pH-mediated synthesis of monodisperse gold nanorods with quantitative yield and molecular level insight
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T Although gold nanorods (GNRs) have been produced with different dimensions and aspect ratios, the current synthesis methods through seed-mediated growth are far from ideal, for instance, the quality (rod yield) and the quantity (gold conversion) cannot be simultaneously satisfied. More critically, there is no molecular level understanding of the growth mechanism. Here, we solved the problem by employing the stoichiometric ratio of reactants and tuning the reactivity of the reductant through adjusting the initial pH value of the growth solution to achieve both good quality and high quantity simultaneously. We also extended our strategy to other enols besides ascorbic acid, such as phenolic compounds, and found that the optimal pH for GNRs synthesis depends on the structure of the individual compound. The mechanistic insight greatly enriches the toolbox of reductants for GNRs growth and makes it possible to synthesize GNRs at both acidic and basic conditions. An interesting phenomenon is that for most of the phenolic compounds we tested, the morphology of the final products follows the same sphere-rod-sphere trend as the initial pH value of the reaction increases, whether it is under acidic or basic conditions, which cannot be explained by any previously proposed mechanism. The effect of pH is mainly attributed to the regulation of the reduction potential of the reductants, and thus the reaction rate. A model has been proposed to explain the dependence of anisotropic growth of GNRs on the concentration gradient of reactants around the seeds, which is decided by both the reaction rate and diffusion rate.
KEYWORDS gold nanorods, gold conversion yield, shape homogeneity, pH value, reduction potential, concentration gradient
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Introduction
Gold nanorods (GNRs) are probably the most well-known anisotropic nanoparticles due to their unique one-dimensional (1D) nanostructures, which allow them to display size- and shape-dependent localized surface plasmon resonance (LSPRs) that cover a broad spectral region in the visible and near-infrared range [1–6]. The diversity of their plasmonic properties facilitates the broad applications in many different fields, such as sensing [4, 7–9], solar harvesting [10], photovoltaics [11], surface enhanced spectroscopies [12], and therapy [13, 14]. However, one prerequisite for the broad applications of GNRs is that they must be synthesized with high quality in a cheap, scalable, and reliable way [15]. Furthermore, in terms of fundamental science studies, the nucleation and growth mechanisms of GNRs remains a huge challenge despite the numerous reports already in the literature. Thus, achieving molecular-level mechanistic insights is a current focus in the community, which calls for new breakthroughs. GNRs were originally prepared in hard templates through electrochemical deposition, such as porous alumina and polycarbonate membranes [16, 17]. This hard template strategy was quickly replaced by the more practical threestep seed-mediated growth method [18–20], which was further developed
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