Optical printing of plasmonic nanoparticles for SERS studies of analytes and thermophoretically trapped biological cell

The optical printing of nanoparticles at pre-defined locations is emerging as an area of intense research due to its potential applications in diverse fields, ranging from photonics to nanodevices. Herein, we demonstrate simultaneous and permanent optical assembly of plasmonic Ag nanoparticles onto the floor as well as the ceiling of a transparent sample chamber by using the optically generated thermal force and the scattering force. The study unravels that, beyond a threshold sample chamber height, thermal force-assisted printing at the floor of the chamber is independent of the sample chamber height. The optical scattering force facilitated printing onto the ceiling relies on the chamber height. By optically printing the plasmonic nanoparticles along with analyte samples, a limit of detection of 323 fM and 82 pM for Rhodamine 6G (Rh-6G) and crystal violet (CV), respectively, is achieved using the surface-enhanced Raman spectroscopic (SERS) technique. Additionally, the method is extended to simultaneous SERS detection of multiple analytes in a mixture. Further, the potential application of the fabricated permanent plasmonic nanoparticle substrate for thermophoretic trapping and SERS studies of biological cells has been illustrated.