Inner-Sphere Heterogeneous Electrode Reactions. Electrocatalysis and Photocatalysis: The Challenge

Electrochemistry at nanoscale electrodes: individual single-walled carbon nanotubes (SWNTs) and SWNT-templated metal nanowires

Individual nanowires (NWs) and native single-walled carbon nanotubes (SWNTs) can be readily used as well-defined nanoscale electrodes (NSEs) for voltammetric analysis. Here, the simple photolithography-free fabrication of submillimeter long Au, Pt, and Pd NWs, with sub-100 nm heights, by templated electrodeposition onto ultralong flow-aligned SWNTs is demonstrated. Both individual Au NWs and SWNTs are employed as NSEs for electron-transfer (ET) kinetic quantification, using cyclic voltammetry (CV), in conjunction with a microcapillary-based electrochemical method. A small capillary with internal diameter in the range 30-70 μm, filled with solution containing a redox-active mediator (FcTMA(+) ((trimethylammonium)methylferrocene), Fe(CN)(6)(4-), or hydrazine) is positioned above the NSE, so that the solution meniscus completes an electrochemical cell. A 3D finite-element model, faithfully reproducing the experimental geometry, is used to both analyze the experimental CVs and derive the rate of heterogeneous ET, using Butler-Volmer kinetics. For a 70 nm height Au NW, intrinsic rate constants, k(0), up to ca. 1 cm s(-1) can be resolved. Using the same experimental configuration the electrochemistry of individual SWNTs can also be accessed. For FcTMA(+/2+) electrolysis the simulated ET kinetic parameters yield very fast ET kinetics (k(0) > 2 ± 1 cm s(-1)). Some deviation between the experimental voltammetry and the idealized model is noted, suggesting that double-layer effects may influence ET at the nanoscale.

Stochastic sensing of single molecules in a nanofluidic electrochemical device

We report the electrochemical detection of individual redox-active molecules as they freely diffuse in solution. Our approach is based on microfabricated nanofluidic devices, wherein repeated reduction and oxidation at two closely spaced electrodes yields a giant sensitivity gain. Single molecules entering and leaving the cavity are revealed as anticorrelated steps in the faradaic current measured simultaneously through the two electrodes. Cross-correlation analysis provides unequivocal evidence of single molecule sensitivity. We further find agreement with numerical simulations of the stochastic signals and analytical results for the distribution of residence times. This new detection capability can serve as a powerful alternative when fluorescent labeling is invasive or impossible. It further enables new fundamental (bio)electrochemical experiments, for example, localized detection of neurotransmitter release, studies of enzymes with redox-active products, and single-cell electrochemical assays. Finally, our lithography-based approach renders the devices suitable for integration in highly parallelized, all-electrical analysis systems.

Elucidating the ionomer-electrified metal interface

The competitive adsorption of Nafion functional groups induce complex potential dependencies (Stark tuning) of vibrational modes of CO adsorbed (CO(ads)) on the Pt of operating fuel cell electrodes. Operando infrared (IR) spectroscopy, polarization modulated IR spectroscopy (PM-IRRAS) of Pt-Nafion interfaces, and attenuated total reflectance IR spectroscopy of bulk Nafion were correlated by density functional theory (DFT) calculated spectra to elucidate Nafion functional group coadsorption responsible for the Stark tuning of CO(ads) on high surface area fuel cell electrodes. The DFT calculations and observed spectra suggest that the side-chain CF3, CF2 groups (i.e., of the backbone and side chain) and the SO3(-) are ordered by the platinum surface. A model of the Nafion-Pt interface with appropriate dihedral and native bond angles, consistent with experimental and calculated spectra, suggest direct adsorption of the CF3 and SO3(-) functional groups on Pt. Such adsorption partially orders the Nafion backbone and/or side-chain CF2 groups relative to the Pt surface. The coadsorption of CF3 is further supported by Mulliken partial charge calculations: The CF3 fluorine atoms have the highest average charge among all types of Nafion fluorine atoms and are second only to the sulfonate oxygen atoms.