Superhydrophobic needles tipped with 2-dimensional arrays of plasmonic colloidal nanoparticles for microdroplet SERS analysis

Aqueous microdroplets strongly attach to metallic needles with superhydrophobic sides and flat hydrophilic tips. Such needles can be made surface-enhanced Raman spectroscopy (SERS)-active by adsorbing a layer of densely packed and uniform plasmonic nanoparticles onto the hydrophilic tip. The resulting particle-tipped needles allow dual enhancement of the Raman signals from microdroplets of low concentration analytes by combining analyte enrichment through solvent evaporation and plasmonic SERS enhancement. The combination of small sample volume, preconcentration, and SERS allows extremely low total amounts of analytes to be detected. Here, we show that with crystal violet as the model analyte, the limit of detection can reach 2 pg. The method can be readily used to fabricate superhydrophobic needles tipped with different types of plasmonic colloidal nanoparticles for enhanced Raman analysis, for example, silver cube tipped needles could be used to detect as little as 2.5 ng of trinitrotoluene.