Abstract
The seed-mediated method is a general procedure for the synthesis of gold nanorods (Au NRs), and reducing agents such as ascorbic acid (AA) and hydroquinone (HQ) are widely used for the growth process. Further, they are mild reducing agents; however, when AA is used, controlling the size of Au NRs with a higher aspect ratio (localized surface plasmon resonance (LSPR) peak, λLmax > 900 nm) is challenging because it results in a faster growth rate of Au NRs. In contrast, when HQ is used, Au NRs with a higher aspect ratio can be synthesized as it slows down the growth rate of the Au NRs and greatly enhanced the λLmax. However, the increase in λLmax is still needs not satisfactory due to the limited enhancement in the aspect ratio of Au NRs due to utilization of single reducing agent. The growth kinetics of the Au NRs can be modulated by controlling the reducing power of the reducing agents. In such scenario, judicious use of two reducing agents such as AA and HQ simultaneously can help us to design Au NRs of higher aspect ratio in a controlled manner due to the optimum growth rate resulting from the combined effect of both the reducing agents. In this study, we investigated the effect of the two reducing agents by controlling the volume ratios. When the growth solution contains both the reducing agents, the growth of Au NRs is first initiated by the fast reduction of Au3+ to Au+ due to stronger reducing power of the AA and when the AA in the growth solution is completely utilized, further growth of the Au NRs continues as a result of the HQ thereby resulting to high aspect ratio Au NRs. Consequently, the LSPR peak (λLmax > 1275 nm) can be tuned by controlling the volume ratios of the reducing agents.
Highlights
Gold nanoparticles are extensively studied noble metal nanoparticles owing to their applications, including in biosensors and photocatalysis, cancer therapy, drug delivery, and antimicrobial activities
This study shows that Au Gold nanorods (NRs) with a high aspect ratio can be obtained with uniformity by controlling the volumes of Ascorbic acid (AA) and HQ, and the LSPR peak (λLmax) can be shifted to the near-infrared radiation (NIR) region to a maximum of approximately 1300 nm, suggesting its potential application in NIR-based photothermal applications
Selecting an appropriate reducing agent is essential so that Au NRs with the desired aspect ratio are synthesized during the growth step
Summary
Gold nanoparticles are extensively studied noble metal nanoparticles owing to their applications, including in biosensors and photocatalysis, cancer therapy, drug delivery, and antimicrobial activities. AA may cause the reduction of some Au ions to grow into relatively short Au NRs because of its stronger reducing power than HQ; subsequently, HQ can gradually reduce the remaining Au ions after AA is completely utilized This can be achieved by controlling the amounts of AA and HQ. We investigated the effect of using both AA and HQ as reducing agents on the aspect ratio of Au NRs, and the effect of the combination of the reducing agents mixed at different ratios, on the growth of the Au NRs. This study shows that Au NRs with a high aspect ratio can be obtained with uniformity by controlling the volumes of AA and HQ, and the λLmax can be shifted to the NIR region to a maximum of approximately 1300 nm, suggesting its potential application in NIR-based photothermal applications
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