ARTICLE pubs.acs.org/Langmuir
Synthesis and Self-Assembly of Highly Monodispersed Quasispherical Gold Nanoparticles Youju Huang and Dong-Hwan Kim* School of Chemical and Biomedical Engineering, Nanyang Technological University, 637457 Singapore
bS Supporting Information ABSTRACT: We report the synthesis of cetyltrimethylammonium bromide (CTAB) assisted seed mediated growth of highly pure and monodispersed quasispherical gold nanoparticles (QAuNPs) and their self-assembly on the silica/glass substrates. The seed-mediated growth approach was modified to prepare sizetunable monodispersed QAuNPs with sizes ranging from 20 to 150 nm. The larger, more uniform seeds and lower CTAB concentration resulted in the formation of relatively large QAuNPs with improved monodispersity (relative standard deviation (RSD) of ∼5 8%) and high purity in their shapes. In addition, CATB-capped QAuNPs can be spontaneously assembled into closely packed and highly aligned superstructures with well-defined mutillayers (two to six layers) on silica substrates. Furthermore, CATB-capped QAuNPs can easily construct density-controllable QAuNP chips by electrostatic self-assembly, showing their promising applications for singlenanoparticle plasmonic sensors.
1. INTRODUCTION In the past few years, due to their unique size- and shapedependent optical properties, gold nanoparticles (AuNPs) and their self-assembled architectures have attracted great interest in various applications, including optoelectronic nanodevices,1,2 catalysts,3,4 biosensors,5,6 and nanomedicine,7,8 which in turn raise a demand on different shapes of AuNPs (spheres,9 rods,10,11 cubes,12,13 cages,14 octahedrons,15 and branched multipods16,17). However, compared with top-down methods (e.g., lithography), bottom-up approaches (e.g., wet chemical synthesis) often meet with difficulty in preparation of size-controlled spherical AuNPs with high purity and monodispersity.18,19 This challenge further creates difficulties during manipulation of AuNPs into dimension-controlled, nano- or microstructured assemblies that are essential components for studying their physical and chemical properties and pursuing further applications.15,20 23 Among two typical wet syntheses reported for the preparation of spherical AuNPs, direct synthesis offers an effective fabrication of spherical AuNPs with relatively narrow distribution (relative standard deviation (RSD) of ∼10 16%) in the size regime of less than 20 nm,9,24,25 whereas the seeding growth allows convenient control of AuNP size by varying the ratio of [HAuCl4]:[seeds].26 30 Recently, the introduction of new reducing agents and stabilizers has enabled the creation of AuNPs in the size range from 15 to 300 nm with adequately narrow size distribution (RSD ∼ 10%).31,32 However, additional modifications are required to construct controlled assembly with precise and predictable dimensions due to the nature of the reducing agents and stabilizers.23,33,34 CTAB has proven to be a versatile capping agent to directly tune the morphology and the dimension of AuNPs10,11,17 and a convenient surfactant to exquisitely tailor the assembly of r 2011 American Chemical Society
AuNPs, i.e., dimension-controlled architectures from one to three dimensions.15,23,35,36 It has however been difficult to prepare the size-tunable spherical AuNPs with high monodispersity and purity because numerous experimental parameters10,11,17,37 39 have interrelated effects on the size and the morphology of synthesized AuNPs. Jana and co-workers prepared spherical AuNPs in the size range from 5 to 40 nm using CTAB.40 The synthesized AuNPs exhibited a broad size distribution (RSD ∼ 10 20%), and approximately 10% of the final product was found to be nonspherical. Rodríguez-Fernandez and co-workers41 prepared quasispherical gold nanoparticles (QAuNPs) with sizes ranging from 12 to 180 nm using, namely, the step-by-step CTAB-assisted seed-mediated growth method. This method created approximately 35% nonspherical shapes and required a purification process. Herein we describe a modified, seed-mediated growth method for high-yield synthesis of QAuNPs and CTAB-driven assembly of QAuNPs. We found that seed size, seed dispersity, CTAB concentration, silver ion, and synthesis temperature have great influence on the dispersity and the purity of the resulting QAuNPs. Improved monodispersity (RSD ∼ 5 8%) and high purity of AuNPs compared with previous reports that adopted CTAB-assisted seed-mediated growth (RSD ∼ 10 20 and ∼ 10% of nonspherical AuNPs) were achieved using the modified, seed-mediated growth method proposed in this work. In addition, due to the nature of CTAB, CATB-capped QAuNPs tend to spontaneously assemble into closely packed and highly aligned superstructures with well-defined mutillayers (two to six layers). Further, the Received: August 11, 2011 Revised: October 8, 2011 Published: October 10, 2011 13861
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Langmuir density of QAuNPs assembled on a solid substrate could be precisely controlled, which holds great promise for surfaceenhanced Raman scattering (SERS) and localized surface plasmonic resonance (LSPR) based sensors.
2. MATERIALS AND METHODS 2.1. Materials and Instrument. Cetyltrimethylammonium bromide, hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O), ascorbic acid, and sodium citrate were purchased from Sigma Aldrich. All other reagents were used as received. Ultraviolet visible (UV vis) absorption spectra were recorded with Shimadzu UV-2450 spectrophotometer in transmission mode. Field-emission scanning electron microscopy (FE-SEM) was performed on the JEOL instrument (JSM6700F) at an acceleration voltage of 5 kV and a working distance between 7 and 8 mm. The ζ potential measurements were made by a ζ potential analyzer (Malvern Instruments Zetasizer). Dark-field imaging on single AuNPs was carried out by an Olympus IX71 inverted microscope coupled with a color digital camera (DS-Fi1-U2 with the NIS Element D software). The detailed information on the dark-field imaging can be found in the previous reports from our group.42,43 2.2. Preparation of Au Seeds (3.5 nm). The Au seeds (3.5 nm) were prepared according to the previous works.37,44 Briefly, 0.6 mL of ice cold 0.1 M NaBH4 was added into a 20 mL aqueous solution containing 2.5 10 4 M HAuCl4 and 2.5 10 4 M trisodium citrate. The orangered color of the solution indicates the formation of AuNPs. The average size of the seed AuNPs was approximately 3.5 ( 0.7 nm.37,44 The Auseed solution was used without modification for further experiments. 2.3. Preparation of Au Seeds (12 nm). The Au seeds (12 nm) were prepared according to the Sastry’s method.45 Briefly, 36 mL of an aqueous solution of 2.5 10 4 M HAuCl4 was prepared in a conical flask. To this solution was added 4 mL of 1 M D-glucose. The mixture was heated to 60 °C in an oil bath. Then, 80 μL of 1 M NaOH was added into the mixture, and the resulting solution was rapidly stirred for 10 s. The color of the solution immediately changed to ruby red, indicating the formation of AuNPs. The average size of the seed AuNPs was approximately 12.2 ( 3.5 nm.37,45 The Au-seed solution was cooled and used without modification for further experiments. 2.4. Preparation of Au Seeds (18 nm). The Au seeds (18 nm) were prepared according to the Frens method.9 Briefly, 100 mL of 2.5 10 4 M HAuCl4 solution was heated to 120 °C in an oil bath under vigorous stirring for 30 min. Then, 10 mL of 1% sodium citrate solution was added into the above solution with continued boiling. After 20 min, the color of the boiled solution changed to ruby red, indicating the formation of AuNPs in the solution. The average diameter of the seed AuNPs was approximately 18 nm.9,31 The Au-seed solution was cooled and used without modification for further experiments. The representative FESEM images and size distributions of Au seeds (18 nm) are shown in S-Figure 1 and S-Figure 2 of the Supporting Information. 2.5. Growth of the Au Seeds. A series of experiments were performed to study the influence of Au-seeds size on the purity of the synthesized AuNPs. Three 20 mL conical flasks were taken. A 9 mL aliquot of the growth solution containing a mixture of 2.5 10 4 M HAuCl4 and 0.01 M CTAB was added to each of these flasks. Then, 50 μL of 0.1 M freshly prepared ascorbic acid was added into each flask followed by gentle stirring for 2 min. Finally 0.5 mL (or 0.02 mL) of Auseed solutions (3.5, 12, and 18 nm) was added into each flask, and the mixtures were kept at 30 °C in a water bath at least 6 h. Another series of experiments were performed to study the influence of CTAB concentration on the purity of synthesized AuNPs. Four 20 mL conical flasks were taken. A 9 mL aliquot of growth solution containing a mixture of 2.5 10 4 M HAuCl4 and CTAB with different concentrations (0.1, 0.05, 0.01, and 0.005 M, respectively) was added to each of these flasks. Then, 50 μL of 0.1 M freshly prepared ascorbic acid was
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added into each flask followed by gentle stirring for 2 min. Finally 0.5 mL of Au-seed solution was added into each flask, and the mixtures were kept at 30 °C in a water bath for at least 6 h. Similarly, the influence of temperature and silver ion on the purity of the synthesized AuNPs was studied. QAuNPs with different sizes ranging from 20 to 150 nm were prepared using one-step seeding growth method by varying the ratios of growth and seed solution. A 200 mL aliquot of growth solution consisting of 0.01 M CTAB and 2.5 10 4 M HAuCl4 was mixed with 1.11 mL of 0.1 M ascorbic acid. The concentration of seed solution was kept at 0.5 mL in each plastic tube while different amounts of growth solution (1, 5, 7, 9, 11, and 13 mL) were added into each tube. Another set of experiments was conducted with a predetermined amount of growth solution (9 mL) in each plastic tube. After gentle stirring for 2 min, different amounts of Au-seed solution (0.2, 0.1, 0.08, 0.05, 0.03, and 0.02 mL) were added into each tube. The solutions were kept at 30 °C in a water bath for at least 6 h.
2.6. Preparation of QAuNP Arrays on a Solid Substrate. The hydroxyl-functionalized glass substrate was immersed into a 10-fold diluted QAuNP suspension. By varying the incubation time of the electrostatic self-assembly between positively charged QAuNPs and hydroxyl-functionalized glass substrate, various assembly densities of QAuNPs on a substrate can be easily obtained.46
3. RESULTS AND DISCUSSION The size and the purity of AuNPs in a seed-mediated method are greatly influenced by the seeds: the larger seeds tend to form the larger particles, and the uniformity of the seeds has an effect on the monodispersity and the purity of the synthesized AuNPs. 37,40,41 Smith and Korgel have studied an effect of the size and the surface charge of Au seeds on resulting Au nanorods.38 They found that negatively charged seeds tend to favor the formation of Au nanorods with minimized variation in aspect ratio. They also found that as the size of seeds increased, the aspect ratio of synthesized nanorods decreased. To prepare relatively large AuNPs, we explored Au seeds in different sizes ranging from 3.5 to 18 nm while the rest of synthetic parameters were unaltered. Parts a c of Figure 1 show SEM images of AuNPs obtained from Au seeds in 18, 12, and 3.5 nm, respectively, showing that larger Au seeds tend to form larger spherical AuNPs with high monodispersity and purity. The smaller the Au seeds that were used, the more byproducts, such as nanorods and nanoplates, were observed. Please note that even with high volume ratio (450 times) between growth solution and Au seeds, small Au seeds, i.e., 12 and 3.5 nm, form nonspherical particles (Figure 1e,f). As far as the nucleation and the growth of AuNPs are concerned, when facets are developed on the surface of a Au seed, which serves as a nucleation center, they tend to grow into nanorods or nanoplates. The probability of the facet formation in large Au seeds was much smaller than that in small ones, as shown in Figure 1, which ultimately improved the monodispersity of the resulting AuNPs. The concentration of CTAB is another important factor affecting the final size and morphology of AuNPs10,11,17,38,40,41 as CTAB molecules at different concentrations tend to bind to different Au crystal faces as reported in previous studies in the case of Au nanorods.17,38 For example, in the high concentration of CTAB, CTAB molecules appear to bind more strongly to the {100} than the {111} faces, leading to formation of Au nanorods, while lower concentrations of CTAB favor the faster formation and deposition of Au0 onto the {111} faces, leading to producing cubic shapes or spherical nanoparticles.11,17 In the present study, 13862
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Figure 1. Representative SEM images of the grown AuNPs using different sizes of Au seeds (9 mL of growth solution containing 0.01 M CTAB, 2.5 10 4 M HAuCl4 and 5.55 10 4 M ascorbic acid mixed with 0.5 mL of 18 (a), 12 (b), and 3.5 nm Au seeds (c), respectively; 9 mL of growth solution containing 0.01 M CTAB, 2.5 10 4 M HAuCl4 and 5.55 10 4 M ascorbic acid mixed with 0.02 mL of 18 (d), 12 (e), and 3.5 nm Au seeds (f), respectively). The scale bars represent 100 nm.
Figure 2. Representative SEM images of AuNPs obtained with 18 nm Au seeds and different concetrations of CTAB (a, 0.1 M; b, 0.05 M; c, 0.01 M; d, 0.005 M.). The scale bars represent 100 nm.
we systematically studied the effect of CTAB concentration on the spherical AuNPs. Figure 2 presents SEM images of AuNPs obtained with various concentrations of CTAB and a constant amount of the seeding solution. It was found that at high concentration of CTAB (equal to or higher than 0.05 M), the Au seeds (18 nm) grow to both spherical and nonspherical particles
such as rods, triangles, and hexagonal nanoplates (Figure 2a,b). The amount of spherical AuNPs obtained by the high CTAB concentration was estimated to be as low as ca. 40 and 70%, respectively. As a result, the UV vis spectra of the synthesized AuNPs (Figure 3), weak and broad peaks at approximately 695 and 728 nm corresponding to nonspherical shapes, are observed 13863
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Table 1. Average diameter, size distribution and Zeta potential of the grown QAuNPs Growth Seed
particle size
Size
Zeta potential
Sample
(mL)
(mL)
(nm)
distribution (%)
(mV)
1
1
0.5
24.5
8.1
44.3
2
5
0.5
37.5
7.5
41.5
3
7
0.5
40.8
6.8
25.6
4
9
0.5
43.6
7.6
44.0
5
11
0.5
47.5
5.8
41.5
6
13
0.5
50.0
7.5
46.0
7
9
0.2
55.4
7.8
28.7
8 9
9 9
0.1 0.08
79.1 94.6
5.5 8.3
29.8 45.7
10
9
0.05
107.7
7.5
35.2
11
9
0.03
125.2
6.6
62.5
12
9
0.02
152.9
5.1
56.5
Figure 3. UV vis spectra of Au colloids at different concetrations of CTAB (a, 0.1 M; b, 0.05 M; c, 0.01 M; d, 0.005 M.).
in addition to the absorption peaks at 530 nm ascribed to spherical AuNPs. At low concentration of CTAB (0.01 M), the seeds (18 nm) favor the growth into spherical shapes (Figure 1c, d). The nonspherical byproducts were hardly found; exhibiting single absorption peaks approximately at 530 nm most likely due to spherical AuNPs. However, at much lower concentration of CTAB (0.005 M), the obtained AuNPs appear to be elongated. The elongated NPs may be ascribed to the fast deposition of Au0 onto Au seeds, leading to slightly irregular growth of NPs. This finding implies that neither high nor low concentration of CTAB benefits homogeneous, high yield growth of spherical AuNPs. We further investigated the effect of temperature and the silver ions on resulting AuNPs. Temperature plays an important role in the reduction rate of Au ions and the subsequent growth rates of AuNPs.47,48 We explored various temperatures ranging from 30 to 80 °C, while maintaining the rest of the parameters unaltered (i.e., 9 mL of growth solution and 0.5 mL of 18 nm Au seeds). As the reaction temperature increased, the growth of Au seeds was expedited, the size of resulting Au spheres increased, and the number of nonspherical AuNPs increased (S-Figure 3 of the Supporting Information), which is in agreement with previous reports.41,49 The formation of nonspherical AuNPs is thought to occur due to anisotropic growth on the seed surface created by surfactant interactions with different crystalline facets and defects on the seed surface. Although there is no clear explanation on the role of silver ions in AuNP synthesis,17 the silver ions have an effect on the morphology of synthesized AuNPs. Sau and Murphy have reported that silver ions can favor the tunable synthesis of various shapes of AuNPs in aqueous solution.17 Nikoobakht and El-Sayed have proposed that silver ions could assist in the template elongation and also tune the length of Au nanorods in classic seed-mediated method.10 In the present study, we found that when silver ions were added into the growth solution, 18 nm Au seeds created irregular particles (S-Figure 4b of the Supporting Information). It can be however seen that an increased addition of silver ions in growth solution has little effect on the size and shape of the obtained AuNPs (S-Figure 4b d). At an increased temperature, the presence of the silver ions led to formation of a number of nanoplates (S-Figure 4e,f). After the above-mentioned optimization procedure, 18 nm Au seeds and 0.01 M CTAB were used in the following experiments to prepare larger spherical AuNPs at 30 °C. The size of synthesized AuNPs can be directly controlled by the ratio between the amount of growth solution and seed;40,41 the higher ratio of growth solution to seeds results in the larger size of synthesized AuNPs.31,40,41 By maintaining the amount of seed solution (0.5 mL) and varying the amount of growth solution (1, 5, 7,
Measured
solution
9, 11, and 13 mL), six sets of spherical AuNPs were obtained via one-step seeding growth method. The synthesized AuNPs corresponding to samples 1 6 in Table 1 are shown in Figure 4 (images a f corresponding to samples 1 6). The measured mean diameters for samples 1 6 are 24.5, 37.5, 40.8, 43.6, 47.5, and 50.0 nm, respectively (Table 1). The synthesized AuNPs exhibit a smooth surface, indicating that the grown AuNPs are not aggregates of smaller units. Figure 5 shows SEM images of AuNPs (images a f corresponding to samples 7 12) prepared by maintaining the growth solution (9 mL) and varying the amount of seed solution (1, 5, 7, 9, 11, and 13 mL). The measured mean diameters of samples 7 12 are 55.4, 79.1, 94.6, 107.7, 125.2, and 152.9 nm, respectively (Table 1). The UV vis absorption spectra, a f in Figure 6A and h m in Figure 6B, correspond to samples 1 6 (SEM in Figure 4) and 7 12 (SEM in Figure 5), respectively. As the size of the QAuNPs increases from 24.5 to 94.6 nm, the absorption peaks are noticeably red-shifted from 523 to 562 nm. When the size of QAuNPs increases to 107.7 nm, the absorption peak (Figure 6B k) is red-shifted to 584 nm and becomes rather broad, yet in a single band. With further increase of the QAuNPs size to 125.2 and 152.9 nm, the main absorption peaks are red-shifted to 670 nm and shoulder peaks appear at approximately 550 nm (Figure 6B, l and m), showing good agreement with previous reports.31,41 The synthesized QAuNPs via our proposed method have narrow size distributions (RSD ∼ 5 8%, S-Figure 5 of the Supporting Information), compared with previous reports by CTAB-assisted, seed-mediated growth (RSD ∼ 10 20%).40,41 More importantly, nonspherical AuNPs were hardly found in our grown QAuNPs. This improved monodispersity of QAuNPs can minimize laborious purification of NPs for further applications. The assembly of AuNPs in controlled architectures or patterns15,23,50,51 is a subject of great importance in nanoscience as the assembled architectures exhibit different collective properties from their counterparts15,50,51 in the case of both individual and bulk ones. It was observed that the CATB-capped QAuNPs could be spontaneously assembled into superstructures with highly aligned patterns by the conventional drop-dry method (Figure 7a). The assembled superstructures have a size ranging from 0.5 to 5 μm and facecentered cubic structures on silicon 13864
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Figure 4. Representative SEM images of the grown QAuNPs with different sizes in diameter. Images a f correspond to samples 1 6. The scale bars represent 100 nm.
Figure 5. Representative SEM images of the grown QAuNPs with different sizes in diameter. Images a f correspond to samples 7 12. The scale bars represent 100 nm.
Figure 6. UV vis spectra of Au colloids with different sizes (A (a f) and B (h m) corresponding to samples 1 6 and 7 12, respectively).
substrates. In addition, the superstructures exhibit multilayers (two to six layers as shown in Figure 7b f). The mechanism of the self-assembly of CTAB-capped QAuNPs into superstructure could be analogous to that of the vertical deposition self-assembly in
which particles in a solution are deposited on a substrate as the solution evaporates.15 Similarly, the formation of superstructures in the CTAB-capped QAuNPs is highly dependent on CTAB concentration, QAuNPs concentration, and inter-QAuNP van der Waals interactions.15,23 In addition, compared with AuNPs stabilized by a layer of citrate,52 which provide a high ζ potential for the prevention of particle aggregation, the CTAB-capped QAuNPs offer interconnected networks in multiple directions, where the interactions among neighboring QAuNP are minimized by CTAB through short-range repulsion and steric hindrance of the surfactant chains.23 A small drop of QAuNPs solution (5 μL) on silica substrate (5 5 mm) gradually evaporates until it reaches a critical concentration, where individual QAuNPs form a threephase contact line (the interface of vapor, substrate, and water). At this stage, the alkyl groups of CATB collapse to the minimum energy conformation, thus reducing the interparticle distance 13865
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Figure 7. Representative SEM images of superstructures assembled by sample 7 (a, low magnification of superstrctures; b f, two, three, four, five, and six layers, respectively). The scale bar for image a represents 2.5 μm, and images b f represent 100 nm.
Figure 8. Representative dark-field images of individual QAuNPs (from sample 7) with different assembly densities. The scale bars represent 5 μm. (The incubation durations for a c are 30, 120, and 300 s, respectively.)
and favoring their aggregation.53 These self-assembled superstructures of QAuNP hold great promise in the SERS- and LSPRbased sensors due to a larger number of hot spots and strong antenna effects.15 In addition, due to the positive charges on the surface of CATB-capped QAuNPs (Table 1), the convenient fabrication of QAuNP arrays can be achieved via electrostatic surfacial selfassembly. In our previous work,54 we have demonstrated an optical array using CTAB-capped Au nanorods for the application of single-nanoparticle plasmonic biosensor. CATB-capped, size-controllable QAuNPs possess tunable surface plasmon wavelengths from 523 to 670 nm. Figure 8 shows QAuNP chips with tunable density of QAuNPs on a glass substrate obtained by electrostatic self-assembly. The interparticle distances can be tuned in the range of 0.5 10 μm, which is desirable for single-nanoparticle plasmonic biosensor.
4. CONCLUSIONS We have shown that the modified one-step, CTAB-assisted, seedmediated growth method can prepare size-tunable QAuNPs with high monidispersity and purity. CATB-capped QAuNPs tend to assemble into closely packed and highly aligned superstructures
with well-defined mutillayers and conveniently form densitycontrollable arrays on solid substrates.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures showing representative field-emission scanning electron microscopy (FESEM) images of 18 nm Au seeds and the grown AuNPs at different temperatures and with use of different amounts of sliver ions, the size distributions of 18 nm Au seeds, and the grown QAuNPs with different sizes. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: 65-67904111. Fax: 65-67911761.
’ ACKNOWLEDGMENT We gratefully acknowledge the Nanyang Technological University for support of this work. We also acknowledge financial support from the Science and Engineering Research Council (Grant 102 152 0014) of Singapore. 13866
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