Photolithographic Technique for Direct Photochemical Modification and Chemical Micropatterning of Surfaces

We describe a photolithographic method for the direct modification and micropatterning of the surface chemical structure of self-assembled monolayers. End-functional azobenzene alkanethiols are designed and synthesized so that, when self-assembled onto gold substrates, an acid-sensitive tert-butyl ester end group is positioned at the air-monolayer interface. Upon exposure to UV radiation in the presence of a photoacid generator, the tert-butyl ester groups are removed in the form of butylene gas to form surface carboxylic acid groups. The orientation of the monolayers and photochemical surface modification reactions are characterized by X-ray photoelectron spectroscopy measurements. The photochemical change from hydrophobic tert-butyl ester groups to hydrophilic carboxylic acid groups causes a significant change in wettability reflected in a water contact angle change from 89 to 28°, respectively. Surface chemical modifications may be patterned on a microscale by simply irradiating the self-assembled monolayers in the presence of a photoacid generator through a photomask. Chemically micropatterned surfaces are shown to be effective templates for patterned surface assembly of amine-terminated polystyrene colloidal particles and fluorescently tagged polystyrene nanoparticles and for the spatial patterning of water droplets. The photochemical changes that occur are constrained structurally to occur only at the air-monolayer surface and require only deep UV radiation and a photoacid generator as reagents. Direct photochemical micropatterning by this method is a simple and direct process that retains the full spatial resolution of deep UV photolithography and can be carried out with conventional photomasking equipment.