Oxidation-state speciation of

The analytical utility of chemically modified microelectrodes for oxidation-state speciation of redox couples by cyclic voltammetry has been explored. Re(I)(DMPE)3]+/Re(II)(DMPE)3]2+, where DMPE = 1,2-bis(dimethylphosphino)ethane, was studied at carbon-fiber microelectrodes of approximately 5 microm in radius coated with Nafion-entrapped solgel-derived silica (Nafion-silica) composite. The results are compared with cyclic voltammetry of Fe(CN)6]3-/Fe(CN)6]4- at bare carbon-fiber microelectrodes. At both microelectrodes, the cathodic and anodic limiting currents are linearly proportional to the concentrations of the reducible and oxidizable species of a redox couple, respectively. The shape of the cyclic voltammogram and the magnitude of the steady-state limiting current are not affected by the potential at which the scan starts. Speciation of both forms of a redox couple could be achieved voltammetrically at the microelectrodes. However, a considerably slower scan rate was required to achieve steady state at the modified electrode because of the smaller diffusion coefficients of Re(I)(DMPE)3]+ and Re(II)(DMPE)3]2+ in the Nafion-silica composite. The detection limit at the modified electrode was considerably lower (5 x 10(-9) M for Re(I)(DMPE)3]+) than at the bare electrode (6 x 10(-5) M for Fe(CN)6]3- and Fe(CN)6]4-) because of the substantial preconcentration of Re(I)(DMPE)3]+ by the Nafion-silica composite.