Patent ID: 7575934

Claim:
A method of detecting the presence of an ligand analyte, comprising (a) exposing the analyte in solution to a fluorescence-detectable composition comprising (i) a magnetic particle having stable north and south-pole magnetic regions, (ii) one or more fluorescent moieties covalently linked to the magnetic particle and having an asymmetric spatial localization with respect to said north- and south-pole regions, and (iii) anti-ligand molecules covalently linked to the magnetic particle, (b) by said exposing binding the ligand analyte to the surface of said magnetic particle, thereby to increase the rotational drag of the composition when placed in an alternating or rotating electromagnetic field, (c) irradiating the composition with a fluorescence-excitation light beam, while rotating the composition in an alternating or rotating electromagnetic field, and (d) determining, from a measured phase lag of fluorescence emission from the composition, relative to the composition prior to ligand binding, the presence and/or amount of ligand bound to the analyte.