Voltammetry of TlI, CdII, CuII, PbII and EuIII at phosphatidylserine-coated mercury electrodes

Abstract

The characteristics of the electrochemical reduction of metal ions at a negatively charged lipid-coated electrode are described. The metal-ion voltammetry is related to the permeability of the electroactive ion in the lipid film and to interactions between the electrolyte and electroactive ions and the lipid surface. The cyclic voltammetry of Tl, Cd, Cu, Pb and Eu was investigated at a phosphatidylserine (PS)-coated mercury electrode in solutions of different pH, ionic composition and ionic concentration. Significant reduction peaks of Tl^I, Cd^II, Cu^II and Pb^II are observed within the potential region where the lipid maintains its stable orientation on the electrode thus these ions are significantly more permeable in PS monolayers than in dioleoyl phosphatidylcholine (DOPC) monolayers. The voltammetric peaks of Cd^II in solution pH 8.1 and the voltammetric peaks of Cu^II and Eu^III in solution pH > 5.0 are characteristic of the irreversible reduction of adsorbed electroactive species whereas those for Tl^I and Pb^II represent the quasi-reversible and irreversible reduction respectively of the dissolved species. With a lowering in solution pH, from 10.4, 8.1, 7.0, 5.8 and 5.6 for Tl^I, Cd^I, Pb^II, Cu^II and Eu^III respectively, the reduction current peaks of all the electroactive ions decrease in height. Specifically, an increased concentration of univalent ion electrolyte and the presence of univalent electrolyte ions with greater binding affinity for PS suppress the Tl voltammograms. In solution pH about one unit below the intrinsic second pK_a value of PS (3.6) or in the presence of Mg²⁺ electrolyte or with small concentrations of La³⁺ ions in a univalent ion electrolyte, the voltammograms of all the electroactive ions begin to resemble those for the reduction of these metal ions at a DOPC coated electrode indicating that the lipid layer has become relatively impermeable to ions. It is evident that in solution pH ca. 8.1, Cd^II is adsorbed on PS. The association of Cd^II with PS decreases with a lowering in solution pH but Eu^III and Cu^II are significantly adsorbed on PS at lower solution pH (5.9) thus Cu^II binds more strongly to PS than Cd^II. In solution pH < 5, the association of Eu^III and Cu^II ions with the PS surface decreases with a lowering in pH. The effects of solution pH on the voltammograms relate directly to the protonation of PS and the lowering of the average monolayer potential. This lessens the electrostatic interaction of electroactive ions with the monolayer surface lowering their concentration in this region and their apparent permeability. It also alters the structure of the lipid. The influence of varying concentration and type of monovalent electrolyte cations on the Tl^I reduction currents is due to both a screening effect on the monolayer potential and a binding of ions to the lipid which lowers the Tl^I concentration adjacent to the lipid surface. Electrolyte Mg²⁺ and small concentrations of La³⁺ in a univalent ion electrolyte bind strongly to the lipid also lowering the electroactive ion concentration at the lipid surface as well as altering the lipid structure.