Electrode microdisk

Miniaturization of electrodes and of electrochemical cells has had considerable Impact on electrochemical sensor design as well as on fundamental electrochemical studies (9,29). Microelectrodes In particular have opened up new avenues for designing electrochemical sensors. Microelectrodes are physically small electrodes (microdisks have radii of 1-3 microns, microbands have widths down to ca 5 nm). Such... 

FIGURE 4-28 Schematic representation of an interdigitated microarray electrode (a) and closely-spaced microdisk electrodes (b). 

In a typical voltammetric experiment, a constant voltage or a slow potential sweep is applied across the ITIES formed in a micrometer-size orifice. If this voltage is sufficiently large to drive some IT (or ET) reaction, a steady-state current response can be observed (Fig. 1) [12]. The diffusion-limited current to a micro-ITIES surrounded by a thick insulating sheath is equivalent to that at an inlaid microdisk electrode, i.e.,... 

The mathematical formulations of the diffusion problems for a micropippette and metal microdisk electrodes are quite similar when the CT process is governed by essentially spherical diffusion in the outer solution. The voltammograms in this case follow the well-known equation of the reversible steady-state wave [Eq. (2)]. However, the peakshaped, non-steady-state voltammograms are obtained when the overall CT rate is controlled by linear diffusion inside the pipette (Fig. 4) [3]. 

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. 

Obviously, the ohmic potential difference does not depend on the distance of the counterelectrode (if, of course, this is sufficiently apart) being situated mainly in the neighbourhood of the ultramicroelectrode. At constant current density it is proportional to its radius. Thus, with decreasing the radius of the electrode the ohmic potential decreases which is one of the main advantages of the ultramicroelectrode, as it makes possible its use in media of rather low conductivity, as, for example, in low permittivity solvents and at very low temperatures. This property is not restricted to spherical electrodes but also other electrodes with a small characteristic dimension like microdisk electrodes behave in the same way. 

Fig. 5.19 Electrodes used in voltammetry. A—dropping mercury electrode (DME). R denotes the reservoir filled with mercury and connected by a plastic tube to the glass capillary at the tip of which the mercury drop is formed. B—ultramicroelectrode (UME). The actual electrode is the microdisk at the tip of a Wollaston wire (a material often used for UME) sealed in the glass tube...

On the other hand, the situation at features smaller than the diffusion layer thickness can be described by analogy with a microdisk electrode [128], The maximum current density for 02 electroreduction (zm(o2)) that may be observed at such a microdisk electrode is as follows ... 

Here F is the Faraday constant C = concentration of dissolved O2, in air-saturated water C = 2.7 x 10-7 mol cm 3 (C will be appreciably less in relatively concentrated heated solutions) the diffusion coefficient D = 2 x 10-5 cm2/s t is the time (s) r is the radius (cm). Figure 16 shows various plots of zm(02) vs. log t for various values of the microdisk electrode radius r. For large values of r, the transport of O2 to the surface follows a linear type of profile for finite times in the absence of stirring. In the case of small values of r, however, steady-state type diffusion conditions apply at shorter times due to the nonplanar nature of the diffusion process involved. Thus, the partial current density for O2 reduction in electroless deposition will tend to be more governed by kinetic factors at small features, while it will tend to be determined by the diffusion layer thickness in the case of large features. 

Fig. 16. Logarithmic plot of im(o2) vs. t for different values of microdisk electrode radius r (equation 64). Adapted from ref. 128.

If the total current can be assumed to be limited by diffusion to the STM tip, Case III is similar to diffusion to a microdisk electrode (one electrode) thin-layer cell (63). Murray and coworkers (66) have shown that for long electrolysis times, diffusion to a planar microdisk electrode TLC can be treated as purely cylindrical diffusion, provided that the layer thickness is much smaller than the disk diameter (66). In contrast to the reversible case discussed above (Case I), the currents in this scenario should decrease gradually with time at a rate that is dependent on the tip radius and the thickness of the interelectrode gap. Thus, for sufficiently narrow tip/sample spacings, diffusion may be constrained sufficiently (ip decayed) at long electrolysis times to permit the imaging of surfaces with STM. 

Yang, W. C., Yu, A. M., and Chen, H. Y. (2001). Applications of a copper microparticle-modified carbon fiber microdisk array electrode for the simultaneous determination of aminoglycoside antibiotics by capillary electrophoresis. /. Chromatogr. A 905, 309—318. 

You, T. Y., Niu, L., Gui, J. Y, Dong, S. J., and Wang, E. K. (1999). Detection of hydrazine, methylhydrazine and isoniazid by capillary electrophoresis with a 4-pyrldyl hydroquinone self-assembled microdisk platinum electrode. /. Pharm. Biomed. Anal. 19, 231—237. 

At an inlaid microdisk electrode, the dependence of the dimensionless net peak current on the sphericity parameter is given by the following equation [31] ... 

Microcylindrical electrodes are easier to constract and maintain than microdisk electrodes [37]. Mass transport to a stationary cylinder in quiescent solution is governed by axisymmetrical cylindrical diffusion. For square-wave voltammetry the shape and position of the net current response are independent of the extent of cyhn-drical diffusion [38]. The experiments were performed with the ferri-ferrocyanide couple using a small platinum wire (25 pm in diameter and 0.5 -1.0 cm in length) as the working electrode [37]. 

Oliveri et al. (2009) presented the development of an artificial tongue based on cyclic voltammetry at Pt microdisk electrodes for the classification of olive oils according to their geographical origin the measurements are made directly in the oil samples, previously mixed with a proper quantity of a RTIL (room temperature ionic liquid). The pattern recognition techniques applied were PCA for data exploration and fc-NN for classification, validating the results by means of a cross-validation procedure with five cancellation groups. 

Figure 12.3 Calculated voltammograms for reversible charge transfer reactions at microdisk electrodes for /(Dtp)1 values of 10 (tallest curve), 8, 6, 4, 3, 2, and 1. Curves were calculated by digital simulation. Points are from Reference 25. [From Ref. 26, reproduced with permission of the copyright holder.]...

Our last example does not involve the rate of a chemical reaction, but instead, the effect of temperature on diffusion rates [25]. One of the motivations for using microelectrodes as in the previous example is to allow fast experiments without appreciable iR drop. When used in the opposite extreme of very small scan rates, microdisk electrodes produce steady-state voltammograms that have the same sigmoidal shape as dc polarograms and RDE voltammograms (cf. Chap. 12). 

Reduction of the solution temperature allows transition from steady-state to peak-shaped response simply by way of the marked diminution of D at low temperatures. Figure 16.5 shows slow-scan cyclic voltammograms obtained at two microdisk electrodes as a function of solution temperature. Between -120 and -140°C there is a particularly clear transition for the 25-pm-diameter electrode as the diffusion-layer thickness becomes less than the disk radius. Also illustrated here is the immense decrease in the limiting currents that is seen over this range of temperatures due to the 100-fold decrease in D. 

Platinum films [27,88], microwires [19,84] and microdisks [12,103] were also employed. Characterisation of electrode fouling and surface regeneration for platinum electrode on an electrophoresis microchip was reported [104], The platinum tip of a scanning electron microscope has also been used for carrying out EC measurements combined in an end-configuration to a CE microchip [76]. 

Yasukawa, T., Ikeya, T. Matsuse, T. Fabrication and characterization of a microvial with a microdisk electrode for cellular measurements. Chem. Sens. 16, 118-120 (2000). 

As illustrated in Figure 3.2, when the microelectrode is distant from the surface by several electrode diameters, a steady-state current, ij., is observed at the tip. The magnitude of the current is the same as that observed for a microdisk in a conventional experiment. When the tip is near a surface, the tip current, ij, differs from ij.oc, and depends on both the distance between the surface and tip, and the chemical nature of the surface. If the interfacial assembly efficiently blocks electron transfer, i.e. it is an electronic insulator, the mediator will not be regenerated, thus causing to be less than unity. If the Red species becomes re-oxidized at... 

Figure 5.6 Cyclic voltammograms obtained for a spontaneously adsorbed [Os(bpy)2 4-tet Cl]+ monolayer on a 5 pm radius gold microdisk electrode where the scan rate is 1333 V s 1. The theoretical fits to the data, using a non-adiabatic electron tunneling model at electrolyte pH values of 0.9 and 6.0, are denoted by O and , respectively. In both cases, k is 27 kj mol-1, while k° is 1.1 x 103 and 1.1 x 104 s 1 at pH values of 0.9 and 6.0, respectively. Reprinted with permission from D. A. Walsh, T. E. Keyes, C. F. Hogan and R. J. Forster, ]. Phys. Chem., 105, 2792 (2000). Copyright (2000) American Chemical Society...

Figure 5.10 Cyclic voltammograms for a spontaneously adsorbed [Os(bpy)2(bpe)Cl]+ monolayer on a 25 pm radius platinum microdisk electrode at a scan rate of 6000 V s-1. The continuous line represents experimental data, while O and denote the best-fit theoretical responses generated by using the Finklea and Chidsey models, respectively. Both theoretical responses correspond to a k° of 9.4 x 103 s 1 and a AC 1 of 11.4 kj mol-1 the supporting electrolyte is aqueous 0.1 M UCIO4. Reprinted with permission from R. J. Forster, P. J. Loughman, E. Figgemeier, A. C. Lees, J. Hjelm and J. G. Vos, Langmuir, 16, 7871 (2000). Copyright (2000) American Chemical Society...

The greatly reduced double-layer capacitance of microelectrodes, associated with their small area, results in electrochemical cells with small RC time constants. For example, for a microdisk the RC time constant is proportional to the radius of the electrode. The small RC constants allow high-speed voltammetric experiments to be performed at the microsecond timescale (scan rates higher than 106V/s) and hence to probe the kinetics of very fast electron transfer and coupling chemical reactions (114) or the dynamic of processes such as exocytosis (e.g., Fig. 4.25). Such high-speed experiments are discussed further in Section 2.1. 

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