Green Synthesis of Triangular Au Nanoplates: Role of Small

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10317-10326

pubs.acs.org/journal/ascecg

Green Synthesis of Triangular Au Nanoplates: Role of Small Molecules Present in Bael Gum Sathiya Balasubramanian and Dhamodharan Raghavachari* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: The green and selective synthesis of triangular nanoplates (NPs) is shown to arise out of the slow rate of reduction and generation of Au(0) from HAuCl4. Toward this purpose, the small molecules present along with the polysaccharide in bael gum (BG) are separated; their structures are identified, and their role in the reduction of HAuCl4 in aqueous solution as well as their possible role in shape direction are studied. The observations suggest that in all the cases studied the slow rate of reduction could be the primary reason for shape selectivity toward formation of NPs, and the role of small molecules is possibly limited to that of a reducing agent. This was further confirmed by carrying out the reduction reaction in some detail by using imperatorin oxide (one of the molecules isolated from BG) at different concentrations. At higher concentrations of imperatorin oxide, the formation of pseudospherical and rodlike particles (instead of smaller NPs) in solution further confirmed the hypothesis. The formation of pseudospherical Au nanoparticles from BG, at high concentration and ambient temperature or relatively lower concentration and high temperature, as well as the formation of NPs from purified BG at ambient temperature reinforce the hypothesis that a moderate reduction rate results in the formation of triangular Au NPs. KEYWORDS: Phytochemicals, Anisotropic nanoparticles, Selective passivation, Rate of reduction, Mechanism of shape selectivity

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INTRODUCTION The unique electrical, magnetic, and optical property of nanoparticles has drawn much attention recently. In particular, in the case of anisotropic nanoparticles, the tunability of surface plasmon resonance (SPR) from the visible to NIR region has unfolded different areas of application such as sensors,1 surfaceenhanced Raman scattering (SERS),2 optical guiding,3,4 biological areas,5 photothermal therapy,6 and drug delivery.7 Different methods have been reported for the synthesis of anisotropic nanoparticles such as seed-mediated,8 photothermal,9 sonochemical,10 polyol synthesis,11 and biological synthesis.12 Among the reported methods, seed-mediated (surfactant-based) synthesis of nanoparticles seems to be the best in terms of tuning the size and shape. The first report on the preferential synthesis of Au nanoplates (NPs) by the reduction of HAuCl4 reports the use of citric acid as the reductant.13 More recently, surfactant-based synthesis has facilitated size tunability,14 monodispersity,14 and selectivity toward triangular nanoplate formation15 as well as conversion to other shapes such as hexagonal and flowerlike.16,17 Using this method, the thickness of NPs could be tuned under certain conditions.18 In contrast, the accomplishment of biological synthetic methods is still in its infant stage. The first report on the synthesis of triangular gold NPs using lemon-grass extract appeared in 2004, and this also demonstrated size tunability. However, the yield of the NPs was 45%.19,20 In the subsequent years, although the use of a number of biological species in the © 2017 American Chemical Society

preparation of NPs has been reported, a method which could produce monodispersed, tunable-size Au NPs in high yield could not be attained using biological species.21−25 Recently, we reported the high-yield synthesis of Au NPs with tunable size using BG (a natural fruit-based gum isolated from bael fruit26) as a reducing-cum-shape-directing agent.27 We could achieve monodispersed nanoparticles if the synthesis was started with monodispersed seeds. In this work, we proposed that the rate of the reduction of HAuCl4 could play an important role in the outcome of the final shape of the nanoparticles.27 Though this paper brought some new insights into the formation mechanism of NPs, the most important and complicated question, i.e., on the structure and role of the active ingredient(s), was left unanswered. The formation of noble-metal nanoparticles such as Au and Ag from the extracts of bael (leaves, fruit) was reported, but a detailed study about the role of different molecules and their electron-donating ability in the formation of different shapes had not been elucidated.28−31 The literature addresses the origin of shape selectivity through a few mechanisms such as the following: selective passivation of a certain crystal facet for further growth through adsorption of a small molecule; stacking faults during the nucleation; kinetic control; and thermodynamic control.32−36 In this context, it is important that the role Received: July 13, 2017 Revised: October 6, 2017 Published: October 9, 2017 10317

DOI: 10.1021/acssuschemeng.7b02346 ACS Sustainable Chem. Eng. 2017, 5, 10317−10326

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ACS Sustainable Chemistry & Engineering Scheme 1. Structure of the Polysaccharide-Constituting BGa

a

Gal stands for galactose, Rha for rhamnose, Ara for arabinose, and GalA for galacturonic acid.

Scheme 2. Structure of the Small Molecules Present in BG38,39

acetic acid and heated to 80 °C for 1 h. Then, it was precipitated using excess acetone. This procedure (dissolution followed by precipitation) was repeated at least five times to obtain pure polysaccharide, as reported earlier.37 The purity of the polysaccharide was inferred from the change of color from brown to colorless as well as through the disappearance of the absorptions above 220 nm in the UV−vis spectrum, which arise due to different chromophores associated with the small molecules present in BG. Extraction of Small Molecules. The supernatant from the above separation process was collected in a round-bottom flask and evaporated using a rotary evaporator to obtain a solid. It was extracted again with different organic solvents in the following order: hexane, chloroform, ethyl acetate, and methanol. The small molecules present in the solvent extracts were evaporated using a rotary evaporator and used individually for the reduction of HAuCl4. Synthesis of Au Nanoparticles Using the Extract of Small Molecules Present in BG. The reduction of HAuCl4 reaction was

of small molecules, present in the natural product, in shape selectivity be assessed to facilitate the movement toward green synthesis of anisotropic nanoparticles. For this purpose, BG consisting of a polysaccharide and a host of small molecules was chosen, and one of the prominent hypotheses associated with shape selectivity arising from the use of the natural product in nanoparticle synthesis, namely, inhibition of growth along a crystal facet due to selective passivation, was examined. An elaborate study on the isolation and identification of molecules present in BG and their role in the formation of Au NPs was investigated, and the results are discussed in this paper.

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EXPERIMENTAL SECTION

Separation and Purification of Polysaccharides from BG. Bael gum (BG) isolated from bael fruit26 was dissolved in 2% v/v 10318


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ACS Sustainable Chemistry & Engineering carried out using different solvent extracts of BG. In a typical experiment, 1 mL of 0.2 wt % aqueous solution of HAuCl4 was added to 10 mg of a solid obtained from one of the different solvents (hexane, ethyl acetate, chloroform, methanol, and the final residue after solvent extraction) in 19 mL of water. The reduction was carried out at 60 °C for 10 h. Among these, the chloroform-soluble component of BG appeared to be specific to the formation of gold triangles, and therefore more detailed experiments were carried out with this extract. Isolation and Identification of Small Organic Molecules Present in the Chloroform Extract of BG. A number of pure compounds were isolated from the chloroform extract using column chromatography. While using hexane and 2% ethyl acetate as eluent, imperatorin was separated as pure crystals. With the gradual increase of the polarity of the eluent (increasing the volume fraction of ethyl acetate), other compounds such as imperatorin oxide, marmesin, and heraclenol were also isolated as pure compounds. The purity of the molecules was ascertained by 1H NMR and single-crystal XRD. Synthesis of Au NPs. Initially, 1 mg of imperatorin crystal was taken in a vial, containing 0.5 mL of 0.2 wt % HAuCl4 solution. It was further diluted to 10 mL using deionized water (milli Q water) and was left at ambient temperature (28 ± 4 °C) for observation. After 3− 4 days, a small quantity of precipitate was observed at the bottom of the vial. The TEM analysis of the precipitate showed the formation of Au NPs. Similarly, other pure crystals such as imperatorin oxide, marmesin, and heraclenol were also used for the reduction of HAuCl4 solution. Synthesis of Au Nanoparticles Using Imperatorin Oxide. A 1 mg portion of imperatorin oxide crystals was dissolved in 9.5 mL of deionized water in a glass vial. Then, 0.5 mL of 0.2 wt % HAuCl4 solution was added, and it was left at ambient temperature for further observation. In 1 or 2 days, precipitate formation at the bottom of the vial was observed. This was further analyzed using SEM. In the same manner, the experiment was repeated by increasing the concentration of imperatorin oxide (3, 5, 10, and 20 mg). With increasing concentration of imperatorin oxide, the solubility of the compound in water was determined to be the issue. Therefore, to increase the solubility, imperatorin oxide solution was placed on a hot plate (60 °C) for 10 min. When the solution turned clear, it was taken out and cooled to room temperature (RT). It was observed that the 10 mg and 20 mg imperatorin oxide solutions looked clear under hot condition but exhibited mild turbidity when cooled to ambient temperature. The turbid solution was used as such for further reaction with HAuCl4. Characterization. UV−vis spectra were recorded using a JASCO UV-530 spectrophotometer. FT-IR was recorded using a JASCO FTIR-4100 instrument. NMR spectra were recorded using a Bruker Avance spectrometer (500 MHz for proton). Powder XRD patterns were obtained using a Bruker D8 Advanced powder X-ray diffractometer equipped with a copper anode (Cu Kα source of wavelength 1.5406 Å). Single-crystal X-ray analysis was carried out with a Bruker X8 Kappa APEXII instrument. Thermogravimetric analysis were carried out with a TA Instruments Q500 Hi-Res TGA instrument. The samples were heated at 10 °C min−1 under flowing N2 atmosphere. Surface-tension measurements in aqueous solutions were carried out with a Dataphysics DCAT 11EC instrument. Dynamic light scattering studies were conducted with a Malvern Zetasizer Nano Series ZS90 instrument. High-resolution scanning electron microscopy (HRSEM) images were obtained using an FEG Quanta 400 scanning electron microscope (FEI). TEM images were obtained using a JEOL3010 transmission electron microscope with an acceleration voltage of 200 kV. AFM results were obtained with an INTEGRA PRIMA instrument under ambient conditions using NT-MDT solver software.

The UV−vis spectrum of BG and pure polysaccharide is shown in Figure 1 along with the photograph of BG and pure

Figure 1. UV−vis spectra of BG and pure polysaccharide along with the photograph of the samples on watch glass.

polysaccharide. The presence of peaks in the region 225−350 nm suggests that BG, as isolated, consists of molecules with chromophores while the absence of the same peaks after purification suggests that it is pure polysaccharide. The results from the detailed spectroscopic studies (Figures S1−S4) are consistent with the structure of the polysaccharide reported earlier and as shown in Scheme 1. The results from the powder X-ray diffraction of the pure polysaccharide are presented in Figure S5. This suggests that it is an amorphous polymer and further indicates the absence of crystalline small molecular impurities that would show sharp diffraction patterns. The thermogravimetric analysis of the pure polysaccharide in nitrogen and air atmosphere is shown in Figure S6. This suggested the presence of polymer and water as evident from the main mass loss in the region 200−500 °C and the small mass loss (∼10%) in the region from ambient temperature to 100 °C. The polysaccharide is decomposed entirely in air atmosphere at 900 °C while significant noncombustible residue could be observed under nitrogen atmosphere at 900 °C. The UV−vis spectrum, proton NMR spectrum, PXRD, and TGA indicate the absence of small molecules in the pure polysaccharide as could be detected by the limits of these methods. BG, as isolated, could reduce HAuCl4 readily at room temperature while the purified BG, containing only the polysaccharide part, required prolonged time to effect a change. The results from the reduction of HAuCl4 with BG (impure as well as pure) of different weight ratios and at different temperatures are summarized in Table 1. The reduction of HAuCl4 with BG, at ambient temperature, in the weight ratio from 1:5 to 1:20 resulted in the formation of triangular NPs24 (Supporting Information, Figures S7 and S8) while spherical particles were obtained when the weight ratio was increased further to 1:100 (Supporting Information, Figure S7). When the temperature of reduction was increased, it was observed that a ∼1:1 mixture of spherical nanoparticles and triangular NPs were formed at 60 °C (an example for the 1:12.5 weight ratio is shown in the Supporting Information, Figure S7) while Au nanospheres were obtained (shown in the Supporting Information, Figure S7) at 90 °C. These experiments suggested that the formation of either spherical particles or NPs could be tailored either by changing the concentration of BG or by

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RESULTS AND DISCUSSION Bael gum is reported to consist of a water-soluble polysaccharide37 and a number of small organic moieties38,39 as shown in Schemes 1 and 2. 10319


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ACS Sustainable Chemistry & Engineering Table 1. Summary of Findings from the Reduction of HAuCl4 under Different Conditions no.

reducing agent

weight ratio HAuCl4:reducing agent

temperature

shape of the nanoparticle

1 2 3 4 5 6

BG BG BG BG purified BG purified BG

1:5 to 1:20 1:100 1:12.5 1:5 to 1:20 1:5 and 1:12.5 1:20

ambient ambient 60 90 ambient 90

triangular NPs spherical 1:1 NPs:spherical spherical broken plates spherical

ref 24 Figure Figure Figure Figure Figure

S7 S7 S7 2 S7

Scheme 3. Separation of Pure Polysaccharide and Small Organic Molecules from BG by Column Chromatography and Results from the Reduction of HAuCl4

variation in temperature (i.e., with variation in the rate of reduction of HAuCl4). For an understanding of the specific role of the constituents of BG consisting of the polysaccharide and the small organic molecules, they were separated, and the details of which are presented in Scheme 3. The purified BG (polysaccharide) was employed as the reducing-cum-stabilizing agent for the reduction of HAuCl4 at room temperature. In this case, the product obtained was not perfect NPs but appeared to be broken plates (Table 1, entry 5). The SEM image of particles synthesized using purified polysaccharide is represented in Figure 2. With an increase in the concentration of the pure

Figure 2. SEM image of Au plates obtained using purified BG as the reducing-cum-stabilizing agent at room temperature.

polysaccharide, the size and fraction of plates decreased while those of the pseudospherical particles increased. The surface 10320


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ACS Sustainable Chemistry & Engineering Table 2. Surface Tension and Hydrodynamic Size of Bael Gum and Pure Polysaccharide concentration (mg/mL)

surface tension of bael gum (mN/m)

average hydrodynamic size (BG; nm)

surface tension of purified polysaccharide (mN/m)

average hydrodynamic size of purified BG (nm)

0.5 1 1.5 2 3

70.22 65.17 51.85 52.52 48.57

714 717 743 798 1044

66.64 66.15 53.72 59.41 58.93

508 544 574 758 994

Figure 3. (a) UV−vis spectra of Au nanoparticles obtained using different solvent extracts of BG. (b, c) TEM image of Au NPs obtained using chloroform extract at 60 °C and at different magnifications.

Figure 4. 1H NMR spectrum of imperatorin.

tension of aqueous solutions of the polysaccharide decreased while the hydrodynamic size as assessed by dynamic light scattering increased with increasing concentration of the polysaccharide (Table 2). If the polysaccharide was to function as a template, with increasing concentration, the formation of bigger nanoplates as well as a greater fraction of nanoplates would be expected on the basis of the hydrodynamic size of the micelle. However, the formation of a greater and greater fraction of pseudospherical nanoparticles is observed with increasing concentration. This result thus excludes the template effect as the possible cause of nanoplate formation. Therefore, the kinetic control (through temperature and concentration variation) could be the dominating factor in shape selectivity. The reduction reaction when carried out at 90 °C with the pure polysaccharide resulted in the formation of spherical Au nanoparticles (Table 1, entry 6; and Figure S7). These results

imply that the polysaccharide, with three hydroxyl groups per repeat unit, enables the reduction of HAuCl4 in a fashion similar to that of hydroxyl-terminated PVP as well as poly(vinyl alcohol), and the kinetic control is probably responsible for the formation of nanoplates at ambient temperature and pseudospherical particles at 90 °C.40−44 For an investigation of the role of the small molecules present in the BG in the formation of NPs, they were isolated as solids after rotary evaporation of the different solvent extracts of BG (such as hexane, chloroform, ethyl acetate, and methanol) and subsequently used in the preparation of Au NPs as detailed in the Experimental Section. The UV−vis spectrum of Au nanoparticles synthesized using different solvent extracts of BG is shown in Figure 3a. The appearance of the SPR peak around 530 nm (Figure 3a) in all the cases implies that small organic molecules soluble in chloroform, 10321


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Figure 5. 1H NMR spectrum of imperatorin oxide.

Figure 6. 1H NMR spectrum of marmesin.

different small molecules present in the chloroform extract was carried out to identify the molecule(s) that could be responsible for the formation of triangular NPs. For this purpose, the CHCl3 extract of BG was further separated using column chromatography, and the structures of each one of the isolated molecules was elucidated using single-crystal XRD and NMR spectroscopy. The NMR spectra of the isolated compounds are presented in Figures 4−7. The first and important compound isolated from the chloroform extract was imperatorin (a well-known drug used in the treatment of cancer and Alzheimer’s disease). Recent research has shown that it can inhibit HIV I replication.46 The next important compound isolated from the chloroform extract was the epoxide of imperatorin (i.e., imperatorin epoxide). It is also known as

ethyl acetate, and methanol reduce HAuCl4 (except hexane). It is important to note here that the chloroform and methanol extracts (as well as the residue left after methanol extraction) resulted in Au nanoparticles that showed two SPR peaks. These are attributed to the transversal and longitudinal electron oscillations in Au NPs.45 The TEM analysis of the nanoparticles formed using the chloroform extract (Figure 3b,c) confirmed the formation of two different shapes (triangular NPs and spherical). These experiments suggested that the small molecules present along with the polysaccharide in BG participate in the reduction of HAuCl4 and possibly in the shape selectivity of the Au nanoparticles formed. The formation of Au NPs in higher yield was observed in the case of the CHCl3 extract of BG. Therefore, the separation of 10322


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Figure 7. 1H NMR spectrum of heraclenol.

reduction of HAuCl4. In each case, the precipitate that was formed consisted of NPs (as assessed by SEM analysis and shown in Figure 9) of thickness ∼4 nm. The supernatant in each case was found to contain spherical particles (by TEM). These experiments establish, unambiguously, that small molecules present in BG are instrumental in the reduction and shape-directing process. As for the direction of the shape of Au nanoparticles, it may be useful to recall the different mechanisms proposed in the literature in the context of the present findings. The different mechanisms are selective passivation of the (111) plane by the specific shape directing agent; stacking faults formation during the nucleation process; slow rate of reduction or kinetic control; and thermodynamic control as stated in the Introduction.32−36 On the basis of the results from the present studies, it appears that two out of the four mechanisms reported in the literature might be more suitable, namely, that the different organic molecules present in BG could selectively passivate the (111) plane, or that the very slow rate of reduction or kinetic control could result in the formation of triangular NPs. If selective passivation is the mechanism, the presence of spherical nanoparticles in the supernatant solution cannot be explained. The alternative explanation for the above observation is that the reaction is slow enough for kinetic control. Recently, it was reported50,51 that the critical supply rate of gold monomers was the required condition for the formation NPs (i.e., three layers per second). If the reaction rate exceeds the above condition, the resultant product should be spherical particles.50,51 The hypothesis that slow reduction could be instrumental in shape selectivity toward triangular NP formation could be further validated by altering the concentration of one of the small molecules significantly present in BG (and in its chloroform extract). For this purpose, imperatorin oxide was chosen as the reducing agent as it could be isolated in greater yield (being also present in higher concentrations in BG), and the reaction was carried out at different concentrations (such as 0.3, 0.5, 1, and 2 mg/mL with respect to HAuCl4). In each case, the formation of a precipitate was observed at the bottom of the

(±)-prangenin, imperatorin oxide, heraclenin, and prengenine. Imperatorin oxide is predicted to play an important role in future organoelectronics;47 is mixed with teflubenzuron to control Plutella xylostella;48 possesses antiplatelet, anticoagulant, and anti-inflammatory activities; and is shown to induce apoptosis significantly in Jurkat leukemia cells.49 By slightly increasing the polarity of the eluent, marmesin (also known as nodakenetin) was separated as pure crystals (used as a natural UV-A filtering product and can act as a novel angiogenesis inhibitor). Subsequently, heraclenol, a diol product formed by opening the epoxide present in heraclenin, was isolated. Though the crystal structure of enantiomers were fortunately obtained, the separation of both isomers could not be achieved successfully. Most of the crystals presented above exhibit antiinflammatory and antimycobacterial properties. The crystal structure of imperatorin, imperatorin oxide, marmesin, and heraclenol (both enantiomeric forms) is presented in Figure 8. All of these molecules (0.1 mg/mL solution in water) were used, in the pure form, for the

Figure 8. Crystal structures of (a) imperatorin, (b) imperatorin oxide, (c) marmesin, and (d) the S and R enantiomeric forms of heraclenol. 10323


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Figure 9. SEM images of Au NPs synthesized using different organic molecules present in BG such as imperatorin, imperatorin oxide, marmesin, and heraclenol (scale bar, 3 μm; 0.1 mg/mL in water).

molecules as well) could form a template and facilitate template-driven synthesis. For investigation of this aspect, the dynamic light scattering of imperatorin oxide in water was studied. This suggested that the hydrodynamic size increased with concentration (337, 423, 488, and 665 nm, respectively, at 0.1, 0.3, 0.5, and 1 mg/mL). The TEM images of micelles and Au nanoparticles formed after the reduction of HAuCl4 with imperatorin oxide in water are shown in the Supporting Information, Figure S9. When the imperatorin oxide concentration was 0.1 mg/mL, micelle formation was not observed by TEM, yet Au triangular NPs are formed (Figure 9). At higher concentrations, spherical micelles are formed (Figure S9), but the shape and size of the nanoparticles are different from those of the micelles. Therefore, on the basis of our experiments, it appears that the rate of reduction of HAuCl4 could play a very crucial role in the shape selectivity toward NPs, but our complete results do not exclude the possibility of the templating effect of the small molecules present in bael gum. With an increase in the concentration of the reducing agent, the rate of production of Au atoms should be greater, and if it exceeds three layers per second, then spherical particles are formed as reported recently.50,51 These experiments clearly implied that rate of reaction could be the decisive factor in determining the shape selectivity toward triangular NP formation, and the specific small molecule(s) present in the natural product may not have a specific role in shape selectivity and may possibly function only as electron donors.

vial. The precipitate contained smaller triangles, and a greater number of hexagonal, unsymmetrical hexagonal, distorted spherical particles and even rods were observed (shown in Figure 10). The same observations were made with heraclenol (data not presented). If the mechanism of formation of NPs proceeded by selective adsorption to the (111) plane, then increasing the concentration of imperatorin oxide should have resulted in smaller Au NPs in high yield, which is not the case. However, it can be argued that imperatorin oxide (and other

Figure 10. TEM images of Au nanoparticles synthesized using different concentrations of imperatorin oxide at ambient temperature: (a) 0.3, (b) 0.5, (c) 1, and (d) 2 mg/mL. 10324


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(4) Maillard, M.; Giorgio, S.; Pileni, M. P. Tuning the Size of Silver Nanodisks with Similar Aspect Ratios: Synthesis and Optical Properties. J. Phys. Chem. B 2003, 107, 2466. (5) Stuart, D. A.; Haes, A. J.; Yonzon, C. R.; Hicks, E. M.; Van Duyne, R. P. Biological Applications of Localised Surface Plasmonic Phenomena. IEE Proc.: Nanobiotechnol. 2005, 152, 13. (6) Wu, G.; Mikhailovsky, A.; Khant, H. A.; Fu, C.; Chiu, W.; Zasadzinski, J. A. Remotely Triggered Liposome Release by NearInfrared Light Absorption via Hollow Gold Nanoshells. J. Am. Chem. Soc. 2008, 130, 8175. (7) Zijlstra, P.; Chon, J. W. M.; Gu, M. Five-dimensional Optical Recording Mediated by Surface Plasmons in Gold Nanorods. Nature 2009, 459, 410. (8) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) using Seed-mediated Growth Method. Chem. Mater. 2003, 15 (10), 1957−1962. (9) Cognet, L.; Berciaud, S.; Lasne, D.; Lounis, B. Photothermal Methods for Single Nonluminescent Nano-objects. Anal. Chem. 2008, 80 (7), 2288−2294. (10) Nag, P.; Banerjee, S.; Lee, Y.; Bumajdad, A.; Lee, Y.; Devi, P. S. Sonochemical Synthesis and Properties of Nanoparticles of FeSbO4. Inorg. Chem. 2012, 51 (2), 844−850. (11) Skrabalak, S. E.; Wiley, B. J.; Kim, M.; Formo, E. V.; Xia, Y. On the Polyol Synthesis of Silver Nanostructures: Glycolaldehyde as a Reducing Agent. Nano Lett. 2008, 8 (7), 2077−2081. (12) Sudhaparimala, S.; Vaishnavi, M. Biological Synthesis of Nano Composite SnO2- ZnO − Screening for Efficient Photocatalytic Degradation and Antimicrobial Activity. Materials Today Proceedings 2016, 3 (6), 2373−2380. (13) Milligan, W. O.; Morriss, R. H. Morphology of Colloidal Gold − A Comparitive Study. J. Am. Chem. Soc. 1964, 86, 3461−3467. (14) Scarabelli, L.; Coronado-Puchau, M.; Giner-Casares, J. J.; Langer, J.; Liz-Marzan, L. M. Monodisperse Gold Nanotriangles: Size Control, Large-Scale Self Assembly, and Performance in SurfaceEnhanced Raman Scattering. ACS Nano 2014, 8, 5833−5842. (15) Chen, L.; Xu, Y.; He, L.; Mi, Y.; Bao, F.; Sun, B.; Zhang, X.; Zhang, Q. High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching. Nano Lett. 2014, 14, 7201− 7206. (16) Cao, Z.; Fu, H.; Kang, L.; Huang, L.; Zhai, T.; Ma, Y.; Yao, J. Rapid Room-temperature Synthesis of Silver Nanoplates with Tunable in-plane Surface Plasmon Resonance from Visible to Near-IR. J. Mater. Chem. 2008, 18, 2673−2678. (17) O’Brien, M. N.; Jones, M. R.; Kohlstedt, K. L.; Schatz, G. C.; Mirkin, C. A. Uniform Circular Disks with Synthetically Tailorable Diameters: Two-dimensional Nanoparticles for Plasmonics. Nano Lett. 2015, 15, 1012−1017. (18) Huang, Y.; Ferhan, A. R.; Gao, Y.; Dandapat, A.; Kim, D. Highyield Synthesis of Triangular Gold Nanoplates with Improved Shape Uniformity, Tunable Edge Length and Thickness. Nanoscale 2014, 6, 6496−6500. (19) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Controlling the Optical Properties of Lemongrass Extract Synthesized Gold Nanotriangles and Potential Application in Infrared-absorbing Optical Coatings. Chem. Mater. 2005, 17, 566−572. (20) Shankar, S.; Rai, A.; Ankamwar, B.; Singh, A.; Absar Ahmad, A.; Sastry, M. Biological Synthesis of Triangular Gold Nanoprisms. Nat. Mater. 2004, 3, 482−488. (21) Ramanathan, R.; O’Mullane, A. P.; Parikh, R. Y.; Smooker, P. M.; Bhargava, S. K.; Bansal, V. Bacterial Kinetics-controlled Shapedirected Biosynthesis of Silver Nanoplates Using. Langmuir 2011, 27 (2), 714−719. (22) Tian, X.; Wang, W.; Cao, G. A Facile Aqueous-phase Route for the Synthesis of Silver Nanoplates. Mater. Lett. 2007, 61, 130−133. (23) Yi, Z.; Li, X.; Xu, X.; Luo, B.; Luo, J.; Wub, W.; Yi, Y.; Tang, Y. Green, Effective Chemical Route for the Synthesis of Silver Nanoplates in Tannic Acid Aqueous Solution. Colloids Surf., A 2011, 392, 131− 136.

CONCLUSIONS The pure polysaccharide in the BG, when utilized as a reducingcum-stabilizing agent, resulted in the formation of NPs at ambient temperature and spherical particles at higher temperature. Some of the small molecules in BG, as in the case of ethyl acetate extract, lead to the formation of spherical nanoparticles at ambient temperature. Some other small molecules, as in the case of chloroform extract, result in the formation of triangular NPs at room temperature and a mixture of triangular NPs as well as spherical nanoparticles at 60 °C. The small molecules that result in the formation of triangular NPs at ambient temperature lead to the formation of pseudospherical particles at higher concentrations and at higher temperature. Thus, small molecules in BG are established to possess a range of electrondonating or reducing ability that in turn results in different rates of reduction of HAuCl4. Thus, the green and selective synthesis of triangular NPs of gold from HAuCl4 is shown to arise out of kinetic control or slow rate of reduction rather than as an exclusive consequence of the presence of the shape-directing molecule(s) present in BG, which could selectively passivate a specific crystal facet. The experiments, unambiguously, establish that the green synthesis of triangular gold NPs could be carried out with the active ingredient in bael gum that provides adequate rate of reduction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02346. FT-IR spectrum, 1H and 13C NMR spectra, HSQC spectrum, powder XRD patterns, TGA results, and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dhamodharan Raghavachari: 0000-0001-9436-1373 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.B. thanks UGC, Government of India for a fellowship. The authors thank Prof. S. Sankaran and the electron microscopy facility of the Department of Materials and Metallurgical Engineering, IIT Madras. The authors wish to thank Ravi of CLRI, Adyar, Chennai, for the AFM studies and Prof. Prasad of the Department of Chemistry, IIT Madras, for providing the access to dynamic light scattering studies. This work was made possible due to the support of IIT Madras.

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REFERENCES

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Green Synthesis of Triangular Au Nanoplates: Role of Small

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