Sandwiched gold/PNIPAm/gold microstructures for smart plasmonics application: towards the high detection limit and Raman quantitative measurements

Abstract

A smart plasmonic sensor, comprising the layer of stimuli-responsive polymer sandwiched between two gold layers, is reported. As a stimuli-responsive material, the monolayer of poly(N-isopropylacrylamide) (PNIPAm) crosslinked globules is used. A quasi-periodic structure of the top gold layer facilitates an efficient excitation and serves as a support of plasmon excitation and propagation. The intermediate layer of PNIPAm efficiently entraps targeted molecules from solutions. The sensor structure was optimized for an efficient light focusing in the “active” PNIPAm layer. The optimization was based on the time-resolved finite-element simulations, which take into account the thickness of gold layers, size of PNIPAm globules and Raman excitation wavelength (780 nm). The prepared structures were characterized using SEM, AFM, UV-Vis refractometry and goniometry. Additional AFM scans were performed in the water at two temperatures corresponding to the collapsed and swollen PNIPAm state. The Raman measurements demonstrate a high detection limit and perfect reproducibility of Raman scattering signal for the prepared sensor. In addition, the use of created SERS structures for the detection of relevant in the medical, biological and safety fields was demonstrated.