Cyclic voltammetry microelectrodes

To predict the electrochemical response when the problem has no known or simple analytical solution, e.g. cyclic voltammetry, microelectrodes, coupled chemical reactions, etc. 

FIGURE 11. Cyclic voltammetries of 68 at a mercury microelectrode, concentration 2x10 m, electrolyte OMF/Bu NI 0.1m, reference electrode Ag/Agl/I" 0.1m, sweep rate 300mVs ... 

Figure 3. Cyclic voltammetry of adjacent electrodes of a poly(I)-coated microelectrode array driven individually and together at 200 mV/s in the region of the oxidative potential of polythiophene in CH3CN/O.I II [11-BU4N] PFg.

Figure 7. Cyclic voltammetry of an interdigitated array of microelectrodes coated with poly(I) in CH3CN/0.I H [n-Bu NlPFg (a) The potential of all eight electrodes is scanned together at 10 mV/s. (b) The potential of electrodes 2,4,6, and 8 is scanned at 10 mV/s while the potential of electrodes 1,3,5, and 7 is held at 0 V vs. Ag+/Ag.

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

An electrochemical sensor using an array microelectrode was tested for the detection of allergens such as mite and cedar pollen (Okochi et ah, 1999). Blood was used in the assay and the release of serotonin, a chemical mediator of allergic response, which is electrochemically oxidized at the potential around 300 mV, was monitored for electrochemical detection by cyclic voltammetry. 

Fig. 7.35. Development of diffusion concentration profiles in ensembles of microelectrodes. Concentration distortions at very short times during chronoamperometry or fast sweep rates during (a) cyclic voltammetry, (b) intermediate times or sweep rates, and (c) long times or slow sweep rates. Voltam-metric responses are shown schematically. (Reprinted from B. R. Scharifker, Microelectrode Techniques in Electrochemistry, in Modem Aspects of Electrochemistry, Vd. 22, J. O M. Bockris, B. E. Conway, and R. E. White, eds., Plenum, 1992, p. 505.)...

Several techniques arising from cyclic voltammetry help the interested reader to peer into the future. Derivative polarograph (di/dV against Vt) increases the sharpness of detection of dissolved radicals and molecular fragments. Microelectrodes can be used with potential sweep circuitry. The use of varying electrical wave forms (instead of the linear potential variation) offers much to be learned in the future. Automation and the use of pattern recognition in mechanism evaluations... 

A cell for cyclic voltammetry (CV) employs three electrodes, a working microelectrode, a counter electrode, and a reference electrode the current flows between the two former, and the potential of the working electrode is determined by measuring the potential difference between that and the reference electrode. 

Stripping is the most sensitive form of voltammetry. In anodic stripping polarography, analyte is concentrated into a single drop of mercury by reduction at a fixed voltage for a fixed time. The potential is then made more positive, and current is measured as analyte is reoxidized. In cyclic voltammetry, a triangular waveform is applied, and cathodic and anodic processes are observed in succession. Microelectrodes fit into small places and their low current allows them to be used in resistive, nonaqueous media. Their low capacitance... 

Cyclic voltammetry is one such electrochemical technique which has found considerable favour amongst coordination chemists. It allows the study of the solution electron-transfer chemistry of a compound on the sub-millisecond to second timescale it has a well developed theoretical basis and is relatively simple and inexpensive. Cyclic voltammetry is a controlled potential technique it is performed at a stationary microelectrode which is in contact with an electrolyte solution containing the species of interest. The potential, E, at the microelectrode is varied linearly with time, t, and at some pre-determined value of E the scan direction is reversed. The current which flows through the cell is measured continuously during the forward and reverse scans and it is the analysis of the resulting i—E response, and its dependence on the scan rate dE/dt, which provides a considerable amount of information. Consider, for example, the idealized behaviour of a compound, M, in an inert electrolyte at an inert microelectrode (Scheme 1). 

There are several excellent articles which deal with the theory and practice of cyclic voltammetry.1-4 Foremost among these is the comprehensive treatise by Bard and Faulkner which gives a thorough description of the theory of controlled potential microelectrode techniques, including cyclic voltammetry.1 Particularly readable accounts of cyclic voltammetry and related techniques are given in Adams book, Electrochemistry at Solid Electrodes ,2 in Pletcher s review3 and in a series of articles which appeared in J. Chem. Educ.e>... 

Another important feature is the relative size of the diffusion layer with respect to the double layer. The diffusion-layer dimensions are proportional to the size of the electrode, and can approach the size of the double layer with electrodes of molecular dimensions [95]. This can also occur in other situations explored with microelectrodes. For example, in solutions of very dilute electrolyte, the diffuse double layer extends several nanometers into solution [62]. Alternatively, very fast cyclic voltammetry results in a very small diffusion layer, which may be of dimensions similar to the double layer [46]. In all these situ-... 

When using microelectrodes to obviate resistance problems, it is convenient to develop a procedure to determine what conditions are required to reduce the error to an acceptable level. The results of such a procedure applied to disk electrodes are shown in Figure 16.6 [45]. In this and the remaining discussion, the technique of cyclic voltammetry is considered, as it is one of the most widely used voltammetric methods. The region of practical working conditions of electrode radius and scan rate is defined by the area set off by lines A, B, and C. 

The introduction of ultramicroelectrodes in the field of voltammetric analysis offers access to cyclic voltammetry experiments that are impossible with conventionally sized macroelectrodes. In addition to analyses in small volumes or at microscopic locations, microelectrodes allow measurements in resistive media and make it possible to perform high scan rate voltammetry [9,10]. 

Cyclic voltammetry proved to be a convenient method to reveal the oxido-reduction properties of the skin and of dermo-cosmetic creams. On the one hand, using microelectrodes, it was for the first time possible to evaluate the antioxidant properties on the skin surface. This simple protocol allowed to study in real time the global oxido-reductive state and to determine several antioxidant species. On the other hand, results showed the effect of oxidative stress on the evolution of the antioxidant properties of dermo-cosmetic products in time. 

In this section, we will show that the stationary responses obtained at microelectrodes are independent of whether the electrochemical technique employed was under controlled potential conditions or under controlled current conditions, and therefore, they show a universal behavior. In other words, the time independence of the I/E curves yields unique responses independently of whether they were obtained from a voltammetric experiment (by applying any variable on time potential), or from chronopotentiometry (by applying any variable on time current). Hence, the equations presented in this section are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. 

All general typical variables considered in this chapter for a particular reaction scheme, for example the half-wave potential, are of fundamental interest for its characterization in any electrochemical technique. Moreover, as indicated in the previous chapter, all the current-potential expressions deduced here under stationary conditions (when microelectrodes are used) are applicable to any multipotential step or sweep electrochemical techniques like Staircase Voltammetry or Cyclic Voltammetry. 

Cyclic Staircase Voltammetry and Cyclic Voltammetry at Electrodes and Microelectrodes of Any Geometry... 

When the electrochemical properties of some materials are analyzed, the timescale of the phenomena involved requires the use of ultrafast voltammetry. Microelectrodes play an essential role for recording voltammograms at scan rates of megavolts-per-seconds, reaching nanoseconds timescales for which the perturbation is short enough, so it propagates only over a very small zone close to the electrode and the diffusion field can be considered almost planar. In these conditions, the current and the interfacial capacitance are proportional to the electrode area, whereas the ohmic drop and the cell time constant decrease linearly with the electrode characteristic dimension. For Cyclic Voltammetry, these can be written in terms of the dimensionless parameters yu and 6 given by... 

Figure 2. A, Solid-state cyclic voltammetry at a Pt microelectrode of a 1 1 mixture of octamethylferrocene (wave at more negative potential) and ferrocenyl ferraazetine (waves at more positive potentials) dissolved in a 4 1 mixture of MEEP/Li[CF S03] before and after exposure to CO. B, Cyclic voltammetry (vs AgNOJAg) at a Pt disk (1-mm diameter) of 0.2 mM la in THF/0.1 M [n-Bu4N]PF, before and after the addition of CO. (Reproduced from ref. 2. Copyright 1990 American Chemical Society.)...

Figure 5. Cyclic voltammetry (500 mV/s) of Au microelectrodes derivatized with a mixture of 4c and 5 at pH 1.4 and pH 6.0. The solutions used were phosphate buffers in 1.0 M NaC104 base electrolyte an SCE reference electrode was used. Reproduced with permission from ref. 1. Copyright 1991 American Association for the Advancment of Science.

Figure 5.22 Cyclic voltammetry data obtained for a 5 pm radius platinum microelectrode modified with a [Ru(bpy)2Qbpy]2+ monolayer following laser excitation at 355 nm, with a scan rate of 3 x 105 V s 1. The surface coverage is 1.1 x 10-10 mol cm 2, the supporting electrolyte is 0.1 M TBABF4 in acetonitrile, and the initial potential is —1.2 V. Reprinted with permission from R. J. Forster and T. E. Keyes, /. Phys. Chem., B, 102,10004 (1998). Copyright (1998) American Chemical Society...

Fig. 14.34. Voltammetry of epinephrine. Background (A, solid line) and signal containing (A, dashed line) currents generated during fast-scan cyclic voltammetry (300 V/s) at a carbon fiber microelectrode r = 5 pm). A background subtracted cyclic voltammogram (B) is produced from the traces shown in A. (Reprinted from Wightman, et al. Chemical Communication, Interface, 5(3) 22, Fig. 2,1996. Reproduced by permission of the Electrochemical Society, Inc.)...

The use of fast scan cyclic voltammetry has already been described (Section 8.6). In general, microelectrodes, in some cases modified by electrocatalysts, are making it possible to learn about biological events on the scale of a single cell. Among the more important achievements (Wightmann, 1996) is the monitoring of dopamine released after stimulation from neurons in the intact brain and involved in neurotransmission. 

The detection of (specifically) dopamine is hindered by the presence in the extracellular fluid of several compounds having redox potentials close to that of dopamine. The technique most likely to succeed here is fast scan cyclic voltammetry (Section 8.6) because the voltamogram provides characteristics that are indicative of the individual compound being monitored. The microelectrodes used have radii of 5 pm, but even this is not small enough to be able to determine dopamine from just one cell. The reacting compounds come from several nerve endings. Nevertheless, the fast scan cyclic voltammetry technique her sufficient time and resolution to allow information to be obtained on the part played by dopamine in neurotransmission in the brain. For example, it answers such questions as does the released dopamine stay at the synapse or does it diffuse in the extracellular fluid to contact other neurons ... 

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