A simple method was developed for the synthesis of copper nanoparticles (CuNPs) with relatively high concentration by using casein, hydrazine, and ascorbic acid as stabilizing, reducing and antioxidant agents, respectively. The obtained nanoparticles were characterized by UV-Vis spectroscopy, transmission electron microscopy, FTIR and X-ray diffraction. The synthesized CuNPs exhibited good antibacterial activity against Gram positive and Gram negative bacterial strains. Notably, they exhibited excellent antibacterial activity against the multi drug resistant Staphylococcus aureus ATCC 43300 strain. Additionally, the optical properties of CuNPs for the first time to our knowledge have been utilized for the selective colorimetric sensing of tryptophan amino acid and Hg²⁺ metal cations in aqueous solution at ppm and ppb level, respectively.