Green Synthesis of Triangular Au Nanoplates: Role of Small

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Green Synthesis of Triangular Au Nanoplates: Role of Small Molecules Present in Bael Gum Sathiya BalaSubramanian, and Dhamodharan Raghavachari ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02346 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Sustainable Chemistry & Engineering

Green Synthesis of Triangular Au Nanoplates: Role of Small Molecules Present in Bael Gum Sathiya Balasubramanian and Dhamodharan Raghavachari* Department of Chemistry I.I.T. Madras, Chennai 600 036, India

*Author for Correspondence E-mail: [email protected]

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”. Towards 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 possible role in shape direction is studied. The observations suggest that in all the cases studied the slow rate of reduction could be the primary reason for shape selectivity towards 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 pseudo spherical and rod like particles (instead of smaller sized NPs) in solution further confirmed the hypothesis. The formation of pseudo spherical 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

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purified BG at ambient temperature reinforces 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. Especially, in the case of anisotropic nanoparticles, the tunability of surface plasmon resonance (SPR) from visible to NIR region has unfolded different areas of application such as sensors,1 surface enhanced Raman scattering (SERS),2 optical guiding,3, photothermal therapy6 and drug delivery7.

4

biological,5

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 synthesis12. Among the reported methods, seed-mediated (surfactants 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

monodispersity14, and selectivity towards triangular nanoplate

formation15 as well as conversion to other shapes such as hexagonal and flower-like.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 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. But, the yield of the NPs was 45 %.19, 20 In the succeeding years, although the use of a number of biological species in the preparation of NPs has been reported, a method which could produce monodispersed, tuneable 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 tuneable 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



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that the rate of the reduction of HAuCl4 could be playing 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., “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 molecule and their electron donating ability in the formation of different shapes had not been elucidated.28-31 Literature addresses the origin of shape selectivity through few mechanisms such as: 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 of small molecules present in natural product in shape selectivity be assessed to facilitate the movement towards 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 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.

Experimental Separation and purification of polysaccharides from BG Bael gum (BG) isolated from bael fruit26 was dissolved in 2 % v/v acetic acid and heated to 80 °C for 1 h. Then, it was precipitated using excess acetone. This procedure (dissolution


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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 colour from brown to colourless as well as through the disappearance of the absorptions above 220 nm in the UV-Visible 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 rotary evaporator to get a solid. It was extracted again with different organic solvents in the 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 carried out using different solvent extracts of BG. In a typical experiment, 1 ml of 0.2 weight % 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 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 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. By gradually increasing the polarity of the eluent (increasing the volume



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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 weight % 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 1 mg of imperatorin oxide crystals were dissolved in 9.5 ml of deionized water in a glass vial. Then, 0.5 ml of 0.2 weight % HAuCl4 solution was added and it was left at ambient temperature for further observation. In a day or two, precipitate formation at the bottom of the vial was observed. This was further analysed using SEM. In the same manner, the experiment was repeated by increasing the concentration of imperatorin oxide (3 mg, 5 mg, 10 mg and 20 mg). With increasing concentration of imperatorin oxide, the solubility of the compound in water turned out to be the issue. Therefore, to increase the solubility, imperatorin oxide solution was placed in hot plate (60 °C) for 10 minutes. When the solution turned clear, it was taken out and cooled to 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.


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Characterization UV-Visible spectra were recorded using JASCO UV-530 spectrophotometer (Japan). FT-IR was recorded using JASCO FT-IR-4100 (Japan). NMR spectra were recorded using Bruker Avance spectrometer (500 MHz for proton). Powder XRD patterns were obtained using Bruker D8 Advanced Powder X-ray Diffractometer equipped with copper anode (Cu Kα source of wavelength1.5406 Å). Single crystal x-ray analysis was carried out with Bruker X8 Kappa APEXII. Thermogravimetric analysis were carried out with TA Instruments Q500 Hi-Res TGA. The samples were heated at 10 °C min-1 under flowing N2 atmosphere. Surface tension measurements in aqueous solutions were carried out with Dataphysics DCAT 11EC. Dynamic Light Scattering studies were conducted with Malvern Zetasizer Nano Series ZS90.

High

Resolution Scanning electron microscopy (HRSEM) images were obtained using FEG Quanta 400 Scanning electron microscope (FEI, USA). TEM images were obtained using JEOL3010 transmission electron microscope with an acceleration voltage of 200 kV. AFM were obtained with INTEGRA PRIMA (Russia) under ambient conditions using NT-MDT solver software (Ireland).



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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.

Scheme 1. Structure of the polysaccharide constituting BG where Gal stands for galactose, Rha for rhamnose, Ara for arabinose and GalA for galactouronic acid. The UV-Visible spectrum of BG and pure polysaccharide is shown in Figure 1 along with the photograph of BG and pure polysaccharide. The presence of peaks in the region 225 to 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 to 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 to 500 °C 8 ACS Paragon Plus Environment

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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-Visible 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.

Scheme 2. Structure of the small molecules present in BG.38, 39



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Figure 1. UV-Visible spectra of BG and pure polysaccharide along with the photograph of the samples on watch glass. 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 temperature are summarized in Table 1. The reduction of HAuCl4 with BG, at ambient temperature, in the weight ratio 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 Figures 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 1:12.5 weight ratio is shown in Supporting Information Figure S7) while Au nanospheres were obtained (shown in 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 variation in temperature (i.e., with variation in the rate of reduction of HAuCl4).


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Table 1. Summary of findings from the reduction of HAuCl4 under different conditions

No

Reducing

Weight ratio

agent

HAuCl4: reducing

Shape of the Temperature

nanoparticle

Reference

agent 1

BG

1:5 to 1:20

Ambient

Triangular NPs

24

2

BG

1:100

Ambient

Spherical

Figure S7

3

BG

1:12.5

60

1:1 NPs: Spherical

Figure S7

4

BG

1:5 to 1:20

90

Spherical

Figure S7

5

Purified BG

1:5 and 1:12.5

Ambient

Broken plates

Figure 2

6

Purified BG

1:20

90

Spherical

Figure S7

To understand 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. By increasing the concentration of the pure polysaccharide the size and fraction of plates decreased while that of the pseudo spherical particles increased. The surface 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 size nanoplates as well as greater fraction of nanoplates would be expected based on the hydrodynamic size of the micelle.



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However the formation of greater and greater fraction of pseudo spherical 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 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 pseudo spherical particles at 90 °C.40-44

Table 2. Surface tension and hydrodynamic size of bael gum and pure polysaccharide. Concentration Surface tension (mg/mL)

Average

Surface tension

Average

of bael gum in

hydrodynamic

of purified

hydrodynamic

mN/m

size in nm (BG)

polysaccharide

size in nm

in mN/m

Purified BG

0.5

70.22

714

66.64

508

1

65.17

717

66.15

544

1.5

51.85

743

53.72

574

2

52.52

798

59.41

758

3

48.57

1044

58.93

994


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Scheme 3. Separation of pure polysaccharide and small organic molecules from BG by column chromatography and results from the reduction of HAuCl4.



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Figure 2. SEM image of Au plates obtained using purified BG as the reducing cum stabilizing agent at room temperature. To investigate 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-Visible 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, 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 (Figures 3b and 3c) 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.


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Figure 3. a) UV-Visible spectra of Au nanoparticles obtained using different solvent extracts of BG; b) and c) TEM image of Au NPs obtained using chloroform extract at 60 °C and at different magnifications. The formation of Au NPs in higher yield was observed in the case of CHCl3 extract of BG. Therefore the separation of 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 molecule isolated was elucidated using single crystal XRD and NMR spectroscopy. The NMR spectra of the compounds isolated are presented in Figures 4 to 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 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 (+/-)-prangenin, imperatorin oxide, heraclenin and prengenine. Imperatorin oxide predicted to play an important role in future organoelctronics47; is mixed with teflubenzuron to control plutella xylostella48; possesses anti-platelet, anti-coagulant, antiinflammatory activities; shown to induce apoptosis significantly in Jurkat leukaemia cells49. By slightly increasing the polarity of the eluent, marmesin (also known as nodakenetin), was



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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 obtained fortunately, the separation of both the isomer could not be achieved successfully. Most of the crystals presented above, exhibit anti-inflammatory and antimycobacterial properties.

Figure 4. 1H NMR spectrum of imperatorin.


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Figure 5. 1H NMR spectrum of imperatorin oxide.



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Figure 6. 1H NMR spectrum of marmesin.


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Figure 7. 1H NMR spectrum of heraclenol.

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 reduction of HAuCl4. In all the cases, 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 all the cases 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 far as 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 (111) plane by the specific shape directing



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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

Figure 8. Crystal structures of a) imperatorin; b) imperatorin oxide; c) marmesin and d) the (S) and (R) enantiomeric forms of heraclenol. Based on the results from the present studies, it appears that two out of the four mechanisms reported in the literature might be more suitable, namely, “the different organic molecules present in BG could selectively passivate the (111) plane” or “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 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


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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). The hypothesis that slow reduction could be instrumental in shape selectivity towards triangular NPs formation, could be further validated by altering the concentration of one of the small molecules present significantly 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 21 ACS Paragon Plus Environment


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also present in higher concentration in BG) and the reaction was carried out at different concentrations (such as 0.3 mg/mL, 0.5 mg/mL, 1 mg/mL and 2 mg/mL with respect to HAuCl4). In all the cases, the formation of a precipitate was observed at the bottom of the vial. The precipitate contained smaller sized triangles and more 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 (111) plane, then, increasing the concentration of imperatorin oxide should have resulted in smaller sized Au NPs in high yield, which is not the case. However it can be argued that imperatorin oxide (and other molecules as well) could form a template and facilitate template-driven synthesis. To investigate this aspect, the dynamic light scattering of imperatorin oxide in water was studied. This suggested that the hydrodynamic size increased with concentration (337 nm, 423 nm, 488 nm 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 is shown in Supporting Information Figure S9. When the imperatorin oxide concentration was 0.1 mg/mL micelle formation was not observed by TEM and 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 that of the micelles. Therefore, based on our experiments, it appears that the rate of reduction of HAuCl4 could be playing a very crucial role in the shape selectivity towards NPs but our results complete do not exclude the possibility of templating effect of the small molecules present in bael gum. By increasing the concentration of the reducing agent, the rate of production of Au atoms should be more and if it exceeds three layers per second then spherical particles are formed as reported recently.50,

51

These experiments


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clearly implied that rate of reaction could be the decisive factor in determining the shape selectivity towards triangular NPs formation and the specific small molecule(s) present in the natural product may not have a specific role in shape selectivity and possibly function only as electron donors.

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

Conclusions The pure polysaccharide in the BG, when utilized as reducing cum 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



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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 pseudo spherical particles at higher concentrations and at higher temperature. Thus small molecules in BG are established to possess a range of electron donating or reducing ability that in turn result 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 shape directing molecule(s) present in BG, which could selectively passivate a specific crystal facet. The experiments, unambiguously, establishes 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.

ASSOCIATED CONTENT Supporting Information FT-IR spectrum of pure polysaccharide,

1

H-NMR and

13

C NMR spectrum of pure

polysaccharide, HSQC spectrum of pure polysaccharide isolated from bael gum, powder XRD pattern and TGA of pure polysaccharide, TEM images of Au nanoparticles formed with different weight ratios of HAuCl4 : BG, PXRD pattern of Au nanoplates, TEM images of micelles and Au nanoparticles formed after the reduction of HAuCl4 with imperatorin oxide in water, Surface tension versus concentration for bael gum and the purified polysachcharide, Dynamic light Scattering data for bael gum and the purified polysaccharide, Hydrodynamic size as determined by dynamic light scattering data for imperatorin oxide of different concentrations in water.


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AUTHOR INFORMATION Corresponding Author Dhamodharan Raghavachari. E-mail: [email protected]

Notes The authors declare no competing financial interests.

Acknowledgements SB thanks UGC, Government of India for 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.

References 1. Dawson, K.; O’Riordan, A. Towards Nanowire (bio)Sensors. J. Phys. Conf. Ser. 2011, 307, 012004, 1-7. 2. Lin, W.; Lu, Y.; Hsu, Y. Au Nanoplates as Robust, Recyclable SERS Substrates for Ultrasensitive Chemical Sensing. Journal of Colloid and Interface Science 2014, 418, 87–94. 3. Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marza´n, L. M.; Mulvaney, P. Gold Nanorods: Synthesis, Characterization and Applications. Coord. Chem. Rev. 2005, 249, 1870.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

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. IEEE 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 Near-Infrared 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. and Morriss, R. H. Morphology of Colloidal Gold – A Comparitive Study. J. Am. Chem. Soc. 1964, 86, 3461−3467.


Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. Scarabelli, L.; Coronado-Puchau, M.; Giner-Casares, J. J.; Langer, J. and Liz-Marzan, L. M. Monodisperse Gold Nanotriangles: Size Control, Large-Scale Self Assembly, and Performance in Surface-Enhanced Raman Scattering. ACS Nano, 2014, 8, 5833-5842. 15. Chen, L.; Xu, Y.; He, L.; Mi, Y.; Bao, F.; Sun, B.; Zhang, X. and 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. High-yield 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 Infraredabsorbing 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. Mat., 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 Shape-directed Biosynthesis of Silver Nanoplates Using Morganella Psychrotolerans. Langmuir 2011, 27(2), 714–719.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

22. Tian, X.; Wang, W.; Cao, G. A Facile Aqueous-phase Route for the Synthesis of Silver Nanoplates. Materials Letters 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 and Surfaces A: Physicochem. Eng. Aspects 2011, 392, 131-136. 24. Roberto Baigorri, R.; García-Mina, J. M.; Aroca, R. F.; Alvarez-Puebla. R. A. Optical Enhancing Properties of Anisotropic Gold Nanoplates Prepared with Different Fractions of a Natural Humic Substance. Chem. Mater. 2008, 20,1516–1521. 25. Xie, J.; Lee, J. Y.; Wang, D. I.C.; Ting, Y. P. Identification of Active Biomolecules in the High-yield Synthesis of Single-crystalline Gold Nanoplates in Algal Solutions. Small 2007, 3, 672–682. 26. Balasubramanian. S.; Bezawada, S. R.; Dhamodharan, R. Facile Aqueous Phase Synthesis of (200) Faceted Au-AgCl Cubes Using Bael Gum and its Activity Toward Oxidation and Detection of o-PDA. ACS Sustainable Chem. Eng. 2016, 4 (7), 2960–2968. 27. Balasubramanian. S.; Bezawada, S. R.; Dhamodharan, R. Green, Selective, Seedless and One-pot Synthesis of Triangular Au Nanoplates of Controlled Size Using Bael Gum and Mechanistic Study. ACS Sustainable Chem. Eng. 2016, 4 (7), 3830–3839; 28. Rao, K. J.; Paria, S. Green Synthesis of Silver Nanoparticles From Aqueous Aegle Marmelos Leaf Extract. Materials Research Bulletin 2013, 48, 628–634. 29. Kumar, K. V. P.; Ghosh, O. S. N.; Balakrishnan, G.; Thirugnanasambantham, P.; Raghavan, S. K.; Viswanath, A. K. Green Synthesis of Zinc Oxysulfide Quantum Dots Using Aegle Marmelos Fruit Extract and Their Cytotoxicity in Hela Cells. RSC Adv. 2015, 5, 16815.


Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30. Angajala, G.; Pavan, P.; Subashini, R. One-Step Biofabrication of Copper Nanoparticles From Aegle Marmelos Correa Aqueous Leaf Extract and Evaluation of its Anti-Inflammatory and Mosquito Larvicidal Efficacy. RSC Adv. 2014, 4, 51459. 31. Rao, K. J.; Paria, S. Green Synthesis of Gold Nanoparticles Using Aqueous Aegle Marmelos Leaf Extract and Their Application for Thiamine Detection. RSC Adv. 2014, 4, 28645. 32. Kawasaki, H.; Nishimura, K.; Arakawa, R. Influence of the Counter Ions of Cetyltrimetylammonium Salts on the Surfactant Adsorption onto Gold Surfaces and the Formation of Gold Nanoparticles. J. Phys. Chem. C 2007, 111, 2683−2690. 33. Lofton, C.; Sigmund, W. Mechanisms Controlling Crystal Habits of Gold and Silver Colloids. Adv. Funct. Mater. 2005, 15, 1197−1208. 34. Lim, B.; Camargo, P. H. C.; Xia, Y. N. Mechanistic Study of the Synthesis of Au Nanotadpoles, Nanokites, and Microplates by Reducing Aqueous HAuCl4 with Poly(vinyl pyrrolidone). Langmuir 2008, 24,10437−10442. 35. Viswanath, B.; Kundu, P.; Halder, A.; Ravishankar, J. Mechanistic Aspects of Shape Selection and Symmetry Breaking During Nanostructure Growth by Wet Chemical Methods. J. Phys. Chem. C 2009, 113, 16866−16883. 36. Viswanath, B.; Kundu, P.; Ravishankar, J. Formation of Two-dimensional Structures by Tuning the Driving Force of Chemical Reactions: An Interpretation of Kinetic Control. J. Colloid Interface Sci. 2009, 330, 211−219. 37. Roy, A.; Mukherjee, A. K.; Rao, C. V. N. Graded Hydrolysis Studies on Bael (Aegle Marmelos) Gum. Carbohydr. Res. 1975, 41, 219-226. 38. Tokitomo, Y.; Shimono, Y.; Kobayashi, A.; and Yamanishi, T. Aroma Components of Bael Fruit (Aegle Marmelos CORREA). Agric. Biol. Chem. 1982, 46 (7), 1873-1877.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

39. Chakthong, S.; Weaaryee, P.; Puangphet, P.; Mahabusarakam, W.; Plodpai, P.; Voravuthikunchai, S. P.; Kanjana-Opas, A., Alkaloid and Coumarins From the Green Fruits of Aegle Marmelos. Phytochemistry 2012, 75, 108-113. 40. Wiley, B.; Sun, Y.; Mayers, B.; and Xia, Y. Shape-Controlled Synthesis of Metal Nanostructures: The Case of Silver. Chemistry-A European Journal 2005, 11, 454-463. 41. Xiong, Y.; Washio, I.; Chen, J.; Cai, H.; Li, Z.-Y.; Xia, Y. Poly(vinyl pyrrolidone): A Dual Functional Reductant and Stabilizer for the Facile Synthesis of Noble Metal Nanoplates in Aqueous Solutions. Langmuir 2006, 22, 8563-8570. 42. Washio, I.; Xiong, Y.; Yin, Y.; Xia, Y. Reduction by the End Groups of Poly(vinyl pyrrolidone): A New and Versatile Route to the Kinetically Controlled Synthesis of Ag Triangular Nanoplates. Adv, Mat. 2006, 18, 1745-1749. 43. Kim, M. H.; Lee, J.-J.; Lee, J.-B.; Choi, K.-Y. Synthesis of Silver Nanoplates with Controlled Shapes by Reducing Silver Nitrate with Poly(vinyl pyrrolidone) in Nmethylpyrrolidone. Cryst. Eng. Comm. 2013, 15, 4660-4666. 44. Kim, M. H.; Kwak, S. K.; Im, S. H.; Lee, J.-B.; Choi, K.-Y.; Byun, D.-J. Maneuvering the Growth of Silver Nanoplates: Use of Halide Ions to Promote Vertical Growth. J. Mater. Chem. C 2014, 2, 6165-6170. 45. Morsin, M.; Salleh. M. M.; Umar. A. A.; M. Z. Gold Nanoplates for a Localized Surface Plasmon Resonance-based Boric Acid Sensor. Sensors 2017, 17(5), 947. 46. Sancho, R.; Márquez, N.; Gómez-Gonzalo, M.; Calzado, M.A.; Bettoni, G.; Coiras, M. T.; Alcamí. J.; López-Cabrer, M.; Appendino, G.; Muñoz, E. Imperatorin Inhibits HIV-1 Replication through a Sp1-dependent Pathway. Journal of Biological Chemistry 2004, 279(36), 37349-59.


Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47. Banday, J. A.; Mirb, F. A.; Kanth, A. H.; Bhat, G. M. Structural and Optical Properties of Heraclenin; A Bio-organic Molecule from Prangos Pabularia, Optik 2013, 124, 4655–4658. 48. Liu, Yu, Shenqing, F. Z. (2016), CN 105994365 A Oct 12, 2016. 49. Gowri, P. Mangala; Haribabu, K.; Kishore, Hara; Manjusha, O.; Biswas, Sukla; Murty, U. S. N. Microbial Transformation of (+)-Heraclenin by Aspergillus niger and Evaluation of its Antiplasmodial and Antimicrobial Activities. Current Science 2011, 100(11), 1706-1711. 50. Tangeysh, B.; Tibbetts, K. M.; Odhner, J. H.; Wayland, B. R.; Levis, R. J. Triangular Gold Nanoplate Growth by Oriented Attachment of Au Seeds Generated by Strong Field Laser Reduction. Nano Lett. 2015, 15, 3377−3382. 51. Alloyeau, D.; Dachraoui, W.; Javed, Y.; Belkahla, H.; Wang, G.; Lecoq, H.; Ammar, S.; Ersen, O.; Wisnet, A.; Gazeau, F.; Ricolleau, C. Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy. Nano Lett. 2015, 15, 2574−2581.



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For Table of Contents Use Only Slow reduction of chloroauric acid with phytochemicals leads to triangular gold nanoplates while fast reduction to pseudo spherical nanoparticles.


Green Synthesis of Triangular Au Nanoplates: Role of Small

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