Eco-friendly Cu-TiO₂ nanoparticles from Citrus limon peel: integrated biological and computational evaluation

The green synthesis of multifunctional nanomaterials from agricultural waste is a major target in sustainable biomedicine. This study reports the eco-friendly, one-pot synthesis of copper-doped titanium dioxide nanoparticles (Cu-TiO₂NPs) using an aqueous peel extract of Citrus limon (Linn.) Burm. f. The formation of the nanoparticles was observed using the ultraviolet-visible spectroscopy, with an absorption edge below 400 nm with a redshift in the visible range (400-650 nm). X-ray diffraction (XRD) confirmed a mixed anatase-rutile crystalline phase, with an average crystallite size of 45.6 nm, and scanning electron microscopy (SEM) showed irregularly shaped agglomerated nanoparticles with an average diameter of 48 ± 0.46 nm. Field Emission Scanning Electron Microscopy (FESEM) revealed irregular, highly agglomerated nanoparticles with a primary size range of 49 ± 07 nm, while X-ray Photoelectron Spectroscopy (XPS) confirmed the oxidation states of Ti, O, and Cu, validating successful doping. Energy-Dispersive X-ray (EDX) analysis further confirmed uniform Cu incorporation into the TiO₂ lattice. In vitro testing showed a dose-dependent antibacterial effect against Staphylococcus aureus (ATCC 9144), Staphylococcus epidermidis (ATCC 12228), and Pseudomonas aeruginosa (ATCC 10145), with minimum inhibitory concentration (MIC) values ranging from 15 to 20 µg/mL. The highest zone of inhibition for Cu-TiO₂NPs was 21.00 ± 0.29 mm against S. epidermidis at 30 µg/mL, similar to the effect observed with ceftriaxone (30 µg/mL). The nanoparticles exhibited high antioxidant capacity, with a ferric reducing antioxidant potential of 67.5 ascorbic acid equivalents (AAE) µg/mL and a 54% 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging capacity at 500 µg/mL. Cytotoxicity was measured using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, which indicated dose-dependent cytotoxicity in human hepatocellular carcinoma (HepG2) cells, with an IC₅₀ of 150 µg/mL after 24 h. The stabilization of the L-ascorbic acid-Cu-TiO₂NPs complex was investigated using Density Functional Theory (DFT), and molecular docking revealed strong interactions with the target bacteria. This study uniquely integrates sustainable waste valorization, comprehensive in vitro bio evaluation, and computational validation of lemon peel-derived Cu-TiO₂NPs for biomedical applications.