Cyclic voltammetry fast scan

In the case of dissociative electron transfer to aromatic compounds, electron transfer is not necessarily concerted with bond dissociation. The substrate 7t-radical-anion may be an intermediate whose existence can be demonstrated by fast scan cyclic voltammetry in aptotic solvents. At fast scan rates, reversible electron transfer occurs. At slower scan rates, die anodic peak height falls and a second reversible electron transfer step appears due to formation of the radical-anion of the compound formed by replacement of the substituent by hydrogen. Cleavage of the... 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

U seful methods for detecting short-lived cationic radicals are fast-scan cyclic voltammetry at a UME (Section 8.4.2) and cyclic voltammetry at low temperatures (Section 8.4.3). It is preferable to prepare electrolyt-... 

Recent studies describe the use of cyclic voltammetry in conjunction with controlled-potential coulometry to study the oxidative reaction mechanisms of benzofuran derivatives [115] and bamipine hydrochloride [116]. The use of fast-scan cyclic voltammetry and linear sweep voltammetry to study the reduction kinetic and thermodynamic parameters of cefazolin and cefmetazole has also been described [117]. Determinations of vitamins have been studied with voltammetric techniques, such as differential pulse voltammetry for vitamin D3 with a rotating glassy carbon electrode [118,119], and cyclic voltammetry and square-wave adsorptive stripping voltammetry for vitamin K3 (menadione) [120]. 

Attention has turned recently to the mechanistic details underlying these processes. Probably the most significant development in this area in recent years is the discovery that the ketyl radical produced by electrochemical reduction of benzaldehyde in buffered neutral ethanol is harder to reduce than benzaldehyde itself46. All previous discussions had assumed that the ketyl radical would be reduced as quickly as it is formed, but fast scan cyclic voltammetry demonstrated the existence of a short-lived intermediate, apparently the ketyl radical. Computer simulation of the voltammograms showed that the radical dimerizes at a rate ca 106 1VD1 s 1. 

Sinusoidal voltammetry (SV) is an EC detection technique that is very similar to fast-scan cyclic voltammetry, differing only in the use of a large-amplitude sine wave as the excitation waveform and analysis performed in the frequency domain. Selectivity is then improved by using not only the applied potential window but also the frequency spectrum generated [28]. Brazill s group has performed a comparison between both constant potential amperometry and sinusoidal voltammetry [98]. 

Another voltammetric method, sinusoidal voltammetry (SV), was also employed. This was a frequency-based electrochemical method, which was found to be more sensitive than the usual constant potential (DC) amperometric detection method. SV has been achieved to detect on-chip separated catecholamines. This method is very similar to fast-scan cyclic voltammetry (CV), except that a... 

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 ... 

Fast-scan cyclic voltammetry (10 mVs-1 to 106 Vs-1) was used to measure the rate constants of C—X cleavage, which are extremely fast. The technique was applied to measure rate constants of the order of submicrosecond half-lives. For example, the radical anions generated at the electrode surface were determined to have a half-life ranging from less than 100 ns in the case ofp-bromoacetophenone to 70 ms for m-nitrobenzyl chloride189. The method complements the redox catalytic method developed by Saveant and co workers190. 

The information that can be obtained with electrochemical detectors is not restricted to quantification. Instead of the conventional use of electrochemical detectors in amperometric mode at fixed potential, electrode arrays with each electrode held at different values of fixed potential can be used, in order to build up chronovoltammograms, three-dimensional current-voltage-time profiles. A 32-microband electrode array has been described for this purpose and applied to phenolic compounds [17] and which permits studying the electrode reaction mechanism at the same time as identification and quantification are carried out. Alternatively, fast voltammetric techniques such as fast-scan cyclic voltammetry or square wave voltammetry can be used to create chronovoltammograms of the eluted components. 

Other forms of voltammetry are as follows (1) fast-scan cyclic voltammetry useful in neuroelectrochemistry (2) nanosecond voltammetry for a 5-pm disk working microelectrode with RC < 1 gs, scan rates of 2.5 MV/s allow for fast kinetics measurements (3) differential-pulse voltammetry with staircase pulses, potential resolutions of 0.04 V and detection limits of 10 8M can be attained (4) anodic (cathodic) stripping voltammetry traces... 

Transient technique — A technique whose response is time dependent and whose time dependence is of primary interest, e.g., -> chronoamperometry, -> cyclic voltammetry (where current is the transient), -> chronopotentiometry and -> coulostatic techniques (where voltage is the transient). A transient technique contrasts with steady-state techniques where the response is time independent [i]. Some good examples are cyclic voltammetry [i, ii] (fast scan cyclic voltammetry), the indirect-laser-induced-temperature-jump (ILIT) method [iii], coulostatics [i]. The faster the transient technique, the more susceptible it is to distortion by -> adsorption of the redox moiety. 

Garris PA, Wightman RM (1995) Regional differences in dopamine release, uptake and diffusion measured by fast-scan cyclic voltammetry. In Boulton A, Baker G, Adams RN (Eds), Neuromethods, Vol. 25, pp. 179-220. Humana Press Inc. 

These considerations highlight the fact that microdialysis and voltammetry, particularly fast-scan cyclic voltammetry, estimate two quite different aspects of DA transmission characterized by different temporal and spatial constants. Microdialysis estimates steady-state levels of extracellular DA and changes in these levels taking place on a minute scale away from DA release sites. Fast-scan voltammetry estimates changes in DA-like signals taking place on a subsecond scale near DA release sites. 

Robinson DL, Venton BJ, Heien ML, Wightman RM (2003) Detecting subsecond dopamine release with fast-scan cyclic voltammetry in vivo. Clin Chem 9(10) 1763—1773. 

These kinetic results are interesting in that they are consistent with the physical reality of the thinned diffusion-layer model introduced above. Moreover it is evident that sonovoltammetry enables fast rate constants to be measured under steady-state conditions at conventionally dimensioned electrodes otherwise these would only be accessible via transient measurements such as fast-scan cyclic voltammetry or using steady-state microelectrode methodology. 

Since the resulting radical cations proved to be highly reactive with lifetimes much below 10 s in acetonitrile (results from fast scan cyclic voltammetry [64]), only irreversible oxidation potentials of the enols and ketones were obtained. Therefore, the data can only be viewed as a good estimate (Table 4). Nevertheless, in agreement with gas-phase results, it is evident that, in solution as well, enol radical cations are more stable than the corresponding ketone ions. Unfortunately, no solution data are so far available for simple aliphatic systems. 

These results, however, do not imply that mechanism C is impossible in general [228]. Recently, sterically hindered enol acetates, where nucleophilic attack (mech. A) and deprotonation (mech. B) on the radical cation stage are suppressed, were synthesized and studied by cyclic voltammetry as well as by product analysis [229]. Accordingly, enol acetates 146-149 undergo loss of CH3CO upon one-electron oxidation and open up a novel route to a-carbonyl cation chemistry. 150-153 rearrange subsequently to the benzofurans 19-21,23. The C-O bond cleavage reaction in 147 is rather slow (k < 10 s as derived from fast-scan cyclic voltammetry studies. 

Garris PA, Ensman R, Poehhnan J, Alexander A, Langley PE, Sandberg SG, Greco PG, Wightman RM, Rebec GV. Wireless transmission of fast-scan cyclic voltammetry at a carbon-fiber microelectrode proof of principle. J. Neurosci. Methods. 2004 140 103-115. 

To improve chemical selectivity, a triangular input waveform can be used that separates compounds into resolvable peaks. This form—cyclic voltammetry—can be carried out with high temporal resolution using high scan rates to allow the waveform to be completed in a short time. In fast-scan cyclic voltammetry (also known as fast cyclic voltammetry), waveforms last around 10 ms, and measurements are typically made every 10-200 ms. 

Although multiple electrochemical techniques exist, those used in freely moving animals are chronoamperometry, differential normal-pulse voltammetry, and fast-scan cyclic voltammetry. Excellent comparisons between these can be found in literature, particularly Troyer et al.[5,7,30] and Robinson el al.[8] and therefore will not be diskussed here. 

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