A method of preparing nanoparticles from desert date can include providing a metal salt solution comprising metal ions; providing desert date extract solution that comprises a reducing agent, and combining the metal ion solution and the desert date extract solution while stirring at a temperature in the range of 25° C. to 100° C. to produce metal or metal oxide nanoparticles. The metal nanoparticles can be gold nanoparticles. The metal oxide nanoparticles can be zinc oxide nanoparticles. The nanoparticles can be used to inhibit the growth or proliferation of a cancer cell and/or microorganisms.