A method of synthesizing copper oxide nanoparticles includes preparing a liquid extract of Rumex vesicarius, dissolving copper salt in the liquid extract to provide a solution with copper nanoparticles, adding a base to the solution with copper nanoparticles to form a precipitate including copper oxide nanoparticles. Copper oxide nanoparticles prepared according to the method are effective photocatalysts for degrading organic dyes and antibacterial agents and exhibit anticancer activities.