Rapid and efficient sonochemical formation of gold nanoparticles under ambient conditions using functional alkoxysilane

Ultrasonics Sonochemistry 20 (2013) 610–617

Contents lists available at SciVerse ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Rapid and efficient sonochemical formation of gold nanoparticles under ambient conditions using functional alkoxysilane Ming-Yuan Wei a,⇑, Leila Famouri b, Lloyd Carroll c, Yongkuk Lee a, Parviz Famouri a,⇑ a Lane Department of Computer Science and Electrical Engineering, Benjamin M. Statler College of Engineering and Mineral Resources, West Virginia University, Morgantown, WV 26506, USA b Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY 10032, USA c Department of Chemistry, West Virginia University, Morgantown, WV 26506, USA

a r t i c l e

i n f o

Article history: Received 19 October 2011 Received in revised form 17 June 2012 Accepted 31 July 2012 Available online 11 August 2012 Keywords: Promotion Silane reagents Quenching Green nanosynthesis Sonochemistry

a b s t r a c t Gold nanoparticles (NPs) are rapidly and efficiently formed under ambient conditions with a novel and highly-efficient sonochemical promoter. Despite of the presence of free oxygen, 3-glycidoxypropyltrimethoxysilane (GPTMS) showed remarkable efficiency in promoting the reduction rate of Au (III) than that of conventional promoters (primary alcohols). This is likely attributed to the formation of a variety of radical scavengers, which are alcoholic products from sonochemical hydrolysis of the epoxide group and methoxysilane moieties of GPTMS under weakly acidic conditions. Interestingly, the promotion is quenched by amine- or thiol-functionalized alkoxysilane, thereby producing marginal amounts of gold NPs. Furthermore, products of hydrolyzed GPTMS were confirmed to attach on the surface of gold NPs by attenuated total reflectance-Fourier transform infrared spectroscopy. However, according to transmission electron microscopy images, gold NPs that were produced in the presence of GPTMS tend to fuse with each other as condensation of silanols occurs, forming worm- or nugget-like gold nanostructures. The use of long chain surfactants (i.e. polyethylene glycol terminated with hydroxyl or carboxyl) inhibited the fusion, leading to mono-dispersed gold NPs. Additionally, the fact that this approach requires neither an ultrasound source with high frequency nor anaerobic conditions provides a huge advantage. These findings could potentially open an avenue for rapid and large-scale green-synthesis of gold NPs in future work. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Gold nanomaterials are one of the most popular substrates applied in catalysis, qualitative and quantitative analysis, and bioapplications, due to their excellent electronic and optical properties, high stability, and high biocompatibility [1–3]. Many sophisticated strategies for the synthesis of gold nanoparticles (NPs) have been developed, since Turkevitch et al introduced citrate reduction of HAuCl4 in 1951 [4]. In general, these strategies include the Brust–Schiffrin method, microemulsion, reversed micelles, polyelectrolytes, seeding growth, and photochemistry [1,3,5–7], by which the parameters of gold NPs could be controlled, including morphology, size, dispersity, surface-ligands, etc. More recently, however, ‘‘green nano-synthesis’’ has been gaining attention, for researchers are more concerning about environmental impact ⇑ Corresponding authors. Current address: Department of Bioengineering, University of Texas at Arlington, 500 UTA Blvd, Arlington, TX 76010, USA. Tel.: +1 817 272 7149; fax: +1 817 272 2251 (M.-Y. Wei), tel.: +1 304 293 9689; fax: +1 304 293 8602 (P. Famouri). E-mail addresses: [email protected] (M.-Y. Wei), [email protected] (P. Famouri). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2012.07.028

and human safety and toxicology effects of chemical wastes in traditional synthesis routes [5,6]. In the presence of sonication, acoustic cavitation could provide extraordinary high energy (10 eV) but possess short duration (1010 s) [8]. It does not affect the chemical species directly, the process of which involves bubble formation, growth, implosion and collapse in solution. At the gas/liquid interface, the energy released from bubble collapse could trigger a chemical reaction to occur. Therefore, sonochemistry has been considered as good alternative strategy to accomplish green synthesis of numerous nanomaterials [9]. Sonochemical synthesis of gold NPs mainly relies on the reduction of Au (III) in aqueous solution by H radicals (from water molecules) [9–17] (Eq. 1), followed by a number of Au (0) develop into gold NPs (Aun) (Eq. (2)).

H2 O !  H þ  OH

ð1Þ

nAuðIIIÞðþby  HÞ ! nAuð0Þ ! Aun

ð2Þ

In order to promote the reaction, a very small amount of primary alcohol (RCH2OH; R@H or alkyl group) is often employed as promoter. It serves as a radical scavenger to produce secondary

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617

611

Scheme 1. Molecular structure of APTES (A), TEOS (B), DEO (C), MPTMS (D), GPTMS (E), PEG-MS (F), and PEG-silane (G).

Fig. 1. UV–Visible absorption spectra of gold NPs prepared in the presence of (i) PEG-MS, (ii) GPTMS, and (iii) PEG-MS and GPTMS. Inset: photographs of gold NPs samples. PEG-MS or GPTMS: 126 mM. Representative TEM images of gold NPs (ii) (B) and (iii) (C and D).

radials [9,12,14,17] (Eq. (3)), which in turn significantly promotes the reduction rate of Au (III) in Eq. (2). In addition, surfactants/ligands are also required to limit the growth of gold NPs and for stabilizing, for example, long chain alcohols, such as polyvinyl alcohol (PVA) [17] or polyethyleneglycol monostearate (PEG-MS) [11,13]. Meanwhile, surfactant-free and reducer-free synthesis of gold NPs has also been reported [16].

RCH2 OH þ  H ! R CHOH þ H2 ðor H2 OÞ

ð3Þ

Nevertheless, the aforementioned approaches usually require a high-frequency ultrasound source (e.g. 5 MHz), as well as anaerobic conditions, in which dissolved oxygen is purged from the solution (e.g. with argon gas) [10–14,16,17]. Otherwise, it could cause an intervening reaction between oxygen and H radical as follows [18]:

2O2 þ 2 H ! 2 HO2 ! H2 O2 þ O2

ð4Þ

To simplify the protocol, we report a rapid and efficient method for the sonochemical synthesis of gold NPs basing on a novel promoter in the present study. GPTMS was selected as the promoter

due to its vast variety of alcoholic products from hydrolysis in acidic solution. Methoxysilane moieties of GPTMS can be fully hydrolyzed under ultrasonic sonication [19,20], yielding a 3-fold higher unit concentration of methanol as well as silanol, from which secondary radials could be provided; however, the epoxide group is very vulnerable to hydrolysis in a fast kinetic reaction (within minutes) [21–23], in which numerous types of radicals could also be formed as long as the epoxy ring is open [24,25]. This could trigger more complicated reactions between methoxysilane and products of hydrolyzed epoxide (e.g. ethanediols). With the help of the highly-efficient performance of GPTMS in promoting the reduction reaction rate towards Au (III), this approach does not require anaerobic conditions or a high-frequency ultrasonic device. To the best of our knowledge, there have been no studies to date that have explored the use of alkoxysilane as a sonochemical promoter to synthesize metal materials under ambient conditions. Gold NPs was synthesized with the promotion of GPTMS in an oxygen-containing aqueous medium in a commercially-available ultrasound cleaner bath (low frequency: 40 kHz). The mechanism of the formation of such GPTMS-induced gold

612

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617

Table 1 Comparison of the efficiency of promoters at the reduction rates of Au (III).

1 2 3 4 5 6 7 8 9 a

PEG-MS GPTMS PEG-MS & GPTMS PEG-silane TEOS APTMS GPTMS DEO Primary alcoholsa

Reduction rate of Au(III)

Functional groups

+ +++ +++ +++ +++ NA (quenched) NA (quenched) +++ +

Oxyethylene, hydroxyl Epoxide, methoxysilane Oxyethylene, hydroxyl, epoxide, methoxysilane Oxyethylene, methoxysilane ethoxysilane Amine, methoxysilane Thoil, methoxysilane Epoxide Hydroxyl

3-fold higher at molar concentration, for 3-fold higher alcoholic products could be generated when alkoxysilane is hydrolyzing.

Fig. 2. (A) UV–Visible absorption spectra of gold NPs (iii) as a function of GPTMS concentrations: from (a) to (d): 0 (dark blue), 14 (pink), 42 (green), 126 (sky blue), and 378 mM (purple). (B) Plot of peak values in (A) as a function of GPTMS concentrations. Inset: photographs of gold NPs samples. PEG-MS: 0.4 mM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

NPs was discussed. Transmission electron microscope (TEM) integrated with an Oxford Inca energy-dispersive silicon-drift X-ray (EDX) spectrometer, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and ultraviolet-visible spectroscopy were utilized to monitor the formation and morphology of the gold NPs.

placed into a Branson 1510 tabletop ultrasonic cleaner bath (Input 160 W, output 70 W, 40 kHz) for sonication for 3 min at 20 °C.

Tetraethyl orthosilicate (TEOS), 3-glycidoxypropyltrimethoxysilane (GPTMS), aminopropyltriethoxysilane (APTES), 3-mercaptopropyl trimethoxysilane (MPTMS), 1,2,7,8-diepoxyoctane (DEO), anhydrous ethanol (200 proof) and 2-proponal was purchased from Sigma-Aldrich Corporate (St. Louis, MO, USA). 2-[Methoxy (polyethyleneoxy) propyl] trimethoxysilane (PEG-silane), (n = 9– 12), tech-90, was purchased from Gelest Inc. (Arlington, VA, USA). Polyethyleneglycol monostearate (PEG-MS, HO (CH2CH2O)nOOC17H35, n = 40) was purchase from Tokyo Chemical Industry (TCI)-America (Portland, OR, USA). Poly ethylene glycol-carboxymethyl (PEG-CM, MW 2 and 20 k) were purchased from Laysan Bio Inc. (Arab, AL, USA).

2.2.2. Characterization of gold nanoparticles After synthesis, the absorption of gold NPs solutions was measured within 1hr and again at 48 h with UV–Visible spectroscopy. Prior to observation by transmission electron microscope (TEM), the solution was centrifuged at 14 k rpm for 30 min, rinsed with water, centrifuged, and then re-dispersed in anhydrous ethanol. A drop of 10 lL of the sample was placed on a 400-Mesh ultrathin carbon film/holey carbon support film TEM grid (from Ted Pella Inc., Redding, CA, USA) and dried overnight. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was used to investigate the surfactants of the gold NPs. The gold NPs solution sample was purified by using centrifugal molecularweight cut-off filters, then re-suspended into water and re-concentrated. This allowed for the removal of free PEG-MS, GPTMS or other molecular species in the suspension. The obtained sample was then measured by ATR-FTIR. Gold nanoparticles were characterized with a JEM-2100 LaB6 transmission electron microscope (TEM) integrated with an Oxford Inca energy-dispersive silicondrift X-ray (EDX) spectrometer and Varian Cary 50 UV–Visible spectroscopy. Fluorescence measurement was carried out with a Hitachi F-7000 fluorescent spectroscopy.

2.2. Methods

3. Results and discussions

2.2.1. Synthesis of gold nanoparticles 8.1 mg PEG-MS was dissolved with 10 mL distilled water. 1800 lL of the PEG-MS solution was transferred to a 19  65 mm Kimble Chase vial, followed by addition of 8 lL of 1% (w/v) HAuCl4 aqueous solution (0.1 mM) and 42 mM of alkoxysilane. The vial containing the mixture was sealed with a closure and subsequently

3.1. GPTMS serves as sonochemical promoter

2. Materials and methods 2.1. Materials

The sonochemical synthesis of gold nanoparticles (NPs) has been well-documented [9], in which Au (0) is generated from the reduction of Au (III) (e.g. HAuCl4) in aqueous solution by radicals of H (from H2O) followed by that a number of Au (0) grow into

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617

613

Fig. 3. (A) UV–Visible absorption spectra of gold NPs prepared in the presence of PEG-MS and DEO. Inset: Photographs of gold NPs sample. (B) UV–Visible absorption spectra of gold NPs prepared in the presence of PEG-MS and alkoxysilanes: APTMS (dark blue), GPTMS (pink), MPTPS (green), TEOS (red), and PEG-silane (purple). Inset: photographs of gold NPs samples. Letters G, T, P, A, and M, marked on the bottles (from left to right) represent GPTMS, TEOS, PEG-silane, APTES, and MPTMS respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

gold NPs (Aun) (Eqs. (1 and 2)). Except for high-frequency ultrasound generator (e.g. 5 MHz), conventional methods using promoters (i.e. primary alcohols) that often rely on anaerobic environments [9,12,14,17], due to the intervening reaction between free oxygen and H radical [18], as shown in Eq. (4). Aiming to offer a convenient alternative, we have developed a rapid and efficient method for the sonochemical synthesis of gold NPs based on a novel promoter. The promotion is so significant because in this method, gold NPs can be formed under ambient conditions (oxygen-containing) with a commercially-available low-frequency ultrasound cleaner bath (40 kHz). For sonochemical synthesis of gold NPs, polyethyleneglycol monostearate (PEG-MS, terminated with hydroxyl, Scheme. 1-F) was employed as an alcoholic promoter to successfully reduce Au (III) in oxygen-free aqueous solution [11,13]. In the presence of oxygen, however, the color of the mixture solution of PEG-MS and Au (III) is barely changed after 3 min sonication (Fig. 1insetA). The UV-visible spectrum showed that negligible amounts of gold NPs were produced, which is likely due to the quenching reaction of hydroxyl radicals in Eq. (4). On the contrary, the mixture solution of GPTMS and Au (III) displayed a strong rose–red color, along with strong absorbance at 534 nm (Fig. 1-A). TEM confirms the formation of worm-like gold NPs in Fig. 1-B. The color of the mixture solution of PEG-MS, GPTMS and Au (III) is pink, and the absorbance peak shifted to 528 nm (Fig. 1-A). In this case, sphere-shaped gold NPs were formed (Fig. 1-C). High resolution 0 TEM reveals that the d111 is 2.23 Å A, and these gold NPs were either single crystal or contained parallel twinning boundaries (Fig. 1-D). EDX confirms the existence of distinct elements including Au, O, and Si. Other primary alcohols (e.g. 2-propanol [12,17], methanol, and ethanol) were also tested (Table 1), and the results were parallel to that of PEG-MS. Overall, the results indicate that GPTMS shows higher efficiency at the sonochemical formation of gold NPs than that of conventional promoters under ambient conditions.

Fig. 4. ATR-FTIR spectra of pure sample of PEG-MS (A), GPTMS (B), and gold NPs (iii) (C). GPTMS: 126 mM, PEG-MS: 0.4 mM.

Taking case (iii) in Fig. 1 as example, the formation of gold NPs as a function of GPTMS concentration was also investigated by UVvisible spectra. The peak value plateaued at a GPTMS concentration of 42 mM (Fig. 2-B). The size of gold NPs was found to decrease from 15 ± 5 to 10 ± 3 nm when the GPTMS concentration increased by 3-fold (Fig. 2-series b and c), which was consistent with the fact that the size of gold NPs is inversely related with the concentration

614

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617

Fig. 5. UV–Visible absorption spectra of gold NPs (ii) as a function of GPTMS concentrations: from (a) to (d): 0 (dark blue), 14 (pink), 42 (green), 126 (sky blue), and 378 mM (purple). After irritation, measurements were carried out within 1 h (A) and after 48 hrs (B); (C) plot of peak values in (A) as a function of GPTMS concentrations; (D) photographs of gold NPs samples. The gray arrow indicates color changing time from <1 h to >48 h. Dashed yellow arrows indicate the direction of increasing GPTMS concentrations; representative TEM images of gold NPs in (B)-pink (E) and -purple (F). At high GPTMS concentration, only one absorbance peak was observed, which redshifts to 537 nm (B-purple). Such red-shift results from aggregation of gold NPs [32], forming bigger gold nuggets according to TEM images (F). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. (A) Photographs of gold NPs prepared in the presence of GPTMS alone (a), with PEG-CM 2 k (b), and PEG-CM 20 k(c). GPTMS: 126 mM. PEG-CM 2 or 20 k: 0.4 mM. (B) UV–Visible absorption spectra of gold NPs. (C, D and E) Representative TEM images of gold NPs. Sample (a): (C); sample (b): (D); sample (c): (E), and the colors of frames indicate the same series sample in (B) as well. Scale bars: top: 100 nm; bottom: 20 nm.

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617

of promoter [9,10]. Therefore, GPTMS was confirmed as the exclusive promoter in our system. In order to reveal the mechanism of promotion, the mixture solution of Au (III) with DEO or several functional alkoxysilanes (in Scheme 1) was also tested. The pH of the mixture solution in case (iii) was measured to be weakly acidic (6). We hypothesized that: (1) a vast variety of alcoholic products could be produced from the sonochemical hydrolysis of GPTMS in acidic solution; (2) such alcoholic products could serve as radical scavenger, promoting the reduction of Au (III) as in Eq. (3). As mentioned in the Introduction section, the complex reaction of sonochemical hydrolysis of GPTMS produces a significantly higher concentration of alcoholic products than that of conventional promoters. As a result, the reduction of Au (III) can be promoted tremendously. Two possible main pathways are involved in the formation of alcoholic products from GPTMS hydrolysis: from the epoxide group and from methoxysilane moieties. Such promotion was observed while using an epoxy compound, 1,2,7,8-diepoxyoctane (DEO), as shown in Fig. 3-A. This implied that the epoxy group of GPTMS participates in the process of promotion. Moreover, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra show that the band of epoxide at 909 cm1 was not found in the gold NPs sample, while the content of –OH band at 3307 cm1 was higher compared to that of pure PEG-MS (Fig. 4). Again, this provides strong evidence that epoxy group is sonochemically hydrolyzed. On the other hand, PEG-silane (containing methoxysilane, Scheme 1-G) was tested to verify the other pathway. It has been conceived that Au (III) can be successfully reduced, and thereby form gold NPs using PEG-silane (Fig. 3-B). In comparison with PEG-MS (which leads to marginal amounts of gold NPs), the functional group methoxysilane took responsibility to the promotion of PEG-silane (in Table 1 at row 1 and 4). This confirmed that the other pathway of sonochemical hydrolysis of GPTMS was correct. Similarly, the use of TEOS (containing ethoxysilane, Scheme 1-B) also yields the formation of gold NPs (Fig. 3-B). Because there is no promotion using primary alcohols (PEG-MS, 2-propanol, methanol, and ethanol), the promoting effect of GPTMS likely stems from the radicals generated from silanols rather than hydroxyls. Therefore, one could conclude that GPTMS has a remarkably higher reductive capacity towards Au (III) than that of PEG-MS and other primary alcohols in the presence of oxygen, which is likely attributed to radials generated from hydrolyzed products of epoxide (e.g. ethanediols) and silanols. Interestingly, there is no color change for the mixture solution once APTMS or MPTMS was employed, and UV–Visible spectra implied that marginal amounts of gold NPs were produced (Fig. 3-B dark blue and green). One possibility is that amine [26,27] or thiol groups [28,29] quenched both of the primary and secondary reducing radials from the hydrolysis of alkoxysilane moieties. The results are contrast to those in most of the organic synthesis strategies that compound with amine or thiol groups and are usually employed as surfactants/ligands for synthesizing and stabilizing gold NPs due to their high affinity binding to gold surface [1,3,7]. The quenching reaction provides useful information for the future design of synthesizing protocols of gold NPs.

615

of silanols occurs, in which linear molecules could be favored to be formed and cross-linked occasionally at gel point under acidic conditions [31]. Consequently, worm-like gold nanostructures were formed according to transmission electron microscopy images (Fig. 1-B and Fig. 5-E). A new absorbance peak at 632 nm was observed (Fig. 5A and B), corresponding to the surface plasmon resonance band rived from the electron oscillation along the long axis [32–34]. When PEG-MS was added into the solution for synthesis (case iii in Fig. 1), it was found that such fusion was inhibited and thus mono-dispersed gold NPs were formed (Fig. 1-C). Characteristic peaks from both of GPTMS (Si–O at 1075, 815 and 778 cm1, blank stars and triangles) and PEG-MS (–OH at 3307 cm1, ester –C@O at 1736 and 1643 cm1, filled stars and triangles) were found by ATRFTIR (Fig. 4-C). This suggests that not only alcoholic products from hydrolyzed GPTMS but also PEG-MS were adsorbing on the surface of gold NPs. The inhibition could be due to the steric effect of adsorbed PEG-MS. To verify this hypothesis, PEG surfactants with varying lengths were tested, as shown in Fig. 6. The length of PEG-CM (MW: 2 and 20 k) was estimated to be 10.1 and 105.4 nm respectively based on that the length of one monomer is 0.18 nm [35]. In this case, worm-like gold NPs were formed when PEG-CM 2 K was used (Fig. 6-D), whereas mono-dispersed sphere-shaped gold NPs were formed when using PEG-CM 20 K (Fig. 6-E). It can be deduced that long spacers could isolate gold

3.2. GPTMS serves as surfactants of gold NPs ATR-FTIR was employed to investigate surfactants on gold NPs in this study. Characteristic peaks from GPTMS were found in comparison with pure samples (Fig. 4-A). This suggests that hydrolyzing products of GPTMS (alcoholic products) adsorb onto the surface of gold NPs, as in the adsorption of alcohols [10,30]. However, such gold NPs (case ii in Fig. 1) stabilized by alcoholic products from hydrolyzed GPTMS can fuse with each other as the condensation

Scheme 2. Illustration of the mechanism of the formation of gold NPs. (A) GPTMS or PEG-MS is playing a dual role: reductant towards Au (III) and surfactants. (B) The possible morphology of gold NPs formed in the three different samples as in Fig. 1.

616

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617

Fig. 7. Representative TEM images of gold NPs prepared in the presence of PEG-MS alone (i) in Fig. 1. Samples were tested after 48 h.

NPs and keep them away from fusing or aggregating with each other. 3.3. Mechanism of the formation of gold NPs In this work, both GPTMS and PEG-MS are playing a dual role: reductant towards Au (III) and surfactant on the surface of gold NPs (Scheme 2-A). Under sonication for 3 min, GTPMS shows remarkably higher efficiency at the reduction of Au (III) than PEG-MS, leading to form gold NPs immediately (Scheme 2-B). Under the same conditions, PEG-MS produces marginal amounts of gold NPs. After sonication, the resulting mixture solution of PEGMS and Au (III) was kept in room temperature for another 48 hrs, and then the color of the solution changed to dark purple. TEM images showed that the size of such gold NPs from the quiescent reaction is estimated to be 25 ± 5 nm, along with plate-like nanostructure (Fig. 7). The formation of plate-like gold nanostructure has also been reported by other groups who reduce Au (III) by PVP polymer [36], polyol [37], citrate [38], and protein [39]. This mechanism is not clearly understood yet and remains debate. One possibility is following a two-step growing process [40], which is preferred for relatively long periods of reduction by PEG-MS (48 h). PEG-MS-induced gold NPs were first formed in the solution, in which the adsorbed PEG-MS molecules block all directions to facilitate the growth of gold NPs except for one (i.e. favoring at {1 1 1} [36]). Afterwards, Au (III) residue in solution continued to be reduced by PEG-MS radicals and/or macroradicals [36], growing gradually toward the direction remaining of the former gold NPs forming a plate-like gold nanostructure. In the absence of long chain surfactants (e.g. PEG-MS or PEGCM), GPTMS-induced gold NPs are likely to aggregate or fuse with each other (as indicated by that the color becomes dark after 48 h in Fig. 5-D), resulting in the formation of worm- or nuggets-like nanostructures (Fig. 5-E and F). This does not result if prepared in the presence of PEG-MS (or PEG-CM) and GPTMS, in which mono-dispersed sphere-shaped gold NPs are likely to form (Fig. 1-C, Fig. 6-E). The inhibition toward the fusion could be in behalf of the steric effects of the adsorbed long chain surfactants. 4. Conclusion Facile sonochemical synthesis of gold NPs in an oxygen-containing aqueous solution was demonstrated. The mechanism of GPTMS-induced gold NPs was discussed. Unlike primary alcohols, GPTMS is able to offer high-efficiency reduction of Au (III), and as a result, efficient formation of gold NPs. This is attributed to radials generated from silanols and products of hydrolyzed epoxides. GPTMS-assisted sonochemical synthesis provides obvious advantages in reaction time and size of gold NPs, compared with that of conventional alcoholic promoters as well as other silanizing

reagents. Since both high-frequency ultrasound sources and anaerobic conditions are not required for this method, it will open an avenue for rapid and large-scale green-synthesis of gold NPs. Further studies are under way in order to reveal more details about the mechanism of the promotion by functional organoalkoxysilanes.

Acknowledgements Financial support for this work was provided by the National Science Foundation grant (NSF-0901303) and by the WV EPSCoR. The authors would like to acknowledge the use of WVU Research Shared Facilities. M.-Y.Wei is also grateful to the equipment support by Dr. Lian-shin Lin in the Department of Civil and Environmental Engineering and Dr. Jianbo Yao in the Division of Animal and Nutritional Sciences.

References [1] M.C. Daniel, D. Astruc, Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology, Chem. Rev. 104 (2004) 293–346. [2] C.R. Patra, R. Bhattacharya, D. Mukhopadhyay, P. Mukherjee, Application of gold nanoparticles for targeted therapy in cancer, J. Biomed. Nanotechnol. 4 (2008) 99–132. [3] Z.X. Wang, L.N. Ma, Gold nanoparticle probes, Coord. Chem. Rev. 253 (2009) 1607–1618. [4] J. Turkevitch, P.C. Stevenson, J. Hillier, A study of nucleation and growth process in the synthesis of colloidal gold, Discuss. Faraday Soc. 11 (1951) 55– 75. [5] J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Toward greener nanosynthesis, Chem. Rev. 107 (2007) 2228–2269. [6] M.B. Dickerson, K.H. Sandhage, R.R. Naik, Protein- and peptide-directed syntheses of inorganic materials, Chem. Rev. 108 (2008) 4935–4978. [7] M. Grzelczak, J. Perez-Juste, P. Mulvaney, L.M. Liz-Marzan, Shape control in gold nanoparticle synthesis, Chem. Soc. Rev. 37 (2008) 1783–1791. [8] K.S. Suslick, Sonochemistry, Science 247 (1990) 1439–1445. [9] J.H. Bang, K.S. Suslick, Applications of ultrasound to the synthesis of nanostructured materials, Adv. Mater. 22 (2010) 1039–1059. [10] R.A. Caruso, M. Ashokkumar, F. Grieser, Sonochemical formation of gold sols, Langmuir 18 (2002) 7831–7836. [11] Y. Mizukoshi, S. Seino, T. Kinoshita, T. Nakagawa, T.A. Yamamoto, S. Tanabe, Selective magnetic separation of sulfur-containing amino acids by sonochemically prepared Au/gamma-Fe2O3 composite nanoparticles, Scr. Mater. 54 (2006) 609–613. [12] Y. Mizukoshi, S. Seino, K. Okitsu, T. Kinoshita, Y.H. Otome, T. Nakagawa, T.A. Yamamoto, Sonochemical preparation of composite nanoparticles of Au/ gamma-Fe2O3 and magnetic separation of glutathione, Ultrason. Sonochem. 12 (2005) 191–195. [13] Y. Mizukoshi, Y. Tsuru, A. Tominaga, S. Seino, N. Masahashi, S. Tanabe, T.A. Yamamoto, Sonochemical immobilization of noble metal nanoparticles on the surface of maghemite: mechanism and morphological control of the products, Ultrason. Sonochem. 15 (2008) 875–880. [14] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Formation of colloidal gold nanoparticles in an ultrasonic field: control of rate of gold(III) reduction and size of formed gold particles, Langmuir 17 (2001) 7717–7720. [15] V.G. Pol, A. Gedanken, J. Calderon-Moreno, Deposition of gold nanoparticles on silica spheres: a sonochemical approach, Chem. Mater. 15 (2003) 1111–1118.

M.-Y. Wei et al. / Ultrasonics Sonochemistry 20 (2013) 610–617 [16] T. Sakai, H. Enomoto, K. Torigoe, H. Sakai, M. Abe, Surfactant- and reducer-free synthesis of gold nanoparticles in aqueous solutions, Colloid Surf. A 347 (2009) 18–26. [17] S. Seino, Y. Matsuoka, T. Kinoshita, T. Nakagawa, T.A. Yamamoto, Dispersibility improvement of gold/iron-oxide composite nanoparticles by polyethylenimine modification, J. Magn. Magn. Mater. 321 (2009) 1404–1407. [18] C.H. Su, P.L. Wu, C.S. Yeh, Sonochemical synthesis of well-dispersed gold nanoparticles at the ice temperature, J. Phys. Chem. B 107 (2003) 14240– 14243. [19] Q. Chen, C. Boothroyd, A.M. Soutar, X.T. Zeng, Sol–gel nanocoating on commercial TiO2 nanopowder using ultrasound, J. Sol-Gel Sci. Technol. 53 (2010) 115–120. [20] N. Zhou, Z.Y. Chen, D.M. Zhang, G.X. Li, Electrochemical assay of human islet amyloid polypeptide and its aggregation, Sensors-Basel 8 (2008) 5987–5995. [21] R.S. Iyer, T.M. Harris, Preparation of aflatoxin-B1 8,9-epoxide using Mchloroperbenzoic acid, Chem. Res. Toxicol. 6 (1993) 313–316. [22] I. Panas, A. Lundin, E. Ahlberg, A mechanistic investigation of ethylene oxide hydrolysis to ethanediol, J. Phys. Chem. A 111 (2007) 9087–9092. [23] M.M.C. Ferreira, R.C.D. Muniz, S.A.A. de Sousa, F.D. Pereira, Theoretical study of acid-catalyzed hydrolysis of epoxides, J. Phys. Chem. A 114 (2010) 5187–5194. [24] M.S. Abaee, V. Hamidi, M.M. Mojtahedi, Ultrasound promoted aminolysis of epoxides in aqueous media: a rapid procedure with no pH adjustment for additive-free synthesis of beta-aminoalcohols, Ultrason. Sonochem. 15 (2008) 823–827. [25] A. Kamal, S.F. Adil, M. Arifuddin, Ultrasonic activated efficient method for the cleavage of epoxides with aromatic amines, Ultrason. Sonochem. 12 (2005) 429–431. [26] U. Pischel, W.M. Nau, Switch-over in photochemical reaction mechanism from hydrogen abstraction to exciplex-induced quenching: interaction of tripletexcited versus singlet-excited acetone versus cumyloxyl radicals with amines, J. Am. Chem. Soc. 123 (2001) 9727–9737. [27] D.A. Armstrong, K.D. Asmus, M. Bonifacic, Oxide radical anion reactivity with aliphatic amino compounds in aqueous solution: comparison of H-atom abstraction from C–H and N–H groups by O and OH, J. Phys. Chem. A 108 (2004) 2238–2246. [28] H. Jiang, H.X. Ju, Electrochemiluminescence sensors for scavengers of hydroxyl radical based on its annihilation in CdSe quantum dots film/peroxide system, Anal. Chem. 79 (2007) 6690–6696.

617

[29] M. Enescu, B. Cardey, Mechanism of cysteine oxidation by a hydroxyl radical: a theoretical study, ChemPhysChem 7 (2006) 912–919. [30] M. Shen, Y. Sun, Y. Han, R. Yao, C.G. Yan, Strong deaggregating effect of a novel polyamino resorcinarene surfactant on gold nanoaggregates under microwave irradiation, Langmuir 24 (2008) 13161–13167. [31] A.M. Buckley, M. Greenblatt, The Sol–gel preparation of silica-gels, J. Chem. Educ. 71 (1994) 599–602. [32] M.A. El-Sayed, X.H. Huang, S. Neretina, Gold nanorods: from synthesis and properties to biological and biomedical applications, Adv. Mater. 21 (2009) 4880–4910. [33] A.B. Yu, L. Kemal, X.C. Jiang, K. Wong, Experiment and theoretical study of poly(vinyl pyrrolidone)-controlled gold nanoparticles, J. Phys. Chem. C 112 (2008) 15656–15664. [34] D. Radziuk, D. Grigoriev, W. Zhang, D.S. Su, H. Mohwald, D. Shchukin, Ultrasound-assisted fusion of preformed gold nanoparticles, J. Phys. Chem. C 114 (2010) 1835–1843. [35] E. Feitosa, W. Brown, K. Wang, P.C.A. Barreleiro, Interaction between poly(ethylene glycol) and C12E8 investigated by dynamic light scattering, time-resolved fluorescence quenching, and calorimetry, Macromolecules 35 (2002) 201–207. [36] C.E. Hoppe, M. Lazzari, I. Pardinas-Blanco, M.A. Lopez-Quintela, One-step synthesis of gold and silver hydrosols using poly(N-vinyl-2-pyrrolidone) as a reducing agent, Langmuir 22 (2006) 7027–7034. [37] C.X. Kan, X.G. Zhu, G.H. Wang, Single-crystalline gold microplates: synthesis, characterization, and thermal stability, J. Phys. Chem. B 110 (2006) 4651– 4656. [38] W.S. Yun, C.S. Ah, Y.J. Yun, H.J. Park, W.J. Kim, D.H. Ha, Size-controlled synthesis of machinable single crystalline gold nanoplates, Chem. Mater. 17 (2005) 5558–5561. [39] J.Y. Lee, J. Xie, D.I.C. Wang, Y.P. Ting, High-yield synthesis of complex gold nanostructures in a fungal system, J. Phys. Chem. C 111 (2007) 16858–16865. [40] J. Biswal, S.P. Ramnani, S. Shirolikar, S. Sabharwal, Synthesis of rectangular plate like gold nanoparticles by in situ generation of seeds by combining both radiation and chemical methods, Radiat. Phys. Chem. 80 (2011) 44–49.