Monolayer Arrays of Au Nanoparticles on Block Copolymer Brush Films for Optical Devices and Biosensors

Gold nanoparticle (AuNP) monolayers possess unique optical properties and are widely used in optical devices and biosensors, sometimes necessitating dense but nonclose-packed monolayers. Current self-assembly methods are generally limited to small surfaces and are often plagued by AuNP aggregation. Here, we use a facile method, suitable for substrates of any size or form, to produce dense, unaggregated, randomly packed AuNP monolayers on brushlike films of polystyrene-block-poly(4-vinylpyridine) (PS-P4VP) obtained by dip-coating flat surfaces from very dilute solutions, where PS forms the brush, and the citrate-stabilized AuNPs interact with the P4VP anchoring layer when the template is incubated in an AuNP colloid. By investigating the effect of molecular weight (M-n), dip-coating solvent, and colloid pH on the characteristics of the adsorbed films and the subsequent deposition of AuNPs, we constructed a morphology map in terms of PS M-n and chain grafting density under favorable pH conditions, showing the region that yields the desired dense well-dispersed AuNP monolayers. Above an upper PS M-n boundary (around 50-60 kg/mol), the PS brush-continuous or in the form of patches-is too thick and rigid for the AuNPs to access the underlying P4VP and is also subject to kinetic effects, thus causing little or nonuniform, even aggregated, AuNP deposition. These findings have important implications for the future employment of dense, unaggregated AuNP monolayers in miniature sensors and as optical coatings in a wide range of optical devices.