Facile Fabrication of Reactive Plasmonic Substrates for Fluorescence Enhancement via Mussel-Inspired Chemistry

Based on the fascinating properties of polydopamine (PDA), a simple strategy was developed for facilely and efficiently fabricating plasmonic substrate with nanohybrid structure (PDA/Metal NPs/PDA). Because of the good reductive ability of PDA, metal nanoparticles such as Ag, Au, or Ag/Au hybrid particles with the good control of size and distribution on plasmonic substrate could be easily achieved. Also, owing to the exceptional self-adhesive nature, the presynthesized monodisperse metal NPs can be directly adsorbed to the surface to enrich plasmon response. Moreover, by carefully tuning the dopamine immersion time, the formed nanohybrid structure PDA/metal NPs/PDA enabled the distance-dependent MEF phenomenon where the distance could be controlled with a resolution of approximately 1 nm. In particular, the covalent (Michael addition or Schiff-base reaction) binding enabled us an easy but efficient way to immobilize a large diverse fluorophores or biomolecules on plasmonic substrate. All these above properties indicated the promising PDA-based plasmonic substrate for fluorescence enhancement. As a demonstration of the good fluorescence enhancement property, thiol-based dye was used. The ca. 5-fold fluorescence intensity enhancement on the plasmonic substrate with pattern structure clearly proved that PDA-based plasmonic substrate was indeed a good reactive platform for fluorescence enhancement. With the assistance of FDTD, the electromagnetic near-field distributions and the radiative power emitted by fluorophores on the substrate were found to be significantly improved, further helpful for explaining our experimental observations. Finally, the optimal set calculations guided us that based on the careful selection of fluorophore and space distance, a better fluorescence enhancement could be achieved for further optical and biological applications. These performed experiments suggested that the PDA-based fabricating protocol is indeed a powerful strategy for creating plasmon substrate that could find a wide range of applications.