the present invention provides a composition and method for functionalizing nanoparticles that enables them to undergo reversible aggregation / deaggregation . the aggregation properties of this new system are reversible and readily monitored by optical absorbance measurements with the possibility of electrical and / or magnetic monitoring as well . the outer portion of the coating material is functionalized with polyethylene glycol entities that facilitate biocompatibility and stability both in solution and in the solid state . also provided are nanoparticles functionalized with rationally designed free radical initiators to effect tailored polymer growth from the surface . these systems may be used for a broad variety of applications , including biosensing with real - time feedback .