High-Throughput Synthesis and Electrochemical Screening of a Library of Modified Electrodes for NADH Oxidation

We report the combinatorial preparation and high-throughput screening of a library of modified electrodes designed to catalyze the oxidation of NADH. Sixty glassy carbon electrodes were covalently modified with ruthenium­(II) or zinc­(II) complexes bearing the redox active 1,10-phenanthroline-5,6-dione (phendione) ligand by electrochemical functionalization using one of four different linkers, followed by attachment of one of five different phendione metal complexes using combinatorial solid-phase synthesis methodology. This gave a library with three replicates of each of 20 different electrode modifications. This library was electrochemically screened in high-throughput (HTP) mode using cyclic voltammetry. The members of the library were evaluated with regard to the surface coverage, midpeak potential, and voltammetric peak separation for the phendione ligand, and their catalytic activity toward NADH oxidation. The surface coverage was found to depend on the length and flexibility of the linker and the geometry of the metal complex. The choices of linker and metal complex were also found to have significant impact on the kinetics of the reaction between the 1,10-phenanthroline-5,6-dione ligand and NADH. The rate constants for the reaction were obtained by analyzing the catalytic currents as a function of NADH concentration and scan rate, and the influence of the surface molecular architecture on the kinetics was evaluated

Scanning Electrochemical Microscopy: Using the Potentiometric Mode of SECM To Study the Mixed Potential Arising from Two Independent Redox Processes

This study demonstrates how the potentiometric mode of the scanning electrochemical microscope (SECM) can be used to sensitively probe and alter the mixed potential due to two independent redox processes provided that the transport of one of the species involved is controlled by diffusion. This is illustrated with the discharge of hydrogen from nanostructured Pd hydride films deposited on the SECM tip. In deareated buffered solutions the open circuit potential of the PdH in equilibrium between its β and α phases (OCP_β→α) does not depend on the tip–substrate distance while in aerated conditions it is found to be controlled by hindered diffusion of oxygen. Chronopotentiometric and amperometric measurements at several tip–substrate distances reveal how the flux of oxygen toward the Pd hydride film determines its potential. Linear sweep voltammetry shows that the polarization resistance increases when the tip approaches an inert substrate. The SECM methodology also demonstrates how dissolved oxygen affects the rate of hydrogen extraction from the Pd lattice. Over a wide potential window, the highly reactive nanostructure promotes the reduction of oxygen which rapidly discharges hydrogen from the PdH. The flux of oxygen toward the tip can be adjusted via hindered diffusion. Approaching the substrate decreases the flux of oxygen, lengthens the hydrogen discharge, and shifts OCP_β→α negatively. The results are consistent with a mixed potential due to the rate of oxygen reduction balancing that of the hydride oxidation. The methodology is generic and applicable to other mixed potential processes in corrosion or catalysis