(647e) Non-Adiabatic Electronic Transitions in Charge Transfer Events at Thin-Film Modified Electrode-Electrolyte Interfaces for the Detection of Dissolved Analytes

Gupta, C., Stanford University
Howe, R. T., Stanford University
Shannon, M. A., University of Illinois at Urbana-Champaign

Monolayer-modified electrode-electrolyte interfaces have long been considered model solid-liquid interfaces for the characterization of the electrochemical properties of specific redox active probes that are either dissolved in the electrolyte or tethered to the monolayer film surface. However, the leakage current across these film-modified electrode-electrolyte interfaces in the absence of redox active moieties has often been subject to empirical description that lacks sufficient experimental validation. In this presentation, we propose to elucidate on a mechanism for charge flow across an electrified monolayer-modified solid-liquid interface where the kinetics of the overall charge transfer process is limited by the rate of the non-adiabatic transition of the transferring electron between two closely spaced energy levels. The extent of the coupling between the initial and final electronic energy levels is shown to determine the actual mechanism of electron transport in the monolayer phase, where tunneling predominates when the energy levels are weakly coupled and a thermal activation/electron hopping like-mechanism operates when the coupling between the initial and final states is large. The experimentally measured leakage current is described by a quasi-continuum charge transport equation that models the observed current as an energy barrier-limited flux of charge. The extent of the coupling between the initial and final electronic states is shown to be a strong function of the applied potential across the electrode-electrolyte interface, and thus, the nature of the transport mechanism can be modulated by the applied bias. Experimentally measured coupling coefficients bear out the proposed hypothesis, and we demonstrate the tunneling and thermal activation characteristics of the leakage current in the weakly and strongly coupled states, respectively. We also demonstrate a capability to characterize molecular analytes in the aqueous electrolyte phase on the basis of their charge, mass or chemical structure from the measured current density when limited by the non-adiabatic electronic transition in the weakly coupled limit. In defining this capability, a label-free, chemistry-specific, electronic detection platform is conceptualized for further development.