Synthesis of Chiral, Crystalline Au-Nanoflower Catalyst Assisting

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Synthesis of Chiral, Crystalline Au-Nanoflower Catalyst Assisting Conversion of Rhodamine-B to Rhodamine-110 and a Single step, One Pot, Eco-Friendly Reduction of Nitroarenes Soumen Basak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5086125 • Publication Date (Web): 26 Dec 2014 Downloaded from http://pubs.acs.org on January 2, 2015

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Synthesis of Chiral, Crystalline Au-Nanoflower Catalyst Assisting Conversion of Rhodamine-B to Rhodamine-110 and a Single step, One Pot, Eco-Friendly Reduction of Nitroarenes

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry jp-2014-086125.R2 Article 24-Dec-2014 Bera, Kallol; Saha Institute of Nuclear Physics, Chemical Sceinces Division Ghosh, Tanmay; Saha Institute of Nuclear Physics, Surface Physics Division Basak, Soumen; Saha Institute of Nuclear Physics, Chemical Sceinces Division

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Synthesis of Chiral, Crystalline Au-Nanoflower Catalyst Assisting Conversion of Rhodamine-B to Rhodamine-110 and a Single step, One Pot, Eco-Friendly Reduction of Nitroarenes Kallol Bera†,‡, Tanmay Ghosh#,‡, and Soumen Basak†,* †

Chemical Sciences Division, #Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, Kolkata 700064, India KEYWORDS. Gold nanoflower; Core-Satellite, Chiral; Rhodamine B; Rhodamine 110; Nitroarene; Aminoarene; Catalysis.

ABSTRACT: Chiral, multipetal gold nanoflowers (AuNFs) with crystalline tips were synthesized in high yield through the reduction of HAuCl4 in water in presence of preformed core-satellite Ag nanoparticle (AgNPs) seeds. High-resolution transmission electron microscopic (HRTEM) images and tomography reveal the 3-dimensional structural profile of the AuNFs. In a first demonstration of this kind, a Hg2+/AuNF combination was shown to catalyze the direct transformation of rhodamine-B to rhodamine-110 in water. The AuNFs were also shown to catalyze reduction of nitroarenes to its corresponding aminoarene with ~90% conversion efficiency by a single step, one pot, eco-friendly protocol. There are numerous opportunities for using these unique AuNFs in catalysis, sensing, imaging, drug delivery and opto-photonics.

Noble metal nanostructures have been extensively studied due to their fascinating optical1-35, electronic7-14, and catalytic properties3, 4, 18, 19, 25-27, 36-45. Assemblies of metallic and inorganic nanoparticles are well-known to yield collective physical properties dependent on particle size3-6, 15, 16, 28, spacing1-7, and higher-order structure18-35. Nanostructures of gold have been of great promise due to their chemical stability5-10, good biocompatibility11-24, and strong SERS enhancement5-9, 15-17, 28. Development of new AuNP carrier systems for controlledrelease and delivery of drugs5,6, genes9,10, or even proteins and bioimaging9,10,18,19 in vitro or in vivo is also of keen interest16,17,22-24. In this article, we report core-satellite Ag nanoparticle seed1,6induced synthesis of stable, 5-mercapto-2-nitrobenzoic acid (MNBA)-protected gold nano flowers

(AuNFs) at room temperature in water. The synthesis can be performed in few hours under mild conditions using readily available precursors. Structural characterization and compositional analysis of these nanoflowers were done by high resolution TEM and associated techniques using a FEI, TECNAI G2 F30, S-TWIN electron microscope operating at 300 kV and equipped with an Orius CCD camera (Gatan Inc.), a HAADF Model 3000 detector (Fischione), an EDS detector (EDAX Inc.) and a post-column Imaging Filter (Quantum SE, Model 963, Gatan Inc.)7. There are several microscopy techniques that provide images of materials across different length scales, but the majority of them are used to record two-dimensional (2D) projections of a three-dimensional (3D) object33. Here we have performed 3D imaging through electron tomography to visualize the actual shape of the nanostructures.



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Core-Satellite AgNPs

AuNFs Scheme 1: Plausible synthetic route for formation of AuNFs and schematic diagram of core-satellite assembly of AgNPs seed [S6]. RESULTS & DISCUSSION Fig. 1 (a & b) shows bright field (BF) TEM images of the gold nanostructures. Each nanostructure appears as an arrangement of objects similar to petals in a flower, all of which diverge from a central position where a spike extends perpendicular to the petal plane. These types of nanostructures look like mermaid flower shown at the inset of Fig. 1(a). High Resolution TEM (HRTEM) image taken from a part of a nanoflower (enclosed by red rectangular box) reveals the underlying lattice planes with spacing of 2.33 Å, corresponding to the (111) plane of FCC gold shown in Fig. 1(c). From the HRTEM image characterization it is confirmed that the nanostructures are crystalline in nature. Fig. 1(e) shows the energy dispersive Xray (EDX) profile (using Au-M energy) along a line across an AuNF, whose scanning transmission electron microscopy/high angle annular dark field (STEM-HAADF) image is shown in

Fig. 1(d). This EDX characterization again confirms gold to be the constituent element of those AuNFs. Performing energy filtered TEM (EFTEM) imaging of an AuNF using Au-N energy; we obtained the elemental map of that AuNF which is shown in Fig 1(g). This elemental map again confirms that the nanostructures are made of gold. We have also carried out the thickness measurement of the AuNFs. Fig. 1(h) displays the relative thickness map of an individual AuNF shown in Fig. 1(f). The line profile shown in Fig. 1(i), across the width of the box indicated in Fig. 1(h), confirms that the thickness is not uniform throughout the AuNF, with the central region being thicker than the surrounding petal regions. The AuNFs showed characteristic plasmon absorption peak (SPR) at 525 nm [Fig. S1 (b)]. All the ligands (MNBA) were conjugated to the nanoshells through Au-thiol binding. FT-IR spectroscopy was used to confirm the presence of MNBA, through the absence of S-H stretching modes (between 2550-2620 cm-1) and the appearance of amine [-NH2] and carboxylic acid [-COOH] vibration modes (at 1233 and 1630 cm-1) [Fig. S2].


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(k)

1.0

15 g/mL AuNFs 30 g/mL AuNFs 50 g/mL AuNFs

0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5

Au[111]

Intensity (a.u.)

(j)

mDeg

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


Au[200] Au[220]

Au[311]

-3.0 -3.5 200

250

300

350

400

450

500

550

600

40

Wavelength (nm)

50

60

2Degree

70

80

Figure 1. (a) and (b) BF-TEM images of AuNFs and a single AuNF, (c) HR-TEM image of a part of an AuNF. (d) STEMHAADF image, (e) EDX line profile. (f) Unfiltered image of a single AuNF. (g) EF-TEM image of that AuNF (h) Relative thickness map. (i) Line profile over the red rectangular box indicated in (h). (j) CD-spectra of chiral AuNFs. (k) Wide-angle XRD profile of AuNFs. The reduction of nitro functionality was further confirmed by UV-Vis spectroscopy [Scheme 2/Fig. 2]. Surface adsorbed MNBA reduction was confirmed by reduction of 408 nm absorption peaks and increment of 300 nm peaks. The reaction is pseudo-first order; the slope of a plot of the natural log of the absorbance at 408 nm yields the apparent reaction rate, kapp, 0.036 ± 0.002 s-1 [Table 1/Fig. S15]. The origin of chirality in gold nanoparticles (AuNPs) is still an open question, despite

extensive theoretical and experimental studies in recent years25,26. CD spectroscopy was used to probe the conformation of the chiral property of NPs. In water, AuNFs showed a negative peak at 270 nm, characteristic of MABA and ~520 nm for SPR. With increasing concentration of AuNFs, the ellipticity minimum was increased (π-π* transition) but the shape of the spectrum was unchanged [Fig. 1 (j)]. The metal core is chiral, as in the asymmetric structure found in AuNFs


The Journal of Physical Chemistry clusters. This result indicates that AuNFs have a chiral geometry25,26. The overall symmetry of the discrete nonchiral MABA-protected AuNPs was chiral and that the sulfur atoms bound to the surface Au atoms were chiral centers. The crystal structures of AuNFs were explored by XRD techniques. In the wide-angle XRD pattern [Fig. 1(k)], five Au diff raction peaks are discerned at 2θ = 37.98, 44.31, 64.45, 77.57° and 81.56 which are assigned to (111), (200), (220), (311) and (222) reflections of the face-centered cubic gold lattice, respectively. The intermediates in the synthesis pathway of AuNFs were electrochemically characterized by cyclic voltametry (CV). Fig. 3 shows that the reduction potentials appear at 0.31 V for Ag-MNBA, − 0.02 V for AgNPs and -0.09 V for AuNFs, respectively. Scheme 2: Schematic representation of reduction of nitro functional group of MNBA on AuNFs surface to corresponding amine (2-amino-5-mercaptobenzoic acid).

3D characterization of nanoparticles, as it can provide the accurate 3D morphology by avoiding Bragg diffraction artifacts inherent in 2D images33. 3 2 1 0

I/A

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-1 -2 -3

Ag-MNBA AgNPs AuNFs

-4 -5 -0.6

-0.4

-0.2

0.0

0.2

0.4

E/V vs Ag/AgCl,Cl

-

0.6

0.8

Figure 3: CV-spectra of Ag-MNBA, AgNPs and AuNFs in Millipore water with a scan rate of 50 mV s-1. Electron tomography was performed in three steps acquisition, reconstruction and visualization. In the acquisition step, a number of BF-TEM images of the AuNF structure were acquired by tilting the stage from -60ο to +60ο in steps of 2ο. These tilt images were then aligned following crosscorrelation technique to correct the shifting of images during acquisition. In the reconstruction step, the tilted images were added together following an iterative back-projection method called Simultaneous Iterative Reconstruction Technique (SIRT) with 20 iterations. The reconstructed image was visualized using Amira 5.3.1 software

Figure 2: Time-dependent UV-Vis spectra of MNBA reduction at the time of NaBH4 induced AuNFs formation in water at 25 ºC. 408 nm peak decreases and 272 nm peak increases with progress of time (0-30 s). The optical properties (specially, surface plasmon resonances) of metal nanoparticles strongly depend on their size and shape1-30. Accurate knowledge of the size and shape becomes especially important in order to realize the quantitative correlation of structure and plasmonic properties of nanoparticles. Contrast variation in BF-TEM 2D images can occur due to three factors: variation in thickness, difference in atomic number and Bragg diffraction at some specific angles. Thus BF images can offer confusing information about the correct morphology (i.e. exact size and shape) of NPs. Electron tomography has been recently recognized as an emerging technique for

Figure 4: The central image is the BF-TEM image of an AuNF. The surrounding images are 3D views of the AuNF at different orientations. (Fig. 4). The BF-TEM image (shown in the middle of the Fig. 4) shows a dark central area enveloped in a fuzzy boundary, with no information available in the direction normal to the image. On the other hand, the 3D images (viewed from differ-


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ent directions) obtained through tomography show the detailed morphology of the AuNFs in all three spatial directions (Video S1). NPs of various materials catalyze many important chemical transformations, including oxidation of hydrocarbons, C-C coupling, cyclization, polymerization, hydrogenation, dehydrogenation, sigmatropic rearrangements and redox reactions36-45. One of the most important breakthroughs in organic synthesis during the past decade has been the use of AuNPs as catalyst, although knowledge of the dependence of its efficacy on the size, shape and capping ligand of AuNPs is still lacking38-45. 2+

Scheme 3: Hg /AuNFs assisted direct interconversion of RhB to rhodamine-110 in water in dark at RT.

We have observed that a mixture of rhodamine-B [RhB] (1 equiv.), AuNFs (1 mg/mL) and Hg2+ (1 equiv.) in water, kept at room temperature in the dark for 8-10 days, emits green fluorescence under UV illumination instead of the customary yellow fluorescence expected from RhB (Scheme 3/S6). This indicates that the above combination of reagents chemically transforms RhB into a new fluorophore having different emission characteristics. The transformed fluorophore was found to have absorption and emission maxima at 495 nm and 520 nm, respectively, both blue shifted from those of RhB (540 nm and 570 nm, respectively) [Fig. S5 (a, b)]. Time-resolved fluorescence measurement resulted in a single exponential decay with lifetime of 3.85 ns [Fig. S5 (c)]. ESI mass spectra revealed the mass of the new compound to be 331.00 [M+H +], which makes it identical to that of rhodamine-110 (exact mass 330.31) [S7]. The structure of the new compound was further explored by 1H-NMR & 13C-NMR [S8, S9]. The results confirmed that the transformed moiety was rhodamine-110 and that the methodology can thus be used for direct interconversion of rhodamine-B to rhodamine-110 with ~70% yield in water at room temperature in the dark. We have attempted to do the transformation of RhB using only AuNFs or only Hg2+ but were unsuccessful in both cases. This indicates the catalytic property of AuNFs. To the best of our knowledge this is the first report of the single step catalytic transformation of RhB to rhodamine-110 by Hg2+/AuNF. The purification step is quite easy as AuNFs absorb Hg2+ and unconverted RhB precipitates out while rhodamine-110 separates out into the aqueous medium [Fig. S6(4)]. Rhodamine-110 is obtained as a reddish orange solid after filtration and lyophilization and can be further purified by column chromatography by utilizing its prominently visible green fluorescence.

A proposed ethanolysisolysis or N-deethylation pathway of RhB is described below: (i) The negative surface charge on the AuNFs40,41 attracts positively charged RhB, facilitating the adsorption of RhB on AuNFs surfaces (formation of πcomplex)42-45. This is indicated by the observed rapid quenching of RhB emission and a 45 nm blue shift of its emission maximum on mixing of RhB with AuNFs. (ii) Oxygenactivated Au(111) surfaces42-44 and co-catalyst Hg2+-generated hydroxyl radical29,30,42-44 attack activated SP3 carbon of C-N bonds on RhB, followed by ethanolysis taking place.36,37,40,41 (iii) Several cycles of this process finally produces rhodamine11046-49 [Scheme 4]. Scheme 4: Proposed AuNF catalyzed ethanolysis mechanism of rhodamine-B.

The nitro group reduction of MNBA (capping agents) inspired us to check whether AuNFs can catalyse reduction of pnitrophenol (PNP) to corresponding amino phenol (AP) or not. PNP, as with other nitro phenols and derivatives, is a common byproduct from the production of pesticides, herbicides, and synthetic dyes51-56. Moreover, the NaBH4 reduction does not proceed in absence of any catalyst. To test the catalytic ability of the synthesized gold nanoflower, we have chosen paranitrophenol as the model reactant. The role of the surface of AuNFs in the catalytic reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) was investigated using ~ 60 nm AuNFs monitored by UV-Vis spectroscopy [Fig. 5]. PNP is a strong visible absorber with a maximum absorbance at 400 nm. As shown in Fig. 5, the reduction of PNP to p-aminophenol (PAP) is evidenced by a decrease in absorbance at 400 nm and a new absorbance growing in at 315 nm associated with formation of PAP both in neutral and alkaline pH. Because the reaction is pseudo-first order in the presence of excess NaBH4, the slope of a plot of the natural log of the absorbance at 400 nm yields the apparent reaction rate, kapp=0.091±0.004 s-1



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(10-100 μg/mL) [Fig. S9]. The catalyst exhibits high activity as the conversion just slightly decreased in the same reaction time (60 s) after running for 4 cycles (Fig. S10). ~ 82 wt % of the Au remained after recycling 4 times. AuNFs catalyzed reduction of aromatic amine is further checked for onitrophenol and m-nitroaniline (Table 1/ see supporting information). It is found that AuNFs can facilely catalyze reduction of both types of nitroarenes [Table 1].

Figure 5: Time-dependent UV-Vis spectra showing pnitrophenol reduction in presence of 50 μg/mL AuNF in aqueous phosphate buffer (pH 7.4) at 27 ºC [0-30 s]. The main peak at 400 nm (characteristic of nitrophenolate ions) decreases with reaction time, whereas a second peak at 300 nm simultaneously increases. An isosbestic point appears at 314 nm. in alkaline condition [Fig. S8]. Thus, we show that the reaction rates can be easily determined by UV−Vis spectroscopy. Initially, we have monitored the apparent rate constant (kapp) of the reaction in presence of varying concentration of AuNFs

Figure 6: Time dependent AuNFs assisted stepwise reduction of 2-hydroxy-5-nitrobenzaldehyde [2-HNB] at pH 7.4, 27οC in PBS. 408 nm peak decreases and 303 nm peak increases with progress of time (0-60 s).

Table 1: Summary of reduction parameters of various nitroarenes to corresponding aminoarenes. Nitroarene

1.

Water Solubility

Aminoarene [Mass]

sparingly soluble

Kapp (s-1)

Reaction Temp. (οC)

Reaction Time (min)

400

water/ PBS ( pH=7.4)

0.091 0.003

25

5

~92

420


0.058 0.004

25

5

~92

0.052 0.002

25

5

λmax (nm)

Reaction Medium

Conversion (% )

109.05

2.

sparingly soluble 109.05

3.

355 sparingly soluble


~90

108.07 4.

sparingly Soluble

406


0.043±0.001

25

5

~87

139.06

Successful completion of reduction of the four molecules (1-4) in Table 1 prompted us to demonstrate AuNF facilitated reduction of a relatively complex nitroarene, 2-hydroxy-5nitrobenzaldehyde (2-HNB), into its corresponding amino derivative. In alkaline buffer 2-HNB showed a double humped absorption peak at 356 nm and 386 nm (due to conjugation with –CHO and –NO2, respectively). NaBH4 can efficiently reduce –CHO to –CH2OH, which is indicated by appearence of a new peak at 406 nm with greater absorbance

(1st Red.). But reduction of 5-NO2 group is not possible even in presence of excess NaBH4 as expected because of its potentially stable nature. The reduction was successfully achieved in presence of AuNF, as indicated by decrease of the 406 nm peak with time (0-60 s). Two isosbestic points were found at 281 nm and 231 nm (2nd Red.) [Fig. 6]. The reaction is pseudo-first order with apparent reaction rate kapp= 0.04±0.001 s-1 [Table 1/Fig. S17]. In Fig. S20, we present a volcano plot of various nanoparticle catalysis [AuNFs, AgNPs, AuNPs] plotted with the experimentally measured reaction rates (Kapp) for


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PNP reduction. From the plot it is evident that AuNFs have high reaction rate than citrate capped AuNPs [~25 nm; control] and core-satellite AgNPs seed. Based on the above observations in presence of AuNF, we propose a plausible mechanism of the electron transfer from AuNF crystals and the catalytic reduction of nitroarene in presence of excess sodium borohydride (Table 1). The energy gap between HOMO and LUMO is much higher in case of nitroarene derivatives and hence NaBH4 cannot reduce the substrate because of its inefficient ability of electron donation. The essence of the reduction process lies in the introduction of AuNF, which proceeds to form a п-complex with nitroarene to foreshorten the energy gap between HOMO-LUMO so that NaBH4 can interconvert nitroarene to corresponding aminoarene with ease (Scheme 5)22.

Scheme 5. Plausible mechanistic pathways of reduction of nitroarene with AuNF. In conclusion, we demonstrate a simple and efficient strategy for synthesis of chiral, multipetal gold nanoflowers (AuNFs) with crystalline tips at high yield through the reduction of HAuCl4 in water, in presence of preformed Ag nanoparticle (AgNPs) seeds without using surfactant. We have also demonstrated the formation of core-satellite non-covalently bound Ag nanoparticle assemblies. The 3D structural profile of the AuNFs was obtained by HRTEM and electron tomography. The facile synthetic methodology can be scaled up easily and should be highly useful for routinely producing new metallic nanocages with large surface areas. The ability to induce chiroptical properties within nanosystems without use of biomolecules may have implications for metamaterials, recognition as well as separation of chiral molecules, and design of nanostructure assemblies. To the best of our knowledge, ours is the first demonstration of Hg2+/AuNF-assisted direct transformation of rhodamine–B to rhodamine-110, a dye widely used as an organic building block for different sensors, catalyst screening and as a fluorescent probe for bio-imaging46-50. We have also developed a new class of gold nanoparticle showing high efficiency of catalyzing reduction of different aromatic nitro functional groups. Importantly, we demonstrate a single step, one pot, and eco-friendly reduction of nitroarenes to corresponding aminoarenes by NaBH4 catalyzed by gold nanoflowers, which opens up its potential application in the design of various industrially important catalysts.

Sodium borohydride (NaBH4), 5,5'-dithiobis-(2-nitrobenzoic acid) [Ellman’s reagent/DTNB], Tris(2-carboxyethyl) phosphine hydrochloride [TCEP], 3-Nitroaniline, 2Nitrophenol, 4-Nitrophenol, 2-hydroxy-5-nitrobenzaldehyde, were purchased from Sigma-Aldrich. Disodium hydrogen orthophosphate dehydrate (Na2HPO4) and Potassium dihydrogen orthophosphate (KH2PO4) were purchased from S.D Fine Chem Ltd (Mumbai, India). Sodium Chloride (NaCl) is obtained from Fisher Scientific (Mumbai, India). Potassium Chloride (KCl) and calcium dihydrate (CaCl2.2H2O) were obtained from SRL (Mumbai, India. Magnesium Sulphate was purchased from Anala R, Glaxo laboratories (Mumbai India). pH measurements were made with a Sartorius PB 20 pH meter. Absorption spectra were collected on Jasco V650 UVvis spectrophotometer (Jasco, Japan). Fluorescence spectra were collected on a Spex Fluoromax 3 spectro fluorometer (Horiba Jovin Yvon, USA). CD spectra were recorded using a Jasco J720 spectropolarimeter (Jasco, Japan). Melting points were determined by using a digital auto melting point apparatus (Scientific International, India). Cuvette cleaning: To eliminate any metal or organic material from the cuvettes, these were cleaned as follows: (1) Rinsed with chromic acid followed by HCl: ethanol (1:1) solution. (2) Rinsed with acetone. (2) Rinsed with distilled water several times and dried. SYNTHESIS A. Preparation of Silver Seed [AgNP]. In a 25 mL beaker, 5, 5’-dithiobis-(2-nitrobenzoic acid) [DTNB] (40 mM, excess) and AgNO3 (10 mM) were stirred in Millipore water (10 mL). 50 mM TCEP was added dropwise until the light yellow solution turned to dark yellow, indicating cleavage of the disulfide bond yielding 5-mercapto-2nitrobenzoic acid (MNBA). An aqueous solution of NaBH4 (50 mM) was then added dropwise with constant stirring until the color turned greenish yellow indicating formation of MNBA/MABA-capped Ag nanoassembly (AgNP seed). B. Preparation of Au-Nanoflowers [AuNFs]. 10 mM HAuCl4 solution (1 mL), 20 mM DTNB (1 mL) and 20 mM TCEP (1 mL) were taken in a 50 mL beaker containing 12 mL water [final volume = 15 mL]. Previously synthesized AgNP seed solution was then added to it until the color turned colorless indicating the partial reduction of HAuCl4 and oxidation of AgNP seed. An ice cold freshly prepared NaBH4 aqueous solution (1 mg, 2 mL, 1.3M) was then added dropwise very carefully and formation of NPs was monitored by UV-Vis spectroscopy. This step is extremely crucial as excess NaBH4 may reduce Ag (I). The color of the solution changed from colorless to light pink and finally to turbid pink at room temperature within 60 min, indicating formation of AuNFs. The solution was kept under stirring condition for 4h. Synthesized Au nanoflowers (~60 nm) were recovered from a 10 mL solution by centrifugation (3000 rpm for 10 min), rinsed three times with water and redispersed in Millipore water.

EXPERIMENTAL SECTION Materials and Methods: All solvents were of analytical grade and used without further purification. Silver nitrate (AgNO3), Auric chloride (HAuCl4),

C. Synthesis of Rhodamine-110. A mixture of rhodamine-B [RhB] (100 μM), AuNFs (1 mg/mL) and Hg2+ (100 μM) in 2 mL water, kept at room



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temperature in the dark for 8-10 days, emits green fluorescence under UV illumination instead of the customary yellow fluorescence expected from RhB. AuNFs absorb Hg2+ and unconverted RhB precipitates out while rhodamine-110 separates out into the aqueous medium. Rhodamine-110 is obtained as a reddish orange solid after filtration and lyophilization and can be further purified by silica gel column chromatography by utilizing its prominently visible green fluorescence. The methodology can be used for direct interconversion of rhodamine-B to rhodamine-110 with 65-70 % yield in water at room temperature in the dark. D. Reduction of nitroarenes. 2 µL of p-nitrophenol/ o-nitrophenol/ m-nitroaniline/ 2hydroxy-5-nitrobenzaldehyde were taken into 2.0 mL aqueous buffer of AuNFs, so that final concentration of the reactant was 100 µM. To the mixed solution of catalyst and reactant, 0.1 mL of NaBH4 was added with stirring to start the reaction (final concentration of NaBH4 = 0.1 M). The progress of the reaction was carried out by UV-Vis spectroscopy at ambient temperature (27 ºC). After the completion of the reaction, the total solution was centrifuged for 60 second to separate the catalyst. It was washed with MilliQ water and then, air dried for further using as catalyst. Calculation of the rate constant. The pseudo-first-order kinetics with respect to the reactants could be applied to our experimental system. The approximately linear shape of the plot of ln (At/A0) [A0 represents the initial and At the absorbance after time t at corresponding wavelength] versus time. As absorbance is proportional to concentration it corresponds to the concentration of the reactants. From, the slop of the linear equation the apparent rate constant (kapp) is calculated.

ASSOCIATED CONTENT Supporting Information. Additional spectroscopic data, TEM images, tomography, NMR. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT KB acknowledges financial support from CSIR, Govt. of India and the BARD project at Saha Institute. We sincerely thank Dr. Biswarup Satpati (SINP), Dr. Amitava De (SINP), Anish KarMahapatra (SINP), Bappaditya Chandra, Manjur Oyasim Akram, Ananya Rakshit, Subha Bakthavatsalam, Ankan Dutta Chowdhury, Pritiranjan Mondal, Debranjan Mondal and Debjit Bhar for their kind help and support.

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SYNOPSIS TOC.

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10 ACS Paragon Plus Environment

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Synthesis of Chiral, Crystalline Au-Nanoflower Catalyst Assisting

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