Electrode, surfactant film

Coordination of NO to the divalent tetrasulfonated phthalocyanine complex [Co(TSPc)]4 results in a complex formally represented as [(NO )Coin(TSPc)]4 kf= 142M-1s-1, KA 3.0 x 105 M-1). When adsorbed to a glassy carbon electrode, [Co(TSPc)]4- catalyzes the oxidation and reduction of NO with catalytic currents detectable even at nanomolar concentrations. Electrochemistry of the same complex in surfactant films has also been studied.905 Bent nitrosyl complexes of the paramagnetic trivalent tropocoronand complex Co(NO)(TC) ((189), R = NO) have also been reported.849... 

A.E.F. Nassar, W.S. Willis, and J.F. Rusling, Electron transfer from electrodes to myoglobin facilitated in surfactant films and blocked by adsorbed biomacromolecules. Anal. Chem. 67, 2386-2392 (1995). 

X.H. Chen, X.S. Peng, J.L. Kong, and J.Q. Deng, Facilitated electron transfer from an electrode to horseradish peroxidase in a biomembrane-like surfactant film. J. Electroanal. Chem. 480, 26-33 (2000). 

Rushng, J.F. and Z. Zhang (2002). Polyion and surfactant films on electrodes for protein electrochemistry. Electroanal. Meth. Bio. Mat. 195-231. 

Jiang, X.-E., L-P. Guo, and X.-G. Du (2003). Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate-surfactant film modified electrode. Talanta 61(3), 247-256. 

Both coulombic and hydrophobic interactions of reactants with adsorbed surfactant on electrodes are important in determining electron transfer kinetics. Reactants in micellar solutions and microemulsions can be preconcentrated into adsorbed surfactant films on electrodes [30], yielding mixed layers of reactants and nonelectroactive surfactants. Coulombic effects in micellar solutions may result in small kinetic enhancements when ionic reactants interact with oppositely charged surfactant adsorbed on electrodes. Partial inhibition of electron transfer can occur by coulombic repulsion if the charge sign on the reactant and adsorbed surfactant are the same. Hydrophobic molecules and ions may show a small amount of preconcentration on the electrode. 

There are several molecular scenarios for the delivery of miceUe-bound reactants into adsorbed surfactant films on electrodes. One possibility is dissociation (Eq. 17) followed by entry of the reactant into the aggregate film on the electrode, orientation near the surface, and electron transfer. Making the analogy between these latter processes and the adsorption rates of 3, entry into the films and orientation is expected to occur on a miUisecond timescale. 

Micellar rate enhancement of thermal and light-initiated biomolecular reactions often occurs via preconcentration of the two reactants in the micelles [3]. In electrochemical catalysis (Scheme 2), the analogous situation can occur for participants in biomolecular reactions coadsorbed into surfactant films on electrodes. Interfacial... 

Equation (20) shows that feobs is enhanced hy compartmentalization of all the reactants into micellar volume Vt< M- In electrochemical catalysis in micellar solutions, large rate enhancements are observed when the reaction occurs in surfactant aggregates on the electrode surface [4, 30]. In this case, the compartmentalization volume where the hiomolecular r.d.s. occurs is that of the surfactant film on the electrode. 

Since the pioneering studies of Hill (8) and Kuwana [9], the most successful electrodes for proteins have been noble metals (Au or Ag) modified with various adsorbates, or materials such as carbon or metal oxides that have natural surface functionalities [10-18]. Conducting metal oxides are often optically transparent, and thus provide additional possibilities for spectral studies, while a further development has been the modification of electrode surfaces with surfactant films [19-21]. Examples of... 

The fourth and fifth sections deal with amphiphiles at solid-liquid interfaces. Section four on Amphiphiles at Electrode Surfaces contains a collection of important applications. The first by Rusling illustrates how surfactant films on electrode surfaces may be fabricated to electrocatalyze a variety of reactions. Koglin and coworkers provide some exciting new surface enhanced Raman spectroscopy results that contravene some of the reigning dogma on how cationic surfactants assemble on anionic surfaces. Kaifer presents a concise review of self-... 

In this paper, we review recent progress in achieving direct electron transfer between electrodes and proteins by using surfactant films. We shall see that these films have been extended successfully to a number of redox proteins and enzymes. Specific applications, including films with catalytic activity, are also discussed. Such enzyme films have future applications as models for investigating the fundamental chemistry of normal and disease state processes, as biosensors, and as catalysts for fine chemical synthesis. 

Table 1 Formal potentials and electron transfer rate constants for thin (0.5-1 nm) myoglobin-surfactant films on PG electrodes in pH 7 buffer containing no protein...

Cyclic and pulsed voltammetric studies of Mb-surfac-tant films have been used to obtain electrochemical parameters such as electron transfer rate constants and formal potentials E° ), i.e. apparent standard potentials under given experimental conditions. Recent work has shown that a Gaussian distribution model for protein molecules with slightly different ° -values fits voltammetric data in thin surfactant films [26, 33]. This model was used with nonlinear regression to extract average ° and values from square wave voltammograms (SWV). Simple models for voltammetry of single species confined to the electrode surface did not fit the data, but formal potentials could be estimated from the midpoints of CV cathodic and anodic peaks. 

Comparisons must take into account that at the present level of analysis, the kinetic constants are reproducible to roughly + 20%. In thin Mb films, values of k follow the trend DHP > DMPC > DDAB, but the dependence is not strong (Table 1). Similarly, the dependence of k on surfactant type was weak for thick liquid crystal films of most of the surfactants in Fig. 2 at pH 5.5 [24]. The dependence of k° on electrode material measured for thick films was also weak, and we can conclude only that Pt, PG > Au, ITO (Table 2). Surfactant films on ITO are much less stable than on the other electrodes [32], so that metal and carbon electrodes are preferable except for spectroelectrochemistry. 

Surfactant films facilitate electron transfer with electrodes for a number of proteins in addition to Mb and cyt P450aam- The protein hemoglobin (Hb) was incorporated... 

---