Electrochemical optical waveguide lightmode spectroscopy (EC-OWLS): a pilot study using evanescent-field optical sensing under voltage control to monitor polycationic polymer adsorption onto indium tin oxide (ITO)-coated waveguide chips

Abstract

A new technique has been developed that combines evanescent-field optical sensing with electrochemical control of surface adsorption processes. This new technique, termed "electrochemical optical waveguide lightmode spectroscopy" (EC-OWLS), proved efficient in monitoring molecular surface adsorption and layer thickness changes of an adsorbed polymer layer examined in situ as a function of potential applied to a waveguide in a pilot study. For optical sensing, a layer of indium tin oxide (ITO) served as both a high-refractive-index waveguide and a conductive electrode. In addition, an electrochemical flow-through fluid cell was provided, which incorporated working, reference, and counter electrodes, and was compatible with the constraints of optical sensing. Poly(L-lysine)-grafted-poly(ethylene glycol) (PLL-g-PEG) served as a model, polycation adsorbate. Adsorption of PLL-g-PEG from aqueous buffer solution increased from 125 to 475 ng/cm(2 )along a sigmoidal path as a function of increasing potential between 0 and 1.5 V versus the Ag reference electrode. Upon buffer rinse, adsorption was partially reversible when a potential of >/=0.93 V was maintained on the ITO waveguide. However, reducing the applied potential back to 0 V before rinsing resulted in irreversible polymer adsorption. PLL-g-PEG modified with biotin demonstrated similar adsorption characteristics, but subsequent streptavidin binding was independent of biotin concentration. Applying positive potentials resulted in increased adsorbed mass, presumably due to polymer chain extension and reorganization in the molecular adlayer.