Adsorption and electrically stimulated desorption of the triblock copolymer poly(propylene sulfide– bl-ethylene glycol) (PPS–PEG) from indium tin oxide (ITO) surfaces

Abstract

Protein-resistant triblock copolymers, poly(propylene sulfide– bl-ethylene glycol) (PPS–PEG) have been previously demonstrated to chemisorb onto gold surfaces forming monolayers that resist non-specific protein adsorption and are stable against oxidation. In this paper, we report on the adsorption of PPS–PEG onto a transparent and electrically conductive substrate, indium tin oxide (ITO). In addition, we demonstrate the controlled desorption of PPS–PEG by applying an electrical stimulus. We have used three complementary surface characterization techniques: variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to analyze the adsorption and electro-desorption of PPS–PEG from an ITO surface. All three methods confirmed the formation of PPS–PEG adlayers on the ITO surfaces. Based on our experimental XPS and ToF-SIMS results as well as former publications, we postulate that the chemisorption of the PPS–PEG on ITO involves direct sulfide–indium (or tin) interactions. When an ascending anodic electrical stimulus was applied to the surface of the modified samples, a gradual and steady polymer removal was observed, with complete loss of the polymeric monolayer at a potential of 2000 mV (referenced to Ag electrode). Anodic polarization did not result in oxidation of the thioether function of the PPS–PEG adlayers, indicating excellent oxidation resistance of PPS–PEG on ITO surfaces. This work is focused on exploiting electrical stimuli for the in situ surface modification under dynamic control.