The use of bimetallic Au(Cu)-coated microelectrodes for improved detection of cystine

Abstract

Au shell – (Au-Cu) core coatings on carbon microdisc electrodes (30 m diameter) have been prepared by a two-step technique whereby Cu particles electrodeposited onto carbon supports(first step) had their surface layers replaced by Au (second step). The latter has been achieved by means of spontaneous partial replacement of the non-precious metal deposits of Cu by Au upon their immersion in the chlorolaurate-based solution: 3 Cu/C + 2 AuCl4 2 Au (Cu)/C + 3 Cu2+ + 8 Cl . The Au-Cu coated microelectrodes, u(Cu)/C, were subsequently used for voltammetric determination of cystine and their performance compared to a bulk Au RDE. The voltammetric picture was similar at both electrodes. A peak corresponding to adsorbed cystine oxidation at ca. +1.2 V vs. Ag/AgCl and a plateau at ca. +1.6 V corresponding to cystine oxidation from the bulk solution under mass transfer control, were recorded. The latter permitted an estimate of the apparent number of electrons associated with cystine oxidation which were calculated to be around 5 in both cases. However, the peak current was found to be much higher at the Au-Cu microelectrodes, even after correction for increased roughness was taken into account. This increase is attributed to an electronic effect of Cu on Au which decreases the affinity of the latter for excessive oxide formation and/or poisonous carbonaceous intermediates of cystine oxidation. Both the peak and plateau currents varied linearly with the concentration range of 1 10-5 – 1 10-4 M cystine and, in the former case, with a detection limit (3 ) estimated to be about 2 10-6 M.