Reduction of Copper (II) To Copper and Copper (I) Oxide Nanoparticles for Antimicrobial Application Using Calamansi (Citrus Microcarpa) Peel Extract

Calamansi (Citrus microcarpa) peel, an abundant agricultural waste in the Philippines, was evaluated as a green reducing agent for synthesizing copper and copper(I) oxide nanoparticles from aqueous Cu(II). A 1000 ppm CuSO₄ solution was mixed with calamansi peel extract (CPE) at varying volume ratios (1:4, 1:2, 3:4), adjusted to pH 8, and reacted for 4 hours under agitation (185 rpm) at temperatures of 40 °C, 60 °C, and 80 °C. Copper reduction was quantified via X-ray fluorescence, and the resulting solids were characterized using X-ray diffraction and SEM-EDX. Increasing both temperature and CPE:Cu solution volume ratio significantly enhanced Cu reduction, with a maximum of 67% at 80 °C and 3:4 ratio. The solids were identified as spherical Cu and Cu₂O nanoparticles with diameters of 42–180 nm. The synthesized Cu/Cu₂O nanoparticles exhibited an 11 mm zone of inhibition against E. coli, demonstrating antimicrobial activity. These findings indicate that calamansi peel extract is an eco-friendly, low-cost alternative for removing Cu(II) from solution and producing antimicrobial copper-based nanoparticles.