Measuring the rates of rapid outer-sphere electron transfer (ET) reactions is of special interest because these mechanistically simple processes are used to check the ET theory.¹   We investigated the kinetics of several fast heterogeneous electron transfer reactions by steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM) and addressed an intriguing question about possible differences between ET dynamics observed at macroscopic and nanometer-sized interfaces.²  The polished Pt electrodes (≥10 nm-radius) were prepared and used as tips for SECM experiments, so that a number of current vs. distance curves and voltammograms were obtained at the same tip without damaging it.  This allowed us to characterize the tip geometry and then use it to obtain an extensive set of experimental data.  In this way, nanoelectrochemical measurements of ET kinetics were made with reproducibility similar to that reported previously for micrometer-sized electrodes.  The SECM current vs. distance curves showed high feedback response indicating that the prepared tips were flat on the nanometer scale and not recessed.  The steady-state voltammograms and approach curves obtained were in agreement with conventional electrochemical theory at all values of the tip radius (a) and tip/substrate separation distance (d).

The kinetic parameters (i.e., the standard rate constant, k°, and the transfer coefficient, α) obtained for four rapid heterogeneous ET reactions were reproducible within ~10% error margin.  The mass transfer rate was changed by more than two orders of magnitude by varying both the tip radius and the tip/substrate separation distance, and the measured k° and α values were essentially independent of both a and d.  For four fast ET reactions (the oxidation of ferrocenemethanol in water and ferrocene in acetonitrile, and the reduction of TCNQ in acetonitrile) the standard rate constants measured at nanoelectrodes were similar to or slightly higher than the values obtained previously at larger electrodes.  The standard rate constant of Ru(NH₃)₆³⁺ reduction in KCl is very fast (17.0 ± 0.9 cm/s) and hard to measure by other electrochemical techniques.  The magnitude of the Frumkin correction for a triple charged cation, Ru(NH₃)₆³⁺  is hard to evaluate quantitatively, and it was also suggested that k° of Ru(NH₃)₆³⁺ reduction increases significantly in the presence of chloride ions.  These factors and the essential adiabaticity of this reaction³ should be responsible for the unusually high k°.

Overall, there is no major difference between ET rate measured at nanoelectrodes and at larger interfaces.  At the same time, the upper limit for the rate constant measurable at nanoelectrodes under steady-state conditions is as high as ~200 cm/s.

We also prepared slightly recessed nanoelectrodes by controlled etching of flat Pt electrodes discussed above.⁴  Using high-frequency (e.g., 2 MHz) ac voltage, the layer of Pt as thin as ≥3 nm was removed to produce a cylindrical cavity inside the insulating glass sheath.  The etched electrodes were characterized by combination of voltammetry and SECM to determine the radius and the effective depth of the recess (l).  Diffusion limiting currents to such electrodes and SECM approach curves were simulated and the simulation results were generalized in the form of analytical approximations.  Although the conductive surface recessed inside glass nanocavity could not be polished and is likely to be rough after etching, the prepared probes exhibited stable and reproducible electrochemical behavior.  Excellent fit between theoretical and experimental approach curves confirmed the validity of the tip shape parameters (a and l) obtained from voltammetry.  The recessed probes are suitable for experiments in both aqueous and non-aqueous solutions including low-polarity solvents like 1,2-dichloroethane. Potential applications include fabrication of nanometer-sized biosensors, experiments with individual molecules in ultra-small volumes, and thin-layer electrochemical studies of charge-transfer reactions at the liquid/liquid interface.  The recessed electrodes can be employed as SECM tips for high resolution imaging, feedback mode and generation/collection experiments.

The PRF support enabled us to start this new project.  A graduate student and a postdoctoral fellow supported part-time by this grant learned a great deal about nanoelectrochemistry and created valuable tools, which are essential for the success of other projects underway in our laboratory.⁵

1.                Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.

2.                Sun, P.; Mirkin, M. V. Anal. Chem. 2006, 78, 6526.

3.                Swaddle, T. W. Chem. Rev. 2005, 105, 2573.

4.                Sun, P.; Mirkin, M. V. Anal. Chem. 2007, 79, 5809.

5.                Sun, P.; Laforge, F. O.; Abeyweera, T. P.; Rotenberg, S. A.; Carpino, J.; Mirkin, M. V. Proc. Nat. Acad. Sci. USA, submitted.