Oxidation state speciation of [ReI(DMPE)3]+/[ReII(DMPE)3]2+ by voltammetry with a chemically modified microelectrode

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 similar to 5 mu m in radius coated with Nafion-entrapped sol-gel-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.