Methods and apparatus are provided herein for assaying biological samples using probes labeled with composite organic-inorganic nanoparticles (COINs) and microspheres with COINs embedded within a polymer matrix to which the probe moiety is attached. COINs are Raman-active nanoparticles made up of aggregated primary metal crystal particles with Raman-active organic compounds adsorbed on the surface in the junctions of aggregated primary metal crystal particles or embedded in the crystal lattice of the primary metal particles. Since COINs intrinsically produce SERS signals upon laser irradiation, COIN-labeled probes are particularly suitable in a variety of methods for assaying biological molecules, most of which are not inherently Raman-active. The invention provides variations of the sandwich immunoassay employing both specific and degenerate binding, methods for reverse phase assay of tissue samples and cell microstructures, in solution displacement and competition assays, and the like. Kits and chips useful for practicing the invention assays are also provided.