Lucía B. Avalle

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.

The oxygen and chlorine evolution reactions at titanium oxide electrodes modified with platinum

Abstract The oxygen (OER) and chlorine (CLER) evolution reactions were used as probes to test the electronic properties of modified electrodes (Ti/TiO 2 -Pt). Both reactions were studied in acidic solutions by means of current-potential (j-E) and impedance measurements under steady state conditions. The effect of dopant concentration, TiO 2 thickness, and the concentration of the electroactive species in the solution was analyzed. A diminution of the electron transfer current with titanium oxide thickness was obtained whereas an increase of the current was found on augmenting Pt amount. At low titanium oxide thickness both inner-sphere reactions behave in the same way as in the case of pure Pt, indicating that platinum sites are the active ones. However, at higher titanium oxide thickness, the platinum sites become less active and the valve metal oxide (TiO 2 ) begins to block OER and CLER reactions. The electrochemical impedance spectroscopy results are compared with those of the current-potential measurements and a good correlation is found between them.

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.

Study of human serum albumin-TiO2 nanocrystalline electrodes interaction by impedance electrochemical spectroscopy

The adsorption of human serum albumin (HSA) onto nanocrystalline TiO(2) electrodes was studied by electrochemical impedance spectroscopy (EIS) in function of pH and electrode potential. The characterization and physico-chemical properties of the TiO(2) electrode were investigated by scanning electron microscopy (SEM), UV-photoelectron spectroscopy (UPS), cyclic voltammetry and capacitance measurements. The impedance response of the particulate TiO(2) electrode/protein interface was fitted using an equivalent circuit model to describe the adsorption process. The adsorbed protein layer, which is formed as soon as the protein is injected into the solution and becomes in contact with the electrode, was investigated as a function of electrode potential and solution pH. The measurements were performed under pseudo-steady-state and steady-state conditions, which gave information about the different states of the system. With the pseudo-steady state measurements, it was possible to determine two rate constants of the protein adsorption process, which correspond to two different states of the protein. The shortest one was associated with the first contact between the protein and the substrate and the second relaxation time, with the protein suffering an structural rearrangement due to the interaction with the TiO(2) electrode. It was detected that at sufficiently long times (approx. 1 h, where the system was under steady state conditions), a quasi-reversible protein adsorption mechanism was established. The measurements performed as a function of frequency under steady-state conditions, an equivalent circuit with a Warburg element gave the better fitting to data taken at -0.585 V closer to the oxide flat band potential and it was associated with protein diffusion. Experimental results obtained at only one frequency as a function of potential could be fitted to a model that takes into account non-specific and probable specific protein adsorption, which renders to be potential- and pH-dependent. Low capacity values were obtained in the whole potential range, which were measured in the presence and in the absence of the protein layer. The capacity dependence on potential and pH were associated with the generation of surface states on TiO(2). A surface state concentration of 4.1x10(18) cm(-2) was obtained by relating the parallel capacitance with oxide surface states arising from the protein-oxide interaction.

Characterization of hafnium oxide films modified by Pt doping

Abstract The Hf/HfO2-Pt electrodes were prepared through galvanostatic platinum deposition from H2PtCl6 solutions followed by potentiodynamic HfO2 growth in 0.5 M H2SO4 solutions. Electron transfer reactions involving a redox couple and impedance investigations were performed in order to obtain information about the electronic conductivity, dielectric properties and thickness of the oxide film as well as the kinetics of the processes taking place on it. X-ray photoelectron spectroscopy (XPS) was used to characterize the composition and dopant distribution profile within the modified film and to correlate the electrochemical behaviour with the structure and composition of the films. A transition from n-type semiconductor to an insulator behaviour was observed at lower Pt content with increase of the final formation potential which is in agreement with platinum distribution through the oxide. At higher Pt content, on the other hand, the oxygen evolution reaction prevails over oxide growth reaction, These results are discussed in terms of a physical model for the system.

ETR on TiO2 films modified by Pt doping

Abstract ETR at Ti/TiO 2 Pt electrodes under different conditions were investigated and a correlation with the electronic properties of the films was made. The effect of doping concentration, film thickness and temperature was analysed. An increase of the catalytic properties of the films was observed for increasing Pt concentration. A diminution of the electron transfer current with the film thickness was obtained. The presence of Pt within the film also produces a decrease of the energy of activation, mainly in the cathodic direction, where it remains almost constant with the potential. The results are interpreted in terms of a direct resonance tunnelling between the redox couple and the underlying metal.