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Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine Gaoxing Su, Chi Yang, and Jun-Jie Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504041f • Publication Date (Web): 18 Dec 2014 Downloaded from http://pubs.acs.org on December 22, 2014
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Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine Gaoxing Su,†, ‡ Chi Yang,*,† and Jun-Jie Zhu*, ‡ †
School of Pharmacy, Nantong University, Nantong 226001, China
‡
State Key Lab of Analytical Chemistry for Life Science, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, China
KEYWORDS: gold nanorods, dopamine, seed-mediated growth, CTAB, longitudinal surface plasmon resonance
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ABSTRACT: Hydroxyphenol compounds are often used as reductants in controlling the growth of nanoparticles. Herein, dopamine was used as an effective reductant in seed-mediated synthesis of gold nanorods (GNRs). The as-prepared GNRs (83×16 nm) were monodisperse and had high degree of purity. The conversion ratio from gold ions to GNRs was around 80%. In addition, dopamine worked as an additive. At a very low concentration of hexadecyltrimethylammonium bromide (CTAB) (0.025 M), thinner and shorter GNRs (60×9 nm) were successfully prepared. By regulating the concentration of silver ions, CTAB, seeds, and reductant, GNRs with longitudinal surface plasmon resonance (LSPR) peaks ranging from 680 to 1030 nm were synthesized. The growth process was tracked using UV-vis-NIR spectroscopy, and it was found that a slow growth rate was beneficial to the formation of GNRs.
INTRODUCTION
Gold nanoparticles (GNPs) have been widely used in biomedicine, catalysis, biosensing, electronic, photonics, etc.1-6 These applications depend on unique physical and chemical properties of the synthesized GNPs, which can be tuned by changing the shape, size, and crystallinity. Many efforts have been taken on the synthesis of anisotropic gold nanostructures, such as nanorods, nanostars, multipods, and so on.7-10 Among them, gold nanorods (GNRs) have attracted the most attention, due to their tunable shape-dependent optical properties.6 Therefore, efficient and reliable synthesis
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of GNRs with a broad range of longitudinal surface plasmon resonance (LSPR) is highly desirable.
Seed-mediated growth is the most widely used method in the synthesis of GNRs, which was first developed by Murphy and El-Sayed.11,12 In their methods, hexadecyltrimethylammonium bromide (CTAB) and silver nitrate were necessary for the formation of rod-like particles; small CTAB-capped GNPs (~4 nm) were used as seeds. GNRs with different aspect ratios were synthesized by varying silver ion concentration or by using binary surfactant mixture. Some other factors, such as temperature,13 pH,14 surfactant types,15-18 concentration of reactants,13,19 additives,20-22 and seed quality,23,24 also influenced the growth of GNRs. By regulating these factors, GNRs with different morphologies and LSPR peaks could be synthesized. Usually, ascorbic acid was employed as a reducing agent, and the GNR growth was very sensitive to its concentration. Low concentration of ascorbic acid led to little GNR growth, while a high concentration led to the formation of spherical particles. Therefore, it is necessary to seek effective reducing agents in seed-mediated synthesis of GNRs.
Polyphenolic compounds are often observed reductants in nature. Although their reduction potentials are lower than ascorbic acid, polyphenolic compounds are able to reduce Au(III) to Au(I) at room temperature. Recently, it was reported that three different phenols were used to synthesize GNRs.25 Dopamine and other hydrophenols were found to be effective reductants in the synthesis of branched GNPs.26 Dopamine 3
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was also used in synthesis of silver,27,28 Zinc Oxide,29 and iron oxide nanoparticles.30 Besides the reducing ability, dopamine and generated polydopamine can be also used as binding agents,31 which is helpful in nanoparticle stabilization and shape control. However, until now, the use of reductive dopamine in seed-mediated growth of GNRs has not been reported.
Herein, we develop a novel route for the controlled synthesis of GNRs using dopamine as a reductant. Incubating the growth solution at 40 °C, monodispersed GNRs were successfully prepared. LSPR peaks and aspect ratios of the synthesized GNRs could be tuned by adjusting the concentration of silver ions, CTAB, seeds, and reductant.
EXPERIMENTAL SECTION
Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4•3H2O, ACS reagent) was purchased from Alfa Aesar. Hexadecyltrimethylammonium bromide (CTAB, >98%), silver nitrate (AgNO3, >99%), sodium borohydride (NaBH4, >98%), dopamine hydrochloride, and ascorbic acid were purchased from Sigma-Aldrich. All chemicals were used without further purification. Ultrapure water (18.2 MΩ•cm, Milli-Q, Millipore) was used in all experiments. All glassware was immersed into aqua regia for 1 h, then washed with ultrapure water several times and dried before use. 4
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Synthesis of GNRs. Seed-mediated method was used to prepare the GNRs. Typically, two steps were included. Firstly, gold seeds were synthesized as reported previously.12 A HAuCl4 solution (250 µL of 10 mM) was added to the CTAB solution (9.75 mL, 0.1 M), then, under vigorous stirring, a freshly prepared NaBH4 solution (0.6 mL, 0.01 M) was injected. The solution color changed immediately from yellow to dark brown. After stirring for 5 min, the mixture solution as seed solution A, was kept for at least 1 h at room temperature before it was used in next step.
Secondly, GNRs were synthesized in a growth solution. A HAuCl4 solution (500 µL of 10 mM) was added to 9.5 mL of the CTAB solution. The mixture solution was incubated at 40 °C for 10 min. Then AgNO3 solution (0.1 M), dopamine hydrochloride solution (0.2 M), and seed solution were added sequentially. The resulting growth solution was mixed thoroughly and kept undisturbed in a water bath set at 40 °C for 3 h.
Synthesis of Seeds with Large Size. Seeds with large size were prepared according to previously reported methods with minor modification.23 A HAuCl4 solution (250 µL of 10 mM) was added to the CTAB solution (6.75 mL, 0.1 M). Under stirring, 50 µL of 0.1 M ascorbic acid solution was added, and next, seed solution A (3 mL) was added. After stirring for 10 min, the obtained solution was centrifuged at 40,000 rpm for 1 h. The pellet was redispersed in 10 mL of 0.1 M CTAB solution. This solution was called seed solution B.
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To synthesize seed solution C, 250 µL of 10 mM HAuCl4 solution was added to the CTAB solution (8.75 mL, 0.1 M). Under stirring, ascorbic acid solution (50 µL, 0.1 M) and seed solution A (1 mL) was added. After additional stirring for 10 min, the obtained solution was centrifuged at 20,000 rpm for 1 h. The pellet was redispersed in 10 mL of 0.1 M CTAB solution, obtaining seed solution C.
LSPR Measurements. After synthesis was finished, the growth solution was diluted for LSPR measurements. The UV-vis-NIR spectra were recorded on a Shimadzu UVmini-1240 spectrometer. TEM Measurements. Before being added to the copper grids, gold seed solutions were centrifuged at 60,000 (A), 40,000 (B), or 20,000 rpm (C) for 1 h and washed once with water. GNRs were obtained by centrifuging the growth solution at 13,000 rpm for 20 min. The pellets were washed once with water and redispersed with water. The size and shape of the obtained nanoparticles were observed by transmission electron microscopy (TEM) (JEM-1011, operating at 100 kV) and high resolution TEM (HRTEM) (JEM-2100, operating at 200 kV).
Gold Concentration Measurements. Before adding the dopamine, a small amount (0.1 mL) of the growth solution was taken, adding to 0.9 mL of aqua regia. After incubation for 1 h, the mixture was diluted for inductively coupled plasma-mass spectrometry (ICP-MS) measurements (Agilent 8800). After the synthesis was finished, the growth solution was centrifuged at 13,000 rpm for 20 min. The pellets were washed with water once and redispersed in 1 mL of water as GNR stock solution. 6
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Then, 10 µL of stock solution was mixed with 990 µL of aqua regia. After incubation for 1 h, the solution was diluted with water for ICP-MS measurements. By calculating the gold contents in the GNR stock solution and growth solution, the yields of GNRs were obtained.
Dopamine Concentration Measurements. After the synthesis was finished, GNRs were removed by centrifugation at 13,000 rpm for 20 min. the supernatant was diluted one time and filtered with 0.45 µm pore size. A 20 µL of the solution was injected into high performance liquid chromatography (HPLC) (Shimadzu LC-20AD) for analysis. A C18 column (5 µm, 2.0 × 150 mm) was used for separation. UV detection (Shimadzu SPD-20A) was performed at 280 nm. The concentration of dopamine was calculated from the calibration curve.
FTIR Spectroscopy. The growth solution was centrifuged at 13,000 rpm for 20 min. The pellet was washed with water twice and dried in vacuum at 50 ˚C for 12 h. The dried GNRs and 200 mg of KBr were ground and pressed to a pellet. Fouriertransform infrared (FTIR) spectroscopy was carried out on a Nicolet iS10 spectrometer (Thermo-Fisher Scientific).
RESULTS AND DISCUSSIONS
Dopamine as a Reductant for the Synthesis of Gold Nanorods. In the growth solution, dopamine hydrochloride was used as a reductant. At 40 °C, Au(III) could be 7
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reduced to Au(I), and the color of the growth solution changed from yellow to light yellow (Figure 1A). This process was confirmed by measuring the absorption of the Au(III)-CTAB complex at 396 nm with UV-vis spectroscopy.32 After dopamine was added for 1 min, the absorption of the growth solution at 396 nm decreased dramatically (Figure 1B). After reaction for 20 min, almost all the Au(III) was reduced. When gold seeds (~3.7 nm, Figure S1) were added, the color of the growth solution changed from yellow to red-brown color within 3 h, indicating that the Au(I) was reduced to Au(0) as well as the formation of GNRs. As shown in the TEM images (Figure 2A, 2B), rod-like monodispersed particles with an aspect ratio of approximately 5 (83×16 nm) were obtained. HRTEM image shows that the obtained GNRs are single-crystalline (Figure 2C). The LSPR peak of the GNRs is around 910 nm as shown in Figure 2D. The concentration of the GNR stock solution was 0.80 mg/mL measured by ICP-MS. The origin gold ion concentration in growth solution was 0.094 mg/mL. Considering their volumes, it was calculated that the conversion ratio from gold ions to GNRs was around 80%. This is a significant improvement, since the conversion ratio was only 15% when ascorbic acid was used as a reductant.33 To investigate the effect of seed size on the growth of GNRs, seeds of about 6 nm and 9 nm in size were synthesized (Figure S1). When the 6 nm seeds were used to synthesize GNRs, a large number of non-well-defined particles mixed with some nanorods were produced (Figure S2A). Furthermore, when the 9 nm seeds were used,
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almost no nanorods could be observed (Figure S2B). Therefore, the seeds larger than 4 nm in size are not suitable for the preparation of GNRs when using dopamine as a reductant. In this reduction reaction, one dopamine molecule could reduce one Au(III) to Au(I). After the synthesis, the amount of dopamine was reduced from 0.105 mmol to 0.092mmol. Because dopamine was oxidized to a dopaminequinone, which is highly reactive to amine groups, resulting in the formation of polydopamine (Figure 1A), there is no molar equivalent between dopamine and Au(III).
Dopamine as an Additive to Decrease the CTAB Concentration in the Growth Solution. Recently, it has been reported that additives play an important role in the monodispersity, morphology, yield, and reproducibility of seeded growth of GNRs.20-22,32 Murray group reported that aromatic additives (salicylic acids or their salts) could intercalate into the CTAB bilayer and control the size and shape of the GNRs.20 Also, the CTAB concentration in the growth solution could be reduced to 0.05 M by using aromatic additives. In our experiments, dopamine with an aromatic structure acts not only as a reductant but also as an additive. Rod-like particles could be formed at lower CTAB concentration. When the CTAB concentration decreased from 0.1 M to 0.075 M, the LSPR peak almost did not change (Figure 3A, S3B). The LSPR peak showed a red-shift to 980 nm when the CTAB concentration decreased to 0.05 M, (Figure 3A, S3A). Furthermore, when the CTAB concentration decreased to
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0.025 M, the LSPR peak continued to red-shift to 1030 nm. TEM image shows the GNRs with an aspect ratio of 6-7 at this concentration (Figure 3C). Compared to the GNRs synthesized at the CTAB concentration of 0.1 M, the GNRs became thinner and shorter with diameter and length of ~9 nm and ~60 nm, respectively. At a lower concentration of CTAB, a higher aspect ratio of GNRs was obtained, and the yield was around 85%. Dopamine or polydopamine are likely to be involved in the interactions between the CTAB and the GNRs, which can control the size and shape of the GNRs. To prove this, FTIR spectrum of the synthesized GNRs was obtained (Figure 4). The strong and wide band at 3445 cm-1 arises from the stretching vibration of hydroxyl groups, whereas the intensity of pure CTAB at this region is weak. The strong absorption at 2918 and 2850 cm-1 are caused by the C-H stretching vibration of methyl and methylene groups of CTAB. The observed small bands at 1710 cm-1 can be attributed to the carboxyl groups. The intensities of the bands at 1626 and 1402 cm1
are stronger than those of pure CTAB, which are caused by the vibrations of
aromatic rings. In the spectrum of pure CTAB, the bands at the region of 1400~1500 cm-1 arise from the C-H bending vibration. In the fingerprint region, the bands at 1100 and 822 cm-1 are contributed from the C-O stretching and in plane C-H bending vibration, which were no appeared in the spectrum of pure CTAB. These results suggest that dopamine or polydopamine exists at the surface of the GNRs. As a result, synthesis of GNRs at a low CTAB concentration can facilitate the purification processes.
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When the CTAB concentration decreased to 0.01 M, big spherical and short rodlike particles appeared (Figure 3B), which suggests that GNRs are not able to be synthesized at this concentration of CTAB.
Tuning the LSPR Peak of the GNRs with the Concentration of Silver Ions. Varying the concentration of silver ions in the growth solution is an often used strategy to tune the aspect ratios and LSPR peaks of GNRs. Different amounts of 0.1 M nitrate silver solution were used in the growth of GNRs. When only 10 µL of 0.1 M nitrate silver solution was added to growth solution, GNRs with the LSPR peak at around 685 nm were synthesized (Figure 5A), corresponding to smallest aspect ratio (~2.5) (Figure 5B, 5H). When the amounts of nitrate silver solution increased to 20 and 40 µL, the LSPR peaks showed a red-shift to 805 and 1000 nm, respectively (Figure 5A), and the corresponding aspect ratios of GNRs increased to ~4.0 (Figure 5C, 5H) and ~5.6 (Figure 5D). By increasing the amounts of silver ions to 70, 100, and 140 µL, however, the aspect ratios of the as-prepared GNRs decreased to ~5.1 (Figure 5E), ~4.0 (Figure 5F), and ~3.3 (Figure 5G), respectively. The corresponding LSPR peaks showed a blue-shift. This tendency is similar to the observation for the preparation of GNRs by using ascorbic acid as a reducing agent.12 Silver ions are necessary for the formation of rod-like particles, however, high silver ion concentration with high ionic strength can decrease the length of GNRs and produce more particles rather than nanorods. High silver ion concentration led to end-cap morphology changes. Figure 5F and 5G show the GNRs change to cylinder or 11
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trapezoid caps, instead of hemisphere caps. The structure is similar to the GNRs synthesized with hydroquinone.34 Effects of the Concentration of the Seeds and Reductant on GNR Growth. Seed concentration is a factor that influences the LSPR peak of the GNRs.34 Figure 6A shows this effect on GNR growth. When the amounts of seed solution A increased from 80 µL to 240 µL, the LSPR peaks showed a gradual shift from 863 to 942 nm, the increase was not linear. When the seed solution increased from 80 µL to 120µL, the LSPR peaks show a 25 nm red-shift, however, when it increased from 200 µL to 240µL, the LSPR peaks only showed a 10 nm red-shift. In this condition, the yield of the GNRs was 83%. TEM images show GNRs synthesized with different amounts of seed solution were high quality, monodisperse and had negligible spherical particles (Figure 6B and S4). Therefore, adjusting seed concentration is a good strategy for tuning the optical properties of GNRs. Reductant concentration is another important parameter in GNR growth.33,34 In the reported method, to obtain high quality GNRs, the molar ratio of ascorbic acid to gold ions was approximately 1:1.33 Figure 6C shows the effect of reductant concentration on GNR growth. When 10 mg of dopamine were used, the LSPR peak of GNRs was 937 nm. However, the TEM image shows the quality of GNRs is poor, which has a mix of spherical particles and different sized rods as shown in Figure 6D. Some black dots could be observed in the TEM image. By amplifying the TEM images, it was found that these black dots were smaller than 5 nm, which was similar to the seed
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size. When 10 mg of dopamine was used, the yield was only around 46%, which was much lower than that of the synthesis when 20 mg of dopamine was used. More seeds did not grow to big particles. It is no doubt these black dots are gold seeds. The LSPR peak showed a maximum red-shift to 955 nm when the amount of dopamine increased to 15 mg (Figure 6C, S5A). After that, further increasing the amounts of dopamine led to the blue-shift of LSPR peaks. When the amount of dopamine increased from 30 to 40 mg, the LSPR peaks showed a 55 nm blue-shift (Figure 6C, S5B, S5C). When the amount of dopamine increased from 20 to 30 mg, however, the LSPR peaks of GNRs showed a negligible shift. Thus, minor variations of the reductant concentration cannot influence the growth of GNRs.
Growth Kinetics of GNRs Synthesized by Dopamine. The growth rate is an important parameter for the morphology of the prepared nanoparticles.32,35-37 The growth process was tracked by UV-vis-NIR absorption spectra (Figure 7A). In the silver-assisted preparation of GNRs, two separate stages were identified according to the changes in the spectra. Firstly, the LSPR peaks show a fast red-shift, then a slow blue-shift,34,38 which agrees with our results. In the first 1.5 h, the LSPR peak redshifted to as far as 1010 nm, an anisotropic structure was created at this stage. When the growth was stopped after 1 h, small nanorods were formed (Figure 7B). In the next 1.5 h, the LSPR peaks blue-shifted to 920 nm, at this stage, the volume of small rods increased gradually (Figure 7C), and the aspect ratios showed a slight decrease. During the reaction, the absorption intensities show consistent increase, indicating the 13
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continuous growth of rods. When the GNRs were kept at room temperature for 12 h, the LSPR peak showed a slight blue-shift. Due to atomic defects on the unstable facets, gold atoms on the nanorod surface would reconstruct and the size of the particles changed slightly, with a LSPR peak shift.39
Although reductive dopamine used in the experiment was largely in excess, the whole growth process lasted for 3 h. The growth rate was much slower than the method using ascorbic acid as a reductant. A slow growth rate is beneficial to the synthesis of high quality nanorods.34 Increasing the growth rate by increasing incubation temperature to 50 °C，the LSPR peak of the GNRs had almost a 100 nm blue-shift (Figure S6), indicating the decrease of the aspect ratios of the GNRs. At 60 °C, the growth rate was too fast to produce rods. A strong absorption at 550 nm in the UV-vis-NIR spectrum of the GNRs suggests the appearance of large amounts of spherical particles. Thus, a weak reductant decreases the growth rate, which controls the growth of GNRs to obtain high quality GNRs.
CONCLUSIONS
In summary, monodispersed GNRs were successfully synthesized using dopamine hydrochloride as a reductant. By replacing ascorbic acid with dopamine, having a surfactant concentration as low as 0.025M, high quality GNRs (60×9 nm) with a LSPR peak at 1030 nm were produced. This was because dopamine has an aromatic structure that can act as an additive to interact with the CTAB bilayers. The 14
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GNR yield was improved to around 80%. This method was less sensitive to reductant concentration, which was 20 or more times higher than the gold concentration in the growth solution. Moreover, the LSPR peaks and aspect ratios could be tuned by varying the concentration of silver ions, seeds, and reductant. The weak reducing ability of dopamine decreased the growth rate of GNRs, which was beneficial for the formation of quality GNRs.
ACKNOWLEDGMENT This work was supported by Natural Science Foundation of China (21405084 and 21305120), and the National Basic Research Program of China (2011CB933502).
ASSOCIATED CONTENT
Supporting Information
Additional TEM images and UV-vis-NIR absorption spectra of GNRs were included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author 15
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*
Tel: 86-513-85051728; E-mail: [email protected] (Chi Yang)
Tel: 86-25-83597204; E-mail: [email protected] (Jun-Jie Zhu)
Notes The authors declare no competing financial interest.
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(17) Wadams, R. C.; Fabris, L.; Vaia, R. A.; Park, K. Time-dependent susceptibility of the growth of gold nanorods to the addition of a cosurfactant. Chem. Mater. 2013, 25, 4772-4780. (18) Ye, X. C.; Zheng, C.; Chen, J.; Gao, Y. Z.; Murray, C. B. Using binary surfactant mixtures to simultaneously improve the dimensional tunability and monodispersity in the seeded growth of gold nanorods. Nano Lett. 2013, 13, 765-771. (19) Sau, T. K.; Murphy, C. J. Seeded high yield synthesis of short Au nanorods in aqueous solution. Langmuir 2004, 20, 6414-6420. (20) Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; Doan-Nguyen, V.; Kang, Y.; Engheta, N.; Kagan, C. R. Improved size-tunable synthesis of monodisperse gold nanorods through the use of aromatic additives. ACS Nano 2012, 6, 2804-2817. (21) Wen, T.; Hu, Z.; Liu, W.; Zhang, H.; Hou, S.; Hu, X.; Wu, X. Copper-ionassisted growth of gold nanorods in seed-mediated growth: significant narrowing of size distribution via tailoring reactivity of seeds. Langmuir 2012, 28, 17517-17523. (22) DuChene, J. S.; Niu, W.; Abendroth, J. M.; Sun, Q.; Zhao, W.; Huo, F.; Wei, W. D. Halide anions as shape-directing agents for obtaining high-quality anisotropic gold nanostructures. Chem. Mater. 2013, 25, 1392-1399. (23) Gole, A.; Murphy, C. J. Seed-mediated synthesis of gold nanorods: role of the size and nature of the seed. Chem. Mater. 2004, 16, 3633-3640. (24) Jiang, X. C.; Pileni, M. P. Gold nanorods: Influence of various parameters as seeds, solvent, surfactant on shape control. Colloids Surf. A: Physicochem. Eng. Aspects 2007, 295, 228-232. (25) Zhang, L.; Xia, K.; Lu, Z.; Li, G.; Chen, J.; Deng, Y.; Li, S.; Zhou, F.; He, N. Efficient and facile synthesis of gold nanorods with finely tunable plasmonic peaks from visible to near-IR range. Chem. Mater. 2014, 26, 1794-1798. (26) Lee, Y.; Park, T. G. Facile fabrication of branched gold nanoparticles by reductive hydroxyphenol derivatives. Langmuir 2011, 27, 2965-2971. 18
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(27) Ma, Y.; Niu, H.; Zhang, X.; Cai, Y. One-step synthesis of silver/dopamine nanoparticles and visual detection of melamine in raw milk. Analyst 2011, 136, 41924196. (28) Liao, Y.; Wang, Y.; Feng, X.; Wang, W.; Xu, F.; Zhang, L. Antibacterial surfaces through dopamine functionalization and silver nanoparticle immobilization. Mater. Chem. Phys. 2010, 121, 534-540. (29) Huang, W.; Jiang, P.; Wei, C.; Zhuang, D.; Shi, J. Low-temperature one-step synthesis of covalently chelated ZnO/dopamine hybrid nanoparticles and their optical properties. J. Mater. Res. 2008, 23, 1946-1952. (30) Shultz, M. D.; Reveles, J. U.; Khanna, S. N.; Carpenter, E. E. Reactive nature of dopamine as a surface functionalization agent in iron oxide nanoparticles. J. Am. Chem. Soc. 2007, 129, 2482-2487. (31) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426-430. (32) Scarabelli, L.; Grzelczak, M.; Liz-Marzán, L. M. Tuning gold nanorod synthesis through prereduction with salicylic acid. Chem. Mater. 2013, 25, 4232-4238. (33) Orendorff, C. J.; Murphy, C. J. Quantitation of metal content in the silverassisted growth of gold nanorods. J. Phys. Chem. B 2006, 110, 3990-3994. (34) Vigderman, L.; Zubarev, E. R. High-yield synthesis of gold nanorods with longitudinal SPR peak greater than 1200 nm using hydroquinone as a reducing agent. Chem. Mater. 2013, 25, 1450-1457. (35) Langille, M. R.; Personick, M. L.; Zhang, J.; Mirkin, C. A. Defining rules for the shape evolution of gold nanoparticles. J. Am. Chem. Soc. 2012, 134, 14542-14554. (36) Gu, J.; Zhang, Y. W.; Tao, F. Shape control of bimetallic nanocatalysts through well-designed colloidal chemistry approaches. Chem. Soc. Rev. 2012, 41, 8050-8065. (37) Huang, M. H.; Chiu, C.-Y. Achieving polyhedral nanocrystal growth with systematic shape control. J. Mater. Chem. A 2013, 1, 8081-8092. (38) Edgar, J. A.; McDonagh, A. M.; Cortie, M. B. Formation of gold nanorods by a stochastic "popcorn" mechanism. ACS Nano 2012, 6, 1116-1125. 19
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(39) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. Surface reconstruction of the unstable {110} surface in gold nanorods. J. Phys. Chem. B 2000, 104, 54175420.
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Figure 1. (A) Chemical scheme of Au(III) was reduced to Au(I) by dopamine and further reduced to Au(0) with the assistant of gold seeds. (B) Kinetics study the reduction of Au(III) by measuring the absorption of Au(III)-CTAB complex at 396 nm. The UV absorption spectra of CTAB solution with Au(III) were recorded before and after dopamine was added.
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Figure 2. TEM images of GNRs fabricated by the reduction of dopamine hydrochloride at 40 °C for three hours. AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water) and seed solution A (160 µL) were added to growth solution subsequently. [CTAB] = 0.1 M. (A) Low magnification; (B) Medium magnification; (C) HRTEM images; (D) UV-vis-NIR Absorption spectra of the synthesized GNRs.
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Figure 3. (A) UV-vis-NIR absorption spectra of the synthesized GNRs at CTAB concentration ranging from 0.01 M to 0.1 M. (B) TEM images of the synthesized GNRs at 0.01 M CTAB solution. (C) TEM images of the synthesized GNRs at 0.025 M CTAB solution. AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water) and Seed solution A (160 µL) were used in preparation.
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Figure 4. FTIR spectra of pure CTAB (black) and synthesized GNRs (blue). GNR preparation parameters: AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water), seed solution A (160 µL), [CTAB] = 0.1 M.
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Figure 5. UV-vis-NIR spectra (A) and representative TEM images (B-G) of GNRs prepared with different amounts of nitrate silver solution (0.1 M): (B) 10 µL, (C) 20 µL, (D) 40 µL, (E) 70 µL, (F) 100 µL, (G) 140 µL. (H) Aspect ratios calculated from TEM images of GNRs by counting 100 particles. Dopamine hydrochloride (20 mg in 0.5 mL water) and Seed solution A (160 µL) were used in preparation.[CTAB] = 0.1 M.
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Figure 6. (A) UV-vis-NIR spectra of GNRs prepared with different amounts of seed solution A. (B) TEM images of GNRs prepared with 240 µL seed solution A. Other preparation parameters: AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water), and [CTAB] = 0.1 M. (C) UV-vis-NIR spectra of GNRs prepared with different amounts of dopamine hydrochloride, which were dissolved in 0.5 mL water. (D) TEM images of GNRs prepared with 10 mg dopamine hydrochloride. Other preparation parameters: AgNO3 (70 µL, 0.1 M), Seed solution A (160 µL), and [CTAB] = 0.1 M.
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Figure 7. (A) UV-vis-NIR spectra of GNRs at different growth stages. (B) TEM images of GNR growth at 1 hour. (C) TEM images of GNR growth at 2 hour. Preparation parameters: AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water), Seed solution A (160 µL), and [CTAB] = 0.1 M.
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Table of Content (TOC)
Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine Gaoxing Su,†, ‡ Chi Yang,*,† Jun-Jie Zhu*, ‡
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Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine Gaoxing Su, Chi Yang, and Jun-Jie Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504041f • Publication Date (Web): 18 Dec 2014 Downloaded from http://pubs.acs.org on December 22, 2014
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Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine Gaoxing Su,†, ‡ Chi Yang,*,† and Jun-Jie Zhu*, ‡ †
School of Pharmacy, Nantong University, Nantong 226001, China
‡
State Key Lab of Analytical Chemistry for Life Science, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, China
KEYWORDS: gold nanorods, dopamine, seed-mediated growth, CTAB, longitudinal surface plasmon resonance
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ABSTRACT: Hydroxyphenol compounds are often used as reductants in controlling the growth of nanoparticles. Herein, dopamine was used as an effective reductant in seed-mediated synthesis of gold nanorods (GNRs). The as-prepared GNRs (83×16 nm) were monodisperse and had high degree of purity. The conversion ratio from gold ions to GNRs was around 80%. In addition, dopamine worked as an additive. At a very low concentration of hexadecyltrimethylammonium bromide (CTAB) (0.025 M), thinner and shorter GNRs (60×9 nm) were successfully prepared. By regulating the concentration of silver ions, CTAB, seeds, and reductant, GNRs with longitudinal surface plasmon resonance (LSPR) peaks ranging from 680 to 1030 nm were synthesized. The growth process was tracked using UV-vis-NIR spectroscopy, and it was found that a slow growth rate was beneficial to the formation of GNRs.
INTRODUCTION
Gold nanoparticles (GNPs) have been widely used in biomedicine, catalysis, biosensing, electronic, photonics, etc.1-6 These applications depend on unique physical and chemical properties of the synthesized GNPs, which can be tuned by changing the shape, size, and crystallinity. Many efforts have been taken on the synthesis of anisotropic gold nanostructures, such as nanorods, nanostars, multipods, and so on.7-10 Among them, gold nanorods (GNRs) have attracted the most attention, due to their tunable shape-dependent optical properties.6 Therefore, efficient and reliable synthesis
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of GNRs with a broad range of longitudinal surface plasmon resonance (LSPR) is highly desirable.
Seed-mediated growth is the most widely used method in the synthesis of GNRs, which was first developed by Murphy and El-Sayed.11,12 In their methods, hexadecyltrimethylammonium bromide (CTAB) and silver nitrate were necessary for the formation of rod-like particles; small CTAB-capped GNPs (~4 nm) were used as seeds. GNRs with different aspect ratios were synthesized by varying silver ion concentration or by using binary surfactant mixture. Some other factors, such as temperature,13 pH,14 surfactant types,15-18 concentration of reactants,13,19 additives,20-22 and seed quality,23,24 also influenced the growth of GNRs. By regulating these factors, GNRs with different morphologies and LSPR peaks could be synthesized. Usually, ascorbic acid was employed as a reducing agent, and the GNR growth was very sensitive to its concentration. Low concentration of ascorbic acid led to little GNR growth, while a high concentration led to the formation of spherical particles. Therefore, it is necessary to seek effective reducing agents in seed-mediated synthesis of GNRs.
Polyphenolic compounds are often observed reductants in nature. Although their reduction potentials are lower than ascorbic acid, polyphenolic compounds are able to reduce Au(III) to Au(I) at room temperature. Recently, it was reported that three different phenols were used to synthesize GNRs.25 Dopamine and other hydrophenols were found to be effective reductants in the synthesis of branched GNPs.26 Dopamine 3
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was also used in synthesis of silver,27,28 Zinc Oxide,29 and iron oxide nanoparticles.30 Besides the reducing ability, dopamine and generated polydopamine can be also used as binding agents,31 which is helpful in nanoparticle stabilization and shape control. However, until now, the use of reductive dopamine in seed-mediated growth of GNRs has not been reported.
Herein, we develop a novel route for the controlled synthesis of GNRs using dopamine as a reductant. Incubating the growth solution at 40 °C, monodispersed GNRs were successfully prepared. LSPR peaks and aspect ratios of the synthesized GNRs could be tuned by adjusting the concentration of silver ions, CTAB, seeds, and reductant.
EXPERIMENTAL SECTION
Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4•3H2O, ACS reagent) was purchased from Alfa Aesar. Hexadecyltrimethylammonium bromide (CTAB, >98%), silver nitrate (AgNO3, >99%), sodium borohydride (NaBH4, >98%), dopamine hydrochloride, and ascorbic acid were purchased from Sigma-Aldrich. All chemicals were used without further purification. Ultrapure water (18.2 MΩ•cm, Milli-Q, Millipore) was used in all experiments. All glassware was immersed into aqua regia for 1 h, then washed with ultrapure water several times and dried before use. 4
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Synthesis of GNRs. Seed-mediated method was used to prepare the GNRs. Typically, two steps were included. Firstly, gold seeds were synthesized as reported previously.12 A HAuCl4 solution (250 µL of 10 mM) was added to the CTAB solution (9.75 mL, 0.1 M), then, under vigorous stirring, a freshly prepared NaBH4 solution (0.6 mL, 0.01 M) was injected. The solution color changed immediately from yellow to dark brown. After stirring for 5 min, the mixture solution as seed solution A, was kept for at least 1 h at room temperature before it was used in next step.
Secondly, GNRs were synthesized in a growth solution. A HAuCl4 solution (500 µL of 10 mM) was added to 9.5 mL of the CTAB solution. The mixture solution was incubated at 40 °C for 10 min. Then AgNO3 solution (0.1 M), dopamine hydrochloride solution (0.2 M), and seed solution were added sequentially. The resulting growth solution was mixed thoroughly and kept undisturbed in a water bath set at 40 °C for 3 h.
Synthesis of Seeds with Large Size. Seeds with large size were prepared according to previously reported methods with minor modification.23 A HAuCl4 solution (250 µL of 10 mM) was added to the CTAB solution (6.75 mL, 0.1 M). Under stirring, 50 µL of 0.1 M ascorbic acid solution was added, and next, seed solution A (3 mL) was added. After stirring for 10 min, the obtained solution was centrifuged at 40,000 rpm for 1 h. The pellet was redispersed in 10 mL of 0.1 M CTAB solution. This solution was called seed solution B.
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To synthesize seed solution C, 250 µL of 10 mM HAuCl4 solution was added to the CTAB solution (8.75 mL, 0.1 M). Under stirring, ascorbic acid solution (50 µL, 0.1 M) and seed solution A (1 mL) was added. After additional stirring for 10 min, the obtained solution was centrifuged at 20,000 rpm for 1 h. The pellet was redispersed in 10 mL of 0.1 M CTAB solution, obtaining seed solution C.
LSPR Measurements. After synthesis was finished, the growth solution was diluted for LSPR measurements. The UV-vis-NIR spectra were recorded on a Shimadzu UVmini-1240 spectrometer. TEM Measurements. Before being added to the copper grids, gold seed solutions were centrifuged at 60,000 (A), 40,000 (B), or 20,000 rpm (C) for 1 h and washed once with water. GNRs were obtained by centrifuging the growth solution at 13,000 rpm for 20 min. The pellets were washed once with water and redispersed with water. The size and shape of the obtained nanoparticles were observed by transmission electron microscopy (TEM) (JEM-1011, operating at 100 kV) and high resolution TEM (HRTEM) (JEM-2100, operating at 200 kV).
Gold Concentration Measurements. Before adding the dopamine, a small amount (0.1 mL) of the growth solution was taken, adding to 0.9 mL of aqua regia. After incubation for 1 h, the mixture was diluted for inductively coupled plasma-mass spectrometry (ICP-MS) measurements (Agilent 8800). After the synthesis was finished, the growth solution was centrifuged at 13,000 rpm for 20 min. The pellets were washed with water once and redispersed in 1 mL of water as GNR stock solution. 6
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Then, 10 µL of stock solution was mixed with 990 µL of aqua regia. After incubation for 1 h, the solution was diluted with water for ICP-MS measurements. By calculating the gold contents in the GNR stock solution and growth solution, the yields of GNRs were obtained.
Dopamine Concentration Measurements. After the synthesis was finished, GNRs were removed by centrifugation at 13,000 rpm for 20 min. the supernatant was diluted one time and filtered with 0.45 µm pore size. A 20 µL of the solution was injected into high performance liquid chromatography (HPLC) (Shimadzu LC-20AD) for analysis. A C18 column (5 µm, 2.0 × 150 mm) was used for separation. UV detection (Shimadzu SPD-20A) was performed at 280 nm. The concentration of dopamine was calculated from the calibration curve.
FTIR Spectroscopy. The growth solution was centrifuged at 13,000 rpm for 20 min. The pellet was washed with water twice and dried in vacuum at 50 ˚C for 12 h. The dried GNRs and 200 mg of KBr were ground and pressed to a pellet. Fouriertransform infrared (FTIR) spectroscopy was carried out on a Nicolet iS10 spectrometer (Thermo-Fisher Scientific).
RESULTS AND DISCUSSIONS
Dopamine as a Reductant for the Synthesis of Gold Nanorods. In the growth solution, dopamine hydrochloride was used as a reductant. At 40 °C, Au(III) could be 7
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reduced to Au(I), and the color of the growth solution changed from yellow to light yellow (Figure 1A). This process was confirmed by measuring the absorption of the Au(III)-CTAB complex at 396 nm with UV-vis spectroscopy.32 After dopamine was added for 1 min, the absorption of the growth solution at 396 nm decreased dramatically (Figure 1B). After reaction for 20 min, almost all the Au(III) was reduced. When gold seeds (~3.7 nm, Figure S1) were added, the color of the growth solution changed from yellow to red-brown color within 3 h, indicating that the Au(I) was reduced to Au(0) as well as the formation of GNRs. As shown in the TEM images (Figure 2A, 2B), rod-like monodispersed particles with an aspect ratio of approximately 5 (83×16 nm) were obtained. HRTEM image shows that the obtained GNRs are single-crystalline (Figure 2C). The LSPR peak of the GNRs is around 910 nm as shown in Figure 2D. The concentration of the GNR stock solution was 0.80 mg/mL measured by ICP-MS. The origin gold ion concentration in growth solution was 0.094 mg/mL. Considering their volumes, it was calculated that the conversion ratio from gold ions to GNRs was around 80%. This is a significant improvement, since the conversion ratio was only 15% when ascorbic acid was used as a reductant.33 To investigate the effect of seed size on the growth of GNRs, seeds of about 6 nm and 9 nm in size were synthesized (Figure S1). When the 6 nm seeds were used to synthesize GNRs, a large number of non-well-defined particles mixed with some nanorods were produced (Figure S2A). Furthermore, when the 9 nm seeds were used,
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almost no nanorods could be observed (Figure S2B). Therefore, the seeds larger than 4 nm in size are not suitable for the preparation of GNRs when using dopamine as a reductant. In this reduction reaction, one dopamine molecule could reduce one Au(III) to Au(I). After the synthesis, the amount of dopamine was reduced from 0.105 mmol to 0.092mmol. Because dopamine was oxidized to a dopaminequinone, which is highly reactive to amine groups, resulting in the formation of polydopamine (Figure 1A), there is no molar equivalent between dopamine and Au(III).
Dopamine as an Additive to Decrease the CTAB Concentration in the Growth Solution. Recently, it has been reported that additives play an important role in the monodispersity, morphology, yield, and reproducibility of seeded growth of GNRs.20-22,32 Murray group reported that aromatic additives (salicylic acids or their salts) could intercalate into the CTAB bilayer and control the size and shape of the GNRs.20 Also, the CTAB concentration in the growth solution could be reduced to 0.05 M by using aromatic additives. In our experiments, dopamine with an aromatic structure acts not only as a reductant but also as an additive. Rod-like particles could be formed at lower CTAB concentration. When the CTAB concentration decreased from 0.1 M to 0.075 M, the LSPR peak almost did not change (Figure 3A, S3B). The LSPR peak showed a red-shift to 980 nm when the CTAB concentration decreased to 0.05 M, (Figure 3A, S3A). Furthermore, when the CTAB concentration decreased to
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0.025 M, the LSPR peak continued to red-shift to 1030 nm. TEM image shows the GNRs with an aspect ratio of 6-7 at this concentration (Figure 3C). Compared to the GNRs synthesized at the CTAB concentration of 0.1 M, the GNRs became thinner and shorter with diameter and length of ~9 nm and ~60 nm, respectively. At a lower concentration of CTAB, a higher aspect ratio of GNRs was obtained, and the yield was around 85%. Dopamine or polydopamine are likely to be involved in the interactions between the CTAB and the GNRs, which can control the size and shape of the GNRs. To prove this, FTIR spectrum of the synthesized GNRs was obtained (Figure 4). The strong and wide band at 3445 cm-1 arises from the stretching vibration of hydroxyl groups, whereas the intensity of pure CTAB at this region is weak. The strong absorption at 2918 and 2850 cm-1 are caused by the C-H stretching vibration of methyl and methylene groups of CTAB. The observed small bands at 1710 cm-1 can be attributed to the carboxyl groups. The intensities of the bands at 1626 and 1402 cm1
are stronger than those of pure CTAB, which are caused by the vibrations of
aromatic rings. In the spectrum of pure CTAB, the bands at the region of 1400~1500 cm-1 arise from the C-H bending vibration. In the fingerprint region, the bands at 1100 and 822 cm-1 are contributed from the C-O stretching and in plane C-H bending vibration, which were no appeared in the spectrum of pure CTAB. These results suggest that dopamine or polydopamine exists at the surface of the GNRs. As a result, synthesis of GNRs at a low CTAB concentration can facilitate the purification processes.
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When the CTAB concentration decreased to 0.01 M, big spherical and short rodlike particles appeared (Figure 3B), which suggests that GNRs are not able to be synthesized at this concentration of CTAB.
Tuning the LSPR Peak of the GNRs with the Concentration of Silver Ions. Varying the concentration of silver ions in the growth solution is an often used strategy to tune the aspect ratios and LSPR peaks of GNRs. Different amounts of 0.1 M nitrate silver solution were used in the growth of GNRs. When only 10 µL of 0.1 M nitrate silver solution was added to growth solution, GNRs with the LSPR peak at around 685 nm were synthesized (Figure 5A), corresponding to smallest aspect ratio (~2.5) (Figure 5B, 5H). When the amounts of nitrate silver solution increased to 20 and 40 µL, the LSPR peaks showed a red-shift to 805 and 1000 nm, respectively (Figure 5A), and the corresponding aspect ratios of GNRs increased to ~4.0 (Figure 5C, 5H) and ~5.6 (Figure 5D). By increasing the amounts of silver ions to 70, 100, and 140 µL, however, the aspect ratios of the as-prepared GNRs decreased to ~5.1 (Figure 5E), ~4.0 (Figure 5F), and ~3.3 (Figure 5G), respectively. The corresponding LSPR peaks showed a blue-shift. This tendency is similar to the observation for the preparation of GNRs by using ascorbic acid as a reducing agent.12 Silver ions are necessary for the formation of rod-like particles, however, high silver ion concentration with high ionic strength can decrease the length of GNRs and produce more particles rather than nanorods. High silver ion concentration led to end-cap morphology changes. Figure 5F and 5G show the GNRs change to cylinder or 11
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trapezoid caps, instead of hemisphere caps. The structure is similar to the GNRs synthesized with hydroquinone.34 Effects of the Concentration of the Seeds and Reductant on GNR Growth. Seed concentration is a factor that influences the LSPR peak of the GNRs.34 Figure 6A shows this effect on GNR growth. When the amounts of seed solution A increased from 80 µL to 240 µL, the LSPR peaks showed a gradual shift from 863 to 942 nm, the increase was not linear. When the seed solution increased from 80 µL to 120µL, the LSPR peaks show a 25 nm red-shift, however, when it increased from 200 µL to 240µL, the LSPR peaks only showed a 10 nm red-shift. In this condition, the yield of the GNRs was 83%. TEM images show GNRs synthesized with different amounts of seed solution were high quality, monodisperse and had negligible spherical particles (Figure 6B and S4). Therefore, adjusting seed concentration is a good strategy for tuning the optical properties of GNRs. Reductant concentration is another important parameter in GNR growth.33,34 In the reported method, to obtain high quality GNRs, the molar ratio of ascorbic acid to gold ions was approximately 1:1.33 Figure 6C shows the effect of reductant concentration on GNR growth. When 10 mg of dopamine were used, the LSPR peak of GNRs was 937 nm. However, the TEM image shows the quality of GNRs is poor, which has a mix of spherical particles and different sized rods as shown in Figure 6D. Some black dots could be observed in the TEM image. By amplifying the TEM images, it was found that these black dots were smaller than 5 nm, which was similar to the seed
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size. When 10 mg of dopamine was used, the yield was only around 46%, which was much lower than that of the synthesis when 20 mg of dopamine was used. More seeds did not grow to big particles. It is no doubt these black dots are gold seeds. The LSPR peak showed a maximum red-shift to 955 nm when the amount of dopamine increased to 15 mg (Figure 6C, S5A). After that, further increasing the amounts of dopamine led to the blue-shift of LSPR peaks. When the amount of dopamine increased from 30 to 40 mg, the LSPR peaks showed a 55 nm blue-shift (Figure 6C, S5B, S5C). When the amount of dopamine increased from 20 to 30 mg, however, the LSPR peaks of GNRs showed a negligible shift. Thus, minor variations of the reductant concentration cannot influence the growth of GNRs.
Growth Kinetics of GNRs Synthesized by Dopamine. The growth rate is an important parameter for the morphology of the prepared nanoparticles.32,35-37 The growth process was tracked by UV-vis-NIR absorption spectra (Figure 7A). In the silver-assisted preparation of GNRs, two separate stages were identified according to the changes in the spectra. Firstly, the LSPR peaks show a fast red-shift, then a slow blue-shift,34,38 which agrees with our results. In the first 1.5 h, the LSPR peak redshifted to as far as 1010 nm, an anisotropic structure was created at this stage. When the growth was stopped after 1 h, small nanorods were formed (Figure 7B). In the next 1.5 h, the LSPR peaks blue-shifted to 920 nm, at this stage, the volume of small rods increased gradually (Figure 7C), and the aspect ratios showed a slight decrease. During the reaction, the absorption intensities show consistent increase, indicating the 13
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continuous growth of rods. When the GNRs were kept at room temperature for 12 h, the LSPR peak showed a slight blue-shift. Due to atomic defects on the unstable facets, gold atoms on the nanorod surface would reconstruct and the size of the particles changed slightly, with a LSPR peak shift.39
Although reductive dopamine used in the experiment was largely in excess, the whole growth process lasted for 3 h. The growth rate was much slower than the method using ascorbic acid as a reductant. A slow growth rate is beneficial to the synthesis of high quality nanorods.34 Increasing the growth rate by increasing incubation temperature to 50 °C，the LSPR peak of the GNRs had almost a 100 nm blue-shift (Figure S6), indicating the decrease of the aspect ratios of the GNRs. At 60 °C, the growth rate was too fast to produce rods. A strong absorption at 550 nm in the UV-vis-NIR spectrum of the GNRs suggests the appearance of large amounts of spherical particles. Thus, a weak reductant decreases the growth rate, which controls the growth of GNRs to obtain high quality GNRs.
CONCLUSIONS
In summary, monodispersed GNRs were successfully synthesized using dopamine hydrochloride as a reductant. By replacing ascorbic acid with dopamine, having a surfactant concentration as low as 0.025M, high quality GNRs (60×9 nm) with a LSPR peak at 1030 nm were produced. This was because dopamine has an aromatic structure that can act as an additive to interact with the CTAB bilayers. The 14
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GNR yield was improved to around 80%. This method was less sensitive to reductant concentration, which was 20 or more times higher than the gold concentration in the growth solution. Moreover, the LSPR peaks and aspect ratios could be tuned by varying the concentration of silver ions, seeds, and reductant. The weak reducing ability of dopamine decreased the growth rate of GNRs, which was beneficial for the formation of quality GNRs.
ACKNOWLEDGMENT This work was supported by Natural Science Foundation of China (21405084 and 21305120), and the National Basic Research Program of China (2011CB933502).
ASSOCIATED CONTENT
Supporting Information
Additional TEM images and UV-vis-NIR absorption spectra of GNRs were included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author 15
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*
Tel: 86-513-85051728; E-mail: [email protected] (Chi Yang)
Tel: 86-25-83597204; E-mail: [email protected] (Jun-Jie Zhu)
Notes The authors declare no competing financial interest.
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(39) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. Surface reconstruction of the unstable {110} surface in gold nanorods. J. Phys. Chem. B 2000, 104, 54175420.
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Figure 1. (A) Chemical scheme of Au(III) was reduced to Au(I) by dopamine and further reduced to Au(0) with the assistant of gold seeds. (B) Kinetics study the reduction of Au(III) by measuring the absorption of Au(III)-CTAB complex at 396 nm. The UV absorption spectra of CTAB solution with Au(III) were recorded before and after dopamine was added.
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Figure 2. TEM images of GNRs fabricated by the reduction of dopamine hydrochloride at 40 °C for three hours. AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water) and seed solution A (160 µL) were added to growth solution subsequently. [CTAB] = 0.1 M. (A) Low magnification; (B) Medium magnification; (C) HRTEM images; (D) UV-vis-NIR Absorption spectra of the synthesized GNRs.
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Figure 3. (A) UV-vis-NIR absorption spectra of the synthesized GNRs at CTAB concentration ranging from 0.01 M to 0.1 M. (B) TEM images of the synthesized GNRs at 0.01 M CTAB solution. (C) TEM images of the synthesized GNRs at 0.025 M CTAB solution. AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water) and Seed solution A (160 µL) were used in preparation.
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GNRs
2918 1100 822 1710
1402 1626 1466
2850 3445 2918
4000
3000 2000 1000 -1 Wavenumber （cm ）
Figure 4. FTIR spectra of pure CTAB (black) and synthesized GNRs (blue). GNR preparation parameters: AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water), seed solution A (160 µL), [CTAB] = 0.1 M.
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Figure 5. UV-vis-NIR spectra (A) and representative TEM images (B-G) of GNRs prepared with different amounts of nitrate silver solution (0.1 M): (B) 10 µL, (C) 20 µL, (D) 40 µL, (E) 70 µL, (F) 100 µL, (G) 140 µL. (H) Aspect ratios calculated from TEM images of GNRs by counting 100 particles. Dopamine hydrochloride (20 mg in 0.5 mL water) and Seed solution A (160 µL) were used in preparation.[CTAB] = 0.1 M.
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Figure 6. (A) UV-vis-NIR spectra of GNRs prepared with different amounts of seed solution A. (B) TEM images of GNRs prepared with 240 µL seed solution A. Other preparation parameters: AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water), and [CTAB] = 0.1 M. (C) UV-vis-NIR spectra of GNRs prepared with different amounts of dopamine hydrochloride, which were dissolved in 0.5 mL water. (D) TEM images of GNRs prepared with 10 mg dopamine hydrochloride. Other preparation parameters: AgNO3 (70 µL, 0.1 M), Seed solution A (160 µL), and [CTAB] = 0.1 M.
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Figure 7. (A) UV-vis-NIR spectra of GNRs at different growth stages. (B) TEM images of GNR growth at 1 hour. (C) TEM images of GNR growth at 2 hour. Preparation parameters: AgNO3 (70 µL, 0.1 M), Dopamine hydrochloride (20 mg in 0.5 mL water), Seed solution A (160 µL), and [CTAB] = 0.1 M.
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Table of Content (TOC)
Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine Gaoxing Su,†, ‡ Chi Yang,*,† Jun-Jie Zhu*, ‡
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