Feedback approach curve

FIG. 13 Experimental approach curve obtained with a 0.4 /xm apparent diameter pyrolytic carbon microelectrode in a 5 mM aqueous solution of hexaamineruthen-ium(III) chloride and 1 M KC1 electrolyte. The substrate is a 100 /xm platinum disk. (Upper dashed curve) Theoretical positive feedback approach curve. (Lower dashed curve) Theoretical negative feedback approach curve. (Inset) Schematic picture of the very end of the carbonized capillary compatible with the experimental approach... 

Typical feedback approach curves for DMPPD oxidation in solutions of pH 10.20, 10.78, and 11.24 are shown in Figure 8, along with the behavior for diffusion-controlled positive and negative feedback. The tip electrode process was defined by Eq. (36), while the unbiased macroscopic substrate was poised at a potential where the reverse reaction occurred at a diffusion-controlled rate ... 

Cornut, R. and Lefrou, C. (2008) New analytical approximation of feedback approach curves with a microdisk SECM tip and irreversible kinetic reaction at the substrate. Journal of Electroaruxlytical Chemistry, 621,178-184. 

Figure 12.16 Experimental approach curves ( ) for the Fe +/Fe + oxidation reaction measured on glassy carbon. The tip (10- jm diameter carbon fiber disk, RG = 10) was held at a potential of -0.5 V vs. Ag/AgCl while the substrate was held at various potentials from )/ = 0.3 V to )/ = 0.75 V from bottom to top. Solid lines are the best fittings using equation 12.4.1 for the rate constant values plotted in the inset graph as a function of t] ( ). Dashed lines are negative and positive feedback approach curves. Linear fit of In (k) vs. t] (solid line in inset graph) gives an intercept value ln(k a/D) = —6.1 and a Tafel slope RT/[(1 — a)nF] = 81 mV/dec, parameters that lead to a standard rate constant k° = 2 X 10 cm sec and a transfer coefficient a = 0.7.

Figure 8. Panel (A) displays the approach curve of the NSOM tip using tapping-mode feedback. Panel (B) displays the approach curve collect using the optical feedback mode. Comparison of the two simultaneously collected approach curves, indicates that the optical feedback approach curve begins to oscillate within micrometers of the surface, while tapping mode feedback remains unaltered until the tip is very elose to the sample surface Reproduced with permission from (Shiku et al. 1999). Copyright 1999 American Chemical Society.

SECM has also been used to evaluate the shape of a nanopipet tip (see Section 1.4). Good fits of SECM negative feedback approach curves between the experimental and theoretical curves can confirm that the ITIES is essentially flat and not recessed, and also provide information about the RG value of the nanopipet. Pipets as small as 8 nm radius with = 1.6 have been characterized in this way. ... 

Diakowski and Kraatz also used [Fe(CN)g]" to study mismatches in a 25 base pair ODN hybridization experiment [144], The negative feedback was greatest for fully matched strands and least for mismatches near the ends of the strands. Their data were reported in terms of the effective rate constant describing the faradaic reaction at the substrate as extracted from feedback approach curves (Chapter 5). Interestingly, a threefold difference between the rate constants at various surfaces with matched/mismatched pairs was observed that was greatly increased to a factor of about 20-fold in the presence of Zn +. This effect is due to the modulation of the charge density of the DNA by the binding of Zn + [138] and appears to be a useful means to enhance the performance of such label-free DNA sensors. 

The redox activity of these cells in the presence of different mediators can be expressed in terms of apparent electron transfer rates and mechanistic pathways can thereby be elucidated using SECM. On the basis of feedback approach curves, it is possible to demonstrate significant differences in the redox activities of nontransformed, highly motile, and metastatic human breast epithelial cells... 

Positive feedback approaching curve in case of fast regeneration process fits within 1% to the equation below if a shielded disc shaped electrode with RG value of 10.0 is used. 

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