Spectroscopic Real-Time Ellipsometry of Putidaredoxin Adsorption on Gold Electrodes

Adsorption of putidaredoxin (Pdx) on gold electrodes was studied using dynamic spectroscopic ellipsometry and differential capacitance. A method to account for the metal surface optical perturbation during protein adsorption at constant potential is described. The method is based on the concept of the charged transition layer which develops between metal substrate and adsorbate. Transition-layer charge was determined from the differential capacitance and the ellipsometry measurements in buffer solutions. A three-phase (metal substrate/protein film/solution) reflecting surface model was amended with a charged metal transition layer as a fourth phase, which allowed the elucidation of the adsorbed Pdx complex index of refraction spectrum. On the basis of this spectrum, a time dependent protein surface concentration was calculated. For short times, this surface concentration was in good agreement with estimates based on mass transport limited adsorption.