Green synthesis of gold nanoparticles from Citrus reticulata peel extract: processing optimisation, phytochemical reduction mechanism, and dual antioxidant-anticancer performance against MCF-7 and AGS cell lines

Gold nanoparticles (AuNPs) were synthesised via a single-step, surfactant-free green route using Citrus reticulata peel extract as sole reducing and stabilising agent. HAuCl4 concentration (6–24 mM) and time (30–120 min) were optimised using SPR intensity and width, with 12 mM HAuCl4 and 60 min yielding intense, narrow SPR. FTIR revealed attenuation and red-shifting of hydroxyl (3408 cm−1) and carbonyl (1750–1600 cm−1) bands, indicating involvement of phenolic and pectic groups in Au3+ bioreduction and capping. XRD confirmed a face-centred cubic gold lattice (JCPDS 04-0784), with a dominant (111) reflection. TEM showed spherical particles (20–45 nm) within an organic corona, while DLS indicated hydrodynamic diameters of 20–55 nm for particles and 80–130 nm for aggregates. Zeta potential was −24.2 mV, indicating stability. AuNPs exhibited concentration-dependent DPPH scavenging (81.3% at 4 mg/mL) and cytotoxicity against MCF-7 and AGS cells, with IC50 values of 23.41 and 34.62 µg/mL, respectively.