Electron Transfer in One- and Zero-Dimensional Nanooptofluidic Devices

Abstract

Working in the same hallway as Prof. Wieckowski and his students for 22 years taught us to appreciate the beauty of heterogeneous electron transfer. We are now pursuing that appreciation through coupled electron transfer and spectroscopic probing of processes in zero- and one-dimensional nanostructures. One-dimensional architectures are explored in two formats: (1) a horizontal geometry in which a small number of nanochannels are arrayed at the top surface of a fluidic device so that optical spectroscopy can be employed to augment studies of electrochemistry at embedded planar electrodes; and (2) vertically-oriented nanopores supporting embedded annular nanoband electrodes (EANEs) in massively parallel high-density arrays. Because transport and electron transfer are intimately coupled, high efficiency (unit efficiency in many cases) electrochemical conversions can be achieved in many structures, and the horizontal geometry allows detailed measurements of the spatiotemporal distributions of electrochemical products for comparison with finite-element modeling.Zero-dimensional architectures are also being studied in two formats: (1) arrays of nanoconfined recessed ring-disk electrodes (RRDEs), in which redox cycling can be carried out at very high efficiency; and (2) electrochemically-active zero-mode waveguides (ZMWs), in which strongly confined optical fields can be coupled to single molecule electron transfer events in order to study the dynamics of single enzyme molecules and single redox-active organic chromophores. Nanoscale separation in RRDE arrays allows ultra-efficient coupling of counterpoised electron transfer processes at the bottom disk and upper ring electrodes in an attoliter-volume nanopore, processes that can be exploited for improving both sensitivity and selectivity in electroanalysis. In the other geometry, the Au optical cladding layer of a ZMW is used as the working electrode in order to couple the local potential to the fluorescence dynamics of molecules exhibiting large changes in emission quantum efficiency with redox state. These experiments allow the distributions of single molecule E eq values and other details of electron transfer reactions to be measured on a single molecule basis.