Figure 1.

Holographic molecular binding assay. (a) Functionalized colloidal spheres are incubated with target molecules that bind to the beads’ surface groups. (b) Conventional analysis requires washing and incubation with fluorescent labels that also bind to the surface-bound target molecules. These labels’ fluorescence is read out in a flow cytometer. (c) Fluid-borne beads travel down a microfluidic channel where they are illuminated by a collimated laser beam. Scattered light interferes with the rest of the beam to form holograms that are recorded with a video camera. (d) Each hologram is fit pixel-by-pixel to predictions of the Lorenz-Mie theory of light scattering to measure the diameter, d_p, and refractive index, n_p, of the associated sphere. This measurement is sufficiently precise to detect the change in diameter associated with molecular binding. (e) Joint probability distribution, ρ(d_p, n_p), of particle diameter and refractive index measurements for biotinylated polystyrene spheres before and after incubation with NeutrAvidin. Each point represents the properties of one bead.