Plasmon-enhanced multiphoton polymer crosslinking for selective modification of plasmonic hotspots

A novel approach to selectively modify narrow sub-areas of metallic nanostructures adjacent to plasmonic hotspots, where strong electromagnetic field amplification occurs upon localized surface plasmon (LSP) excitation, is reported. In contrast to plasmon-driven polymerization, it relies on plasmonically enhanced multiphoton crosslinking (MPC) of polymer chains carrying photoactive moieties. When attached to metallic nanostructures and irradiated with a femtosecond near-infrared beam resonantly coupled with LSPs, the enhanced field intensity locally exceeds the threshold and initiates MPC at plasmonic hotspots. This concept is demonstrated using gold nanoparticle arrays coated with two specifically designed polymers. Local MPC of a poly(N,N-dimethyl acrylamide)-based copolymer with an anthraquinone crosslinker is shown via atomic force microscopy. Additionally, MPC is tested with a thermoresponsive poly(N-isopropyl acrylamide)-based terpolymer. The reversible thermal collapsing and swelling of the MPC-formed hydrogel at specific nanoparticle locations are confirmed by polarization-resolved LSPR spectroscopy. These hybrid metallic/hydrogel materials can be further post-modified offering attractive characteristics for future spectroscopic/bioanalytical applications.