Osvaldo R. Cámara

Adsorption of human serum albumin (HSA) onto colloidal TiO2 particles, Part I.

The adsorption of human serum albumin (HSA) onto colloidal TiO2 (P25 Degussa) particles was studied in NaCl electrolyte at different solution pH and ionic strength. The HSA-TiO2 interactions were studied using adsorption isotherms and the electrokinetic properties of HSA-covered TiO2 particles were monitored by electrophoretic mobility measurements. The adsorption behavior shows a remarkable dependence of the maximum coverage degree on pH and was almost independent of the ionic strength. Other characteristic features such as maximum adsorption values at the protein isoelectric point (IEP approximately 4.7) and low-affinity isotherms that showed surface saturation even under unfavorable electrostatic conditions (at pH values far away from the HSA IEP and TiO2 PZC) were observed. Structural and electrostatic effects can explain the diminution of HSA adsorption under these conditions, assuming that protein molecules behave as soft particles. Adsorption reactions are discussed, taking into account acid-base functional groups of the protein and the surface oxide in different pH ranges, considering various types of interactions.

Hydrogen evolution reaction on anodic titanium oxide films

Abstract Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H 2 SO 4 . The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HClO 4 or H 2 SO 4 solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. The log i vs E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F . The influence of the physico-chemical properties of the oxide films on the HER has also been studied from the changes in the current—potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.

Influence of the hydrogen evolution reaction on the anodic titanium oxide film properties

Abstract The influence on the physico-chemical properties of electroformed titanium oxide film produced by the hydrogen evolution reaction (HER) was studied. The relation between the cathodic treatment in the HER potential range and the modifications produced in the oxide film was established from current density and differential capacitance measurements, as a function of potential. The products of the oxide film dissolution were identified when this was subject to a prolonged cathodic treatment. The formation of Ti(III) species was found. Depending on the kind of applied cathodic treatment, these species precipitated on the electrode surface or diffused to the bulk of the solution.

Potentiodynamic behaviour of mechanically polished titanium electrodes

Abstract The behaviour of titanium electrodes mechanically polished and/or anodically polarized at low positive potential in solutions at constant ionic strength between pH 0.3 and 11.0 is reported. The oxide electroformation potential on a mechanically polished electrode shows a complex dependence on the bulk solution pH. This dependence is similar to that obtained through acid-base titration with titanium as the indicating electrode. The formation of hydroxo-complexes on the spontaneously formed titanium oxide offers a possible explanation for the oxide electroformation potential dependence on pH. Anodic and cathodic wide current peaks are obtained between the potential of the hydrogen evolution and that of the massive oxide electroformation; the corresponding redox system becomes evident at pH 4.0 from the first potentiodynamic cycle. An interpretation of these processes involving the participation of non stoichiometric oxides and hydrogen ions is attempted.

Open circuit potential measurements with Ti/TiO2 electrodes

The open circuit potential (OCP) vs pH response of Ti/TiO2 electrodes prepared by thermal and electrochemical oxidation of metallic titanium was measured in KNO3 aqueous solutions in order to establish whether the slopes of the OCP-pH curves can be used as a measure of the variation of surface potential (ψo) of TiO2 with the pH of the aqueous solution. For comparison purposes, ψo—pH slopes were also evaluated from surface charge-pH data obtained by acid-base potentiometric titrations of TiO2 dispersions. Ti/TiO2 electrodes showed a linear OCP-pH response in the range of ph 5–10 with slopes of −0.039 ± 0.005 V ph−1. The OCP-pH dependence of Ti/TiO2 electrodes was lower than that predicted thermodynamically. According to the triple-layer model, used to describe the oxide/aqueous solution interface, the OCP-pH slopes obtained for Ti/TiO2 electrodes cannot be identified with the variation of ψo with the pH. Calculations with this model indicated that neither the single nor the double extrapolation methods used to evaluate the intrinsic ionization constant of oxide surface sites render realistic values of these parameters for the TiO2 surface. The analysis of surface charge—pH data and the use of the triple-layer model to predict experimental values indicated that the ψo—pH slopes of TiO2 surfaces must be greater (in absolute magnitude) than 0.041 V pH−1.

Electrochemical behaviour of human serum albumin–TiO2 nanocrystalline electrodes studied as a function of pH: Part 1. Voltammetric response

The interaction of human serum albumin (HSA) with the surface of nanocrystalline Ti/TiO2 electrodes was studied using electrochemical techniques in 0.1 M NaCl at different solution pHs. The adsorbed protein layer that is formed immediately when the protein becomes in contact with the electrode was investigated as a function of solution pH and electrode potential. The surface electrochemical properties were studied by cyclic voltammetry in the presence and absence of HSA. The total charge of the voltammogram obtained at pH 3.5 shows a linear behaviour with the reciprocal of sweep rate from which the rate determining step of the Ti(IV)/Ti(III) process could be determined. The voltammetric charge values obtained in the presence of the protein suggested that the protein blocks part of the electrochemically active surface. The experimental results are explained with a model that takes into account both non-specific and specific protein/substrate interaction, which renders the system potential and solution pH dependent.

Semiconducting properties of TiO2 films thermally formed at 400°C

Titanium oxide films, TiO2, were prepared on metallic titanium substrates employing a thermal treatment at 400° C under normal air atmosphere, with various annealing times (15, 25 and 45 min). Cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to study the electrochemical and electrical characteristics of these films. The potential range for the voltammetric measurements was −0.80 to 8.00 V vs MSE (mercurous sulfate reference electrode). An analysis of the capacitance values of these semiconducting oxide films gave information about their electronic characteristics. From Mott-Schottky plots the donor concentration (ND) and the flat band potential (Efb) were obtained. ND values ranged from 14 × 1022 cm−3 to 3.3 × 1022 cm−3, while Efb values ranged from −0.40 to −0.98 V vs MSE, depending on the heating times for the oxide growth. The thickness of the space charge region, calculated from the minimum value of the capacitance at high band bending, varied between 1.45 and 2.13 rim.

Correlation between the electrochemical properties of colloidal oxides and supported oxides on metallic substrates

Mixed oxide electrodes of Ti(IV) and Cr(III) were prepared by calcining Cr(OH)3 layers deposited on metallic titanium supports. This treatment produced a mixed oxide film of TiO2−Cr2O3 covered with a layer of pure Cr2O3. The electrochemical response (cyclic voltammetry) shows the presence of two or three oxidation peaks depending on the electrode preparation conditions. One peak may be interpreted as the oxidation of Cr(III) to Cr(VI) species and the appearance of other peaks is due to the presence of chromium atoms in oxidation states higher than (III). The results of chemical analyses, electrophoretic mobilities and acid-base potentiometric titrations on calcined Cr(OH)3 powders shows that the calcination step in air produced the decomposition of the Cr(III) hydroxide to the Cr(VI) oxide. Soluble Cr(VI) compounds were found in equilibrium with the suspended powder oxide which markedly affected the shape of the titrations curves. From the amount of Cr(VI) present in solution it was possible to correct the experimental σ0-pH curves. These corrected data indicated that Cr(VI) soluble species adsorbed at the Cr2O3/electrolyte interface.