Local Oxidation of Polystyrene by Scanning Electrochemical Microscopy

Polystyrene (PS) surfaces are modified locally by an electrochemically generated oxidant such as Ag(II) in concentrated nitric acid. The oxidant is generated at the probe of a scanning electrochemical microscope (SECM), which allows one to investigate the kinetics of the etching by the SECM. The etching kinetics is deduced from approach curves and transient current-time response with the probe held in the vicinity of (i) massive PS or (ii) thin PS films obtained by controlled radical polymerization (atom-transfer radical polymerization, ATRP) on a gold surface. The etching kinetics is interpreted by taking into account both the homogeneous chemical instability of Ag(II) and finite heterogeneous kinetics at the substrate. The PS oxidation occurs in a limited number of active sites and the effective rate constant is limited by the permeation of the mediator within the oxidized PS structure. With ATRP-grown PS layers, the completion of the PS etching is manifested by direct electron transfer to the Au surface. The kinetic study is then used to interpret the extent of PS patterning surface as a function of the silver concentration, the ultramicroelectrode scanning rate and the tip/substrate distance.