A study of L-cysteine adsorption on gold via electrochemical desorption and copper(II) ion complexation

Abstract

The adsorption of L-cysteine on gold electrodes was studied by the methods of electrochemical oxidative desorption and Cu²⁺ complexation. Comparisons with L-cystine adsorption, via the oxidative method, revealed lower saturation surface coverages for L-cystine (1.7 ± 0.17 nmol cm^–2) than for L-cysteine (2.7 ± 0.11 nmol cm^–2), perhaps due to an incomplete cleavage of the S–S bond on adsorption and consequent effects on molecular orientation. Electrochemistry of solution-phase redox probes at L-cysteine monolayer electrodes have shown a porous open monolayer structure amenable to solvent, electrolyte and probe molecule permeation. Both the L-cysteine- and the L-cystine-derived electrodes were capable of complexation of Cu²⁺ ions from solution which were then detectable by cyclic voltammetry. The binding constant of Cu²⁺ ions to the L-cysteine monolayer was estimated to be 2 × 10⁵ L mol^–1 while the molar ratios of L-cysteine to Cu²⁺ on L-cysteine- and L-cystine-derived films were 2∶1 and 2.5∶1, respectively. Detection of Cu²⁺ down to 10^–7 M was possible with cyclic voltammetry, indicating the possible use of such simple films for trace metal analysis.