Fabrication of polymer colloidal/Au composite nanofilms for stable and reusable SERS-active substrates with highly-dense hotspots

Abstract

Two-dimensional (2D) nanostructured arrays are regarded as attractive materials for surface-enhanced Raman scattering (SERS)-active substrates because of their excellent stability and signal reproducibility. We present a novel approach for fabricating stable and reusable 2D polymer colloidal monolayer on a SERS-active nanostructured Au substrate with high-density hotspots. Poly(styrene-co-N-isopropylacrylamide)@polyacrylic acid (PSN@PAA) polymer particles with core@shell structure were synthesized by a two-step polymerization. Using the soft microgel polymer particles as building blocks, a 2D close-packed PSN@PAA colloid nanofilm was fabricated at air–water interface. An ultra-thin Au nanofilm was sputtered onto the 2D PSN@PAA colloidal monolayer by a simple and efficient sputter-deposition technique. The aggregation structure of Au nanoparticles on the surface of the 2D microgel nanolayer could be regulated by shrinking the polymer microgels in methanol. Furthermore, the electrostatic interaction of sodium citrate on nanoparticles and the ripening process to Au nanoclusters could facilitate the formation of crevices. The region of Au nano-islands and nanocaps formed on the surface of the soft 2D polymer monolayer could generate very dense enhanced electromagnetic field hotspots, which can greatly enhance the Raman signals of R6G adsorbed on the substrate. The excellent reusability of the PSN@PAA/Au composite nanofilm was mainly due to the preferable adsorption of probe molecules on the 2D soft microgel particles. Moreover, the shrinkage of polymer microgel particles in the ethanol solvent made it easy to desorb the probe molecules from the substrate surface. Therefore, the macroscopic uniformity of the nanofilms ensured the reproducibility of SERS signals, providing an ideal candidate for SERS applications.