Insight into the Role of Ag in the Seed-Mediated Growth of Gold Nanorods: Implications for Biomedical Applications

The synthesis of gold nanorods (AuNRs) with a specific length/diameter aspect ratio is crucial for their use in imaging, sensing, drug delivery, and biological applications. However, the most commonly used silver-aided seed-mediated synthesis method still suffers from poor outcome predictability and hence, in an overall sense, reproducibility. To address this information gap, the mechanism of the seed-mediated synthesis has been investigated, particularly with regard to the possible existence of limiting reagents or intermediates in the reaction. The key silver intermediate which controls the AuNR aspect ratio has thus been identified as a CTA–Ag–Br complex. The AuNR growth solution preparation process has been systematically investigated and the solubility of the CTA–Ag–Br complex is established to be the limiting agent in the preparation and growth of the resulting AuNR. The sequence of reagent addition is shown to also be a determinant in the evolution of a resulting gold nanorod. The importance of the CTA–Ag–Br complex in nanorod syntheses is supported by the observation of gold NP formation when a reductant (ascorbic acid) is added before the CTA–Ag–Br complex has formed. This result informs the understanding of the role of the silver ion in the AuNR synthesis and provides a much-needed entry to the synthesis of AuNRs with custom aspect ratios, including those with much sought after large aspect ratios. This result will benefit research involving AuNRs, especially that in biomedical applications.