The research presented here is to understand the synthesis mechanism of gold nanorods (AuNRs) from a kinetic perspective by considering it as a diffusion-controlled reaction, investigate the relationship between the reaction/diffusion speed and the anisotropic growth, and thus optimize the synthesis to improve the shape uniformity and synthesis reproducibility of AuNRs. Although various AuNRs have been produced with different aspect ratio, current synthesis methods through seed-mediated growth are far from ideal, sharing the same drawbacks, such as low yield of gold conversion (~20%), poor shape uniformity and reproducibility, due to a lack of understanding of the reaction mechanism. While the mechanism of the anisotropic growth of AuNRs is not clear yet, the experimental detail in the literatures shows evidence that it is a diffusion-controlled reaction (for example, no stirring, low temperature and reaction rate dependent), where the final products depend on the relative rate of transport of reactants through the reaction medium and the rate of chemical reduction of metal ion. As a diffusion-controlled reaction, both the diffusion rate and reduction rate are important. Basically, the reduction rate depends on both the reactivity and concentration of the reactants (gold salts and ascorbic acid (AA)). The concentration of AA is commonly utilized to control the reduction rate. For example, to decrease the reduction rate, the ratio of AA to gold ion has been optimized to be 1.1 which is far below the stoichiometric ratio (1.5), with extremely poor yield of gold conversion (~20%). To achieve a high gold conversion yield, it is better to keep stoichiometric ratio (1.5) but tuning the reduction rate by reactivity. As a polyol compound, the reactivity of AA depends on the pH of the system. At stoichiometric ratio (1.5), the optimal pH range for AA has been discovered to get prefect AuNRs with improved uniformity, reproducibility and gold conversion yield (&gt; 80%). The gold conversion yield can be improved further at higher ratio of AA to gold ion than the stoichiometry. With the same idea, the PI has extended the reducing agent to other polyol compounds, such as phenol, hydroquinone, catechol, resorcinol, and phloroglucinol. Other kinetic controls were also studied and a model has been created to correlate these factors together.

Kinetic control of the seed-mediated growth of gold nanorods

256th National Meeting (2018)

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