Negative feedback imaging

While the experiments in these studies may approach the ultimate resolution of SECM imaging, the main emphasis in this chapter is put on the utilization of biochemical reactions for imaging formation. Relevant applications of the negative feedback imaging mode are therefore only briefly summarized in Sec. II.A. 

Nugues and Denuault have also used SECM to study diffusive transport in the pores of dentine. In these experiments, the concentration of Fe(CN)g" was 1 mM in the receptor compartment and 100 mM in the donor compartment, corresponding to the combined tip collection negative feedback imaging mode. With these solution conditions, the SECM image represents a convolution of... 

In the negative feedback mode, the concentration of redox species is the same on both sides of the membrane no concentration gradient exists between the donor and receptor compartments to drive diffusive transport across the pore (11-13). This mode of imaging can be used to obtain topographical maps of the surface where the variation in the SECM tip current arises from differences in the tip-to-sample separation (28). When the tip is far from the sample surface, a steady-state current is measured at the tip,... 

To image the localized enzymatic activity in the feedback mode would be even more difficult than in the case of the enzyme-modified PPy lines (vide supra). There is negative feedback above the unaffected long-chain SAM and positive feedback above areas from which the long-chain alkanethiolate had been desorbed (see Fig. 2 for the principle). The cystaminium-bound enzyme layer is less effective for inhibition of the reaction of ferro-cinium derivatives (64) at the Au surface than the densely packed long-chain alkanethiolate layer. Therefore, positive feedback would be observed above the enzyme-modified region due to the electrochemical feedback at the Au surface, although the positive feedback may be reduced to some extent compared to a bare Au surface. 

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