Electrochemical microscopy, scanning

FIGURE 2-16 Design of a scanning electrochemical microscope. (Reproduced with 

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 

FIGURE 2-17 Principles of SECM. (a) Tip far from the substrate surface diffusion of O leads to steady-state current. (b) Tip near a conductive substrate positive feedback of O. (c) Tip near the insulating substrate hindered diffusion of O. c = concentration a = radius of tip. (Reproduced with permission from reference 55.) 

FIGURE 2-18 SECM image of a gold minigrid surface. (Reproduced with permission from reference 57.) 

The scanning tunneling microscope (STM) has led to several other variants (61). Particularly attractive for electrochemical studies is scanning electrochemical microscopy (SECM) (62-65). In SECM, faradaic currents at an ultramicroelectrode tip are measured while the tip is moved (by a piezoelectric controller) in close proximity to the substrate surface that is immersed in a solution containing an electroactive species (Fig. 2.17). These tip currents are a function of the conductivity and chemical nature of the substrate, as well as of the tip-substrate distance. The images thus obtained offer valuable insights into the microdistribution of the electrochemical and chemical activity, as well 

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SECM Scanning electrochemical microscopy [40] An STM serves as microelectrode to reduce electroactive species Electrochemical reactions on surfaces... 

Fulian Q, Fisher A C and Denuault G 1999 Applications of the boundary element method in electrochemistry scanning electrochemical microscopy, part 2 J. Phys. Chem. B 103 4393... 

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... 

Zhou F and Bard A J 1994 Detection of the electrohydrodimerization intermediate acrylonitrile radical-anion by scanning electrochemical microscopy J. Am. Chem. See. 116 393... 

Barker A L, Gonsalves M, Maepherson J V, Slevin C J and Unwin P R 1999 Scanning electrochemical microscopy beyond the solid/liquid interface Anal. Chim. Acta 385 223... 

Such approximation is valid when the thickness of the polymeric layer is small compared to die thickness of die crystal, and the measured frequency change is small with respect to the resonant frequency of the unloaded crystal. Mass changes up to 0.05% of die crystal mass commonly meet this approximation. In die absence of molecular specificity, EQCM cannot be used for molecular-level characterization of surfaces. Electrochemical quartz crystal microbalance devices also hold promise for the task of affinity-based chemical sensing, as they allow simultaneous measurements of both tile mass and die current. The principles and capabilities of EQCM have been reviewed (67,68). The combination of EQCM widi scanning electrochemical microscopy has also been reported recently for studying die dissolution and etching of various thin films (69). The recent development of a multichannel quartz crystal microbalance (70), based on arrays of resonators, should further enhance die scope and power of EQCM. 

Salicylate, 182 Sauebrey equation, 53 Scanning electrochemical microscopy, 49, 163... 

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... 

Fernandez JL, Walsh DA, Bard AJ. 2005b. Thermodynamic guidelines for the design of bimetallic catalysts for oxygen electroreduction and rapid screening by scanning electrochemical microscopy. M-Co (M Pd, Ag, Au). J Am Chem Soc 127 357-365. 

Fernandez JL, White JM, Sun YM, Tang WJ, Henkelman G, Bard AJ. 2006. Characterization and theory of electrocatalysts based on scanning electrochemical microscopy screening methods. Langmuir 22 10426-10431. 

Scanning Electrochemical Microscopy as a Local Probe of Chemical Processes at Liquid Interfaces... 

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. 

Bard, A.J. and Mirkin, M.V. (eds) (2001) Scanning Electrochemical Microscopy, Marcel Dekker, New York. 

A. Pailleret, J. Oni, S. Reiter, S. Isik, M. Etienne, F. Bedioui, and W. Schuhmann, In situ formation and scanning electrochemical microscopy assisted positioning of NO-sensors above human umbilical vein endothelial cells for the detection of nitric oxide release. Electrochem. Commun. 5, 847-852 (2003). 

Wittstock G, Burchardt M, Pust SE, Shen Y, Zhao C (2007) Scanning electrochemical microscopy for direct imaging of reaction rates. Angew Chem Int Ed 46 1584—1617... 

There are many possible reaction pathways between acrylonitrile and adiponitrile and, in each, there are several possible rate-determining steps. None of the reaction intermediates has yet been detected electrochemically or spectroscopically thus indicating very fast chemical processes with intermediates of half-lives of < 10-5 s. Bard and Feiming Zhou [104a] have recently detected the CH2 = CHCNT radical by Scanning Electrochemical Microscopy (SCEM) using a 2.5 fim radius Au electrode (1.5 mol CH2 = CHCN in MeCN/TBAPF6). The dimerization rate has been determined to 6.107 M-1 S l. 

Topics discussed above are some basic principles and techniques in voltammetry. Voltammetry in the frequency domain where i-E response is obtained at different frequencies from a single experiment known as AC voltammetry or impedance spectroscopy is well established. The use of ultramicroelectrodes in scanning electrochemical microscopy to scan surface redox sites is becoming useful in nanoresearch. There have been extensive efforts made to modify electrodes with enzymes for biosensor development. Wherever an analyte undergoes a redox reaction, voltammetry can be used as the primary sensing technique. Microsensor design and development has recently received... 

UMEs decrease the effects of non-Earadaic currents and of the iR drop. At usual timescales, diffusional transport becomes stationary after short settling times, and the enhanced mass transport leads to a decrease of reaction effects. On the other hand, in voltammetry very high scan rates (i up to 10 Vs ) become accessible, which is important for the study of very fast chemical steps. For organic reactions, minimization of the iR drop is of practical value and highly nonpolar solvents (e.g. benzene or hexane [8]) have been used with low or vanishing concentrations of supporting electrolyte. In scanning electrochemical microscopy (SECM [70]), the small size of UMEs is exploited to locahze electrode processes in the gm scale. 

A. J. Bard, E.-R. E. Fan, M. Mirkin, Scanning electrochemical microscopy in Physical Electrochemistry. Principles, Methods, and Applications (Ed. I. Rubinstein), Monographs in Edectroanalyhcal Chemistry and Electrochemistry, Marcel Dekker, New York, 1995, pp. 209-242. 

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