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Three-dimensional feedback-driven trapping of a single nanoparticle or molecule in aqueous solution with a confocal fluorescence microscope.

Control of electroosmotic flows in a two-layer microfluidic device with crossed channels is used to counteract Brownian diffusion in aqueous solution for three-dimensional trapping of a single nanoparticle or molecule within the probe volume of a confocal fluorescence microscope. A field programmable gate array sorts and counts photons into four channels synchronous with laser pulses in four beams focused to waists slightly offset from the center of the confocal volume and uses the counts to update voltages between the four fluidic inlets every 13.5 µs. Trapping is demonstrated for 40 nm nanoparticles for up to 240 s, 20 nm nanoparticles for up to 25 s, and single molecules of streptavidin-Alexa 647 for up to 1.2 s.