In Situ Studies of Surface-Plasmon-Resonance-Coupling Sensor Mediated by Stimuli-Sensitive Polymer Linker

Hybrid plasmonic nanostructures comprising gold nanoparticle (AuNP) arrays separated from Au substrate through a temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) linker layer are constructed, and unique plasmonic-coupling-based surface plasmon resonance (SPR) sensing properties are investigated. The optical properties of the model system are investigated by in situ and scan-mode SPR analysis. The swelling-shrinking transitions in the polymer linker brush are studied by in situ contact-mode atomic force microscopy at two different temperatures in water. It is revealed that the thickness of the PNIPAM layer is decreased from 30 to 14 nm by increasing the temperature from 20 to 32 °C. For the first time the dependence of the coupling behavior in AuNPs is investigated with controlled density on the temperature in a quantitative manner in terms of the change in SPR signals. The device containing AuNPs with optimized AuNP density shows 3.2-times enhanced sensitivity compared with the control Au film-PNIPAM sample. The refractive index sensing performance of the Au film-PNIPAM-AuNPs is greater than that of Au film-PNIPAM by 19% when the PNIPAM chains have a collapsed conformation above lower critical solution temperature. A plasmonic coupling sensing device is designed, comprising AuNPs separated from the Au substrate in a surface plasmon resonance (SPR) spectrometer through a thermoresponsive polymer layer. The optical properties of the stimuli-responsive sensing devices are investigated by in situ and scan-mode SPR analysis. The Au film-PNIPAM-AuNP system shows markedly enhanced sensitivity toward refractive index sensing due to SPR coupling.