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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
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.