Electrochemical studies of the adsorption behavior of serum proteins on titanium

Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used to examine the adsorption behavior of bovine serum albumin (BSA) and bovine fibrinogen on titanium in phosphate buffer pH 7.4, over the temperature range 295-343 K. It was shown that the surface charge density is directly proportional to the amount of the adsorbed protein (surface concentration), thus indicating that the adsorption is accompanied by the transfer of charge, i.e. chemisorption. On the other hand, the resulting adsorption pseudocapacitance obtained under the potentiostatic conditions not only depends on the protein surface concentration but also is a very complex function of parameters that are, in turn, dependent on structural, physical, and chemical properties of the proteins. Both techniques were shown to be very sensitive to the conformational behavior of the proteins. The adsorption of BSA onto a Ti surface resulted in a bimodal isotherm at all the temperatures studied, while the adsorption of fibrinogen resulted in a single saturation plateau. The adsorption process was modeled with a Langmuir adsorption isotherm. It was found that fibrinogen exhibits more than twice the affinity for adsorption onto a Ti surface compared to BSA. At lower surface coverage, adsorption appears to be mainly surface binding rate limited. The calculated standard Gibbs energies of adsorption also suggested a very strong adsorption of both proteins through a chemisorption process. The adsorption process for both proteins was found to be endothermic, resulting from the excess energetics required for the disruption of intramolecular interactions relative to those involved in the formation of protein-metal interactions, i.e. chemisorption at the electrode surface. In addition, adsorption of BSA onto a Ti surface at low concentrations was shown to be an entropically controlled process, also suggesting structural unfolding of the protein occurs at the electrode surface.