Towards modulating near-field plasmonic coupling for enhanced optical spectroscopy

Metallic nanostructures allow for strong enhancement of field intensity by plasmonic effects and offer efficient means for the amplification of weak optical spectroscopy signals. Typically, the metallic nanostructures are made static. A possible route to expand the spectrum of applications and performance of plasmon-enhanced spectroscopy tools is pursued, based on responsive hydrogel materials that act as artificial muscles and provide on-demand, reversible reconfiguration of plasmonic hotspots. Hydrogels are three-dimensional polymer networks with the ability to intake large amounts of water. Some classes of responsive hydrogels can be reversibly toggled between two states – swollen and collapsed – by modulating their temperature T. In this work, we use poly(N-isopropylacrylamide)-based responsive terpolymers (pNIPAAm) and we disperse polystyrene (PS) nanoparticles in the hydrogels, allowing precise control on the temperature-induced changes of the swelling ratio and allowing for a mechanically more rigid structure . This controlled actuation mechanism finds various applications in plasmonic nanomaterials. Here we present the concept of a microscopic responsive hydrogel structure that allows the modulation of the distance between metallic nanoparticles and a flat metal surface, for reversible near-field coupling and formation of a gap mode. The plasmonic coupling can be exploited for probing of molecules, by plasmonically-enhanced optical spectroscopy.