Negative Feedback Mode

FIGURE 14.3 Schematic of negative and positive feedback situation in SECM. 

2 [17]. Accordingly, a negative FB mode can be used to determine the tip-substrate distance and the substrate topography based on the values of the tip current at different (x, y) locations. 

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A phenomenon called negative feedback mode is well known in SECM.84 85 When a microelectrode measuring a current from a redox species is approaching... 

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,... 

Scanning electrochemical microscopy (SECM) sustains great interest for biomolecular recognition detection [29, 30]. This comes from the versatility of SECM methodology that offers versatile detection principles, e.g., positive or negative feedback modes together with collection mode that are compatible with unlabeUed hybridization detection as well as with redox amplification strategies of DNA hybridization [31, 32]. 

Self-assembled spherical electrodes have been successfully used as SECM amperomet-ric probes both in positive and negative feedback modes (la,b) (Chapter 12). A very close tip-substrate separation could be attained due to the great smoothness of the self-assembled structures. 

The SECM positive-negative feedback mode can also be achieved based on FIT reactions at the L/L interface (121). Figure 17.3.14 illustrates the principle. The tip current can be produced by transferring K+, facilitated by DB18C6, from the aqueous phase inside the micropipet to the outer DCE phase. When the tip is biased at a sufficiently positive potential, the tip current is limited by the diffusion of DB18C6 to the interface. As the tip approaches the bottom aqueous phase, is released from the complex and transferred to... 

The modeling sequence of the SECM problem is similar to that used above for a microdisk. Additional changes are required to take into account mediator diffusion from behind the shield. The following Multiphysics model can be used to simulate negative feedback mode (insulating substrate) by solving the diffusion problem from Section 5.2.2.1, that is. Equation 5.26 with boundary conditions given by Equations 5.27, 5.28b, and 5.29. 

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