Nanoporous Thin Films Formed from Photocleavable Diblock Copolymers on Gold Substrates Modified with Thiolate...

Nanoporous thin films formed on electrodes are considered functional elements of electrochemical sensing systems, thus motivating methods for their development. We report a preparative strategy detailing the effects of surface modification of gold substrates with thiolate self-assembled monolayers (SAMs) on the properties of nanoporous thin films derived from polystyrene-block-poly(ethylene oxide) having a photocleavable o-nitrobenzyl ester junction (PS-hν-PEO). Two PS-hν-PEO having similar PEO volume fractions (≈0.2) but different molecular weights (10 and 23 kg/mol) were used to prepare films (30-100 nm thick) spin-cast on gold substrates unmodified and modified with cysteamine, thioctic acid, and 6-hydroxy-1-hexanethiol SAMs. Solvent vapor annealing followed by PEO removal led to the formation of nanopores with average diameters of 12 and 19 nm from the smaller and larger PS-hν-PEO, respectively. Cyclic voltammograms of 1,1'-ferrocenedimethanol showed that nanoporous films on cysteamine SAMs afforded nanopores reaching the underlying substrates at higher density than those on the other substrates. This result was attributed to balanced affinity of the cysteamine SAM surface with PS and PEO, which enhanced the vertical orientation of PEO microdomains. The generation of carboxyl groups associated with the photocleavage reaction was revealed by pH-dependent changes in the voltammogram of Fe(CN)₆^3- that reflected electrostatic effects regulated by the protonation state of the carboxyl groups. The SAMs underneath the nanoporous films could be replaced by treatment with a thiol solution, as verified by voltammograms of l-ascorbic acid. These results suggest that thiolate SAM modification provides a simple means to control the interfacial orientation of PEO microdomains in thin PS-hν-PEO films.