Acetylthiocholine/acetylcholine and thiocholine/choline electrochemical biosensors/sensors based on an organically modified sol–gel glass enzyme reactor and graphite paste electrode

Electrochemical sensors for acetylthiocholine and acetylcholine are described. The non-mediated electrochemistry of acetylthiocholine and thiocholine is studied on the surface of graphite paste electrode and results show that acetylthiocholine is directly oxidized/reduced at >0.32 V vs. Ag/AgCl in both acidic and basic medium. In basic medium, both cathodic and anodic peak currents are less as compared to that of the same amount in acidic medium, which shows that the kinetics of non-enzymatic hydrolysis of acetylcholine into electroactive thiocholine is faster in acidic medium and slower in basic medium. Thiocholine is directly oxidized/reduced at >0.35 V vs. Ag/AgCl with relatively larger anodic current compared to cathodic peak current similar to that of acetylcholine results recorded in acidic medium (pH 6.0). The electrochemical sensor/biosensors for acetylthiocholine/acetylcholine and thiocholine/choline are developed using two enzyme reactors: (1) acetylcholinesterase (AChE) encapsulated organically modified sol–gel glass, and (2) choline oxidase (ChO) immobilized within mediators (tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF), and dimethyl ferrocene (dmFc))-modified graphite paste electrodes. The AChE-immobilized into organically modified sol–gel glass behaves as the reactor for enzymatic hydrolysis of acetylthiocholine/acetylcholine into thiocholine/choline, whereas mediator- and ChO-modified paste electrodes are used for the detection of thiocholine/choline through mediated mechanism. The electrochemistry of AChE-generated thiocholine is studied at the mediator-modified electrodes in the presence and absence of ChO. It is observed that thiocholine undergoes both mediated and non-mediated oxidation in the absence of ChO as well as oxidation through enzyme-catalyzed mediated reactions. The results based on cyclic voltammetry on the oxidation of thiocholine at the surface of mediator-modified electrodes in the presence and absence of ChO are reported. In the presence of the ChO large anodic current is observed near the mediator's redox potentials as compared to the anodic current in the absence of enzyme, which shows mediated bioelectrochemistry of thiocholine. The typical response curves for the detection of thiocholine/choline using mediators and ChO-modified electrodes below 0.24 V vs. Ag/AgCl in 0.1 M Tris–HCl buffer pH 8.0 are reported. Comparative analytical performance on the mediated electrochemical responses of the biosensors is discussed.     

Development of electrochemical micro machining for air-lubricated hydrodynamic bearings

 A specially-built EMM (Electrochemical Micro Machining)/PECM (Pulse Electrochemical Machining) cell, a electrode tool filled with non-conducting material, a electrolyte flow control system and a small & stable gap control unit are developed to achieve accurate dimensions of spindle recesses. Two electrolytes, aqueous sodium nitrate and aqueous sodium chloride are investigated in this study. The former electrolyte with few pits on the surface of workpiece has better machine-ability than the latter one with many pits on the surface of workpiece. It is easier to control the machining depth precisely with pulse electrical current than direct electrical current. This paper also presents an identification method for the machining depth by in-process analysis of applied electrical current and interelectrode gap size. The interelectrode gap characteristics, including pulse electrical current, effective volumetric electrochemical equivalent and electrolyte conductivity variations, are analyzed using the model and experimental results. Received: 5 July 2001/Accepted: 11 December 2001 This work was supported by Korea Research Foundation Grant (KRF-2001-041-E00095) Paper presented at the 12th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 28–29 June, 2001.     

State-of-the-art monitoring of fuel acidity

Development of a novel iridium oxide (IrOx) based acidity sensor for off-line monitoring of fuel acidity is described. The sensor works in the potentiometric mode using an IrOx electrode as an indicating electrode and a Ag/AgCl or Ag/Ag₂O—reference electrode. The data show that the IrOx sensor responds to compounds present in fuel that have acid–base character. Using an off-line IrOx sensor, it is possible to determine the acidity of different fuels and discriminate between unstressed and thermally stressed fuels. It is possible to correlate the response of an IrOx sensor with the total acid numbers of different fuels. Experimental results also indicate that the low fuel conductance, the material used for sensor encapsulation, and/or the type of reference electrode may influence the response time of the IrOx sensor. Finally, the IrOx response has been demonstrated to be faster, better defined, more accurate and more reproducible than a glass electrode response for titrations of non-aqueous solutions.     

DC humidity sensing properties of BaTiO3 nanofiber sensors with different electrode materials

Barium titanate (BaTiO₃) nanofibers were synthesized by electrospinning and calcination techniques. Two direct current (DC) humidity sensors with different electrodes (Al and Ag) were fabricated by loading BaTiO₃ nanofibers as the sensing material. Compared with the Al electrode sensor, the Ag electrode sensor exhibits larger sensitivity and quicker response/recovery. The current of Al electrode sensor increases from 4.08 × 10⁻⁹ to 1.68 × 10⁻⁷ A when the sensor is switched from 11% to 95% relative humidity (RH), while the values are 2.19 × 10⁻⁹ and 3.29 × 10⁻⁷ A for the Ag electrode sensor, respectively. The corresponding response and recovery times are 30 and 9 s for Al electrode sensor, and 20 and 3 s for Ag electrode sensor, respectively. These results make BaTiO₃ nanofiber-based DC humidity sensors good candidates for practical application. Simultaneously, the comparison of sensors with different electrode materials may offer an effective route for designing and optimizing humidity sensors.     

Synergistic effects of micro/nano modifications on electrodes for microfluidic electrochemical ELISA

Microfluidic electrochemical sensing has been considered to be highly efficient. However, we showed, by using numerical simulations in this study, that a planar electrode formed on the bottom of a microchannel is exposed to only a small fraction of analytes in amperometric detection. We also showed that three-dimensional (3D) micropillar electrodes significantly improve the detection current. The practical performance was evaluated using 3D micropillar electrodes fabricated by photolithography. The output current increased as the diameters of the micropillars decreased, as predicted by the simulations. It is noteworthy that the current enhancements obtained with the 3D electrodes were larger than those expected from an increase in the surface area. Further increase in current was achieved by electrical deposition of nanoporous gold-black onto the surface of the 3D electrode: when a 3D electrode with micropillars 30 μm in diameter was used, the output current was approximately 20 times that obtained with a 2D electrode without modification. The applicability of the micropillar electrodes was demonstrated in electrochemical enzyme-linked immunosorbent assay (ELISA) of bone metabolic marker proteins. Although an increase in the surface area of the electrode leads to more noise in general, there is no significant difference in the signal-to-noise ratio between the modified 3D electrode and the 2D electrode without modification in the ELISA experiments. This nanoporous micropillar electrode could potentially be a useful component for the development of on-site diagnosis systems.     

Screen-printed carbon electrode for choline based on MnO2 nanoparticles and choline oxidase/polyelectrolyte layers

This paper presents the amperometric biosensor that determines choline and cholinesterase activity using a screen printed graphite electrode. In order to detect H₂O₂ we have blanket modified the electrode material with manganese dioxide nanoparticles layer. Using layer-by-layer technique on the developed hydrogen peroxide sensitive electrode surface choline oxidase was incorporated into the interpolyelectrolyte nanofilm. Its ability to serve as a detector of choline in bulk analysis and cholinesterase assay was investigated. We examined the interferences from red-ox species and heavy metals in the blood and in the environmental sample matrixes. The sensor exhibited a linear increase of the amperometric signal at the concentration of choline ranging from 1.3 × 10⁻⁷ to 1.0 × 10⁻⁴ M, with a detection limit (evaluated as 3σ) of 130 nM and a sensitivity of 103 mA M⁻¹ cm⁻² under optimized potential applied (480 mV vs. Ag/AgCl). The biosensor retained its activity for more than 10 consecutive measurements and kept 75% of initial activity for three weeks of storage at 4 °C. The R.S.D. was determined as 1.9% for a choline concentration of 10⁻⁴ M (n = 10) with a typical response time of about 10 s. The developed choline biosensor was applied for butyrylcholinesterase assay showing a detection limit of 5 pM (3σ). We used the biosensor to develop the cholinesterase inhibitor assay. Detection limit for chlorpyrifos was estimated as 50 pM.     

Capacitive humidity sensor design based on anodic aluminum oxide

Capacitive humidity sensors based on anodic aluminum oxide (AAO), a material used as a sensing layer for high sensitivity, were investigated. The AAO film has many nanosized pores, giving it a large surface area for absorbing water vapor. Effects of design factors and heating were investigated. A thick porous layer or big pore diameter increases sensitivity because of increase in contact surface area. An electrode of rectangular spiral-shaped type tends to have a slightly higher hysteresis than the interdigitated type, but the rectangular spiral-shaped type is efficient and sensitive if the hysteresis and nonlinearity are reduced by controlling design factors or heating. Although heating reduced the sensitivity, it improved performance parameters such as nonlinearity, hysteresis, response time and temperature dependence. Also, a porous electrode would show a higher sensitivity than a nonporous electrode because of the larger surface area.     

Development of a screen-printed cholesterol biosensor: Comparing the performance of gold and platinum as the working electrode material and fabrication using a self-assembly approach

Gold (Au) and platinum (Pt) were used as the working electrode material to detect cholesterol in solution through enzymatically generated hydrogen peroxide (H₂O₂). Both gold and platinum were capable of detecting cholesterol through the electrochemical oxidation of H₂O₂, and could be used as the working electrode material. By comparison, however, Au was preferable over Pt in terms of higher response current and better sensitivity. Therefore, Au was chosen as the working electrode material for the fabrication of a thick-film screen-printed cholesterol biosensor consisting of three electrodes on an alumina substrate (working: Au, reference: Ag/AgCl, and counter: Au). The immobilization of the enzyme cholesterol oxidase (ChOx, E.C. 1.1.3.6) on the Au working electrode was achieved using a self-assembly approach. A thiol, 3-mercaptopropionic acid (MPA), was self-assembled onto the gold working electrode forming a thin organic layer that served as the anchor for the enzyme immobilization. 1-Ethyl-3(3-dimethylamino propyl)carbodiimide methiodide (EDC) was then used to immobilize the enzyme ChOx covalently on the gold working electrode through the carbodiimide coupling between the carboxyl (–COOH) groups of the self-assembled MPA layer and the amino (–NH₂) groups of the enzyme. Electrochemical measurements showed that this biosensor responded well to cholesterol, confirming that the self-assembly immobilization method was effective. The reproducibility, the interference, and the storage stability of the biosensor were studied and assessed.     

Preparation of ferroelectric ceramics in a film structure and their photovoltaic properties

This paper reports the formation of a new-layered film structure and the highly improved photovoltaic output of the lead lanthanum zirconate titanate (PLZT) employed. The new structural design is described using an upper top transparent indium tin oxide (ITO) electrode. The PLZT film structure exhibited V and μA output. The photovoltaic current of the PLZT film per unit width was more than 10² times larger than that of bulk PLZT, while the photovoltaic voltage per unit thickness in the layered film structure was almost the same as that in bulk ceramics and single crystals. These differences are due to the characteristics of the film structure and configuration of the electrode. The PLZT film also has the advantage of easily controllable parameters: film thickness, illuminated area, and illumination intensity. A simple model is used for the phenomenological explanation of the improved photovoltaic effect of the PLZT film.     

平面微电极阵列的研制和表征

作者用微电子平面工艺制备了多种平面电极阵列,研究了它们的电化学特性.实验证明:微电极阵列能极大地提高响应电流并保持单一微电极的全部优点,即尺寸越小,响应时间越短,电流密度越大,能使用的电位扫描述率越高.并又共价键合葡萄糖氧化酶,研制成功具有良好性能的微型化葡萄糖传感器,它的响应电流比现有报导的单一微电极的葡萄糖传感器大十多倍.     

Differential pulse voltammetric determination of N-acetylcysteine by the electrocatalytic oxidation at the surface of carbon nanotube-paste electrode modified with cobalt salophen complexes

The preparation and electrochemical performance of the carbon nanotube-paste electrode modified with salophen complexes of cobalt(III) perchlorate, with various substituents on the salophen ligand, as well as their electrocatalytic activity toward the oxidation of N-acetylcysteine (NAC) is investigated. Several Schiff base complexes containing various nucleophilic and electrophilic functional groups were prepared, and their electrochemical characteristics for the electro-oxidation of NAC were evaluated using cyclic and differential pulse voltammetry (CV and DPV). The results revealed, the modified electrodes show an efficient and selective electrocatalytic activity toward the anodic oxidation of NAC among biologically important compounds in buffered solutions at pH of 7.0. The best voltammetric responses were obtained for a carbon-paste electrode (CPE) modified with a salophen complex containing para-methoxy groups on its salicylaldehyde ring. The analytical response of the modified electrode for response to other sulfhydryl compounds (e.g., cysteine, penicillamine, captopril and mercaptopropionyl glycine) in comparison to NAC was investigated by CV and DPV methods. The DPV method was applied as a sensitive method for the quantitative detection of the trace amounts of NAC. A linear dynamic range from 1 × 10⁻⁷ to 1 × 10⁻⁴ M with calibration sensitivity of 0.0646 μA/μM is resulted for NAC in DPV measurements. The detection limit was 5 × 10⁻⁸ M, which is remarkably lower than those reported previously for NAC using other modified electrodes. The results of voltammetric determinations show a very good reproducibility, and the R.S.D. for the slope of the calibration curve, based on 10 measurements in a period of two months, was <3.9%. The detection system provides very stable electrochemical responses toward NAC, makes it very suitable for using in pharmaceutical and clinical measurements.     

Electrocatalytic characteristics of electrodes based on ferritin/carbon nanotube composites for biofuel cells

A single wall carbon nanotube (SWNT)/ferritin/glucose oxidase (GOx) layer on a glassy carbon electrode acting as a biofuel cell anode was fabricated using an SWNT/ferritin composite as an electron transfer mediator from the enzyme to the electrode. In the absence of glucose, the SWNT/ferritin/GOx composite showed a higher current response than an SWNT/GOx composite, and the electrocatalytic oxidation of glucose on the anode increased linearly with increasing concentration of glucose. The highly distributed SWNT/ferritin composite as a platform for enzyme immobilization resulted in an enhanced electrocatalytic activity towards glucose. The SWNT/ferritin composite showed an enhanced electron transfer from the enzyme to the electrode; therefore, SWNT/ferritin/GOx composites can be used as an anode in biofuel cells.     

Design of a highly sensitive and responsive electrode for particulate matter monitoring

In a previous sensor device based on a Pt|Sn_0.9In_0.1P₂O₇|Pt electrode system, electrochemical carbon oxidation was realized over the surface of a working electrode, enabling continuous carbon monitoring with self-regeneration of the sensor. Specific reactivity of the active oxygen species for carbon was demonstrated along with a high current efficiency. However, this reaction did not occur over the entire electrode layer, limiting the sensing properties and giving a low sensitivity and response speed. The main reason for carbon oxidation being limited to the surface region was the lack of proton conduction in the working electrode. To overcome this problem, we added proton-conducting Sn_0.9In_0.1P₂O₇ particles as an ionomer to the working electrode. The Sn_0.9In_0.1P₂O₇ ionomer was successfully distributed over the working electrode at the level of a few micrometers, providing reaction sites for carbon oxidation over the entire electrode layer. The resulting amperometric sensor provided a sensitivity to carbon that was 1.4 times greater than that of the ionomer-free working electrode, as determined by the current at which carbon oxidation had ceased. Moreover, carbon floating in a glass container could be transported to reaction sites present on the external surface of the working electrode, allowing for a large current and a rapid response.     

Frequency-dependent electrostatic actuation in microfluidic MEMS

Electrostatic actuators exhibit fast response times and are easily integrated into microsystems because they can be fabricated with standard IC micromachining processes and materials. Although electrostatic actuators have been used extensively in "dry" MEMS, they have received less attention in microfluidic systems probably because of challenges such as electrolysis, anodization, and electrode polarization. Here we demonstrate that ac drive signals can be used to prevent electrode polarization, and thus enable electrostatic actuation in many liquids, at potentials low enough to avoid electrochemistry. We measure the frequency response of an interdigitated silicon comb-drive actuator in liquids spanning a decade of dielectric permittivities and four decades of conductivity, and present a simple theory that predicts the characteristic actuation frequency. The analysis demonstrates the importance of the native oxide on silicon actuator response, and suggests that the actuation frequency can be shifted by controlling the thickness of the oxide. For native silicon devices, actuation is predicted at frequencies less than 10 MHz, in electrolytes of ionic strength up to 100 mmol/L, and thus electrostatic actuation may be feasible in many bioMEMS and other microfluidic applications.     

High‐transmittance in‐plane switching liquid crystal display device driven by three‐level voltages

We propose a single‐layered electrode structure using three voltage levels instead of two to achieve high transmittance in an in‐plane switching liquid crystal display device. The proposed structure consists of two pixel electrodes and one common electrode. By using three voltage levels, we can generate an in‐plane electric field higher than that in a conventional in‐plane switching device. We confirmed that by using the proposed structure, the transmittance of a liquid crystal device can be increased from 29% to 35% at a slightly lower operating voltage without using the double‐layered electrode structure required for the fringe‐field switching mode. The transmittance of the proposed device is higher than that of the fringe‐field switching device.     

Sensitive electrochemical determination of acetaminophen in pharmaceutical formulations at multiwalled carbon nanotube-alumina-coated silica nanocomposite modified electrode

A highly sensitive electrochemical sensor for the determination of acetaminophen at the multiwalled carbon nanotube-alumina-coated silica (MWCNT-ACS) nanocomposite modified glassy carbon electrode is reported. The morphology of the MWCNT-ACS nanocomposite was characterized by field emission scanning electron microscopy. The electrocatalytic properties of the MWCNT-ACS nanocomposite modified glassy carbon electrode were characterized by cyclic voltammetry and square-wave voltammetry in the presence of acetaminophen. The MWCNT-ACS nanocomposite modified glassy carbon electrode exhibited the abilities to raise the current response and to decrease the electrooxidation potential. In cyclic voltammetric responses, the oxidation peak current of acetaminophen obtained at the MWCNT-ACS modified glassy carbon electrode was 100 times greater than that of bare glassy carbon electrode. The MWCNT-ACS nanocomposite modified glassy carbon electrode for the determination of acetaminophen displayed a sensitivity of 376.5 A M⁻¹ cm⁻² and a detection limit of 0.05 μM using square-wave voltammetry. The analytical applicability of the developed method was achieved by analyzing the content of acetaminophen in five commercial drugs without pretreatment.     

H2 sensing properties of diode-type gas sensors fabricated with Ti- and/or Nb-based materials

A thermally oxidized TiO₂ or Nb₂O₅ film equipped with a top Pd film electrode and a bottom Ti or Nb plate electrode (Pd/MO(n)/M, MO: oxide film, M: metal plate, n: annealing temperature (°C)) has been investigated as a diode-type H₂ sensor under air or N₂ atmosphere. Pd/TiO₂(n)/Ti sensors showed relatively poor H₂ sensing properties in air, in comparison with Pd/anodic-TiO₂(n)/Ti sensors constructed with an anodized TiO₂ film equipped with a top Pd film electrode and a bottom Ti plate electrode, which were reported in our previous studies. On the other hand, Pd/Nb₂O₅(n)/Nb sensors showed relatively larger H₂ response with fast response and recovery speeds than Pd/TiO₂(n)/Ti sensors in air under high forward bias conditions. A Pd/Nb₂O₅(450)/Ti sensor, which was fabricated by radio-frequency magnetron sputtering of Nb metal on a Ti substrate followed by thermal oxidation at 450 °C, showed the largest H₂ response and relatively fast response and recovery speeds in air, among the sensors tested. In addition, H₂ response of the Pd/Nb₂O₅(450)/Ti sensor in air was much lower than that in N₂, but the logarithm of H₂ response was almost proportional to the logarithm of H₂ concentration in a wide range of H₂ concentration (10–8000 ppm) in air, and the H₂ sensitivity in air was much higher than that in N₂.     

One-step fabrication of a polyaniline nanofiber vapor sensor

A single-step, bottom-up technique has been used to fabricate sensors, based on conducting polymer nanofibers. A small amount of an aqueous solution containing aniline, a dopant, and an oxidant was placed on an interdigitated electrode array. Ultraviolet (UV)-irradiation of the solutions affected polymerization, yielding a highly porous film of polyaniline nanofibers with a mean diameter of around 100 nm and a length on the order of 1 μm. Solutions that were not irradiated formed bulk-like polyaniline (PANI) films. Nanofibers and bulk polyaniline sensors were exposed to chloroform, a weak proton donor; to toluene, a vapor that causes polymer swelling; and to triethylamine, which alters the doping level. Because of their higher surface areas, the response times of the fiber sensors were about a factor of 2 faster, with the current variations up to 4 times larger than those of the bulk polyaniline sensors. These results suggest methods for the advancement of simple and environment-friendly production of organic nanofiber-based sensors and electronic devices.     

Effect of surface charges of nanoparticles on response current of enzyme electrode for single use

The response current of glucose oxidase (GOD) electrode has been enhanced by the nanoparticles (NPs), but the mechanism of enhancement remains unclear. The effect of surface charges of NPs on the response current of GOD-electrode enhanced by NPs was studied by using the electrophoresis and the determination of zeta potential. The results indicate that, besides the inherent surface effect of NPs, the surface charges are essential for the enhancement of response current of enzyme electrode. More GOD molecules can be adsorbed on the surface of SiO₂ NPs, because GOD molecules hold surface charge whose property was opposite to that of SiO₂ NPs, but the same as that of Au NPs. When Au NPs and SiO₂ NPs are mixed with the ratio of 1:1 in mol, the combined particles can carry out both functions of the two kinds of NPs, and enhance response performance of GOD-electrode greatly.     

Bioelectrochemical sensor based on PQQ-dependent glucose dehydrogenase

Bioelectrochemical responses of pyrroloquinoline quinone (PQQ) dependent glucose dehydrogenase (PQQ-GDH) have been studied with the use of two principal electrode configurations: (i) glassy carbon electrode, coated with Nafion layer, containing sorbed N-methylphenazonium (NMP), and overcoated by an enzyme layer, crosslinked by glutaraldehyde, and (ii) glassy carbon electrode, containing an electropolymerized layer of either Toluidine blue, or o-phenylenediamine, or pyrrole, and overcoated by a crosslinked enzyme layer. When operated within the potential window of 0.0–0.3 V, Nafion-coated and NMP-soaked bioelectrode shows an anodic current response to glucose without the presence of electron transfer mediator in solution. The current–concentration profile obtained resemble to that, typical for enzyme catalyzed reaction. Other configurations studied showed bioelectrochemical response to glucose only in the presence of soluble mediator NMP. Without any mediator, no electrochemical responses have been registered. It indicates that direct electron transfer between PQQ-GDH and all types of electrodes modified during current work is undetectable.