Morphological Transition during Reversible Aqueous and Organic

ARTICLE pubs.acs.org/JPCC

Morphological Transition during Reversible Aqueous and Organic Phase Transfer of Gold Nanostructures Synthesized by Tyrosine-Based Amphiphiles Enakshi Dinda, Mrinmoy Biswas, and Tarun K. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

bS Supporting Information ABSTRACT: We describe the successful synthesis of multipod-shaped gold (Au) nanostructures by an in situ reduction approach using a series of tyrosine-based redox active amphiphiles (conjugates of stearic/palmitic/myristic/lauric/capric acid with tyrosine) as reducing-cum-structure directing agents under alkaline condition. High-resolution transmission electron microscopy (HRTEM) confirms that the oriented attachment of the initially formed spherical Au nanoparticles (NPs) through {111} planes give rise to the formation of multi-pod-shaped morphology. We also demonstrate that the successful phase transfer of amphiphile-capped Au nanomultipods from water to nonpolar organics can be achieved by simple HCl treatment. We discover that there is a complete morphological transition in the Au nanostructures from multipod to spherical upon transfer to nonpolar organics. The water-to-organic phase transfer efficiency decreases with a decrease in the length of alkyl chain of the amphiphiles. The reversibility of the phase transfer is probed by the addition of aqueous NaOH solution to organic phase containing spherical Au NPs. This toluene-to-water transfer is also associated with a reversible shape change from spherical to multi-pod-shaped Au nanostructures. The reversible phase transfer and the associated shape transitions are monitored visually as well as via UV vis spectroscopy and TEM analysis. HRTEM results show that the Au nanomultipods and the spherical Au NPs obtained after phase transfer are bounded by {111} planes. The mechanism of formation of Au nanostructures and their reversible phase transfer with associated shape transition is proposed and discussed.

’ INTRODUCTION Gold (Au) nanoparticles (NPs) are fascinating owing to their interesting size- and shape-dependent physicochemical and optoelectronic properties.1 Owing such properties, Au NPs have been widely exploited for use in catalysis,2,3 surface-enhanced Raman scattering (SERS),4 photonics,5 biolabeling,6 bioimaging,7 sensing,8,9 and so on. Therefore, an ultimate goal is to develop the synthetic skill to manipulate the size and morphology of Au nanostructures in such a fashion that their optical and electronic properties can be tuned for further applications in nanotechnology.1 To date, there are several synthetic strategies, such as templatebased,10 12 micelle-based,13,14 and seed-mediated growth,15,16 that have been developed to control the shape as well as size of the Au nanostructures. Besides these techniques, the in situ reduction method proved itself to be an interesting approach for preparing Au nanostructures of different shapes using various reducing-cum-capping agents in aqueous medium.3,8,17 23 Furthermore, some groups including ours have also synthesized differentshaped Au nanostructures via similar in situ reduction approach using green reducing-cum-capping agents such as plant extract,22 24 bovine serum albumine (BSA),17,25 ionic liquids,26,27 peptides,28 30 and amino acid-based amphiphiles.20,31 Note that Au and Ag nanostructures of various morphologies such as r 2011 American Chemical Society

sphere,29 triangle,23,32 polygons,3,32 and so on were produced in this in situ approach. The branched Au nanostructures have also been prepared by this approach2,21,31,33 because they are especially important because of striking supramolecular structure, high surface area, and admirable connectivity between the different lobes of a single particle. However, our group has mainly focused on the synthesis of the branched Au nanostructures via in situ reduction approach using ecofriendly molecules such as small peptides/tryptophan-based amphiphiles.21,31,33 These molecules are biocompatible. So, maybe one could use the formed amphiphile Au nanostructure conjugates in bioanalytical applications. All of the above-mentioned approaches are dealing with the synthesis of Au nanostructures in aqueous medium because many applications require the water-dispersible NPs.34 However, the use of organic medium to synthesis of Au NPs is advantageous because the monodispersity of the obtained NPs is usually higher compared with those prepared in aqueous medium.35 Furthermore, the organic Au NPs have versatility in terms of their applications such as catalysis of organic reaction in nonpolar Received: July 2, 2011 Revised: August 18, 2011 Published: August 22, 2011 18518

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The Journal of Physical Chemistry C solvents,36 solvent-dependent optical properties37 and are facile to further surface modification with organically soluble functional groups to fine-tune their properties.38,39 Also, organic Au NPs usually produced new structural materials such as hexagonal-closed-packed array of particles upon solvent evaporation.40 To fabricate the organic optoelectronic devices, we need to disperse Au NPs in organic solvents to facilitate the processing. Brust was the first to produce organic Au NPs by a two-step protocol in organic media using a phase transfer catalyst and an alkanethiol.41 After this invention, a range of synthetic approaches have been developed to prepare Au NPs in the organic medium using alkanethiol,42,43 aromatic thiol,44 dialkyl disulfides,45 thiolated cyclodextrine,46 and alkyl-amine47 51 as capping agents. Surface modification of preformed Au NPs by suitable hydrophobic ligand for phase transfer from aqueous to organic phase is also considered as an alternative method for producing organic Au NPs.35,52 A similar technique has also been used to transfer metal NPs from aqueous to organic phase and vice versa using various ligands and surfactants.35,53 56 We have recently reported the preparation Au NPs by the in situ reduction approach using tryptophan-based amphiphiles and their efficient transfer from aqueous to organic medium by simple acid treatment,20 but the example of reversible phase transfer of Au nanostructure is still very rare in the literature. Besides these, the formation of monodisperse spherical Au NPs in toluene medium from polydisperse particles via digestive ripening process has also been reported.57,58 However, to date, it is hard to find any report that deals with the change of morphology of preformed Au nanostructures during their phase transfer from aqueous to organic or vice versa. In the present work, we report the generation of multi-podshaped Au nanostructures using a series of newly designed tyrosine-based redox active amphiphiles (conjugates of stearic/ palmitic/myristic/lauric/capric acid with tyrosine) as reducingcum-structure directing agents. The amphiphile-capped Au nanostructures are efficiently transferred from aqueous to organic phase by simple HCl treatment. It is also shown that the chain length associated with the amphiphiles has a prominent effect on the shape of the obtained Au nanostructures as well as their efficiency of transfer from aqueous to organic phase. It is noticed that the multipod geometry of Au nanostructures is converted to a spherical one during the aqueous-to-organic phase transfer process. This shape conversion is reversible; that is, the spherical NPs are converted back to multipod geometry during their transfer from organic to aqueous phase by treating with NaOH solution. The details of the formation and shape transition mechanism of Au nanostructures are described based on UV vis spectroscopic and HRTEM image analysis.

’ EXPERIMENTAL SECTION Materials. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O) was purchased from Aldrich. Stearic acid [CH3-(CH2)16COOH], palmitic acid [CH3-(CH2)14-COOH], myristic acid [CH3-(CH2)12-COOH], lauric acid [CH3-(CH2)10-COOH], capric acid [CH3-(CH2)8-COOH], and sodium hydroxide (NaOH) were purchased from E. Merck. L-Tyrosine (Tyr), dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) and hydrochloric acid (HCl) were purchased from SRL. All of these chemicals were used as received. All aqueous solutions were prepared with triple-distilled water. Freshly distilled reagent

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Scheme 1

grade organic solvents were used for all synthetic process as well as in phase-transfer process. Synthesis of Tyrosine-Based Redox Active Amphiphiles. A series of five different tyrosine-based redox active amphiphiles (conjugates of tyrosine and long chain fatty acid, see Scheme 1 for chemical structures) were synthesized by following conventional solution-phase racemization-free fragmentation/condensation strategy, as mentioned in our previous reports.20,31,59 These amphiphiles are abbreviated as C17H35-Tyr, C15H31-Tyr, C13H27-Tyr, C11H23-Tyr, and C9H19-Tyr (Scheme 1). In brief, first, the methyl ester of tyrosine was coupled to a long chain fatty acid (stearic/palmitic/myristic/lauric/capric acid) to produce methyl ester of amphiphiles. The obtained methyl esters of amphiphiles were then purified by column chromatography and finally saponified to generate the actual amphiphiles. The details of the reaction recipe and characterization of the synthesized amphiphiles are provided in the Supporting Information (SI). Synthesis of Au Nanostructures Using tyrosine amphiphiles. In a typical reaction, the methanolic solution of an amphiphile, C17H35-Tyr (40 mM, 1 mL), was added to 8.0 mL of triple-distilled water, and the mixture was made alkaline (pH ∼11) by adding NaOH solution so that the amphiphile dissolved 18519

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The Journal of Physical Chemistry C

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Table 1. Reaction Recipe for the Preparation of Au Nanostructures Using Different Amphiphilesa amphiphiles C17H35-Tyr

sample IDb

[amphiphiles]/[HAuCl4]

λmax,t (nm)c

λmax,t (nm)d 534

C17H35-Tyr-Au-4

4

574

C17H35-Tyr-Au-8

8

582

colore

colorf

bluish black

red

bluish black

C17H35-Tyr-Au-1.3

1.3

530

C15H31-Tyr

C15H31-Tyr-Au-4

4

538

541

red violet

red

C13H27-Tyr

C13H27-Tyr-Au-4

4

562

531

blue

red

C11H23-Tyr

C11H23-Tyr-Au-4

4

585

532

blue

red

C9H19-Tyr

C9H19-Tyr-Au-4

4

600

blue

a [Amphiphile] = 4.0 10 3 M. b C17H35-Tyr-Au represents the amphiphile C17H35-Tyr and Au system. The numerical value represents the molar ratio (R) = [amphiphile]/[HAuCl4]. c λmax = Maximum wavelength of the SPR band for the Au nanostructures in water. d λmax = Maximum wavelength of the SPR band for the Au NPs in toluene. e Color of the aqueous colloidal suspension of Au nanostructures. f Color of the colloidal suspension of Au NPs in toluene after phase transfer.

completely. An aqueous solution of HAuCl4 (10 mM, 1 mL) was then added dropwise to the above mixture with constant magnetic stirring. In this case, the molar ratio (R) of amphiphile (C17H35-Tyr) to HAuCl4 is 4 (Table 1). The resultant solution was further stirred for another three days at ambient temperature. The color of the colloid started to change from pale-yellow to bluish black after 1 min of the final addition of the aqueous HAuCl4, indicating the formation of Au nanostructures. Two similar reaction sets were also carried out using different molar ratios of amphiphile (C17H35-Tyr) to HAuCl4, that is, R = 8 and 1.3. (For a detailed recipe, see Table 1.) Similarly, we also used other amphiphiles (C15H31-Tyr, C13H27-Tyr, C11H23-Tyr, and C9H19-Tyr) to generate Au nanostructures at R = 4 to examine whether there is any effect of length of alkyl chain associated with the amphiphiles on the morphology (Table 1) of formed Au nanostructures. In all of these cases, the color of the obtained colloids is dark blue, violet, or bluish black, which is dependent on the nature of amphiphiles used in the reaction (Table 1). These as-prepared suspensions of amphiphile-Au nanoconjugates were used in the subsequent transfer process from aqueous to organic phase. Phase Transfer of tyrosine amphiphile-Capped Au Nanostructures from Aqueous to Nonpolar Organics. Typically, 1.5 mL of an as-prepared aqueous colloidal suspension of amphiphile-capped Au nanostructures (C17H35-Tyr-Au-4, see Table 1) was mixed with 2 mL of toluene in a clean glass vial. This biphasic mixture containing a transparent toluene on the top and blue colored hydrosol at the bottom was vortexed for 2 min, which results in the formation of a blackish-white-colored emulsion (Figure 1). An aqueous HCl solution (500 μL, 2 N) was then added to bring the pH of the emulsion to ∼2, and the resultant emulsion was vortexed for a further 2 min. During vortexing, we found that the color of the emulsion changed from blackish white to faint red. The mixture was kept undisturbed for another 2 h. The transfer of C17H35-Tyr-Au-4 nanoconjugate from water to toluene can be visualized by the change of color of both the aqueous as well as toluene phase after separation of two layers (Figure 1). Furthermore, we also used other organic solvents such as xylene, benzene, hexane, ethylbenzene, and cyclohexane to transfer the sample C17H35-Tyr-Au-4 from water to these solvents by similar procedure. Amphiphile-Au nanostructures samples (C15H31-Tyr-Au-4, C13H27-Tyr-Au-4, C11H23-Tyr-Au4, and C9H19-Tyr-Au-4; see Table 1) prepared with respective amphiphiles were also checked for phase transfer from aqueous to toluene phase by similar procedure (Figure 1). All amphiphile-Au

Figure 1. Photograph showing different stages during phase transfer of Au nanomultipods from water to toluene for different samples (A) C17H35-Tyr-Au-4, (B) C15H31-Tyr-Au-4, (C) C13H27-Tyr-Au-4, (D) C11H23-Tyr-Au-4, and (E) C9H19-Tyr-Au-4.

nanostructures samples’ suspensions in toluene phases were collected for further analysis. Reversible Phase Transfer of Amphiphile-Capped Au Nanostructures. For reversible phase transfer, the first 2 mL of the obtained red-colored colloidal suspension of a representative sample C17H35-Tyr-Au-4 in toluene was mixed with 2 mL of water and made the solution alkaline (pH ∼12) by the addition of 2 N NaOH solution. The mixture was intensively shaken for 2 min by a mechanical shaker and was allowed to stand for 2 h, until the two layers were separated. Again, after this step, the obtained blue-colored aqueous colloidal suspension of sample C17H35Tyr-Au-4 (1.5 mL) was mixed with toluene (2 mL) with a mechanical shaker, its pH was decreased to ∼1.5 by the addition of a requisite amount of 2 N HCl, and it was kept undisturbed for 2 h until the two layers were separated.

’ CHARACTERIZATION NMR Study. 1H NMR spectra of all amphiphiles and their

methyl esters were acquired in CDCl3 (1 10 mmol) and mixture of CDCl3 and DMSO-d6 on a Bruker DPX 300 MHz spectrometer. 18520


The Journal of Physical Chemistry C ESI Mass Study. The ESI mass spectra of the purified amphiphiles and their methyl esters were recorded from a methanol solution on a quadrupole time-of-flight (Q-Tof) Micro YA263 mass spectrometer. C H N Analysis. The elemental analyses of the purified amphiphiles were carried out using a Perkin-Elmer 2400 series II CHN analyzer. UV vis Absorption Spectroscopic Study. UV vis absorption spectra of all amphiphile-Au nanostructures’ suspension in both aqueous and nonaqueous media were acquired using a Hewlett-Packard 8453 UV vis spectrophotometer. Transmission Electron Microscopic Study. For transmission electron microscopic (TEM) analysis, one drop of the asprepared suspension of the aqueous or nonaqueous suspension of amphiphile-Au nanostructures was placed on a carbon-coated copper grid and was allowed to air-dry. The grid was then observed under a JEOL JEM2010 high-resolution transmission electron microscope (HRTEM) and imaged at an accelerating voltage of 200 kV. FTIR Spectroscopic Study. FTIR spectra of centrifuged/ washed/dried sample C17H35-Trp-Au-4 from both the aqueous and toluene phase were recorded in a KBr pellet using a Parkin Elmer FTIR Spectrum-400 spectrometer. Pellets were prepared by mixing the corresponding dried sample with KBr in a 1:100 (w/w) ratio. X-ray Diffraction Study. For X-ray diffraction (XRD) analysis, the aqueous amphiphiles Au nanoconjugate suspension was first concentrated by centrifugation and was deposited on a microscopic glass slide as thin-film, followed by drying in air. The diffractogram of the nanoconjugate film was then recorded by using a Bruker AXS D8 Advance diffractometer at an accelerating voltage of 40 kV using Cu Kα (λ = 1.5405 Å) as the X-ray radiation source.

’ RESULTS AND DISCUSSION Synthesis of Au Nanostructures Using Tyrosine Amphiphiles in Water. A series of five different tyrosine amphiphiles of

varying alkyl chain length were employed in this study. These amphiphiles are conjugates of fatty acids (such as stearic, palmitic, myristic, lauric, capric acid) and an amino acid, tyrosine. It has been reported by us and others that the tyrosine moiety can reduce Au(III) ion to metallic Au atoms.29,60 Therefore, it is expected that these amphiphiles could be used to generate Au NPs from gold precursor because they contain a tyrosine moiety. This is indeed the case when the aqueous solution of HAuCl4 was added dropwise to the alkaline solution of amphiphile (C17H35Tyr); the yellow color of the solution turns to bluish black, indicating the formation of Au nanostructures (Table 1). This sample is prepared at a molar ratio of amphiphile to HAuCl4 (R) = 4. Similarly, the other four amphiphiles (C15H31-Tyr, C13H27Tyr, C11H23-Tyr, and C9H19-Tyr) were also used to generate Au nanostructures at R = 4, maintaining the similar reaction condition (Table 1). The aim behind these experiments was to understand the effect of chain length associated with the amphiphiles on the morphology or the size of the formed Au nanostructures. The color of the colloidal suspension, which is appearing because of the formation of nanostructured Au, depends on the nature of the amphiphiles. For example, the colors of the colloidal suspension of the sample C17H35-TyrAu-4 and C15H31-Tyr-Au-4 (Table 1) were bluish black and violet, respectively, whereas, the suspensions of rest of the three

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Figure 2. UV vis absorption spectra of the as-prepared aqueous suspension of different Au nanostructure samples: (a) C17H35-TyrAu-4, (b) C15H31-Tyr-Au-4, (c) C13H27-Tyr-Au-4, (d) C11H23-Tyr-Au4, and (e) C9H19-Tyr-Au-4. The spectra were recorded after 3 days of reaction.

samples, C13H27-Tyr-Au-4, C11H23-Tyr-Au-4, and C9H19-TyrAu-4, are blue in color (Table 1). The variation in color may be due to the formation of different shaped Au nanostructures when prepared with different amphiphiles. The UV vis absorption spectra of samples C17H35-Tyr-Au-4, C15H31-Tyr-Au-4, C13H27-Tyr-Au-4, C11H23-Tyr-Au-4, and C9H19-Tyr-Au-4 exhibit only one broad surface plasmon resonance (SPR) band centered at 574, 538, 562, 585, and 600 nm, respectively (Figure 2). The exhibition of such broad SPR band may be attributed to the formation of either anisotropic Au nanostructures or spherical particles with high polydispersity or the aggregated spherical NPs.61,62 The TEM images (Figure 3) of the samples clearly reveal the formation of nonspherical Au nanostructures as expected from their UV vis spectra (Figure 2a e). It is also clear that the morphology of the obtained Au nanostructures is somewhat dependent on the nature of the amphiphiles used for their preparation. The TEM image of the as-prepared sample C17H35Tyr-Au-4 shows mostly multipod-like nanostructures (Figure 3A). One could say that these multipods look like nanodendrites. The diameter of the multipods (the maximum possible distance between the two-extreme pods of single multi-pod-shaped particles) is in the range of 35 to 50 nm, and the particles are quite polydisperse in nature. From the enlarged view of a single Au nanomultipod, it seems that its branches are formed by the attachment of initially formed smaller spherical seed particles (inset of Figure 3A). Some of such spherical seed particles (marked by the white solid circles) can be seen in the same image (Figure 3A). Previously, we have reported such multi-podshaped Au nanostructures prepared using tryptophan-based amphiphiles.31 The formation mechanism of such multi-podshaped Au nanostructures will be discussed later based on our previously reported mechanism.31 Figure 3B E shows the TEM images of the samples C15H31-Tyr-Au-4, C13H27-Tyr-Au-4, C11H23-Tyr-Au-4, and C9H19-Tyr-Au-4, respectively. It should be noted that as the alkyl chain length of the amphiphiles 18521



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Figure 3. TEM images of the different as-prepared Au nanostructures’ samples prepared using different amphiphiles: (A) C17H35-Tyr-Au-4, (B) C15H31-Tyr-Au-4, (C) C13H27-Tyr-Au-4, (D) C11H23-Tyr-Au-4, and (E) C9H19-Tyr-Au-4. The images were taken after 3 days of reaction. The insets show the enlarged view of a portion of the corresponding images.

increases, the length of obtained pods in a multi-pod-shaped Au nanostructures also increases. The TEM images of C15H31-TyrAu-4 and C13H27-Tyr-Au-4 (Figure 3B,C, respectively) also reveal multi-pod-shaped Au nanostructures along with some spherical Au NPs, as marked by the white solid circles. It seems that the size of the multipod for the sample C15H31-Tyr-Au-4 is larger than the multipods of C13H27-Tyr-Au-4. The maximum possible distance between the two-extreme ends of a single multipod is in the range of 20 to 35 nm for sample C15H31Tyr-Au-4, whereas the value is 10 to 20 nm for the sample C13H27-Tyr-Au-4. Note that for C13H27-Tyr-Au-4 the population of multipod is relatively low (Figure 3C) compared with particles with spherical, bipod, and tripod geometry. It should

also be noted that nanomultipods of sample C13H27-Tyr-Au-4 are not as prominent, as observed for the above two samples (compare Figure 3A,B with Figure 3C). Again, in sample C13H27-Tyr-Au-4, the population of spherical NPs is much higher than that of the sample C15H31-Tyr-Au-4 (compare Figure 3B,C). In addition, the population of spherical NPs in the sample C15H31-Tyr-Au-4 is much higher than that in the sample C17H35-Tyr-Au-4 (compare Figure 3A,B). The nanomultipods of C15H31-Tyr-Au-4 and C13H27-Tyr-Au-4 may also form from the initially formed seed particles, as can be understood from the enlarged view of one of such (inset of the Figure 3B,C). The samples C11H23-Tyr-Au-4 and C9H19-TyrAu-4 also contain multi-pod-shaped particles, but the pods are 18522



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Figure 5. TEM image of the sample C17H35-Tyr-Au-4 taken out after 1 h of reaction.

Figure 4. HRTEM images of one portion of a single Au nanomultipod of different samples from aqueous medium: (A) C17H35-Tyr-Au-4, (B) C15H31-Tyr-Au-4, (C) C13H27-Tyr-Au-4, and (D) C11H23-Tyr-Au-4. The images were recorded after 3 days of reaction. (E H) Magnified HRTEM images taken from a specified portion of the multipod in panels A D, respectively, showing the formation of twin structure of the respective sample.

not as prominent as observed for the other sample (compare Figure 3A with Figure 3D,E). However, the enlarged view of a single multipod of the samples C11H23-Tyr-Au-4 and C9H19Tyr-Au-4 indicate that they are also formed by the combination of initially formed spherical seed NPs (insets of Figure 3D,E, respectively). The HRTEM analysis was performed on the Au nanomultipod to check further their crystalline nature. The HRTEM image of the sample C17H35-Tyr-Au-4 reveals that the multipods are single-crystalline in nature and are formed through the oriented

attachment of the initially formed small seed particles in the Æ111æ direction as the interplanar spacing is 0.23 nm (Figure 4A). Furthermore, it is clear that the lattice fringes are perfectly aligned. The HRTEM images of other three samples C15H31Tyr-Au-4, C13H27-Tyr-Au-4, and C11H23-Tyr-Au-4 also reveal that these multipods contain aligned lattice fringes with an interplanar spacing of 0.23 nm corresponding to the {111} planes of metallic gold, as shown in Figure 4B D, respectively. Therefore, it can be concluded that the Au multipods prepared with different amphiphiles are grown along the Æ111æ direction from the initially formed seed particles. To confirm the issue that the multipods are grown from the initially formed spherical seed particles, we performed the following experiments. A small portion of suspension of a representative sample, C17H35-Tyr-Au-4, was withdrawn after 1 h of reaction and was examined via TEM. TEM image shows the formation of mostly spherical NPs along with few multipods, as indicated by the white circles (Figure 5). This clearly indicates that in the early stage of reaction, mostly spherical NPs are formed, which eventually combined through oriented attachment mechanism to form multi-pod-shaped particles during aging of the reaction mixture, as previously mentioned here as well as in our previous report.31 The growth will be discussed later in the mechanism section. Furthermore, we also prepared two samples C17H35-Tyr-Au-8 and C17H35-Tyr-Au-1.3 (Table 1) by varying the molar ratio (R) of amphiphile (C17H35-Tyr) to HAuCl4 to check whether there is any effect of R on the morphology of the obtained Au nanostructures. The colloidal suspensions of these two samples show blackish blue and red color, respectively (Table 1). The UV vis spectra of the sample C17H35-Tyr-Au-8 exhibit one broad SPR band centered at 582 nm (Figure S1a of the SI), indicating the formation of Au NPs of nonspherical geometry.62 However, a single sharp SPR band centered at 530 nm was observed for C17H35-Tyr-Au-1.3 (Figure S1b of the SI), which may indicates the formation of spherical Au NPs.62 Previously, we have also observed the formation of spherical Au NPs when prepared with tyrosine amphiphile at lower R value (= 1.3).31 The TEM image of the sample C17H35-Tyr-Au-8 reveals similar nanomultipods (Figure S2A in the SI), as expected from their UV vis spectrum (Figure S1a of the SI). A few spherical NPs are also present (indicated by white solid circle in Figure S2A of the SI) along with nanomultipods. The enlarged view (inset of 18523



Figure 6. XRD patterns of the different Au nanostructures in aqueous medium prepared by using different amphiphiles: (a) C17H35-Tyr-Au-4, (b) C15H31-Tyr-Au-4, (c) C13H27-Tyr-Au-4, (d) C11H23-Tyr-Au-4, and (e) C9H19-Tyr-Au-4. The XRD were recorded after 3 days of reaction.

Figure S2A in the SI) of a multipod also indicates that they are grown by the attachment of initially formed spherical seed NPs. Again, the sample C17H35-Tyr-Au-1.3 shows only spherical Au NPs with average size 3.5 ( 1.4 nm but no multi-pod-shaped particles. In our previous report, we explained the detailed growth mechanism of formation of such multipod and spherical Au nanostructures when tyrosine amphiphiles were used at varying R values.31 We believe a similar mechanism is operating in this case. The XRD patterns (Figure 6) of all five samples (C17H35-TyrAu-4, C15H31-Tyr-Au-4, C13H27-Tyr-Au-4, C11H23-Tyr-Au-4, and C9H19-Tyr-Au-4) exhibit the most intense peak at 2θ = 38.2, which is corresponding to the (111) lattice plane of the face-centered cubic (fcc) metallic Au (JCPDS card no. 04-0784). The other three peaks for all samples appearing at 2θ = 44.4, 64.2, and 77.1 are corresponding to (200), (220), and (311) lattice planes, respectively. The entire peak belongs to (200), (220), and (311) and appeared to be weaker than the peak obtained corresponding to {111} plane, indicating that the growth of Au multipod occurs along the Æ111æ direction. The value (Table S1 in the SI) of the intensity ratio of the peaks corresponding to {111} and {200} planes are very high compared with the standard value (1.93) (JCPDS card no. 04-0784). These results also indicate that the Au nanomultipods from all samples are bounded by {111} facets. Phase Transfer of tyrosine amphiphile-Coated Au Nanostructures. We were able to phase transfer as-prepared Au nanomultipods of different samples, C17H35-Tyr-Au-4, C15H31Tyr-Au-4, C13H27-Tyr-Au-4, and C11H23-Tyr-Au-4, from water to nonpolar organics such as toluene simply by treating their aqueous suspension with HCl. Figure 1 shows photographs of the different stages during transfer of Au nanomultipods of different samples from water to toluene. First, the addition of toluene to the blue-colored aqueous colloidal Au suspension results in two liquid bilayers with the clear colorless toluene on top. Because of prolonged vortexing, the liquid bilayers turn into a grayish emulsion. The emulsion was then acidified by the addition of HCl solution. Finally, upon standing, the emulsion breaks into two layers again in which the red color upper toluene

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Figure 7. UV vis absorption spectra of the colloidal suspension of Au NPs in toluene after phase transfer for different samples: (a) C17H35Tyr-Au-4, (b) C15H31-Tyr-Au-4, (c) C13H27-Tyr-Au-4, (d) C11H23Tyr-Au-4.

layer contains Au nanostructures, leaving a colorless aqueous layer at the bottom of the vial. Note that we were unable to transfer the sample C9H19-Tyr-Au-4 from water to toluene probably due to shorter alkyl chain associated with the amphiphile (C9H19-Tyr). The UV vis spectrum of the sample C17H35-Tyr-Au-4 in toluene exhibits a single sharp SPR absorption band at 534 nm, indicating the presence of only spherical Au NPs (Figure 7a).20,61,62 We also noticed that the position of this band in toluene was blue-shifted from 574 nm corresponding to the Au nano-multipods in aqueous medium, as previously mentioned above (compare Figure 2a with Figure 7a). This shift in SPR band is probably due to the change of shape of Au nanostructures from multipod to spherical geometry when they are transferred from water to toluene. The TEM analysis indeed shows such a change in morphology during phase transfer, the details of which will be discussed later in this section. Similarly, the UV vis spectra of the samples C15H31-Tyr-Au-4 and C13H27-Tyr-Au-4 in the toluene also exhibit a single sharp SPR band centered at 541 and 531 nm, respectively, after transfer from water (Figure 7b,c). Note that for the sample C15H31-Tyr-Au-4, we did not observe any noticeable shift in the λmax of SPR band of Au nanostructures due to this transfer (compare Figure 2b with Figure 7b). We do not know the exact reason for this anomaly; however, for the sample C13H27-Tyr-Au-4, a shift of λmax from 562 to 531 nm was observed because of this transfer (compare Figure 2c with Figure 7c). The UV visible spectra of the remaining aqueous phase after transfer of each sample show no trace of any SPR signal of GNPs (Figure S3 of the SI). It can be concluded that the Au nanostructures prepared with amphiphiles of long alkyl chain (samples C17H35-Tyr-Au-4, C15H31-Tyr-Au-4, and C13H27-TyrAu-4) can be completely transferred from water to toluene phase by HCl treatment. However, in the case of transfer of the sample C11H23-Tyr-Au-4 from water to toluene, it is noticed that some of the Au NPs were stuck on the inner wall of the glass vials (Figure 1D), some lie at the interphase of the two layers, and the rest are well-dispersed, making the ruby red coloration of toluene layer, but the aqueous layer after phase transfer of Au nanostructures 18524



Figure 8. TEM images of different samples from toluene media after phase transfer: (A) C17H35-Tyr-Au-4, (B) C15H31-Tyr-Au-4, (C) C13H27-Tyr-Au-4, and (D) C11H23-Tyr-Au-4. The insets show the enlarged view of Au NPs of the corresponding images.

to toluene phase becomes colorless with no trace of Au NPs as verified from UV vis spectrum. (See Figure S3d of the SI.) A blue shift in the λmax of SPR of Au nanostructures from 585 to 532 nm was also noticed because of this transfer (compare Figure 2d with Figure 7d). However, as mentioned above, we were unable to transfer the sample C9H19-Tyr-Au-4 from aqueous to toluene phase (Figure 1E). It is worth mentioning that after HCl treatment, all Au NPs that stayed at the interphase of the two layers left both the aqueous and toluene phases colorless (Figure 1E). Therefore, it can be summarized that the length of the alkyl chain of the amphiphiles has a prominent effect on this transfer process. As the chain length decreases, the efficiency of transfer of the Au nanostructures from the aqueous to the toluene phase decreases. Zhu et al. have reported that the phase transfer of surfactantcapped Au NPs across the water toluene interphase were dependent on the alkyl chain length associated with the surfactant.52 They have also reported that the Au NPs capped with surfactant with decyl or even lower chain length than decyl cannot be transferred from aqueous to toluene phase at all.52 Both the spectral as well as the visual results suggested that the shape of the multipod-shaped Au nanostructures might be changed when they were transferred from the aqueous to toluene phase. To check the morphology of Au nanostructures after transfer to toluene, we examined the transferred samples via TEM. The TEM images of the samples C17H35-Tyr-Au-4, C15H31-Tyr-Au4, C13H27-Tyr-Au-4, and C11H23-Tyr-Au-4 from toluene showed that the morphologies were totally changed from multipod to spherical geometry because of this transfer (Figure 8). No podshaped NPs were observed in these images (Figure 8). The inset of Figure 8 shows the enlarged view of the spherical Au NPs of all samples. The average diameters of the spherical NPs of the samples C17H35-Tyr-Au-4, C15H31-Tyr-Au-4, C13H27-Tyr-Au-4, and C11H23-Tyr-Au-4 in toluene were measured to be 10.5 ( 2.8,

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Figure 9. HRTEM images of a single spherical Au NP of different samples from toluene after phase transfer: (A) C17H35-Tyr-Au-4, (B) C15H31-Tyr-Au-4, (C) C13H27-Tyr-Au-4, and (D) C11H23-Tyr-Au-4. The images show the interplanar spacing of spherical nanoparticles is 0.23 nm for all cases.

4.7 ( 0.9, 4.4 ( 1, and 6.5 ( 1.1 nm, as shown in Figure 8A D, respectively. It is worth mentioning that as the length of the alkyl chain of the amphiphiles decreases the size of the obtained spherical Au NPs in toluene phase also decreases, except for the sample C11H23-Tyr-Au-4 where the size of Au NPs was increased (compare Figure 8A D). Note that the sample C11H23-Tyr-Au4 was partially transferred to toluene after HCl treatment, and most of the aggregated Au NPs were left at the interphase of the two layers (Figure 1D). Therefore, it seems that the sample C11H23-Tyr-Au-4 in the toluene contains some aggregated Au NPs, and as a result, the average particle size increases. Prasad et al. have reported that during digestive ripening of Au nanostructures, as the alkyl chain length of alkanethiol increases, the size of as obtained Au NPs after ripening increases.58 The HRTEM images of a single spherical NP of these samples from toluene medium clearly show the presence of perfectly aligned lattice fringes (indicated by white solid line) with an interplanar spacing of 0.23 nm corresponding to the {111} lattice planes of metallic gold (Figure 9). Therefore, the crystalline patterns of the Au nanostructures were unaltered before and after the phase transfer process. TEM results further confirm that the Au nanostructures are undergoing shape transformation from multipod to spherical geometry due to phase transfer from aqueous to toluene. The detailed mechanism for phase transfer and the associated shape transformation will be discussed later. Similarly, we were also able to transfer sample C17H35-Tyr-Au4 (representative case) from aqueous to the other nonpolar organics such as xylene, benzene, cyclohexane, ethyl benzene, and hexane in a similar fashion. The color of the colloidal suspension was changed from blue to ruby red, and the aqueous layer was left colorless, as previously observed. (See Figure S4 in the SI.) The UV vis spectra of the sample C17H35-TyrAu-4 in different organics such as xylene, benzene, cyclohexane, 18525



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Figure 11. TEM images of the colloidal suspension of Au NPs of the sample C17H35-Tyr-Au-4 after phase transfer in: (A) xylene and (B) ethylbenzene. The insets show the enlarged view of the Au NPs of the corresponding images.

Figure 10. UV vis absorption spectra of the colloidal suspension of Au NPs of the sample C17H35-Tyr-Au-4 after phase transfer to different organic: (a) xylene, (b) benzene, (c) cyclohexane, (d) ethyl benzene, and (e) hexane.

ethylbenzene, and hexane showed the appearance of only one sharp SPR band centered at 534, 533, 533, 537, and 533 nm, respectively (Figure 10a e) indicating the change in morphology of Au nanomultipods to spherical geometry, as observed in the case of toluene. For all cases, it is noticed that the positions of λmax of the SPR bands are blue-shifted corresponding to the asprepared aqueous colloidal suspension of Au nanostructures of the same sample (C17H35-Tyr-Au-4) (compare Figure 2a with Figure 10). After phase transfer of Au nanomultipods (of the sample C17H35-Tyr-Au-4) to nonpolar organics other than toluene in a similar fashion, the remaining aqueous phases have no traces of Au NPs, as confirmed by UV vis spectra. (See Figure S5 of the SI.) It is worth mentioning that after transfer the stability of colloidal Au NPs in these solvents is not as good as that of toluene. We do not know the exact reason for this observation. Surprisingly, other research groups have mostly used toluene as a nonpolar organic to transfer Au NPs from water.41,50 As representative cases, we performed the TEM analysis of the sample C17H35-Tyr-Au-4 in xylene and ethylbenzene after transfer process, as shown in Figure 11A,B, respectively. The Au NPs obtained from xylene and ethylbenzene also show spherical morphology. No single pod-shaped particles are present in the image, which confirms the fact that the shape transition is also occurring when they are transferred to other nonpolar organics than toluene. The average particles sizes of the Au NPs (sample C17H35-Tyr-Au-4) from ethylbenzene and xylene are calculated to be 8.5 ( 1.6 and 9.0 ( 1.6 nm, respectively (Figure 11A,B). Note that the centrifuged, washed, and redispersed sample C17H35-Tyr-Au-4 was settled at the interface of the two layers when it were subjected to phase transfer in a similar fashion. (See Figure S6A in the SI.) However, the addition of excess amphiphiles (C17H35-Tyr) with this purified sample C17H35-Tyr-Au-4 before transfer at pH ∼11 can bring the Au nanostructures from aqueous to toluene layer. (See Figure S6B in the SI.) The spectrum of the redispersed sample exhibits an SPR at ∼604 nm (Figure S7a of the SI). However, the spectrum of the toluene layer does not exhibit any SPR band due to Au NPs after similar treatment

(Figure S7b in the SI). However, after the addition of excess amphiphiles, the transfer of Au nanostructures to toluene is confirmed from visual color change of the toluene layer (Figure S6B in the SI) to red, and the UV vis spectrum exhibits a single sharp SPR band at 530 nm due to the transference of Au NPs in the toluene phase (Figure S7c in the SI). This result further confirms that the presence of amphiphile on the Au NPs’ surface as well as in the transfer medium is necessary to observe the shape change of Au nanostructures and their transfer. Reversible Phase Transfer. When the red color suspension of spherical Au NPs (sample C17H35-Tyr-Au-4) in toluene was mixed with an aqueous alkaline solution (pH ∼12) and shaken vigorously, some of the Au NPs were transferred from toluene to aqueous medium, and some were settled at the interface of the two layers. We noticed that the aqueous layer turns blue, and the toluene phase is left colorless. Figure 12 shows photographs of the different stages during reversible phase transfer of Au nanostructures. The UV vis spectrum of Au NPs in aqueous layer after reversible transfer showed a continuous long rising SPR band originating from 554 nm (Figure 13a). The UV visible spectrum of the remaining toluene after transfer shows no trace of any SPR signal of Au NPs (Figure S9 in the SI). Furthermore, when this blue color aqueous suspension of Au nanostructures, as obtained after reversible phase transfer, was further mixed with HCl solution in presence of toluene (pH ∼1 to 1.5) and vigorously shaken, we noticed the transfer of Au nanostructures from water to toluene. The color of the toluene layer becomes violet (for color change, see Figure 12), and the aqueous layer is left colorless, as confirmed by the UV visible spectra (Figure S9 in the SI). Note that some of the Au NPs were also settled at the interface of the two layers; as a result, the color intensity of the toluene layer is less compared with that previously observed. The Au NPs, as obtained after reversible phase transfer in the toluene layer, exhibited a sharp SPR absorption band centered at 534 nm (Figure 13b). The TEM image of the sample C17H35-Tyr-Au-4 obtained from water after reversible phase transfer showed the presence of multi-pod-shaped particles (Figure 14A). However, the multipods are not as prominent, as observed in the first cycle of the as-prepared sample C17H35-Tyr-Au-4 (compare Figure 3A with Figure 14A). Some spherical NPs are also present along with the multipods, as indicated by the black circles (Figure 14A). The enlarged view also shows the multipod formation in aqueous phase after reversible phase transfer from toluene (inset of Figure 14A). The HRTEM image of this Au nanomultipods is also grown along the Æ111æ direction as the interplanar spacing is 18526



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Figure 12. Photographs showing different stages of reversible phase transfer of Au nanostructures (C17H35-Tyr-Au-4) from toluene to water and vice versa.

Figure 14. TEM images of the Au nanostructures (A) aqueous medium and (B) toluene after 2nd cycles. The insets show the enlarged view of a portion of the corresponding images. The bottom inset of image A shows the interplanar spacing of Au nanomultipods is 0.23 nm.

Figure 13. UV vis absorption spectra of the colloidal suspension of Au NPs for the sample C17H35-Tyr-Au-4 after reversible phase transfer to water (a) and from water to toluene (b) in the second cycle.

measured to be 0.23 nm (bottom inset of Figure 14A). The TEM image of the sample in toluene after reversible transfer again shows Au NPs of only spherical geometry with no pod-shaped particles (Figure 14B). Note that the obtained spherical NPs are not uniform in shape, as observed after first cycle of transfer (compare Figure 8A with Figure 14B). The enlarged view of these NPs from toluene after reversible phase transfer also has spherical morphology, indicating the clear transformation of shape from multipod to sphere. The mechanism of reversible phase transfer will also be discussed below. Proposed Mechanism. The above-mentioned results tell us that tyrosine amphiphiles produced multi-pod-shaped Au nanostructures in the aqueous alkaline solution. In this case, the formation of multi-pod-shaped Au nanostructures can be explained in the following way. It is known from our previous of reducing Au(III) to metallic Au NPs.29,60 Therefore, in this case, the tyrosinate moiety of amphiphiles reduces Au(III) to Au(0), which eventually combine to form Au nanoclusters. The amphiphile present in the system adsorbed to certain planes of the formed Au nanoclusters probably through the interaction of hydroxyl group of the tyrosinate moiety of amphiphile and directed the growth to form Au nanomultipods. To confirm the anchoring of amphiphile molecules on the surface of Au nanostructures in aqueous medium, we performed FTIR characterization (Figure S8a of the SI) on a representative sample C17H35-Tyr-Au-4 after purification (centrifuged, washed, and dried). For comparison, we also acquired the FTIR spectrum of sodium salt of amphiphiles (C17H35-Tyr). (See Figure S8b of the SI.) All characteristic peaks of sample C17H35-Tyr-Au-4 and

sodium salt of amphiphile (C17H35-Tyr) are well-matched, as summarized in Table 2, indicating the adsorption of tyrosine amphiphile. In our previous report, we showed the adsorption of tyrosine amphiphile onto Au NPs via FTIR spectroscopy.20,31 The formation of Au nanomultipods may be attributed from the orientated attachment of the initially formed spherical seed NPs, as can be preliminary understood from the above TEM results and as has also been discussed in our previous report.31,63,64 The mechanism of formation of multi-pod-shaped nanostructures using tryptophan-based amphiphiles has been described in detail in our previous paper.31 We believe a similar mechanism is also operating in this case. In brief, it is a known fact that the order of stability of the individual facets of the fcc geometry is {111} > {100} > {110} according to their increasing surface energy.65 Thus, in this case, it should be expected that the amphiphile molecules adsorb more onto {110}/{100} facets of the initially formed Au NPs for stabilizing the said planes. The lower energetic {111} planes are not sufficiently capped with the amphiphiles molecules. Therefore, the low energetic {111} facets of Au NPs are vulnerable to further growth. Therefore, during growth of a crystal, the seed particles are accommodated and grow along the Æ111æ direction, as we confirmed from the HRTEM analysis of amphiphile-capped Au nanostructures described above (Figure 4). These images clearly show the perfectly aligned lattice fringes (as marked by white solid line) with an interplanar spacing of 0.23 nm corresponding to the {111} plane of fcc metallic Au. The magnified HRTEM images (Figure 4E H) also show the formation of a twin boundary at the interface, which is evidence of formation of branched nanostructures through oriented attachment.18,31 The facet of growth along Æ111æ was supported by the XRD results of different multi-pod-shaped samples obtained from aqueous media (Figure 6). It has been reported by our group that the amino-acid-based amphiphiles with long hydrophobic tail exist as micellar structures 18527



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Table 2. FTIR Peak Assignment (inverse centimeters) for the Amphiphile (C17H35-Tyr) and Amphiphile-Gold Nanoconjugate (Sample C17H35-Tyr-Au-4) Obtained from Different Solvents C17H35-Tyr

sodium salt of C17H35-Tyr

C17H35-Tyr-Au-4 isolated from water

3438, 3352 2916, 2850

3435, 3352 2916, 2850

1586, 1466

1586, 1462

1623, 1516

1623, 1511

3300 2915, 2850

3300 2916, 2850

1718 1648, 1544

C17H35-Tyr-Au-4 isolated from toluene

1715

peak assignment OH OH CH2 CdO COO

1648, 1540

amide I + II

Scheme 2

in aqueous alkaline solution.20 It has also been proposed in that report that the formed spherical Au NPs by these amphiphiles are stabilized through the micelles present in the solution. Therefore, in this case, we may assume that the formed Au nanomultipods are stabilized by the micelles, as obtained from the aggregation of tyrosine amphiphiles in aqueous medium, as shown pictorially in Scheme 2. There is a possibility that tyrosine amphiphile molecules themselves can anchor the formed multipods, but, in such case, it is difficult get a stable dispersion of nanomultipods as the long hydrophobic tail of amphiphile molecules faces the water. Therefore, we believe that the nanomultipods are stabilized through the micelles of tyrosine amphiphiles whose polar head groups interact with the Au surface and hydrophobic tails stay in the core of the micelle (Scheme 2). However, during the transfer of Au nano-multi-pods from water to nonpolar organics, the carboxylate/phenolate moieties of the amphiphiles in the micelle in aqueous medium or that adsorbed onto nanomultipod surface is protonated upon acidification and converted to carboxylic/phenolic groups, respectively. As a result, the solubility of amphiphiles in the aqueous phase decreases, but its solubility in the organic phases increases. Therefore, the protonated hydrophobic amphiphiles would be transferred to nonpolar organics along with the anchored Au nanostructures. However, in the nonpolar organic phase, the micellar structure breaks into individual amphiphile molecule.

The amphiphile molecule contains a carboxylic acid and phenolic OH group that have the capability to anchor to the Au nanostructures surface. Karg et al. very recently demonstrated the phase transfer of hydrophilic Au NPs from aqueous to different organic solvents by suitable hydrophobic ligands.66 In this stage, in nonpolar organic phase, the relatively large multipod-shaped particles break up into spherical NPs in the presence of the tyrosine amphiphile ligands via room-temperature digestive ripening process (Scheme 2). The as-formed spherical Au NPs are then stabilized through the anchoring of the amphiphile molecules containing carboxylic and phenolic groups (Scheme 2). The presence of amphiphile on the Au NPs surface was confirmed from FTIR characterization of purified Au NPs isolated from toluene (centrifuged/washed/dried) and compared with that of neat amphiphiles (C17H35-Tyr). (For spectra, see Figure S8c,d, respectively, in the SI.) Table 2 clearly reveals that the characteristic peaks are well-matched, indicating adsorption of amphiphile on the Au NP surface. Previously, Stoeva et al. reported that large polyhedral Au NPs can be converted to small spherical particles upon stirring with alkanethiol at room temperature.58 According to them, the presence of defects (e.g., twins) in the large particles causes great strain, and they can serve as attacking sites for thiol ligand to break into small particles. In our case, the HRTEM image of nanomultipod from all samples (C17H35-Tyr-Au-4, C15H31-Tyr-Au-4, C13H27-Tyr-Au-4, and 18528


The Journal of Physical Chemistry C C11H23-Tyr-Au-4) also showed the presence of a twin boundary (Figure 4E H). The amphiphiles present in the organic phase may attack these defect sites and can break this structure into spherical structures. Recently, Pileni and coworkers showed that worm-like palladium clusters could be broken down to spherical NPs just by stirring with dodecanethiol via room-temperature digestive ripening.67 Kalidindi and Jagirdar reported the generation of monodiperse magnesium NPs from bigger particles using hexadecylamine by a similar process.68 During reversible transfer from toluene to water, the addition of aqueous NaOH results in the deprotonation of carboxylic/phenolic OH groups of amphiphile present on the Au NPs and in solution. As a result, the hydrophobic character of amphiphiles decreases, and their solubility in toluene also decreases, but because of their hydrophilic nature, they become more soluble in water, and that drives the transfer of amphiphiles along with spherical Au NPs to water. Now, we assume that the transferred spherical Au NPs in water can grow to form multi-pod-shaped particles probably through a similar oriented attachment mechanism, as previously described.

’ CONCLUSIONS Redox-active tyrosine-based amphiphiles were successfully used to synthesize multi-pod-shaped Au nanostructures; however, other shapes such as spherical Au NPs can also be prepared by varying the molar ratio of amphiphile to gold salt. HRTEM and XRD results showed that the nanomultipods were formed by oriented attachment of the initially formed spherical Au NPs through {111} crystallographic planes. We have demonstrated a simple yet efficient method to completely transfer multi-podshaped Au nanostructures from the aqueous to nonpolar organic phase by simple acidification with HCl. Surprisingly, we have observed (visually as well as via UV vis spectroscopy and TEM imaging) a complete morphological transition from multipod to spherical geometry upon such a transfer from aqueous to nonpolar organics. Both the Au nanomultipods and the spherical Au NPs obtained after phase transfer were bounded by {111} planes. Reversible phase transfer to and from the aqueous phase was completely achieved through the adjustment of pH to acidic or alkaline using HCl or NaOH. This reversible phase transfer was associated with a shape transformation of Au nanostructures from multipod to spherical geometry and vice versa. The length of the alkyl chain of amphiphiles has a predominant effect on the morphology as well as the transfer efficiency of the amphiphilecoated Au nanostructures to nonpolar organics. This method of phase transfer and associated shape transition are much simpler and greener because (i) no externally added ligands such as amine or thiol is needed, (ii) no refluxing condition is required, and (iii) it can be done by simple HCl treatment. This method can also be applied for the generation of other metal NPs in both the aqueous and nonaqueous phases. ’ ASSOCIATED CONTENT

bS

Supporting Information. The details of the synthesis and characterization of methyl ester of amphiphiles and amphiphiles via NMR, ESI-mass and elemental analysis, UV vis spectra, TEM images, HRTEM images, photographs of phasetransferred Au nanomultipods in various nonpolar solvents, and FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 91-33-2473 2805.

’ ACKNOWLEDGMENT E.D. and M.B. thank the CSIR, Government of India for providing fellowships. We acknowledge the financial support from the DST, New Delhi under the Nanoscience and Nanotechnology Initiative. ’ REFERENCES (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (2) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141–7143. (3) Rashid, M. H.; Mandal, T. K. Adv. Funct. Mater. 2008, 18, 2261–2271. (4) Zhao, J.; Pinchuk, A. O.; McMahon, J. M.; Li, S. Z.; Ausman, L. K.; Atkinson, A. L.; Schatz, G. C. Acc. Chem. Res. 2008, 41, 1710–1720. (5) Zhang, L.; Chen, H. J.; Wang, J. F.; Li, Y. A. F.; Wang, J. A.; Sang, Y.; Xiao, S. J.; Zhan, L.; Huang, C. Z. Small 2010, 6, 2001–2009. (6) Boisselier, E.; Astruc, D. Chem. Soc. Rev. 2009, 38, 1759–1782. (7) Sharma, P.; Brown, S.; Walter, G.; Santra, S.; Moudgil, B. Adv. Colloid Interface Sci. 2006, 123, 471–485. (8) Rashid, M. H.; Bhattacharjee, R. R.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 9684–9693. (9) Alkilany, A. M.; Murphy, C. J. J. Nanopart. Res. 2010, 12, 2313–2333. (10) Ohshiro, T.; Zako, T.; Watanabe-Tamaki, R.; Tanaka, T.; Maeda, M. Chem. Commun. 2010, 46, 6132–6134. (11) Li, Z.; Kubel, C.; Parvulescu, V. I.; Richards, R. ACS Nano 2008, 2, 1205–1212. (12) Ma, Y. R.; Qi, L. M. J. Colloid Interface Sci. 2009, 335, 1–10. (13) Yoo, H.; Sharma, J.; Yeh, H. C.; Martinez, J. S. Chem. Commun. 2010, 46, 6813–6815. (14) Watanabe, S.; Nakano, S.; Imai, C.; Laskar, I. R.; Komura, T.; Hadano, S.; Iyoda, T. Chem. Lett. 2010, 39, 902–904. (15) Zhang, Y.; Xu, F. G.; Sun, Y. J.; Guo, C. L.; Cui, K.; Shi, Y.; Wen, Z. W.; Li, Z. Chem.—Eur. J. 2010, 16, 9248–9256. (16) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414–6420. (17) Murawala, P.; Phadnis, S. M.; Bhonde, R. R.; Prasad, B. L. V. Colloids Surf., B 2009, 73, 224–228. (18) Rashid, M. H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750–16760. (19) Sanpui, P.; Pandey, S. B.; Ghosh, S. S.; Chattopadhyay, A. J. Colloid Interface Sci. 2008, 326, 129–137. (20) Si, S.; Dinda, E.; Mandal, T. K. Chem.—Eur. J. 2007, 13, 9850–9861. (21) Si, S.; Dinda, E.; Mandal, T. K. J. Nanosci. Nanotechnol. 2008, 8, 5934–5941. (22) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Chem. Mater. 2005, 17, 566–572. (23) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482–488. (24) Ghodake, G. S.; Deshpande, N. G.; Lee, Y. P.; Jin, E. S. Colloids Surf., B 2010, 75, 584–589. (25) Singh, A. V.; Bandgar, B. M.; Kasture, M.; Prasad, B. L. V.; Sastry, M. J. Mater. Chem. 2005, 15, 5115–5121. (26) Dinda, E.; Si, S.; Kotal, A.; Mandal, T. K. Chem.—Eur. J. 2008, 14, 5528–5537. (27) Dinda, E.; Rashid, M. H.; Biswas, M.; Mandal, T. K. Langmuir 2010, 26, 17568. (28) Mitra, R. N.; Das, P. K. J. Phys. Chem. C 2008, 112, 8159–8166. (29) Si, S.; Bhattacharjee, R. R.; Banerjee, A.; Mandal, T. K. Chem.— Eur. J. 2006, 12, 1256–1265. (30) Si, S.; Mandal, T. K. Chem.—Eur. J. 2007, 13, 3160–3168. 18529



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(31) Dinda, E.; Rashid, M. H.; Mandal, T. K. Cryst. Growth Des. 2010, 10, 2421–2433. (32) Jana, D.; De, G. J. Mater. Chem. 2011, 21, 6072–6078. (33) Palui, G.; Ray, S.; Banerjee, A. J. Mater. Chem. 2009, 19, 3457–3468. (34) Davis, S. C.; Klabunde, K. J. Chem. Rev. 1982, 82, 153–208. (35) Sastry, M. Curr. Sci. 2003, 85, 1735–1745. (36) Andrews, M. P.; Ozin, G. A. J. Phys. Chem. 1986, 90, 2929–2938. (37) Mulvaney, P. Langmuir 1996, 12, 788–800. (38) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem. B 1999, 103, 7238–7246. (39) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602–610. (40) Yang, J.; Ying, J. Y. Nat. Mater. 2009, 8, 683–689. (41) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801–802. (42) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036–7041. (43) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428–433. (44) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51–57. (45) Porter, L. A.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378–7386. (46) Liu, J.; Xu, R. L.; Kaifer, A. E. Langmuir 1998, 14, 7337–7339. (47) Kumar, A.; Mukherjee, P.; Guha, A.; Adyantaya, S. D.; Mandale, A. B.; Kumar, R.; Sastry, M. Langmuir 2000, 16, 9775–9783. (48) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723–4730. (49) Mayya, K. S.; Caruso, F. Langmuir 2003, 19, 6987–6993. (50) Yang, J.; Lee, J. Y.; Deivaraj, T. C.; Too, H. P. J. Colloid Interface Sci. 2004, 277, 95–99. (51) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911–8916. (52) Zhu, H. F.; Tao, C.; Zheng, S. P.; Wu, S. K.; Li, J. B. Colloids Surf., A 2005, 256, 17–20. (53) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427–3430. (54) Gittins, D. J.; Caruso, F. ChemPhysChem 2002, 3, 110–113. (55) Gittins, D. J.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001–3004. (56) Kulkarni, A. P.; Munechika, K.; Noone, K. M.; Smith, J. M.; Ginger, D. S. Langmuir 2009, 25, 7932–7939. (57) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515–7520. (58) Stoeva, S. I.; Zaikovski, V.; Prasad, B. L. V.; Stoimenov, P. K.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2005, 21, 10280–10283. (59) Bodanszky, M.; Bodanszky, A. The Practice of Peptide Synthesis, 2nd ed.; Springer-Verlag: Berlin, 1994. (60) Zhou, Y.; Chen, W. X.; Itoh, H.; Naka, K.; Ni, Q. Q.; Yamane, H.; Chujo, Y. Chem. Commun. 2001, 2518–2519. (61) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (62) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (63) Bakshi, M. S. Langmuir 2009, 25, 12697–12705. (64) Wang, L.; Yamauchi, Y. Chem. Mater. 2009, 21, 3562–3569. (65) Xiang, Y.; Wu, X.; Liu, D.; Jiang, X.; Chu, W.; Li, Z.; Ma, Y.; Zhou, W.; Xie, S. Nano Lett. 2006, 6, 2290–2294. (66) Karg, M.; Schelero, N.; Oppel, C.; Gradzielski, M.; Hellweg, T.; Klitzing, R. V. Chem.—Eur. J. 2011, 17, 4648–4654. (67) Naoe, K.; Petit, C.; Pileni, M. P. J. Phys. Chem. C 2007, 111, 16249–16254. (68) Kalidindi, S. B.; Jagirdar, B. R. Inorg. Chem. 2009, 48, 4524–4529.

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