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 TlI, CdII, CuII and PbII 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 CdII in solution pH 8.1 and the voltammetric peaks of CuII and EuIII in solution pH > 5.0 are characteristic of the irreversible reduction of adsorbed electroactive species whereas those for TlI and PbII 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 TlI, CdI, PbII, CuII and EuIII 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 pKa 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, CdII is adsorbed on PS. The association of CdII with PS decreases with a lowering in solution pH but EuIII and CuII are significantly adsorbed on PS at lower solution pH (5.9) thus CuII binds more strongly to PS than CdII. In solution pH < 5, the association of EuIII and CuII 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 TlI reduction currents is due to both a screening effect on the monolayer potential and a binding of ions to the lipid which lowers the TlI 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.