A composition and method of detecting an analyte in a sample using the composition, the composition including: a liquid that includes water; a plurality of first hollow glass bubbles in the liquid; a plurality of covalently attached first affinity groups that are covalently attached to at least some of the plurality of first hollow glass bubbles; and a plurality of first detector compound molecules not covalently bonded to the plurality of first hollow glass bubbles; wherein the first detector compound molecules include a first detectable group that is detected at a first wavelength; and wherein the first hollow glass bubbles have: a density of less than 0.60 gram/mole; a span of less than 1.0; and a plurality of covalently attached first affinity groups.