Functionalized silica microspheres that contain carboxylic acid, amine or streptavidin groups are available from Polysciences Inc. Nominal diameters are 0.5, 1 and 5 μm to accommodate specific assays. The carboxylic and amine materials are supplied as 100% solids, and the streptavidin types are offered as a 1% dispersion in water. The company says that, compared with polymer versions, the silica microspheres feature a lower refractive index, greater density for easier settling, and a hydrophilic surface that reduces the nonspecific binding of hydrophobic ligands.

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