DOI: 10.1021/cg101212y
Hyperbranched Polyglycidol-Assisted Green Approach to the Fabrication of Morphology-Tunable Gold Architectures
2010, Vol. 10 5319–5326
Haiqing Li, Jungkyu Jo, Jing Wang, Lidong Zhang, and Il Kim* The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea Received September 13, 2010; Revised Manuscript Received October 21, 2010
ABSTRACT: Biocompatible hyperbranched polyglycidol (HBP) has been demonstrated to be an effective reducing and stabilizing agent for the fabrication of gold architectures with easily tuned morphologies, including nanospheres and hexagonal and triangular nanoplates. All the fabrication reactions are conducted in water at room temperature in the absence of any additional reducing agents and surfactants; thus, they are green and energy-efficient processes. Morphological, spectral, and structural properties associated with the formation processes of gold nanoplates have been systematically examined, and a plausible formation mechanism of such nanoplates was also proposed. Additionally, the HBP-stabilized gold nanoplates provide excellent surface-enhanced Raman scattering substrates for the detection of HBP molecules, which can be extended to the analysis of other organic species.
Introduction Nanometer-sized gold architectures have attracted tremendous interest because of their distinctive physical and chemical properties, and their promising applications in catalysis, photoelectronic devices, and biomedicine.1-3 Such fascinating properties and applications strongly depend on the morphology and size of gold structures. To date, a variety of gold architectures with well-controlled morphology, such as rods,4 wires,5 cubes,6 boxes,7 disks,8 and prisms,9 have been widely achieved. Among many different shapes of gold particles, gold plates make up a particularly attractive class of structures because of their unique optical features. Although many pioneering methods such as the polyol process,10 a photochemical strategy,11 and a thermal approach12 have been developed for the synthesis of gold nano- or microplates, most preparation protocols involve either harsh reaction conditions such as high temperature and/or the use of toxic organic solvents or additional reduction agents and surfactants. These unavoidably complicated procedures increase the preparation cost and lead to the formation of injurant-containing gold materials. Therefore, simple, cost-effective, and green solutions for the fabrication of gold nanostructures with tunable morphologies are strongly desired. Hyperbranched polyglycidol (HBP) has been shown to be a type of highly biocompatible polymer, which can be conveniently synthesized in a single step and in a high yield.13,14 HBP macromolecules consist of dendritic units, linear polyether segments, and numerous terminal hydroxyl groups, which provides a void-containing three-dimensional architecture (Scheme 1). Such unique molecular structures build an electronegative environment derived from the numerous electron-rich oxygen atoms,15,16 which makes HBP particularly suited for capping metal ions. More surprisingly, in this study, we have found that the HBP-capped Au(III) ions can be reduced in situ to yield stable Au architectures, without the
addition of any other reducing and stabilizing agents. Moreover, the morphologies of those resulting Au architectures were easily controlled from nanospheres to hexagonal and triangular nanoplates via simple changes in the reaction conditions, such as the concentration of chloroauric acid (HAuCl4) and the amount of NaBr. Note that all the reactions were conducted in water at room temperature, under normal pressure, and in the absence of any additional surfactants. Therefore, our current protocol is a simple, energy-efficient, and green strategy for the synthesis of gold architectures with easily tuned morphologies. Experimental Section
*To whom correspondence should be addressed. E-mail: ilkim@ pusan.ac.kr.
Synthesis of HBP. HBP macromolecules were synthesized via an anionic ring-opening multibranching polymerization.17,18 Polymerization was conducted in a reactor equipped with a mechanical stirrer and a dosing pump under a nitrogen atmosphere. A 50 mL aliquot of glycidol (Aldrich) was slowly added to the reactor containing trimethylolpropane partially deprotonated (10%) with a sodium methylate solution (3.7 M in methanol, Aldrich) at 95 °C over 12 h. After completion of the reaction (without excess epoxide), the product was dissolved in methanol and neutralized by filtration over cation-exchange resin. The polymer was triply precipitated from a methanol solution into acetone and subsequently dried for 20 h at 80 °C under vacuum. The resulting HBPs applied in this study have a number average molecular weight (Mn) of 3400 g/mol determined with the 13C NMR spectrum of HBP:17 1H NMR (300 MHz, DMSO-d6) δH 4.9 (singlet, OH), 3.7 (multiplet, CH), 2.8-3.6 (multiplet, CH2); 13C NMR (75 MHz, DMSO-d6) δC 81 (CH), 80 (CH), 74 (CH2), 73 (CH2), 72.4 (CH2), 71.2 (CH2), 68.9 (CHOH), 63.8 (CH2OH), 61.6 (CH2OH). Preparation of Au Particles with Tunable Morphologies. All the following reactions were conducted in water at ambient temperature and under normal pressure. In a typical process, 0.5 mL of an HBP aqueous solution (Mn=3400 g/mol, 10 mg/mL) and a prescribed amount (0.10, 0.15, 0.20, 0.25, and 0.30 mL) of an aqueous HAuCl4 solution (10 mM) were sequentially injected into a vial containing a prescribed amount of deionied water (0.40, 0.35, 0.30, 0.25, and 0.2 mL) under magnetic stirring. After a suitable period of stirring, the resulting gold colloids were purified by dialysis against deionied water using cellulose acetate dialysis tubing [molecular weight cutoff (MWCO) of 12000 g/mol].
r 2010 American Chemical Society
Published on Web 11/08/2010
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Scheme 1. Architecture of the HBP Macromolecule (left) and HBP Optimized Geometry Structure (right) Generated with the Discovery Model and Compass Force Field System, Using Materials Studio Release 4.2 for Windows
To synthesize triangular gold nanoplates, 0.5 mL of an HBP aqueous solution (Mn = 3400 g/mol, 10 mg/mL) and 0.25 mL of an aqueous HAuCl4 solution were sequentially injected into a vial containing 0.2 mL of water and 0.05 mL of an aqueous NaBr solution (20 mM) under magnetic stirring for 20 h. The gold nanoplates that were produced were purified by dialysis against deionied water using cellulose acetate dialysis tubing (MWCO of 12000 g/mol). Following similar procedures, 6.0 and 9.0 mM NaBr in the reaction system were used to investigate the effect of NaBr concentration on the morphology of Au nanoplates. Characterizations. UV-vis absorption spectra of the samples were recorded at room temperature on a UV-1650PC apparatus (Shimadzu). Samples for transmission electron microscopy (TEM) were deposited onto carbon-coated copper electron microscope grids and dried in air. The size, shape, and fine structures of gold nanostructures were investigated using a MODEL H-7600 (Hitachi) low-resolution and a JEOL 1200 EX high-resolution TEM (HRTEM) microscope. Selective area electron diffraction (SAED) patterns of gold plates were achieved with a HRTEM microscope. The structures of the products were examined by X-ray diffraction (XRD) with an automatic Philips powder diffractometer using nickel-filtered Cu KR radiation. The diffraction pattern was collected in the 2θ range of 15-90° in steps of 0.02° and a counting time of 2 s/step. Atomic force microscopy (AFM) images of the gold plates were obtained using an Innova scanning probe microscope (Veeco, Plainview, NY). Raman measurements were taken with a Renishaw System 1000 Raman imaging microscopy system (Renishaw PLC) equipped with a 25 mW (1064 nm) He-Ne laser (model 127-25RP, Spectra-Physics) and a Peltier-cooled CCD detector. A 50 objective (NA=0.80) mounted on an Olympus BH-2 microscope was used to focus the laser onto a spot approximately 1 μm in diameter and to collect the back-scattered light from the samples.
Results and Discussion Figure 1 shows the morphologies of Au nanostructures obtained in the presence of different concentrations of HAuCl4 while the HBP concentration was kept unchanged (10 mg/mL). When 1.0 mM HAuCl4 is used, the resulting Au nanoparticles exhibit regularly spherical structure with uniform size [6.8 ( 1.5 nm in diameter (Figure 1a)]. When the concentration is increased to 1.5 mM, the gold nanoparticles with rather irregular morphologies are obtained (Figure 1b).
When the concentration of HAuCl4 is increased to 2.0 mM, the resultant gold structures become more regular. Some gold plates are observed together with the large particles (Figure 1c). More surprisingly, when the concentration of HAuCl4 is further increased to 2.5 mM, large amounts of hexagonal gold plates (∼85% content in the produced nanoplates) are produced (Figure 1d), which is similar to the recently reported results.19 Figure 1e shows the HRTEM image of an individual hexagonal gold nanoplate and its corresponding SAED pattern. In the SAED pattern, the hexagonal symmetry of the diffraction spot array reveals that the gold nanoplate is a single crystal with a preferential growth direction along the gold (111) plane.20,21 Determined by AFM analysis, the thickness of the gold plates obtained with 2.5 mM of HAuCl4 is 10.3 nm (Figure 1f). In contrast, when 3.0 mM HAuCl4 is used, some triangular nanoplates are generated in the sample, where the content of hexagonal nanoplates in the produced nanoplates decreases to 70% (Figure 1g,h). The optical properties of the as-prepared gold nanostructures with varied morphologies ranging from nanospheres to micoplates shown in Figure 1 were also investigated by means of UV-vis absorption analysis (Figure 2). The uniform gold nanoparticles exhibited a sharp absorbance band at 520 nm, characteristic of the presence of well-isolated and uniform gold nanospheres. As for the sample obtained with 1.5 mM HAuCl4, a relatively broad band centered at 546 nm was presented, evidence of the formation of irregular gold particles with a broad size distribution. However, for the samples composed of hexagonal gold plates, two absorbance bands associated with transverse (shorter wavelength) and longitudinal (longer wavelength) surface plasmon resonance (SPR) can be recognized in the spectra. The presence of such a quite broad longitudinal SPR band is derived from the strong dipoledipole interactions among the nanoplates that are massively oriented to form larger plates, which is typical for the gold architectures with platelet anisotropy.22,23 To demonstrate the process of formation of gold hexagonal microplates (Figure 2e,f), the morphologies of as-synthesized samples taken at different reaction times during the reaction
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Figure 1. TEM images of gold nanostructures prepared by using (a) 1.0, (b) 1.5 and (c) 2.0 mM of HAuCl4; (d) SEM image of gold sample prepared by using 2.5 mM of HAuCl4, (e) HRTEM image and SAED pattern, as well as (f) AFM image of a typical hexagonal gold nanoplate selected from sample (d); (g) SEM and (h) TEM images of gold nanoplates fabricated by using 3.0 mM of HAuCl4.
were monitored by TEM (Figure 3). Many clusters are formed after the reaction is performed for 10 min. In such clusters, a few randomly distributed gold nanoparticles can be visualized (Figure 3a). When the reaction time is extended to 20 min, those clusters grow to much larger particles, in which the single gold nanoparticle is hardly identified, indicating the formation of clusters with much higher gold nanoparticle content (Figure 3b). When the reaction time is increased to 60 min, some two-dimensional gold plates with an undefined shape are formed. In the meantime, some irregular gold particles are also generated (Figure 3c). Surprisingly, when the reaction time is increased to 75 min, neat and regular hexagonal gold plates are successfully obtained (Figure 3d). The formation of these well-defined hexagonal gold plates was
also traced by UV-vis spectra (Figure 3e). As the reaction proceeds, the absorbance band intensity at both the lower and higher wavelengths gradually increases over 60 min. However, after the reaction is performed for 75 min, those band intensities are significantly increased and exhibited the characteristic SPR properties of gold plates. On the basis of these observations, a plausible formation mechanism of hexagonal gold nanospheres and gold nanoplates is briefly presented as follows (Scheme 2). HBP molecules possess three-dimensionally void-containing structures, in which the numerous electron-rich oxygen atoms build an electron-negative environment. When the HAuCl4 is added to the aqueous HBP solution, noncovalent, electrostatic (ion dipole) interactions exist among the electropositive metal ions,
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electron-rich oxygen atoms on the ether chains, and the adjacent hydroxyl groups of inter- or intra-HBP molecules. Such interactions not only force the metal ions to be tightly trapped within the voids of single HBP molecules to form a metal ion-HBP complex but also attract the adjacent HBP molecules to generate metal ion-HBP supercomplexes. In such complexes, many hydroxyl groups constitute a polyol environment with a certain alcohol concentration. Following an analogous polyol process,24,25 those capped metal ions within HBP complexes are reduced in situ and form numerous fine-sized gold nanoclusters, leading to the formation of grain-sized HBP-passivated gold nanoparticles. With the reaction proceeding in the presence of a lower precursor concentration, more Au(III) ions were reduced. The produced Au species with zero valence were
Figure 2. UV-vis spectra of gold nanostructures obtained in the presence of different concentrations of HAuCl4.
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successively deposited onto the preformed grain-sized gold clusters within HBPs, resulting in the isotropic growth of those HBP-passivated gold nanoparticles into well-defined gold nanospheres (Figure 1a). If the reaction is conducted in the presence of a higher concentration of the metal precursor, such grain-sized gold nanoparticles tend to aggregate and form larger clusters, which can be evidenced by the formation of gold nanoparticle-containing clusters at the early stage of the reaction (Figure 3a). In addition to the reaction proceeding, such existing small nanoparticles keep growing. In the mean time, more gold nanoparticles are generated within those clusters. These facts are responsible for the formation of clusters with a higher gold content (Figure 3b) and the gradual increase in the absorbance band intensity of the corresponding samples (Figure 3e). Because of the high surface energy of the as-prepared gold nanoparticles, they tend to coalesce and finally form larger gold plates. Previous reports have demonstrated that the halide ions play important roles in the formation of gold architectures with interesting morphologies.26 We have also investigated the effects of the Br- ions on the morphology of gold architectures. When 1.0 mM Br- is involved in the reaction system that is the same as that for the preparation of hexagonal gold plates (Figure 1d), to our surprise, a large amount of triangular gold plates together with some gold nanoparticles is obtained (Figure 4a). When the employed concentration of Br- ions is further increased to 3.0 mM, the resulting products exhibit well-defined triangular structures with quite sharp tips (Figure 4b). Determined by TEM observation, the edge length of these triangular gold plates ranged from 300 to 1400 nm, which is similar to that of the sample obtained using 1.0 mM Brions. The statistic content of such well-defined triangular nanoplates is around 95%, which is higher than the levels in the reported results.10,22 AFM images shown in Figure 4c
Figure 3. TEM images of the as-prepared gold samples collected at different reaction times during the reaction for the synthesis of hexagonal gold nanoplates by using 2.5 mM HAuCl4: (a) 10, (b) 20, (c) 35, and (d) 75 min. (e) UV-vis spectra of such samples taken at different reaction times.
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Scheme 2. Illustration of the Formation of Gold Nanostructures with Easily Tuned Morphologies Ranging from Nanospheres to Hexagonal and Triangular Nanoplates Using HBPs as Reducing and Stabilizing Agents
indicate that the gold plates possess flat surfaces with a uniform thickness of 5.1 nm. The SAED pattern of a typical triangular nanoplate (inset of Figure 4b) shows hexagonally arranged diffraction spots with 6-fold symmetry, characteristic of a (111)-oriented single-crystal gold nanoplate (Figure 4d).20,21 However, when the concentration of Br- ions is increased to 9.0 mM, a large amount of gold particles with a rather broad size distribution is produced. Only a few triangle-like plates with irregular edges are visualized (Figure 4e). The UV-vis spectra of samples obtained with 1.0 and 3.0 mM Br- ions exhibit typical longitudinal SPR properties, characteristic of the presence of gold architectures with a platelet anisotropy. The sample prepared with 9.0 mM Br- ions mainly shows broad absorbance bands situated at ∼600 nm, corresponding to the formation of a large amount of gold particles with a broad size distribution (Figure 4f). Therefore, the utilization of a suitable amount of Br- ions is favorable for the formation of well-defined triangular gold plates with a high yield. The use of an overly high concentration of Br- ions tends to generate edge-lost gold plates in a rather low yield because of the sculpting effects of halide ions, which is consistent with previous reports.26,27 Note that the reaction conditions for the fabrication of triangular plates are exactly the same as those for the hexagonal plates (Figure 1d), except for the involvement of Br- ions.
Hence, the Br- ions play a key role in achieving triangular gold plates. The dynamic processes for the generation of triangular gold plates (Figure 4b) were also investigated by TEM analyses. As is shown in Figure 5a, when the reaction is performed for 2 h, gold nanostructures with varied morphologies, including prism, hexagon, and spheres, are formed. Note that no asformed clusters are observed in this reaction system, which is quite different from the formation process of hexagonal gold plates (Figure 3a). The involvement of NaBr might disturb the formation of HBP-metal ion supercomplexes. After reaction for 3.5 h, much larger gold architectures with different shapes and a broad size distribution are visualized (Figure 5b). When the reaction time is further increased to 5 h, a large amount of truncated triangular gold plates together with numerous small nanoparticles is formed (Figure 5c). Most of those plates possess blunt edges. Interestingly, after the reaction has proceeded for 5 h, well-defined triangular gold plates with sharp edges are obtained in high yield, while a few small nanoparticles are observed (Figure 5d). On the basis of these facts, the formation of such triangular nanoplates could follow an Ostwald ripening process, in which the newly generated small gold nanoparticles were adsorbed and digested on the surfaces of as-formed larger plates, making the truncated triangular nanoplates anisotropically grow into well-defined triangular architectures (Scheme 2).
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Figure 4. TEM images of gold nanoplates obtained in the presence of (a) 1.0 and (b) 3.0 mM NaBr. (c) AFM image and (d) SAED pattern of a typical triangular nanoplate selected from sample b. (e) TEM image of products obtained with 9.0 mM NaBr. (f) UV-vis spectra of gold samples prepared with different concentrations of NaBr.
Figure 5. TEM images of the as-synthesized gold samples recorded at different reaction times during the reaction for the synthesis of triangular gold nanoplates using 3.0 mM NaBr: (a) 2, (b) 3.5, (c) 4, and (d) 5 h. (e) UV-vis spectra of such samples recorded at different reaction times.
The UV-vis spectra of the corresponding samples recorded at different reaction times are also recorded for the trace of the formation processes of triangular gold plates. As shown in Figure 5e, at the early stage of the reaction (
