Polymer-coated gold island films as localized plasmon transducers for gas sensing

Ultrathin (typically < or = 10-nm thick) gold island films evaporated on transparent substrates show a prominent localized surface plasmon (SP) extinction in the visible-to-NIR range. Changes in the dielectric properties of the contacting medium influence the SP absorption band, providing a scheme for optical sensing based on refractive index change. In the present work, the gas sensing capability of gold island based localized surface plasmon resonance (LSPR) transducers was explored using polymeric coatings as the active interface. LSPR transducers were fabricated by spin-coating of polystyrene (PS) or polystyrene sulfonic acid, sodium salt (PSS) onto 5-nm-thick (nominal thickness) gold island films evaporated on silanized glass and annealed. Detailed characterization of the transducers was carried out using high-resolution scanning electron microscopy, cross-sectional transmission electron microscopy, and in situ atomic force microscopy under controlled atmosphere. The hydrophobic PS film exhibits swelling and significant thickness increase upon exposure to chloroform vapor and little or no change in water vapor, whereas the hydrophilic PSS film shows the opposite behavior when exposed to the same vapors. Polymer swelling upon absorption of vapors of good solvents shows a net effect of lowering the effective refractive index in the vicinity of the gold islands, manifested as a characteristic decrease of the SP band intensity and a blue shift of the band maximum. The response, measured for four different vapors, is fast (approximately 15 s) and reversible. It is shown that gold island systems coated with polymeric films can be applied to vapor recognition in an array configuration.