Enzymatic activity of surface-immobilized horseradish peroxidase confined to micrometer- to nanometer-scale structures in nanocapillary array membranes

The charge transport processes in thin insulating films separating a gold electrode and an electrolyte solution are characterized using a constitutive admittance expression that accounts for conduction, diffusion, and polarization of charge within the films. We specifically investigate cases in which the electrolyte solution does not contain electroactive ions. The general impedance response of all gold−monolayer−electrolyte systems to an applied potential, for systems in which the electrolyte does not contain any redox-active species, suggests the existence of a potential regime where the current is limited by the rate of charge transport through the monolayer phase and a regime limited by the rate of charge transfer at the monolayer−electrolyte interface. The monolayer free charge density that appears as a parameter in the admittance expression is evaluated from the measured admittance in both regimes. This calculated parameter describes two field-dependent mechanisms of charge transport in the monolayer phase, for potentials where charge transport limits the flow of charge through the insulating films. These charge transport mechanisms follow well-characterized solid-state mechanistic models of charge conduction; namely, Ohmic conduction at low electric fields and space−charge-limited transport at higher electric fields. Quantum mechanical tunneling effects are also observed at large (109 V/m) electric fields in the monolayer. For potential regimes in which the charge transfer is rate-limiting, the evaluation of the monolayer free charge density from the impedance response results in current densities that are described by the thermal activation of reacting species over a free energy barrier at the monolayer−electrolyte interface. At low electric fields, the rate-limiting process involves the thermally activated reorganization of the solvent molecules, in accordance with the Marcus theory, and for higher fields, the observed current is limited by the thermal hopping of transferring electrons over an electrostatic potential energy barrier within the monolayer phase. The charge transport and charge transfer mechanisms are also shown to be dependent on physical and chemical interactions between the monolayer functional group and the electrolyte constituents at the Stern layer. These interactions are described using empirical parameters obtained from the mechanistic expressions for these charge transport and transfer processes, and the effect of varying electrolyte properties on these parameters is examined in detail here