Raman-spectroscopy and of optically trapped nanoparticles

Abstract

The present work begins a large layer of experiments on the study of Raman radiation from extracellular vesicles. It is a promising method that provides unique information about the global biomolecular composition of a single vesicle or a small number of vesicles. Two physical phenomena are present in this work, Raman scattering and optical trapping. In Raman scattering, the scattered radiation depends on the molecular structure of the scatterer. The amount of energy that is lost or gained in such interactions is determined by molecular vibrations and oscillations. By collecting and analyzing the spectra of the scattered light, the structure of a molecule can be identified. Objects represented as small dielectric spheres interact with the electric field created by the light wave due to the dipole moment induced on the sphere. As a result of the interaction of this dipole with the electric field of the electromagnetic wave, the object moves along the electric field gradient. In addition to the gradient force, the object is also affected by the force caused by the pressure (reflection) of light from its surface. These two effects can be used to trap and control micro- and nanoparticles. Confocal Raman tweezer microscope has been designed and constructed. A focused laser beam has been used to optically trap polystyrene beads and excite Raman-scattering. The Ramanscattered light has been transmitted to an optical spectrometer. As the PS beads are in a buffersolution, a considerable Raman-background is present and has to be subtracted. Due to this and the fact that the Raman-scattering is weak, major part of the project is to optimise the signalto-noise ratio by choosing a good design and experimental testing. Raman spectra are obtained of nanometer-sized polystyrene particles trapped by a laser tweezer. It provides sufficient sensitivity for the Raman measurement of the trapped nanoparticles. The set-up and allows us to determine their molecular structure. The presented studies were performed to investigate the validity of Raman tweezer microspectroscopy. This set-up can be used in the future experiments with biological nanoscale particles such as extracellular vesicles.