Green synthesis of gold nanoparticles from Lilium ciliatum leaf and flower extracts through phenolic composition, process optimization, and colloidal properties

Plant-mediated synthesis of gold nanoparticles (AuNPs) represents a sustainable alternative to conventional chemical methods; however, reproducibility remains a key limitation due to variability in plant composition and reaction parameters. In this study, an integrated extraction-to-nanoparticle approach was developed using Lilium ciliatum P.H.Davis, an endemic species from the Eastern Black Sea region of Türkiye, to establish a composition–process–colloid relationship for plant resource valorization. Ethanolic supercritical CO₂ extraction was applied separately to leaves and flowers at 150 bar and 40 °C, and the resulting extracts were characterized by HPLC–DAD. Leaf extracts were dominated by flavonoids, particularly rutin (13.83 mg L⁻¹) and quercetin (9.79 mg L⁻¹), with a total phenolic content of 34.96 mg L⁻¹, whereas flower extracts exhibited a higher phenolic content (65.08 mg L⁻¹), mainly composed of gallic acid (19.88 mg L⁻¹), protocatechuic acid (15.84 mg L⁻¹), and chlorogenic acid (11.00 mg L⁻¹). Initial screening showed that 1% (w/v) aqueous extracts did not produce reproducible AuNPs; therefore, optimization was performed using 2% (w/v) extracts by varying extract volume, HAuCl₄·3 H₂O concentration, microwave power, irradiation time, PEG-200 addition, and pH. AuNP formation was confirmed by surface plasmon resonance bands at 520–560 nm, TEM, DLS, zeta potential, and XRD analyses. The zeta potentials of PEG-200 stabilized leaf- and flower-derived AuNPs were − 18.3 ± 1.504 mV and − 21.9 ± 0.529 mV, respectively, indicating moderate colloidal stability with heterogeneous size distributions. XRD patterns confirmed crystalline face-centered cubic Au with Scherrer crystallite sizes of 6.19–7.10 nm. Despite lower colloidal stability, flower-derived AuNPs exhibited stronger antioxidant activity (IC₅₀ 1.267 ± 0.074 mg mL⁻¹) than leaf-derived AuNPs (3.180 ± 0.225 mg mL⁻¹), consistent with their higher phenolic loading. These findings suggest that tissue-specific phytochemistry may influence the formation, colloidal behavior, and antioxidant-related properties of plant-mediated AuNPs.