Positive Feedback Mode

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

Martin RD, Unwin PR (1997) Scanning electrochemical microscopy theory and experiment for the positive feedback mode with unequal diflusion coefficients of the redox mediator couple. J Electroantil Chem 439 123—136... 

FIGURE 5.3 Effect of y on the chronoamperometric characteristics for the positive feedback process at a tip/substrate separation, L=0.2. The reduced form initially present in solution is oxidized at the tip and regenerated at the substrate. Normalized tip current is plotted as a function of dimensionless time, %=tD la -. (Reprinted from/. Electroanal. Chem., 439, Martin, R.D. and Unwin, RR., Scanning electrochemical microscopy Theory and experiment for the positive feedback mode with unequal diffusion coefficients of the redox mediator couple, 123-136, Copyright 1997, with permission from Elsevier Science S.A.)... 

Steady-State SECM images were also obtained in RTIL nsing nanometer-sized tips [16]. In addition, a chronoamperometric tip response in the total positive feedback mode was studied to determine the unequal diffusion coefficients of the oxidized and reduced forms of redox couples in RTILs, for example, the O2/O2 couple [17,18] and a series of nitro-derivative compounds [19]. The smaller diffusion coefficients of the radical anions generated from neutral oxidants were ascribed to electrostatic interactions with RTIL cations. 

Nakano, K., Nakamura, K., Iwamoto, K., Soh, N., Imato, T. Positive-feedback-mode scanning electrochemical microscopy imaging of redox-active DNA-poly(l,4-benzoquinone) conjugate film deposited on carbon fiber electrode for micrometer-sized hybridization biosensor applications. J Electroanal Chem 2009, 628, 113-118. 

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