Fabrication of Arrays of Plasmonic nanostructures for Biosensor Applications

Using interferometric lithography (IL) in a double exposure process a library of different arrays of gold nanostructures was formed. Self-assembled monolayer resists were patterned by exposure to an interferogram formed using a Lloyd's mirror interferometer, in which a sample and a mirror are placed at an angle  relative to each other, and etched using mercaptoethylamine in ethanol. Patterning using a single exposure yielded gold nanowires with pitches in the range 110 nm – 270 nm and lengths of up to 1 cm. The pitch of these lines was controlled by changing the angle between the sample and the mirror in the interferometer. Using a double exposure process in which the sample is rotated between exposures, it was possible to fabricate arrays with a variety of different geometries. After annealing, extinction spectra were significantly changed and exhibited strong plasmon bands. It was found that the energy of the localized surface plasmon resonance (LSPR) was controlled by changing the pitch in the interferometer, the etch time, and the geometry of the arrays. The LSPR peak was typically in the range 500 nm to 750 nm. The extinction spectra of samples fabricated using small values of  yielded strong plasmon bands, the gold nanostructure arrays functioned as label-free LSPR sensors. After the immobilisation of streptavidin and IgG on the gold nanostructure arrays, a red shift was observed in the position of the LSPR. The adsorption kinetics of streptavidin on biotinylated gold surfaces were characterised by spectroscopic ellipsometry, and the measured shifts in the position of the plasmon band following binding of streptavidin to biotinylated gold nanostructure arrays was found to be closely correlated with the adsorption kinetic data obtained using spectroscopic ellipsometry.

Poly cysteine methacrylate (PCysMA) brushes were grown from gold nanostructure arrays that had been functionalised by adsorption of a brominated thiol using atom transfer radical polymerisation with the bromine acting as an initiator. The thickness of the brush layer was controlled by controlling the polymerisation time, while the kinetics of brush growth were measured using spectroscopic ellipsometry and AFM. PCysMA brushes exhibited pH-responsive behaviour: at neutral pH, the polymer is zwitterionic, while at pH <2 and >12, the polymer acquired a net charge, leading to increased repulsion between surface-grafted chains and an increase in swelling away from the substrate.