Cyclic voltammetry of the reductive desorption of thiol-modified metal electrodes is shown to be a useful and convenient tool for elucidating the state of the adsorbed monolayers, such as the intermolecular interaction of adsorbed molecules, the mixing state of two-component self-assembled monolayers, and the surface diffusion of adsorbed molecules. The application of the reductive desorption to the engineering of the self-assembled monolayer in nanometer scale has also been exemplified.

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The distribution process of an objective ion (i^z+) with a counter ion (x^z-) between water (W) and an organic solution (O) was assumed as that i^z+ and x^z- transfer at the W|O interface individually, depending on their own standard Gibbs free energies (ΔG⁰_tr) for the transfer from W to O and maintaining the electroneutrality of both phases. Also, theoretical equations were derived for the quantitative expression of the distribution ratios. (D) of i^z+ in both the absence and presence of a special complexing neutral ligand (Y) in O. The derived equations were composed of ΔG⁰_tr of i^z+ and x^z-, ion pair formation constants (K_ip) of i^z+ with x^z- in W and O, the stability constant (K_st) of the complex [(iY_p)^z+] in O and K_ip of (iY_p)^z+ with x^z- in O. The D calculated using the derived equations and constants such as ΔG⁰_tr, K_ip and K_st, which were determined with the aid of voltammetry for the transfer of the ion at the W|O interface and conductometry, agreed well with the D determined experimentally by ion pair extraction. This fact means that the distribution process assumed in the present work is reasonable. Based on a theoretical consideration of the distribution process mentioned above, a method to determine ΔG⁰_tr of highly hydrophilic metal ions, which could not be determined by previous methods, was proposed. The D and the selectivity in the distribution (Γ_i,j) at the W|O interface were connected quantitatively to the potential (E_ISE) and selectivity coefficient (K^pot_i,j) at an ion selective electrode (ISE) of the liquid membrane type while taking into account that not only D and Γ_i,j, but also E_ISE and K^pot_i,j could be expressed by using ΔG⁰_tr, K_st and K_ip when the ISE membrane was regarded as O.

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The present paper describes analytical aspects of electrochemical ultramicrosensors constructed using glass capillaries having a tip diameter of 0.5∼12 μm. The constructed sensors included open capillary electrodes with the sampling ability of analytes based on electrokinetic phenomena and capillarity, membrane sensors in combination with planar bilayer lipid membranes (BLMs) and excised biomembrane sensors. The open capillary sensor had abilities of electrokinetic sampling and charge-selective detection of analyte ions. Selective detection of dopamine in a neutral solution after being transported into an inner acidic solution of the open capillary sensor was also demonstrated. The constructed capillary membrane sensors include (i) a novel sensor that mimics the mode of signal transduction displayed by G protein-linked receptors with BLMs containing receptors and a single gramicidin channel, (ii) an excised biomembrane sensor that allows the detection of arachidonic acid selectively over its metabolites and other fatty acids, and (iii) L-glutamate sensors using BLMs and biomembranes containing a glutamate receptor. The response characteristics of these capillary sensors were studied in terms of the working principle, dynamic range, sensitivity and selectivity. The application of these sensors to detection of signaling molecules in mouse brain slices was also demonstrated.

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Two examples of an electrochemical gene-sensing system are reported. They are built based on the chemistry of a DNA probe, a DNA-modified electrode and, especially, DNA conjugate formation through a DNA-DNA and a DNA-small molecule interactions to assure electrochemcial sensing. A 16-mer oligodeoxynucleotide (ODN) having five successive phophorothioate units on its 5'-terminus (s16) was prepared and covalently immobilized onto gold electrode surfaces through chemisorption. A ferrocenyl ODN (f12) was synthesized by a coupling reaction between amino-terminated ODN with an activated ester of ferrocenecarboxylic acid. The f12 was subsequently annealed with its complementary strand, p19, which contains the complemental sequence to s16. The resulting double-strand, p19-f12, was used for hybridization experiments. A treatment of the s16-modified electrode turned it to be electroactive due to s16-p19-f12 sandwich-type complex formation. Cyclic voltammetric (CV) measurements in an aqueous KCl solution showed reversible redox waves due to the redox reaction of the ferrocenyl moiety. On the other hand, a s16-modified electrode treated with the mismatch control, m19-f12, also showed redox waves, but only slightly (ca. 5% to p19-f12). These results indicate that the present sensing system is fundamentally applicable to the electrochemcial detection of specific genes. Next, a ferrocenyl derivative of psoralen (FcPso) is described. Psoralen is a class of intercalator which forms a photoadduct with the pyrimidine bases upon UV radiation; FcPso is expected to be useful for the electrochemical tagging of DNA. FcPso was synthesized by a reaction of 4'-chloromethylated 4,5',8-trimethylpsoralen with N,N-dimethylaminomethylferrocene. A 5'-terminally thiolated 12- mer ODN (e12) was immobilized on a gold electrode surface through chemisorption. The e12-electrode was annealed with the complementary strand, t12. Although CVs measured in an aqueous KCl solution only gave capacitive currents, a FcPso treatment turned the electrode system redox actively; the CV peak currents due to the Faradaic reaction of the ferrocene moiety showed a linear dependence on the sweep rate of the electrode potential, indicating a surface process. On the other hand, almost no faradaic response was associated for FcPso treatment of the e12-electrode. The potential application of the electrochemcial detection of a specific gene was successfully demonstrated. As a basis of gene-sensor applications, a detailed characterization of the immobilization chemistry is also presented. The results concerning IR spectral measurements and a microgravimetric analysis using a quartz-crystal microbalance are described.

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The quantitative characterization of oxide films formed on copper is an important subject in corrosion research. So far, chronopotentiometry has been most frequently applied to the selective determination of cuprous oxide (Cu₂O) and cupric oxide (CuO) in copper oxide films. However, it is time-consuming, and has a serious problem that there are two different doctrines about which oxide is reduced first. In this study, we prepared two standard samples of Cu₂O film (Dumet wires) and CuO film (by treating a copper sheet with a commercial oxidizing agent) and also a real sample in which the two oxides coexist on a copper sheet. We then performed quantitative analyses of the oxide films using a recently proposed voltammetric technique, i.e., double-sweep cyclic voltammetry (DSCV). It was found that the use of a strongly alkaline solution (i.e., 6 M KOH+1 M LiOH) allowed a perfect separation between two cathodic peaks due to the reductions of Cu₂O and CuO. One of the peaks, appearing at a less negative potential, was due to the reduction of CuO to Cu, whereas the other peak, appearing at a more negative potential, was due to the reduction of Cu₂O to Cu. These peak assignments have been justified by analyses of standard samples by X-ray diffraction, inert-gas fusion analysis for oxygen, and scanning ion microscopy (SIM). Also, the thickness of the oxide layer of each standard sample, calculated from the peak area, was in fair agreement with that estimated by either gas analysis or the SIM image.

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A simple and highly sensitive method has been developed for the determination of nitrite at the nM level by adsorptive cathodic stripping voltammetry with a graphite electrode in an acidic solution. The optimized experimental conditions were as follows: nitrite was diazotized with sulfanilamide and coupled the resulting diazonium salt with 1-naphthylamine for 25 min to produce an azo dye; the azo compound was accumulated on a graphite electrode with stirring for 5 min on an open circuit; the deposit was then cathodically stripped in the potential range 0.2 to -0.2 V vs. SCE at a scan rate of 50 mV/s by a differential pulse mode in 0.4 M hydrochloric acid. A single well-defined cathodic peak was obtained at around -0.05 V vs. SCE, and nitrite was determined from the peak height in the stripping voltammogram. The calibration graph was linear over a concentration range of 1.1∼217 nM of nitrite (correlation coefficient>0.999) with a relative standard deviation (n=5) of 1.1% for 65 nM of nitrite. The detection limit (3σ), calculated from repeated determinations (n=20) of a blank solution, was 0.15 nM for an accumulation time of 5 min. The possible contamination due to the laboratory atmosphere was evaluated. By applying a non-electrolytic accumulation step, the maximum permissible concentrations of foreign elements in the determination of nitrite increased extremely. The proposed method was successfully applied to the determination of trace nitrite in commercial hydrochloric acid reagents. The time required for the whole procedure was within 35 min.

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The adsorption of vitamins at a mercury electrode was investigated by measuring differential capacity-time curves with a flow injection method, differential capacity-potential curves and cyclic voltammograms at a hanging mercury drop electrode. An experiment to measure the differential capacity-time curves was done as follows. A supporting electrolyte solution (0.05 M Na₂SO₄) was placed in a reservoir. Teflon tubing was used to connect the detector to a peristaltic pump and an injection valve. Solutions of vitamins were introduced via the injection valve. The adsorption of vitamins was almost irreversible. The adsorption of riboflavin and its reduced form (leucoflavin) was irreversible except at -0.25 V and -0.7 V. The less adsorption of leucoflavin i.d.fferential capacity-time curves is interpreted as follows. In the experiment involving the flow injection method, the molecules of riboflavin carried to the electrode are adsorbed and reduced to leucoflavin. Because the adsorption structures of riboflavin and leucoflavin are different, it takes time for the leucoflavin molecules to be adsorbed. During this time a part of the leucoflavin is carried away from the electrode. The adsorption of Vitamin B₁₂ is irreversible between -0.1 V and -1.4 V. Vitamin B₁₂ is considered to be one of the strongly adsorbed molecules at a mercury electrode. In the differential capacity-time curves between -0.4 V and -1.0 V, kinks are observed. The kinks are considered to be due to the different adsorption structures (flat and perpendicular) of vitamin B₁₂. The reduced form of thioctic acid i.d.sorbed. In the differential capacity-time curves, two adsorption structures are found. One is reversible adsorption with (CH₂)₄COOH to the electrode and the other is irreversible adsorption with S atms to the electrode. It is considered that the irreversible adsorption of vitamins is favorable for adsorptive stripping voltammetry.

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A novel glucose sensor base on an osmium-complex modified polypyrrole was prepared and evaluated. Four new-type osmium-complexes ([Os(bpy)₂(py(4)-bpy)]^2+/3+, [Os(bpy)₂(py(3)-bpy)]^2+/3+, [Os(bpy)₂(pyro-pri)Cl]^+/2+, and [Os(bpy)₂(vi-bpy)]^2+/3+) were synthesized, and the kinetic parameters analyzed. A hydrophilic redox polymer with an osmium-complex was modified by electrolytic copolymerization on the surface of a platinum electrode. The effects of the polymer concentration, times of the scan, dissolved oxygen, and concomitant compounds, and a calibration curve for measurements of glucose were analyzed. The catalytic currents of synthesized osmium-complexes were observed. The relationship between glucose and the oxidation current was examined using the sensor under the optimum conditions. A linear relationship was obtained over the range of 0.9∼100 mmol dm^-3 glucose. It was measured in the presence of ascorbic acid or uric acid. These compounds did not affect the response current. The lifetime of the sensor was also examined. The analytical values were constant for 9 days when the sensor was stored in a pH 7.07 phosphate buffer solution at 4°C.

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The concentration of anionic polyelectrolytes was indirectly evaluated from the electrode oxidation of a ferrocenyl group of the title cationic surfactant, which adsorbed at a carbon paste electrode (CPE) to associate with anionic polyelectrolyte. The adsorptive ion-association complex between anionic polyelectrolyte and the ferrocenyl cationic surfactant was accumulated at the electrode in the absence of an applied potential. The adsorption wave which appeared in the voltammogram at CPE was larger than that at a glassy carbon electrode, and the peak current was observed at a more positive site than that for the voltammogram of only ferrocenyl cationic surfactant in 0.1 mol dm^-3 NaCl solution. A second adsorption wave, which appeared in the measurement of an anionic surfactant using the ferrocenyl cationic surfactant, was not obtained. After 10 min of accumulation, the level of heparin of 10^-6 eq. mol dm^-3 could be measured by a linear relationship between the concentration and the peak current. The dependence of peak current on the concentration of poly(styren sulfonate), however, was not linear. In addition, the peak current decreased with the concentration at a high concentration. The relative standard deviation for each peak height was less than 5.4% for heparin and 5.8% for poly(styren sulfonate) in five runs.

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A residual chlorine detector for flow analysis was developed with a platinum oxide sheet electrode. The detector consists of a working electrode (platinum oxide sheet), a counter electrode (platinum wire) and a reference electrode {saturated calomel electrode (SCE)}. A sample solution containing residual chlorine (sodium hypochlorite and monochloramine) was pumped at a flow rate of 0.7 ml min^-1 and mixed with a buffer solution (pH 9.8, sodium carbonate) containing potassium iodide. The residual chlorine reacted with potassium iodide, producing iodine in a mixing coil. After the mixed solution was led into the detector, the produced iodine was reduced with the working electrode at +0.125 V vs. SCE. The response time was about 6 min. A linear relationship between the concentration of residual chlorine and reducing current (response) was obtained in the concentration range of 0.04 to 2.6 mgCl₂ l^-1. The relative standard deviation was about 2.5% (n=8) for 1.0 mgCl₂ l^-1. The lifetime of the detector was about several hundred times of experiments. A monochloramine was the reaction product of sodium hypochlorite with ammonium chloride. When the potassium iodide was not added to the buffer solution, the reducing current was restricted to the concentration of the sodium hypochlorite. The concentration of monochloramine was equivalent to the difference between two responses.

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Multilayer films composed of poly(ethyleneime) (PEI) and uricase were prepared by an alternate deposition of PEI and uricase on the surface of a platinum (Pt) electrode for developing amperometric uric acid sensors. PEI and uricase were successfully built into a multilayer film, and the loading of uricase in the film was evaluated using a quartz-crystal microbalance. The Pt electrode coated with the PEI/uricase multilayer film functioned as an amperometric biosensor senstive to uric acid. The output current of the sensor depended linearly on the number of PEI/uricase layers in the film.

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The magnitude of the steady-state current by a sulfhydryl compound at a dialysis membrane (molecular cut off: 5000) coverd cobalt phthalocyanine-embedded electrode was inversly proportional to the molecular weight. That is, the currents by 1 μM glutathione (GSH), glutamylcysteine and cysteine at 200 mM and at pH 8.3 were 0.07, 0.14 and 0.39 nA, respectively. The activity of γ-glutamyltransferase (γ-GT) in serum was determined by measuring the current increase for a given time due to the degradation of GSH to cysteinylglysine in a 0.1 M tris buffer of pH 8.3 containing 1.2 mM GSH, 50 mM glysylglysine and 5 mM EDTA. The activities of γ-GT determined amperometrically using GSH as a substrate for sixteen serum samples were in good agreement with those determined by a colorimetric method using γ-glutamy1-3-carboxy-4-nitroanilide (Glucana) a as substrate (r=0.98).

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Two novel ruthenium (II) complexes (Ru^II) having four positive charges were prepared so as to be strongly incorporated into a Nafion film. Complex A is a bis-Ru^II complex bound with a long alkyl chain and Complex B is a Ru^II having two pyridinium pendants. A flow-through detector using Ru^II/Nafion-coated Au electrdodes was applied for the determination of amino acids. The Complex A electrode more strongly responded to proline than did the Ru (bpy)₃²⁺ electrode, and showed a more stable response than did the Ru (bpy)₃²⁺ electrode. On the other hand, the Complex B electrode showed almost the same response profile for amino acids as did the Ru (bpy)₃²⁺ electrode. The response intensity was 1/60-times less strong than that of the Complex A electrode, perhaps due to the consumption of a strong reducing agent which may have been generated from oxidation of amino acids.

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