Articles
DOI: 10.1002/cphc.201402697
Sonochemical Synthesis of Gold Nanoparticles by Using High Intensity Focused Ultrasound Nor Saadah Mohd Yusof [a, b] and Muthupandian Ashokkumar*[a, c] The sonochemical synthesis of gold nanoparticles (GNPs) with different shapes and size distributions by using high-intensity focused ultrasound (HIFU) operating at 463 kHz is reported. GNP formation proceeds through the reduction of Au3 + to Au0 by radicals generated by acoustic cavitation. TEM images reveal that GNPs show irregular shapes at 30 W, are primarily icosahedral at 50 W and form a significant amount of nanorods at 70 W. The size of GNPs decreases with increasing acoustic power with a narrower size distribution. Sonochemilumines-
cence images help in the understanding of the effect of HIFU in controlling the size and shapes of GNPs. The number of radicals that form and the mechanical forces that are generated control the shape and size of the GNPs. UV/Vis spectra and TEM images are used to propose a possible mechanism for the observed effects. The results presented demonstrate, for the first time, that the HIFU system can be used to synthesise sizeand shape-controlled metal nanoparticles.
1. Introduction agent.[19] This process led to the formation of gold seed particles of about 2 nm. These seed nanoparticles were then grown to various sized GNPs with precise control over their size in the range of 2–8 nm by using decyl amine as a reducing agent. A number of other techniques have been developed to synthesise size-controlled GNPs by using similar strategies.[20–22] Interestingly, Huang et al. used naturally occurring plant polyphenol to synthesise GNPs in the size range of about 2 nm.[22] The polyphenol has a dual role as a reducing agent and as a stabilising agent. As mentioned earlier, the shapes of GNPs play an essential role in their physical and functional properties. This is evident by the increasing number of studies that have focused on synthesising shape-controlled GNPs.[18, 23–26] Some pioneering work in this area was reported by Murphy and co-workers.[18, 23, 27] In one of their studies, a surfactant-mediated three-step process was successfully developed to synthesise GNPs as nanorods.[23] Non-spherical seed GNPs were initially prepared and then added to a solution containing gold ions and ascorbic acid in the presence of a surfactant to achieve nanorod formation.[23] The mechanism involved twinning and preferential binding of cetyltrimethylammonium bromide (CTAB) molecules on the (100) crystal face. Grzelczak et al. reviewed various synthetic methods for the generation of shape-controlled GNPs.[24] In general, the choice of seed particles, the nature of the stabilising and reducing agents, and the Au3 + /seed ratio play a significant role in controlling the shape of GNPs. Sonochemical synthesis of metal nanoparticles has been extensively studied in recent years.[28–36] The interaction between sound waves and bubbles in aqueous solution leads to acoustic cavitation: the growth and collapse of gas/vapour bubbles in an acoustic field.[37] The collapse of cavitation bubbles generates extreme temperatures within cavitation bubbles that ultimately result in the formation of highly reactive radicals. For
There is broad interest in developing novel techniques for synthesising metal nanoparticles owing to their unique physical and functional properties and wide applications, ranging from catalysis to biomedical.[1–12] The synthesis of gold nanoparticles (GNPs) has received special attention because they exhibit extraordinary optical,[13] electronic,[14] molecular recognition[15] and catalytic properties.[7] The physical and functional properties of GNPs depend not only on their size and size distribution, but also on their shapes.[16, 17] A variety of methods have been reported to tailor the size and shape of GNPs. Sau and Murphy synthesised size-controlled GNPs in a two-step chemical reduction technique.[18] In the first step, 5–20 nm seed GNPs were prepared by the photochemical reduction of gold ions.[18] The seeds then were grown to 20–100 nm GNPs by a chemical reduction method with ascorbic acid as a reducing agent.[18] The size and size distribution of the GNPs was controlled by varying the ratio between the gold ions and seed particles.[18] Yang et al. also used the Au3 + /seed ratio to control the size and size distribution of GNPs in a different approach.[19] In a complex process, HAuCl4 was dissolved in toluene and Au3 + reduction was initiated by NaBH4 in the presence of dodecanethiol as a stabilising
[a] N. S. M. Yusof , Prof. M. Ashokkumar School of Chemistry, University of Melbourne Parkville, VIC 3010 (Australia) E-mail: [email protected] [b] N. S. M. Yusof Department of Chemistry, University of Malaya Kuala Lumpur (Malaysia) [c] Prof. M. Ashokkumar Chemistry Department, King Abdulaziz University Jeddah, Saudi Arabia Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402697.
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Articles example, homolysis of water molecules within bubbles due to high-temperature conditions generates HC and HOC radicals (generally referred to as primary radicals). In the presence of suitable surface-active solutes, secondary reducing radicals can be produced. Such reducing radicals have been used to reduce metal ions to metal atoms.[38] The aggregation of metal atoms leads to the formation of nanoparticles. A number of studies have reported the synthesis of GNPs by using sonochemically generated secondary radicals.[38] In a typical study, the sonication of an aqueous solution containing gold chloride ions leads to the formation of GNPs. Okitsu et al. reported the size-controlled formation of GNPs by manipulating the acoustic parameters, namely, sonication power and frequency.[29] A higher rate of reduction resulted in the formation of smaller GNPs. Because the rate of reduction is directly dependant on the radicals present, which is governed by the ultrasound frequency used, a correlation between ultrasound frequency and the size of GNPs has been established. Although studies on the sonochemical synthesis of size-controlled GNPs are available in the literature, the control of shapes of GNPs by a sonochemical method has not been widely found in the literature. Hence, the main aim of this study is to explore the possibility of synthesising shape-controlled GNPs by a sonochemical procedure. For this purpose, high-intensity focused ultrasound (HIFU) has been used. HIFU is chosen for two reasons: 1) there are no reports available on the HIFU-mediated synthesis of GNPs, and 2) HIFU can deliver strong mechanical/shear forces as well as a high number of radicals in the sonicated medium that may result in size- and shape-controlled GNPs. HIFU is used in medicine for therapeutic and diagnostic applications.[39–42] It is most popular in therapeutic medicine, such as for the ablation of malignant and metastatic tumours, thrombolysis and targeted drug- or genedelivery deep within the human body.[43–46] In addition, the focus of HIFU can be controlled to a well-defined volume. The use of HIFU in medical procedures lies behind the use of mechanical energy to generate heat and cavitation bubbles at the focal area, resulting in tissue lesion.[47] Herein, we have used HIFU for the sonochemical synthesis of GNPs to control their size and shape. It is important to mention that HIFU used in this study was custom built for chemical reactions and operated at much lower frequency and at a power level higher than that used in medicine. Sonochemiluminescence (SCL), UV/Vis spectra and TEM images have been recorded to understand the formation mechanism of GNPs of various sizes and shapes.
Figure 1. Colour changes observed upon HIFU sonication of solutions of gold chloride at f = 463 kHz and P = 30, 50 and 70 W.
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The reason behind the colour changes observed are discussed throughout the Results and Discussion section. It is known that the colour of GNPs in bulk solutions reflects their morphologies at the nanoscale.[13, 48, 49] The gradual colour changes observed during sonication is due to the formation and growth of GNPs to different sizes and shapes as governed by the experimental conditions. To quantitatively follow this, the concentration of Au3 + was monitored at l = 217 nm by using UV/Vis absorption spectroscopy. An example of the spectral changes observed at 70 W is shown in Figure 2 a. As the Au3 + absorption (l = 217 nm) decreased, a new absorption band in the wavelength range of l = 500–600 nm appeared and grew during sonication. This is the plasmon absorption band of GNPs.[28, 50–53] Further analysis of this absorption band for each power level is discussed later. The band at l = 217 nm was then converted to Au3 + concentration ([AuCl4]), and the results are plotted in Figure 2 b. Figure 2 b shows the amount of Au3 + reduced as a function of sonication time for different acoustic power levels. The plots clearly show that the rate of reduction depends upon the power used. A faster reduction is observed at higher power. This seems logical as the sonication of an aqueous solution at higher acoustic power levels generates a higher number of cavitation bubbles, and therefore, a higher amount of radicals. It is also possible that a greater number of radicals are generated per bubble at higher acoustic power levels due to the larger size (Rmax) of the cavitation bubbles.[37] All GNP samples were analysed by using TEM, and the images are shown in Figure 3 along with the size distributions of the GNPs. For clarity, let us first focus on the differences in their size distribution.
Upon sonication, the colourless solution of gold chloride (AuCl4) went through gradual colour changes to pink and then to red for 30 W, purple for 50 W and blue for 70 W (Figure 1). Such colour changes are due to the formation and growth of GNPs by the reduction of Au3 + ions (AuCl4) to formAu0. The reduction of Au3 + ions is aided by the sonochemically generated secondary radicals through Reactions (1)–(4):[38] ChemPhysChem 0000, 00, 0 – 0
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2. Results and Discussion
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Articles possible reasons for different shapes of GNPs are discussed later. Considering the size and size distribution, the size of GNPs is dependent upon the rate of reduction.[29] A higher rate of reduction favours the formation of smaller GNPs and vice versa. Okitsu et al. showed a direct correlation between the rate of reduction of Au3 + ions and the size of GNPs, for which the rate of radical production was controlled by varying the ultrasonic frequencies.[29] At 70 W, it can be expected that the amount of secondary reducing radicals produced is higher. Therefore, the GNP size is smaller with a narrower size distribution at 70 W. At 30 W, the size distribution was broader than the other two power settings. This may also be due to a slower reduction rate, leading to competition between nucleation and growth processes that occur simultaneously. The correlation between the rate of reduction and GNP sizes can be easily understood by the schematic diagram shown in Figure 4. In Figure 4 i, there are 20 nAuCl4 [n = number of Au0 (gold nuclei) molecules needed to form one Aun] and 2 nR (nR = radicals needed to reduce n number of Au3 + to Au0). The 2 nR act by reducing 2 nAuCl4 to form 2 Aun. The reduction process continues in a similar manner, leading to the formation of new nuclei and the growth of existing nuclei. If the number of nR is low (at low acoustic power levels), growth of existing nuclei dominates and results in the formation of a small Figure 2. a) UV/Vis spectra of sonicated solution at 70 W. b) Au3 + reduction number of larger GNPs. At higher power levels, nR is higher upon HIFU sonication at various power levels (squares: 30 W, triangles: 50 W, circles 70 W). and hence the nucleation process dominates over the growth process (Figure 4 ii), resulting in the formation of a large number of small GNPs. Thus, the most important reason for the variation in GNP size is the difference in the rates of nucleation and growth processes, which, in turn, depends upon the concentration of the reducing agent.[54] The implosion of cavitating bubbles under specifc experimental conditions results in the emission of light, which is known as sonoluminescence (SL).[53, 55] Due to the high temperature within the cavitation bubbles, hydroxyl radicals (HOC) are also generated from the homolysis of water [Reaction (1)].[53, 55] If luminol is present in the solution, HOC radicals react with luminol and generate an intermediate that can chemiluminesce.[56] This is referred to as SCL.[53, 56] SCL imaging was performed to gain further insight into the cavitation activity at various power levels. The cell used and the SCL images observed for the HIFU system are shown in Figure 5. It is evident from the SCL image in Figure 5 that there are five regions of cavitation activity due to the establishment of a local standing wave pattern in the Figure 3. Particle-size distribution and TEM images for the GNPs synthesised by means of sonicated solution. 1) Region I is the focal area of the HIFU at 463 kHz and 30 (a), 50 (b) and 70 W (c). HIFU system where no SCL is observed. This region is where a strong mechanical force is generated due to the focussed ultrasound waves.[57] The diameter of Region I The GNPs were largest for 30 W, followed by 50 W and smallest for 70 W. For 30 W, the size ranges from 12 to 104 nm and was measured to be 0.57 cm. 2) Region II is the bubble-rich the size distribution shows an irregular pattern. For 50 W, the area or the antinode where cavitation bubbles accumulate due size ranges from 8 to 22 nm and shows a mono-modal distrito Bjerknes forces; therefore, a strong SCL activity can be obbution, whereas for 70 W the spherical GNP is from 3 to served. 3) Region III is a nodal region with no SCL activity. 14 nm. In addition, nanorods were also found at 70 W. The 4) Region IV is the antinode of active bubbles with clear SCL ChemPhysChem 0000, 00, 0 – 0
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Articles At 25 W, very weak SCL is observed for Region II. No sign of active bubbles was observed in other regions due to the very low sonication power. With an increase in acoustic power, more active bubbles formed. This was evident from the increasingly pronounced regions of SCL activity from 30 to 50 W. The SCL profile intensity is most defined with clear nodes and antinodes at 50 W. The standing waves are well defined and undisturbed by the strong mechanical forces from Region I. However, from 50 to 80 W, the SCL intensity is visually found to decrease. Moreover, the nodes and antinodes of Region II, and the standing waves in Region V, become less defined. This is due to the stronFigure 4. Illustration showing the gold reduction processes in the presence of a relatively ger mechanical force from Region I that forced the small number of reducing radicals (i) and a relatively large number of reducing radicals displacement of the cavitation bubbles. It is evident (ii). nAuCl4 is the number of gold chloride molecules needed to form one gold nuclei from these images that a decrease in the amount of after reduction, nR is the number of radicals needed to reduce one nAuCl4 , Aun is the HOC radicals occurs at very high power levels, which gold nuclei and GNP represents gold nanoparticles. contradicts the trend observed for Au3 + reduction shown in Figure 2. We quanitified the amount of HOC radicals generated under these experimental conditions by using the Weisler method and observed a reduction in HOC radicals at higher power levels.[58] However, this contradictory observation could be explained based on the mechanism involved in Au3 + reduction. Previous studies showed that reducing radicals generated within cavitation bubbles were primarily responsible for the Au3 + reduction process. Although there is a reduction in the number of active cavitation bubbles and primary radical production, the overall amount of secondary radicals generated may still be high at higher power levels. Both SCL and Weisler methods provide information on primary radicals only. However, a previous study has reported that there is no correlation between SL intenFigure 5. SCL profile for HIFU sonication of a solution of luminol (left) in a custom-made sity (maximum bubble temperature) and sonochemicell (right). The width of the glass cell is 4 cm. cal activity.[59] Another important possibility to take into account is the ability of gold nuclei to catalyse activity. 5) Region V is a secondary standing-wave region estabthe reduction of Au3 + ions. Therefore, a higher number of lished within the neck region of the custom-made cell. nuclei generated at higher power levels (higher nucleation The SCL images were taken at increasing acoustic power rate) may also increase the rate of reduction, as observed at from 25 to 80 W (Figure 6). Under the experimental conditions 70 W (Figure 2). In conclusion, a clear trend of size distribution used, no SCL emission was captured at 20 W, and therefore, it dependence on sonication power is observed in this study. was not included herein. The maximum power for the unit was This proves that HIFU can also be used to engineer size-specif100 W, and therefore, the experiment was limited to 80 W. ic GNPs.
Figure 6. SCL profiles upon HIFU irradiation from 25 to 80 W.
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Articles More interestingly, the shapes of synthesised GNPs are also significantly different for all three power levels. This is evident from the TEM images shown in Figure 3. At 30 W (Figure 3 a), the shapes could not be defined because the image consisted of particles with irregular shapes, whereas, for 50 W (Figure 3 b), the shape of most of the particles were icosahedral. Surprisingly, about 50 % of the particles formed at 70 W were nanorods, and the remaining particles showed a spherical shape with very small flat surfaces (Figure 3 c). This can be clearly seen by the smooth surface of the particle shown in Figure 3 c. It is known that the flat surface of the particles possesses higher reactivity for one-directional growth; therefore, preferably forming nanorods.[60] As discussed earlier, an increase in acoustic power from 30 to 50 W caused an increase in active cavitation bubbles followed by a slight decrease in the active cavitation region. If the rate of radical formation were responsible for the difference in the shapes, previous studies should have observed such trends. However, such shape changes were not reported in early studies in which the rate of radical production was correlated with the size and size distribution of GNPs. This suggests that HIFU is unique in generating nanorods at higher power levels. One very clear and interesting characteristic of HIFU is the changes to the strength of mechanical forces as a function of acoustic power. The SCL images shown in Figure 5 clearly suggest that the mechanical forces increase with an increase in acoustic power, as discussed above, as evidenced from the changes to the cavitation field. This observation suggests that the strong mechanical forces generated at higher power settings might be responsible for the significant changes in the shapes of GNPs. To see if the mechanical forces were responsible for the nanorod shape of the GNPs, chemically synthesised 3–5 nm GNP seeds were sonicated at 20 kHz by using a 3 mm horn at 160 W of applied power. At this setting, the shear is strong, but a relatively low amount of radical is generated. The TEM image is shown in Figure S1 in the Supporting Information. The formation of nanorods from the action of the strong shear could be observed under the experimental conditions used. However, the size of the nanorods was larger than that synthesised by using HIFU due to the lower amount of radicals generated at 20 kHz. Based on the observed data and discussion provided above, a possible mechanism for the observed trends in size and shape of GNPs is speculated below. Changes in the size and shape of GNP are reflected by their colour, which can be closely monitored by the shift in the absorption maxima (lmax) of the GNPs. The UV/Vis spectra of GNPs are shown in Figure 2 a. It is important to mention that an increase in the absorption without a significant change in its lmax generally indicates an increase in the number of particles of similar size.[52] Also, a shift in the absoption of GNPs means a change in their size and/or shape.[52] The absorption spectra shown in Figure 2 a clearly show a shift in lmax. Similar observations were made for sonication at 50 and 70 W (Figure S2 in the Supporting Information). For clarity, the shift in lmax is plotted against sonication time in Figure 7. From Figure 7, three distinct regions (a weak shift, a strong shift and almost constant lmax) can be observed for all acoustic power levels. In general, there are three ChemPhysChem 0000, 00, 0 – 0
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Figure 7. Shift of maximum absorption wavelength (lmax) as a function of sonication time for 30 (&), 50 (~) and 70 W (*).
steps involved in the formation of GNPs: nucleation, growth and coagulation.[52, 61, 62] All three processes may occur simultaneously with different processes dominating.[62] Nucleation is mainly driven from the reduction process by the reducing radicals.[54, 62] When this process reaches supersaturation, the nucleation process will effectively stop, whereas growth will mainly continue.[62] Growth can occur by both an autocatalytic process on the surface of gold nuclei (surface reduction) and by reducing radicals.[54, 63] The Au0 species formed combine to form gold nuclei, Aun.[54, 62] A limited number of gold nuclei (Aun) are formed during the nucleation process and a maximum number of nuclei is reached under specific experimental conditions.[62] The growth of particles may occur due to agglomoration between gold nuclei under suitable conditions.[54, 64] Region I (Figure 7), where the lmax shift is very small, corresponds to the domination of the nucleation process over growth. There is a very minor change in the size of GNP nuclei, but the number of nuclei increases. Region II is where the growth process dominates over the nucleation process, as indicated by a strong shift in lmax. Region III is almost a plateau, which indicates that the GNPs have reached their maximum size. These processes are schmatically shown in Figure 8. The conse-
Figure 8. Mechanism of GNP nucleation and possible growth routes.
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Articles quence of Au3 + reduction by the secondary radicals generated during acoustic cavitation is the formation of Au0. As this process continues, few Au0 species combine to form the GNP nuclei (Aun). There are three possible routes for GNP growth under HIFU irradiation. Firstly (Figure 8 I), is the further reduction and nucleation processes by the secondary radicals. The second (Figure 8 II) is reduction by the autocatalytic reaction of the existing nuclei, resulting in immediate growth of the nuclei. The third (Figure 8 III) is the combination of few nuclei, as a result of the strong mechanical forces generated at the focal area that pushes them together. The significance of the three routes are dependent on the experimental conditions. For 30 W, nuclei formation is slower because fewer radicals are generated. Due to the slow reduction of Au0, the autocatalytic growth plays an important role here, along with the formation of new nuclei. Nucleation that occurs over a longer time, as well as significant growth at the surface of existing nuclei, could explain the wide size distribution and variety of shapes of GNP formed. For 50 W, the radicals were generated in relatively large quantities; therefore, the radical-driven reduction process dominated. At the same time, the mechanical force is still contained within the first region (Figure 5), so there is no mechanical effect on the nucleation and growth processes. Therefore, they form icosahedra with a narrow size distribution. However, for 70 W, the third (Figure 8 III) process plays a very important role in the formation of nanorods. This is evident from the SCL image (Figure 5) as well as the TEM images in Figure 3 c. The mechanical forces push the spherical nuclei together, resulting in the formation of nanorods.
sodium hydroxide (Merck, 98 %), ammonium molybdate tetrahydrate (Sigma Aldrich, 99 %), potassium hydrogen phthalate (Ajax, 99.95 %) and luminol (Sigma Aldrich, 97 %) were also of the highest available commercialised purity, and therefore, no purification was needed. All solutions were dissolved in Milli-Q water with a conductivity of < 106 S cm1 and surface tension of about 72.0 mN m1 at 25 8C.
HIFU A custom-built HIFU unit was used in this study (Figure 9). The ultrasound wave was transmitted by an 8 cm concave transducer at either 463 kHz or 1.61 MHz. Only the lower frequency was used in this study.
Figure 9. Illustration of the HIFU unit used in this study.
The shape and specified length of each side on the HIFU unit were chosen so that the sample could be placed where the ultrasound wave was focused and there were no reflected waves (dashed lines in Figure 9) interfering with the focused region of the sample. Water was used elsewhere as a medium for the propagation of ultrasound waves. A custom-made cell was used so that the ultrasound wave could go through a flat glass surface to the sample solution.
3. Conclusions We successfully used a HIFU system to generate GNPs of various sizes and shapes. UV/Vis absorption spectroscopy and TEM were used to analyse the morphology of the GNPs generated. The trends observed for the size and size distribution of GNPs as a function of acoustic power could be understood based on the amount of secondary radicals generated during acoustic cavitation. The most interesting observation in our study was the difference in the shape of GNPs at various power levels. Irregular shapes were observed at low power levels. Icosahedra were observed at a medium power level, and significant amount of nanorods were generated at a higher power level. A possible mechanism, involving the rate of radical production and mechanical forces, was proposed. The results presented herein may initiate a new synthetic route for generating shape-controlled metal nanoparticles.
Methods Water (50 mL) was placed in a custom-made glass cell and irradiated with HIFU operating at a frequency of 463 kHz and at 30, 50 or 70 W of applied power. We did not measure the calorimetric power because the glass cell was immersed in water, as shown in Figure 1, which led to heat dissipation to the surrounding medium. For this reason, we only reported the applied power for this study. For SCL imaging, 0.1 mm luminol (50 mL) was prepared in 0.1 m NaOH and exposed to HIFU. The SCL images were recorded by using a Sony a57 camera with an exposure time of 60 s in a dark room.
Experimental Section
An aqueous solution containing 0.2 mm HAuCl4·3 H2O, 0.1 m isopropanol and 0.01 m SDS (stabilising agent) was prepared for the gold ion reduction experiments. This solution (50 mL) was placed in the glass cell in HIFU setup (Figure 1) and sonicated at 463 kHz with varying applied power. Visual observations of colour changes during sonication were recorded for all experiments. Absorption
Materials HAuCl4·3 H2O) was purchased from Sigma Aldrich with 99.8 % purity and used as received. Isopropanol, sodium dodecyl sulfate (SDS; Ajax, 99 %), potassium iodide (APS, Sigma Aldrich, 99 %),
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Articles spectra of solutions of AuCl4 before and during sonication were measured by using a Shimadzu UV-1601 spectrometer. The solution of AuCl4 showed a band at l = 217 nm, and the formation of gold (Au0) nanoparticles was monitored in the wavelength range of l = 450 to 650 nm. GNPs synthesised were analysed by using a Tecnai TF30 transmission electron microscope from FEI, Eindhoven, the Netherlands. Samples of GNP were prepared by placing a drop of sonicated solution on a copper grid covered with a thin carbon film. It was then left on a filter paper to dry.
[29] K. Okitsu, M. Ashokkumar, F. Grieser, J. Phys. Chem. B 2005, 109, 20673 – 20675. [30] K. Okitsu, Y. Mizukoshi, T. A. Yamamoto, Y. Maeda, Y. Nagata, Mater. Lett. 2007, 61, 3429 – 3431. [31] K. Okitsu, H. Bandow, Y. Maeda, Y. Nagata, Chem. Mater. 1996, 8, 315 – 317. [32] K. Okitsu, K. Sharyo, R. Nishimura, Langmuir 2009, 25, 7786 – 7790. [33] K. Okitsu, B. M. Teo, M. Ashokkumar, F. Grieser, Aust. J. Chem. 2005, 58, 667 – 670. [34] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Langmuir 2001, 17, 7717 – 7720. [35] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Y. Yoo, Bull. Chem. Soc. Jpn. 2002, 75, 2289 – 2296. [36] R. A. Caruso, M. Ashokkumar, F. Grieser, Langmuir 2002, 18, 7831 – 7836. [37] M. Ashokkumar, T. J. Mason, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley, New York, 2000. [38] L. Dharmarathne, M. Ashokkumar, F. Grieser, J. Phys. Chem. A 2012, 116, 7775 – 7782. [39] K. W. Kim, J. Y. Lee, J. M. Lee, Y. S. Jeon, Y. S. Choi, J. Park, H. Kim, J. K. Han, B. I. Choi, J. Med. Ultrasound 2014, 22, 99 – 105. [40] A. L. Malcolm, G. R. ter Haar, Ultrasound Med. Biol. 1996, 22, 659 – 669. [41] N. H. Hoang, H. Y. Murad, S. H. Ratnayaka, C. Chen, D. B. Khismatullin, Ultrasound Med. Biol. 2014, 40, 1869 – 1881. [42] V. S. Dogra, M. Zhang, S. Bhatt, Ultrasound Clin. 2009, 4, 307 – 321. [43] J. Y. Chapelon, M. Ribault, F. Vernier, R. Souchon, A. Gelet, Eur. J. Ultrasound 1999, 9, 31 – 38. [44] N. T. Sanghvi, R. S. Foster, R. Bihrle, R. Casey, T. Uchida, M. H. Phillips, J. Syrus, A. V. Zaitsev, K. W. Marich, F. J. Fry, Eur. J. Ultrasound 1999, 9, 19 – 29. [45] S. Throff, C. Chaussy, G. Vallancien, W. Wieland, H. J. Kiel, A. Le Duc, F. Desgrandchamps, J. J. De La Rosette, A. Gelet, J. Endourol. 2003, 17, 673 – 677. [46] A. Blana, B. Walter, S. Rogenhofer, W. F. Wieland, Urology 2004, 63, 297 – 300. [47] R. E. Apfel, C. K. Holland, Ultrasound Med. Biol. 1991, 17, 179 – 185. [48] S. Aldrich, Gold Nanoparticles: Properties and Applications, http://www.sigmaaldrich.com/materials-science/nanomaterials/goldnanoparticles.html. [49] Nanopartz, Nanopartz Gold Nanoparticles for In Vitro Dark Field Imaging, http://www.nanopartz.com/invitro_imaging_darkfield.asp. [50] W. Haiss, N. T. K. Thanh, J. Aveyard, D. G. Fernig, Anal. Chem. 2007, 79, 4215 – 4221. [51] W. Shi, J. Casas, M. Venkataramasubramani, L. Tang, ISRN Nanomaterials 2012, 2012, 659043. [52] J. Polte, R. Kraehnert, M. Radtke, U. Reinholz, H. Riesemeier, A. F. Thnemann, F. Emmerling, J. Phys. Conf. Ser. 2010, 247, 012051. [53] E. N. Harvey, X. Harvey, J. Am. Chem. Soc. 1939, 61, 2392 – 2398. [54] J. Turkevich, P. C. Stevenson, J. Hillier, Discuss. Faraday Soc. 1951, 11, 55 – 75. [55] S. Putterman, Scientific Am. 1995, p. 6. [56] F. Schultes, Z. Phys. Chem. 1934, 27, 421 – 424. [57] Z. Wang, J. Bai, F. Li, Y. Du, S. Wen, K. Hu, G. Xu, P. Ma, N. Yin, W. Chen, F. Wu, R. Feng, Ultrasound Med. Biol. 2003, 29, 6. [58] A. Weissler, J. Am. Chem. Soc. 1959, 81, 5. [59] M. Ashokkumar, F. Grieser, J. Am. Chem. Soc. 2005, 127, 5326 – 5327. [60] A. R. Tao, S. Habas, P. Yang, Small 2008, 4, 310 – 325. [61] S. Kumar, K. S. Gandhi, R. Kumar, Ind. Eng. Chem. Res. 2007, 46, 3128 – 3136. [62] C. Li, D. Li, G. Wan, J. Xu, W. Hou, Nanoscale Res. Lett. 2011, 6, 440 – 449. [63] A. I. Bulavchenko, A. T. Arymbaeva, V. V. Tatarchuk, Russ. J. Phys. Chem. 2008, 82, 801 – 806. [64] V. S. Cabeza, S. Kuhn, A. A. Kulkarni, K. F. Jensen, Langmuir 2012, 28, 7007 – 7013.
All experiments were repeated at least three times to ensure the reproducibility of the data.
Acknowledegements N.S.M.Y. acknowledges the University of Malaya for the award of Bright Sparks-SLAI scholarships and grant UM.C/HIR/MOHE/sc/07. We acknowledge Dr. Eric Hanssen and Dr. Meifang Zhou for the TEM images. Keywords: gold nanoparticles · high intensity focused ultrasound · shape control · size control · sonochemistry [1] N. Toshima, in Nanoscale Materials (Eds.: L. Liz-Marzn, P. Kamat), Kluwer Academic Pub., London, 2003, pp. 79 – 96. [2] M. T. Reetz, in Nanoparticles and Catalysis, Wiley-VCH, Weinheim, 2008, pp. 253 – 277. [3] B. R. Cuenya, Acc. Chem. Res. 2013, 46, 1682 – 1691. [4] H. Xu, K. S. Suslick, ACS Nano 2010, 4, 3209 – 3214. [5] S. Andreescu, M. Ornatska, J. Erlichman, A. Estevez, J. C. Leiter, in Fine Particles in Medicine and Pharmacy (Ed.: E. Matijevic´), Springer, New York, 2012, pp. 57 – 100. [6] H. Xu, B. W. Zeiger, K. S. Suslick, Chem. Soc. Rev. 2013, 42, 2555 – 2567. [7] D. T. Thompson, Nano Today 2007, 2, 40 – 43. [8] R. A. Sperling, P. Rivera Gil, F. Zhang, M. Zanella, W. J. Parak, Chem. Soc. Rev. 2008, 37, 1896 – 1908. [9] P. Ghosh, G. Han, M. De, C. K. Kim, V. M. Rotello, Adv. Drug Delivery Rev. 2008, 60, 1307 – 1315. [10] R. K. Gupta, G. Ying, M. P. Srinivasan, P. S. Lee, J. Phys. Chem. B 2012, 116, 9784 – 9790. [11] R. Sardar, A. M. Funston, P. Mulvaney, R. W. Murray, Langmuir 2009, 25, 13840 – 13851. [12] K. Lee, V. P. Drachev, J. Irudayaraj, ACS Nano 2011, 5, 2109 – 2117. [13] X. Huang, M. A. El-Sayed, J. Adv. Res. 2010, 1, 13 – 28. [14] T. Teranishi, C. R. Chim. 2003, 6, 979 – 987. [15] M. Sakono, T. Zako, S. Drakulic, J. M. Valpuesta, M. Yohda, M. Maeda, Chem. Phys. Lett. 2010, 501, 108 – 112. [16] C. L. Nehl, J. H. Hafner, J. Mater. Chem. 2008, 18, 2415 – 2419. [17] C. Xue, Y. Xue, L. Dai, A. Urbas, Q. Li, Adv. Opt. Mater. 2013, 1, 581 – 587. [18] T. Sau, C. Murphy, J. Am. Chem. Soc. 2004, 126, 8648 – 8649. [19] Y. Yang, Y. Yan, W. Wang, J. Li, Nanotechnology 2008, 19, 175603. [20] N. R. Jana, L. Gearheart, C. J. Murphy, Langmuir 2001, 17, 6782 – 6786. [21] J. Song, D. Kim, D. Lee, Langmuir 2011, 27, 13854 – 13860. [22] X. Huang, H. Wu, X. Liao, B. Shi, Green Chem. 2010, 12, 395 – 399. [23] A. Gole, C. Murphy, Chem. Mater. 2004, 16, 3633 – 3640. [24] M. Grzelczak, J. Perez-Juste, P. Mulvaney, L. M. Liz-Marzan, Chem. Soc. Rev. 2008, 37, 1783 – 1791. [25] I. H. El-Sayed, X. Huang, M. A. El-Sayed, Cancer Lett. 2006, 239, 129 – 135. [26] Y. Zhou, C. Y. Wang, Y. R. Zhu, Z. Y. Chen, Chem. Mater. 1999, 11, 2310 – 2312. [27] N. R. Jana, L. Gearheart, C. J. Murphy, Chem. Mater. 2001, 13, 2313 – 2322. [28] Y. Nagata, Y. Mizukoshi, K. Okitsu, Y. Maeda, Radiat. Res. 1996, 146, 333 – 338.
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ARTICLES N. S. M. Yusof , M. Ashokkumar*
The physical and chemical effects generated by high-intensity focused ultrasound are used to generate size- and shape-controlled gold nanoparticles
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DOI: 10.1002/cphc.201402697
Sonochemical Synthesis of Gold Nanoparticles by Using High Intensity Focused Ultrasound Nor Saadah Mohd Yusof [a, b] and Muthupandian Ashokkumar*[a, c] The sonochemical synthesis of gold nanoparticles (GNPs) with different shapes and size distributions by using high-intensity focused ultrasound (HIFU) operating at 463 kHz is reported. GNP formation proceeds through the reduction of Au3 + to Au0 by radicals generated by acoustic cavitation. TEM images reveal that GNPs show irregular shapes at 30 W, are primarily icosahedral at 50 W and form a significant amount of nanorods at 70 W. The size of GNPs decreases with increasing acoustic power with a narrower size distribution. Sonochemilumines-
cence images help in the understanding of the effect of HIFU in controlling the size and shapes of GNPs. The number of radicals that form and the mechanical forces that are generated control the shape and size of the GNPs. UV/Vis spectra and TEM images are used to propose a possible mechanism for the observed effects. The results presented demonstrate, for the first time, that the HIFU system can be used to synthesise sizeand shape-controlled metal nanoparticles.
1. Introduction agent.[19] This process led to the formation of gold seed particles of about 2 nm. These seed nanoparticles were then grown to various sized GNPs with precise control over their size in the range of 2–8 nm by using decyl amine as a reducing agent. A number of other techniques have been developed to synthesise size-controlled GNPs by using similar strategies.[20–22] Interestingly, Huang et al. used naturally occurring plant polyphenol to synthesise GNPs in the size range of about 2 nm.[22] The polyphenol has a dual role as a reducing agent and as a stabilising agent. As mentioned earlier, the shapes of GNPs play an essential role in their physical and functional properties. This is evident by the increasing number of studies that have focused on synthesising shape-controlled GNPs.[18, 23–26] Some pioneering work in this area was reported by Murphy and co-workers.[18, 23, 27] In one of their studies, a surfactant-mediated three-step process was successfully developed to synthesise GNPs as nanorods.[23] Non-spherical seed GNPs were initially prepared and then added to a solution containing gold ions and ascorbic acid in the presence of a surfactant to achieve nanorod formation.[23] The mechanism involved twinning and preferential binding of cetyltrimethylammonium bromide (CTAB) molecules on the (100) crystal face. Grzelczak et al. reviewed various synthetic methods for the generation of shape-controlled GNPs.[24] In general, the choice of seed particles, the nature of the stabilising and reducing agents, and the Au3 + /seed ratio play a significant role in controlling the shape of GNPs. Sonochemical synthesis of metal nanoparticles has been extensively studied in recent years.[28–36] The interaction between sound waves and bubbles in aqueous solution leads to acoustic cavitation: the growth and collapse of gas/vapour bubbles in an acoustic field.[37] The collapse of cavitation bubbles generates extreme temperatures within cavitation bubbles that ultimately result in the formation of highly reactive radicals. For
There is broad interest in developing novel techniques for synthesising metal nanoparticles owing to their unique physical and functional properties and wide applications, ranging from catalysis to biomedical.[1–12] The synthesis of gold nanoparticles (GNPs) has received special attention because they exhibit extraordinary optical,[13] electronic,[14] molecular recognition[15] and catalytic properties.[7] The physical and functional properties of GNPs depend not only on their size and size distribution, but also on their shapes.[16, 17] A variety of methods have been reported to tailor the size and shape of GNPs. Sau and Murphy synthesised size-controlled GNPs in a two-step chemical reduction technique.[18] In the first step, 5–20 nm seed GNPs were prepared by the photochemical reduction of gold ions.[18] The seeds then were grown to 20–100 nm GNPs by a chemical reduction method with ascorbic acid as a reducing agent.[18] The size and size distribution of the GNPs was controlled by varying the ratio between the gold ions and seed particles.[18] Yang et al. also used the Au3 + /seed ratio to control the size and size distribution of GNPs in a different approach.[19] In a complex process, HAuCl4 was dissolved in toluene and Au3 + reduction was initiated by NaBH4 in the presence of dodecanethiol as a stabilising
[a] N. S. M. Yusof , Prof. M. Ashokkumar School of Chemistry, University of Melbourne Parkville, VIC 3010 (Australia) E-mail: [email protected] [b] N. S. M. Yusof Department of Chemistry, University of Malaya Kuala Lumpur (Malaysia) [c] Prof. M. Ashokkumar Chemistry Department, King Abdulaziz University Jeddah, Saudi Arabia Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402697.
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Articles example, homolysis of water molecules within bubbles due to high-temperature conditions generates HC and HOC radicals (generally referred to as primary radicals). In the presence of suitable surface-active solutes, secondary reducing radicals can be produced. Such reducing radicals have been used to reduce metal ions to metal atoms.[38] The aggregation of metal atoms leads to the formation of nanoparticles. A number of studies have reported the synthesis of GNPs by using sonochemically generated secondary radicals.[38] In a typical study, the sonication of an aqueous solution containing gold chloride ions leads to the formation of GNPs. Okitsu et al. reported the size-controlled formation of GNPs by manipulating the acoustic parameters, namely, sonication power and frequency.[29] A higher rate of reduction resulted in the formation of smaller GNPs. Because the rate of reduction is directly dependant on the radicals present, which is governed by the ultrasound frequency used, a correlation between ultrasound frequency and the size of GNPs has been established. Although studies on the sonochemical synthesis of size-controlled GNPs are available in the literature, the control of shapes of GNPs by a sonochemical method has not been widely found in the literature. Hence, the main aim of this study is to explore the possibility of synthesising shape-controlled GNPs by a sonochemical procedure. For this purpose, high-intensity focused ultrasound (HIFU) has been used. HIFU is chosen for two reasons: 1) there are no reports available on the HIFU-mediated synthesis of GNPs, and 2) HIFU can deliver strong mechanical/shear forces as well as a high number of radicals in the sonicated medium that may result in size- and shape-controlled GNPs. HIFU is used in medicine for therapeutic and diagnostic applications.[39–42] It is most popular in therapeutic medicine, such as for the ablation of malignant and metastatic tumours, thrombolysis and targeted drug- or genedelivery deep within the human body.[43–46] In addition, the focus of HIFU can be controlled to a well-defined volume. The use of HIFU in medical procedures lies behind the use of mechanical energy to generate heat and cavitation bubbles at the focal area, resulting in tissue lesion.[47] Herein, we have used HIFU for the sonochemical synthesis of GNPs to control their size and shape. It is important to mention that HIFU used in this study was custom built for chemical reactions and operated at much lower frequency and at a power level higher than that used in medicine. Sonochemiluminescence (SCL), UV/Vis spectra and TEM images have been recorded to understand the formation mechanism of GNPs of various sizes and shapes.
Figure 1. Colour changes observed upon HIFU sonication of solutions of gold chloride at f = 463 kHz and P = 30, 50 and 70 W.
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isopropanol ! RC and other secondary reducing radicals
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The reason behind the colour changes observed are discussed throughout the Results and Discussion section. It is known that the colour of GNPs in bulk solutions reflects their morphologies at the nanoscale.[13, 48, 49] The gradual colour changes observed during sonication is due to the formation and growth of GNPs to different sizes and shapes as governed by the experimental conditions. To quantitatively follow this, the concentration of Au3 + was monitored at l = 217 nm by using UV/Vis absorption spectroscopy. An example of the spectral changes observed at 70 W is shown in Figure 2 a. As the Au3 + absorption (l = 217 nm) decreased, a new absorption band in the wavelength range of l = 500–600 nm appeared and grew during sonication. This is the plasmon absorption band of GNPs.[28, 50–53] Further analysis of this absorption band for each power level is discussed later. The band at l = 217 nm was then converted to Au3 + concentration ([AuCl4]), and the results are plotted in Figure 2 b. Figure 2 b shows the amount of Au3 + reduced as a function of sonication time for different acoustic power levels. The plots clearly show that the rate of reduction depends upon the power used. A faster reduction is observed at higher power. This seems logical as the sonication of an aqueous solution at higher acoustic power levels generates a higher number of cavitation bubbles, and therefore, a higher amount of radicals. It is also possible that a greater number of radicals are generated per bubble at higher acoustic power levels due to the larger size (Rmax) of the cavitation bubbles.[37] All GNP samples were analysed by using TEM, and the images are shown in Figure 3 along with the size distributions of the GNPs. For clarity, let us first focus on the differences in their size distribution.
Upon sonication, the colourless solution of gold chloride (AuCl4) went through gradual colour changes to pink and then to red for 30 W, purple for 50 W and blue for 70 W (Figure 1). Such colour changes are due to the formation and growth of GNPs by the reduction of Au3 + ions (AuCl4) to formAu0. The reduction of Au3 + ions is aided by the sonochemically generated secondary radicals through Reactions (1)–(4):[38] ChemPhysChem 0000, 00, 0 – 0
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Articles possible reasons for different shapes of GNPs are discussed later. Considering the size and size distribution, the size of GNPs is dependent upon the rate of reduction.[29] A higher rate of reduction favours the formation of smaller GNPs and vice versa. Okitsu et al. showed a direct correlation between the rate of reduction of Au3 + ions and the size of GNPs, for which the rate of radical production was controlled by varying the ultrasonic frequencies.[29] At 70 W, it can be expected that the amount of secondary reducing radicals produced is higher. Therefore, the GNP size is smaller with a narrower size distribution at 70 W. At 30 W, the size distribution was broader than the other two power settings. This may also be due to a slower reduction rate, leading to competition between nucleation and growth processes that occur simultaneously. The correlation between the rate of reduction and GNP sizes can be easily understood by the schematic diagram shown in Figure 4. In Figure 4 i, there are 20 nAuCl4 [n = number of Au0 (gold nuclei) molecules needed to form one Aun] and 2 nR (nR = radicals needed to reduce n number of Au3 + to Au0). The 2 nR act by reducing 2 nAuCl4 to form 2 Aun. The reduction process continues in a similar manner, leading to the formation of new nuclei and the growth of existing nuclei. If the number of nR is low (at low acoustic power levels), growth of existing nuclei dominates and results in the formation of a small Figure 2. a) UV/Vis spectra of sonicated solution at 70 W. b) Au3 + reduction number of larger GNPs. At higher power levels, nR is higher upon HIFU sonication at various power levels (squares: 30 W, triangles: 50 W, circles 70 W). and hence the nucleation process dominates over the growth process (Figure 4 ii), resulting in the formation of a large number of small GNPs. Thus, the most important reason for the variation in GNP size is the difference in the rates of nucleation and growth processes, which, in turn, depends upon the concentration of the reducing agent.[54] The implosion of cavitating bubbles under specifc experimental conditions results in the emission of light, which is known as sonoluminescence (SL).[53, 55] Due to the high temperature within the cavitation bubbles, hydroxyl radicals (HOC) are also generated from the homolysis of water [Reaction (1)].[53, 55] If luminol is present in the solution, HOC radicals react with luminol and generate an intermediate that can chemiluminesce.[56] This is referred to as SCL.[53, 56] SCL imaging was performed to gain further insight into the cavitation activity at various power levels. The cell used and the SCL images observed for the HIFU system are shown in Figure 5. It is evident from the SCL image in Figure 5 that there are five regions of cavitation activity due to the establishment of a local standing wave pattern in the Figure 3. Particle-size distribution and TEM images for the GNPs synthesised by means of sonicated solution. 1) Region I is the focal area of the HIFU at 463 kHz and 30 (a), 50 (b) and 70 W (c). HIFU system where no SCL is observed. This region is where a strong mechanical force is generated due to the focussed ultrasound waves.[57] The diameter of Region I The GNPs were largest for 30 W, followed by 50 W and smallest for 70 W. For 30 W, the size ranges from 12 to 104 nm and was measured to be 0.57 cm. 2) Region II is the bubble-rich the size distribution shows an irregular pattern. For 50 W, the area or the antinode where cavitation bubbles accumulate due size ranges from 8 to 22 nm and shows a mono-modal distrito Bjerknes forces; therefore, a strong SCL activity can be obbution, whereas for 70 W the spherical GNP is from 3 to served. 3) Region III is a nodal region with no SCL activity. 14 nm. In addition, nanorods were also found at 70 W. The 4) Region IV is the antinode of active bubbles with clear SCL ChemPhysChem 0000, 00, 0 – 0
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Articles At 25 W, very weak SCL is observed for Region II. No sign of active bubbles was observed in other regions due to the very low sonication power. With an increase in acoustic power, more active bubbles formed. This was evident from the increasingly pronounced regions of SCL activity from 30 to 50 W. The SCL profile intensity is most defined with clear nodes and antinodes at 50 W. The standing waves are well defined and undisturbed by the strong mechanical forces from Region I. However, from 50 to 80 W, the SCL intensity is visually found to decrease. Moreover, the nodes and antinodes of Region II, and the standing waves in Region V, become less defined. This is due to the stronFigure 4. Illustration showing the gold reduction processes in the presence of a relatively ger mechanical force from Region I that forced the small number of reducing radicals (i) and a relatively large number of reducing radicals displacement of the cavitation bubbles. It is evident (ii). nAuCl4 is the number of gold chloride molecules needed to form one gold nuclei from these images that a decrease in the amount of after reduction, nR is the number of radicals needed to reduce one nAuCl4 , Aun is the HOC radicals occurs at very high power levels, which gold nuclei and GNP represents gold nanoparticles. contradicts the trend observed for Au3 + reduction shown in Figure 2. We quanitified the amount of HOC radicals generated under these experimental conditions by using the Weisler method and observed a reduction in HOC radicals at higher power levels.[58] However, this contradictory observation could be explained based on the mechanism involved in Au3 + reduction. Previous studies showed that reducing radicals generated within cavitation bubbles were primarily responsible for the Au3 + reduction process. Although there is a reduction in the number of active cavitation bubbles and primary radical production, the overall amount of secondary radicals generated may still be high at higher power levels. Both SCL and Weisler methods provide information on primary radicals only. However, a previous study has reported that there is no correlation between SL intenFigure 5. SCL profile for HIFU sonication of a solution of luminol (left) in a custom-made sity (maximum bubble temperature) and sonochemicell (right). The width of the glass cell is 4 cm. cal activity.[59] Another important possibility to take into account is the ability of gold nuclei to catalyse activity. 5) Region V is a secondary standing-wave region estabthe reduction of Au3 + ions. Therefore, a higher number of lished within the neck region of the custom-made cell. nuclei generated at higher power levels (higher nucleation The SCL images were taken at increasing acoustic power rate) may also increase the rate of reduction, as observed at from 25 to 80 W (Figure 6). Under the experimental conditions 70 W (Figure 2). In conclusion, a clear trend of size distribution used, no SCL emission was captured at 20 W, and therefore, it dependence on sonication power is observed in this study. was not included herein. The maximum power for the unit was This proves that HIFU can also be used to engineer size-specif100 W, and therefore, the experiment was limited to 80 W. ic GNPs.
Figure 6. SCL profiles upon HIFU irradiation from 25 to 80 W.
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Articles More interestingly, the shapes of synthesised GNPs are also significantly different for all three power levels. This is evident from the TEM images shown in Figure 3. At 30 W (Figure 3 a), the shapes could not be defined because the image consisted of particles with irregular shapes, whereas, for 50 W (Figure 3 b), the shape of most of the particles were icosahedral. Surprisingly, about 50 % of the particles formed at 70 W were nanorods, and the remaining particles showed a spherical shape with very small flat surfaces (Figure 3 c). This can be clearly seen by the smooth surface of the particle shown in Figure 3 c. It is known that the flat surface of the particles possesses higher reactivity for one-directional growth; therefore, preferably forming nanorods.[60] As discussed earlier, an increase in acoustic power from 30 to 50 W caused an increase in active cavitation bubbles followed by a slight decrease in the active cavitation region. If the rate of radical formation were responsible for the difference in the shapes, previous studies should have observed such trends. However, such shape changes were not reported in early studies in which the rate of radical production was correlated with the size and size distribution of GNPs. This suggests that HIFU is unique in generating nanorods at higher power levels. One very clear and interesting characteristic of HIFU is the changes to the strength of mechanical forces as a function of acoustic power. The SCL images shown in Figure 5 clearly suggest that the mechanical forces increase with an increase in acoustic power, as discussed above, as evidenced from the changes to the cavitation field. This observation suggests that the strong mechanical forces generated at higher power settings might be responsible for the significant changes in the shapes of GNPs. To see if the mechanical forces were responsible for the nanorod shape of the GNPs, chemically synthesised 3–5 nm GNP seeds were sonicated at 20 kHz by using a 3 mm horn at 160 W of applied power. At this setting, the shear is strong, but a relatively low amount of radical is generated. The TEM image is shown in Figure S1 in the Supporting Information. The formation of nanorods from the action of the strong shear could be observed under the experimental conditions used. However, the size of the nanorods was larger than that synthesised by using HIFU due to the lower amount of radicals generated at 20 kHz. Based on the observed data and discussion provided above, a possible mechanism for the observed trends in size and shape of GNPs is speculated below. Changes in the size and shape of GNP are reflected by their colour, which can be closely monitored by the shift in the absorption maxima (lmax) of the GNPs. The UV/Vis spectra of GNPs are shown in Figure 2 a. It is important to mention that an increase in the absorption without a significant change in its lmax generally indicates an increase in the number of particles of similar size.[52] Also, a shift in the absoption of GNPs means a change in their size and/or shape.[52] The absorption spectra shown in Figure 2 a clearly show a shift in lmax. Similar observations were made for sonication at 50 and 70 W (Figure S2 in the Supporting Information). For clarity, the shift in lmax is plotted against sonication time in Figure 7. From Figure 7, three distinct regions (a weak shift, a strong shift and almost constant lmax) can be observed for all acoustic power levels. In general, there are three ChemPhysChem 0000, 00, 0 – 0
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Figure 7. Shift of maximum absorption wavelength (lmax) as a function of sonication time for 30 (&), 50 (~) and 70 W (*).
steps involved in the formation of GNPs: nucleation, growth and coagulation.[52, 61, 62] All three processes may occur simultaneously with different processes dominating.[62] Nucleation is mainly driven from the reduction process by the reducing radicals.[54, 62] When this process reaches supersaturation, the nucleation process will effectively stop, whereas growth will mainly continue.[62] Growth can occur by both an autocatalytic process on the surface of gold nuclei (surface reduction) and by reducing radicals.[54, 63] The Au0 species formed combine to form gold nuclei, Aun.[54, 62] A limited number of gold nuclei (Aun) are formed during the nucleation process and a maximum number of nuclei is reached under specific experimental conditions.[62] The growth of particles may occur due to agglomoration between gold nuclei under suitable conditions.[54, 64] Region I (Figure 7), where the lmax shift is very small, corresponds to the domination of the nucleation process over growth. There is a very minor change in the size of GNP nuclei, but the number of nuclei increases. Region II is where the growth process dominates over the nucleation process, as indicated by a strong shift in lmax. Region III is almost a plateau, which indicates that the GNPs have reached their maximum size. These processes are schmatically shown in Figure 8. The conse-
Figure 8. Mechanism of GNP nucleation and possible growth routes.
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Articles quence of Au3 + reduction by the secondary radicals generated during acoustic cavitation is the formation of Au0. As this process continues, few Au0 species combine to form the GNP nuclei (Aun). There are three possible routes for GNP growth under HIFU irradiation. Firstly (Figure 8 I), is the further reduction and nucleation processes by the secondary radicals. The second (Figure 8 II) is reduction by the autocatalytic reaction of the existing nuclei, resulting in immediate growth of the nuclei. The third (Figure 8 III) is the combination of few nuclei, as a result of the strong mechanical forces generated at the focal area that pushes them together. The significance of the three routes are dependent on the experimental conditions. For 30 W, nuclei formation is slower because fewer radicals are generated. Due to the slow reduction of Au0, the autocatalytic growth plays an important role here, along with the formation of new nuclei. Nucleation that occurs over a longer time, as well as significant growth at the surface of existing nuclei, could explain the wide size distribution and variety of shapes of GNP formed. For 50 W, the radicals were generated in relatively large quantities; therefore, the radical-driven reduction process dominated. At the same time, the mechanical force is still contained within the first region (Figure 5), so there is no mechanical effect on the nucleation and growth processes. Therefore, they form icosahedra with a narrow size distribution. However, for 70 W, the third (Figure 8 III) process plays a very important role in the formation of nanorods. This is evident from the SCL image (Figure 5) as well as the TEM images in Figure 3 c. The mechanical forces push the spherical nuclei together, resulting in the formation of nanorods.
sodium hydroxide (Merck, 98 %), ammonium molybdate tetrahydrate (Sigma Aldrich, 99 %), potassium hydrogen phthalate (Ajax, 99.95 %) and luminol (Sigma Aldrich, 97 %) were also of the highest available commercialised purity, and therefore, no purification was needed. All solutions were dissolved in Milli-Q water with a conductivity of < 106 S cm1 and surface tension of about 72.0 mN m1 at 25 8C.
HIFU A custom-built HIFU unit was used in this study (Figure 9). The ultrasound wave was transmitted by an 8 cm concave transducer at either 463 kHz or 1.61 MHz. Only the lower frequency was used in this study.
Figure 9. Illustration of the HIFU unit used in this study.
The shape and specified length of each side on the HIFU unit were chosen so that the sample could be placed where the ultrasound wave was focused and there were no reflected waves (dashed lines in Figure 9) interfering with the focused region of the sample. Water was used elsewhere as a medium for the propagation of ultrasound waves. A custom-made cell was used so that the ultrasound wave could go through a flat glass surface to the sample solution.
3. Conclusions We successfully used a HIFU system to generate GNPs of various sizes and shapes. UV/Vis absorption spectroscopy and TEM were used to analyse the morphology of the GNPs generated. The trends observed for the size and size distribution of GNPs as a function of acoustic power could be understood based on the amount of secondary radicals generated during acoustic cavitation. The most interesting observation in our study was the difference in the shape of GNPs at various power levels. Irregular shapes were observed at low power levels. Icosahedra were observed at a medium power level, and significant amount of nanorods were generated at a higher power level. A possible mechanism, involving the rate of radical production and mechanical forces, was proposed. The results presented herein may initiate a new synthetic route for generating shape-controlled metal nanoparticles.
Methods Water (50 mL) was placed in a custom-made glass cell and irradiated with HIFU operating at a frequency of 463 kHz and at 30, 50 or 70 W of applied power. We did not measure the calorimetric power because the glass cell was immersed in water, as shown in Figure 1, which led to heat dissipation to the surrounding medium. For this reason, we only reported the applied power for this study. For SCL imaging, 0.1 mm luminol (50 mL) was prepared in 0.1 m NaOH and exposed to HIFU. The SCL images were recorded by using a Sony a57 camera with an exposure time of 60 s in a dark room.
Experimental Section
An aqueous solution containing 0.2 mm HAuCl4·3 H2O, 0.1 m isopropanol and 0.01 m SDS (stabilising agent) was prepared for the gold ion reduction experiments. This solution (50 mL) was placed in the glass cell in HIFU setup (Figure 1) and sonicated at 463 kHz with varying applied power. Visual observations of colour changes during sonication were recorded for all experiments. Absorption
Materials HAuCl4·3 H2O) was purchased from Sigma Aldrich with 99.8 % purity and used as received. Isopropanol, sodium dodecyl sulfate (SDS; Ajax, 99 %), potassium iodide (APS, Sigma Aldrich, 99 %),
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Articles spectra of solutions of AuCl4 before and during sonication were measured by using a Shimadzu UV-1601 spectrometer. The solution of AuCl4 showed a band at l = 217 nm, and the formation of gold (Au0) nanoparticles was monitored in the wavelength range of l = 450 to 650 nm. GNPs synthesised were analysed by using a Tecnai TF30 transmission electron microscope from FEI, Eindhoven, the Netherlands. Samples of GNP were prepared by placing a drop of sonicated solution on a copper grid covered with a thin carbon film. It was then left on a filter paper to dry.
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Received: October 8, 2014 Published online on && &&, 2014
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ARTICLES N. S. M. Yusof , M. Ashokkumar*
The physical and chemical effects generated by high-intensity focused ultrasound are used to generate size- and shape-controlled gold nanoparticles
&& – && Sonochemical Synthesis of Gold Nanoparticles by Using High Intensity Focused Ultrasound
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