Green hydrothermal synthesis and optimization of gold/silver core-shell nanoparticles using thyme extract

Gold/Silver (Au@Ag) core-shell nanoparticles were successfully synthesized via a green hydrothermal method using thyme extract at temperatures of 100, 125, and 150 °C. Structural characterization confirmed well-defined core-shell morphology, with TEM revealing a uniform Ag core (~ 30 nm) and Au shell (~ 29 nm), yielding a total particle diameter of 88–92 nm. XRD analysis verified face-centered cubic (FCC) crystallinity for both metals, with no alloying phases. UV-vis spectroscopy demonstrated tunable plasmonic properties, showing a systematic blue shift from 543.2 nm (100 °C) to 537.4 nm (150 °C) due to controlled shell growth. Optimal synthesis conditions at 150 °C produced nanoparticles with exceptional colloidal stability, as evidenced by a zeta potential of − 25.3 ± 1.2 mV and a narrow size distribution (92.4 ± 3.2 nm, PDI = 0.21 ± 0.03). DLS measurements further confirmed minimal aggregation, with a dominant monodisperse peak at ~ 90 nm and only a minor secondary population (> 1000 nm). The hydrothermal approach enabled precise control over nanoparticle composition and optical properties, offering a sustainable and scalable alternative to conventional chemical reduction methods. These findings highlight the potential of thyme-mediated synthesis for producing thermally stable, plasmonically tunable Au@Ag nanoparticles for catalytic, sensing, and biomedical applications.