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C. Haiden, T. Wopelka, M. Jech, F. Keplinger, M.J. Vellekoop:
"Sizing of metallic nanoparticles confined to a microfluidic film applying dark field particle tracking";
Langmuir, 30 (2014), 31; S. 9607 - 9615.

We present Brownian motion-based sizing of individual submicron and nanoparticles in liquid samples. The advantage of our approach is that particles can freely diffuse in a 10μm thin liquid film and are therefore always within the focal depth of a low numerical aperture objective. Particles are visualized with dark field microscopy and the resulting diffraction-limited spots are tracked over a wide field of view of several hundred micrometers. Consequently, it is ascertained that long 2D-trajectories are acquired, which leads to significantly increased particle sizing precision. The hydrodynamic diameters of metal particles with nominal sizes ranging from 70nm to 200nm in aqueous solution were determined by tracking for up to two minutes and it was investigated if the diffusion characteristics were influenced by the proximity of substrates. This was not the case and the estimated diameters were in good agreement with the values obtained by electron microscopy, thus validating the particle sizing principle. Close particle sizes in a sample mixture were resolved well thanks to the high sizing precision. Furthermore, we measured a sample mixture to demonstrate the distinction of close particle sizes and performed the conjugation of a model protein (BSA) on the nanoparticle surface. An average increase of the radius of 9nm was determined, which corresponds to the size of the BSA protein.

http://dx.doi.org/10.1021/la5016675