Formation of Some Cysteine-Containing Peptide Monolayers on Au Electrodes and Their Applications for Metal Ion Sensing and Electrocatalytic Reactions

Abstract

The use of biological molecules that already exist in nature offers advantages over using artificially made molecules in developing biosensors or electrocatalysts. Since those molecules have already been best adopted by nature to carry out specific functions, a lot of practical benefit could be acquired by utilizing or mimicking them rather than synthesizing complex molecules. In this regard, amino acids and peptides are favorite systems in metal ion sensing in a sense that they can act as very effective and often specific ligands for a variety of metal ions. While amine and carboxyl groups of amino acids participate in complexation, an amide nitrogen atom is also involved in complexation in the case of peptides. This often forms a chelate and a peptide-metal ion binding becomes significantly stronger as a result. Detailed accounts have been treated by Sigel and Martin and other authors. Despite the fact that amino acids and peptides readily form complexes with metal ions, electrochemical study of those complexes is scarce. It was not until recently when a first report by Yang et al. appeared for the electrochemical metal ion detection. They synthesized a tripeptide system, GlyGly-His, on top of an alkanethiol self-assembled Au electrode to detect copper ions. Since then, many papers regarding various electrochemical aspects of a peptide-metal binding have been published. Here we present our preliminary results of metal ion sensing and electrocatalytic reactions using cysteine-containing simple peptide monolayers constructed on a Au surface. Cysteine was chemisorbed on the Au surface via Au-S bonding and other amino acids were formed by the usual peptide synthesis method. This way two types of peptide monolayers were prepared: With a Au/ GSH-His system (GSH: glutathione, γ-L-glutamyl-L-cysteinyl-glycine), copper ions of nano molar concentration were detected by accumulating Cu on the peptide monolayer. With a Au/Cys-Cys-Cys/M(M = Cu, Fe) system, electrocatalytic oxidation of ascorbic acid (AA) and reduction of hydrogen peroxide were performed. Figure 1 shows cyclic voltammograms of a Au/GSH-His/ Cu system after immersing a Au/GSH-His electrode in a Cu-containing solution and transferring into the Cu-free electrolyte. Fairly stable voltammograms were obtained. Without copper ion, no faradaic current was observed. Redox peaks at +0.33 and +0.010 V are assigned to the 1-e− redox reactions of a Cu/Cu couple as indicated in other studies. With only GSH layer on Au, current slightly decreased upon multi potential cycling. This means copper ions form a more stable complex with histidine in which imidazole moiety of histidine is involved in complexation. Full copper ion accumulation was complete after ca. 10 min. Treating the Au/GSH-His/Cu electrode with EDTA solution, copper ions were completely removed from the modified