Synthesis of Homogeneous Gold Nanorods through the Optimized Multi-step Seed-Mediated Growth Method

The uniformity of nanoparticles is critical to their properties and applications, especially for anisotropic nanoparticles such as gold nanorods (GNRs). The uniformity of GNRs mainly includes three different aspects: shape, optical, and size. The shape uniformity of GNRs has been improved above 95% by using the silver-assisted seeded growth method, and their optical uniformity can be improved by using organic additives in the synthesis or by post-synthetic laser irradiation. Compared with the shape and optical uniformity, the size uniformity of GNRs still needs more effort to improve. Here, we demonstrated that the size uniformity of GNRs can be effectively improved by separating the symmetry breaking of seeds from subsequent growth into different steps, called the multi-step growth method. Compared with the traditional one-step growth, the full width at half maximum (fwhm) of the longitudinal localized surface plasmon resonance (L-LSPR) peak and the standard deviation of length, width, and aspect ratio of GNRs after just one more step growth has been greatly improved from 182 nm, 9.9%, 9.6%, and 14.7% to 82 nm, 4.8%, 4.3%, and 7.3%, respectively. In addition, the size and L-LSPR absorption of GNRs obtained from multi-step growth can be tuned in a broad range by the amount of seeds, the silver ion content, and the pH value of the growth solution in each step. This multi-step growth method allows us to distinguish between the effects of each experimental parameter on symmetry breaking and anisotropic growth, for example, silver ion is primarily responsible for symmetry breaking of seeds while pH value has a more significant effect on anisotropic growth, which provides new insights into the growth mechanism of GNRs. We believe that this approach will further facilitate the research of GNRs into practical applications that rely on the high uniformity of GNRs.