Feedback mode of SECM

Recently [8b,30], a new IT feedback mode of SECM was introduced, in which the tip process is a simple or assisted ion transfer. In this mode, a micropipette filled with solvent (e.g., aqueous) immiscible with the outer solution (e.g., organic) serves as an SECM tip. 

The feedback mode of SECM operation is most suited for probing heterogeneous charge-transfer reactions. Electron transfer at the metal-solution interface was the first chemical reaction probed by SECM. An important advantage of this technique for studies of charge transfers at... 

Kinetics of ET is of primary importance for most electrochemical applications ranging from fuel cells and batteries to biosensors to solar cells to molecular electronics. To measure the fast ET kinetics under steady-state conditions, one needs a technique with the sufficiently high mass transfer rate and negligibly small uncompensated resistive potential drop in solution (IR-drop). The feedback mode of SECM meets both requirements. 

In an EC2j process, the initial ET step is followed by a second-order irreversible homogeneous reaction. For example, the feedback mode of SECM was employed to study the reductive hydrodimerization of the dimethyl fumarate (DF) radical anion [22]. The experiments were carried out in solutions containing either 5.15 or 11.5 mM DF and 0.1 M tetrabutylammonium tetrafluoroborate in A,A,-dimethyl form amide (DMF). The increase in the feedback current with increasing concentration of DF indicated that the homogeneous step involved in this process is not a first-order reaction. The analysis of the data based on the EC2 theory yielded the fc2 values of 180M-1 s-1 and 160M-1 s-1 for two different concentrations. Another second order reaction studied by the TG/SC mode was oxidative dimerization of 4-nitrophenolate (ArO-) in acetonitrile [23]. In this experiment, the tip was placed at a fixed distance from the substrate. The d value was determined from the positive feedback current of benzoquinone, which did not interfere with the reaction of interest. The dimerization rate constant of (1.2 0.3) x 108 M x s-1 was obtained for different concentrations of ArO-. 

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. 

The feedback mode of SECM was used to map the conductivity of a fluoroborate-doped polypyrrole film and to evaluate its potential dependence [158]. An ultramicroelectrode (UME), 12.5 pm in diameter, was used to detect the redox mediator more or less concentrated in the UME vicinity, depending on the ability of the polymer film to regenerate it, thus providing the potential region where the polypyrrole film is electronically conducting. 

SECM was also used to evaluate the conductivity changes of polybithiophene microstructures deposited by a laser-assisted microstructuring deposition technique as a function of their doping level, controlled by fixing the potential value at the polymer sample [ 163]. In order to achieve this purpose, the feedback mode of SECM was used in an electrolytic solution containing [IrfCNjg]. ... 

SECM has been proven a powerful multipurpose technique for the investigation of a variety of surfaces, interfaces, and processes [176, 177]. Hapiot et at. studied the electrochemistry of the Fe(CN)6 redox couple on HOPG using feedback mode of SECM (Figure 2.18a), in which Fe(CN)g (Ox) in bulk solution was reduced to Fe(CN)g (Red) at the tip and this could undergo conversion back to Ox at the carbon sample, depending on the surface ET kinetics. There was no... 

Figure 2.18 (a) Schematic for feedback mode of SECM. SECM steady-state normalized current-distance approach curves with a gold disc ultramicroelectrode (UME) tip (radius 6 pm) for Fe(CN)g reduction toward (b) HOPG and (c) glassy carbon, where i... 

Figure 9.29 Feedback modes of SECM operations. The UME tip is poised at a potential such that R will oxidize to O. (a) UME is freely hanging in the redox solution, ij = (b) UME...

Deposition of condncting polymers can be accomplished by either direct or feedback modes of SECM. In an earlier study, SECM was used to deposit polyaniline on a Pt substrate by scanning a UME tip in thin, ionically conductive Nafion films coated on the Pt surface (127). Anilinium ions were incorporated into Nafion films and the Pt substrate was biased positively, which caused the electropolymerization of aniline. The resolution of the polyaniline deposition is controlled by the tip size, the thickness of Nafion films, and the electric field distribution. 

Work has also been reported on hnman breast cells (171) and Rhodobacter sphaeroides (172). In these stndies, the feedback mode of SECM was used to look at the regeneration reaction of different mediators when exposed to cells. These studies provide useful information abont the permeability of the membrane to a wide variety of redox couples. A theoretical treatment enabled kinetic information about these processes (173) to be extracted. SECM has also been used to distinguish between normal and malignant human breast cells (174) with corroboration by optical and fluorescence microscopy measurements. 

In fact, the feedback mode of SECM is essential for the in-situ characterization of the geometry and size of a nanopipet-supported ITIES tip, which is not compatible with electron microscopy in vacuum. For this purpose, the feedback effect on sharp SECM tips with small rja values has been described quantitatively by empirical equations for a few or any " values of rja. 

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