Precursor interaction with electrode

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... 

Monolayers as modifiers on electrode surfaces are prepared by sorption techniques. Self-assembled monolayer s (SAMs) form by spontaneous adsorption of precursors on the electrode substrate with an ordered orientation by interaction of certain molecule areas protruding from the sorption plane [109—112] most popular are SAMs prepared by thiol compounds on gold surfaces, but also SAM formation on CPEs is possible. Langmuir-Blodgett layers (LBLs) are monolayers made by surfactants by adsorption from the surface of a solution [113]. Highly ordered multistacked structures from polyelectrolytes can be obtained by layer-by-layer deposition, where the ensuing plane of the stack can be immobilized by sorption of a countercharged polyelectrolyte [114-116]. 

These situations are depicted in Fig. 1 for a multistep reaction Ox -I- ne = R. In this figure, A denotes the transition state and P and S, respectively, denote the precursor and successor states that precede and follow the transition state. The full Kne represents the energetics of the process for a large distance between reactive species and electrode. If the reacting species come closer to the electrode surface and these species have attractive interactions with the electrode surface, the dashed and dotted lines are obtained, where the dotted line represents a stronger overlap than the dashed line. In this simplest model, the transition state corresponds to the same reaction coordinate. 

Lactate is a small biological molecule that functions as metabolite in the mitochondria and a precursor to pyruvate in the citric acid cycle [87]. In 1997, Bardea et al. developed a lactate BFC using NAD -dependent lactate dehydrogenase (LDH) [88]. They introduced a new method for enzyme immobilization that enabled better oxidation of the substrate and allowed the enzyme to have eleetrieal eontact with the electrode. Covalently linked PQQ and native NAD" form a monolayer on gold electrodes to induce affinity interactions with cross-linked NAD -dependent LDH. In 2001, Katz et al. further improved on the concept of this anode, coupling it with a cytochrome c oxidase cathode to produce a self-powered biosensor that is active only in the presence of the anode s substrate, lactate [89]. With this addition, the cell is completely dependent on substrate for voltage and current output, which is ideal for BFCs. 

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

Because the formation of an inner-sphere precursor state involves specific chemical interactions between the reactant and the electrode surface, it is difficult to calculate the precursor-complex equilibrium constant. However, such states can be sufficiently stable so that the electrode coverage by the precursor complex approaches unity (i.e., a monolayer of adsorbed reactant is formed). In these circumstances, the observed rate becomes independent of the bulk reactant concentration, and k, can be obtained directly from combined with the estimated close-packed surface concentration. An analogous situation exists for stable precursor complexes formed in homogeneous solution ( 12.3.3.1). 

Additional alterations in the work terms with the electrode material for outer-sphere reactions may arise from discreteness-of-charge effects or from differences in the nature of the reactant-solvent interactions in the bulk solution and at the reaction plane. Thus metals that strongly chemisorb inner-layer solvent (e.g., HjO at Pt) also may alter the solvent structure in the vicinity of the outer plane, thereby influencing k bs variations in the stability of the outer-sphere precursor (and successor) states. Such an effect has been invoked to explain the substantial decreases (up to ca. 10 -fold) in the rate constants for some transition-metal aquo couples seen when changing the electrode materiaf from Hg to more hydrophilic metals such as Pt. Much milder substrate effects are observed for the electroreduction of more weakly solvated ammine complexes . 

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