Sol-Gel Modified Electrodes

Amorphous sol-gel materials have become increasingly appealing as electrode modifiers in the past 20 years (113). The initial interest in these materials was based on applications of their optical characteristics (silica, zirconia, and other transparent oxides) (113). As electrochemists sought convenient methods for combining different metal oxides in a crystalline product, their interest in sol-gel materials was sparked. 

Sol-gel materials possess attractive properties which can be exploited as electrode modifiers in electrochemical detection schemes (114, 115). These properties include high adsorption capacity, acid/base functionality, and thermal stability. Moreover, the inclusion of functional groups, enzymes, and proteins in sol-gels is easily achieved, expanding the possible applications. 

The term sol-gel is used to describe a broad class of solid structures created through gelation of a colloidal suspension (i.e sol) (113). Upon dehydration, a xerogel, or dry gel state forms. Subsequent heat treatment can be used to remove unreacted organic residues, increase stability and density, or introduce crystallinity (113). 

The chemical composition of sol-gels is typically a basic inorganic oxide. A common example of a sol-gel is silica gel, which has particular utility as stationary phase supports in chromatographic columns. The sol-gel materials used for electrode modification are typically created from two different classes of material inert and redox active. Many different types of sol-gels are derived from these classes, as outlined in Table 8.7 (113). 

The first class, inert inorganic sol-gels, are not redox active (e.g., silica, alumina, and zirconia) (113). In the gel form these materials have a large surface area, high ion-exchange capacity (due to surface hydroxyls), and exhibit good adhesion to metal oxide and ceramic supports. 

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Figure 16. The electrochemical response of a GOx ferrocene-sol-gel-modified electrode in the presence of glucose cyclic voltammogram before (a) and after (b) the addition of 10 mM glucose solution phosphate buffer, pH 5.6 potential scan rate, 10 mV s . Adapted from Ref [116a] with permission.

Figure 41. Square-wave voltammograms for 1.0 itiM Fe(CN)6 , 0.1 M KCl at (1) a bare graphite electrode, (2) a sol-gel-modified electrode and (3) a sol-gel-PDMAAC-modified electrode after a 10 min exposure time.

Figure 17. Schematic representation of electrocatalytic reduction of H202 at gold nanoparticles (Aunano) embedded in MTMOS silicate sol-gel modified electrode.

Figure 13. Effect of solution pH on pure sol—gel modified electrode in 2.8 mM [Ru(bipy)3], O.lMKCl. Working electrode sol-gel modified graphite film coating at 3500 rpm, 2 min immersion time 10 min. Error bars show standard deviation for four different electrodes. [Adapted...

After the addition of all the components necessary to obtain an electrode surface with the characteristics sought, sol-gel can be differently deposited onto a solid electrode surface for the formation of a film [211,216,217] or modeled in various configurations including bulky cylinders, rods, or fibers [218]. Alternatively, sol-gel-modified electrodes are produced by the screen-printing fabrication technique [210, 214]. 

A few examples of the huge number of applications of sol-gel-modified electrodes in electroanalysis are listed in Table 7.9, which also tries to pick up the advantages of such a material in different analytical contexts. 

The ion-sensor together with double junction reference electrode was dipped in the stirred electrochemical cell with a working volume of 15 ml. The electrode potential was monitored with a Orion pH meter model SA 520 and recorded. At the steady-state potentiometric response, varying concentrations of the ionic solution ( KCl, NH4CI, NaCl) were inj ted into the cell and the new steady-state potential was recorded. The measurements were made with the sol-gel modified electrode with and without the ionsensing membrane. 

The properties of sol-gels that are useful in electroanalysis include abrasion and corrosion resistance, ionic conductivity, optical transparency, and electrochromism. These, along with the ease with which sol-gels can be constructed with variable chemical composition have led to their use in electrocatalysis (116), sensing, and as robust reference electrodes (113). Other applications are summarized in references (113,115). Table 1 in (115) includes an in-depth list of useful sol-gel modified electrodes. 

Niu and Lee [86] Hypoxanthine Carp Xanthine oxidase (XO)/ bulk modified Sol-gel graphite electrode/-0.95 V Benzyl viologen... 

Meyer and coworkers investigated the photophysical behavior of vinyl containing Ru(II) and Os(II) complexes electropolymerized into the channels of silica sol-gel modified ITO electrodes. The monomeric complexes, [Ru(vbpy)3]2+ and [Os(vbpy)3]2+ (vbpy = 4-methyl-4/-vinyl-2,2/-bipyridine), have excited state lifetimes of approximately 900 and 60 ns, respectively. Incorporation into the sol-gel pores and polymerization (reductive polymerization initiated at the ITO electrode) results in chromophores that exhibit a remarkably small amount of self-quenching and have domains that reflect relatively isolated chromophores with excited state lifetimes longer than the solution values [125]. 

Sol-gels containing electroactive species have been used in the development of both amperometric and potentiometric electrodes. Films coated with anionic poly-(dimethyldiallylammonium chloride) (PDMDAAC) and cationic poly(vinylsulfonic acid) were used to concentrate Ru(bpy)3 + and the hexacyanoferrate anion, respectively, for use as amperometric electrodes [208a]. The detection limit by square-wave voltammetry improved by up to 50-fold compared with uncovered electrodes. In Figure 41, curve 1 corresponds to a bare graphite electrode, curve 2 to a sol-gel-covered electrode and curve 3 to a sol-gel-PDMDAAC-modified electrode after 10 min of exposure to Fe(CN)g. ... 

Prasad, B.B. Madhuri, R. Tiwari, M.P. Sharma, P.S. (2010c). Electrochemical sensor for folic acid based on a hyperbranched molecularly imprinted polymer-immobilized sol-gel-modified pencil graphite electrode. Sensors and Actuators, B Chemical, 146, 321-330. 

The solution of PVC casting membrane was made in dried THF of the (xxnposition PVC fine powder (56 mg) dibenzo-18-crown-6 (1 mg) dibutyl phfiialate (0.12 ml) tetraphenyl borate (0.75 mg) THF (1.3 ml). After complete dissoluticm of the membrane material 70 pi of the solution was added to the recessed depth of the electrode body covered with ferrocene-linked sol-gel glass film. The solvent (THF) was allowed to evaporate slowly over a 20-h period at room temperature (25 C). On complete evaporation of the solvent a transparent smooth layer of the sensing meml ane remained at the sur ce of modified sol-gel glass electrode. The resulting electrode was conditioned for 12 h in 1 M KCl solution. 

Solar energy, 6, 488 surface modified electrodes, 6, 30 Sol-Gel process fast reactor fuel, 6, 924 Solid state reactions, 1, 463-471 fraction of reaction, 1, 464 geometric, 1, 464 growth, 1, 464 nucleation, 1, 464 rate laws, 1,464 Solochrome black T metallochromic indicators, 1,555 Solubility... 

S.3.2 Sol-Gel Encapsulation of Reactive Species Another new and attractive route for tailoring electrode surfaces involves the low-temperature encapsulation of recognition species within sol-gel films (41,42). Such ceramic films are prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH3)4 under acidic or basic condensation, followed by polycondensation of the hydroxylated monomer to form a three-dimensional interconnected porous network. The resulting porous glass-like material can physically retain the desired modifier but permits its interaction with the analyte that diffuses into the matrix. Besides their ability to entrap the modifier, sol-gel processes offer tunability of the physical characteristics... 

Another electro-oxidation example catalyzed by bimetallic nanoparticles was reported by D Souza and Sam-path [206]. They prepared Pd-core/Pt-shell bimetallic nanoparticles in a single step in the form of sols, gels, and monoliths, using organically modified silicates, and demonstrated electrocatalysis of ascorbic acid oxidation. Steady-state response of Pd/Pt bimetallic nanoparticles-modified glassy-carbon electrode for ascorbic acid oxidation was rather fast, of the order of a few tens of seconds, and the linearity was observed between the electric current and the concentration of ascorbic acid. 

Fig. 2.12 Electrode modified with vitamin B12 by the sol-gel process and electrolysis of benzyl bromide at the electrode surface. Adapted from [61], H. Shimakoshi et al., Dalton Trans,...

Audebert, P. and Sanches, C. (1994) Modified electrodes from hydrophobic alkoxide silica gels — insertion of electroactive compounds as glucose oxidase. Journal of Sol-Gel Science and Technology, 2, 809-812. 

Tan, X.C.,Tian, Y.X., Cai, P.X. andZou, X.Y. (2005) Glucose biosensor based on glucose oxidase immobilized in sol—gel chitosan/silica hybrid composite film on Prussian blue modified glass carbon electrode. Analytical and Bioanalytical Chemistry, 381, 500-507. 

P.N. Deepa and S.S. Narayanan, Sol-gel coated Prussian blue modified electrode for electrocatalytic oxidation and amperometric determination of thiols. Bull. Electrochem. 17, 259-264 (2001). 

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