Brian J. Birch

Voltammetric behaviour of ascorbic acid at a graphite—epoxy composite electrode chemically modified with cobalt phthalocyanine and its amperometric determination in multivitamin preparations

Abstract Cyclic voltammetry was used to investigate the electrochemical behaviour of ascorbic acid at a carbon—epoxy composite electrode modified with the electron mediator cobalt phthalocyanine. The modified electrode reduced the overpotential necessary for the oxidation of the vitamin by approximately 150 mV to 0.21 V vs. The saturated calomel electrode; the process was dependent on the pH of the supporting electrolyte, but independent of ionic strength over the range studied. The relative standard deviation (r.s.d.) of the peak heights of the cyclic voltammograms was 0.81% for a 1 × 10−4 M ascorbic acid solution (n = 7). The optimum supporting electrolyte was found to be 0.05 M phosphate buffer (pH 5). Amperometry in stirred solutions was then done at an applied potential of +0.25 V. The limit of detection was 0.65 ng ml−1 and the calibration graph was linear in the range 175 ng ml−1−50 μg ml−1. The method was used to determine ascorbic acid concentrations in single- and multivitamin preparations; the recovery was 97.86% for the vitamin added to one preparation. The r.s.d. for the analyses in these samples was about 5%. For comparison, the vitamin was also determined in these tablets using LC with UV detection at 254 nm; the correlation coefficient for the levels determined was 0.9989 (p = 0.0007).

Development of screen-printed carbon electrodes, chemically modified with cobalt phthalocyanine, for electrochemical sensor applications

Abstract A method is described for the production of screen-printed graphite electrodes and also for similar electrodes chemically modified with the electrocatalyst cobalt phthalocyanine. Using cyclic voltammetry, the electrochemical behaviour of these electrodes towards ascorbic acid, reduced glutatione and coenzyme A (CoA-SH) was investigated. The modified electrodes were found to give significant decreases in the over-potential required for the oxidation of these species at carbon electrodes. The useful electrochemical window for the unmodified carbon film electrodes was −1.08 V to +0.85 V vs. SCE, using 1 μA background current cut-off points. Amperometry in stirred solutions was used to investigate the hydrodynamic behaviour of the electrodes and their calibration performance. The limits of detection for ascorbic acid and reduced glutathione at the modified films were 5 × 10−8 and 1 × 10−7 M, respectively. The calibration graphs were also linear up to 2 mM concentrations of both analytes. Using differential-pulse voltammetry, linear calibration graphs were obtained for both species up to 2.5 mM. This technique was also used to assess the reproducibility of the electrode manufacture; the coefficient of variation was 2.8% for 1.49 mM ascorbic acid and 6.9% for 0.92 mM reduced glutathione.

Development of an improved carbon electrode chemically modified with cobalt phthalocyanine as a re-usable sensor for glutathione

Cyclic voltammetry, differential-pulse voltammetry (DPV), amperometry and flow injection with electrochemical detection were used to investigate the electrochemical behaviour of reduced glutathione (GSH) at a carbon paste electrode and also at a carbon paste electrode doped with cobalt phthalocyanine (CPC). The modified electrode was found to reduce greatly the overpotential necessary for the oxidation of GSH at the electrode surface; the process was dependent on the ionic strength and pH of the supporting electrolyte. The oxidation of GSH in the presence of CPC was found to occur at two potentials, 0.27 and 0.75 V versus a saturated calomel electrode. Using DPV the first wave was found to give optimum sensitivity for the measurement of GSH concentations in the range 2.5 × 10–6–6.25 × 10–5M; whereas the second was preferential for GSH concentrations of 1.25 × 10–4–2.5 × 10–3M. Electrode deterioration rendered the other techniques unsuitable for routine quantitative analyses. Similar investigations were also carried out on a carbon-epoxy resin composite electrode containing CPC; the various techniques were used to optimise the electrode response for the determination of GSH in biological fluids. This electrode was found to be superior to the graphite paste electrode as GSH could be determined in flowing or stirred solutions. The modified composite electrode was stable and could be used over a period of at least 10 h. The limits of detection were 10 ng ml–1 and 450 pg injected on column for amperometry and high-performance liquid chromatography with electrochemical detection (LCEC), respectively. Preliminary studies using LCEC have shown that the method is suitable for the determination of circulating levels of GSH in human plasma.

Voltammetric behaviour of screen-printed carbon electrodes, chemically modified with selected mediators, and their application as sensors for the determination of reduced glutathione

The evaluation of screen-printed carbon electrodes, chemically modified with selected ferrocene, phthalocyanine and hexacyanoferrate(III) derivative mediators for the determination of reduced glutathione (GSH), is described. Cyclic voltammetry was used to investigate the effect of pH on the electrochemical behaviour of these mediators incorporated in the disposable electrodes. Values of the electron-transfer coefficient (αna) were calculated for the oxidation of the mediators in phosphate buffer solution and solutions containing 0.48 mmol dm–3 GSH. Amperometry in stirred solutions was used to construct hydrodynamic voltammograms for each of the modified electrodes; these voltammograms were used to elucidate their steady-state behaviour. Both electrochemical techniques were used to calculate the reduction in overpotential for the oxidation of GSH; these calculations were performed for all of the chemically modified electrodes. Amperometry in stirred solutions was used as the technique for quantitative measurements, to determine the calibration response factors (µA mmol–1 dm3) and limits of detection for selected mediators towards GSH. The appropriate applied potentials were selected by reference to the hydrodynamic waves; a range of values were investigated to find the potential that gave the maximum sensitivity. The most promising mediators were cobalt phthalocyanine, iron phthalocyanine and ferrocenecarbaldehyde.

Voltammetric and amperometric behaviour of uric acid at bare and surface-modified screen-printed electrodes: studies towards a disposable uric acid sensor

Systematic voltammetric and amperometric studies have been undertaken to examine the electrochemical behaviour of uric acid at bare and surface-modified screen-printed electrodes. The precision of the electrode manufacture was determined by cyclic voltammetry with a 1.0 × 10–4 mol dm–3 uric acid solution and was calculated to be 6.0%(n= 5). Several strategies were investigated in an attempt to eliminate interference from ascorbic acid. These involved coating the electrode surface with Nafion, or the enzyme L-ascorbic acid oxidase. The latter was immobilized using one of two methods: either by a simple adsorption process, or by cross-linking with bovine serum albumin and glutaraldehyde. The amperometric response at the surface-adsorbed enzyme electrode for uric acid was linear over the concentration range from 5.08 × 10–6 to 1.51 × 10–4 mol dm–3; the limit of detection was 2.54 × 10–6 mol dm–3 using a full-scale deflection of 0.5 µA. This modified electrode exhibited no response to ascorbic acid at levels up to 0.53 mmol dm–3. The electrode modified by cross-linking the enzyme to the surface showed no response to ascorbic acid concentrations of up to 0.093 mmol dm–3.

Development of amperometric sensors for uric acid based on chemically modified graphite–epoxy resin and screen-printed electrodes containing cobalt phthalocyanine

Cyclic voltammetry, hydrodynamic voltammetry (HDV) and amperometry were used to investigate the behaviour of uric acid at carbon(graphite)–epoxy resin electrodes modified with the following electrocatalysts: ferrocenes, phthalocyanines, phenoxazines, phenazines and hexacyanoferrate species. As no direct electrocatalytic oxidation of uric acid was apparent, enzyme-coupled mediation was studied using uricase with electrodes modified with ferrocene, cobalt phthalocyanine (CoPC) and Meldolas Blue, in the presence and absence of oxygen. Again, no electrocatalysis was observed so we exploited the specificity of the enzyme uricase, which is known to produce hydrogen peroxide from uric acid. This product has been shown to undergo electrocatalysis at modified electrodes, and preferentially traverses certain membranes, so this rationale was adopted for the ensuing studies. Cyclic voltammetry and amperometry were used to study the electrochemical behaviour of H2O2 in 0.05 mol dm–3phosphate buffer (pH 9.3). Hydrogen peroxide was found to be electroactive at substantially lower potentials (<600 mV) at CoPC-modified electrodes than at unmodified electrodes. The precision of surface–surface reproducibility of epoxy resin electrodes was calculated by cyclic voltammetry to be 7.3%(n= 5). Further studies were performed with and without cellulose acetate (CA) membranes using amperometry in stirred solution. Epoxy resin electrodes were simply dropcoated with CA. The membrane functioned as an effective barrier against a range of potential interferences, but allowed the selective detection of enzymically generated H2O2 from urate. This methodology was then translated to screen-printed electrodes (SPEs). Cyclic voltammetry was first used to establish the precision of electrode manufacture; this was 4.9%(n= 5). The SPEs were then modified by placing each inside a tube, one end of which was covered with a solvent cast (CA) membrance. This electrode–polymer configuration furnished similar permselective properties as the drop-coated devices. The solvent cast membranes were treated with various concentrations of uricase; when the membrane was loaded with 0.48 U of enzyme a measurable H2O2 signal was obtained at the CoPC–SPE for a solution containing 0.18 mmol dm–3 uric acid.

Determination of glutathione in human plasma using high-performance liquid chromatography with electrochemical detection with a carbon-epoxy resin composite electrode chemically modified with cobalt phthalocyanine

High-performance liquid chromatography with electrochemical detection (LCEC), incorporating a novel carbon-epoxy resin working electrode modified with cobalt phthalocyanine, has been employed for preliminary studies directed towards the determination of normal circulating levels of reduced glutathione (GSH) in human plasma. The mobile phase consisted of 0.05 M phosphate buffer (pH 3) containing 0.1% m/m ethylenediaminetetraacetic acid (EDTA); the calibration graph was linear in the range 0.24-30.7 ng of GSH injected. The mean recovery of GSH added to a control serum over the physiological concentration range (0.38-3.07 ng ml-1) was 99%; this was achieved following a simple sample pre-treatment method, prior to LCEC, involving chelation of divalent cations with EDTA and subsequent acidification with orthophosphoric acid. Using the LCEC method, the mean circulating level of GSH in plasma, found in three normal subjects, was 2.69 microM, GSH; this indicates that the method might be applicable to the determination of depressed circulating levels of GSH.

Electrochemical determination of silver in photographic solutions using fixed-volume single-use sensors

Electrochemical methods are described for the determination of silver ions in photographic processing solutions using a generic sensor design. The sensor consists of three electrodes (working, auxiliary and pseudo-reference) screen-printed with carbon ink. Silver can be measured between 100 µg l–1 and 5 g l–1 encompassing the concentrations of silver in the various stages of photoprocessing from fixer to effluent. Square-wave voltammetry and chronoamperometry were used for low and high silver concentrations, respectively.

Flow injection analysis with tubular membrane ion-selective electrodes in the presence of anionic surfactants

Calcium ion-selective electrodes based on calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} with trioctyl phosphate in poly(vinyl chloride) have improved resistance to interference by anionic surfactants when used in flow injection analysis. Tubular membrane electrodes have been used in flow injection analysis for determining calcium ion levels in selected potable and raw water and the data agree well with determinations by atomic-absorption spectroscopy and EDTA titration.Tubular membrane electrodes with improved membrane sensor have the potential for the determination of free calcium ions for a concentration as low as 10–4M in the presence of a moderate background concentration of detergent, but this is poor compared with the concentration of ⩽10–5M, which can be attained by the conventional type electrode with the same membrane system under static conditions.

Determination of reduced glutathione in human whole blood by high-performance liquid chromatography with electrochemical detection by a graphite-epoxy resin electrode chemically modified with cobalt phthalocyanine.

High-performance liquid chromatography with electrochemical detection by means of a re-polishable graphite-epoxy resin composite electrode, modified with the electrocatalyst cobalt phthalocyanine, has been used for the determination of reduced glutathione (GSH) in 25-mul samples of whole blood. The mobile phase was O.O5M phosphate buffer (pH 2.3) containing 1 mM EDTA, used in conjunction with a Waters muBondapak ODS chromatography column. The use of the electrocatalyst reduced the overpotential for the oxidation of GSH at a carbon electrode by approximately 750 mV, and the applied potential used was +0.5 V vs. Ag/AgCl. The mean recovery of GSH added during the sample pretreatment step was 95%; the assay imprecision was 1-2% for triplicate analyses of the whole blood samples.