Microdisk

FIGURE 4-24 Cyclic voltanimograms for the oxidation of ferrocene at a 6 pm platinum microdisk at different scan rates. (Reproduced with permission from reference 84.)... 

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.,... 

FIG. 2 Schematic representation of different microhole geometries, (a) Recessed microdisk interface, spherical-linear, linear-spherical diffusion, (b) quasi-inlaid microdisk interface, spherical-spherical diffusion, (c) Long microhole with quasi-inlaid interface, spherical-linear diffusion. (Reprinted with permission from Ref. 13. Copyright 1999 Elsevier Science S.A.)... 

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.

Fig. 13.1 Photonic sensors that are either based on MNF sensing ((a),(b), (d), (e), (f)) or use MNF for the input and output connection (c), (g) (j). (a) straight MNF sensor with surrounding evanescent field (b) straight MNF sensor coated with bio or chemical layer and surrounding evanescent field (c) generic structure of MNF based optical sensor with surrounding evanescent field (d) straight MNF sensor (e) MNF loop resonator (MLR) sensor (f) MNF coil resonator (MCR) sensor (g) MNF/microsphere sensor (h) MNF/microdisk sensor (i) MNF/micro cylinder sensor (j) MNF/microcapillary sensor (k) a sensor composed of an MNF coupled to a series of microcylinders (optical fibers)...

The principle of an MNF performance as an optical sensor is based on the variation of the MNF transmission spectrum in response to changes in the ambient medium. More advanced photonic sensors exploiting MNFs allow to enhance these variations and/or to make them more selective. This section briefly reviews the basic theory of simplest resonant photonic sensors. In these devices, an MNF either form an optical resonator or serve as an input and output connection to an optical resonator. These devices include the MLR23-26, the MCR8,26-33, the MNF/micro-sphere resonator34-45, the MNF/microdisk resonators46-49, and the MNF/microcy-linder resonator18,50-54. 

MNF Loop Resonator, MNF Microsphere Resonator, and MNF /Microdisk Resonator... 

An MLR, which is illustrated in Fig. 13.le, is a miniature version of a fiber loop resonator created from an MNF. An MNF/microsphere resonator and MNF/ microdisk resonator, which are illustrated in Fig. 13.lg, h, consist of an MNF coupled to a microsphere or a microdisk. The excited WGMs are localized in the neighborhood of the microsphere (microdisk) circumference situated in the plane of symmetry of these devices. The transmission power of an MLR, a microsphere, or a microdisk sensor, P(/1), near the resonance wavelength 20 can be found in the form similar to the transmission power of a ring, a disk, and a microsphere resonator66,67 ... 

Generally, the resonances in transmission spectrum can overlap. Then, the transmission spectrum of these devices depends on the coupling of resonant mode to the MNF/MNF or MNF/microsphere (microdisk) as well as on the coupling of different WGMs to each other in the MNF/microsphere contact region. 

An MNF/microcylinder sensor exploits WGMs resonances in a cylinder (optical fiber), which are excited by an MNF. The arrangement of an MNF and a cylinder is shown in Fig. 13.li. As opposed to the WGM in a microsphere and microdisk considered in Sect. 13.3.1, the beam launched from the MNF into the cylinder spreads along the cylinder surface and eventually vanishes, even if there is no loss. The theory of resonant transmission of the MNF/microcylinder sensor was developed in Ref. 18. The resonant transmission power of this device can be modeled by a self-interference of a Gaussian beam that made n turns along the cylinder circumference ... 

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. 

Figure 3.25. Laser resonators applicable to molecular glasses A = microdroplet, B = microdisk, C = ring laser, D = vertical cavity distributed bragg laser, E = distributed feedback laser, F = random laser.

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. 

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