Microholes

Most electrochemical studies at the micro-ITIES were focused on ion transfer processes. Simple ion transfer reactions at the micropipette are characterized by an asymmetrical diffusion field. The transfer of ions out of the pipette (ejection) is controlled by essentially linear diffusion inside its narrow shaft, whereas the transfer into the pipette (injection) produces a spherical diffusion field in the external solution. In contrast, the diffusion field at a microhole-supported ITIES is approximately symmetrical. Thus, the theoretical descriptions for these two types of micro-ITIES are somewhat different. 

The shape of steady-state voltammograms depends strongly on the geometry of the microhole [13,14], Wilke and Zerihun presented a model to describe diffusion-controlled IT through a microhole [15], In that model, a cylindrical microhole is assumed to be filled with the organic phase, so that a planar liquid-liquid interface is located at the aqueous phase side of the membrane. Assuming that the diffusion is linear inside the cylindrical pore and spherical outside [Fig. 2(a)], the expression for the steady-state IT voltammo-gram is... 

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 theory has been verified by voltammetric measurements using different hole diameters and by electrochemical simulations [13,15]. The plot of the half-wave potential versus log[(4d/7rr)-I-1] yielded a straight line with a slope of 60 mV (Fig. 3), but the experimental points deviated from the theory for small radii. Equations (3) to (5) show that the half-wave potential depends on the hole radius, the film thickness, the interface position within the hole, and the diffusion coefficient values. When d is rather large or the diffusion coefficient in the organic phase is very low, steady-state diffusion in the organic phase cannot be achieved because of the linear diffusion field within the microcylinder [Fig. 2(c)]. Although no analytical solution has been reported for non-steady-state IT across the microhole, the simulations reported in Ref. 13 showed that the diffusion field is asymmetrical, and concentration profiles are similar to those in micropipettes (see... 

The concentration of the transferred ion in organic solution inside the pore can become much higher than its concentration in the bulk aqueous phase [15]. (This is likely to happen if r <5c d.) In this case, the transferred ion may react with an oppositely charged ion from the supporting electrolyte to form a precipitate that can plug the microhole. This may be one of the reasons why steady-state measurements at the microhole-supported ITIES are typically not very accurate and reproducible [16]. Another problem with microhole voltammetry is that the exact location of the interface within the hole is unknown. The uncertainty of and 4, values affects the reliability of the evaluation of the formal transfer potential from Eq. (5). The latter value is essential for the quantitative analysis of IT kinetics [17]. Because of the above problems no quantitative kinetic measurements employing microhole ITIES have been reported to date and the theory for kinetically controlled CT reactions has yet to be developed. 

The use of micropipette electrodes for quantitative voltammetric measurements of ion transfer (IT) and electron transfer (ET) reactions at the ITIES requires knowledge of geometry of the liquid interface. For the micrometer-sized micropipettes, both the orifice radius and the thickness of the pipette wall can be measured microscopically. A typical error of the microscopic determination of a radius was estimated to be 0.5/am for a micropipette and 1 /am for a microhole [24]. 

Quinn et al. studied ET at micro-ITIES supported by micropipettes or microholes [16]. The studied systems involved ferri/ferrocyanide redox couple in aqueous phase and ferrocene, dimethylferrocene, or TCNQ in either DCE or o-nitrophenyl octyl ether. Sigmoidal, steady-state voltammograms were obtained for ET at the water-DCE interface supported at a micropipette. The semilogarithmic plot of E versus log[(/(j — /)//] was... 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

A microhole-based ITIES has been used by Osborne et al. for amperometric determination of ionic species in aqueous solutions [12]. They studied the assisted ammonium transfer with DB1816 at the water-DCE interface. Because the concentration of iono-phore in the organic phase was high, the measured steady-state current was proportional to the concentration of ammonium in the aqueous phase. The time required to reach a steady state was relatively short (e.g., 5 s for an 11/xm hole). A linear relationship was found between the steady-state plateau current and the ammonium concentration over the range 1 to 500/aM. 

In one report, the development of a cellulose acetate decoupler has improved the LOD of dopamine further to 25 nM [332]. In another, a microhole array was constructed in a PET chip and used as an electrical decoupler (see Figure 7.22). The array, which was perpendicular to the separation channel, was 1 mm away from the working electrode. Decoupling was successful for 10-pm holes used in... 

FIGURE 7.22 (Top) Schematic representation of the microchip layout used for the CE separation with EC detection. (Bottom) Schematic representation of a cross section at the end of the separation channel comprising the microhole array decoupler, the working electrode, and the silver/AgCl reference electrode [191]. Reprinted with permission from Elsevier Science. 

There can be no doubt that one of the most interesting applications of the results from studies of bilayers is to biomembranes and other biostructures, Investigations of the stability and permeability of biomembranes are particularly of great interest. For instance, it is known [425-427] that a possible mechanism of the transfer of permeant (e.g. water dissolved ions, etc.) across a bilayer biomembrane is the passage of the permeant through microholes in the membrane. The statistically distributed holes in the bilayer biomembrane, formed by the mechanism of nucleation described, may thus turn out to be very important for the permeability of such biomembranes. 

The good agreement between theoretical and experimental results of hole-mediated permeability of foam bilayers to air allows the determination of the permeability coefficient of bilayers of both ionics and nonionics. Though the mechanism of hole-mediated permeation of foam bilayers is not entirely clarified, its efficiency for lower surfactants concentrations in a wide range of temperatures is firmly established. This finding is in strong support of the basic idea of the existence of randomly nucleated microholes in the amphiphile bilayer. 

Luo et al. [61] demonstrated a simple method for the preparation of SERS active Ag nanostructures substrates by deposition of Ag nanoparticles into the designed Si holes. The morphologies of the Ag nanostructures were observed with SEM. The diameters of the Ag nanoparticles were found to be 40-60 nm. With increasing deposition time, flower-like Ag nanostructure commenced crystallization to form near the edge of the bottom surface of the Si microholes. These Ag nanostructures exhibited strong SERS enhancement, which provided an excellent platform for monitoring the R6G molecules by SERS technology [62]. 

Shimizu, Y. and Mori la, K. (1990) Microhole array electrode as a glucose sensor. Analytical Chemistry, 62 (14), 1498. 

SACE technology can be used for flexible glass microstructuring. Channel-like microstructures and microholes can be obtained. Two examples are illustrated in Fig. 1.4. The channel microstructure was machined with a cylindrical 90 pm... 

Figure 1.4 Micrographs of a SACE-machined channel-like structure (left) and a microhole (right) in Pyrex glass. Reprinted from [128] with permission from Elsevier.

The diameters of the microholes were chosen according to experimentally known over-cut (see Section 6.2.6). For simplification, it was further assumed that the lateral space between the tool-electrode and the glass is filled with the gas film. 

Figure 5.8d shows the radial temperature distribution at the drilling depth of 300 pm. The tool reaches a temperature of around 500°C, as measured experimentally by Kellog [70] and Basak and Ghosh [6,7]. The marked points are the microhole diameters. The temperature at the border of the microhole is the machining temperature. It is remarkable that for all simulated voltages, the machining temperature Tm is found to be similar (around 500-600°C). 

The shapes depicted in Fig. 5.8a-c are typical for microholes obtained by drilling at high depth or, more generally, when melting is the dominant mechanism for material removal. These effects are not restricted to glass but are also observed in other materials such as ceramics [104]. 

As discussed below, the hydrodynamic regime is responsible for the increase in the machining over-cut and for the formation of heat affected zones around the microhole. This is an undesired effect and one should try to avoid machining in this situation. Strategies to reduce this regime are presented in Chapter 7. 

Figure 6.6 Mean entrance diameter of microholes obtained by SACE glass gravity-feed drilling as a function of the machining voltage and the drilling depth for a 0.4 mm stainless steel tool-cathode. Reprinted from [84] with the permission of the Journal of Micromechanics and Microengineering.

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