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.
In this work, we report on a polymer toolkit for fabrication of hydrogel microstructures by means of one- and two-photon crosslinking process. Using home-built and commercial multi-photon lithography setups, we show the ability to prepare hydrogel structures attached to a solid substrate from developed copolymer or terpolymer layers. These polymers bear a photo-active benzophenone or anthraquinone moieties and additional groups enabling their chemical post-modification with biofunctional molecules. The crosslinked polymer networks can swell in water forming hydrogel structures with a geometry controlled by the UV or two-photon crosslinking imposed by a focused beam scanned over the substrate. In conjunction with post-modification, these micro-structured materials may serve in biosensing and other emerging applications such as responsive hydrogel-based miniature actuators supporting microfluidic devices and micromachines.
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