P.Tuñón Blanco

Poly(o-aminophenol)-modified bienzyme carbon paste electrode for the detection of uric acid

A reagentless uric acid selective biosensor constructed by immobilising uricase and horseradish peroxidase (HRP) in carbon paste without the addition of an electron transfer mediator is described. The response of the electrode is based on the enzymatic reduction of hydrogen peroxide in the presence of uric acid. Uricase and HRP were dispersed in the carbon paste and the optimum paste mixture was determined. Poly(o-aminophenol) was electropolymerised at the working surface area of the electrode acting as a conducting polymer layer. Cyclic voltammetry was used to characterise the permselective characteristics of the polymer layer. At an applied potential of 50 mV vs. Ag/AgCl, a linear response was obtained up to 1 x 10(-4) M, with a limit of detection of 3 x 10(-6) M. The sensor had a response time of 37 s. a calibration precision of 2.2% (n = 4) and an estimated sample frequency of 20 h(-1). Responses to the analyte of interest were pH dependent. The sensor was incorporated into a flow injection system for the qualification of uric acid in human serum. Results compared favourably with a standard spectrophotometric method.

Amperometric detection of ethanol with poly-(o-phenylenediamine)-modified enzyme electrodes

Abstract Poly-(o-phenylenediamine was electropolymerized on the surface of an alcohol dehydrogenase-nicotimanide adenine dinucleotide-modified carbon paste electrode to yield an electrochemical biosensor for ethanol. Efficient electrocatalytic oxidation, at low applied potentials, of the enzymatically produced NADH by the conducting film provided a current that was dependent on ethanol concentration. The influence of various experimental variables was explored to optimize the analytical performance. Linear responses for ethanol in the range 3 × 10−8 − 3 × 10−6 M and a response time of 20 s were obtained. The trend in sensitivity towards different alcohols is in agreement with the known biospecificity of yeast ADH. Ethanol sensor was applied to the analysis of different alcohol beverages, resulting in excellent accuracy and precision.

Adsorptive Stripping Voltammetric Behaviour of Folic Acid

The effective adsorption of folic acid at mercury electrodes is demonsrated. The adsorptive stripping response is evaluated with respect to experimental parameters such as the preconcentration time and potential, bulk concentration, stirring rate and others. The accumulation curves at different concentrations of folic acid show slopes proportional to the bulk concentration. At a certain stage (40% of the equilibrium surface coverage), the change of the morphology of the adsorption peak indicates that interactions between adsorbed molecules are substantial. The Effect of some surfactants present in the medium on the adsorbed folic acid stripping signal is studied. In the presence of a 700-fold excess of gelatin, the folic acid signal is 40% of its initial value (gelatin-free solution). The ac voltammetric mode applied to the stripping of adsorbed folic acid reveals the most sensitive response, allowing 1×10−11M folic acid to be detected by using 10 min of accumulation time.

A comparative study of different adenine derivatives for the electrocatalytic oxidation of β-nicotinamide adenine dinucleotide

Abstract The electrochemical oxidation of the adenine moiety in different adenine derivatives, including adenosine, adenosinemonophosphate (AMP), adenosinediphosphate (ADP) and adenosinetriphosphate (ATP), has been studied at pyrolytic graphite electrodes in aqueous solution in the pH range 5–11. All these compounds yield strongly adsorbed electroactive oxidation products with reversible behavior and formal potentials close to 0 V at pH 9.0. These oxidation products show strong electrocatalytic activity for the oxidation of the reduced form of β-nicotinamide adenine dinucleotide (NADH). The electrochemical properties of the electrocatalyst obtained by oxidation of the corresponding parent nucleotide were investigated by cyclic voltammetry. The influence of the structure of the parent compound on the electrocatalytic activity of the corresponding product toward NADH oxidation was studied also. The kinetics of the electrocatalytic reactions were evaluated from rotating disk electrode measurements. These modified graphite electrodes can be used as NADH transducers in biosensors for substrates of dehydrogenase enzymes and also for the measurement of enzymatic activity.

Simultaneous adsorptive stripping voltammetric determination of riboflavin and folic acid in multivitamin preparations

Abstract The adsorptive stripping voltammetric behaviour of folic acid and riboflavin was studied at a hanging mercury drop electrode by phase-selective a.c. voltammetry. In 0.1 M sodium acetate buffer (pH 5.0) a cathodic scan gave peaks at −0.29 and −0.55 V vs. Ag/AgCl for riboflavin and folic acid, respectively. The adsorptive stripping response was evaluated with respect to concentration dependence and preconcentration time. Both compounds can be simultaneous determined with a relative standard deviation of 1.4% at 5 × 10 −8 M riboflavin and 0.74% at 10 −8 M folic acid. The method compared favourably with liquid chromatography with UV detection and was succesfully applied to the simultaneous determination of both compounds in multivitamin preparations. The average contents of riboflavin and folic acid were found to be 15.19 mg ± 2.1% and 1.8 mg ± 2.6%, respectively.

An electroanalytical study of the anticancer drug dacarbazine

The electrochemical behaviour of dacarbazine [5-(3,3-dimethyl-1-triazenyl) imidazole-4-carboxamide; DTIC] was investigated by Tast and differential pulse polarography (d.p.p.) at the dropping mercury electrode, by cyclic and differential pulse voltammetry at the hanging mercury drop electrode and by anodic voltammetry at the glassy carbon electrode. Calibration graphs were obtained for 2×10−8−2×10−5 M DTIC by d.p.p., for 5×10−9−1×10−5 M by adsorptive stripping voltammetry ar a hanging mercury drop electrode, and for 1−10×10−5 M by high-performance liquid chromatography with oxidative amperometric detection at a glassy carbon electrode. The methods are compared and applied to determine DTIC added to blood serum after a simple clean-up procedure.

Cathodic stripping voltammetry of 5-fluorouracil

An investigation has been carried out into the anodic differential-pulse polarographic and cathodic stripping voltammetric behaviour of the anticancer drug 5-fluorouracil at mercury indicator electrodes. The anodic peak, corresponding to the formation of a mercury salt, has been used for the determination of the drug in formulations with a relative standard deviation of 0.5%. The cathodic stripping voltammetric behaviour of this mercury salt can be used to monitor 10–7–10–8M concentrations of 5-fluorouracil in a borax-HClO4 buffer, pH 7.8. The interference of Cl– with the cathodic stripping signals is only evident at concentrations of Cl– four orders of magnitude higher than the concentration of the drug and uric acid was found not to interfere at up to equal concentrations of 5-fluorouracil. The cathodic stripping method has been applied to the determination of 5-fluorouracil in human serum.

Adsorptive stripping voltammetry on mercury-coated carbon fibre ultramicroelectrodes

Abstract The conditions for the electrodeposition of mercury on the surface of carbon fibres to permit the sensitive and reproducible voltammetry of folic acid and mitoxantrone (MXT) were investigated. The accumulation behaviour of both molecules on a mercury film ultramicroelectrode was studied and the possibility of carrying out a.c. adsorptive stripping analysis using mercury-coated carbon ultramicroelectrodes was demonstrated. Optimum conditions for both the deposition of mercury and a.c. monitoring are described. This procedure gave rise to a linear calibration graph from 5.0 x 10−10 to 2.0 x 10−8 M, a detection limit of 5.0 x 10−10 M and a relative standard deviation (R.S.D.) of 5.05% at 5.0 x 10−9 M (n = 10 ) for the determination of MXT. For folic acid a linear calibration graph from 1.0 x 10−9 to 5.0 x 10−8 M, a detection limit of 9.0 x 10−10 M and an R.S.D. of 1.44% at 4.0 x 10−8 M (n = 10) were found. Both compounds can be determined directly in biological samples at physiological levels without any separation or clean-up procedure.

Immobilized enzyme electrode for the determination of l-lactate in food samples

Abstract A sensitive immobilized enzyme electrode for l -lactate is described. Toluidine blue O (TBO) and lactate dehydrogenase (LDH) were co-immobilized onto the surface of a graphite electrode by means of a dialysis membrane. l -lactate is oxidized to pyruvate by LDH in the presence of nicotinamide adenine dinucleotide NAD+ which is then reduced to NADH. The NADH formed is electrocatalytically oxidized by TBO and the reduced mediator is detected at 0 V vs. a Ag AgCl reference electrode. Different dialysis membranes were tested in order to improve stability. The most stable enzyme electrode was fabricated with a benzoylated membrane, showing a half-life of eight days. This amperometric electrode responds fast and linearly to l -lactate in a concentration range 10−6 − 6 × 10−5 M. The limit of detection is 4 × 10−7M. The enzyme electrode was applied to the determination of l -lactate in food samples. Results were compared to those obtained using a spectrophotometric method, showing a good agreement.

Electrooxidation of methadone on carbon paste electrodes

The oxidation of methadone on carbon paste electrodes has been studied using voltammetric techniques under semi-infinite linear diffusion and hydrodynamic conditions. A simple “in situ” electrode pretreatment is proposed to obtain a good reproducibility of the current signal. The diffusion coefficient (0.19 × 10−5 cm2 s−1), the charge transfer coefficient (β = 0.30) and the charge transfer rate constant (K = 1.2 × 10−11 cm s−1) are reported. A mechanism involving two electrons and two protons is proposed for methadone oxidation, which is in agreement with reported data concerning other aliphatic amines.