Decentralized core-shell Au/Ag bimetallic nanostructures prepared via green approach for catalytic and antimicrobial applications

Synthesis of noble metallic nanomaterials via green routes using various plant extracts has witnessed a remarkable rise in applications and research interest over the past decade. These biogenic routes are devoid of toxic chemicals, harsh preparation environments, and external capping agents, coupled with their cost-effectiveness, have attracted researchers to adopt green chemistry in nanotechnology. In this study, we synthesized Au, Ag, and Au/Ag nanoparticles (NPs) using a facile method employing an extract derived from a mixture of spices. Optical, morphological, and structural analysis is carried out using UV-visible spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDAX), Fourier- transform infrared spectroscopy (FTIR), and X-ray diffraction studies (XRD), respectively. The Zeta potential of the prepared NPs was determined using dynamic light scattering spectroscopy (DLS). Optimization of pH to 10 and 8 was performed for the synthesis of Ag and Au NPs, respectively, crucial for precise tuning of their optical absorption spectra. Finite difference time domain (FDTD) simulations validate the experimental findings. The Ag NPs exhibit primarily uniform and spherical size and shape, whereas the Au NPs display a variety of shapes including spheres, rods, and triangular plates. This variation is attributed to the influence of pH and concentration of precursors. TEM analysis confirmed the formation of Au/Ag bimetallic core-shell nanostructures (NSs), characterized by a decentralized gold core. XRD confirmed the formation of FCC crystallites. FTIR spectra identified the possible biological functional groups responsible for reduction and capping. Furthermore, a negative zeta potential exceeding 20 mV confirmed the stable formation of NPs. These NPs were capped by negatively charged functional groups, thereby avoiding agglomeration. Both mono and bimetallic NSs exhibited enhanced catalytic activity for reducing 4-nitrophenol in the presence of sodium borohydride (NaBH₄). Furthermore, Ag and Au/Ag NPs have exhibited antibacterial properties against specific bacteria, which are discussed in detail.