Electrochemical determination of thiols at single-wall carbon nanotubes and PQQ modified electrodes

Abstract

The electrocatalytic oxidation of thiols has been observed at a glassy carbon (GC) electrode coated with a single-wall carbon nanotube (SWNT) film. Fourteen thiols including L-cysteine (CySH) and glutathione were tested using the SWNT/GC electrode, and the cyclic voltammetry (CV) showed that each thiol was oxidized at much less positive potential than those at other electrodes such as bare GC and diamond electrodes. The SWNT/GC electrode was also modified with pyrroloquinoline quinone (PQQ) which showed a further improvement of the catalytic behavior of the SWNT/GC electrode: e.g. the oxidation peak current of CySH was observed at 0.27 V vs. Ag/AgCl in pH 7.5 phosphate buffer. The amperometic responses at these electrodes showed a linear relationship with the substrate concentration in a 10(-6)-10(-3) M range and 10(-6)-10(-7) M detection limits for several thiols including CySH, L-homocysteine, N-acetyl-L-cysteine, L-penicillamine and glutathione. These electrodes show a response time of 2-3 s and storage stabilities over 3 weeks. A PQQ/SWNT/GC electrode has been successfully applied for the assay of both L-cysteine and N-acetyl-L-cysteine in the dietary supplement.

Multi-vanadium substituted polyoxometalates as efficient electrocatalysts for the oxidation of l-cysteine at low potential on glassy carbon electrodes

“…Also associated with this strategy are several other beneficial improvements including the increase in selectivity, the decrease of surface fouling and the decrease of surface oxide formation. Remarkable achievements were obtained recently with boron-doped diamond electrodes [2,7,11] and also with carbon nanotube materials, unmodified or modified with various catalysts [1][2][3]9].…”

“…The potential step programs were from 0 V to + 0.5 V and back to 0 V for As 2 V 2 IV VW 18 and from-0.2 V to + 0.5 V and back to surface. This feature must be constrasted with the known slowness and sluggishness of the oxidation of thiols at solid electrodes, including glassy carbon, on which the reaction requires at least 1.0 V to be effective [1,7].…”

“…It is used in medicine and food industries. As a consequence, its oxidation and accurate determination at low potential constitute a valuable task and the search for sensitive and selective methods in these purposes continues unabated [1][2][3][4]. In this context, electrochemical techniques at solid electrodes appear among the favourites, probably because, simultaneously with detection, they provide interesting insights into the redox behaviours of cysteine.…”

“…L-cysteine is recognized as one of the most important among marker thiols in biological systems [1][2][3][4]. It is used in medicine and food industries.…”

“…Unfortunately, direct oxidation of thiols including cysteine, at classical solid electrodes surfaces (Pt, Au, Graphite, Carbon) is kinetically slow and requires a large overpotential to be effective. Hence the necessity to study electrocatalytic processes that might decrease the overpotential [1][2][3][4][5][6][7][8][9][10][11][12][13]. Also associated with this strategy are several other beneficial improvements including the increase in selectivity, the decrease of surface fouling and the decrease of surface oxide formation.…”