Voltage-Tunable Volume Transitions in Nanoscale Films of Poly(hydroxyethyl methacrylate) Surfaces Grafted onto Gold

Abstract

Surface grafting of a polymerizable monomer onto Au was used to produce nanometer-scale planar hydrogel films with controllable volume. A self-assembled monolayer of 11-mercaptoundecanoic acid on a planar Au surface was activated through water-soluble carbodiimide and N-hyroxysuccinimide followed by reaction with 2-aminomethacrylate to produce a methacrylate-terminated surface layer, which readily polymerized under UV radiation in the presence of hydroxyethyl methacrylate monomer, ethylene glycol dimethacrylate cross-linker, and a photoinitiator. The reaction steps were characterized by external reflection mode Fourier transform IR spectroscopy. Under controlled UV exposure, thin (3 nm < d < 10 nm) hydrogel films were obtained from 1:1 ethanol/H(2)O. Surface plasmon resonance measurements were used to characterize both the synthesis of the hydrogel and the potential-induced volume changes. The nanometer-scale hydrogels thus produced undergo reproducible changes in thickness, when a potential is applied across the film. Thickness changes increasing with applied potential were obtained for both voltages in the range |V(appl)| </= 600 mV. In NaCl, electrolyte films swell with application of negative potentials and shrink with positive potentials, due to the imbibing or extrusion of hydrated Na(+) ions, respectively. Thickness changes as large as 50% can be achieved. An increase in the cross-linker content results in thicker films, but at the cost of dramatically restricted swelling. Response times are generally faster for smaller applied potentials, as expected if the volume change results from mass transport of electrolyte.