Synthesis of Different Aspect-Ratios of Fixed Width Gold Nanorods

Abstract

Gold nanorods (Au NRs) have been widely used in the optics, electricity, informatics, and biomedical fields in recent years. However, Au NRs with specialized requirements cannot be prepared by conventional methods. For instance, in photothermal therapy, Au NRs with high aspect ratios (ARs) are desirable for increasing tissue penetration and reducing the burning of human skin during treatment. However, when their ARs were adjusted to match the laser used in second near-infrared windows (NIR-II), the length and width of the Au NRs simultaneously increased. This increase in width reduces its photothermal conversion efficiency. Unfortunately, tuning the ARs of Au NRs at a fixed width requires complex procedures. In this study, we developed a new seeded-growth method to synthesize different ARs of fixed width Au NRs (FW-Au NRs). To the best of our knowledge, this is the first study to adjust the length of FW-Au NRs by introducing lauryl alcohol (LA) molecules into the traditional seeded growth method. Moreover, the length span of FW²³-Au, FW¹⁴-Au, and FW^6.5-Au NRs (the superscript numbers denote the width of Au NRs in nm) was adjusted between 130 and 38.4 nm, 109 and 26.4 nm, and 16 and 46 nm, respectively, by judiciously selecting the corresponding reaction conditions. Notably, the lengths of the Au NRs can be readily achieved at a fixed width over a wide range. In addition, their ARs were tuned at a fixed width by adjusting only their length, instead of simultaneously varying their length and width. In addition, their widths were maintained between 6.5 and 23 nm by adjusting [AgNO₃] between 0.24 and 0.30 mmol·L⁻¹ in the presence of LA. Furthermore, the synergetic effect of Ag⁺ and LA on the density of the cetyltrimethylammonium (CTA)-Br-Ag⁺ complexes distributed on the facets of added Au-NP seeds, which can impact their symmetry-breaking efficiency (SBE) and the particle number of Au-NP seeds that grow into final Au NRs, is key to the synthesis of FW-Au NRs. The results of this study offer a flexible and reliable method to tune the length of Au NRs with a fixed width and pave the way for achieving an on-demand synthesis of Au NRs, especially for cancer photothermal therapy.

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