Protein Adsorption Kinetics under an Applied Electric Field:  An Optical Waveguide Lightmode Spectroscopy Study

The controlled surface placement of protein molecules represents a crucial step toward many new biotechnological devices and processes. A promising means of directing the structure and formation rate of an adsorbed protein layer is through an applied electric potential difference. We present here a method for continuously measuring the protein adsorption under a direct current voltage using optical waveguide lightmode spectroscopy. An indium tin oxide-coated waveguiding sensor chip serves as the anode and adsorbing substrate, and a platinum counter electrode serves as the cathode in a parallel plate arrangement. For (negatively charged) human serum albumin in either pure water or N-[2-hydroxyethyl]piperazine-N‘-ethanesulfonic acid (HEPES) buffer, we find the transport-limited and initial surface-limited rates of adsorption to significantly increase with the applied potential. For (positively charged) horse heart cytochrome c, we observe no influence of the voltage on the transport-limited adsorption rate in either solvent and a decrease with the voltage in the initial surface-limited rate in a HEPES (but not a pure water) solvent. Interestingly, we find the rate of adsorption at moderate to high surface density to greatly increase with the voltage for both proteins; this effect is more pronounced in water than in HEPES. We attribute this enhanced adsorption to contact between electrode and protein patches of complementary charge, leading to more oriented and efficiently packed adsorbed molecules and, in the case of high voltage, to multilayer formation.