Synthesis and characterization of ultrasmall copper nanoparticles (Cu@CuOx). (a) Schematic diagram of the synthesis of Cu@CuOx-ECL1i, Cu@CuOx-ECL1i-Gem, and 64Cu radiolabeled nanoparticles, (b) STEM of Cu@CuOx-ECL1i, (c) Number average hydrodynamic diameter of Cu@CuOx-ECL1i, (d) In vitro TA-PEG-Gem release profiles of Cu@CuOx-ECL1i-Gem under physiological and acidic conditions.