Liquid Bridge-Induced SERS Hotspots: Nitrobenzene Grafting on Silver-Gold Nanoparticles

The crafting process of nitrobenzene on silver@gold nanoparticles from a solution of 4-nitrobenzenediazonium-tetrafluoroborate in acetonitrile strongly depends on the silver to gold ratio. The investigation of SERS substrates consisting of gold, silver, and silver@gold nanoparticles, prepared by employing a helium droplet-based synthesis approach, provides insight into the underlying plasmon-enhancement mechanism. Gold reacts slowly with the diazonium salt; however, the reaction can be accelerated by photoexcitation at the surface plasmon resonance. In contrast, silver reacts spontaneously, which prevents plasmon-induced local grafting. UV/vis absorption spectroscopy and Raman spectroscopy indicate that, in the case of silver, SERS hotspot formation is mediated by acetonitrile liquid bridges that initially connect the nanoparticles. This provides a novel approach to simultaneously generating and selectively functionalizing SERS hotspots.