Ultramicroelectrodes, advantages

What are the advantages of using ultramicroelectrodes for electrochemical measurements ... 

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. 

However, advantageous applications of micro- and ultramicroelectrodes are not limited to fundamental investigations. Such electrodes open up possibilities for work in very low concentrations of solute. Whatever can be done at a planar electrode can be done at a concentration about a thousand times lower by using an ultramicroelectrode without reaching the limiting diffusion current. This means that one could even obtain responses from solutes of 1 ppb (assuming a measured current density of 1 pA cm-2). 

Earlier it was pointed out that the use of ultramicroelectrodes could also give a several hundred times increase in iL compared with the diffusion-free currents at planar electrodes. The advantage of increasing the ability to measure at higher current densities by using short times in a transient technique with a planar electrode is that the magnitude of the currents is normal and is not forced down to the difficult-to-measure picoampere region that microelectrodes require. 

The conventional voltammetric indicator electrodes are 0.5-5 mm in diameter. However, ultramicroelectrodes (UME) [8] that have dimensions of 1-20 pm, are also used as indicator electrodes. The tiny electrodes have some definite advantages over conventional ones ... 

The above mentioned advantages mean micro and ultramicroelectrodes can be used in very low supported media. 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

It can be a further advantage of microelectrodes that they often increase the electrode resistance to bulk resistance ratio Rei/Rbuik- This is so because Re 1 frequently scales with the inverse area of the electrode, whereas the bulk resistance between a circular microelectrode and a counter-electrode is proportional to the inverse microelectrode diameter dme (see Sec. 4.1). Hence Rei/Rb iik ocbulk resistance decreases with decreasing microelectrode diameter. This is particularly helpful in order to investigate electrode polarization phenomena below the detection limit in experiments using macroscopic electrodes. (The reduced importance of the electrolyte resistance is also one of the reasons for ultramicroelectrodes to be applied in liquid electrochemistry [33, 34].)... 

In the SECM measurements, an ultramicroelectrode tip is used as a probe for ET occurring at an O/W interface. The electrode potential is controlled by a three-electrode potentiostat, whereas the potential drop across the O/W interface is usually determined by adding a common ion to both phases (except for the recent study [23] using an externally polarized interface). This sets the SECM measurements free from the restriction of the potential window. It should also be noted that in ordinary SECM measurements, all electrodes are in a single phase, so that it is possible to avoid the problems of IR drop and charging current. These advantages of SECM have been realized in kinetic studies of ET at O/W interfaces. 

The development of ultramicroelectrodes with characteristic physical dimensions below 25 pm has allowed the implementation of faster transients in recent years, as discussed in Section 2.4. For CA and DPSC this means that a smaller step time x can be employed, while there is no advantage to a larger t. Rather, steady-state currents are attained here, owing to the contribution from spherical diffusion for the small electrodes. However, by combination of the use of ultramicroelectrodes and microelectrodes, the useful time window of the techniques is widened considerably. Compared to scanning techniques such as linear sweep voltammetry and cyclic voltammetry, described in the following, the step techniques have the advantage that the responses are independent of heterogeneous kinetics if the potential is properly adjusted. The result is that fewer parameters need to be adjusted for the determination of rate constants. 

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. 

The cell in Figure 2 is a typical apparatus used in LL studies. However, recently small interfaces, called here microinterfaces, were shown to have some experimental advantage. The purpose of this modification was to use the same advantage that the ultramicroelectrodes have. Ultramicroelectrodes help to overcome solution resistance difficulties that originate from a potential shift due to an uncompensated iR drop. As the interfacial area becomes smaller, the diffusion geometry becomes a spherically symmetric process, which means that the ratio of charge transport current versus solution resistance increases and, ultimately, renders the iR drop minimal. In ITIES studies, restriction of the interfacial area and use of a current amplifier for voltammetric studies is a viable alternative to a four-electrode potentiostat. 

As mentioned already (Sect. 2.1.4.1), at short times scales, i.e. fast scan rates, LSV or CV curves at ultramicroelectrodes still attain the conventional peak shape related to linear diffusion. Under these conditions, another advantage of such electrodes becomes apparent the small electrode area, resulting in a small double layer capacity (hence a small time constant RCi), and a small current i (hence a small iR drop). Artifacts due to double layer charging and uncompensated iR drop become less prominent, and consequently high scan rates up to 10 V s can be used [39] as compared to the limit of a few 10Vs at conventional electrodes. Furthermore, specific techniques allow the recording of i /f-drop free voltammograms even at i > 10 V s [50]. 

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]. 

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