We have obtained a Raman spectrum of single nanometer-sized particles (nano-particles) trapped by laser tweezers for the first time. The microscope used in this study is a new version of our Raman tweezers microscope (RTM); it contains an oil-immersed objective lens with high numerical aperture to increase the force of the optical radiation pressure of the near-infrared laser beam to trap single organic and biological nanoparticles and provides sufficient sensitivity for the Raman measurement of the trapped nanoparticles. The confocal arrangement in the system completely eliminates the Raman signal of the oil under the objective lens. The RTM provides a Raman spectrum of a single polystyrene latex bead about 40 nm in diameter with an exposure time of 3 s and allows us to determine its molecular species. Furthermore, the RTM enables us to count the number of nanoparticles in the focal spot of the laser beam from the Raman spectra of trapped nanoparticles.
Katsuhiro Ajito and Keiichi Torimitsu, "Single Nanoparticle Trapping Using a Raman Tweezers Microscope," Appl. Spectrosc. 56, 541-544 (2002)