Mechanistic Insights into the Nucleation and Growth of Bimetallic Gold Nano-Stars

Gold nanoparticles (GNPs) are renowned for their chemical and physical stability, reproducibility, ease of modification, and biocompatibility. Their synthesis through various methods allows for the creation of a wide range of structures, morphologies, porosities, and shapes. Bimetallic nanoparticles (BMNPs) represent an advanced class of nanostructures with enhanced technological properties, which vary based on their size, shape, composition, and structure. In particular, gold and silver-based BMNPs are highly valued for their superior sensitivity as surface-enhanced Raman scattering (SERS) substrates, benefiting from the synergistic enhancement of the properties of both elements. The combination of gold with silver and silver chloride (Ag-AgCl) has demonstrated superior absorbance and localized surface plasmon resonance (LSPR) properties compared to Ag-AgCl alone, along with increased stability and reproducibility, making them ideal for use in catalysis and sensing applications.<br/>Gold nanostars (GNSs) are extensively studied for their applications in biosensing, bioimaging, and photothermal therapy, due to their enhanced local optical and electromagnetic properties, particularly in their star-like protrusions. A simple, rapid, and efficient method to produce GNSs is a one-pot, seedless, bottom-up synthesis. This method involves the reduction of HAuCl₄ and AgNO₃ by ascorbic acid (AA) under acidic conditions, avoiding the use of toxic materials. The final morphology of GNSs depends on several factors, including the gold-to-silver ratio, reaction time, pH, and the amount of AA. However, the exact mechanism of GNS synthesis using HAuCl₄, AgNO₃, and AA is not yet fully understood. It is hypothesized that gold and silver distribute evenly throughout the particles, forming an alloy composition.<br/>In our study, we investigated the complex process of nucleation and growth of these bimetallic GNSs. We focused on the effects of temperature and the timing of silver ion introduction on the morphology, size, particle concentration, stability, and optical properties of the BMNPs. Chemical analysis was conducted using high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) and 3D tomography in a high-resolution transmission electron microscope (HR-TEM), providing detailed insights into the synthesized GNSs. Additionally, synchrotron high-resolution powder X-ray diffraction and other experimental data helped us propose a nucleation and growth mechanism for GNSs. We developed a theoretical model and calculated the energy barriers for nucleation and the energy required for particle growth.