Electrochemical scanning microscope

Kranz C, Wittstock G, Wohlschlager FI and Schumann W 1997 Imaging of microstructured biochemically active surfaces by means of scanning electrochemical microscope Electrochim. Acta 42 3105... 

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

FIGURE 36.6 Typical configuration of a scanning electrochemical microscope. In this case the solution contains Ox species (mediators) that are reduced on the active part of the micro-electrode, yielding the reduced Red species. A possible reaction of the Red species with the substrate, with the reaction rate. illustrated. 

Bard AJ, Fan FRF, Pierce DT, Unwin PR, Wipf DO, Zhou FM. 1991. Chemical imaging of surfaces with the scanning electrochemical microscope. Science 254 68-74. 

Alternatively, a higher rate of mass transport in steady-state measurements with a larger UME can be obtained by using it as a tip in the scanning electrochemical microscope (SECM). The SECM has typically been employed for probing interfacial ET reactions [29]. Recently, micropipettes have been used as SECM probes (see Section IV.B below) [8b,30]. Although the possibility of probing simple and assisted IT at ITIES by this technique was demonstrated, no actual kinetic measurements have yet been reported. 

Perhaps the most important experimental progress made recently in electrochemistry was the introduction of a scanning electrochemical microscope (SECM). Tsionsky et al. have used SECM to study also the rate of ET across a lipid monolayer at the water-benzene interface [48,49]. The presence of the monolayer decreased the rate of ET, being the decrease more significant for longer hydrocarbon chains and larger lipid concentration in solution. It was thus concluded that the ET reaction does not occur at defect sites in the lipid monolayer. 

The kinetics of AgGl dissolution in aqueous solutions without supporting electrolyte have been studied utilizing well-defined and high mass transport properties of the scanning electrochemical microscope [376]. An ultramicroelectrode probe positioned close to the AgGl surface was used to induce and monitor dissolution of the salt via reduction of Ag+ from the initially saturated solution. 

D. Mandler, Micro- and nanopatterning using the scanning electrochemical microscope. In A.J. Bard and M.V. Mirkin (Eds.), Scanning Electrochemical Microscopy, Marcel Dekker, New York, Basel, 2001, pp. 593-627. 

The scanning electrochemical microscope (SECM) consisted of a positioning system from Marzhauser (Wetzlar, Germany), a bipotentiostat CH701 (CH Instruments, Austin, TX, USA) and a homemade control software. 

A third screening method for arrays of electrocatalysts was recently introduced by Hillier and coworkers [15, 29, 30]. Using a scanning electrochemical microscope (SECM), a microelectrode tip is moved over an electrocatalyst array. The resulting electrochemical feedback currents are measured and used to generate an activity map of the electrocatalyst library. This method does not require individual electronic addressability for each electrocatalyst... 

Light emission by ECL at scanning electrochemical microscope (SECM) tips is also under current development [68], Bard and coworkers have demonstrated that ECL can be generated at SECM tips when [(bpy)3Ru]2+ is used as... 

Figure 15 Diagram of scanning electrochemical microscope for ECL-based detection. ECL generated at an ultramicroelectrode via annihilation of R and R+ and detected with a photomultiplier. (From Ref. 85. With permission from the American Chemical Society.)...

Turyan, I., T. Matsue, and D. Mandler. 2000. Patterning and characterization of surfaces with organic and biological molecules by the scanning electrochemical microscope. Anal. Chem. 72 3431-3435. 

Treutler, T.H., and G. Wittstock. 2003. Combination of an electrochemical tunneling microscope (ECSTM) and a scanning electrochemical microscope (SECM) Application for tip-induced modification of self-assembled monolayers. Electrochim. Acta 48 2923-2932. 

Figure 3.2 Operation of the scanning electrochemical microscope illustrating diffusion of a redox mediator to the microscope tip (a) when the tip is far from any surface (b) when the tip is near an insulator and the insulating surface blocks transport of the mediator to the tip (c) when the tip is near a conducting surface that regenerates the mediator, this causing an enhanced current...

Fig. 18 Scanning electrochemical microscope (SECM) surface images of bare gold electrode top) and AcMn5b modified gold electrode bottom) containing 5 mM ferrocyanide as redox mediator in phosphate buffer. Reproduced with permission from [76]...

The SECM can be used in the feedback mode to probe lateral mass-charge transfer [79-83]. The theory of SECM feedback surveyed in Section IV.A.2 assumes that the substrate surface is uniformly reactive. When lateral mass and/ or charge transfer occurs on the substrate surface, or within a thin film, the surface reactivity of the substrate becomes non-uniform and the SECM feedback theory must be modified. Unwin and Bard [79] developed the theory for adsorption-desorption of a redox species at the substrate that allowed for surface diffusion of the adsorbate. They introduced a new approach, the scanning electrochemical microscope induced desorption (SECMID), as a way to probe surface diffusion. The set of differential equations for the diffusion problem comprise Eqs. (8a,b), and Eq. (26), which relates the redox concentration at the substrate surface and the surface coverage by adsorbed species... 

The main purpose of this contribution, however, is to review recent advances in solid state ionics achieved by means of microelectrodes, i.e. electrodes whose size is in the micrometer range (typically 1-250 pm). In liquid electrolytes (ultra)-microelectrodes are rather common and applied for several reasons they exhibit a very fast response in voltametric studies, facilitate the investigation of fast charge transfer reactions and strongly reduce the importance of ohmic drops in the electrolyte, thus allowing e.g. measurements in low-conductive electrolytes [33, 34], Microelectrodes are also employed to localize reactions on electrodes and to scan electrochemical properties of electrode surfaces (scanning electrochemical microscope [35, 36]) further developments refer to arrays of microelectrodes, e.g. for (partly spatially resolved) electroanalysis [37-39], applications in bioelectrochemistry and medicine [40, 41] or spatially resolved pH measurements [42], Reviews on these and other applications of microelectrodes are, for example, given in Ref. [33, 34, 43-47],... 

Jayaraman, S. Hillier, A. C. Screening the Reactivity of PtxRuy and PtxRuyMoz Catalysts toward the Hydrogen Oxidation Reaction with the Scanning Electrochemical Microscope. Journal of Physical Chemistry B 2003 107(22) 5221-5230. 

Bipotentiostat — An instrument that can control the potential of two independent -> working electrodes. A - reference electrode and an -> auxiliary electrode are also needed therefore the cell is of the four-electrode type. Bipotentiostats are most often employed in electrochemical work with rotating ring-disk electrodes and scanning electrochemical microscopes. They are also needed for monitoring the electrode-reaction products with probe electrodes that are independently polarized. All major producers of electrochemical equipment offer this type of potentiostat. The instruments that can control the potential of more than two working electrodes are called multipotentiostats. 

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. 

Edge diffusion — The diffusion edge is the fictional boundary between the diffusion layer and the bulk. It is a convenient concept when an extending diffusion later reaches an object to react, e.g., at a pair electrode and a -> scanning electrochemical microscope [i]. Ref. [i] Bard AJ, Faulkner LR (2001) Electrochemical methods, 2nd edn. Wiley, New York, p 669... 

---