Ligand-Directed Formation of Gold Tetrapod Nanostructures

Article pubs.acs.org/JPCC

Ligand-Directed Formation of Gold Tetrapod Nanostructures Haining Liu,† Yaolin Xu,† Ying Qin,‡ Wesley Sanderson,† Dorothy Crowley,† C. Heath Turner,*,† and Yuping Bao*,† †

Department of Chemical and Biological Engineering and ‡Alabama Innovation and Mentoring of Entrepreneurs, The University of Alabama, Tuscaloosa, Alabama 35487, United States ABSTRACT: Branched gold nanoparticles are synthesized via a soft-template-directed process using a biological buffer, 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES). These branched Au nanoparticles are mainly tetrapods and show distinct absorption in the range of 700−1000 nm. A combined experimental and computational study suggests that at high concentration, the HEPES molecules self-assemble into structures with long-range order serving as soft templates to direct the formation of the anisotropic gold nanoparticles. Detailed analyses of surface chemistry and structure indicate the formation of a molecular bilayer structure for the stabilization of the branched Au nanostructures. Our densityfunctional theory (DFT) calculations predict that the sulfonate group of the HEPES molecules prefers to bind to the Au surfaces, while the free hydroxyl groups facilitate the self-assembly and bilayer formation through the formation of hydrogen bonds. By comparing three different buffer molecules, our study demonstrates the critical importance of ligand chemistry in the directed formation of anisotropic metallic nanoparticles. lamonium bromide (CTAB) via overgrowth conditions;25 (b) branched nanopods from the reduction of HAuCl4 in large excess of 4-dimethylaminopyridine;9 (c) Au multipods formed by reducing HAuCl4 with L-ascorbic acid in the presence of CTAB and silver seeds;26 and (d) a mixture of branched Au nanoparticles synthesized in Good’s buffers.27 In addition to these experimental studies, computational methods have previously been employed to understand the formation details and growth mechanisms of spherical Au nanoparticles.28−31 For example, the fundamental interactions between gold nanoparticles and formyloxyl radicals have been studied using density functional theory (DFT) methods in order to better understand the role of citrate in gold nanoparticle synthesis.28 In addition, binding-energy calculations using DFT methods have shed light on the formation of gold nanoparticles in fluorine-containing ionic liquids.29 Furthermore, the growth mechanism of a 3-thiopheneacetic acid-capped nanoparticle was investigated by performing DFT calculations and molecular dynamics simulations.31 Here, our calculations are intended to capture the features responsible for anisotropic nanoparticle growth. Besides the NIR absorption, the biocompatibility of the surface-capping molecules of the Au nanoparticles is also critical for in vivo cancer therapy. For example, toxicity concerns have been raised about CTAB,32,33 which has been the primary surface coating for Au nanorods. Therefore, it is

1. INTRODUCTION Anisotropic gold (Au) nanoparticles have shown great potential in various applications, such as shape-dependent sensing1 and catalysis.2,3 Driven by the potential applications, many anisotropic Au nanoparticles have been reported,4 including nanorods,5 nanowires,6 nanostars,7 nanoprism,8 and branched nanoparticles.9−11 The shape control of Au nanoparticles allows one to tune the surface plasmon resonance over a wide range.12 In addition to sensing and catalytic applications, Au nanoparticles have shown great promise in photothermal cancer therapy (i.e., thermal destruction of cancer cells),13 where the light absorbed by the Au nanoparticles is quickly and efficiently converted to localized heat.13 The efficiency of the heat generation in vivo by Au nanoparticles strongly depends on the absorption regions of the Au nanostructures because lights have different tissue penetration depth depending on the wavelengths.14 In general, Au nanostructures with absorption in the range of 700−1100 nm are preferred because light in the nearinfrared region (NIR) can penetrate soft tissue better. Several different shapes of Au nanoparticles have been previously studied for photothermal cancer therapies,13 such as Au nanoshells,15−19 nanocages,20 nanorods,21 nanocrosses,22 and nanohexapods.23 A direct comparison of branched Au nanostructures with Au nanocages and Au nanorods suggests that branched Au nanostructures could be more effective in photothermal conversion24 and photothermal therapy.23 Several synthetic approaches have been reported for generating branched Au nanoparticles, including (a) the formation of planar multiarmed Au nanostructures by the reduction of HAuCl4 with L-ascorbic acid in the presence of cetyltrimethy© XXXX American Chemical Society

Received: June 13, 2013 Revised: July 30, 2013

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dx.doi.org/10.1021/jp405859x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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slab model calculations. For the Au20 cluster model, the geometry optimization and frequency calculations were performed using the M06-2X functional.37 The LANL2DZ basis set was used for Au, while the 6-31+G(d,p) basis set was used for the rest of the atoms. At this level of theory, the calculated frequencies of the complex, in which we are interested, were found to agree very well with the experiment. To obtain the binding energies of the HEPES and EPPS dimers, geometry optimizations were performed using the polarizable continuum model (PCM)38 with a dielectric constant (ε) of 78.36 to model the aqueous solution. Again, the M06-2X functional in conjunction with the 6-31+G(d,p) basis set was used. All of the reported energies in this paper include zero-point vibrational energy (ZPVE) corrections, which were obtained from the frequency calculations. The projector-augmented wave (PAW)39,40 method in conjunction with the generalized gradient approximation of the Perdew−Burke−Ernzerhof (PBE)41 functional was used for the slab calculations. The energy cutoff for the plane-wave basis set was set to 400 eV. The Au(111) surface was modeled as a (6 × 4) supercell and was composed of three layers. A 30 Å vacuum slab was created in the z direction, which is large enough to prevent the interaction between the images. The Brillouin-zone integration was sampled by 3 × 3 × 1 k-points using the Monhorst-Pack scheme.42 The bottom gold layer was constrained during geometry optimization, while the rest of the atoms were allowed to fully relax.

highly desirable to synthesize Au nanostructures with broad NIR absorption using more biocompatible molecules. In this paper, we report a facile approach for the synthesis of Au tetrapods using a biological buffer molecule, 2-[4-(2hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES). These branched Au nanoparticles show broader absorption in the range of 700−1000 nm. Furthermore, our combined experimental and computational approach provides deep insight into the formation mechanism of the branched Au nanoparticles. Mainly, our detailed studies on the synthetic parameters suggest that HEPES molecules at high concentrations self-assemble into structures with long-range order, and these structures serve as a soft template to direct the formation of the branched Au nanoparticles. The surface analyses suggest the existence of a molecular bilayer structure on the Au nanostructure surfaces, and this bilayer effectively stabilizes the nanoparticles in the buffer solution. Our DFT calculations predict that the sulfonate group of the HEPES molecules preferably binds to the Au surface, while the free hydroxyl groups facilitate the self-assembly and bilayer formation through the formation of hydrogen bonds.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS Chemicals. 2-[4-(2-Hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES, VWR), 4-(2-hydroxyethyl)-1piperazinepropanesulfonic acid (EPPS), 1,4-piperazinediethanesulfonic acid (PIPES), gold(III) chloride (HAuCl4, 0.2 wt %, Sigma Aldrich), and sodium hydroxide (99%, VWR) were used. Synthesis of Au Tetrapods. Aqueous HEPES solution (3 M) was first prepared by dissolving HEPES powder (360 g) in 500 mL deionized water, and the buffer solution was subsequently adjusted to pH 7 using 1 M NaOH. Buffer solutions at other concentrations (2 M, 1 M, 100 mM, and 10 mM) were prepared by diluting the 3 M solution with DI water. To synthesize Au branched Au nanoparticles, each buffer solution was simply mixed with HAuCl4 solution at the desired HAuCl4 to HEPES ratios. The reaction mixture was then kept in a temperature-controlled shaking incubator. Samples were taken at 10 min, 1 h, and 4 h. Studies of Synthetic Parameters. To understand the Au nanostructure formation, several synthetic parameters were studied, including the buffer concentration (10 mM, 100 mM, 1 M, 2 M, and 3 M), HEPES to HAuCl4 ratios (6000:1, 5000:1, 4000:1, 3000:1, 2500:1, 2000:1, 1500:1, and 1000:1), reaction time (10 min, 1 h, and 4 h), and reaction temperature (25, 37, and 45 °C). Characterization. The morphology and the size of the Au nanoparticles were examined with transmission electron microscopy (TEM). The surface chemistry of the nanoparticles was studied by Fourier transform infrared spectroscopy (FTIR) spectroscopy. The surface charges of the nanoparticles in aqueous solution were measured using a Zetasizer nano series dynamic light scattering (DLS) instrument. The UV−vis spectra were collected on a Shimadzu UV−vis spectrophotometer (UV-1700 series). Thermogravimetric analysis (TGA) was conducted to study the weight percentage of HEPES molecules on Au nanoparticle surfaces using a TA Instruments TGA 2950 thermogravimetric analyzer (New Castle, DE) under a nitrogen atmosphere at a constant heating rate of 5 °C min−1 from room temperature to 800 °C. Computational Modeling. The Gaussian 0934 program was used for cluster model calculations, and the Vienna ab initio simulation package (VASP)35,36 program was used for the

3. RESULTS AND DISCUSSION Figure 1a shows a TEM image of the branched Au nanoparticles synthesized in the HEPES buffer solution (3 M, pH 7) at a HEPES to Au molar ratio of 3000 to 1. At this

Figure 1. (a) TEM images of branched Au nanoparticles, (b) HRTEM image of a tetrapod, (c) HRTEM tip of a pod, and (d) XRD pattern and peak intensity. B



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narrow absorption) also reduces the need for a specific laser frequency. We performed further experimental studies to understand the effects of synthetic parameters on the formation of the tetrapod nanostructures, and these parameters include the HEPES concentration, molar ratio of HEPES to Au, reaction temperature, and reaction time. Figure 3 shows TEM images of the Au nanoparticles produced at different buffer concentrations (10 mM, 100 mM, 1 M, and 2 M). The experiments were all performed at the same HEPES to Au ratio (3000:1). A clear morphological change of the Au nanoparticles is observed as the concentration of the buffer molecules is increased. The branched Au nanostructures become more and more evident as the concentrations of the HEPES molecules are increased. At lower HEPES concentrations (10 and 100 mM), the majority of the products are irregular nanospheres, but some branched Au nanoparticles are produced in the 100 mM HEPES reaction solution. With increased buffer concentration, more and more branched Au nanostructures are observed (Figure 3c and d). Nanoparticles produced at higher concentrations, such as 2 M, are very similar to those produced at 3 M concentrations as shown in Figure 1a. The shape evolution of the Au nanoparticles in solution is reflected by their UV−vis absorption spectra (Figure 4a). When the majority of the nanoparticles are irregular nanospheres (10 mM HEPES concentration), a broad absorption peak at around 585 nm is observed. With the formation of branched Au nanostructures, an additional absorption peak at around 850 nm appears. Further increase in the buffer concentration (2 and 3 M) leads to a broad absorption in the range of 700−1000 nm, and this absorption is well-correlated with the branched Au nanostructures observed in the TEM images. This experimental observation is supported by previous studies where Au nanoflowers46 and nanostars47 were formed at HEPES concentrations of lower than 100 mM. In addition to the HEPES concentration, the molar ratios of HEPES to HAuCl4 also play an important role in the formation of the Au tetrapods. Because of the characteristic shapedependent absorption of the branched Au nanoparticles, the UV−vis spectrum was used as the primary tool to evaluate the nanostructure formation. Figure 4b shows the UV−vis spectra of Au nanostructures generated in 2 M HEPES solution at different HEPES to HAuCl4 molar ratios. At lower HEPES to HAuCl4 ratios (1000:1 and 1500:1), two additional absorption peaks around 600 and 800 nm are observed (along with the 520 nm absorption), and these peaks likely result from the different shaped nanoparticles similar to the products at lower buffer concentrations. With increasing HEPES to HAuCl4 molar ratios, the two absorption peaks at higher wavelengths merge together and become centered at 800 nm. Once the molar ratio of HEPES to HAuCl4 is above 2500 to 1, the absorption spectra remain fairly consistent indicating that similar shapes of Au nanostructures are produced. The effects of different HEPES to Au(III) molar ratios were observed at lower HEPES concentrations (5048 and 10049 mM) as well, where Au spheres and fused nonbranched Au nanoparticles were produced. The reaction reaches equilibrium rapidly. After 10 min, the characteristic broad absorption spectrum is clearly seen, and little change is observed with increased reaction times. Figure 5 shows the reaction-time-dependent UV−vis absorption spectra of the Au nanoparticles for 2 and 3 M HEPES concentrations at a molar ratio of HEPES to HAuCl4 of 3000 to 1. Samples at

reaction condition, a mixture of Au nanoparticles with various branched nanostructures was generated by simply mixing the HEPES buffer and the HAuCl4 solution. Among these branched Au nanoparticles, the Au tetrapods were the primary nanostructures identified using the TEM images. A yield analysis of a typical reaction on the basis of TEM images showed a rough distribution of various components as follows: tetrapods (65%), five or more tips (6%), two or three tips (15%), and spheres and other irregular shapes (14%). The lengths of the branches are roughly 20 nm, but the diameters of the pods decrease from the center (11 nm) to the tip (6 nm). Figure 1b and c shows an enlarged image of a tetrapod and a high-resolution transmission electron microscopy (HRTEM) image of the tip, respectively. The crystalline structure can be clearly seen from the defined lattice planes but with clear stacking faults. Because of the stacking faults, it is rather difficult to identify the growth direction of the crystal. The calculated interspacing of the tip gives a distance of 2.33 Å corresponding to the (111) planes. The X-ray diffraction (XRD) pattern and the respective peak intensity of the branched Au nanoparticles agree well with the bulk Au face-centered cubic crystal structure of typical (111), (200), (220), and (311) peaks (Figure 1d). The peak at 47.5° is from the carbon film substrate, and the other three peaks (52°, 59°, and 62.5°) are assigned to sodium sulfonate from computer database matching.43 The relatively strong peaks of sodium sulfonate suggest that additional sulfonate groups with long-range order are present in addition to the sulfonate groups on the Au nanoparticle surfaces. The sodium counterions of the sulfonate groups were introduced into the buffer solution as sodium hydroxide to adjust the pH of the buffer solution. The Au tetrapods in solution exhibit a bright blue color (Figure 2a) distinct from the characteristic dark red color of

Figure 2. The Au tetrapod solution: (a) photograph and (b) UV−vis absorption spectrum.

spherical Au nanoparticles.44 The bright blue color has been observed for hollow Au nanoshells45 and Au nanorods with certain aspect ratios.5 The blue color of the Au tetrapod solution is a result of light absorption in the visible range. The UV−vis spectrum of the tetrapod solution shows a small peak at 520 nm and a strong broad peak in the range of 700−1000 nm. The small absorption peak at 520 nm can be assigned to the transverse plasmon absorption of each pod tip similar to Au nanorods. The broad and strong absorption in the range of 700−1000 nm is a result of the longitudinal plasmon resonance, which is due to the high aspect ratio of the branches. The peak broadness is likely due to the heterogeneity in the lengths and diameters of the branches. This broad NIR absorption of the Au tetrapods makes them excellent candidates for photothermal therapy, and the broad absorption (rather than sharp C



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Figure 3. Au nanoparticles generated at different HEPES concentrations: (a) 10 mM, (b) 100 mM, (c) 1 M, and (d) 2 M.

Figure 4. UV−vis spectra of Au nanoparticles as a function of (a) HEPES concentration and (b) molar ratios of HEPES to HAuCl4.

addition, the samples that precipitated out of the reaction right after mixing could not be redispersed into water likely because of the lack of the well-packed HEPES stabilization. On the basis of our studies of the synthetic parameters, we propose a growth mechanism of soft-template-directed nanostructure formation, where the HEPES molecules selfassemble with long-range order and direct the formation of the Au tetrapods. The self-assembly behavior of the HEPES molecules is more pronounced at higher concentrations, and this conclusion is supported by the formation of the Au tetrapods at high HEPES concentrations and with high HEPES to Au molar ratios. The HEPES molecule has two accessible functional groups: a sulfonate group (RSO3−) and a hydroxyl group (−OH), both of which can interact with the Au nanostructure surfaces. Our FTIR spectra of the Au nanostructures and free HEPES molecules were used to evaluate the interaction between the HEPES molecules and the Au nanoparticle surfaces.

Figure 5. Time-dependent UV−vis spectra of Au nanoparticles at HEPES concentrations of (a) 2 M and (b) 3 M with the molar ratio of HEPES to HAuCl4 being 3000 to 1.

early reaction stages were not captured because of the time requirement to isolate Au nanoparticles from the HEPES environments limiting the access of very early stage samples. In D



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The interaction between HEPES and the Au surface was studied by FTIR, where the HEPES-stabilized Au nanoparticles were precipitated out of solution and were washed three times with DI water to remove free HEPES molecules. Figure 6

Figure 7. The DFT optimized structures of the HEPES−Au20 complex corresponding to (a) Au-sulfonate binding and (b) Au-hydroxyl binding. (c) The free HEPES molecule. The tables indicate the predicted vibrational modes corresponding to a and c. Figure 6. FTIR spectra of HEPES-stabilized Au nanoparticles and free HEPES molecules.

reasonably well with the experimental FTIR spectra providing additional insights into the interactions between HEPES and the Au surface. For example, the strongest peak at 1231.5 cm−1 of the experimental IR spectra of the isolated HEPES molecule agreed well with the calculated frequency at 1230.8 cm−1. By visualizing this vibrational mode in our model, this wavenumber was found to correspond to the asymmetric stretching of the SO bond. However, in the simulated IR spectra of the HEPES−Au20 cluster, this peak could not be clearly identified. In experiment, this peak is also significantly reduced in the FTIR spectrum of the HEPES-stabilized Au nanoparticles (as compared to the HEPES control spectrum), and this difference provides additional evidence that the sulfonate bond directly interacts with the Au surface. Also in the calculated frequencies, both the HEPES control and the HEPES−Au nanocluster models show the piperazine ring stretching or −OH rocking peaks (1034.8 and 1039.9 cm−1, respectively) and the C−N bond stretching (1173.8 and 1175.2 cm−1). These observations suggest that the immediate chemical environments for these bonds were not changed and are unlikely interacting with the Au surfaces, and these features also agree well with the experimental FTIR spectra. The preference of the sulfonate group to interact with the Au surface is further supported by DFT calculations using a slab model to represent the Au surface. The optimized structures of two possible binding configurations are shown in Figure 8. In Figure 8a, the HEPES molecule is arranged vertically relative to the Au surface, and all three sulfonate oxygen atoms are able to directly interact with the Au surface (at distances of 2.53, 3.06, and 2.52 Å) leading to a binding energy of 82.6 kcal mol−1. In Figure 8b, the hydroxyl end of the HEPES molecule interacts with the Au surface leading to a binding energy of 70.6 kcal mol−1. Similar to the cluster model (Figure 7), these calculations further support the preferable binding of the sulfonate group to the Au surface. With the sulfonate group preferably binding to the Au nanoparticle surface and the hydroxyl group protruding from the nanoparticle surface, it is expected that the hydroxyl groups would provide a hydrophilic surface and would assist Au tetrapod dispersion in water. Further, the hydroxyl group termination should lead to a nearly neutral or slightly negatively charged surface. However, the zeta potential measurement of

shows the FTIR spectra of the HEPES-stabilized Au nanoparticles and free HEPES molecules collected as powder samples. The FTIR spectrum of the free HEPES shows several distinct peaks at 1034, 1163, 1231, and 1459 cm −1 corresponding to the piperazine ring stretching or −O−H rocking, −C−N bond stretching, −SO bond stretching, and −CH2 scissoring, respectively. For the FTIR spectrum of HEPES-stabilized Au nanoparticles, the −SO bond stretching peak is significantly reduced, but the −OH rocking or piperazine ring stretching and C−N stretching remain unchanged. This observation suggests that the sulfonate group likely interacts with the Au nanoparticle surfaces leaving the −OH groups facing outward. To verify the vibrational modes of these key peaks observed in the experimental FTIR spectra, we compared these experimental results against the DFT generated spectra from our model systems. In the models, we employed a tetrahedral Au20 cluster as the substrate to investigate the possible interfacial interactions between HEPES and gold. Two possible binding modes between HEPES and the Au20 substrate were obtained, and the optimized structures are shown in Figure 7a and b. In Figure 7a, the HEPES molecule is located on the top of the Au cluster with the sulfonate oxygen interacting with the Au surface at distances of 2.58−2.60 Å. In the other possible binding configuration (Figure 7b), the hydroxyl oxygen of the HEPES interacts with the Au surface at a distance of 2.90 Å. However, it is predicted to be 18.1 kcal mol−1 higher in energy than the configuration shown in Figure 7a indicating a strong thermodynamic preference for the sulfonate group of HEPES to interact with the Au surface. Thus, the optimized structures of the lowest energy HEPES−Au20 cluster model and the free HEPES molecule (Figure 7a and c, respectively) were then used for IR frequency calculations to make comparisons with the experimental spectra. The dominant wavenumber peaks and their corresponding vibrational modes are also provided in Figure 7. While it is impossible to match every peak observed in the experimental IR spectra with the DFT calculated wave numbers, the dominant peaks from the calculations matched E



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Figure 8. The optimized structures of a single HEPES molecule adsorbed on a periodic Au surface: (a) sulfonate binding configuration and (b) hydroxyl binding configuration.

Figure 9. (a) Zeta-potential plot of HEPES-stabilized Au tetrapod solution, (b) a diagram of the proposed HEPES bilayer structure, and (c ) TGA plot of HEPES-stabilized Au tetrapods.

Figure 10. (a) Chemical structures of HEPES, EPPS, and PIPES; (b) the optimized structure of a HEPES dimer; and (c) the optimized structure of an EPPS dimer. Key distances are shown in angstroms.

To further confirm the bilayer hypothesis, we performed a TGA study on the HEPES stabilized Au tetrapods, which were precipitated out of the solution and which were washed three times with DI water to remove the free HEPES molecules. A roughly 6% weight loss was observed (as shown in Figure 9c), which is very close to our theoretical estimate (∼5%) by assuming that the surface of the Au tetrapods was covered with a HEPES bilayer at a concentration of 5 mols/nm2 (predicted from DFT calculations on a planar Au surface). To further understand the energetics of self-assembly and bilayer formation of the HEPES molecules, we compared the behavior of HEPES with that of two other molecules with similar chemical structures, 4-(2-hydroxyethyl)-1-piperazinepropanesulfonic acid (EPPS) and 1,4-piperazinediethanesul-

the branched Au nanoparticles suggests a highly negatively charged surface (Figure 9a), which does not suggest that the surfaces are terminated with hydroxyl groups. From XRD measurements (Figure 1d), we observe relatively strong peaks of sodium sulfonate, which suggests that additional sulfonate groups are present with long-range order in addition to the sulfonate groups on the Au nanoparticle surfaces. Therefore, we propose that during synthesis there is a bilayer structure of HEPES molecules formed facilitated by the hydrogen bonding among neighboring hydroxyl groups as illustrated in Figure 9b. This bilayer molecular structure is very similar to the CTAB bilayer structure on Au nanorod surfaces,5 where the bilayer formation results from the hydrophobic interaction between the hydrocarbon chains. F



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fonic acid (PIPES) (Figure 10a). The EPPS molecule has exactly the same functional groups as HEPES but with an additional −CH2− group between the piperazine ring and the sulfonic acid group. Under identical experimental conditions, similar Au tetrapod nanoparticles were produced with the EPPS solution, but the reaction rate was much lower as indicated by the slow color change of the reaction solution (30 min for EPPS versus 10 min for HEPES). Interestingly, however, the PIPES buffer solution did not produce Au tetrapods. Instead, large nanoparticle aggregates were observed, and the nanoparticles quickly precipitated out of the reaction solution. This observation suggests that PIPES is not effective at stabilizing the Au nanoparticles. The PIPES molecule also has a very similar structure to HEPES, but the hydroxyl group is replaced by a sulfonate group resulting in sulfate termination at both ends of the molecule. In the absence of hydroxyl groups, it is impossible for PIPES to form similar self-assembled structures via hydrogen bonds (as predicted for HEPES). This thus explains the fact that the PIPES buffer solution fails to generate Au tetrapods. Energetic differences between HEPES and EPPS during the nanoparticle formation process can be identified by calculating the binding energies between HEPES−HEPES and EPPS− EPPS pairs (as a first approximation of the bilayer energetics). We did not attempt to calculate PIPES−PIPES binding energies because of the strong electrostatic repulsion between two sulfonate end groups and because of the lack of hydrogenbonding interactions. The optimized structures of the HEPES and EPPS pairs (using the polarizable continuum model (PCM)38 for an aqueous solvent) are shown in Figure 10b and c. In both of these complexes, a hydrogen bond is formed between the hydroxyl groups with an equivalent separation distance of 1.92 Å. However, the calculated binding energy between the HEPES dimer (25.7 kJ mol−1) is much higher than that of the EPPS dimer (−1.0 kJ mol−1). This binding energy difference is likely due to the conformational strain in the EPPS dimer (because of the additional −CH2− group) to form the hydrogen bond resulting in an energetic penalty. The large difference in the binding energy is at least one plausible reason explaining the experimental difficulty of forming self-assembled EPPS structures, and this is reflected by the slow reaction rate during the nanostructure synthesis.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]; phone: 205-348-9869; fax: 205348-7558 (Y.B.). E-mail: [email protected]; phone: 205-3481733; fax: 205-348-7558 (C.H.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by The National Science Foundation (DMR-0907204, DMR-1149931, and CTS-0747690). We acknowledge the UA Central Analytical Facility (CAF) and the Biological Science Department for the use of TEM. We thank the Alabama Supercomputer Authority for providing additional computational resources.

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4. CONCLUSIONS In summary, we report a facile approach for synthesizing branched Au nanoparticles with absorption in the range of 700−1000 nm using biological buffer molecules. Our results indicate that the high concentration of HEPES molecules selfassemble into bilayer structures and serve as a soft template for the formation of branched Au nanoparticles. In conjunction with DFT calculations, we conclude that the sulfonate groups directly interact with the Au surfaces, and the hydroxyl groups further interact with other HEPES molecules forming a bilayer structure via a hydrogen-bond network. Such a bilayer structure is important for stabilizing the branched Au nanostructures. Furthermore, the broad absorption of these nanostructures makes them excellent candidates for photothermal cancer therapy. G



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