Scanning probe microscopy, electroactive

Summary, Scanning probe microscopy studies of electrodes chemically modified with electroactive transition metal complexes are described. Emphasis is placed on scanning tunneling microscopy and electrochemical scanning tunneling microscopy studies of their structure and dynamics of formation and on electrochemical force spectroscopy studies of their electrochemical potential dependent chemical properties. 

R. Nyffenegger, E. Ammann, H. Siegenthaler, R. Kotz, and O. Haas, In-situ scanning probe microscopy for the measurement of thickness changes in an electroactive polymer. Electrochim. Acta, 40, 1411 (1995). 

P. Haring, R. Kotz, G. Repphun, O. Haas, and H. Siegenthaler, In situ scanning probe microscopy investigations of electroactive films. Appl. Phys. A Mater. Sci. Process., 66, S481 (1998). 

Forrer P, Repphun G, Schmidt E, Siegenthaler H (1997) Electroactive polymers an electrochemical and in situ scanning probe microscopy study. In Jerkiewicz G, Soriaga MP, Uosaki K, Wieckowski A (eds) Solid-liquid electrochemical interfaces (ACS Symp Ser 656). American Chemical Society, Washington, DC, p210 Malhotra BD, Chaubey A, Singh SP (2006) Anal Chem Acta 578 59 Biallozor S, Kupniewska A (2005) Synth Met 155 443... 

Scanning electrochemical cell microscopy (SECCM) is a recent innovation [161, 167] in electrochemical scanning probe microscopy [168] that has been proven particularly powerful for visualizing electroactivity. In the case of HOPG, the SECCM response also informs on the location of the measurement, that is, basal... 

Scanning electrochemical microscopy can also be applied to study localized biological activity, as desired, for example, for in-situ characterization of biosensors (59,60). In this mode, the tip is used to probe the biological generation or consumption of electroactive species, for example, the product of an enzymatic surface reaction. The utility of potentiometric (pH-selective) tips has also been... 

A variety of other techniques have been used to investigate ion transport in conducting polymers. The concentrations of ions in the polymer or the solution phase have been monitored by a variety of in situ and ex situ techniques,8 such as radiotracer studies,188 X-ray photoelectron spectroscopy (XPS),189 potentiometry,154 and Rutherford backscatter-ing.190 The probe-beam deflection method, in which changes in the density of the solution close to the polymer surface are monitored, provides valuable data on transient ion transport.191 Rotating-disk voltammetry, using an electroactive probe ion, provides very direct and reliable data, but its utility is very limited.156,19 193 Scanning electrochemical microscopy has also been used.194... 

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. 

Scanning electrochemical microscopy seeks to overcome the lack of sensitivity and selectivity of the probe tip in STM and AFM to the substrate identity and chemical composition. It does this by using both tip and substrate as independent working electrodes in an electrochemical cell, which therefore also includes auxiliary and reference electrodes. The tip is a metal microelectrode with only the tip active (usually a metal wire in a glass sheath). At large distances from the substrate, in an electrolyte solution containing an electroactive species the mass-transport-limited current is therefore... 

Formation or consumption of reacting species at the electrode surface causes concentration distribution of electroactive species in the solution phase during electrolysis. Equi-concentration contours stand for a concentration profile. A concentration profile can be measured by detecting current or potential by use of a small probe electrode at various locations near a target large electrode. A typical method is scanning electrochemical microscopy. See also diffusion layer, - scanning electrochemical microscope. 

Surface excesses of electroactive species are often examined by methods sensitive to the faradaic reactions of the adsorbed species. Cyclic voltammetry, chronocoulometry, polarography, and thin layer methods are all useful in this regard. Discussions of their application to this type of problem are provided in Section 14.3. In addition to these electrochemical methods for studying the solid electrode/electrolyte interface, there has been intense activity in the utilization of spectroscopic and microscopic methods (e.g., surface enhanced Raman spectroscopy, infrared spectroscopy, scanning tunneling microscopy) as probes of the electrode surface region these are discussed in Chapters 16 and 17. 

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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