The present invention provides nanoparticles including a metallic core having a length along each axis of from 1 to 100 nanometers and a
coating disposed on at least part of the surface of the metallic core, wherein the
coating comprises polydopamine, along with methods for making and using such nanoparticles. The metallic core may be gold, silver or
iron oxide and the polydopamine
coating may have other substances bound to it, such as silver,
targeting ligands or antibodies, or other therapeutic or imaging contrast agents. The disclosed nanoparticles can be targeted to cells for treating
cancer or bacterial infections, and for use in diagnostic imaging.