No abstract

Ag/AgCl reference electrodes fabricated from pulled quartz tubes with orifice radii of 20 nm to 20 µm were characterized in KCl solutions of different concentrations by cyclic voltammetry. Linear current-voltage (i-V) dependence was observed with micropipet electrodes (with micrometer-sized tips) with the same concentration (0.01-1 M) of KCl inside and outside the pipet, but current rectification was found at nanopipet electrodes at KCl concentrations of e0.1 M (with nanometer-sized tips). This is attributed to formation of a diffuse electrical double layer within the tip orifice. The effects of electrode size, electrolyte concentration, and solution pH on the nonlinear i-V behavior of these electrodes were investigated. A model for the observed behavior shows the rectification to be caused by the permselectivity in the tip region and the geometric asymmetry of the tip orifice. This phenomenon may be important in studies of ion transport in charged channels and porous membranes.

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.