Unraveling the Seedless Growth of Gold Nanostars through Fractional Factorial Design

Abstract

Gold nanostars (AuNSs) are anisotropic nanoparticles that display morphology-tunable optical properties. Most AuNS synthetic methods require a two-step procedure and the use of cytotoxic and/or strongly bound surfactants, which complicate biomedical application. The seedless synthesis of AuNSs overcomes those limitations by using biocompatible Good’s buffers, such as HEPES, as reducing and stabilizing agents. However, the simplicity of this one-pot protocol (i.e., two reagents and one solution) can result in high batch-to-batch variability, as several factors are thought to affect the growth of the nanostars. In this paper, we used a fractional factorial design of experiments to identify five factors (i.e., HEPES concentration, Au3+ concentration, temperature, pH, and stirring) that control the growth, morphology, and optical properties of AuNSs. Notably, we identified several secondary interactions, most prominently, temperature–Au3+ concentration and pH–HEPES concentration, for which simultaneous changes of both factors induced larger effects than the sum of effects of each factor alone. These results provide new insights into the growth of AuNSs, explaining some of the discrepancies in the literature regarding the impact of certain factors and highlighting the importance of characterizing secondary interaction effects to improve the synthesis of gold nanoparticles with desired properties.