Nanoelectrode voltammetry

Hoeben, F.J.M., et al. Toward single-enzyme molecule electrochemistry [NiFe]-hydrogenase protein film voltammetry at nanoelectrodes. ACS Nano 2(12), 2497-2504 (2008)... 

Nanoelectrode ensembles (NEEs) (see also Chapter 10 of this handbook) are nanotech-based electroanalytical tools which find application in a variety of fields ranging from electroanalysis to sensors (86) and electronics (7). They are fabricated by growing metal nanowires in the pores of a template, typically a PC nanoporous manbrane. The density of the pores in the template determines the number of Au-disk nanoelectrode elanents per cm of NEE surface and, correspondingly, the average distance between the nanoelectrode elements. Such electrode systems proved to be valuable tools for trace determinations and kinetics studies by simply using cyclic voltammetry (CV) (5, 69, 86, 97). 

One of the earliest CNT-tipped biological sensor reported was an SWCNT-coated carbon fiber nanoelectrode (100-300 nm tip diameter, as shown in Figure 7.2b). Using cyclic voltammetry, the CNT-tipped electrode could detect dopamine, epinephrine, and norepinephrine at concentrations on an order of magnitude lower than noncoated probes. The demonstration was significant because the dimensions of the CNT-tipped probe would make it possible to study the functions of living cells and tissue with a minimal level of intrusion. Additionally, the pencil-like shape of the probe readily fits the standard ceU physiology equipment and facilitated its use. 

Rodgers, P. J., S. Amemiya, Y. Wang, and M. V. Mirkin, Nanopipet voltammetry of common ions across the liquid-liquid interface. Theory and limitations in kinetic analysis of nanoelectrode voltammograms, Anal. Chem., Vol. 82, 2010 pp. 84-90. 

As with microelectrodes, diffusive transport to nanoelectrodes on conventional voltammetric timescales is dominated by convergent, as opposed to planar, diffusion. Therefore, for a simple electron transfer process, the voltammetric response at steady state is characterised by a sigmoidal shape. Simulation of such voltammetry requires solution of the diffusion equation typically with a Nemstian or Butler-Vofiner boundary condition for the rate of electron transfer at the electrode surface, depending on its reversibility. For simple, uniformly accessible, electrode geometries analytical solutions of these equations are available, and so for a disk electrode we obtain the familiar equation for the current (iiim) in the limit of diffusion control ... 

More recent works have demonstrated the application of numerical simulation to exploring non-uniformly accessible 3D nanoelectrode geometries. For example, Streeter and Compton employed the finite difference approach to examine diffusion limited currents at isolated spheroidal and hemispheroidal nanoparticle electrodes immobilized on inert substrates. Building on this. Ward et al. used numerical methods to simulate isolated spherical nanoparticle voltammetry in the limit of irreversible electron transfer kinetics and derived a simple expression describing the voltammetric wave-shape ... 

The situation becomes more complicated for nanoelectrodes arrays since they typically have a total footprint in the order of microns, and hence even when adjacent diffusion fields fully overlap, behaviour akin to that at a single microelectrode is still observed. The extension of the diffusion domain approximation to nanoelectrode arrays was explored by Godino et al, who compared simulated voltammetry generated by 2D and 3D modelling with... 

The study of fundamental electron transfer processes at nanoelectrodes has also been extended to the field of bioelectrochemistry, notably in the elucidation of enzyme electron transfer kinetics and mechanism via protein film voltammetry. This typically involves immobilizing a film of redox active enzymes onto an electrode such that electronic contact is achieved between the enzyme active site and the underlying surface, enabling voltammetry to... 

FIGURE 2.15 Schematic illustration of the different ET probability for an electroactive molecule on a large electrode and a spherical nanoelectrode. (Reprinted with permission from Limon-Petersen, J.G., Streeter, L, Rees, N.V., and Compton, R.G., Quantitative voltammetry in weakly supported media Effects of the applied overpotential and supporting electrolyte concentration on the one electron oxidation of ferrocene in acetonitrile, J. Phys. Chem. C, 2009, 113, 333-337. Copyright 2009 American Chemical Society.)... 

Swisher, L. Z., Syed, L. U., Prior, A. M. et al. 2013. Electrochemical protease biosensor based on enhanced AC voltammetry using carbon nanofiber nanoelectrode arrays. /. Phys. Chem. C 117 4268-4277. 

FIGURE 15.2 Characterization of a nanoelectrode by voltammetry and SECM. (a) Slow (red curve) and fast (blue curve) scan voltammograms of 1 mM FcCHjOH at the 52 nm polished Pt electrode, v=50 mV/s (red) and 50 V/s (blue), (b) Expalmental (symbols) and theoretical (solid line) current-distance curves obtained with the same electrode as in (a) approaching an evaporated Au substrate. 

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