Microparticle-Modified Electrodes

The voltammetric behavior of surface-immobilized microparticles of redox active solid materials has been extensively studied by the groups of Scholz (Greifswald, Germany), Bond (Melbourne, Australia), Grygar (Rez, Czech Republic), Komorsky-Lovric and Lovric (Zagreb, Croatia), Domenech-Carbo (Valencia, Spain), Marken (Bath, UK), and others. Theoretical aspects, however, have been addressed only in some reports. Recently, the Compton group (Oxford, UK) made several reports on the theory of microparticle-modified electrodes, and these will mainly be discussed at this point. 

The Diffusion Categories Following a classification in a recent review [42], the situation of the microparticle-modified electrode can be described and treated as that of a spatially heterogeneous, or partially blocked, electrode. The solid macroelectrode is partially covered with microparticles, which - for the sake of theoretically... 

The experimental arrangement outlined in Scheme 2 of Fig. 14.1 is relevant when a microparticle-modified electrode is coated with a thin layer of ionic liquid before being placed in contact with aqueous solution. The results of modelling of the [trans-Mnf process in both the ionic liquid dissolved and adhered states have been described in detail by Zhang and Bond [23] and compared to results obtained by microchemical methods at an electrode/ionic hquid/aqueous electrolyte interface under the conditions of outlined in Scheme 2. 

Fig. 14.13 Schematic representation of the principal processes assumed to influence the voltammetric response when a microparticle-modified electrode is placed in contact with an ionic liquid and when dissolved electrogenerated species Ox (ionic liquid) undergoes a square reaction scheme. Adapted with permission fi om Zhang et al., J Phys Chem B. 2004, 108, 7363-Till [13]. Copyright 2013, American Chemical Society...

Yang, W. C., Yu, A. M., and Chen, H. Y. (2001). Applications of a copper microparticle-modified carbon fiber microdisk array electrode for the simultaneous determination of aminoglycoside antibiotics by capillary electrophoresis. /. Chromatogr. A 905, 309—318. 

These drawbacks can be avoided to a large extent, using the voltammetry of microparticles—a technique involving solid state electrochemistry where down to about 10 to 10 mol of sample [74-78] can be transferred by abrasion into the surface of an inert electrode, usually paraffin-impregnated graphite electrodes, and the electrode is later immersed in a suitable electrolyte for recording its voltam-metric response. The response of this sample-modified electrode, consisting of the reduction or oxidation of the solid materials, becomes phase-characteristic. 

Solid-state electrochemistry — is traditionally seen as that branch of electrochemistry which concerns (a) the -> charge transport processes in -> solid electrolytes, and (b) the electrode processes in - insertion electrodes (see also -> insertion electrochemistry). More recently, also any other electrochemical reactions of solid compounds and materials are considered as part of solid state electrochemistry. Solid-state electrochemical systems are of great importance in many fields of science and technology including -> batteries, - fuel cells, - electrocatalysis, -> photoelectrochemistry, - sensors, and - corrosion. There are many different experimental approaches and types of applicable compounds. In general, solid-state electrochemical studies can be performed on thin solid films (- surface-modified electrodes), microparticles (-> voltammetry of immobilized microparticles), and even with millimeter-size bulk materials immobilized on electrode surfaces or investigated with use of ultramicroelectrodes. The actual measurements can be performed with liquid or solid electrolytes. 

Description of electrocatalytic processes in such modified electrodes can be derived from the intersection between the theory of Andrieux and Saveant (1980, 1988) for mediated electrocatalysis in redox polymers and those for metal oxide electrocatalysis (Lyons et al., 1992,1994 Attard, 2001 Pleus and Schulte, 2001) and the recent models for the voltammetry of microparticles given by Lovric and Scholz (1997, 1999) and Oldham (1998) and combined by Schroder et al. (2000). 

Following this pioneer work, similar experiments were done by several groups, with various modified electrodes Pt microparticles electrodeposited into polyaniline films [61], platinum particles nanodispersed into PPy films [54], but in this latter case the electrocatalytic activity is very poor, which certainly comes from the fact that the electropolymerization process embedded the colloidal Pt particles, so that they were not directly accessible to the reactive species. 

Yin, H., Meng, X., Su, H., Xu, M., and Ai, S. (2012) Electrochemical determination of theophylline in foodstuff, tea and soft drinks based on urchin-like CdSe microparticles modified glassy carbon electrode. Food Chem., 134 (2), 1225-1230. 

Equation (44) is the polarization curve equation for a modified inert electrode for y = 1. It is valid for inert substrates modified by active microparticles or nanoparticles as well as by 2D and 3D islands of active metal. 

Solid electrodes, modified with microparticles attached onto the surface, can be assumed as the result of the usual abrasive transfer by rubbing, for example, a graphite rod over nanogram amounts of the respective sample. These adhered microparticles are randomly distributed over the surface, and the whole set-up can be understood and theoretically treated as random array of microdisk electrodes (unlike regular arrays, see Section 6.3.2.3) (Figure 6.12). 

The following results are applicable to any electrode modified with a sparse distribution of microparticles. The mass transport to a single, diffusionally independent microparticle can (in theory) be treated on an equal basis as a microparticle within an independent diffusional zone in the experimental time scale with respect to its neighbors. Therefore, many theoretical results produced for microparticle arrays of diffusional categories 1 and 2 (see Section 6.3.2.2.2) are also valid for single particles. 

Glassy carbon felt electrodes modified by electrodeposited poly(pyrrole-viologen) films containing electroprecipitated microparticles of precious metals like Pt, Pd, Rh or Ru have been shown to be suitable for the electrocatalytical hydrogenation of several organic substrates including benzonitrile, in acidic aqueous solution. Pd exhibited the highest current efficiency and yields for benzylamine formation when compared with Pt and Rh. 

Besides methanol and ethanol, only a few other small molecules (HCOOH, HCHO, CO), have been oxidized at electron conducting polymer electrodes modified by incorporation of platinum microparticles. The first study on formic acid oxidation at Pt particles dispersed in a PAni matrix was carried out, as early as 1986, by Gholamian et al. [57], They found that the incorporation of 100 pg cm of Pt into PAni was sufficient to enhance considerably the oxidation rate of formic acid (ten-fold increase). The cyclic voltammograms recorded with 0.5 M HCOOH in 0,5 M H2SO4 displayed an enhanced oxidation current particularly for the first oxidation peak at 0.2 V/SCE, attributed to the oxidation of the weakly adsorbed intermediate (reactive species). The second peak, at 0.6 V/SCE, attributed to the oxidation of the strongly chemisorbed... 

S.M. Golabi and A. Nozad, Electrocatalytic oxidation of methanol on electrodes modified by platinum microparticles dispersed into poly(o-phenylenediamine), J. Electroanal. Chem., 521, 161-167 (2002). 

L. Coche and J.C. Moutet, Electrocatalytic hydrogenation of organic compounds on carbon electrodes modified by precious metal microparticles in redix active polymer films, J. Am. 

Fig. 14.1 Schematic illustration of two chanically modified working electrode-solution interface configurations used in ionic liquids studies. Scheme 1 An array of microparticles adhered to an electrode in contact with bulk ionic liquid. Scheme 2 An electrode-microparticle array-ionic liquid layer-aqueous electrolyte microchemical configuration. Adapted with permission from Zhang et al., Anal. Chem. 2003, 75, 6938-6948 [23]. Copyright 2013, American Chemical Society...

Fig. 14.8a reveals that when the electrode is modified with a high mass of trans-Mn microparticles and a thick layer of [C4mim][PF6], a well-defined, diffusion-controlled, one-electron reversible oxidation process is observed. Once again, aside firom the current magnitude, the voltammogram obtained under microchemical conditions (Fig. 14.8a) is indistinguishable from that obtained from the adhered trans-Mn microparticles in contact with bulk [C4mim][PF6] (Fig. 14.7). 

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