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 me thod to account for the metal surface optical perturbation during prot ein 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 i n buffer solutions. A three-phase (metal substrate/protein film/soluti on) reflecting surface model was amended with a charged metal transiti on layer as a fourth phase, which allowed the elucidation of the adsor bed Pdx complex index of refraction spectrum. On the basis of this spe ctrum, 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.