Scanning Electrochemical Microscopy feedback

Selzer Y and Manler D 2000 Scanning electrochemical microscopy. Theory of the feedback mode for hemispherical ultramicroelectrodes steady-state and transient behavior Anal. Chem. 72 2383... 

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Alpuche-Aviles, M.A., and D.O. Wipf. 2001. Impedance feedback control for scanning electrochemical microscopy. Anal. Chem. 73 4873-4881. 

Scanning electrochemical microscopy (SECM the same abbreviation is also used for the device, i.e., the microscope) is often compared (and sometimes confused) with scanning tunneling microscopy (STM), which was pioneered by Binning and Rohrer in the early 1980s [1]. While both techniques make use of a mobile conductive microprobe, their principles and capabilities are totally different. The most widely used SECM probes are micrometer-sized ampero-metric ultramicroelectrodes (UMEs), which were introduced by Wightman and co-workers 1980 [2]. They are suitable for quantitative electrochemical experiments, and the well-developed theory is available for data analysis. Several groups employed small and mobile electrochemical probes to make measurements within the diffusion layer [3], to examine and modify electrode surfaces [4, 5], However, the SECM technique, as we know it, only became possible after the introduction of the feedback concept [6, 7],... 

Wipf, D. O. and Bard, A. J. (1991a), Scanning electrochemical microscopy.7. Effect of heterogeneous electron-transfer rate at the substrate on the tip feedback current. J. Electrochem. Soc., 138(2) 469 174. 

Scanning electrochemical microscopy (SECM) — Figure 1. SECM principles approach curves on insulating (a) and conductive (b) substrates. Negative feedback is observed on an insulating substrate and positive feedback is observed on a conductive substrate... 

Scanning electrochemical microscopy (SECM) - Direct mode - Feedback mode - Generation/collection mode Scanning reference electrode technique (SRET) Scanning vibrating electrode technique (SVET) Scanning photoelectrochemical microscopy (SPECM) Scanning electrochemical induced desorption (SECMID)... 

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. 

Kwak, J. Bard, A. J., Scanning electrochemical microscopy - theory of the feedback mode, Anal. Chem. 1989, 61, 1221-1227... 

FIG. 3 Basic principles of scanning electrochemical microscopy (SECM) (A) far from the substrate, diffusion leads to a steady-state current, iT (B) near a conductive substrate, feedback diffusion leads to iT > iT 0O (C) near an insulating substrate, hindered diffusion leads to iT < iT 0O. (Reprinted with permission from A. J. Bard, G. Denuault, C. Lee, D. Mandler, and D. O. Wipf, Acc. Chem. Res. 23, 357 (1990). Copyright 1990 American Chemical Society.)... 

K wak J and Bard A J 1989 Scanning electrochemical microscopy— theory of the feedback mode Anal. Chem. 61 1221... 

Unwin P R and Bard A J 1991 Scanning electrochemical microscopy—theory and application of the feedback mode to the measurement of following chemical-reaction rates in electrode processes J. Phys. Chem. 95 7814... 

Fig. 3.25a-c Basic principles of scanning electrochemical microscopy a the small tip is far from the substrate, lU-tramicroelectrode behavior, steady state current, b near a conductive substrate, feedback diffusion leads to It > It, ==>, c near an insulating substrate, hindered difiusion leads to Ij < It,o -If,CO = 4nFDca, where n is the charge number of the electrode reaction, F is the Faraday constant, D is the difiusion coefficient, c is the concentration and a is the radius of the microdisk electrode, which is usually less than 20 Xm [15], (Reproduced with the permission of the American Chemical Society)... 

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]. 

Scanning Electrochemical Microscopy (SECM), Fig. 3 Mechanism of lateral electron transfer that is responsible for a positive feedback current in cases where the conductive surface is unbiased... 

Mandler D, Bard AJ (1989) Scanning electrochemical microscopy - the application of the feedback mode for high-resolution copper etching. J Electrochem Soc 136(10) 3143-3144. doi 10.1149/ 1.2096416... 

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... 

Bard AJ, Mirkin MV, Unwin PR, Wipf DO (1992) Scanning electrochemical microscopy. 12. Theory and experiment of the feedback mode with finite heterogeneous electron-transfer kinetics and arbitrary substrate size. J Phys Chem 96 1861-1868... 

Xiong, H., J. Guo, and S. Amemiya, Probing heterogeneous electron transfer at an unbiased conductor by scanning electrochemical microscopy in the feedback mode. Anal. Chem., Vol. 79, 2007 pp. 2735-2744. 

Fig. 20.31 Basic principles of scanning electrochemical microscopy (SECM). (A) Hemispherical diffusion far from electrode (B) feedback diffusion near a conductive electrode (C) hindered diffusion near an insulating electrode surface. The probe s electroactive area is shaded in the diagrams. (Reproduced with permission from Ref. 157.)...

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