Green Synthesis of AuNPs Using Red Propolis: Physicochemical Characterization and Selective Biological Cell Response

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Gold nanoparticles (AuNPs) have attracted considerable interest for biomedical applications due to their biocompatibility, tunable physicochemical properties, and potential for surface functionalization. In this study, purified Brazilian red propolis (RP) was employed as both reducing and stabilizing agent for the green synthesis of AuNPs. Initially, RP was characterized by total polyphenol content, antioxidant activity, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). AuNPs were synthesized at pH 7.0 and 12.0 and characterized by ultraviolet‐visible (UV–vis) spectroscopy, dynamic light scattering (DLS), zeta potential (ZP) measurements, nanoparticle tracking analysis (NTA), transmission electron microscopy (TEM), and small‐angle x‐ray scattering. The synthesized NPs exhibited a characteristic band at 540 nm and predominantly spherical morphology, with particle sizes ranging from 5 to 20 nm. AuNPs synthesized at pH 7.0 displayed more favorable physicochemical characteristics, including a narrower LSPR band and improved colloidal behavior. The biological activity of purified RP, citrate‐stabilized (CS) AuNPs, and RP‐functionalized AuNPs was evaluated in colorectal cancer (HCT‐116) and non‐tumor fibroblast (L929) cell lines. RP‐functionalized AuNPs significantly reduced HCT‐116 cell viability after 24, 48, and 72 h of exposure while maintaining high cytocompatibility toward L929 cells, indicating a selective biological response. These findings highlight the potential of purified RP‐mediated AuNPs as promising nanoplatforms for biomedical applications.