Amino Acid-Based Redox Active Amphiphiles to In Situ Synthesize

DOI: 10.1021/cg100281z

Amino Acid-Based Redox Active Amphiphiles to In Situ Synthesize Gold Nanostructures: From Sphere to Multipod

2010, Vol. 10 2421–2433

Enakshi Dinda, Md. Harunar Rashid, and Tarun K. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received March 2, 2010; Revised Manuscript Received April 1, 2010

ABSTRACT: Gold (Au) nanostructures of controllable shapes (spherical and multipod) were synthesized by an in situ reduction technique using newly designed amino acid-based redox active amphiphiles without any additional template in alkaline condition. These amphiphiles are the conjugates of fatty acids (e.g., caproic acid, caprylic acid, and capric acid) and a redox active amino acid (tryptophan). The nature of amphiphiles (especially the length of the alkyl chain associated with fatty acids) and the molar ratio (R) of amphiphile to HAuCl4 have a significant effect on the morphology of the formed Au nanostructures. For example, the caproic acid- and caprylic acid-based amphiphiles produce mostly multipod-shaped Au nanostructures at a value of R in the range 4-8. However, these amphiphiles produced only spherical Au nanoparticles (NPs) at R in the range 1.3-2.0. However, the capric acid-based amphiphile generates only spherical Au NPs with an average diameter in the range 3.5-6.2 nm, no matter what the value of R is. FTIR and thermogravimetric analysis confirmed the capping of Au nanostructures with amphiphile. XRD results indicated the formation of highly crystalline spherical and multipod-shaped Au nanostructures that were bounded by {111} facets. The mechanisms of formation of spherical/multipod-shaped Au nanostructures are discussed based on the high-resolution transmission electron microscopic (HRTEM) and time-dependent UV-vis spectroscopic studies. HRTEM analysis revealed that the multipod-shaped Au branches were formed by oriented attachment of the initially formed spherical Au NPs of smaller size and were grown along the Æ111æ direction. HRTEM results further indicated that spherical Au NPs were also grown along the Æ111æ direction.

*Corresponding author. Fax: 91-33-2473 2805, E-mail: psutkm@mahendra. iacs.res.in.

Branched Au nanostructures, including multipods and dendritic and star-shaped nanostructures, were also interesting due to their attractive supramolecular structure, large surface area, and excellent connectivity among the different parts of the nanomaterials, which are mostly prepared by using a conventional reducing agent in conjugation with a template such as polymer or surfactant.12,14,15,29,32-36 On the other hand, the preparation of dendritic or branched Au nanostructures by in situ reduction techniques2,22,37,38 is very limited. Recently, we reported the preparation of spongy Au nanostructures and dendritic silver (Ag) nanostructures by a template-less in situ reduction approach using several citrate salts containing different cations.2,33 Realizing the need to develop ecofriendly routes, several research groups reported the preparation of anisotropic Au nanostructures via an in situ reduction approach using biomolecules such as plant extracts,18 protein,39 globular proteins like bovine serum albumin (BSA),40-43 peptides,44 and ionic liquids.45 Recently, our group also reported spherical and anisotropic Au NPs by an in situ approach using varieties of compounds such as oligopeptides,23,30,46,47 biocompatible macromolecules,21 and ascorbic acid-based ionic liquids.22 A similar approach has also been used to prepare Au nanostructures using multifunctional peptides as a reducing-cumcapping agent.48,49 However, reports on the preparation of branched or multipod-shaped Au nanostructures using simple small peptides are very rare, although there was a report of the preparation of branched Au nanostructures using a more complex peptide molecule.37 We recently showed that redox-active amphiphiles consisting of a fatty acid (e.g., stearic, lauric, or oleic acid) and an amino acid (e.g., tyrosine or tryptophan) produced mostly

r 2010 American Chemical Society

Published on Web 04/20/2010

Introduction In recent years, anisotropic gold (Au) nanostructures have fascinated scientists because of their immense importance in materials science and nanotechnology, as those nanostructures have interesting size- and shape-dependent physicochemical and optoelectronic properties which find application in a variety of fields like nanodevice,1 catalysis,2-4 surface enhanced Raman scattering (SERS),5 chemical and biological sensing applications,6,7 and optics.1,8 Consequently, nowadays, different chemical strategies have been adopted to control either the size or the shape of Au NPs.9-14 Among them, mainly two strategies were widely used to prepare such Au NPs. The first involves the reduction of gold salts using conventional reducing agents such as citric acid, ascorbic acid, and sodium borohydride (NaBH4) in the presence of templates/capping agents13,15 where the post synthetic method is sometimes necessary to remove the template. The second strategy is the in situ reduction technique, which has recently been adopted by many researchers16-19 including our groups6,20-23 to prepare Au NPs of different shapes using different compounds such as organic ligands,6,19 biomolecules,18,21 or polymers16,17,20 as simultaneous reducingcum-capping agents. Besides these two major strategies, seedmediated growth has also been successfully used to prepare anisotropic Au nanostructures.24-27 Au nanostructures of a variety of shapes ranging from wires,28,29 rods, 24-27 and cubes/polyhedrons4,30 to triangular plates19,31 and prisms18 have been prepared by these strategies.

pubs.acs.org/crystal

2422

Crystal Growth & Design, Vol. 10, No. 5, 2010

Dinda et al.

spherical Au nanoparticles (NPs) in alkaline medium that could be efficiently transferred from aqueous to nonaqueous solvent by simple acid treatment.23 However, the effect of chain lengths of fatty acids associated with such types of amphiphiles on the morphology of the formed Au NPs has not been explored in detail. Thus, in the present work, we report the use of three newly designed redox-active amphiphiles (conjugates of caproic, caprylic, and capric acid with redox active tryptophan; for chemical structure, see Scheme 1) as reducing-cum-capping agent for the preparation of differentshaped Au nanostructures. Moreover, it is shown that the morphology of Au NPs could be controlled (spherical to multipod) simply by varying the alkyl chain length of amphiphiles used, as well as by varying the molar ratios of amphiphiles to HAuCl4. The mechanisms of formation of differently shaped Au nanostructures are discussed on the basis of results of UV-vis spectroscopy and HRTEM. Experimental Section Materials. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4 3 3H2O) was purchased from Sigma-Aldrich, USA. Caproic acid [CH3-(CH2)4-COOH], caprylic acid [CH3-(CH2)6-COOH], and capric acid [CH3-(CH2)8-COOH] were purchased from E. Merck, India.

Scheme 1

L-Tryptophan (Trp), dicyclohexylcarbodiimide (DCC), and 1-hydro-

xybenzotriazole (HOBt) were purchased from SRL, India. All these compounds were used as received. All the aqueous solutions were prepared with triple-distilled water. Freshly distilled reagent-grade organic solvents were used for all synthetic purposes. Synthesis of Redox-Active Amphiphiles. Three newly designed redox-active amphiphiles (conjugates of a fatty acid and an amino acid), caproic-Trp-COOH (C5H11-Trp), caprylic-Trp-COOH (C7H15Trp), and capric-Trp-COOH (C9H19-Trp) were synthesized for generation of Au nanostructures, the chemical structures of which are depicted in Scheme 1. The preparation of these amphiphiles involves the synthesis of their methyl ester (caproic-Trp-COOMe, caprylic-Trp-COOMe, and capric-Trp-COOMe) followed by their saponification. The methyl esters of these amphiphiles were synthesized by a conventional solution-phase method using a racemizationfree fragmentation/condensation strategy.50 In general, the C terminus of tryptophan was first protected via esterification with a methyl group, followed by coupling with a fatty acid (such as either caproic acid or caprylic acid or capric acid) by using a DCC/HOBt mixture. These esters were finally saponified to obtain the actual amphipiles. The detailed synthesis procedures of methyl ester of amphiphiles and actual amphiphiles were provided in the Supporting Information (SI). Preparation of Au Nanostructures Using Redox-Active Amphiphiles. In a typical preparation of Au nanostructures, a solution of amphiphile, C7H15-Trp in methanol (1 mL, 40 mM) was diluted with 8.5 mL of triple-distilled water so that the final concentration of amphiphile was 4 mM and the pH of the reaction medium was adjusted to alkaline (pH ≈ 11.5) by adding NaOH for complete solubilization of the amphiphile at room temperature. An aqueous solution of HAuCl4 (0.5 mL, 10 mM) was then added dropwise to the above solution with constant magnetic stirring. In this case, the molar ratio of amphiphile to HAuCl4, i.e., R = [C7H15-Trp]/ [HAuCl4] = 8. The stirring was further continued for 15 min at ambient temperature (25 C). We designated this reaction set and the formed Au nanostructures as C7H15-Trp-Au-8. Five more similar sets of reactions were also carried out by varying the molar ratio of C7H15-Trp to HAuCl4, i.e., R=5.3, 4.0, 2.7, 2.0, and 1.3 (see Table 1 for detailed recipe). During the course of these reactions, we observed a color change of the reaction mixture from yellow to blue or ruby red depending upon the molar ratio R (see Table 1) used. This visual color change indicated the formation of colloidal Au nanostructures. It is worthwhile to mention that the concentration of amphiphiles (C7H15-Trp) was kept constant at 4.0 10-3 M for all the reaction sets. Thus, the variation of molar ratio (R) reflects only the variation in the concentration of HAuCl4. We also prepared Au NPs using the other two amphiphiles, C5H11-Trp and C9H19-Trp, having different alkyl chain length under the reaction conditions similar to that used in the case of amphiphile, C7H15-Trp. The details of reaction recipes for the preparation of different Au NPs samples using amphiphiles C5H11-Trp and C9H19-Trp are depicted in Tables 2 and 3, respectively. The as-prepared colloidal suspensions of Au nanostructures (without centrifugation) were used for UV-vis spectroscopic, microscopic, and diffractometric studies. For FTIR and TGA analysis, the Au nanostructures samples were first isolated by centrifugation followed by washing with triple-distilled water. The whole process was repeated several times (∼3 times) until the

Table 1. Reaction Recipe for Preparation of Different Au Nanostructure Samples Using [C7H15-Trp] = 4.0 10-3 M for All Cases [HAuCl4] (M)

sample ID a

C7H15-Trp-Au-8 C7H15-Trp-Au-5.3a C7H15-Trp-Au-4a C7H15-Trp-Au-2.7a C7H15-Trp-Au-2a C7H15-Trp-Au-1.3a a

-4

5.0 10 7.5 10-4 1.0 10-3 1.5 10-3 2.0 10-3 3.0 10-3

[C7H15-Trp]/ [HAuCl4]

λmax,t/λmax,lc (nm)

morphology

DTEM/DXRDd (nm)

colorb

8.0 5.3 4.0 2.7 2.0 1.3

523/845 527/860 525/817 520/740 527 527

multipod multipod multipod spherical and multipod spherical and multipod spherical

ND/ND ND/ND ND/ND ND/ND ND/ND 10.6 ( 2.9/12.4

blue blue blue blue violet red

C7H15-Trp-Au represents amphiphile, C7H15-Trp and Au system, and the numerical values represent the molar ratio (R) = [C7H15-Trp]/[HAuCl4]. Color of the final Au NPs colloid. c λmax,t and λmax,l are the maximum wavelength of shorter and longer SPR bands, respectively. d DTEM=average diameter obtained from TEM. DXRD=diameter obtained from XRD data applying Scherer equation. ND=not possible to determine. b

Article


2423

Table 2. Reaction Recipe for Preparation of Au Nanostructures of Different Shapes Using [C5H11-Trp] = 4.0 10-3 M for All Cases sample ID

[HAuCl4] (M) a

C5H11-Trp-Au-8 C5H11-Trp-Au-5.3a C5H11-Trp-Au-4a C5H11-Trp-Au-2.7a C5H11-Trp-Au-2a C5H11-Trp-Au-1.3a

-4

5.0 10 7.5 10-4 1.0 10-3 1.5 10-3 2.0 10-3 3.0 10-3

[C5H11-Trp]/[HAuCl4]

λmaxc (nm)

morphology

DTEM/DXRDd (nm)

colorb

8.0 5.3 4.0 2.7 2.0 1.3

577 563 562 567 533 533

multipod NC multipod NC spherical spherical

ND/ND NC/NC ND/ND NC/NC 4.7 ( 1.5/6.5 10.8 ( 1.5/7.6

blue blue blue blue violet red

a

C5H11-Trp-Au represents amphiphile, C5H11-Trp and Au system, and the numerical values represents the molar ratio (R) = [C5H11-Trp]/[HAuCl4]. Color of the final colloidal solution of formed Au nanostructures. c λmax = maximum wavelength of SPR band. d DTEM = average diameter obtained from TEM. DXRD = diameter obtained from XRD data applying Scherer equation. ND = not possible to determine. NC = Not checked. b

Table 3. Reaction Recipe for Preparation of Au NPs Using [C9H19-Trp] = 4.0 10-3 M for All Samples [HAuCl4] (M)

sample ID a

C9H19-Trp-Au-8 C9H19-Trp-Au-5.3a C9H19-Trp-Au-4a C9H19-Trp-Au-2.7a C9H19-Trp-Au-2a C9H19-Trp-Au-1.3a

-4

5.0 10 7.5 10-4 1.0 10-3 1.5 10-3 2.0 10-3 3.0 10-3

[C9H19-Trp]/[HAuCl4]

λmaxc (nm)

morphology

DTEM/DXRDd (nm)

colorb

8.0 5.3 4.0 2.7 2.0 1.3

524 525 524 524 524 526

spherical NC spherical NC NC spherical

4.9 ( 0.7/4.8 NC/4.2 3.7 ( 0.7/4.5 NC/4.6 NC/5.3 5.5 ( 2.9/6.2

red red red red red red

a C9H19-Trp-Au represents amphiphile, C9H19-Trp and Au system, and the numerical value represents the molar ratio (R) = [C9H19-Trp]/[HAuCl4]. Color of the final colloidal solution of formed Au NPs. c λmax = maximum wavelength of SPR band. d DTEM = average diameter obtained from TEM. DXRD = diameter obtained from XRD data applying Scherer equation. ND = not possible to determine. NC = Not checked. b

supernatant is free from traces of alkali (tested with litmus paper) and to remove any physically adsorbed (loosely bound) amphiphiles on the surface of Au nanostructures. Finally, the residue in the centrifugation tube was further dried in a freeze-dryer to remove a trace amount of water, and the resultant mass was then used for FTIR and TGA analysis.

Characterization 1

NMR Study. H NMR studies of all the amphiphiles and their methyl esters were carried out in CDCl3 (1-10 mM) using a Bruker DPX 300 MHz spectrometer. ESI Mass Spectrometry Study. The ESI mass spectra of the as-synthesized amphiphiles, C5H11-Trp, C7H15-Trp, and C9H19-Trp, were recorded from a methanol solution in 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. Thermogravimetric Analysis. The purified and dried samples of amphiphile coated Au nanostructures (amphiphileAu NPs) and neat amphiphiles were subjected to thermal analysis using a TA SDT Q600 instrument at a heating rate of 20 C min-1 under N2 atmosphere. UV-vis Absorption Spectroscopy Study. UV-vis absorption spectra of all the amphiphile-Au nanostructures’ suspension were acquired in a Hewlett-Packard 8453 UV-vis spectrophotometer. Transmission Electron Microscopy (TEM) Study. For TEM study, one drop of the as-prepared aqueous suspension of the 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 at an accelerating voltage of 200 kV. Fourier-Transform Infrared (FTIR) Spectroscopy Study. FTIR spectra of neat amphiphile, C7H15-Trp, and the centrifuged/washed/dried C7H15-Trp-Au nanostructure conjugates were recorded in a KBr pellet using a Shimadzu FTIR8400S spectrometer. Pellets were prepared by mixing the corresponding dried sample with KBr in a 1:100 (w/w) ratio.

X-ray Diffraction (XRD) Study. For XRD analysis, the concentrated suspensions, as obtained via centrifugation, of amphiphile-Au nanoconjugate was deposited on a microscopic glass slide as thin film and air-dried. The diffractogram of the Au nanostructure conjugate was then recorded by using a Bruker AXS D8 Advance diffractometer at an accelerating voltage of 40 kV using a CuKR (λ=1.5405 A˚) as the X-ray radiation source. Results and Discussion Synthesis of Au Nanostructures Using Different RedoxActive Amphiphiles. Three different newly designed redoxactive amphiphiles (see Scheme 1) were used for the preparation of either spherical or multipod-shaped Au nanostructures. These amphiphiles were prepared by following our earlier method,23 which was based on the conventional solutionphase coupling of the respective amino acid and fatty acid by using a racemization-free fragmentation/condensation strategy (see page S1 of the Supporting Information for details of synthesis). The synthesis of Au nanostructures is simply achieved by mixing aqueous solution of HAuCl4 and methanolic solution of the as-synthesized/purified amphiphiles (C5H11-Trp, C7H15-Trp, and C9H19-Trp) in alkaline condition (pH ≈ 11) at ambient temperature. The formation of differently shaped Au nanostructures was primarily monitored visually from the change of the color of the reaction mixture from yellow to blue or red (see Tables 1-3). The color of the formed colloidal Au nanostructures depends on the molar ratio of amphiphile to HAuCl4 used as well as nature of amphiphile. This difference in color of the final colloid is probably due to the formation of either spherical (red-colored colloid) or pod-shaped Au nanostructures (blue/violet-colored colloid) under different reaction conditions, as explained by other researchers.51 The different morphology of the obtained Au nanostructures under different reaction conditions will be discussed in more detail later in this section. Au Nanostructures Prepared Using Caprylic Acid-Based Amphiphile, (C7H15-Trp). A series of different C7H15-TrpAu nanoconjugate samples were prepared at different molar

2424


ratios (R) of C7H15-Trp to HAuCl4 (e.g., R=8.0, 5.3, 4.0, 2.7, 2.0, and 1.3) under alkaline condition (pH ≈ 11) (see Table1). Figure 1 shows a set of UV-vis spectra of the as-prepared

Figure 1. UV-vis absorption spectra of as-prepared suspensions of different C7H15-Trp-Au conjugate samples: (a) C7H15-Trp-Au-8, (b) C7H15-Trp-Au-5.3, (c) C7H15-Trp-Au-4, (d) C7H15-Trp-Au-2.7, (e) C7H15-Trp-Au-2, and (f) C7H15-Trp-Au-1.3. The spectra are recorded after 15 min of reaction.

Dinda et al.

colloidal suspension of these C7H15-Trp-Au nanoconjugate samples. The UV-vis spectra of samples C7H15-Trp-Au-8, C7H15-Trp-Au-5.3, and C7H15-Trp-Au-4, prepared at R=8, 5.3, and 4, respectively, clearly show two distinguishable surface plasmon resonance (SPR) absorption bands (at shorter-wavelength and longer-wavelength), might be indicating the formation of nonspherical Au nanostructures [see curves (a-c) of Figure 1].52 All three samples exhibit a shorter-wavelength SPR band at around 525 nm, whereas longer-wavelength SPR bands exhibited by these samples are centered at 850, 855, and 817 nm for samples C7H15-Trp-Au-8, C7H15-Trp-Au-5.3, and C7H15-Trp-Au-4, respectively. These results may indicate the formation of nonspherical Au nanostructures as also reported by many researchers including our group.6,34,35 In fact, we observed multipodshaped morphology of sample C7H15-Trp-Au-4 via TEM analysis, the details of which will be discussed later in this section. Chen’s group, as well as other research groups, has also observed two SPR bands for their pod-like and branched Au nanostructures.14,34,35 However, the sample C7H15-Trp-Au-2.7 (see Table 1) prepared at a lower value of R (=2.7), exhibits a major strong peak at 520 nm accompanied by a very weak peak at 740 nm (Figure 1d), which could be called a shoulder peak. This result also indicates the formation of nonspherical Au nanostructures as reported earlier.34,35 However, it is interesting to note that the spectra of samples C7H15-Trp-Au-2 and C7H15-Trp-Au-1.3, prepared at further lower values of R (R = 2 and 1.3), exhibit only a sharp SPR absorption band, and both of them are centered at 527 nm [Figure 1, curves (e,f)] indicating possibly

Figure 2. TEM images of different Au nanostructure samples prepared using amphiphile, C7H15-Trp, at different R-values: (A) C7H15-TrpAu-8, (B) C7H15-Trp-Au-4, (C) C7H15-Trp-Au-2, and (D) C7H15-Trp-Au-1.3. The images were recorded after 15 min of reaction.

Article

the presence of spherical Au NPs.47,53 Thus, these results clearly reveal that the variation of R-values affected the SPR properties of the formed Au nanostructures. As SPR properties of Au nanostructures are directly related to their shape, therefore, from these results, we may conclude that both spherical and nonspherical Au nanostructures are formed when the amphiphile, C7H15-Trp, is used. Also, one can obtained differently shaped (spherical or nonspherical) Au nanostructures simply by varying the molar ratio (R) of C7H15-Trp to HAuCl4. To ascertain the morphology of the above-mentioned Au nanostructures, four representative C7H15-Trp-Au nanoconjugate samples (C7H15-Trp-Au-8, C7H15-Trp-Au-4, C7H15-Trp-Au-2, and C7H15-Trp-Au-1.3) (see Table 1) were examined through TEM. The TEM images of these samples (see Figure 2) revealed the formation of differently shaped (spherical or nonspherical) Au nanostructures as expected from their UV-vis results. For example, the sample C7H15Trp-Au-8, prepared at R = 8.0, shows the formation of mostly multipod-shaped Au nanostructures along with very few spherical Au NPs (some of them are indicated by the white circle in Figure 2A). One could say that these multipod nanostructures look like nanodendrites. Morphologically, these multipods are different from those reported by earlier researchers.11,14,15,35 However, the diameter of spherical Au NPs that are present in the image is about 5.9 nm (see Figure 2A). From the enlarged view of a single Au multipod (bottom inset in Figure 2A), it seems that these Au branches are formed through the attachment of the initially formed small spherical NPs. Such small NPs can be seen in the image (see Figure 2A). These spherical NPs are the seed particles. These Au multipods had a single crystal structure and were formed through the oriented attachment of small seed NPs along the Æ111æ direction as revealed by HRTEM, the details of which will be discussed later in this section. It should be noted that this result supported the corresponding UV-vis spectrum (see Figure 1a) that showed two distinguishable SPR bands. We also examined the morphology of sample C7H15-TrpAu-4 via TEM, as the UV-vis spectrum is slightly different from that of sample C7H15-Trp-Au-8 but is quite similar to that of sample C7H15-Trp-Au-5.3 (compare Figure 1a-c). TEM image of C7H15-Trp-Au-4 (see Figure 2B) also shows the formation of similar multipods as well as some spherical Au NPs (D ≈ 5.7 nm, as some of them are indicated by circles in Figure 2B). The diameters (maximum possible distance between two pods of single nanostrucutred gold multipod) of multipods are in the range 15-32 nm and are quite polydisperse in nature. However, the population of such spherical NPs is relatively lower than that observed in the case of sample C7H15-Trp-Au-8 (compare Figure 2A and B). The enlarged view of a single multipod (bottom inset of Figure 2B) also indicates similar attachment of smaller Au NPs as also supported by the HRTEM results that will be discussed later. HRTEM results also revealed that these Au branches are also single crystalline in nature. The sample C7H15-Trp-Au-5.3 also shows formation of mainly multipod-like nanostructures (see Figure S1A in the SI). Furthermore, the samples C7H15-Trp-Au-2 and C7H15Trp-Au-1.3 (see Table 1) were also examined by TEM. As mentioned above, their absorption spectra were almost similar to that of only one SPR (527 nm) (see Figure 1e,f). However, the color of the colloidal C7H15-Trp-Au-2 nanoconjugate is violet (see Table 1). The TEM image of the


2425

Figure 3. XRD patterns of different Au nanostructure samples prepared using amphiphiles C7H15-Trp at different R-values: (a) C7H15Trp-Au-8, (b) C7H15-Trp-Au-5.3, (c) C7H15-Trp-Au-4, (d) C7H15Trp-Au-2.7, (e) C7H15-Trp-Au-2, and (f) C7H15-Trp-Au-1.3.

sample C7H15-Trp-Au-2 displays mostly spherical particles (D ≈ 5.6 nm) along with some pod-shaped structures (see Figure 2C). A high-magnification image of one such podshaped nanostructure is presented in the top inset of Figure 2C. This image indicated that two spherical NPs are combined to form a bipod-shaped nanostructure. Note that the sample, C7H15-Trp-Au-2.7, also contains mostly spherical particles along with some pod-shaped Au nanostrucutures (see Figure S1B of the SI). However, the sample C7H15-TrpAu-1.3, prepared at R = 1.3 (see Table 1), showed the presence of only spherical Au NPs (Figure 2D). No pod-like structure was observed in any places of the TEM grid containing this sample. The size distribution of these spherical Au NPs was shown by a histogram (see Figure S2 of the SI). The average diameter of these NPs was measured to be 10.65 ( 2.95 nm. X-ray diffraction (XRD) was performed to further investigate the crystalline nature of these spherical and multipodshaped C7H15-Trp-Au nanoconjugates. The XRD patterns of all six samples of Table 1 are presented in Figure 3. All samples exhibit four peaks with 2θ values of 38.18, 44.6, 64.2, and 77.1 that are assigned to (111), (200), (220), and (311) lattice planes of face-centered cubic (fcc) metallic Au (JCPDF card no.04-0784), respectively. The cell parameter a of one representative Au nanostructure sample is 4.07 A˚ (calculated from the obtained XRD), which is very close to the reported value of 4.08 for fcc gold (JCPDF card no. 04-0784). In the entire XRD patterns, the intensity of the peaks belonging to (200), (220), and (311) planes are weaker than that of the (111) plane. The intensity ratio of the peaks corresponding to the planes {111} and {200} for all these samples are provided in Table S1 in page S6 of the Supporting

2426


Figure 4. UV-vis absorption spectra of as-prepared suspension of different Au nanostructure samples prepared using amphiphile, C5H11-Trp: (a) C5H11-Trp-Au-8, (b) C5H11-Trp-Au-5.3, (c) C5H11Trp-Au-4, (d) C5H11-Trp-Au-2.7, (e) C5H11-Trp-Au-2, and (f) C5H11Trp-Au-1.3. The spectra are recorded after 15 min of reaction.

Information. For multipod samples, these values are very high compared to the standard value of 1.93 for fcc Au (JCPDF file no. 4-0784). For spherical Au NPs, the ratio values are also high compared to the standard value (1.93). These results indicated that the formed multipod and spherical Au nanostructures are bounded by {111} facets. The XRD data of these Au nanostructure samples are also indicative of their crystalline nature. The diameter of the sample C7H15-Trp-Au-1.3 was found to be 12.4 nm (Table 1) as measured by its XRD data by applying the Scherer equation,54 which matched well with that (10.65 ( 2.95 nm) (Table 1) obtained from TEM (Figure 2D) analysis. In order to study the effect of alkyl chain length of fatty acid moiety of the amphiphile on the morphology of the formed Au nanostructure, we have chosen two other amphiphiles that are the conjugates of caproic acid/tryptophan (C5H11-Trp) and capric acid/tryptophan (C9H19-Trp). The alkyl chain length of the former and latter amphiphiles is, respectively, lower and higher than that of amphiphile C7H15-Trp, which is a conjugate of caprylic acid/tryptophan. Au Nanostructures Prepared Using Caproic Acid-Based Amphiphile, (C5H11-Trp). In this case, we have also prepared six different Au nanostructures (see Table 2) using amphiphile C5H11-Trp under the reaction conditions similar to those used for the preparation of Au nanostructures with C7H15-Trp as mentioned in Table 1. Interestingly, the change of color of the reaction mixture was similar to that observed in the case of Au nanostructures synthesized with C7H15Trp. The UV-vis spectra of the as-prepared suspension of all the samples in Table 2 are shown in Figure 4. Surprisingly, the natures of spectral patterns are quite different from those observed for C7H15-Trp-Au nanoconjugate samples (compare Figure 4 and Figure 1). For example, the spectra of samples C5H11-Trp-Au-8, C5H11-Trp-Au-5.3, C5H11Trp-Au-4, and C5H11-Trp-Au-2.7 exhibited only one broad SPR band at 577, 563, 562, and 567 nm, respectively (Figure 4a-d), whereas the corresponding C7H15-Trp-Au nanoconjugate

Dinda et al.

samples showed two SPR band as mentioned above (compare Figure 4 and Figure 1). The exhibition of such a broad SPR band may be attributed to the formation of Au nanostructures of either nonspherical morphology or spherical particles with high polydispersity or aggregated spherical particles.35,55 Although, in our case, the TEM image of some of these representative samples showed the formation of nonspherical Au nanostructures, the details of morphology will be discussed later in this section. On the other hand, samples C5H11-Trp-Au-2 and C5H11-Trp-Au-1.3 show only one sharp SPR band, and both are centered at 533 nm (see Figure 4e,f). However, these SPR bands are comparatively sharper than those of samples C5H11-Trp-Au-8, C5H11-TrpAu-5.3, C5H11-Trp-Au-4, and C5H11-Trp-Au-2.7. Figure 4 also clearly demonstrates that, as the R-values decrease, the SPR band of the obtained samples become sharper. We also noticed a similar trend in the spectra of Au nanostructure samples prepared with C7H15-Trp at different R-values. The spectral results of samples C5H11-Trp-Au-2 and C5H11-TrpAu-1.3 might indicate the formation of spherical NPs with low polydispersity. Figure 5A-D represents the typical TEM images of four representative samples, C5H11-Trp-Au-8, C5H11-Trp-Au-4, C5H11-Trp-Au-2, and C5H11-Trp-Au-1.3 (see Table 2). The sample C5H11-Trp-Au-8 again showed the formation of multipod-like Au nanostructures (Figure 5A), but the pods are not as prominent as that of sample C7H11-Trp-Au-8. Also, in this case, the Au branches contain a smaller number of pods. However, the branching in these pod-like Au nanostructures is very clear from the magnified image provided in the bottom inset of Figure 5A. We also observed some spherical Au NPs (as indicated by circles in Figure 5A) along with multipod-like nanostructures, as in the case of C7H15-Trp-Au-8 (see Figure 2A). The sample C5H11-TrpAu-4, prepared at a value of R=4, again shows the presence of similar pod-shaped nanostructures with few spherical NPs (indicated by circle in Figure 5B) as observed for sample C5H11-Trp-Au-8 (see Figure 5A). It seems that the podshaped nanostructures of samples C5H11-Trp-Au-8 and C5H11-Trp-Au-4 are also formed by the attachment of small spherical NPs, as revealed by high-magnification images shown in the bottom insets of Figure 5A and B, respectively. The HRTEM image of one such structure of sample C5H11Trp-Au-4 also revealed that it had a single crystal structure, mostly bounded by the {111} plane, as will also be discussed in more detail later. Furthermore, the presence of these nonspherical pod-like nanostructures might be responsible for the exhibition of such a broad SPR band by the corresponding samples as shown above in Figure 4a and c. Interestingly, the obtained samples, C5H11-Trp-Au-2 and C5H11-Trp-Au-1.3, at lower R values (see Table 2) showed the presence of mostly spherical NPs (see Figure 5C,D). However, the sample C5H11-Trp-Au-2 (Figure 5C) shows very few aggregated bipod- and tripod-like nanostructures (as indicated by a circle). The high-magnification image of one such nanostructure again clearly revealed that this nanostructure is formed by the attachment of small Au NPs. The average diameters of the spherical C5H11-TrpAu-2 and C5H11-Trp-Au-1.3 conjugates are 4.7 ( 1.5 and 10.82 ( 1.5 nm, respectively, as measured from TEM analysis (see Table 2). The histograms of particle size distributions of these two samples (C5H11-Trp-Au-2 and C5H11-Trp-Au-1.3) are depicted in Figure S3A,B of the SI, respectively. These spherical Au NPs are responsible for

Article


2427

Figure 5. TEM images of different Au nanostructure samples prepared using amphiphile, C5H11-Trp, at different R-values: (A) C5H11-TrpAu-8, (B) C5H11-Trp-Au-4, (C) C5H11-Trp-Au-2, and (D) C5H11-Trp-Au-1.3. The images were recorded after 15 min of reaction.

generation of a single strong SPR band (λmax =533 nm) as shown above in Figure 4e,f. XRD patterns of these C5H11-Trp-Au nanoconjugates (Figure 6) show all the characteristic diffraction peaks centered at 2θ=38.18, 44.6, 64.6, and 77.1 corresponding to (111), (200), (220), and (311) lattice planes, respectively, of fcc metallic gold (JCPDF card no. 04-0784). The XRD data revealed the crystalline nature of the formed Au nanostructures. It is also clear that the peak due to the (111) plane is the most intense among the four observed peaks, and these results match well with the XRD results of Au nanostructures prepared using C7H15-Trp. The intensity ratios of the peaks corresponding to the planes {111} and {200} for all these nanoconjugate samples are provided in Table S2 in page S6 of the SI. These results again indicate that these Au nanostructures are also bounded by {111} facets. As the samples C5H11-Trp-Au-2 and C5H11-Trp-Au-1.3 show spherical morphology, their sizes can be measured from XRD data applying the Scherer equation54 and are found to be 6.5 and 7.6 nm, respectively. These diameters match well with those obtained from via TEM measurement (see Table 2). Au Nanoparticles Prepared Using Capric Acid-Based Amphiphile, C9H19-Trp. Table 3 describes six different Au NP samples prepared using capric acid-based amphiphiles, C9H19-Trp, in order to understand further the role of alkyl chain associated with the amphiphiles on their morphology. The reaction conditions for these samples are similar to those used for samples prepared using two other amphiphiles, C7H15-Trp and C5H11-Trp (see Tables 1-3). It should be noted that, in this case, all the formed Au NP colloids were

Figure 6. XRD patterns of different Au nanostructure samples prepared with amphiphile, C5H11-Trp, at different R-values: (a) C5H11Trp-Au-8, (b) C5H11-Trp-Au-5.3, (c) C5H11-Trp-Au-4, (d) C5H11Trp-Au-2.7, (e) C5H11-Trp-Au-2, and (f) C5H11-Trp-Au-1.3.

red in color and are not dependent on the value of R used, indicating the formation of only spherical Au NPs. However, as mentioned above, the color of the Au nanostructure samples prepared using amphiphiles C7H15-Trp and C5H11-Trp were indeed dependent on the value of R used

2428


Dinda et al.

(see Tables 1 and 2). The red-colored colloids were observed only when the Au NPs were prepared only at lower values of R (see samples C7H15-Trp-Au-1.3, C5H11-Trp-Au-1.3). The UV-vis spectra of all these Au NP samples in their colloidal form exhibit a single SPR band (Figure 7). The λmax values (524-526 nm) of these SPR peaks are almost independent of the type of Au NP samples that are prepared using different values of R (see Table 3). The SPR band can be attributed to the formation of spherical Au NPs of smaller sizes.53 Thus, we do not expect the formation of Au NPs of any morphology other than spherical due to this variation of R-values. Therefore, we only examine the morphology of a few representative samples via TEM. As expected, the TEM image clearly shows spherical Au NPs (Figure 8) with average diameters of 4.9 ( 0.7 (Figure 8A), 3.7 ( 0.7 (Figure 8B), and 5.5 ( 2.9 nm (Figure 8C) for samples C9H19-Trp-Au-8, C9H19-Trp-Au-4, and C9H19-Trp-Au-1.3, respectively. Enlarged views of particles of each sample also revealed that the particles are indeed spherical in nature (see bottom insets of panels A-C of Figure 8). The histogram of particle size distribution of these Au NPs samples were depicted in Figure S4 of the SI.

XRD patterns of all the C9H19-Trp-Au conjugate NPs show four broad diffraction peaks centered at 2θ =38.18, 44.6, 64.2, and 77.1 (Figure 9) that are assigned to (111), (200), (220), and (311) lattice planes, respectively, of fcc Au (JCPDF card no. 04-0784). The intensity ratios of peaks belonging to the {111} and {200} planes of these samples revealed the formation of crystalline Au NPs, which are bounded by the {111} plane (see Table S3 in page S7 in the SI). Applying the Scherer equation,54 we again calculated the size of these samples from the XRD data (see Table 3). The results show that the average diameter of these Au NPs prepared at varying molar ratio (R) of C9H19-Trp to HAuCl4 are in the range 4-6 nm. In general, for these samples, these diameters as determined from XRD matched well with that obtained from the TEM measurement (see Table 3). FTIR Spectroscopy Study. To confirm the adsorption of amphiphiles on the surface of the formed Au NPs, FTIR characterization was performed on a representative sample C7H15-Trp-Au-1.3 after purification (centrifugation, washing, and drying) (see Figure S5a in SI). For comparison, we also recorded the FTIR spectra of neat amphiphiles, C7H15Trp, which are shown in Figure S5b in SI. The positions of the entire characteristic peaks assigned to neat amphiphiles

Figure 7. UV-vis absorption spectra of as-prepared suspensions of different Au NPs prepared with amphiphile, C9H19-Trp: (a) C9H19Trp-Au-8, (b) C9H19-Trp-Au-5.3, (c) C9H19-Trp-Au-4, (d) C9H19Trp-Au-2.7, (e) C9H19-Trp-Au-2, and (f) C9H19-Trp-Au-1.3. The spectra were recorded after 15 min of reaction.

Figure 9. XRD patterns of different Au NP samples prepared with amphiphile, C9H19-Trp, at different R-values: (a) C9H19-Trp-Au-8, (b) C9H19-Trp-Au-5.3, (c) C9H19-Trp-Au-4, (d) C9H19-Trp-Au-2.7, (e) C9H19-Trp-Au-2, and (f) C9H19-Trp-Au-1.3.

Figure 8. TEM images of representative samples prepared with C9H19-Trp: (A) C9H19-Trp-Au-8, (B) C9H19-Trp-Au-4, and (C) C9H19-TrpAu-1.3. The images were recorded after 15 min of reaction.

Article


2429

Table 4. FTIR Peak Assignment of the Amphiphile, C7H15-Trp and C7H15-Trp-Au-1.3 Nanoconjugate samples

>CdO

-COO-

-C(NH)dO

>CH2

Indole NH

C7H15-Trp-Au-1.3 C7H15-Trp

1710

1407 -

1631 1620

2921, 2854 2930, 2850

3433 3320, 3410

Scheme 2

are very close to those of the amphiphile-Au nanocomposites as summarized in Table 4. FTIR data confirmed the anchoring of amphiphiles on the surface of Au NPs. In our report, we have also used FTIR spectroscopy to confirm the adsorption of amphiphile of Au NP surfaces.23 Thermogravimetric Analysis. Further, to examine to what extent the amphiphiles are adsorbed on the surface of Au NPs, thermogravimetric analysis (TGA) was performed on some of the representative nanoconjugate samples (C7H15Trp-Au-5.3, C7H15-Trp-Au-2.7, and C7H15-Trp-Au-1.3), as well as on neat amphiphiles (C7H15-Trp) (see Figure S6 in SI). The TGA thermogram of the neat amphiphile (C7H15-Trp) shows that the decomposition starts at 227 C and continues up to 450 C and a weight loss of 83% was registered between 150 and 500 C (Figure S6a in SI), whereas incorporation of Au NPs decreases the decomposition temperature to 150 C of the amphiphile (C7H15-Trp) in the nanoconjugate C7H15-Trp-Au and a significant weight loss of ∼20% was recorded for the sample C7H15-Trp-Au-1.3 in the same temperature range (Figure S6b in SI), and whereas the weight losses registered for the samples C7H15-Trp-Au-2.7 and C7H15-Trp-Au-5.3 were ∼8% and ∼12%, respectively, in the same temperature range (Figure S6c and Figure S6d in SI). From these weight loss values, the calculated amounts of amphiphiles present on nanoconjugate samples, C7H15Trp-Au-1.3, C7H15-Trp-Au-2.7, and C7H15-Trp-Au-5.3, were ∼27%, ∼10%, and ∼15%, respectively. It is very clear from these results that the amounts of amphiphile adsorption do not follow any trend. However, we do not know the exact reason behind this discrepancy, but these results confirmed the adsorption of amphiphiles on the surface of Au NPs. Proposed Mechanism for the Formation of Spherical and Multipod-Shaped Au NPs. The above-mentioned results showed that the amphiphiles, C5H11-Trp and C7H15-Trp, both produce Au nanostructures of both spherical and multipod-shaped morphology depending on the values of molar ratio (R) of amphiphile to gold salt used, whereas the amphiphile C9H19-Trp produced only spherical Au NPs no matter what the value of R is. The above-mentioned TEM results also indicated that Au multipods were formed by the attachment of initially formed small spherical Au seeds. The formation of such seed particles, in this case, is explained on the basis of our earlier reports.23,30,47 The tryptophan moiety

of the amphiphile reduces Au(III) to metallic Au atoms that eventually combine to form Au clusters. The adsorption of amphiphile on the formed Au surface occurs through the interaction of the indole moiety of tryptophan residue present in the amphiphile (see Scheme 2). Note that the adsorption of amphiphile onto Au NP surfaces was confirmed via FTIR spectroscopy and TGA analysis as described above (SI Figures S5 and S6). Further, to investigate the mechanism of formation of these Au nanostructures, we studied their kinetics of formation via UV-vis spectroscopy as well as TEM. The timedependent absorption spectra (see Figure 10A) of C7H15Trp-Au-8 (a representative sample with multipod-shaped morphology) were acquired during their formation. The spectra of sample C7H15-Trp-Au-8, acquired after 1.4 and 32 s, show the generation of a weak, broad absorption band centered at 525 nm (see Figure 10Aa and Ab, respectively), which is the characteristic SPR peak of only spherical Au NPs.51 We could assume these particles as primary seed NPs, but at this early stage, we are unable to take the TEM image, because we were not able to freeze the reaction at this stage, as the reaction is very fast. However, the spectrum taken after 1 min of reaction exhibits a small hump at 740 nm along with the peak at 525 nm (Figure 10Ac). This might be an indication of formation of Au NPs of nonspherical morphology. The time-dependent absorption spectra of sample C7H15-Trp-Au-4 (see Figure 10B) also show similar results to those observed for sample C7H15Trp-Au-8 (Figure 10A). At this very early stage (1.4 s) of reaction, a broad SPR absorption band was observed at 523 nm (see Figure 10Ba), but the spectrum of the final reaction mixture after 15 min of reaction showed both longer- and shorter-wavelength SPR absorption peaks (Figure 1c). This might be due to growing multipod-shaped gold nanostructures. The position of this second peak redshifted with time and appeared at 817 nm after 15 min of reaction as mentioned above in Figure 1c. Again, the kinetics of formation of Au nanostructures of samples C7H15-Trp-Au-8 and C7H15-Trp-Au-4 were studied by measuring the changes in absorbance values of the SPR band maxima at 525 and 750 nm of Au NPs as a function of time (see Figure S7A and Figure S7B, respectively, in Page S11 of the SI). These plots clearly indicate that the rate of reaction is indeed very fast at early stages, but the reaction

2430


Dinda et al.

Figure 10. (A) Evolution of absorption spectra during the formation of Au nanostructures for the sample C7H15-Trp-Au-8. Spectra were recorded after (a) 1.4, (b) 32, (c) 62, (d) 92, (e) 122, (f) 152, (g) 272, (h) 422, and (i) 572 s of reaction. (B) Evolution of absorption spectra during the formation of Au nanostructures for the sample C7H15-Trp-Au-4. Spectra were recorded after (a) 1.4, (b) 22, (c) 47, (d) 72, (e) 97, (f) 122, (g) 147, (h) 272, and (i) 497 s of reaction.

Scheme 3

completed almost within 15 min of time. This is the reason we recorded the final spectra (Figure 1a,c) and TEM (Figure 2A,B) after 15 min of time. These results may indicate the formation of small, spherical Au NPs (seeds) at a very early stage of reaction that eventually assembled at a very fast rate into pod-shaped Au nanostructures, and the process attains saturation almost after 15 min. As a result, we ended up with mostly multipod-shaped nanostructures along with some spherical NPs, as some of them can be seen in the TEM image of samples C7H15-Trp-Au-8 (Figure 2A) and C7H15-Trp-Au-4 (Figure 2B). Thus, one can easily say that the used amphiphiles must play an important role during the formation of these anisotropic branched Au

nanostructures from the initially formed seed particles as also reported by earlier researchers.56,57 To provide further light on this issue, we recorded the TEM image of the sample C7H15-Trp-Au-4 taken out after 1 min of reaction. The sample, at this stage, contained spherical Au NPs (D ≈ 5 nm) along with some pod-shaped Au nanostructures (see Figure S8 of the SI). The bottom inset of Figure S8 (SI) shows the high-magnification image of one such pod-shaped Au NP, which clearly indicates that this was indeed formed by the fusion of small Au particles even at this early stage. We believe that the formation of this type of metal nanostructure is a result of assembly of small, spherical NPs,

Article


2431

Figure 12. (A) HRTEM images of the sample C7H15-Trp-Au-4 taken after 1 min of reaction. (B) Magnified HRTEM images of a portion showing the twin structure formation.

Figure 11. HRTEM images of a single multipod structure for the samples: (A) C7H15-Trp-Au-8, (B) C7H15-Trp-Au-4, and (C) C5H11Trp-Au-4. In all cases, the measured interplanar spacing was 0.23 nm indicating that preferential growth occurred along Æ111æ directions. These images were recorded after 15 min of reaction. (D-F) Magnified HRTEM images taken from a specified portion of the multipod in panels A, B, and C, respectively, showing the formation of twin structures of the respective sample.

which has also been reported by many research groups.16,24,33,58 The formation of the Au multipods with the amphiphiles C7H15-Trp/C5H11-Trp is actually attributed to the oriented attachment mechanism operating between the initially formed spherical Au NPs seed (see Scheme 3). It is known that the stability of individual facets of fcc geometry decreases in the order {111} > {100} > {110} according to their increasing surface energy.59 Thus, in this case, the amphiphile (C7H15-Trp/C5H11-Trp) molecules would preferentially adsorb more onto {110}/{100} facets of the initially formed Au NPs and stabilize the said planes (see case-1 of Scheme 3).24,59,60 The FTIR and TGA results, as mentioned above, clearly show the adsorption of amphiphile on the Au surface. Therefore, during the crystal growth, the low-energy {111} plane of initially formed spherical Au seed is more favorable for accommodating other Au seed than the seeds with highenergy {110}/{100} planes (see case-1 of Scheme 3), which resulted in the formation of multipod-shaped Au branches. The HRTEM image of single multipod-shaped NPs of sample C7H15-Trp-Au-8 (see Figure 11A) clearly shows the perfectly aligned lattice fringes (indicated by white solid line) with an interplanar spacing of 0.23 nm corresponding to the {111} plane of metallic gold. The HRTEM images of the

multipod-shaped Au branches obtained from the samples C7H15-Trp-Au-4 (see Figure 11B) and C5H11-Trp-Au-4 (see Figure 11C) also showed that the branches contained aligned lattice fringes and were predominantly bounded by {111} facets. The HRTEM image of a Au bipod of sample C7H15-TrpAu-4 obtained after 1 min of reaction also shows the clear lattice fringes that are aligned along the Æ111æ direction (Figure 12A). Thus, these results clearly revealed that branches of the Au multipod grew along the {111} direction as also indicated by XRD results mentioned above. This type of aligned lattice fringe also indicated that the Au branches are formed through the oriented attachment of the initially formed spherical Au nanocrystallites as indicated by highmagnification images of multipod-shaped Au NPs mentioned above. In high-magnification HRTEM images of all these samples (see Figure 11D-F), we found the formation of a twin boundary at the interface that is evidence of formation of branched nanostructures through oriented attachment.61,62 Again, such twin structures were also found in the branched Au nanostructures formed at the earlier stage (after 1 min) of reaction (Figure 12B). Earlier, we had reported this type of growth in the case of formation of dendritic Ag nanostructures.33 Furthermore, the spectral evolution of a representative sample C7H15-Trp-Au-1.3 showed the presence of only one SPR peak centered at 520 nm, but no second SPR peak at higher wavelength at any stage of reaction (Figure 13A). This indicates the formation of only spherical Au NPs and no pod-like nanostructures at any stage of reaction. As mentioned above, TEM results of this sample and the other samples that are prepared at 1.3 e R < 2 with amphiphiles (C7H15-Trp/C5H11-Trp) showed the presence of only spherical Au NPs (see Figure 2D and Figure 5C,D). For these samples, the initial rate of nuclei formation is very high, as the concentration of HAuCl4 is large and comparable to the concentration of amphiphiles (Table 1 and Table 2). The kinetics of formation of the spherical Au NPs clearly indicates that the rate is indeed very fast (Figure 13B). Due to the production of a higher amount of primary spherical nanocrystallites from such a fast reduction reaction, many of the planes of the initially formed Au nanocrystals remain uncapped by the amphiphile molecule and vulnerable to further growth in all directions.63 Therefore, one should not expect growth in any one particular plane. As a result, solely spherical Au NPs were obtained as shown schematically in case-II of Scheme 3. Note that the HRTEM images of spherical Au NPs of the samples

2432


Dinda et al.

Figure 13. (A) Evolution of absorption spectra during the formation of Au NPs (sample C7H15-Trp-Au-1.3). Spectra were recorded after (a) 1.4, (b) 15, (c) 30, (d) 45, (e) 65, (f) 80, (g) 130, and (h) 180 s of reaction. (B) Plots showing the variation of absorbance of SPR bands at 525 and 800 nm for sample C7H15-Trp-Au-1.3 with time.

C5H11-Trp-Au-1.3 (see Figure S9A in SI) and C7H15-TrpAu-1.3 (see Figure S9B of the SI) also reveal its single crystalline nature with lattice spacing of 0.23 nm that corresponds to the {111} plane of fcc metallic gold. Again, the above-mentioned TEM results showed that the amphiphile C9H19-Trp produces only spherical Au NPs (Figure 8) no matter what the value of R is. In this case, the formation chemistry of initial spherical Au NP seeds is similar to that of the other two amphiphiles (C7H15-Trp and C5H11-Trp), since all the amphiphiles have similar a headgroup that contains the tryptophan moiety. However, the growth mechanism is probably different, since the amphiphile C9H19-Trp has a longer hydrophobic tail compared to the other two amphiphiles (C7H15-Trp and C5H11Trp). It is known that a longer hydrophobic chain faced stronger expulsion from the aqueous phase than a shorter one would. So, they possess a greater potential for interfacial adsorption.64-66 Because of this phenomenon, C9H19-Trp molecules have the ability to bind tightly and compactly not only to higher-energy low-density {100} and {110} planes, but also to some extent on {111} planes (see case-III of Scheme 3). This resulted in the formation of mainly spherical Au NPs, but not any other anisotropic morphology. This is probably the reason we obtained spherical Au NPs for amphiphile C9H19-Trp no matter what the value of R is. The HRTEM image of one such spherical Au NPs for the sample (samples C9H19-Trp-8, C9H19-Trp-4, and C9H19-Trp-1.3) clearly revealed that it had a single crystal structure and was bounded by {111} facets (see Figure S10 in SI).

predominant effect of the molar ratio (R) ([amphiphile]/ [HAuCl4]), as well as the nature of amphiphiles on the morphology of the formed Au nanostructures. The slight change in the chain length associated with the amphiphiles can produce differently shaped gold nanostructures, from multipod to spherical; e.g., the amphiphiles C5H11-Trp and C7H15-Trp produced mostly multipod-shaped Au nanostrucutres at R-values in the range 4-8, whereas these two amphiphiles produced only spherical Au NPs at the value of R in the range 1.3-2. However, interestingly, we found that the amphiphile C9H19-Trp generated only spherical Au NPs with very low dimensions (3.6-6.2 nm), no matter what the value of R is. XRD analysis indicated that the formed Au nanostructures are highly crystalline and are bounded by {111} facets. HRTEM analysis and time-dependent UV-vis spectral study indicated that the multipod Au nanostructures might be formed by the assembly of smaller-sized spherical seed nanoparticles via oriented attachment mechanism. The HRTEM results also revealed that these Au nanostructures grew along the Æ111æ direction. The mechanisms of formation of Au nanostructures of all obtained shapes were explained schematically on the basis of TEM and UV-vis spectral results. Acknowledgment. E. D. and M. H. R. thank Council of Scientific and Industrial Research (India) for financial support. We thank the reviewer who made constructive criticism and suggested additional experimentation which were duly performed as described in the text. This research, in part, was supported by the grants from CSIR, India. Thanks are also due to the financial support from the DST, New Delhi, under the Nanoscience and Nanotechnology Initiative.

Conclusions In conclusion, we prepared three different newly designed redox active amphiphiles that are conjugates of fatty acids such as caprylic, caproic, and capric acid and an amino acid such as tryptophan. These amphiphiles were successfully utilized to prepare spherical and multipod-shaped Au nanostructures at ambient temperature by the in situ reduction of HAuCl4 in aqueous alkaline condition without any additional template. It was further shown that there was a

Supporting Information Available: Synthesis and characterization via NMR, ESI-mass, and elemental analysis of methyl ester of amphiphiles and amphiphiles, the histogram of particle size distribution of spherical nanoparticles obtained for different samples, kinetic plot of absorbance vs time, HRTEM images of spherical particles, FTIR spectra, TGA thermograms, TEM images of Au NPs formed after 1 min, tables describing the intensity ratios of corresponding the planes {111} and {200} from XRD data. This material is available free of charge via the Internet at http://pubs. acs.org.

Article


References (1) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. Adv. Mater. 2001, 13, 1501. (2) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141. (3) Kanaoka, S.; Yagi, N.; Fukuyama, Y.; Aoshima, S.; Tsunoyama, H.; Tsukuda, T.; Sakurai, H. J. Am. Chem. Soc. 2007, 129, 12060. (4) Rashid, M. H.; Mandal, T. K. Adv. Funct. Mater. 2008, 18, 2261. (5) Kwon, K.; Lee, K. Y.; Lee, Y. W.; Kim, M.; Heo, J.; Ahn, S. J.; Han, S. W. J. Phys. Chem. C 2007, 111, 1161. (6) Rashid, M. H.; Bhattacharjee, R. R.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 9684. (7) Li, C. Z.; Male, K. B.; Hrapovic, S.; Luong, J. H. T. Chem. Commun. 2005, 3924. (8) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (9) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (10) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (11) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (12) Kuo, C. H.; Huang, M. H. Langmuir 2005, 21, 2012. (13) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B 1998, 102, 9985. (14) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (15) Chen, H. M.; Hsin, C. F.; Liu, R. S.; Lee, J. F.; Jang, L. Y. J. Phys. Chem. C 2007, 111, 5909. (16) Lim, B.; Camargo, P. H. C.; Xia, Y. Langmuir 2008, 24, 10437. (17) Pardinas-Blanco, I.; Hoppe, C. E.; Pieiro-Redondo, Y.; LpezQuintela, M. A.; Rivas, J. Langmuir 2008, 24, 983. (18) Shankar, S. S.; Rai, A.; Ankamwar, B.; Sing, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482. (19) Shao, Y.; Jin, Y.; Dong, S. Chem. Commun. 2004, 1104. (20) Bhattacharjee, R. R.; Chakraborty, M.; Mandal, T. K. J. Nanosci. Nanotechnol. 2004, 4, 844. (21) Bhattacharjee, R. R.; Rashid, M. H.; Mandal, T. K. J. Nanosci. Nanotechnol. 2008, 8, 3610. (22) Dinda, E.; Si, S.; Kotal, A.; Mandal, T. K. Chem.;Eur. J. 2008, 14, 5528. (23) Si, S.; Dinda, E.; Mandal, T. K. Chem.;Eur. J. 2007, 13, 9850. (24) Bakshi, M. S. Langmuir 2009, 25, 12697. (25) Bakshi, M. S.; Sachar, S.; Kaur, G.; Bhandari, P.; Kaur, G.; Biesinger, M. C.; Possmayer, F.; Petersen, N. O. Cryst. Growth Des. 2008, 8, 1713. (26) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633. (27) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (28) Vasilev, K.; Zhu, T.; Wilms, M.; Gillies, G.; Lieberwirth, I.; Mittler, S.; Knoll, W.; Kreiter, M. Langmuir 2005, 21, 12399. (29) Chen, J.; J. Wiley, B. J.; Xia, Y. Langmuir 2007, 23, 4120. (30) Si, S.; Dinda, E.; Mandal, T. K. J. Nanosci. Nanotechnol. 2008, 8, 5934. (31) Elnathan, R.; Kantaev, R.; Patolsky, F. Nano Lett. 2008, 8, 3964. (32) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (33) Rashid, M. H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750. (34) Wu, H. Y.; Liu, M.; Huang, M. H. J. Phys. Chem. B 2006, 110, 19291.

2433

(35) Xiao, Y.; Shlyahovsky, B.; Popov, I.; Pavlov, V.; Willner, I. Langmuir 2005, 21, 5659. (36) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391. (37) Palui, G.; Ray, S.; Banerjee, A. J. Mater. Chem. 2009, 19, 3457. (38) Li, Z.; Liu, Z.; Zhang, J.; Han, B.; Du, J.; Gao, Y.; Jiang, T. J. Phys. Chem. B 2005, 109, 14445. (39) Lu, L.; Ai, K.; Ozaki, Y. Langmuir 2008, 24, 1058. (40) Xie, J.; Lee, J. Y.; Wang, D. I. C. J. Phys. Chem. C 2007, 111, 10226. (41) Basu, N.; Bhattacharya, R.; Mukherjee, P. Biomed. Mater. 2008, 3, 034105. (42) Housni, A.; Ahmed, M.; Liu, S.; Narain, R. J. Phys. Chem. C 2008, 112, 12282. (43) Murawala, P.; Phadnis, S. M.; Bhonde, R. R.; Prasad, B. L. V. Colloids Surf., B 2009, 73, 224. (44) Selvakannan, P.; Swami, A.; Srisathiyanarayanan, D.; Shirude, P. S.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2004, 20, 7825. (45) Zhu, J.; Shen, Y.; Xie, A.; Qiu, L.; Zhang, Q.; Zhang, S. J. Phys. Chem. C 2007, 111, 7629. (46) Si, S.; Bhattacharjee, R. R.; Banerjee, A.; Mandal, T. K. Chem.; Eur. J. 2006, 12, 1256. (47) Si, S.; Mandal, T. K. Chem.;Eur. J. 2007, 13, 3160. (48) Slocik, J. M.; Naik, R. R.; Stone, M. O.; Wright, D. W. J. Mater. Chem. 2005, 15, 749. (49) Slocik, J. M.; Stone, M. O.; Naik, R. R. Small 2005, 1, 1048. (50) Bodanszky, M.; Bodanszky, A. The Practice of Peptide Synthesis, 2nd ed.; Springer-Verlag: Berlin, 1994. (51) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (52) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Chem. Mater. 2005, 17, 566. (53) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (54) Nuffield, E. W. X-ray diffraction methods; Wiley: New York, 1966. (55) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (56) Viswanath, B.; Kundu, P.; Halder, A.; Ravishankar, N. J. Phys. Chem. C 2009, 113, 16866. (57) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (58) Wang, L.; Yamauchi, Y. Chem. Mater. 2009, 21, 3562. (59) Xiang, Y.; Wu, X.; Liu, D.; Jiang, X.; Chu, W.; Li, Z.; Ma, Y.; Zhou, W.; Xie, S. Nano Lett. 2006, 6, 2290. (60) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 5417. (61) Lim, B.; Jiang, M.; Yu, T.; Camargo, P. H. C.; Xia, Y. Nano Res. 2010, 3, 69. (62) Lu, L.; Kobayashi, A.; Kikkawa, Y.; Tawa, K.; Ozaki, Y. J. Phys. Chem. B 2006, 110, 23234. (63) Bakshi, M. S.; Possmayer, F.; Petersen, N. O. J. Phys. Chem. C 2007, 111, 14113. (64) Li, F.; Rosen, M. J.; Sulthana, S. B. Langmuir 2001, 17, 1037. (65) Matsuoka, K.; Yoshimura, T.; Shikimoto, T.; Hamada, J.; Yamawaki, M.; Honda, C.; Endo, K. Langmuir 2007, 23, 10990. (66) Yoshimura, T.; Ohno, A.; Esumi, K. Langmuir 2006, 22, 4643.

Amino Acid-Based Redox Active Amphiphiles to In Situ Synthesize

Recommend Documents