Scanning Electrochemical Microscopy SECM

Scanning electrochemical microscopy (SECM) (see Chapter 12) combines useful features of UMEs and thin-layer cells. The mass transfer rate in SECM is a function of the tip-substrate distance d. Eor an UME far from a substrate, the mass transfer coefficient, m D/a, while for the tip near a conductive substrate (d a),m D/d. By decreasing the tip-substrate distance, the mass transport rate can be increased sufficiently for quantitative characterization of the ET kinetics, preserving the advantages of steady-state methods, i.e., the absence of problems associated with ohmic drop, adsorption, and charging current. Eor example, with 

D -10 cmVsec, = 0.1 /on corresponds to m = 1 cm/sec. This gives the upper limit for the determinable rate constant of abont 10 cm/sec. 

SECM theory has been developed for extraction of kinetic parameters of heterogeneous ET reactions occnrring at both tip and substrate electrodes. If the mediator regeneration at the substrate is diffusion-controlled, the finite kinetics at the tip can be extracted from steady-state tip voltammograms (15). Conversely, when the tip process is diffusion-controlled, the finite kinetics at the substrate can be extracted from the tip current vs. distance curves (16). For the former situation, two approximate equations have been proposed for calculating the tip current at any potential and separation distance values (15). However, more recent simulations (17) showed that both equations in reference (15) are not very accurate and may yield underestimated k° values. Equation (15.18) is a significantly better approximation  

The rate constant of an irreversible heterogeneous reaction occurring at the substrate ( ) can be extracted by fitting an experimental current-distance curve to equation (15.20) (16)  

The nanopipet- and micropipet-supported ITIES have been successfully used as SECM tips to demonstrate several advantages in comparison to 

In fact, the feedback mode of SECM is essential for the in-situ characterization of the geometry and size of a nanopipet-supported ITIES tip, which is not compatible with electron microscopy in vacuum. For this purpose, the feedback effect on sharp SECM tips with small rja values has been described quantitatively by empirical equations for a few or any values of rja. 

Nanopipet-supported ITIES tips were employed to probe the kinetics of fast CT reactions at the ITIES. For instance, fast ET reactions at the ITIES tip was studied under the positive feedback condition of the ET mode (Fig. 9a).Specifically, a 206nm-radius pipet was filled with the aqueous solution of Fe(EDTA) and Fe(EDTA) and was immersed in the DCE solution of 7,7,8,8-tetracyanoquinodimethane (TCNQ). The tip reaction is given by 

The fast kinetics of a facilitated IT reaction at the macroscopic DCE/ water interface was studied using nanopipet-supported ITIES tips in the egress IT mode (Fig. 9c). ° Elegantly, this challenging measurement was 

In comparison to the aforementioned examples, an order of magnitude smaller ITIES tip was used to quantitatively image the single nanopores of a 

Fundamentals. A microelectrode with a small diameter (e.g. 10-20 pm, such an electrode is sometimes also called ultramicroelectrode (UME) [112-116]) is exposed to an electrolyte solution containing an electrochemically active substance. The electrode potential is adjusted to a value sufficiently negative to drive the electrochemical reaction O -h riQ R under diffusion control. Diffusion of reactive species to the electrode surface is hemispherical instead of planar, as in the case of large electrodes. The current I flowing across the solid/electrolyte solution interface of the microelectrode tip quickly reaches a steady state value /xa = nFDcr with n as the number of electrons transferred in the electrochemical reaction step, F the Faraday constant, D the diffusion coefficient of the reacting species, c its concentration and r the tip radius. The experimental setup is pictured schematically in Fig. 7.10. 

When the UME is moved close to an insulating surface, the current drops to a lower value Ij because the surface and the insulating sheath of the UME block transport of active species O. This effect is sometimes called negative feedback and is further enhanced by the fact that no reoxidation of R can occur at insulating parts of the surface. Approaching a conductive surface kept at an electrode potential where reoxidation of R is possible causes an opposite effect (positive feedback) and Ij is enhanced with a closer distance. Both possibilities are schematically depicted in Fig. 7.11. A similar effect may be observed with an unbiased (not kept at any specific potential, but instead at open circuit) surface. Because the large surface area is in contact with the solution containing a supply of O, the surface electrode potential is essentially controlled by the Nernst equation. At the potential established by the concentration of O, the reduced species R created at the UME will be reoxidized, whereas further O is reduced elsewhere on the surface. 

An AC voltage can be applied to the UME and a counter electrode (AC-SECM). The AC current response can be evaluated and it can provide information about local surface conductivity of the surface under investigation [123-125]. This setup has been applied to interrogate living cells [126]. Enhanced spatial resolution may be obtained by using a shear force-based distance control to operate the UME at submicrometer distance. 

Instrumentation. A suitable microelectrode [119] or nanoelectrode [127] is attached to a piezo-driven micropositioner. It is connected as the working electrode with a potentiostat. A counter electrode and a reference electrode are wired in a three-electrode arrangement. Investigations with conducting substrates require the use of a bipotentiostat. The surface to be investigated is immersed into the electrochemical cell together with the other electrodes. The position of the microelectrode and the flowing current are controlled and monitored by a computer equipped with 

Lateral charge propagation in a monolayer of polyaniline has been monitored with an SECM [129] kinetic data could be extracted by modeling. The charge transfer between a dissolved redox mediator and polyalkylterthiophene films has been studied [130]. In the oxidized (/ -doped) state of the film, redox reactions proceeded at the film/solution interface, not inside the film. In the reduced state the film behaved like a completely passivating film and penetration of redox mediator ions into the film was obviously completely inhibited. 

In 1995, Bard and his group published a series of papers on the application of SECM to the stndy of electron-transfer reactions at ITIES [260-262]. Here, the classic scanning microtip electrode was replaced by a micropipette filled with an 

FIGURE 1.30 (a) Scanning electron photomicrograph of a microhole array in a PET film covered with a PVC-NPOE gel cast at 70°C. (b) The PVC gel can be seen filling the holes, (c) Steady-state voltammogram for choline transfer with concentrations of 0, 0.1, 0.2, 0.3...0.9, 1.0 mM at the microhole array illustrated on the left. (Lee, H. J., P. D. Beattie, B. J. Seddon, M. D. Osborne, and H. H. Girault, 1991, J Electro anal Chem, Vol. 440, p. 73. Used with permission.) 

FIGURE 1.31 Schematic presentation of the setup for thelTIES measurement at a single microhole at pH 5.35. 

Of course, it is now possible to control the polarization of the ITIES either by using a drop on a solid electrode as pioneered by Zhang et al. [268], or by using a 5-electrode potentiostat. 

As discussed earlier (see Section 1.4.5), SECM has been extremely useful in studies of assisted-ion-transfer reactions following the seminal work of Shao and Mirkin to study the transfer of K assisted by the presence of di-benzo-18-crown-6 in the organic phase [269]. 

A few years ago Bard and his group developed the technique called scanning electrochemical microscopy (SECM) which makes possible a spatial analysi,s of charge transfer processes [9]. In this method an additional tip electrode of a diameter of about 2 pm is used as well as the three other electrodes (semiconductor, counter and reference electrode). Assuming that a redox system is reduced at the semiconductor, then the reduced species can be re-oxidized at the tip electrode, the latter being polarized positively with respect to the redox potential. The corresponding tip current / [ is proportional to the local concentration of the product formed at the semiconductor surface and therefore also to the corresponding local semiconductor current, provided 

This technique has been applied to the examination of polymer surfaces [9]. 

Measurements of Surface Recombination and Minority Carrier injection 

The surface recombination is defined by Eq. (2.47) in Chapter 2. It can be measured by the thin slice method as described below. This method involves mainly a p-n junction at the rear of the electrode in addition to the ohmic contact mentioned in Section 4.1. Surface recombination is measured after the excitation of electron-hole pairs near the electrode surface. They can diffuse not only toward the electrode surface but also toward the p-n junction at the rear. The electron-hole pairs are separated at the p-n junction which leads to a corresponding short-circuit current across this junction. As already discussed in Section 2.5, this current is proportional to the light intensity and, therefore, to the density of electron-hole pairs created by light excitation. If there is some additional surface recombination in a certain potential range, then fewer electron-hole pairs reach the rear p-n junction. A quantitative relation between the short-circuit current at the rear p-n junction and light intensity and surface recombination can be derived as follows. 

The generation rate depends on the light intensity I, and on the absorption coefficient a. The short-circuit current is defined as 

The surface recombination velocity can be introduced by a further boundary condition. At the semiconductor-Uquid interface, the recombination rate is given by 

Using Eqs. (4.2b)-(4.4), restricting their application to a small penetration depth of light (a i ) and assuming that d one obtains by solving Eq. (4.2a)  

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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. 

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. 

Scanned probe microscopies (SPM) that are capable of measuring either current or electrical potential are promising for in situ characterization of nanoscale energy storage cells. Mass transfer, electrical conductivity, and the electrochemical activity of anode and cathode materials can be directly quantified by these techniques. Two examples of this class of SPM are scanning electrochemical microscopy (SECM) and current-sensing atomic force microscopy (CAFM), both of which are commercially available. 

The preparation and application of SAM systems patterned by STM and their use in catalysis was demonstrated by Wittstock and Schuhmann [123]. The patterning (local desorption) of SAMs from alkane thiols on gold was performed by scanning electrochemical microscopy (SECM), followed by the assembly of an amino-deriva-tized disulfide and coupling of glucose oxidase to form a catalytically active pattern of the enzyme. The enzymatic activity could be monitored/imaged by SECM. 

Nanostructured materials have also been formed by scanning tunneling microscopy (STM) [24], scanning electrochemical microscopy (SECM) [25], and atomic force microscopy (AFM) [26], Recent reports on the modification of atomic sites at bare surfaces by STM [27] and the formation of nanometer-scale defects by STM [28] and AFM [29] illustrate the power of these techniques. 

Use of Electrochemical Quartz Crystal Microbalance (EQCM) 281 Use of Scanning Electrochemical Microscopy (SECM) 281 References 284... 

Analysis of the activity of f-galactosidase from E. Coli by scanning electrochemical microscopy (SECM)... 

The first micro-ITIES were introduced in 1986, using a glass micropipette which was pulled down to a fine tip of around 25 pm to support the interface [66-71]. The smaller size of micropipettes or microcapillaries is advantageous for sensor applications, providing the possibility of studying microenvironments as living cells, and it can also be used as a probe in scanning electrochemical microscopy (SECM) [72]. 

Zhang, J., C.J. Slevin, C. Morton, P. Scott, D.J. Walton, and P.J. Unwin. 2001. New approach for measuring lateral diffusion in Langmuir monolayers by scanning electrochemical microscopy (SECM) Theory and application. J. Phys. Chem, B 105 11120-11130. 

Bard and co-workers have developed the technique of Scanning Electrochemical Microscopy (SECM) [3], to provide information about the redox activity of a wide variety of assemblies. In common with STM, SECM uses high-resolution piezoelectric elements to scan a microelectrode tip across the interface of interest. However, in SECM the microelectrode acts as a working electrode in an electrochemical cell that contains a redox-active species. A redox reaction occurs at the microelectrode, e.g. Ox + ne = Red, and by monitoring the current generated at the tip, the surface can be mapped in terms of its redox activity. 

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... 

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],... 

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