Shuichi Suzuki

A catechol electrode based on spinach leaves

Abstract Minced spinach leaf (Spinacea oleracea) has a high activity of catechol oxidase (dimerizing) (EC 1.1.3.14), which is utilized for the determination of catechol by coupling the spinach tissue with a Clark oxygen electrode. The calibration graph for catechol is linear over the range 2×10−5–8×10−4 M (RSD 3%). The sensor retains its enzyme activity for at least 18 days. 4-Methylcatechol and glycolate interfere; glucose and ascorbate do not.

Mediated glucose sensor using a cylindrical microelectrode

The performance of a cylindrical enzyme electrode with a soluble mediator was analysed theoretically. The normalized current response was calculated at various values of both the electrode radius (a) to enzyme layer thickness (l) ratio, a/l, and the relative catalytic activities, σS and σM { = (kcat[E]l2/KMS,MDS,M)12}. The calculated results demonstrated that at high mediator concentration, larger a/l ratio and increased relative catalytic activity for the substrate, σS, a wider linear range could be obtained for a calculated glucose calibration graph. However, the linearity was found to be less dependent on a/l than σS. Alternatively, at high glucose concentration, the mediator concentration — response curve calculated at low a/l values and a high catalytic activity for the mediator, σM, indicated a wide linear range. Immobilized glucose oxidase (GOD) cylindrical microelectrodes were fabricated and their characteristics were evaluated by using 1,4-benzoquinone as electron mediator. GOD was immobilized in a photo-cross-linkable polymer on two types of cylindrical microelectrodes of 2 and 25 μm diameter. The linear ranges of the observed calibration graphs were wider than that obtained using a disc electrode of 1 mm diameter. This was probably due to the larger σS values obtained with the glucose sensors of 2 and 25 μm diameter. Moreover, the response of the 2-μm glucose sensor based on hydrogen peroxide detection was compared with that using mediators. This result showed that the wider measurable range was obtained using mediators.

Flow-injection determination of L-ascorbate with cucumber juice as carrier

Cucumber (Cucumis sativus L.) juice has a high l-ascorbate oxidase activity and can be used as the carrier solution in an amperometric flow-injection system for the determination of l-ascorbate. The determination time is 1 min. The calibration graph is linear over the range 5×10−4–7×10−3 M (RSD 4%). No enzyme purification is needed. The juice solution retains its activity for 8 days with recycling.

Flow-injection determination of polyphenols using a plant juice carrier

Abstract Banana pulp and spinach leaves exhibit enzyme acitivities of polyphenol oxidase and catechol oxidase (dimerizing), respectively, and the extracted solutions of these plants were employed as carriers for the determination of polyphenols. Spinach leaf juice was found to be superior to banana juice in terms of its long stability and broad linear rang of the calibration graphs for dopamine (1 × 10−4–6 × 10−3 M), catechol (2 × 10−4–8 × 10−3 M) and dl-epinephrine (2 × 10−4–6 × 10−3 M). Each peak current returned to the original baseline completely within about 5 min when the flow-rate was 0.65 ml min−1. The enzume activity of spinach leaf solution containing 0.3% of sodium azide did not vary for at least 7 days and it was found that this solution was useful as a carrier in a flow-injection system for the determination of polyphenols.

Selective coulometric determination of uric acid in human urine using uricase

Abstract The coulometric determination of uric acid in human urine is done using porous carbon felt electrodes containing electrolyte. The diluted human urine is dropped on the carbon felt surface and uric acid is completely electrically oxidized in a few minutes. The coulombs consumed by interferences contained in human urine are determined by measuring the electrolytic oxidation of the same diluted human urine to which uricase is added. The current efficiencies of uric acid are nearly 100% (RSD

Determination of urea by ion chromatography with an immobilized urease reactor

Abstract An immobilized urease reactor can be used with ion chromatography for the simultaneous determination of urea, and sodium, potassium and ammonium ions. The conversion of urea to ammonium ion was found to be 76.5%. The calibration graph for urea was linear over the range 1 × 10−5−1 × 10−3 M (RSD 3%). The method was applied to human urine and a chemical fertilizer.

Flow-coulometric detector for uric acid in human urine

Abstract A novel flow-coulometric detector integrating an immobilized uricase reactor and an electrolytic cell was fabricated and used for the determination of uric acid in human urine. The procedure is based on the measurement of the total charge with and without passing the sample through an enzyme reactor which allows the complete conversion of the electro-active uric acid in electro-inactive products. The amount of uric acid is linearly related to the difference between the two total charges. The current efficiencies for 1 × 10−4-1 × 10−3 M uric acid were found to be nearly 100% (r.s.d.

Concentration-step amperometric biosensors using thin enzyme reactors

Abstract The thin carbon felt impregnated with a glucose oxidase solution or an immobilized ascorbate oxidase carbon felt was placed on the gas permeable membrane of a Clark Oxygen electrode and concentration-step bio-amperometry of glucose and vitamin C ( l -ascorbic acid and dehydroascorbic acid) was carried out. The decreased current peaks are proportional to the amount of substrate, the relative standard deviations of the peak values were found to be below 5% and the lifetime of the immobilized ascorbate oxidase reactor was extended to 90 days. The steady-state amplified concentration-step amperometric current response of vitamin C using a cyclic reaction of vitamin C was obtained and the amplification factor of vitamin C was about 3 when pH 7.5 buffer solution with 5 × 10 −3 M dithiothreithol was impregnated in the carbon felt.

Self-driven coulometry without an external electric source

Abstract During the usual electrolysis, an electric power supply is needed to transfer electrons between the electroactive species and an electrode. If the substance in the sample solution is to be oxidized (reduced) without an external electric power supply, the potential of the half-cell containing the sample solution must be sufficiently lower (higher) than that of the half-cell in which the counter electrode is placed. If on short-circuiting both half-cells electrolysis in the sample-containing half-cell occurs spontaneously, i.e., without external power supply, the cell is so-called self-driven. This concept has been used in a coulometric cell with a carbon felt electrode impregnated with electrolyte not containing an electroactive species and a carbon felt counter electrode impregnated with hexacyanoferrate(III) solution. Determinations of l -ascorbic acid and NADH carried out with nearly 100% discharging efficiencies and it was demonstrated that self-driven coulometry exhibited the same properties as conventional electrolytic coulometry using an external electric power supply.

Characteristics of porous polymer composite columns prepared by radiation cast-polymerization

Porous polymer composite columns having porous structure were prepared by radiation cast-polymerization of hydrophilic monomers at low temperature and their characteristics were studied. The porosity of the polymer increased with decreasing monomer concentration. The elution time of water in the polymer column increased with increasing monomer concentration and with decreasing irradiation temperature. The elution time was dependent on the degree of hydration of the polymer. The polymer with a degree of hydration of 0.2 to 0.4 gave the minimum elution time. The elution time decreased with the addition of porous inorganic substances.