The invention provides assay particles useful, for example, for detecting analytes and binding molecule interactions. One type of assay particle includes a core portion encased by a shell portion, wherein the shell portion comprises an inorganic phosphor that binds selectively to a target molecule. Another type of an assay particle includes a core portion encased by a shell portion, and a coat portion covering the shell portion, wherein the coat portion comprises an inorganic phosphor that binds selectively to a target molecule. A further type of assay particle includes a core portion encased by a shell portion, and a coat portion covering the shell portion, wherein the coat portion comprises an inorganic phosphor and a target selective binding moiety, and wherein the assay particle is buoyant in aqueous media. An additional type of assay particle includes a core portion encased by a shell portion, and a coat portion covering the shell portion, wherein the shell portion comprises an inorganic phosphor and the coat portion comprises a target selective binding moiety, and wherein the assay particle is buoyant in aqueous media. Also provided are kits and related methods.