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Manipulating and probing the spatio-temporal dynamics of nanoparticles near surfaces

In this report, we combine total internal reflection-fluorescence correlation spectroscopy (TIR-FCS) with a single optical trap to simultaneously manipulate and measure the dynamics of individual molecules near the substrate-solution interface. As a proof of principle, polystyrene particles (84 nm in diameter) are used as a model system to test our approach in studying their diffusion properties near surfaces, which are treated with polyethylene glycol 8000, bovine serum albumin or sodium hydroxide. The evanescent field of 543 nm excitation propagates ~100 nm into the solution, and the fluorescence detection is spatially confined by a 25 or 50 μm pinhole that is parfocal with the specimen plane. The optical trap is generated using a cw Ti:sapphire laser at 780 nm. Our results indicate that the particles' diffusion is influenced by surface interactions, which might have further implications on biomembrane studies. Furthermore, the observed translational diffusion of individual particles can be manipulated using an optical trap. By combining the single molecule sensitivity of TIR-FCS with a noninvasive manipulation method, such as optical trapping, we will be able to probe molecular dynamics in biomimetic systems and living cells.