Capacitance and Open Circuit Potential on SemiconductorElectrodes: further evidence of a strong dependence with protein adsorption.

GONZALO PILAR, LUCÍA B. AVALLE, OSVALDO CÁMARA, FABIANA Y. OLIVA

Cancún, México

Congreso; International Society of Electrochemistry; 2006

ISE

The adsorption behaviour of serum albumins on different Ti/TiO2 electrodes and its effect on the electrode surface have been studied in 0.1 M NaCl solution at different protein concentration and temperature. Electrochemical Impedance Spectroscopy measurements at different applied potentials as well as open circuit potential studies were performed in presence and in absence of protein.
Electrode capacitance as a function of potential was calculated in order to obtain information about the double layer structure and the adsorption processes occurring around the semiconductor flat band potential (-0.2 V vs. SCE). Figure 1 shows the time dependence of the electrode open circuit potential (ocp) and the effect of the injection of 0.1g/L HSA into the electrochemical cell. The shift in the electrode ocp towards more negative potential values after protein addition is equivalent to the effect of the interface alcalization observed for Ti/TiO2 electrodes (1-2). Therefore, this effect could be interpreted as the protein adsorption occurring through the chemical reaction involving deprotonated carboxyl and/or protonated amino functional groups through electrostatic
interaction or hydrogen bonding with mono- and di-coordinated surface -OH groups postulated in (3). These reactions introduces changes in the interface compatible with surface states generation in the oxide film (4,5). The impedance behaviour of the electrode interface in presence of the protein has a strong potential dependence near the Ti/TiO2 flat band potential. The capacitance of the system obtained from the data fitting, using the equivalent circuit R(RQ) in the frequency range of 0.5 to 5.103 Hz is shown in Figure 2 and 3. It can be observed a strong dependence of the capacitance with more negative electrode polarization and the increase in protein bulk concentration. For protein concentration up to 0.1 g/L the response of the system is an increase in the capacitance values with respect to the capacitance values obtained for the bare electrode. This response could imply the generation of surface states as a consequence of the protein-surface interaction. For protein bulk concentration higher than 0.1 g/L a decrease in the capacitance values is observed which could not be explained considering only the surface states generation approach. Under these conditions, the adsorbate/adsorbate interactions could be
playing an important role, influencing the capacitance of the system.
To get further insight in the adsorption process occurring at protein concentrations higher than 0.1g/L, the capacitance response of the system is evaluated removing the protein in solution after the formation of a protein layer on the electrode surface.
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