Ultramicroelectrodes transients

Selzer Y and Manler D 2000 Scanning electrochemical microscopy. Theory of the feedback mode for hemispherical ultramicroelectrodes steady-state and transient behavior Anal. Chem. 72 2383... 

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

Values of A , and k may be extracted from the polarographic data, although the treatment is complex. Examples of its use to measure the rate constants for certain redox reactions are given in Refs. 339 and 340 which should be consulted for full experimental details. The values obtained are in reasonable agreement with those from stopped-flow and other methods. The technique has still not been used much to collect rate constants for homogenous reactions. The availability of ultramicroelectrodes has enabled cyclic voltammograms to be recorded at speeds as high as 10 Vs". Transients with very short lifetimes (< ps) and their reaction rates may be characterised. ... 

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 Qf E curve for a reversible two-electron transfer taking place in a monolayer is independent of time (i.e., it has a stationary character) and, therefore, is independent of the potential-time waveform applied to the electrode, as in the case of a reversible one-electron transfer reaction. It is also important to highlight that the normalized charge, has a identical expression to that for the normalized transient current 7 v N obtained for solution soluble species when the NPV technique is applied to an electrode with any geometry (see curves in Fig. 3.16, and Eq. (3.141)), and also to the normalized stationary current obtained for solution soluble species when any potential-time waveform is applied for ultramicroelectrodes with any geometry. 

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. 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

G. Denault, M. Mirkin, and A. J. Bard [/. Electroanai Chem., 308, 27 (1991)] suggested that by normalizing the diffusion-limited transient current, /, obtained at an ultramicroelectrode at short times, by the steady-state current, one can determine the diffusion coefficient, Z), without knowledge of the number electrons involved in the electrode reaction, n, or the bulk concentration of the reactant, C. ... 

Fig. 1.3 Time line of ECL 1964-1965, first experiments 1965, theory 1966, transients 1969, magnetic field effects 1972, Ruffipy) 1977, oxalate 1981, aqueous 1982, Ruibpy) polymer and persulfate 1984, Ru(bpy) label 1987, tri-n-propylamine (TPA) 1989, bioassay 1993, ultramicroelectrodes 1998, laser action 2002, semiconductive nanocrystals (Reprinted with permission from Ref. [1]. Copyright 2008 American Chemical Society)...

Quantitative investigations of the kinetics of these polymerization a-coupling steps have been performed by fast-scan voltammetry on ultramicroelectrodes in order to estimate the lifetime of the transient species. The rate constants for the dimerization of thiophene are greater than 10 M s, while the lifetime of oligothiophene radical cations increases with chain length the rate constant for unsubstituted thiophene tetramer is close to 10 and that of the pentamer is below 10 M s [18]. These studies have... 

Calculation by Chronoamperometry using an ultramicroelectrode based on the time-dependent current respmise, i, following a potential step [19]. Theoretical transients are calculated according to the Aoki model which is given by the expression ... 

ABSTRACT. Several aspects of electrochemistry at ultramicroelectrodes are presented and discussed in relevance to their application to the analysis of chemical reactivity. The limits of fast scan cyclic voltammetry are examined, and the method shown to allow kinetic investigations in the nanosecond time scale. On the other hand, the dual nature of steady state at ultramicroelectrodes is explained, and it is shown how steady state currents may be used, in combination with transient chronoamperometry, for the determination of absolute electron stoichiometries in voltammetric methods. Finally the interest of electrochemistry in highly resistive conditions for discussion and investigation of chemical reactivity is presented. 

Finally, to conclude this introduction, and to avoid any possible confusion in the terminology, we wish to define briefly what is an ultramicroelectrode (at least in our sense ). When their interfacial properties are to be considered identical with those of any other electrode of a larger dimension, ultramicroelectrodes must remain much larger than the double layer thickness. This sets a lower dimension of a few tens of A for ultramicroelectrodes.On the other hand, if diffusional steady state voltammetry has to be observed without significant interference of convection, they must be smaller than convective layers, which sets an upper limit of a few tens of /im. Between these limits, all ultramicroelectrodes possess identical intrinsic physico-chemical properties. However, their behavior (viz. ohmic drop, steady state or transient currents, etc) obviously depends on the medium and the time-scale considered. ... 

The difficulty in determining an electron consumption stems from the fact that only nxD (steady state current plateau at a disk ultramicroelectrode of radius Vq) or nxD (transient chronoamperometry) values can be determined from voltanunetric experiments. However, whenever one is able to measure both parameters within the same time scale (viz, for chron. stcad st. oth n and D values ensue. The above results for pyrylium... 

The redox potential of interest to understand the biological effects of flavan-3-ols is the one related to phenoxyl radical-phenate couple, as this potential is roughly 1 V lower than the potential of the phenoxyl radical-phenol couple, which furthermore may transiently involve the oxidation of the aromatic atoms. Standard potential can be measured by electrochemistry [49] or pulse radiolysis [40 4]. However, determining the redox potential of polyphenolic compounds is a real challenge since for these methods the measurement must be faster than the subsequent reactions induced by the oxidation of the phenol group in order to obtain the thermodynamic value. By using ultramicroelectrodes (electrodes with a micrometer diameter), it has been shown that a very high scan rate, up to 1 milUon... 

Fig. 4.2 (a) Hemispherical diffusion to a microdisk ultramicroelectrode, (b) A steady-state cyclic voltammogram recorded using an ultramicroelectrode, (e) A transient cyclic voltammogram recorded at a macroelectrode... 

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