A hanging plasmonic droplet: three-dimensional SERS hotspots for a highly sensitive multiplex detection of amino acids.

A novel and simple platform for an efficient SERS detection was demonstrated in a hanging plasmonic droplet of Ag sols by virtue of three-dimensional (3D) SERS hotspots. The platform was a single droplet of concentrated Ag sols with analytes hung on a hydrophobically treated transparent cover glass. Liquid adhesive force, solvent evaporation, and hydrophobic interaction induced a concentration of dense nanoparticles into a 3D space. The generated 3D hotspots enable the highly-sensitive detection of ultratrace analytes and simultaneous multiplex identification of different amino acids, which are well-known molecules with low Raman cross-sections. This platform demonstrates an excellent mechanical stability and is suitable for the excellent examination of ultratrace analytes, with detection limits as low as 0.01 fmol of dye molecules and 5 pmol of amino acids with extremely low Raman cross-sections.