RESUMO

The site geometry preference of CO binding on stepped Pt single crystals in alkaline solution was investigated by in situ FTIR spectroscopy. The surfaces of the Pt single crystals consisted of different width (111) terraces, interrupted by (110) or (100) monatomic steps. Experiments carried out with CO adsorbed exclusively on the top of the steps revealed that only linearly bonded CO formed on the (110) steps, while two CO binding geometries (linear and bridge) were observed on the (100) steps. On one hand, for CO adsorbed only on the steps, the positions of the bands corresponding to linearly bonded CO were similar, regardless of the density of steps, suggesting the existence of an interaction between COads only along the line of the steps. On the other hand, for full CO coverage, the CO stretching frequencies and the geometry of bound CO were sensitive to the width of the (111) terraces and the step orientations. Consequently, the CO binding sites favored linearly bonded CO for surfaces consisting of shorter (111) terraces and (110) steps. Bridge-bonded CO was favored on surfaces consisting of shorter (111) terraces interrupted by (100) steps. In order to understand the origin of the preference of CO binding sites, the results were compared to the corresponding behavior in acid media, which revealed that, in addition to the effect inherent to the Pt surface, the charge on the metal side in an aqueous environment should be taken into consideration. The analysis suggested that the CO adlayers formed at full coverage in acidic and alkaline media had different structures. On the other hand, the structure of the layer of CO adsorbed only at the steps was independent of pH.

RESUMO

Amphotericin B (AMB) is an antifungal drug that binds to ergosterol and forms pores at the cell membrane, causing the loss of ions. In addition, AMB induces the accumulation of reactive oxygen species (ROS), and although these molecules have multiple deleterious effects on fungal cells, their specific role in the action mechanism of AMB remains unknown. In this work, we studied the role of ROS in the action mechanism of AMB. We determined the intracellular induction of ROS in 44 isolates of different pathogenic yeast species (Candida albicans, Candida parapsilosis, Candida glabrata, Candida tropicalis, Candida krusei, Cryptococcus neoformans, and Cryptococcus gattii). We also characterized the production of ROS in AMB-resistant isolates. We found that AMB induces the formation of ROS in all the species tested. The inhibition of the mitochondrial respiratory chain by rotenone blocked the induction of ROS by AMB and provided protection from the killing action of the antifungal. Moreover, this phenomenon was absent in strains that displayed resistance to AMB. These strains showed an alteration in the respiration rate and mitochondrial membrane potential and also had higher catalase activity than that of the AMB-susceptible strains. Consistently, AMB failed to induce protein carbonylation in the resistant strains. Our data demonstrate that the production of ROS by AMB is a universal and important action mechanism that is correlated with the fungicidal effect and might explain the low rate of resistance to the molecule. Finally, these data provide an opportunity to design new strategies to improve the efficacy of this antifungal.

Assuntos

RESUMO

The electrochemical reactivity of catechol-derived adlayers is reported at platinum (Pt) single-crystal electrodes. Pt(111) and stepped vicinal surfaces are used as model surfaces possessing well-ordered nanometer-sized Pt(111) terraces ranging from 0.4 to 12 nm. The electrochemical experiments were designed to probe how the control of monatomic step-density and of atomic-level step structure can be used to modulate molecule-molecule interactions during self-assembly of aromatic-derived organic monolayers at metallic single-crystal electrode surfaces. A hard sphere model of surfaces and a simplified band formation model are used as a theoretical framework for interpretation of experimental results. The experimental results reveal (i) that supramolecular electrochemical effects may be confined, propagated, or modulated by the choice of atomic level crystallographic features (i.e.monatomic steps), deliberately introduced at metallic substrate surfaces, suggesting (ii) that substrate-defect engineering may be used to tune the macroscopic electronic properties of aromatic molecular adlayers and of smaller molecular aggregates.

RESUMO

Ethanol oxidation on platinum stepped surfaces vicinal to the (111) pole modified by tin has been studied to determine the role of this adatom in the oxidation mechanism. Tin has been slowly deposited so that the initial stages of the deposition take place on the step, and deposition on the terrace only occurs when the step has been completely decorated. Voltammetric and chronoamperometric experiments demonstrate that tin on the step catalyzes the oxidation. The maximum enhancement is found when the step is completely decorated by tin. FTIR experiments using normal and isotopically labeled ethanol have been used to elucidate the effect of the tin adatoms in the mechanism. The obtained results indicate that the role of tin is double: (i) when the surface has sites capable of breaking the C-C bond of the molecule, that is, when the step sites are not completely covered by tin, it promotes the oxidation of CO formed from the molecular fragments to CO(2) through a bifunctional mechanism and (ii) it catalyzes the oxidation of ethanol to acetic acid.

RESUMO

This manuscript compares the electrochemically controlled adsorption of hydroquinone-derived adlayers and their reductive desorption from nanometer-sized Pt(111) domains present on the surface (i) of model stepped single-crystal electrodes and (ii) of preferentially oriented Pt nanoparticles. The results obtained using a stepped surface series, i.e., Pt(S)[(n - 1)(111)x(110)], suggest that in the presence of 2 mM H(2)Q((aq)) the electrochemically detected desorption-adsorption process takes place selectively from ordered Pt(111) domains present as terraces, while being precluded at other available surface sites, i.e., Pt(110) steps, where adsorption takes place irreversibly. This domain-selective electroanalytical detection scheme is employed later to selectively monitor desorption-adsorption of hydroquinone-derived adlayers from ordered, nanometer-scaled Pt(111) domains on the surface of preferentially oriented Pt nanoparticles, confirming the existence of well-ordered (111) domains on the surface of the Pt nanoparticles. A good correlation is noted between the electrochemical behavior at well-ordered Pt(hkl) surfaces and at preferentially oriented Pt nanoparticles. Key learnings and potential applications are discussed. The results demonstrate the technical feasibility of performing domain-selective decapping of nanoparticles by handle of an externally controlled parameter, i.e., the applied potential.

RESUMO

Ethanol oxidation has been studied on stepped platinum single crystal electrodes in acid media using electrochemical and Fourier transform infrared (FTIR) techniques. The electrodes used belong to two different series of stepped surfaces: those having (111) terraces with (100) monoatomic steps and those with (111) terraces with (110) monoatomic steps. The behaviors of the two series of stepped surfaces for the oxidation of ethanol are very different. On the one hand, the presence of (100) steps on the (111) terraces provides no significant enhancement of the activity of the surfaces. On the other hand, (110) steps have a double effect on the ethanol oxidation reaction. At potentials below 0.7 V, the step catalyzes the C-C bond cleavage and also the oxidation of the adsorbed CO species formed. At higher potentials, the step is not only able to break the C-C bond, but also to catalyze the oxidation of ethanol to acetic acid and acetaldehyde. The highest catalytic activity from voltammetry for ethanol oxidation was obtained with the Pt(554) electrode.

RESUMO

The electrochemical reactivity of hydroquinone-derived adlayers (Q((ads))) is compared at basal Pt(hkl) single-crystal surfaces, revealing that the electrochemically controlled desorption of Q((ads)) is a highly selective surface reaction. At well-ordered Pt(111) single-crystal surfaces, classical electrochemical methods are combined with in situ SNIFTIRS measurements to demonstrate that the reductive desorption of Q((ads)) and their full oxidative readsorption can be achieved, even in the presence of hydroquinone solution (H(2)Q(aq)), by controlling the potential of Pt(111) electrodes. At well-ordered Pt(111) domains, the presence of vertically adsorbed molecules within the Q((ads)) adlayer is deduced from the spectroelectrochemical SNIFTIRS measurements. The desorption mechanism, detected voltammetrically at Pt(111) electrodes, is precluded at well-ordered Pt(110) and Pt(100) single-crystal electrodes immersed in hydroquinone-containing solutions, requiring the presence of well-ordered Pt(111) surface domains in order to be detected. In clean supporting electrolyte, the partial desorption of Q((ads)) layers may take place, but predominantly from minority surface imperfections at Pt(110) and Pt(100) via a different mechanism than at Pt(111) surface domains.

RESUMO

Ethanol oxidation has been studied on Pt(111), Pt(100) and Pt(110) electrodes in order to investigate the effect of the surface structure and adsorbing anions using electrochemical and FTIR techniques. The results indicate that the surface structure and anion adsorption affect significantly the reactivity of the electrode. Thus, the main product of the oxidation of ethanol on the Pt(111) electrode is acetic acid, and acetaldehyde is formed as secondary product. Moreover, the amount of CO formed is very small, and probably associated with the defects present on the electrode surface. For that reason, the amount of CO2 is also small. This electrode has the highest catalytic activity for the formation of acetic acid in perchloric acid. However, the formation of acetic acid is inhibited by the presence of specifically adsorbed anions, such as (bi)sulfate or acetate, which is the result of the formation of acetic acid. On the other hand, CO is readily formed at low potentials on the Pt(100) electrode, blocking completely the surface. Between 0.65 and 0.80 V, the CO layer is oxidized and the production of acetaldehyde and acetic acid is detected. The Pt(110) electrode displays the highest catalytic activity for the splitting of the C-C bond. Reactions giving rise to CO formation, from either ethanol or acetaldehyde, occur at high rate at any potential. On the other hand, the oxidation of acetaldehyde to acetic acid has probably the lower reaction rate of the three basal planes.

Assuntos