Production of aqueous spherical gold nanoparticles using conventional ultrasonic bath
© Lee et al.; licensee Springer. 2012
Received: 18 May 2012
Accepted: 27 July 2012
Published: 27 July 2012
A conventional ultrasonic bath was used to examine the feasibility of forming aqueous spherical gold nanoparticles (GNPs) under atmospheric conditions. The effects of ultrasonic energy on the size and morphology of GNPs were also investigated. Highly monodispersed spherical GNPs were successfully synthesised by sodium citrate reduction in a conventional ultrasonic bath, without an additional heater or magnetic stirrer, as evidenced by ultraviolet–visible spectra and transmission electron microscopy. Ultrasonic energy was shown to be a key parameter for producing spherical GNPs of tunable sizes (20 to 50 nm). A proposed scheme for understanding the role of ultrasonic energy in the formation and growth of GNPs was discussed. The simple single-step method using just a conventional ultrasonic bath as demonstrated in this study offers new opportunities in the production of aqueous suspensions of monodispersed spherical GNPs.
KeywordsSpherical gold nanoparticle Ultrasonic bath Ultrasonic energy
Gold nanoparticles (GNPs) have generated much interest due to their unique and attractive physical and chemical properties, such as high thermal and electrical conductivity, photothermal effects, tunable size and shape dependent optical properties, chemical stability, biocompatibility and facile functionalisation, and are used in a wide range of applications including material science, catalysis, biomedicine, and quantum dots technology [1–6]. Since the first scientific research on the formation of gold colloids by the reduction of gold trichloride by phosphorus was published by Faraday in 1857 , various methods for the synthesis of colloidal gold have been used, such as chemical methods [8–11]. The well-known Turkevich method [8, 12] is the simplest way to produce aqueous suspensions of monodispersed GNPs with good stability .
Sonochemistry has also been used to synthesise colloidal gold since the pioneering work on the formation of GNPs using ultrasonic sound was carried out in 1980 . Extensive studies on the sonochemical production of GNPs have been performed to investigate the effects of many synthesis variables on the size of GNPs [14–21]. These studies show that most GNPs have been synthesised in non-aqueous solutions using a high intensity ultrasonic generator. For example, various alcohols were used as the base fluid, reducing agent, and stabiliser in the greater part of sonochemical works [14–18, 20]. However, with regard to GNPs synthesised in aqueous solutions, there has only been limited research. It has been reported that the rates of the formation of GNPs in pure water were approximately zero without any additives such as surfactants, water-soluble polymers and aliphatic alcohols and ketones under atmospheric conditions, resulting in only a small amount of synthesised GNPs that were unstable and coagulated within several hours . Horn or cup-horn type ultrasonic generators were used in previous studies [14–20] to apply sufficient ultrasound energy to cause the pyrolysis of fluid molecules. An appropriate ultrasonic energy is required to induce collapsing gas bubbles with high temperature (in excess of 4,000 K) [22, 23]. It also has been reported that the reduction of gold (III) occurred when using a high intensity ultrasonic generator, but this did not occur when using a conventional ultrasonic bath .
The present study is interested in producing GNPs in water, not in an organic medium, because GNPs used in biological applications are in water. Hence, it is of practical interest to synthesise aqueous GNPs in a simpler but more consistent process using a conventional ultrasonic bath instead of a horn or cup-horn type ultrasonic apparatus. The importance of the synthesis of aqueous GNPs is well-summarised by Ji et al.  as follows: (1) High-quality gold nanocrystals have been synthesised in non-aqueous solutions under elevated temperatures; (2) however, from a green chemistry standpoint, all non-aqueous synthetic schemes are far from ideal; (3) water may eventually become a plausible medium for the growth of high-quality nanocrystals with various compositions ; and (4) this attractive future will likely come with systematic and quantitative studies of some carefully chosen aqueous model systems such as aqueous gold nanocrystals synthesised by citrate reduction. In addition, a conventional ultrasonic bath may become a simple apparatus for the production of consistent quality spherical GNPs in aqueous solutions. However, Nagata et al.  and Okitsu et al.  have shown that it is barely possible to synthesise stable GNPs in pure water using a conventional ultrasonic bath under atmospheric conditions.
Recently, Chen and Wen  proposed a novel ultrasonic-aided method for the synthesis of aqueous gold nanofluids containing both spherical and plate-shaped GNPs and demonstrated that their shape and size were controllable. They synthesised aqueous gold nanofluids containing spherical GNPs by the conventional citrate reduction method, and then placed the gold nanofluids in an ultrasonic bath to study the effect of sonication time on nanoparticle size. They also synthesised aqueous gold nanofluids containing plate-shaped GNPs by citrate reduction of chloroauric acid (HAuCl4) solutions immersed in an ultrasonic bath at room temperature to study the effect of sonication time on the morphology of the gold materials produced.
In this work, only a conventional ultrasonic bath was used without an additional heater or magnetic stirrer under atmospheric conditions to examine the feasibility of forming aqueous GNPs by sodium citrate reduction. Although ultrasonication was used in this study in Chen and Wen , its effect is quite different. Since we produced spherical-shaped GNPs in the presence of ultrasonication, we were able to see the effects of sonication on the formation of spherical-shaped GNPs. In contrast, such effects were essentially non-existent in the investigation of Chen and Wen  because they sonicated aqueous spherical-shaped GNPs that had been formed and grown in the absence of ultrasonication. We also examined the effects of ultrasonic energy on the size and morphology of GNPs and discussed a proposed scheme for understanding the role of ultrasonic energy in the formation and growth of spherical GNPs in an ultrasonic bath. The present study shows for the first time that aqueous spherical GNPs can be produced by sodium citrate reduction in a conventional ultrasonic bath without any additional heater or magnetic stirrer. This single-step synthesis of aqueous GNPs using a conventional ultrasonic bath allows us to investigate the effects of sonication time and ultrasonic energy on the formation and growth of GNPs.
Production of gold nanofluids using an ultrasonic bath
Our method to produce spherical-shaped GNPs is unique, mainly because the starting materials we used and the shape of GNPs we produced are quite different from those described in the experimental study of Chen and Wen . First, the starting materials to which we applied sonication to make spherical-shaped GNPs were chloroauric acid, sodium citrate and water. In contrast, the starting materials to which Chen and Wen  applied sonication were aqueous 20 nm spherical-shaped GNPs that were already synthesised in the absence of ultrasonication by the conventional CR method. Therefore, it can be inferred that ultrasonication did not affect either the formation or growth of spherical-shaped GNPs. Second, when ultrasonication was used from the initial CR stage, the results obtained by our method and that used by Chen and Wen  are different. The GNPs produced by the other method were plate-shaped rather than spherical-shaped. In contrast, we produced spherical-shaped GNPs. It should be noted that the primary aim of our work was to synthesise aqueous spherical GNPs in a conventional ultrasonic bath. The difference in the shape of GNPs was due to the different reaction temperatures and molar ratios of HAuCl4 to sodium citrate as follows: (1) We maintained the reaction temperature at 80 °C to 85 °C during the entire process of synthesis of GNPs in our study, while Chen and Wen  performed experiments at 25 °C and exposed the resultant solutions to natural light for 16 h further; and (2) we set the molar ratio between HAuCl4 and sodium citrate at 1:3.5 based on the study by Ji et al. ; while Chen and Wen  used a different molar ratio based on the study by Huang et al. . As a consequence, we produced spherical-shaped GNPs, while Chen and Wen  produced plate-shaped GNPs. Since the focus of our study was on the spherical-shaped GNPs, we did not use their data on plate-shaped GNPs, although they are interesting and important.
Our method has two primary advantages over that of Chen and Wen . First, our results presented in the next section show that ultrasonication alone is very effective in the synthesis of spherical GNPs. In this way, a heater/stirrer is not necessary to initiate nanoparticle formation and growth. Thus, we have developed a simplified method to produce size-tunable spherical GNPs. Second, because this simplified method involves fewer steps compared to the procedure used by Chen and Wen , it is a highly reproducible method for making consistent quality spherical GNPs.
Results and discussion
In summary, highly monodispersed spherical GNPs were produced by the sodium citrate reduction method using a conventional ultrasonic bath without an additional heater or magnetic stirrer. It was found that the sonication energy has a significant effect on the particle size and morphology of GNPs for a fixed ultrasonic power and frequency. Thus, this study shows the importance of ultrasonic energy in the ultrasonic-induced production of water-soluble GNPs of tunable sizes (20 to 50 nm) by citrate reduction. A hypothetical scheme for understanding the role of ultrasonic energy on the size of water-soluble GNPs was discussed. The single-step method using a conventional ultrasonic bath developed in this study offers new opportunities to synthesise aqueous suspensions of monodispersed spherical GNPs without a magnetic stirrer.
The use of ultrasonication without any additional heating and stirring devices is both technologically and scientifically important. Since our results successfully demonstrated that ultrasonication alone is very effective in the synthesis of spherical GNPs, we have developed a simplified method to produce spherical GNPs. Furthermore, because this simplified method involves fewer steps compared to the procedure used by Chen and Wen , it is a highly reproducible method for making spherical GNPs of consistent quality. It can, hence, be expected to produce a large volume of consistent quality spherical-shaped GNPs.
Similar experiments using a conventional ultrasonic bath should be performed for other variables, such as the reduction temperature and quantity of sodium citrate.
Transmission electron microscopy
This work was supported by the National Research Foundation of Korea Grant funded by the Korean government (NRF-2011-0013579).
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