The invention is directed to a particle system comprising a plurality of three-dimensional particles, each particle having an interior region defining a three-dimensional cavity or void and an exterior region, wherein the cavity or void is open to an external environment of the three-dimensional particle and wherein the cavity or void has a volume between about 1 pL and about 125 nL and a molecular capture region disposed on the interior region of the particles configured to capture cells, and to a method of performing an assay using the particle system.