Efficient and Facile Synthesis of Gold Nanorods with Finely Tunable

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Efficient and Facile Synthesis of Gold Nanorods with Finely Tunable Plasmonic Peaks from Visible to Near-IR Range Liming Zhang,†,‡,∥ Kai Xia,†,∥ Zhuoxuan Lu,†,‡,∥ Guopeng Li,‡ Juan Chen,‡ Yan Deng,† Song Li,† Feimeng Zhou,§ and Nongyue He*,†,‡ †

Hunan Key Laboratory of Green Packaging and Application of Biological Nanotechnology, Hunan University of Technology, Zhuzhou 412008, P. R. China ‡ State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, P. R. China § Department of Chemistry and Biochemistry, California State University, Los Angeles, California 90032, United States S Supporting Information *

ABSTRACT: Although gold nanorods (GNRs) have been prepared with a wide range of methods for their uses as novel diagnostic and therapeutic agents, the synthesis of monodispersed GNRs with high yields and size tunability still requires further improvements. We report on a simple one-pot method for preparing highly monodispersed GNRs using phenols (e.g., hydroquinone, 1,2,3-trihydroxybenzene, and 1,2,4-trihydroxybenzene) as the reducing agent and NaBH4 as the initiating reactant. Finetuning of the LSPR peak position of phenols-reduced GNRs from 550 to 1150 nm is accomplished by regulating the silver ion concentrations. The size of GNRs produced via phenols reduction can also be controlled by changing the NaBH4 concentration. By systematically optimizing the concentrations of the reagents involved in the one-pot synthesis of GNRs, the yield (in many cases exceeding 90%) is significantly higher than that prepared with the commonly used reductant (e.g., ascorbic acid). The improved efficiency and controllability cut down the cost and time involved in GNRs production.

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peak position over 1000 nm.20,27 However, this method needs the introduction of additional surfactant. What’s more, the morphology and uniformity of as-prepared GNRs need further to be improved. Using a silver-free, seed-mediated procedure is also possible for the synthesis of GNRs with LSPR over 1000 nm, but spherical and platelet-shaped gold nanoparticles are also produced.9,28,29 In the presence of additives of aromatic compounds, Murray and co-workers developed monodispersed GNRs with LSPR peaks longer than 1000 nm.30 Recently, Zubarev and Vigderman used hydroquinone to prepare GNRs.31 Such a method greatly increases the yield and produced GNRs with LSPR peaks longer than 1000 nm. On-pot synthesis is much simpler than the traditional seedmediated method. In such a method, GNRs are synthesized by directly adding NaBH4 in a growth solution including the gold ion precursor, silver ions, CTAB, and ascorbic acid.32 The inherent limitation is that the prepared GNRs have relatively low monodispersity32 and the LSPR peaks are not tunable.33 Elsayed and co-workers, by adjusting solution pH and optimizing the amount of NaBH4,34 significantly improved the monodispersity of GNRs. However, to our knowledge, the tunability of the LSPR peak was not demonstrated. Furthermore, the

INTRODUCTION Gold nanorods (GNRs) have attracted considerable interest as novel diagnostic and therapeutic agents owing to their unique shape-dependent optical and electronic properties.1−14 Generally, the seed-mediated method15−21 and photochemical method22−24 are used, the former of which, pioneered by Murphy and co-workers18,21 and El-sayed and co-workers,20 is more popular. Two steps are involved in the seed-mediated method: (1) fresh preparation of gold seeds using NaBH4 as the reducing agent and (2) expansion of gold seeds into GNRs in a growth solution. One limitation of this method is that gold seeds could not be preserved over an extended period. Another problem is that the yield is rather low (only ∼15% of precursor gold ions are converted to metallic gold).20,25 The requirement of freshly prepared seeds and the low yield both increase the production cost. It is also noteworthy that the longitudinal localized surface plasmon resonance (LSPR) peak position of GNRs prepared is not longer than 850 nm.20 Considering that many interesting optical properties of GNRs depend on their sizes, reliable synthesis of GNRs with a finely and broadly tunable range of LSPR peak is highly desirable.5,26 Many efforts to prepare GNRs with an LSPR peak at longer wavelengths have proven successful. El-Sayed and co-workers improved their original method by using a mixed hexadecyltrimethylammonium bromide/benzyldimethylammonium chloride hydrate (CTAB/BDAC) surfactant, producing GNRs with the LSPR © 2014 American Chemical Society

Received: September 18, 2013 Revised: January 31, 2014 Published: February 14, 2014 1794

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was maintained at 30 °C in an incubator. The growth of GNRs is triggered by adding sodium borohydride. Phenols used in our experiment include hydroquinone, 1,2,3-trihydroxybenzene, and 1,2,4-trihydroxybenzene. TEM images display monodispersed and rod-like gold nanoparticles, while spherical and cubic nanoparticles are almost negligible (Figure 1a, b, and c).

yield in producing GNRs is not higher than that generated by the seed-mediated procedure. This limitation probably stems from the use of ascorbic acid as the reducing agent.20 We envision that the use of a milder reductant in one-pot synthesis should improve both the yield and the tunability of the resultant GNRs. We explored three different phenols (hydroquinone, 1,2,3-trihydroxybenzene, and 1,2,4-trihydroxybenzene) as reducing agents and systematically varied the concentrations of the key reagents in the synthesis (NaBH4, silver ions). Remarkable yields (in many cases exceeding 90%) were routinely obtained with GNR LSPR peaks adjustable across a broad range (from 550 nm to over 1000 nm). Our approach offers a convenient and cost-effective way to prepare high yield and uniform GNRs.

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EXPERIMENTAL SECTION Materials. Chloroauric acid, silver nitrate, NaBH4, hydroquinone, 1,2,3-trihydroxybenzene, and 1,2,4-trihydroxybenzene were purchased from Sinopharm Chemicals Reagent Co. Ltd. (China); CTAB was purchased from Sigma-Aldrich. Characterization. The morphologies and sizes of nanoparticles were characterized with a JEM-2100 TEM microscope. Vis−NIR spectrometric measurements were taken on a Shimadzu spectrophotometer. ICP-MS was measured with a PerkinElmer spectrometer. Synthesis of GNRs. To prepare the growth solution, HAuCl4 (0.01 M), silver nitrate (0.02 M), and phenols were sequentially added into 7.125 mL of CTAB (0.11 M) solution under slight shaking, followed by standing for 5 min. Then, NaBH4 (0.498 mM) solution was added and kept standing for over 12 h (final reactant concentrations are given in the figure and table captions). All experiments were conducted inside a 30 °C incubator. LSPR and TEM Measurements. GNRs were collected by centrifuging at 16 000g for 20 min and washing with distilled water. The precipitations were redispersed with distilled water, and LSPR spectra were measured on a spectrophotometer. TEM images of GNRs were collected from copper meshes cast with solution containing the redispersed precipitates. Inductively Coupled Plasma-Mass Spectrometry (ICPMS) Measurement. GNRs were collected by centrifuging and were washed with distilled water three times. The precipitations were dissolved with aqua regia and diluted with distilled water. The concentrations of gold ion were measured with ICP-MS.

Figure 1. TEM images of GNRs produced by reduction of (a) hydroquinone, (b) 1,2,3-trihydroxybenzene, and (c) 1,2,4-trihydroxybenzene reduced GNRs. HR-TEM images and vis−NIR spectra of the corresponding GNRs are shown in d−f and g−i, respectively. Other preparative parameters are (a) [CTAB] = 0.1 M, [HAuCl4] = 0.40 mM, [AgNO3] = 0.21 mM, [hydroquinone] = 5.26 mM, and [NaBH4] = 0.0017 mM; (b) [CTAB] = 0.1 M, [HAuCl4] = 0.40 mM, [AgNO3] = 0.11 mM, [1,2,3-trihydroxybenzene] = 0.36 mM, and [NaBH4] = 0.0078 mM; (c) [CTAB] = 0.1 M, [HAuCl4] = 0.40 mM, [AgNO3] = 0.053 mM, [1,2,3-trihydroxybenzene] = 0.8 mM, and [NaBH4] = 0.00067 mM, and total volume of the reactant is 7.6 mL.

For comparison, GNRs generated by ascorbic acid reduction were imaged with TEM. Many non-rod-shaped particles were found (Figure S1). Fast Fourier transform (FFT) of highresolution transmission electron microscopy (HR-TEM) images reveals that the phenol-reduced GNRs are singlecrystalline (Figure 1d−f). Vis−NIR spectra of GNRs prepared by different phenols are shown in Figure 1g−i. The GNRs with the longest LSPR wavelength (>1000 nm) were prepared when hydroquinone was used as the reducing agent. The use of 1,2,3trihydroxybenzene or 1,2,4-trihydroxybenzene as the reducing agents produced GNRs that generally possess the maximum LSPR peaks longer than 800 nm but shorter than 1000 nm. In contrast, the LSPR wavelength of GNRs produced with ascorbic acid as the reductant is shorter than 800 nm (Figure S1).32 It is recently reported that some aromatic additives may effectively mediate the binding between the CTAB bilayer and certain facets of growing GNRs, facilitating the improvement of the GNR monodispersity.30 Therefore, phenols in our experiments may play a dual role: (1) reducing Au3+ and aiding the growth of the GNRs and (2) mediating the binding between CTAB and GNRs. That the hydroquinone reduction produced GNRs with the highest aspect ratio and the longest LSPR wavelength suggests that this reducer is most effective in mediating the binding of CTAB and the facets of GNRs. Tuning the Aspect Ratio and the LSPR Peak of GNRs with Silver Ion Concentration. The unique optical characters of GNRs are governed by their aspect ratios,

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RESULTS AND DISCUSSION Synthesis of GNRs. As shown in Scheme 1, a growth solution containing CTAB, gold ion, silver nitrate, and phenols Scheme 1. Schematic of One-Pot Synthesis of GNRs Produced by Phenol Reduction

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the GNR LSPR peak is dependent on both the silver ion and NaBH4 concentrations. It should also be mentioned that excessive silver ion concentration (>0.182 mM) can cause a blue shift of the LSPR peak. The decrease in the aspect ratio at a high silver ion concentration has been reported for ascorbic acid as a reductant, and possibly attributed to the effect of ionic strength.20 We also investigated the effect of silver ion concentration on the aspect ratio of GNRs produced via 1,2,3-trihydroxybenzene or 1,2,4-trihydroxybenzene reduction. The aspect ratios of 1,2,3-trihydroxybenzene-reduced GNRs can also be fine-tuned by the silver ion concentration. Briefly, when the concentration of silver ions is increased from 0.055 to 0.16 mM, the aspect ratio of GNRs augments from 2.6 to 4.9. The corresponding LSPR peaks gradually shift from 668 to 900 nm (Figure S3). Again, at an exceedingly high silver ion concentration (0.325 mM), a lower aspect ratio (4.4), a blue shift of the LSPR peak (853 nm), and poorer uniformity in the GNR morphology were observed (Figure S3). A systematic tuning of the LSPR peak of 1,2,3-trihydroxybenzene-reduced GNRs by adding different silver ion concentrations (from 0.026 to 0.26 mM) is shown in Figure S4. The LSPR peak gradually increases from 620 to 900 nm. Essentially, the same trend was observed for the GNRs generated with the 1,2,4-trihydroxybenzene reduction (data not shown). Tuning the GNR Sizes with NaBH4 Concentration. The NaBH4 concentration is known to govern the number of crystal nuclei, which for a given gold ion concentration dictates the ultimate GNR dimension.18 Therefore, we also examined the effect of NaBH4 concentration on the GNR sizes. Figure 4

which are in turn affected by the silver ion concentration.17,18,30,31 We attempted to fine-tune the aspect ratio of GNRs by regulating the silver ion concentration. Figure 2

Figure 2. TEM images of GNRs produced via quinone reduction at different silver ion concentrations: (a) 0.08, (b) 0.16, (c) 0.26, and (d) 0.37 mM. Other preparative parameters are [CTAB] = 0.1 M, [HAuCl4] = 0.40 mM, [hydroquinone] = 5.26 mM, and [NaBH4] = 0.0017 mM. The total solution volume was 7.6 mL.

shows the TEM images of GNRs produced via hydroquinone reduction in the presence of 0.0017 mM NaBH4 and various concentrations of silver ion. GNRs with the smallest aspect ratio (∼1.8) were synthesized at the lowest silver ion concentration (0.08 mM). When silver ion concentrations were increased to 0.16, 0.26, and 0.37 mM, the corresponding aspect ratios of the resultant GNRs augmented to 3.5, 5.6, and 6.8. The corresponding LSPR peaks were observed at 598, 773, 976, and 1110 nm (Figure S2). However, when silver nitrate concentration is as high as 0.37 mM, the monodispersity of the resultant GNRs deteriorated, and some non-rod-shaped nanoparticles were observed (Figure 2d). Figure 3 shows the

Figure 3. Vis−NIR spectra of GNRs at different silver ion concentrations. Other GNR preparation conditions: [CTAB] = 0.1 M, [HAuCl4] = 0.40 mM, [hydroquinone] = 5.26 mM, and [NaBH4] = 0.017 mM. (Total reaction volume was 7.6 mL.)

LSPR peaks of hydroquinone-reduced GNRs at different silver ion concentrations and 0.017 mM of NaBH4 (which is 10 times as high as that used for obtaining Figure 2). The LSPR peaks can be fine-tuned across a broad spectral range (from 550 to 1110 nm) by judiciously choosing the amount of silver ions. It should be mentioned that the silver ion concentration needed is much lower compared to that in Figure 2 for the synthesis of GNRs with comparable LSPR peak position, suggesting that

Figure 4. TEM images of GNRs produced via 1,2,3-hydroxybenzene reduction at different NaBH4 concentrations: (a) 0.0013, (b) 0.0026, (c) 0.0039, and (d) 0.0078 mM. Other preparative parameters are [CTAB] = 0.1 M, [HAuCl4] = 0.40 mM, [AgNO3] = 0.11 mM, and [1,2,3-trihydroxybenzene] = 0.36 mM. Panel e depicts the LSPR peaks of GNRs shown in panels a−d. 1796

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weaken the interaction between CTAB and the facets of growing GNRs. GNR Yield. Ascorbic acid is a stronger reducing agent than the three phenols used in our work. Consequently, in the standard seed-mediated method, a rapid depletion of ascorbic acid could occur, which is probably the cause of the low reaction yield.25 Although adding excess ascorbic acid improves the yield, the rapid and uncontrolled seed growth results in the formation of other non-rod-shaped gold nanoparticles. Our one-pot synthesis utilizes the mild reduction power of phenols, which improves both the yield and the GNR quality. We used ICP-MS to verify whether phenols are better reducing agents for improving the yield. GNRs were purified by three cycles of centrifugation to remove any unreacted gold ions. As shown in Table 1, the yields of phenol-reduced GNRs are all more than 60% and, in many cases, exceed 90%. Such yields are 2.5−4-fold greater than that obtained with ascorbic acid as the reducing agent. It should be mentioned that CTAB may have a remarkably high affinity to Au ions, which can contribute toward slow secondary growth mechanisms.35,36 Therefore, we did a control experiment. We first washed the hydroquinonereduced GNRs by centrifugation three times. Then, we redispersed the precipitates and again centrifuged and collected precipitates and supernatants separately. Supernatants were directly diluted with distilled water, while GNRs in the precipitates were digested by aqua regia and diluted to optimal concentrations. ICP-MS measurements were conducted for both the supernatants and the diluted aqua-regia-digested GNRs solutions. We found that Au in the supernatants is detectable, but the amount is negligible (less than 0.1%), compared with that in the precipitates. This result suggests that a few Au ions may actually be adsorbed on the CTAB layer.

shows TEM images of GNRs produced via 1,2,3-trihydroxybenzene reduction. When the NaBH4 concentration is increased from 0.0013 to 0.0078 mM, the diameter decreased from 45.2 to 13.1 nm and the length from 111 to 57.8 nm (Figure 4a and Figure S8). The GNR volume decreases inversely with the NaBH4 concentration (Figure 5). A similar

Figure 5. The volume of GNRs produced via 1,2,3-trihydroxybenzene reduction plotted against the NaBH4 concentration.

trend was also observed for GNRs produced via hydroquinone or 1,2,4-trihydroxybenzene reduction (Figures S6 and S7). These results clearly demonstrate that it is feasible to prepare GNRs with desired sizes by adjusting the amounts of NaBH4. It should be noted that the NaBH4 concentration affects not only the GNR sizes but also the positions of the LSPR peaks. A drastic red shift of the LSPR peak of the 1,2,3-trihydroxybenzene-reduced GNRs was observed when the concentration of NaBH4 was increased from 0.0013 to 0.0039 mM (Figure 4e). Further increase of the NaBH4 concentration leads to a negligible shift in the LSPR peak, because the aspect ratio of GNRs no longer changes. It should be mentioned that NaBH4 solution is not stable in aqueous solution and may lead to GNRs with ill-defined morphology under some conditions (e.g., preservation for an extended time). We next studied the effect of using different batches of NaBH4 solution on the morphology of GNRs. Our results indicate that three batches of freshly prepared NaBH4 using ice water leads to a negligible change in the morphology and LSPR of GNRs (Figure S9). In addition to the NaBH4 concentration, incubation temperature was also found to affect the LSPR peak position and the GNR morphology. Higher temperature decreases the size of GNRs (Figure S10). This may be ascribed to the higher efficiency of producing seeds at higher temperatures. Furthermore, a significant blue shift of the LSPR peak occurs at higher temperatures. We hypothesize that a higher temperature may

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CONCLUSION In summary, we have developed an efficient and facile one-pot method for preparing high monodispersed GNRs using phenols as reductants and NaBH4 as the initiating reactant. Fine-tuning of the LSPR peaks of GNRs can be achieved across a broad range (from 550 to 1110 nm) by varying the silver ion concentrations. Moreover, tuning the size of the GNRs can be accomplished by changing the NaBH4 concentration. GNRs prepared by our method possess much higher yields than those produced with ascorbic acid. This new one-pot method has great potential for preparing high-quality GNRs for a range of applications.

Table 1. Yields of GNRs at Different Reagent Concentrations with a Constant HAuCl4 Concentration (0.4 mM) concentrations (mM) reducing agent hydroquinone

1,2,3-trihydroxybenzene

1,2,4-trihydroxybenzene

ascorbic acid

silver ion

NaBH4

reducing agent

LSPR

TEM figures

yields (%)

0.08 0.16 0.263 0.11 0.11 0.32 0.027 0.055 0.11 0.11

0.0017 0.0017 0.0017 0.0039 0.0026 0.0039 0.00066 0.00066 0.00066 0.00066

5.26 5.26 5.26 0.36 0.36 0.36 0.80 0.80 0.80 0.60

598 771 976 858 780 845 705 835 861 763

2a 2b 2c 4c 4b S 3d S5a S5b S5c S1

100 94.0 99.2 91.9 100 63.0 76.1 99.2 84.8 24.7

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ASSOCIATED CONTENT

S Supporting Information *

Seed-mediated GNRs preparation by using trihydroxybenzene as reductant; Forming dogbone-like GNRs using trihydroxybenzene as reducer; TEM image of ascorbic acid−reduced GNRs; hydroquinone-reduced GNRs at 0.00033 and 0.0017 mM NaBH4, and GNRs produced with 1,2,3-trihydroxybenzene or 1,2,4-trihydroxybenzene reductions GNRs at different AgNO3 and NaBH4 concentrations; TEMs of GNRs produced at different temperatures; VIS-NIR spectra of hydroquinoneand 1,2,3-trihydroxybenzene-reduced GNRs at different AgNO3 concentrations; Statistical analysis of the GNR sizes and a fitting curve using LSPR as a function of AR; TEM images and LSPR spectra of GNRs initiated by three different batches of freshly prepared NaBH4 solutions; TEM images and LSPR spectra of GNRs produced only with different temperatures.This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

Joint first authors, contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by two 973 Projects (No. 2010CB933903 and No. 2014CB744501), NSFC (No. 61271056, No. 21205036 and No. 61301039), Hunan Provincial Natural Science Foundation of China (No. 12JJ4049 and No. 13JJ4091), Scientific Research Fund of Hunan Provincial Education Department (No. 13A003). F. Z. also acknowledges partial support from the NSF-Center for Research Excellence in Science and Technology (NSF HRD0932421). We are grateful for the help from Prof. Litao Sun and Dr. Yilong Zhou on the HR-TEM measurements at the College of Electronic Science & Engineering at Southeast University.

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