Mass transport ultramicroelectrodes

The UMEs used in bioarrays can be divided into three types disk, ring, and strip electrodes. The theory of the disk, ring, and strip UMEs has been extensively studied [97-100], Due to the edge effect, the profile of the mass diffusion to the ultramicroelectrode surface is three dimensional, and can significantly enhance the mass transportation in comparison to the conventional large electrode with one-dimensional mass transportation. The steady-state measurement at a planar UME can be expressed as... 

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. 

The unique mass transport properties of ultramicroelectrodes (discussed in Section 4.5.4) are attributed to shrinkage of the electrode radius. 

The RDE technique has found widespread use in analytical electrochemistry because of an excellent signal-to-noise ratio resulting from the enhanced mass transport. The RDE method has also been employed for monitoring concentrations in kinetic applications [59], as described for ultramicroelectrodes [60] and in the determination of the stoichiometry for electron-transfer reactions by means of redox titration [61]. The latter procedure will be described next. 

The use of ultramicroelectrodes in electrochemical microscopy has been the subject of several books and reviews (7,8). These electrodes provide very high rates of mass transport, since the effective mass transfer coefficient m (9) is inversely proportional to the active radius a (10). For a microdisk electrode m can be approximated as... 

So far SECM applications have been considered where enzymes immobilized at a surface catalyze redox reactions of low molecular weight compounds. The reaction products are detected at the ultramicroelectrode tip under diffusion-controlled conditions. This approach requires that the biochemically active layer continuously generate or consume redox active (for amperometric detection) or charged (for potentiometric detection) species. Since the tip signal depends on the diffusion coefficients and/or convective effects as well as the local concentration, it is possible to image localized mass transport phenomena instead of localized chemical fluxes (Chapter 9). In a general sense it is a process that is recorded with lateral resolution. 

This approach has been employed, for example, in determining the steady-state uncompensated resistance at an ultramicroelectrode (28) and the solution resistance between an ion-selective electrode tip and a surface in a scanning electrochemical microscope (29, 30). It also is sometimes possible to model the mass transport and kinetics in an electrochemical system by a network of electrical components (31, 32). Since there are a number of computer programs (e.g., SPICE) for the analysis of electric circuits, this approach can be convenient for certain electrochemical problems. 

One advantage of ultramicroelectrodes (Section 5.3) is that mass transport to the electrode by radial diffusion is high, even in the absence of convection. For a microdisk electrode of radius r, the mass-transfer coefficient is ... 

Amperometric electrodes that are extremely thin (<1 pm diameter) are called ultramicroelectrodes and have a number of advantages over conventional electrodes. Being narrower than the diffusion rate thickness, mass transport is enhanced, the signal-to-noise ratio is improved and the measurements can be made in resistive matrices such as nonaqueous solvents. These have huge applications in medicine as they can lit inside a living cell. Carbon fibre electrodes are coated in insulating polymer and plated with a thin layer of metal at the exposed tip to prevent fouling of the carbon itself. These can then be used to measure analytes of interest in various cells and membranes of the human body. 

Usual conditions for LSV or CV experiments require a quiet solution in order to allow undisturbed development of the diffusion layer at the electrode. Some groups, however, have purposely used the interplay between diffusion and convection in electrolytes flowing in a channel or similar devices [23]. In these experiments (see also Chapter 2.4), mass transport to the electrode surface is dramatically enhanced. A steady state develops [54] with a diffusion layer of constant thickness. Thus, such conditions are in some way similar to the use of ultramicroelectrodes. Hydro-dynamic voltammetry is advantageous in studying processes (heterogeneous electron transfer, homogeneous kinetics) that are faster than mass transport under usual CV or LSV conditions. A recent review provides several examples [22]. 

Linear sweep voltammetry at ultramicroelectrode disks of radius r < 10 pm under mass transport control, usually achieved at scan rates <50 mV s , provides a limiting current /l that depends directly on D [25] ... 

The ultramicroelectrodes allow the investigation in highly resistive media due to extensive decrease of the ohmic drop. However, the decrease of the supporting electrolyte concentration results in an increase of the migration contribution to the mass transport in the region adjacent to the electrode. 

The high diflfusivity at the nanoscale also enhances diffusion of a liquid into the nanosolid [189]. Powder nanosolids as electrodes in chemical sensors show much improved diffusion efficiency (10-10" ) [189]. Further, the powder ultramicroelectrode can significantly enhance the mass transportation rate from solution to the nanosolids surface, being irrespective of particular catalytic material [190, 191]. 

Amatore et al. developed a theoretical framework to describe the electrochanical responses of ultramicroelectrode ensemble and NEEs by considering mass transport for assemblies of microdisk and microband electrodes. Lee et al. used finite element simulation to solve 3D diffusion equations and found that a collection of 10 pm diameter microdisk electrodes required a separation distance of more than 40R to exhibit a sigmoidal simulated CV response typical for radial diffusion. " CV response typical of reversible linear diffusion at macroelectrodes was observed when the separation distance was less than 6R . Assemblies of microelectrodes for which the separation distances were between 6R and 4QR exhibited peak-shaped simulated CVs indicative of a mixture of radial and linear diffusion behavior. Thus, 12/ seems to be too small a separation distance for the design of ideal microelectrode arrays. 

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