Disclosed herein is a composition for ultrasensitive bioassay applications. The composition includes a plurality of dispersible, nanoparticles having a size less than 500 nm. The nanoparticles contain a metal catalyst or a metal catalyst precursor. The nanoparticles are conjugated to at least one biospecific binding reactant that is selectively reactive with a target analyte. The composition includes a dispersing medium. A method and a kit for conducting bioassays are described.