Sensors based on carbon paste in electrochemical analysis: A review with particular emphasis on the period 1990-1993

Chemical and biochemical sensors based on advances in materials chemistry

The advance of materials chemistry has influenced the design of analytical sensors, especially those using spectroscopic or electrochemical methods for generating the signal. New methods of immobilizing enzymes, chromophores, and electron-transfer catalysts have resulted from initiatives in materials science. Systems based on sol-gel chemistry are especially noteworthy in this regard, but other important materials for chemical and biochemical sensors include zeolites, organic polymers, and various conducting composites. Applications cited include determinations of inorganic ions, gases, neurotransmitters, alcohols, carbohydrates, amino acids, proteins, and DNA.

Electrochemical study of some 2-mercapto-5-R-ammino-1,3,4-thiadiazole derivatives using carbon paste electrodes

The electrochemical study of some 2-mercapto-5-R-ammino-1,3,4-thiadiazole derivatives was made by cyclic and linear sweep voltammetry using a carbon paste electrode (CPE, graphite/solid paraffin ratio 2:1) as working electrode and an Ag/AgCl reference electrode. The current-potential curves were recorded in anodic polarisation in -0.1 and +1.3 V range using aqueous solutions and different buffers (between pH 1.2 and 10.0), with 20 or 50 mV s(-1) sweep rate. The oxidation peak appears between +0.65 and +0.70 V due to disulphides formation. The 5-phenyl derivative has two oxidation peaks, the first at +0.45 +/- 0.03 V and the second at +0.65 +/- 0.03 V. The oxidation potentials are pH dependent, decreasing from 0.9 +/- 0.1 V at pH 1.2 to 0.6 +/- 0.1 V at a pH between 8.0 and 10.0. In some potential ranges depending on pKa of molecules the oxidation potential and oxidation current are pH independent. Simple, precise and accurate voltammetric methods for the determination of these compounds were developed and validated in 2.5 x 10(-6)-7.5 x 10(-4) mol l(-1) concentration ranges. The detection limits were 2.3 micromol l(-1) for 5-ammino-, 12.3 micromol l(-1) for 5-acetylammino-, 11.6 micromol l(-1) for 5-allylammino-, and 1.2 micromol l(-1) for 5-phenylammino-2-mercapto-1,3,4 thiadiazole derivatives.

The voltammetric study of 2-mercapto-5-phenylammino-1,3,4-thiadiazole using carbon paste electrodes

A rapid and accurate voltammetric method for the quantitative determination of 2-mercapto-5-phenylammino-1,3,4-thiadiazole (MPATD) with carbon paste electrodes (CPE) has been developed. The study was made by cyclic voltammetry between -0.4 and +0.6 V with 50 mV s(-1) sweep rate in aqueous solution. After successive oxidation/reduction cycles we found a total oxidation of MPATD at +0.45 V. As the compound is oxidated, the reduction current peak increases at +0.13 V, indicating an irreversible process. Following only the oxidation process in the -0.1 to +0.6 V range, we investigated the optimum scan rates at different current densities and pH values (realised with buffers, pH between 1.0 and 10.0) with CPE versus Ag/AgCl reference electrode using linear sweep voltammetry. We found a good linear relation between the current peak height and concentration in a 2.5 x 10(-9)-1.25 x 10(-7) mol ml(-1). This method allows the quantitative detection of the MPATD as it or from dosage forms and biological media.

Determination of L-phenylalanine based on an NADH-detecting biosensor

An enzyme carbon paste electrode containing three different enzymes was developed for the determination of L-phenylalanine. This sensor is based on the enzymatic/electrochemical recycling of tyrosinase in combination with salicylate hydroxylase and L-phenylalanine dehydrogenase (PADH). The enzymes salicylate hydroxylase and tyrosinase were coimmobilized first in a carbon paste electrode for the sensitive detection of NADH. The principle of the bienzyme scheme is as follows: the first enzyme, salicylate hydroxylase, converts salicylate to catechol in the presence of oxygen and NADH. The second enzyme, tyrosinase, then oxidizes the catechol to o-quinone, which is electrochemically detected and reduced back to catechol at the electrode at an Eappl = -50 mV vs Ag/AgCl. This results in an amplified signal due to the recycling of the catechol and o-quinone between tyrosinase and the surface of the electrode. Prior to adding PADH, the salicylate hydroxylase-tyrosinase carbon paste electrode was characterized in terms of its sensitivity to NADH, pH dependence, buffer composition, interferences, and stability. Interference from ascorbic acid and uric acid was found to be minimal. Human serum was used to investigate whether this bienzyme system was suitable for the detection of NADH in serum and blood samples. The sensitivity for NADH was increased by a factor of 33 times using the bienzyme amplification scheme (electroreduction of o-quinone at Eappl = -50 mV) as opposed to the salicylate hydroxylase single-enzyme system (at which catechol would have been oxidized at Eappl = +150 mV vs Ag/AgCl). The detection limit for NADH achieved by the bienzyme carbon paste electrode was 1 vs 100 microM for the single-enzyme carbon paste electrode. The salicylate hydroxylase-tyrosinase system was then coupled with phenylalanine dehydrogenase for L-phenylalanine determination. This multienzyme sensor was able to achieve a linear range of 20-150 microM and a detection limit of 5 microM for L-phenylalanine. The sensitivity is sufficient since the reference clinical range for L-phenylalanine is 78-206 microM.