Electrochemical characterization of nanoporous films fabricated from a polystyrene-poly(methylmethacrylate) diblock copolymer: monitoring the removal of the PMMA domains and exploring the functional groups on the nanopore surface

Cyclic voltammetry (CV) was used to assess fabrication of a nanoporous film from a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA) and also to explore the surface functional groups on the resulting nanopores. Polymer films containing vertically aligned cylindrical nanoscale pores (ca. 10 nm in pore radius, 20-30 nm in film thickness) were prepared on gold substrates by removing the cylindrical PMMA domains from PS-b-PMMA films via UV irradiation and subsequent acetic acid treatment. CV measurements provided a simple means for monitoring the extent of the removal of the PMMA domains and for assessing the formation of a recessed nanodisk-array electrode (RNE) structure. The resulting RNEs exhibited a decrease in redox current of anionic Fe(CN)6(3-) with increasing solution pH from 4.6 to 6.3 and a negligible change in CV of uncharged 1,1'-ferrocenedimethanol. The decrease in redox current of Fe(CN)6(3-) at the higher pH was due to electrostatic repulsion between Fe(CN)6(3-) and the electrical double layer formed in the neighborhood of the negatively charged nanopore surface. Indeed, the reduction of effective pore radius measured from CVs of Fe(CN)6(3-) was correlated to the change in the thickness of the electrical double layer. The pH range that showed the decrease in redox current of Fe(CN)6(3-) was consistent with the presence of -COOH groups on the nanopore surface, although they were not detected using Fourier transform infrared spectra of etched PS-b-PMMA films.