Biosensing under an applied voltage using optical waveguide lightmode spectroscopy.

An applied dc voltage offers a means of controlling immobilization during biosensor fabrication and detection during biosensing application. We present a method to directly and continuously measure the adsorption of biomacromolecules or other polyelectrolytes, under an applied potential difference, based on optical waveguide lightmode spectroscopy (OWLS). An indium tin oxide (ITO) film of thickness ca. 10 nm coated onto a silicon titanium oxide (STO) waveguiding film serves as the working (sensing) electrode. We observe the effective refractive index of the 0th transverse electric guided mode to increase significantly in the presence of an applied potential due to charging of the interfacial double layer and, possibly, modest electrochemical oxidation. Adsorption from solution onto the ITO electrode is detected by a further increase in the effective refractive index. We achieve accurate detection by employing an optical model in which the STO and ITO layers are combined into a single waveguiding film. No improvement is found using models treating the ITO as a separate layer, either dielectric or conducting. Using this method, we find the adsorption of human serum albumin and horse heart cytochrome c to be considerably enhanced in the presence of an applied potential exceeding 1 V. We attribute this behavior to adsorption at positions on the protein molecules of complementary charge.