Combining confocal microscopy and optical tweezers, we map out the spatial distribution of the particle concentrations of quantum dots, fluorescent HIV pseudo virus particles and polystyrene nanospheres in an optical trap. By analyzing the Boltzmann distribution of local particle concentrations, we obtain the two-dimension single particle trapping potential profile at the center of the optical trap in the direction perpendicular to the beam propagation. We compare the trapping potential energies of pseudo HIV vesicles and same-sized polystyrene spheres. We also compare the trapping potential energy of polystyrene spheres of a focused Gaussian beam and two modes of cylindrical vector beams.
Fluorescence correlation spectroscopy is one of the most sensitive methods for biomolecule and nanoparticle studies. By
fluorescently labeling the target particles, statistical analysis of the fluorescence fluctuation inside the focal volume of a
laser beam yield information of the concentration and diffusion of the target particles in the illuminated volume.
However, the application of FCS is limited by its detection range of 1010-1014 particles/mL. To overcome this sensitivity
threshold on the low concentration end, we designed a hybrid system that augments FCS with optical trapping. By using
the optical gradient force from a second laser focused to the same illuminated volume, we were able to show that the
local concentration of particles can be enriched significantly, thus extending the useful range of FCS. In this work, we
describe this novel hybrid optical method for nanoparticle detection by first considering freely diffusing particles about
the illuminating volume, and then compare the results to nanoparticles under the influence of the optical trapping laser.
Analysis of trapped particle number permits measurement of trapping energy as well as determine ambient concentration
out of the trap. Furthermore, the hindered diffusion of trapped particles due to optical forces will be discussed.
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