Layered Double Hydroxide Nanosheets on Plasmonic Arrays of Al Nanocylinders for Optical Sensing

Layered double hydroxides (LDHs) are a class of materials that act as adsorbents of anionic molecules because of their positively charged layered structure. One appealing application is optical sensors where the LDH acts as a host material to adsorb and concentrate the probes for low-threshold detection. Herein, we demonstrate a prototype of an optical sensing system combining the nanosized LDHs (nano-LDHs) and the plasmonic substrate. The nano-LDHs are obtained as a colloidal dispersion in an aqueous solution with a high concentration via epoxide-driven alkalization. The nano-LDHs adsorb fluorescein molecules, a model analyte, faster than the micrometer-sized coarse LDHs prepared by a standard method, thanks to their nanosized geometry and highly homogeneous dispersion in water. After concentrating fluorescein molecules by adsorbing them onto the surface, the nano-LDHs are deposited on the plasmonic substrate consisting of a periodic array of aluminum nanocylinders for the fluorescence detection of fluorescein. The substrate enhances the fluorescence from the nano-LDH film with fluorescein up to 18 times by resonating both the excitation and emission light with plasmonic modes. Given the fast adsorption of the analyte by nano-LDHs and the benefit of fluorescence enhancement by the plasmonic substrate to reduce the detection threshold, we believe that this strategy of concentrating analytes by nano-LDHs and detecting them by fluorescence enhancement on the plasmonic substrate is inspiring in building sensing systems.