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Colloidal spheres are pervasive in natural, industrial and biomedical applications. Here, we describe a rapid and precise method for measuring individual colloidal spheres' radii and refractive indexes as they flow down a microfluidic channel, while simultaneously tracking their three dimensional positions with nanometer resolution. Based on quantitative analysis (1) of images obtained with holographic video microscopy (3,2), this technique is exceptionally robust against motion blurring (4,5) and offers near-real-time performance through hardware accelerated image analysis. Its subnanometer resolution for particle sizing is fine enough to detect the presence of molecular coatings on the surface of micrometer-scale colloidal beads and so can be used for label-free molecular binding assays. We demonstrate this by detecting the binding of neutravidin to biotinylated polystyrene spheres.

David Grier 2009-07-17