could be pulled and picked up within a few sec. This cell selection system was thought to be useful for the high throughput experiment using single-cells.

A flow-injection amperometric system for evaluation of the activity of an immobilized α-amylase column was constructed and the system was applied to detect of calcium (II) ions based on the apoenzyme reactivation method. The α-amylase activity was amperometrically evaluated by using a double-layered column which consisted of α-glucosidase and pyranose oxidase. The detection of calcium (II) ions, being the cofactor of α-amylase, was realized by measuring the reactivation of apo-type α-amylase column, which was the recognition element of Ca(II) ions. Maleate buffer (0.1 M, pH 6.0) as a carrier solution was continuously pumped through the system, and sample solutions were introduced into the system via rotary injection valves. The catalytic activity of the multi-enzyme column was assessed by injecting 0.5% starch solution as a substrate. Calcium(II) ions in 1.0 mM levels were detected by using the system based on the apoenzyme reactivation method.

A calorimetric flow-injection analysis (FIA) with an immobilized thermolysin column was applied to determination of zinc (II) ions based on the apoenzyme reactivation method. The immobilization yield and the enzyme stability were significantly improved by using NaBr solution for dissolving thermolysin. The catalytic activity of the enzyme-packed column was assessed by injecting 1.0 ml of synthetic oligopeptide solution as a substrate into the system. The column was regenerated on exposure to 100 mM EDTA solution and reversibly reactivated with addition of zinc (II) ions. The reactivation ratio depended on the concentration of zinc (II) ions solution injected. Thus, reactivation ratio was evaluated by injection of various concentrations of zinc(II) ions, and thereby, zinc(II) ions were determined. The zinc(II) ions in a range of 10 to 100 μM were measured.

A catalytic combustion type gas sensor with low heat capacity employing Pd/γ-Al₂O₃ was fabricated on a silicon substrate by micromachining and screen-printing. The sensor showed a large response peak before reaching a steady-state value upon pulse-heating for 0.2 s or 0.4 s with a cycle of 1-30 s when volatile organic compounds such as alcohol and acid were contained in the environment, while hydrogen and methane did not show such a response peak but a simple increase to a steady-state value. The peak height or area increased with increasing the non-heating period (0.6-29.6 s) and also with the concentration of target gases. The unique peak response has likely arisen from the combustion of the volatile organic molecules adsorbed on the catalytic sensor material during the non-heating period. The response mechanism was supported by temperature programmed desorption experiments of ethanol.

The purification capabilities of four kinds of foliage plants for formaldehyde, which is a main indoor air pollutant, are examined. The purification capability (P_α) is introduced using a tin oxide gas sensor-output. P_α is derived by dividing the peak value of the sensor-output by the full-width at half maximum. Formaldehyde is one of the typical chemical substances which cause sick-building syndrome. The abilities of the following plants are examined: rubber plant, Boston fern, golden pothos and snake plant. The purification capabilities of these plants for formaldehyde in the amount of 10 ppm are about 19, 15, 23 and 12 respectively. The number becomes greater as the capability is higher. There are several features in P_α characteristics as a function of formaldehyde concentration. Namely, the P_α characteristics for rubber plant and Boston fern can be shown by a linear increasing function. The characteristics are measured until the concentration of 40 ppm. The characteristic of golden pothos is almost constant and the characteristic of snake plant has a maximum value at the concentration of 20 ppm.

SnO₂ based gas sensors have been investigated for the detection of aliphatic carboxylic acids (C₂~C₈). Among SnO₂ sensors modified with foreign metal oxide, MoO₃-, and WO₃-SnO₂ sensors showed high sensitivities to aliphatic carboxylic acids, although both sensors exhibited different properties from each other. Catalytic properties of SnO₂ based sensor materials have been also evaluated in the oxidation of carboxylic acid in order to be associated with sensing properties. Compared with pure SnO₂, the catalytic activity was decreased and the partial oxidation was favored for WO₃-SnO₂. For MoO₃-SnO₂, the partial oxidation was promoted with the similar activity to pure SnO₂, and the cyclic compound like maleic anhydride was produced in the oxidation of carboxylic acid with carbon number larger than 5. These results mean that the consumption of adsorbed oxygen on the surface of sensor material is decreased due to the addition of WO₃ and MoO₃. It was found that the catalytic activity was too high and most of carboxylic acid were oxidized at sensor surface for pure SnO₂ sensor. On the other hand, for WO₃- and MoO₃-SnO₂ sensors, the catalytic activity was decreased and the partial oxidation was preferred. This causes the consumption of adsorbed oxygen inside the sensor, inducing the enhancement of sensitivity.

TiO₂ powder was prepared by employing a modified sol-gel method with Ti(NO₃)₄ and polyethylene glycol having an average molecular weight of 6000. The as-prepared powder was in anatase phase with a crystallite size of 4.4 nm, and showed a high specific surface area of 214 m² g^-1 involving a small fraction of ordered mesopores with a d-spacing of ca. 4.1 nm. Calcination of the powder at 500°C for 1 h led to fracture of the ordered mesoporous strucutre along with a partial phase transition to rutile and a decrease in specific surface area to 25.5 m² g^-1 so that the ordered mesoporous structure was not thermally stable enough. Phosphoric acid (PA) treatment was found to be effective for improving the thermal stability of the ordered mesoporous structure. In addition, no phase transition and little change in crystallite size were observed, and a high specific surface area of 217 m² g^-1 was maintained even after calcination at 500°C for 1 h. Sintering of the calcined powder at 550°C for 3 h resulted in a decrease in specific surface area to 157 m² g^-1 in the case of the PA-treated powder, but the usefulness of the PA treatment to restrict the growth of TiO₂ crystallites was again confirmed. Sensitivities of the sensor fabricated with the PA-treated powder to H₂ and CO was not so high as those of the sensor fabricated with the untreated powder, irrespective of its high specific surface area, small crystallite size and ordered mesoporous structure involved. Thus, the PA treatment was effective for restricting the crystallite growth and then for maintainig the ordered mesoporous structure, but it may result in inactivation of surface sites for gas detection.

NO_x sensing properties of semiconductor heterolayer sensors fabricated by slide-off transfer printing have been investigated. A WO₃ single layer sensor showed much higher sensitivity to NO₂ than that to NO. Loading of Ru or Pt on the surface of WO₃ led to a slight improvement of both the sensitivity and the response speed to NO, but the response speed to NO₂ was decreased especially by the Ru loading. In contrast, loading of Au improved notably the sensitivities and the response behavior to both NO₂ and NO. Stacking of a 0.5Pt-Al₂O₃ (0.5 wt% Pt-loaded Al₂O₃) layer over a 0.5Au-WO₃ layer showed little effect on the NO_x sensitivities, whereas double or triple stacking resulted in a remarkable enhancement of NO and NO₂ sensitivities at 350°C. However, the NO₂ sensitivity of these sensors was superior to the NO sensitivity. On the other hand, the NO₂ sensitivity of WO₃ was decreased by the stacking of a Pt-Al₂O₃ catalyst layer, whereas the NO sensitivity increased. Such changes in NO_x sensitivity became more significant when the loading amount of Pt in the catalyst layer increased from 0.5 to 1.0 wt%. As a result, it was found that the 1.0Pt-Al₂O₃/WO₃ heterolayer sensor exhibited comparable sensitivity to both NO₂ and NO at 350°C.

An amperometric sensor based on pH-dependent electrode reaction of RuO₂ electrode for the detection of acidic and basic gases has been developed. The sensor shows good characteristics of the response and stability for the sensing of CO₂ and NH₃. The detection mechanisms of the sensing reactions were studied by using different pH buffer solutions and X-ray photoelectron spectrometry (XPS). The response currents were hardly observed for acidic and basic gases, such as CO₂ and NH₃, respectively, by using the sensor containing a pH buffer solution. The steady current is concerned with variation of pH of buffer electrolyte. By measuring the XPS spectrum of RuO₂ electrode surface after acidic or basic gas flowing, the changes of the oxidation states of RuO₂ was observed. The results proved that the detection mechanism of the sensor is based on the pH dependent electrode reaction of RuO₂ which response current is determined by the concentration of hydrogen ion or hydroxide ion induced by acidic and basic gases.

Genetically engineered thermostable pyrroquinoline quinone glucose dehydrogenase (S415CGDH) was used for labeling probe DNA and amperometric DNA sensor was constructed and utilized for the detection of PCR amplified Salmonella virulence invA gene. The invA gene from Salmonella which accounts for many cases of food poisoning was targeted and the DNA bearing a specific sequence complementary to the invA gene was immobilized onto an Au electrode as a capture DNA S415CGDH labeled probe DNA was hybridized with the immobilized DNA at 60°C for 10 min and then the resulting electric current generated from S415CGDH by glucose addition was measured. The electric current was obtained when S415CGDH was used for labeling probe DNA but not when the native enzyme was used. The sensor response increased with the addition of glucose and 4.0 × 10^-9 M of the S415CGDH labeled target DNA was detected in the presence of 29 mM glucose. The detection of PCR product was also investigated and it was successfully detected using asymmetric PCR product with sandwich method.

A solid state CO₂ sensor operative at room temperature, developed by incorporating a metal oxide sensing electrode and a metal carbonate auxiliary phase into a NASICON (Na₃Zr₂Si₂PO₁₂)-based electrochemical cell, suffers from a humidity-dependent shift of the EMF response to CO2. In an attempt to reduce the humidity dependence, binary carbonate systems of Li₂CO₃-BaCO₃ and Na₂CO₃-BaCO₃ at various compositions were tested for the auxiliary phase of the device. The melting-and-quenching method conventionally used for attaching the carbonate was not applicable well to the present type device due to the corrosion of NASICON surface. The humidity-least dependent device could be obtained when the Li₂CO₃-BaCO₃ composite (1:2 in molar ratio) was adhered to NASICON by calcination at 500°C (non melting method). The device could respond to 300-3000 ppm CO₂ quite well almost independent of relative humidity (RH) beyond 30%. In the lower range of RH, however, the EMF response commenced to shift and the CO₂ sensing capability was completely lost in the dry atmosphere. A planar type CO₂ sensor was fabricated successfully, which could work stably under the conditions of 30% RH and above.

A field effect transistor (FET) device fitted with a WO₃ layer over the gate area exhibited almost ideal FET behavior at 150 and 180°C in air containing various concentrations of NO₂. Under the conditions of fixed source-drain voltage (3.0 V) and fixed drain current (450 μA), the gate-source voltage (V_GS) was found to increase linearly with an increase in the logarithm of NO₂ concentration over the range of several tens to 700 ppb NO₂, proving its potentiality to work as an environmental NO₂ sensor. However the times of 90%-response and -recovery to switching-on and -off 50 ppb NO₂ were as long as 10 and 25 min even at 180°C, respectively. Cross sensitivity test revealed that the device was totally insensitive to CO₂, but rather sensitive to NO. Water vapor was found to give a serious disturbance to the device: The Vgs response to NO₂ as well as its dependence on NO₂ concentration changed with a change in relative humidity.

Polyelecctrolyte multilayer (PEM) films composed of poly (allylamine hydrochloride) and poly (potassium vinyl sulfate) were employed for improving the dynamic range of ascorbic acid sensors fabricated by coating the surface of a gold (Au) electrode with a ferrocene-containing film. The modified electrode can be used to determine ascorbic acid up to ca. 20 mM, while the upper detection limit of PEM film-free sensor was ca. 3 mM.