Direct electron transfer (DET) from bare spectrographic graphite (SPGE) or 3-mercaptopropionic acid-modified gold (MPA-gold) electrodes to Trachyderma tsunodae bilirubin oxidase (BOD) was studied under anaerobic and aerobic conditions by cyclic voltammetry and chronoamperometry. On cyclic voltammograms nonturnover Faradaic signals with midpoint potentials of about 700 mV and 400 mV were clearly observed corresponding to redox transformations of the T1 site and the T2/T3 cluster of the enzyme, respectively. The immobilized BOD was differently oriented on the two electrodes and its catalysis of O(2)-electroreduction was also massively different. On SPGE, where most of the enzyme was oriented with the T1 copper site proximal to the carbon with a quite slow ET process, well-pronounced DET-bioelectroreduction of O(2) was observed, starting already at >700 mV vs. NHE. In contrast, on MPA-gold most of the enzyme was oriented with its T2/T3 copper cluster proximal to the metal. Indeed, there was little DET-based catalysis of O(2)-electroreduction, even though the ET between the MPA-gold and the T2/T3 copper cluster of BOD was similar to that observed for the T1 site at SPGE. When BOD actively catalyzes the O(2)-electroreduction, the redox potential of its T1 site is 690 mV vs. NHE and that of one of its T2/T3 copper centers is 390 mV vs. NHE. The redox potential of the T2/T3 copper cluster of a resting form of BOD is suggested to be about 360 mV vs. NHE. These values, combined with the observed biocatalytic behavior, strongly suggest an uphill intra-molecular electron transfer from the T1 site to the T2/T3 cluster during the catalytic turnover of the enzyme.

The desorption of 2-mercaptoethanesulfonate (MES) spontaneously adsorbed on Au(111) has been studied by using both potential-step and voltammetric experiments. From the amount of gold oxide formed during the oxidation sweep in the fingerprint region it is shown that the adsorption process induces structural changes of the gold surface. It is also shown that together with the reductive desorption of MES ions a concomitant faradaic process occurs. The results suggest that this process is connected to the reduction of solvent on the structurally modified gold electrode. The reductive desorption process of MES undergone at more negative potentials is characterized by a single peak in the voltammetric response and the presence of a maximum in the chronoamperogram. It is shown that the logarithm of the maximum current, the time at which the maximum current appears, and the peak width at half height depend linearly on potential. An experimental protocol for the desorption/readsorption of MES based on a potential-step experiment followed by cyclic voltammetry is outlined as an appropriate tool to analyze simultaneously the desorption of adsorbed and readsorbed MES. A linear dependence between the two desorption peak potentials and the surface concentration of adsorbate was observed. Changes in the environment surrounding the adsorbed moities and in the potential of zero charge of the electrode are shown to be the factors ruling this dependence.

A new method is proposed for the determination of the potential of zero charge of gold electrodes modified with thiol monolayers. It makes use of the immersion technique, in combination with a vapor deposition protocol to build the thiol monolayers. As compared to previous methods, the present approach provides more accurate results, particularly in the case of long-chain alkanethiol monolayers, and it is applicable to any monolayer irrespective of its degree of hydrophilicity. Results are presented for a series of 12 alkanethiol monolayers and for 11-mercaptoundecanol and 11-mercaptoundecanoic acid monolayers. Good agreement is found between the variation of the potential of zero charge along the alkanethiol series with the corresponding change of the surface work function. The potential of zero charge of the 11-mercaptoundecanoic acid monolayer is shown to depend on the extent of dissociation of the acid, thus opening the possibility of applying this type of measurements to the study of surface ionization processes.

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A model for the potential distribution at the interface between a gold electrode modified by a self-assembled monolayer, in which some units terminate in an ionizable head group, and an electrolyte solution is presented. Estimation of the local potential at an ionizable head group is based on the hexagonal array model by Barlow and Macdonald in which the role of the nearby dielectric discontinuities is considered. The effect of the electrode potential on ionization of the head groups and on the differential capacity of the interface is estimated by using this model, and the results are compared with those ignoring the three-dimensional nature of the potential distribution and those obtained from the cutoff disc model. Capacity data obtained recently at gold electrodes modified by mercaptopyridine and mercaptoaniline are analyzed on the basis of the model presented.

The formation and reductive desorption of self-assembled monolayers of 6-mercaptohexanol on mercury has been studied by using cathodic stripping voltammetry and capacitative transients, including the possibility of expanding or contracting the electrode area at the end of the preconcentration step. Experimental evidence shows the existence of three sequential stages during the formation of a thiol self-assembled monolayer. Each of these stages can be associated to the presence of (i) a low surface density state of oxidized thiol molecules, characterized by a single electrodimerization wave, (ii) a high surface density state, characterized by the emergence of a second voltammetric wave, and (iii) an ordered monolayer, which gives rise to a voltammetric spike. On the basis of electrode expansion experiments, a method is described to determine the surface concentrations of oxidized products, which does not require a baseline subtraction of the voltammograms to account for the nonfaradaic current. Quantitative voltammetric fits are consistent with the initial formation of a mixture of noninteracting monomers and dimers of oxidized thiol. The value of the maximum surface concentration and the ability to block the Ru(NH 3 ) 6 3+ electron transfer reveal that oxidized thiol molecules adopt a nearly perpendicular orientation in the high surface density state, which hampers ionic permeation. A theoretical model is proposed to account for the observed voltammetric behavior. The transition from the lower to the higher surface density states is modeled as a chemical step involving the exchange of metal free sites. Capacitative transients are also interpreted in terms of the three-stages model.

Packing restrictions and hydrophobic interactions are likely to lead to a spatial distribution of redox centers in electroactive monolayers. A mean field analysis of the electrochemical implications of spatial redox dispersion in SAMs, including the possibility of surface ion pair formation, has been carried out. The boundary value problem associated with a layered distribution of potential-induced charges has been solved by using the orthogonal collocation technique under equilibrium conditions. Spreading of the redox centers into a 3D dielectric slab results in broader and asymmetric voltammograms, reflecting a layer-by-layer redox conversion. It is also shown that the voltammetric shape is sensitive to the specific features of the spatial redox distribution, and theoretical requirements for the appearance of asymmetric broadening are examined in terms of the electrostatic properties of the monolayer. It is suggested that this type of spatial inhomogeneity may cause some of the broad and asymmetric voltammetric shapes that often characterize the electrochemical behavior of electroactive SAMs, and that some structural information can be gained from the analysis of these voltammograms, as long as electrolyte ions do not permeate the organic monolayer. The effect of surface ion association on the voltammetric features is also examined, and it is interpreted in terms of the distinct sensitivity of the potential at each redox plane with respect to the local counterion concentration. Comparison is made with the experimental results of Chidsey et al. (J. Am. Chem. Soc. 1990, 112, 4301) for the oxidation of FcCO 2 (CH 2 ) 11 SH/CH 3 (CH 2 ) 9 SH and Fc(CH 2 ) 16 SH/CH 3 (CH 2 ) 15 SH mixed monolayers.

The influence of ion pairing on the potential distribution at an electrode modified with a redox-active selfassembled monolayer is considered. The voltammetric response of the film is modeled as a function of its dielectric properties and the extent of association between the redox centers and counterions in solution. On the basis of the scheme of squares, a general treatment for any number of association steps is introduced. In addition, the effect of the counterion selective permeation on the voltammetric features is presented. It is shown that ion pairing and double-layer effects are strongly coupled, and may lead in some situations to the same variation of the peak potential with electrolyte concentration. The possibility of obtaining association parameters from voltammetric experiments is discussed. Comparison is made with Rowe and Creager's data on the oxidation of mixed monolayers of ferrocene-n-hexanethiol and n-alkanethiols.