Green synthesis and characterization of alloy nanoparticles using plant extracts

Abstract

Medicinal plant-mediated synthesis represents an environmentally friendly approach that is currently gaining increased attention due to its potential as an alternative to conventional toxic reducing agents, such as sodium borohydride and hydrazine, commonly employed in chemical and physical methods. Medicinal plants, rich in diverse bioactive phytochemicals, are especially intriguing for their potential role in bioreduction and stabilization of phytogenic nanoparticles with remarkable therapeutic properties. This pioneering study introduces the efficient synthesis of novel Au-Pd bimetallic nanoparticles alongside monometallic gold (Au) and palladium nanoparticles (PdNPs). The synthesis process utilizes Aspalathus linearis (Burm.f.) R. Dahlgren, commonly known as green rooibos, as well as its pure bioactive compound, Aspalathin (C-glucosyl dihydrochalcone). The biomolecules within the plant extract serve as both reducing and capping agents, facilitating the reduction of Au and Pd metals and resulting in the formation of distinct AuNPs, PdNPs, and Au-Pd bimetallic nanoparticles. Comprehensive characterization of the synthesized nanomaterials was conducted through Ultraviolet-Visible (UV-Vis) Spectroscopy, Dynamic Light Scattering (DLS) Analysis, High-Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED), Scanning-Transmission Electron Microscopy-High Angle Annular Dark Field (STEM-HAADF) and Attenuated Total Reflection-Fourier-Transform Infrared Spectroscopy (ATR-FTIR). The initiation of Au nanoparticle formation was visually evident through a noticeable colour change in the solution from pale yellow to deep purple, further confirmed by UV-Vis spectroscopy. The UV-Vis spectra exhibited Surface Plasmon Resonance (SPR) peaks between 530 nm and 540 nm for AuNPs, while no SPR band was observed for PdNPs due to the absence of free electrons in the outer shell. However, a blue shift in the SPR peak of Au-Pd bimetallic nanoparticles was observed compared to that of AuNPs. DLS analysis revealed a hydrodynamic size range of 42 nm to 80 nm for the synthesized nanoparticles, with Zeta Potential ranging between -18.2 to -31.6 mV, indicating minimal to moderately stable nanoparticles. HRTEM coupled with SAED was employed to assess particle size, morphology, and crystallinity, revealing mostly spherical nanoparticles with occasional triangular shapes of varying sizes. STEM-HAADF mapping elucidated the structural configuration of the Au-Pd bimetallic nanoparticles. Cytotoxicity testing on cell lines, coupled with cellular uptake analysis, demonstrated that none of the tested samples exhibited significant cytotoxic effects; instead, they showcased cell proliferation against the selected cell lines. This green synthesis approach proves to be convenient and simple, with potential applications extending to the synthesis of various nanomaterials, promising new advancements in technological and biomedical fields.