Publications

Abstract Graphene and its derivatives are generally portrayed as electron transfer enhancers that effectively boost the electrochemical response of classic electrodes for applications in renewable energy, electronics, or analysis (amongst others). However, a number of fundamental studies have challenged this view. In certain reports, not only could no beneficial effect be demonstrated, but the opposite was concluded. If we want to advance towards a more rational design of high-performance electrode devices, these discrepancies need to be cleared and the fundamental aspects of electron transfer reactions through graphene-electrodes further understood. The present study contributes to this cause by exploring the relationships between the structure and morphological appearance of graphene films and their electrochemical performance in fundamental proof-of-concept experiments. The results unveil that important differences in the structure and morphology of the films (which are tightly related to the composition and load of graphene materials) govern the electrochemical response of the modified electrodes. Thereby, a possible explanation for the apparently contradictory conclusions reported in the literature is provided.

Abstract Chitosan is a natural polysaccharide which strongly interacts with whey proteins in solution. Certain protein-polyelectrolyte complexes and electrostatically assembled thin films have been shown to be good platforms for the preservation of globular and small proteins in their native state mainly due to the hydrophilic nature of these polyelectrolytes and to the lack of physical space for embedded proteins to relax. As a consequence, the use of natural polyelectrolytes in new nanocomposite materials for medicine, chemical analysis, catalysis, etc. has exploded in the last few years. Nevertheless, in many of these applications proteins are immobilized in more open structures without physical restrictions to undergo conformational changes. In this work, we investigate the nature of these transitions for a model whey protein, beta-LG, on a chitosan-decorated Au surface in a scenario for which protein-surface interactions compete with boosted protein-protein non-coulombic forces. The adsorption kinetics, protein mass uptake (plus associated water), and flexibility of the adsorbed layers have been followed in situ by the quartz crystal microbalance with dissipation monitoring (QCM-D). Further ex situ characterization has been performed by atomic force microscopy (AFM) and non-invasive scanning electron microscopy (SEM). The balance between both types of interactions yielded surfaces heavily loaded with protein and water in which orientational transitions seemed restricted. The kinetics of the process was registered in a wide range of concentrations and successfully fitted to a double exponential equation derived from the RSA theory accounting for the establishment of slow post-adsorption conformational transitions.

Abstract A new methodology for the covalent functionalization of a SAM of 11-amino-1-undecanethiol, previously adsorbed on polycrystalline Au, was successfully applied in order to immobilize a beta-CD layer on surface. A two steps synthetic strategy is proposed based on the activation of the SAM with di-(N-succinimidyl) carbonate for the further inclusion of the beta-CD. The modification of the SAM was followed by PM-FTIRRAS, AFM imaging, CV, and EIS which confirmed the introduction of beta-CD layer. The AFM images allowed the identification of homogeneously distributed beta-CD aggregates over the Au grain microstructure. The electrode was characterized in the presence of electroactive species in solution, with the ability to form inclusion complexes with the beta-CD cavity. Contrary to the reported for other thiolated CD derivative films, the results of this study showed the formation of well-packed and compact structures which strongly reduce non-specific adsorption phenomena. The redox response of the probes at the beta-CD electrode was shown to appear at higher potentials with respect to the response at bare Au. Good correlation was found between the increase of the hydroquinone oxidation peak and, both, the scan rate used in CV experiments (typical behavior of surface-confined species) and the hydroquinone concentration. In the case of dopamine, the processes seem to shifted out of the potential window of SAM stability. The results suggest that this problem could be overcome by improving the design of the device.

Abstract The electrochemical oxidation of CO has been studied on Pt(S)[(n - 1)(1 0 0) x (1 1 0)] electrodes to investigate the effect of the step density in the reaction. This series shows two different trends for long (n >= 7) and short terraces. For long terraces, the voltammetric peak shifts towards higher potential as the step density increases, unlike the behaviour observed for other stepped surfaces, which exhibit the opposite behaviour in agreement with the Smoluchowski effect. For short terraces, the "normal" behaviour is observed, that is, as the step density increases the peak shifts towards lower potentials. Chronoamperometric measurements were used to determine rate constants and Tafel slopes using the mean field Langmuir-Hinselwood kinetics. Rate constants follow the same trends as the peak potentials in voltammetry. A Tafel slope of 75 +/- 4 mV has been obtained for the surfaces with long terraces whereas a value of the surfaces with short terraces showed a value of 100-120 mV is obtained. This change of slopes is interpreted as a change in the electrochemical behaviour of the species involved in the mechanism, probably, a change in the adsorption isotherm of adsorbed OH. Pt(5 10) electrode exhibits an intermediate behaviour between those of long and short terraces with two different peaks that can be associated with both behaviours previously described.

Abstract The desorption of a self-assembled monolayer (SAM) of 11-amino-1-undecanethiol (AUT) formed on an Au polycrystalline electrode was investigated with the purpose of establishing the most efficient method for SAM removal prior to electrode re-Use. The cleanliness of the surface was evaluated by assessing the characteristics (and their reproducibility) of a newly prepared SAM (on a freshly cleaned electrode) in the presence of Ru(NH3)(6)(3+/2+) probe species. The simple flame annealing of the modified electrodes showed poor reproducibility. Later, cleaning treatments based on previous reports about the electrochemical desorption of SAMs, were investigated in NaOH, HClO4, and KCl solutions. The anodic removal in alkaline and chloride solutions was not efficient enough due to the effect of the Au surface oxidation and dissolution. The commonly used reductive desorption in alkaline solution did also not offer a high efficiency (in contrast with the behavior described in those reports) probably due to the low stability of the thiolate molecules at high-pH values. In addition, such procedures did not provide a way to visualize the evolution of the process. The oxidative desorption in perchloric acid showed a higher efficiency at each single desorption cycle, and allowed to monitor the extension of the removal by comparing the cyclic voltammograms obtained in that medium with the expected fingerprint for the bare Au. However the voltammetric characterization of new AUT films, prepared on such electrochemically treated surfaces by re-incubation in the AUT solution, showed that the conditions of substrate cleanliness and smoothness necessaries to reproduce a close-packed and compact AUT monolayer were not reached. A new cleaning strategy based on the coupled use of oxidative removal in HClO4 0.1 M and flame annealing showed to be efficient and reproducible, providing the proper substrate precursors for the formation of highly ordered AUT films. The results also showed that the methodology works well in the removal of other SAMs like the one formed by the neutral 1-dodecanethiol (1-DT).

Abstract Self-assembled monolayers (SAMs) of 11-amino-1-undecanethiol (AUT) have been prepared on polycrystalline Au by immersion of the corresponding surfaces in an AUT 1 mM solution in pure ethanol. The films were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and quartz crystal microbalance (QCM). Results of CV and EIS experiments in NaClO4 solution agree with the fast formation of a well-packed film with low current density (hundreds of nA/cm(2)) and capacitance values around 2 mu F/cm(2). The films were stable between -0.7 and 0.7 V (vs Ag/AgCl/NaCl sat.). Average values of 1.6 nm and 26 degrees were obtained for the film thickness and the tilt chain angle in the potential region under study. The kinetic analysis of the adsorption process, monitored in situ by the QCM technique, showed that it occurs in two stages: a fast Langmuir type adsorption step (k(1) = 0.1047 min(-1)), followed by a much slower process of molecular rearrangement (k(2) = 0.0020 min(-1)). AFM operated in tapping mode did not reveal any morphological changes on the surface after immersion in AUT discarding multilayer growth. The electron transfer (ET) of Fe(CN)(6)(-3) and Ru(NH3)(6)(+3) species in solution through the AUT layer was investigated by CV and EIS. A mechanism of selective permeation of the electroactive species across the monolayer, controlled by the nature of the electrostatic interactions established at the SAM-solution interface, explains the experimental data obtained and previously reported in the literature for ET processes through substituted SAMs. Analysis of the film structure according to theoretical models (commonly used for SAM characterization) to our experimental data, led to contradictory results clearly affected by the nature of the unconsidered electrostatic interactions (values of theta = 0.80 and 0.99 were obtained in the presence of Fe(CN)(6)(-3) and Ru(NH3)(6)(+3), respectively, in KCl electrolyte using the Amatore model).

Abstract Oxygen reduction on platinum single crystals has been studied in perchloric and sulfuric acid media using a hanging meniscus rotating disk electrode configuration. The surfaces studied belong to the [01 (1) over bar] zone and can be classified in two different series, i.e., surfaces with terraces with (111) symmetry and (100) steps and surfaces with (100) terraces and (111) steps. In sulfuric acid media, the highest catalytic activity is found for the Pt(211) electrode whereas the Pt(111) electrode exhibits an anomalously low j(0) value. Conversely, the extrapolated value for the catalytic activity of Pt(111) obtained from the series of stepped surfaces having (111) terraces is much higher than that obtained experimentally. The comparison with the results obtained in perchloric acid solutions indicates that the anomalously low catalytic activity of the Pt(111) electrode in sulfuric acid solutions is related to the formation of an ordered adlayer of specifically adsorbed (bi)sulfate anions. For practical purposes, the effect of the step density and the (bi)sulfate specific adsorption is small, except for the Pt(111) electrode.