Polydopamine nave gained popularity as a coating material for nanoparticles,and yet it has been demonstrated that polydopamine is capable of fixing transition metal ions via covalent interaction.In this work,we coated a thin layer of dopamine onto the surface of gold nanoparticles then the targeting moiety of folate was conjugated onto the nanocomplex via polyethylene glycol with a terminal group of thiol,taking the advantage of the reactive surface of polydopamine.Finally,the nanoparticle was incubated with radionuclide 64Cu,which transformed it into a β+-emitting PET probe.We have shown using mouse model that the probe would accumulate in the tumor region.This strategy offers an easy way to transform common nanoparticles into PET probes,and can be extended to other nanoparticles such as iron oxide and titanium dioxide nanoparticles.