20January2017

Nano-Micro Letters

Rapid Seedless Synthesis of Gold Nanoplates with Micro-scaled Edge Length in a High Yield and Their Application in SERS

Sheng Chen1,2, Pengyu Xu1,2, Yue Li,1,2, Junfei Xue1,2, Song Han2, Weihui Ou2, Yaping Ding1, Weihai Ni2,*

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Nano-Micro Letters, October 2016, Volume 8, Issue 4, pp 328–335

First Online: 11 May 2016 (Article)

DOI: 10.1007/s40820-016-0092-6

*Corresponding author. E-mail: whni2012@sinano.ac.cn

 

Abstract

 


Figure discription XXX
We report a facile and reproducible approach towards rapid seedless synthesis of single crystalline gold nanoplates with edge length on the order of microns. The reaction is carried out by reducing gold ions with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB). Reaction temperature and molar ratio of CTAB/Au are critical for the formation of gold nanoplates in a high yield, which are respectively optimized to be 85°Cand 6. The highest yield that can be achieved is 60% at the optimized condition. The synthesis to achieve the micro-scaled gold nanoplates can be finished in less than 1 h under proper reaction conditions. Therefore, the reported synthesis approach is time- and cost-effective. The gold nanoplates were further employed as the surface-enhanced Raman scattering (SERS) substrates and investigated individually. Interestingly, only those adsorbed with gold nanoparticles exhibit pronounced Raman signals of probe molecules, where a maximum enhancement factor of 1.7 × 107 was obtained. The obtained Raman enhancement can be ascribed to the plasmon coupling between the gold nanoplate and nanoparticle adsorbed on it.

 

Keywords

Gold nanoplates; Seedless synthesis; SERS; CTAB

 

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1 Introduction

 

Noble metalnanocrystalshave attracted a great amount of attention because of their unique light absorbing and scattering properties due to the localized surface plasmon resonance[1-3].Wet chemical approaches has been developed towards the synthesis of avariety of metal nanocrystals[4,5],such as nanospheres[6],nanorods[7,8],nanoplates[9,10],nanowires[11,12] etc.Comparing to 0D and 1D counterparts, 2D anisotropic nanocrystals, such as gold nanoplateshave large surface areas, sharp corners and edges which can provide high enhancement of electric field[13-17], and therefore, achieve extensive applications includingbio-imaging[18],nanodevices[19],and surface-enhanced Raman scattering (SERS)[20], etc.

The growth of gold nanoplates can be directed by either temples orcapping agents. By providing constrictions in a 2D space or dimension, planar substrates[14] and interfaces in lamellar bilayer membranes[21] have been used as effective temples for growth of gold nanoplates. Alternatively, capping agents can preferentially adsorb on a specific surface of goldso that the adsorption of gold ions to this surface is blocked and the growth is restricted on a planar direction. These agents should be surfactants[22], polymers[23-26], biomolecules[27], and halide ions[28,29]. Among them, cetyltrimethylammonium bromide (CTAB) is one of the most frequently used surfactants for the growth of gold nanoplates, which can be easily adsorbed on the surface of gold through complexing with halide ions. For example, Mirkin group developed the seed-mediated growth of small gold nanoplates using CTAB as the capping agent[30].Although the seed-mediated synthesis process effectively prohibits secondary nucleation and easily controls size and shape of the final product, it involves multistep growth of seeds.To solve this problem, Huanggroupdeveloped a seedless approach to synthesizegold nanoplates in presence of CTABvia thermal reduction, where reaction solutions were preheated before they were mixed together to ensure the control of the size distribution[22,31].However, long preheating time will lead to time-consuming and inefficient. Recently, high-yield synthesis of gold nanoplates with submicron edge length was reported where iodide ions were used as both the capping and etching agent[28].However,rapid synthesisof gold nanoplates in micro-scaled edge length with highyield, simplicityand low-coststill remainchallenging.

Herein, we report a facile and reproducibleapproachofrapid seedless synthesis of singlecrystalline gold nanoplates withedge length in micron order of magnitude. The reaction wascarried out by reducing gold ions with ascorbic acid in the presence of CTAB.The reaction temperature and molar ratio of CTAB/Au on the products were examined in detail. The SERS properties of as-synthesized gold nanoplateswere alsoinvestigated.

 

2 Experimental

 

2.1  Chemicals

 

Hydrogen tetrachloroaurate tetrahydrate (HAuCl4•3H2O), L-ascorbic acid (AA), cetyltrimethylammonium bromide (CTAB), and4-Mercaptophenol (Mph) werepurchased from Sigma-Aldrich.All chemicals and reagents were used without any further purification. Ultrapure water was obtained from a Milli-Q system (18.2 MΩcm-1).

 

2.2  Synthesis of Gold Nanoplates

 

A typical synthesis procedure isas the following:First,100 µL of 0.1 M HAuCl4 was added in 3 mLof 0.02M CTAB aqueous solution in a plastic tube, andthe mixturewas left undisturbed for several minutes.Then 100 µL of 0.1 M AA was added to the mixture, followed by rapid inversion for 10sec.The resultantsolution was immediately placed ina water bathof 85°C and kept undisturbedfor about 1 h. The productswere washedby centrifugation at 4000 rpm for 10 min and finallydispersed in deionizedwater.

 

2.3  Preparation ofSERS Substrates

 

The SERS substrate was prepared as the following:Gold nanoplatesolution was drop-casted onto a clean siliconsubstrate. The substrate was rinsed and blowndryby nitrogen gas. After that, it was immersed into a solution of gold nanoparticles for several minutes, allowing deposition of gold nanoparticleson the gold nanoplates. Afterthoroughly rinsedwithwaterfor several times, it was immersed into a solution of 0.01 M Mph for 3 h. The substratewascarefully rinsedandblown dry by nitrogen gas before the SERS measurement. A cross bar was finallymarked on the substrate for locating the gold nanoplate and investigating them individuallyunder the optical microscope and SEM.

 

2.4  Characterizations

 

The extinction spectra of the gold nanoplateswere obtainedby an Agilent Cary 60 UV-Vis spectrophotometer using a cuvette with 0.5 cm path length. Themorphology of the gold nanoplates was characterized by Hitachi S-4800 field emission and FEI Quanta 250 FEG SEMs. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance powder X-ray diffractometer at a scanning rate of 4° min-1, using Cu-Kα radiation (λ=1.54056 Å).Raman scattering spectrawere measured on amicro-Raman system (HR evolution 260, Horiba). The sample was excited at 633 nm and 4 mW in the Raman measurement. The Raman scattered lightwas collected withan Olympus objective (100X, N.A.=0.9, W.D.=1 mm). Raman spectra were recorded usinga grating of 600 lines per mm with anintegration time of 15 sec.

 

3 Results and Discussion

 

As shown in Fig. 1, the synthesis of the gold nanoplates is very straightforward. To be specific, aqueous solutions of HAuCl4 and CTABwerefirstly mixed andleft undisturbed at room temperature forseveral minutes.The color of the mixture solutionslowly changed to red brown owing to the formation of AuBr4-complex ions[25,32,33].After that, a reducing reagent of AAwas injected, which quickly makes the mixture to be colorless.The reaction solution was kept undisturbed in a water bath of 85°Cfor 1 h, and some precipitation of the resultant products was foundat the bottom of the glassvial.

Figure 1 Schematic illustration of the synthesis of the gold nanoplates

Fig. 1 Schematic illustration of the synthesis of the gold nanoplates.

Figure 2a shows the extinction spectra of as-prepared gold nanoplates in an aqueous solution and the inset compares the color of the solution before and after the synthesis reaction. The gold nanoplates exhibit a broad absorption bandstarting from 500 nm, which can beascribed to the dipole and quadrupole plasmon resonances[25,34,35].The morphologies of resultant productswere examined by SEM (see Fig.2b) and statistically analyzed in Fig. S1 ofSupporting Information (SI). Though a smallamount of spherical nanoparticles were observed as well, the resultant plate-like products were mainly composed of 8.5% triangular and 91.5% equilateral hexagonal nanoplates with average edge lengthsof 3.5 µm,and thickness around 114 nm (obtained from the tiltedSEM image shown in the inset), which strongly confirms the comparatively monodisperse and uniformnanoplates.

The XRD pattern of gold nanoplates was recorded using a quartz substrate(Fig.3).The diffraction peaks are assigned to (111), (200), (220), and (311) planes of faced-centered-cubic structure of Au (PDF No. 04-0784). Note that the intensity of Au (111)peakis much stronger than those of (200), (220), and (311).The diffraction intensity ratio of (200)/(111) is extremely lower than the standard file (0.0051 versus 0.52). ThisXRD resultclearly indicates thatthe as-synthesized gold naonoplates are singlecrystalline and possessa preferred plane of (111).

Figure 2 a The extinction spectrum of gold nanoplates in aqueous solution. The inset shows the photographs of the solutions before and after the growth of gold nanoplates. b The SEM image of gold nanoplates. The inset shows a cross-section SEM image of the gold nanoplates.

Fig. 2a The extinction spectrum of gold nanoplates in aqueous solution. The inset shows the photographs of the solutions before and after the growth of gold nanoplates. b The SEM image of gold nanoplates. The inset shows a cross-section SEM image of the gold nanoplates.

 Fig. 3 XRD pattern of as-prepared gold nanoplates deposited on quartz

Fig. 3 XRD pattern of as-prepared gold nanoplates deposited on quartz

To achievea high yield, the synthesis conditions that affect the growth of gold nanoplates were finely optimized. It is known that the reaction temperature plays an important role in the formation of anisotropic nanostructures[36,37].Therefore, in our experiment thereaction temperaturewas varied from 25 to 95°C, as illustrated in Fig.4.Figure 4a shows the morphologies of resultant products synthesized at room temperature (25°C) wheremost of them are identified as plate- and sphere-like particles. Both the yield and the edge length of the nanoplates start to increase when the temperature is increased. Figure 4b, c shows the products synthesized at45and 65°C, respectively.At 85°C, a large amount of hexagonal nanoplates with edge length about ~3.5 µmcan be found where a highest yield of 60% is obtained (Fig.2b).The extinction spectrum suggeststhat the further increase of the reaction temperature to 95°Cleads to both inhomogeneous size distribution of the gold nanoplates and decrease of the yieldto 53% (Fig.4d).These results indicate that the most preferable reaction temperature for the high-yield synthesis of the gold nanoplates is 85°C. Extinction spectra of the Au products grown at different temperature are shown in Fig. S1, Supporting Information (SI). In contrast to the turbid appearance of the solutions obtained at 25 and 45 °C, those at 65, 85, and 95 °C exhibit features of plasmon resonances belonging tothe gold nanoplates, which is consistent with the SEM images. The gold nanoplate solution obtained at 85 °C shows the highest extinction intensity among the three ones, suggesting the highest yield.

 Fig. 4 SEM images of the samples prepared at:a25 °C, b45 °C, c65 °C, andd 95 °C. The molar ratio of Br-/Au is fixed at 6:1.

Fig. 4 SEM images of the samples prepared at:a25 °C, b45 °C, c65 °C, andd 95 °C. The molar ratio of Br-/Au is fixed at 6:1.

Besides the reaction temperature, the molar ratio of Br-/Au has great influence on the formation of the gold nanoplates. Figure 5 shows the gold nanoplates synthesized at various Br-/Au ratio by adding different volumes of 0.2 M NaBr to the reaction solution. The synthesis was carried out in a water bath of 85 °C for 1 h. As shown in Fig.5a, when molar ratio of Br-/Au is 1:1, the final products are almost nanoparticles and no plate-like nanocrystals were found. Nanoplates start to appear in the resultant products when molar ratio of Br-/Au is increased to 3:1 (Fig. 5b). When molar ratio of Br-/Au is tuned to 6:1,nanoplates dominate in the resultant products (Fig. 2b). The fraction of nanoplates start to decrease when molar ratio of Br-/Au is further increased (Fig. 5c,d). The results indicate that the Br-/Au molar ratio of 6:1is the most preferable for the formation of nanoplates. Also, CTA+ as the capping agentis necessary in the synthesis, in which a proper concentration of 20 mM was used. The results were also confirmed by the extinction spectra (Fig. S1b). A prominent plasmon peak at around 550 nm can be found for the solutions obtained at Br-/Au ratios of 1:1 and 3:1, indicating the presence of considerable amount of spherical gold nanoparticles in the solutions. This peak becomes less observable at Br-/Au ratios of 6:1, and the spectrum in the near infrared increases, suggesting high yield of nanoplates. The extinction intensity drops when the ratio is further increased to 12:1 and 30:1.

 Fig. 5 SEM images of the samples prepared with different molar ratio of Br-/Au: a1:1, b3:1, c12:1, and d30:1. The reaction temperature is fixed at 85°C.

Fig. 5 SEM images of the samples prepared with different molar ratio of Br-/Au: a1:1, b3:1, c12:1, and d30:1. The reaction temperature is fixed at 85°C.

It is generally believed that the formation pathway of nanoplates is“kinetically controlled”along with the surfactant as capping agent or template-like CTAB and PVP [33,38].On the basis of the experimental evidence, the reaction temperature of 85°C and the Br-/Au molar ratio of 6:1are optimal conditions for obtaininghigh-yield gold nanoplates.To investigate this reaction route, aseries of experimental investigations on the formation process werecarried out through sampling gold nanoplates with increasing reaction time. Figure 6 shows the size and shape of gold nanoplates at different increasing reaction time. It can be found that small plate-like nuclei is formed in less than 5 min (Fig. 6a).They grow bigger intonanoplates with micrometer edge length within 30 min (Fig. 6b), and a high yield is obtained after 60 min (Fig. 6c). The experiment was performed at the preferable reaction temperature and concentration of Br- and CTA+.This result indicates that at preferable reaction condition a minimum 1 h’s reaction time is essential for the synthesis of gold nanoplates in a high yield.

 Fig. 6 SEM images of the gold nanoplates obtained aless than 5 min, bwithin 30 min, and cafter 60 min.

Fig. 6 SEM images of the gold nanoplates obtained aless than 5 min, bwithin 30 min, and cafter 60 min.

Recently, many endeavors have been devotedto the fabrication ofefficient SERS substrates.For example, the nanoscaled gaps provided by sandwichedstructures can greatly augment the Raman signals[39-43].Individual gold nanoplates, however, are rarely used as SERS substrates alone due to their flat surface. Here, we demonstrate that the gold nanoplate adsorbed with gold nanoparticles can be used as efficient SERS substrates with high enhancement factors.The Raman measurements were performed under excitation of 633 nm where Mph was used as the probing molecule. A same objective was used for excitation and collection of the Raman scattering light. Figure 7a, b shows a typical gold nanoplate adsorbed with a gold nanoparticle. Under the optical microscope, the same nanoplate was located and positioned in the center of the optical view (Fig. 7c). The focal point of the excitation laser at 633 nm was thereafter positioned exactly on the nanoparticle (Fig. 7d). Raman responses from gold nanoplates with and without adsorbed nanoparticles are measured and compared (Fig. 8a).Raman spectra from the individual corresponding gold nanoplates were shown in Fig.8b. The Raman signals from nanoplates without nanoparticles adsorbedare so weak that no peaks can be identifiedon the spectrum (cases 1 and 2).It is also the case when a nanoparticle is adsorbed on the edge of the nanoplate (case 3). This result is related to the lack of hot spots no matter what shapes of these nanoplates are. Interestingly, strong Raman signalsare observable when some nanoparticles are adsorbed on the upper surface of the nanoplates (cases 4, 5, and 6) and the excitation laser beam is focused on the nanoparticles. In such cases, hot spots are formed in the gap between nanoparticles and nanoplates, which greatly enhances the Raman response because of the interparticle electromagnetic coupling [44-46].Characteristic Raman peaks of Mph can be found on the measured spectra at 830, 1014, 1083, 1175, and 1600 cm-1. Among these peaks 830, 1014, and 1083 cm-1 are respectively assigned to C-H wagging, C-C bending and C-S stretching, and 1175 and 1600 cm-1 are belong to C-H bending modes[44,47,48].Note that the Raman spectra from the SERS substrates are slightly different from the bulk (Fig.8b).

 Fig. 7a The SEM images of gold nanoplates with a nanoparticle adsorbed. b Zoomed-in SEM image of (a). c Corresponding optical image with the nanoplate highlighted in the blue circle. dThe focal point of the excitation laser positioned on the nanoparticle.

Fig. 7a The SEM images of gold nanoplates with a nanoparticle adsorbed. b Zoomed-in SEM image of (a). c Corresponding optical image with the nanoplate highlighted in the blue circle. dThe focal point of the excitation laser positioned on the nanoparticle.

 Fig. 8a SEM images of gold nanoplates without nanoparticles adsorbed (cases 1 and 2), that with a nanoparticle adsorbed on the side (case 3), and those with nanoparticles adsorbed on the surface (case 4, 5, and 6). The nanoparticles are indicated by red arrows. The scale bars are 2 µm. b Raman spectra of bulk MPh and corresponding nanoplates in cases from 1 to 6.

Fig. 8a SEM images of gold nanoplates without nanoparticles adsorbed (cases 1 and 2), that with a nanoparticle adsorbed on the side (case 3), and those with nanoparticles adsorbed on the surface (case 4, 5, and 6). The nanoparticles are indicated by red arrows. The scale bars are 2 µm. b Raman spectra of bulk MPh and corresponding nanoplates in cases from 1 to 6.

Enhancement factors (EFs) of the samples are evaluated by the equation of EF=(ISERS/IBulk)(NBulk/NSERS), where ISERS and IBulk represent the Raman intensity measured on the SERS substrate and bulk Mph, respectively. NSERS and NBulk are the numbers of Mph moleculesadsorbed on the SERS and bulk samples inside of the laser spot. The EFs of SERS peaks at 1011, 1081, 1492, and 1599 cm-1 are evaluated to be 1.5×107, 4.0×106, 1.7×107, and 8.1×106, respectively. These results suggest that the gold nanoplates can be used as ideal SERS substrates for detecting Raman analytes.

 

4 Conclusions

 

In summary, we successfully developed a simple but effective route to the synthesis ofsingle crystalline gold nanoplateswithedge length on the order ofmicrons. Optimized reaction temperature and molar ratio of CTAB/Au are found to be respective 85°C and 6:1 for the formation of gold nanoplates in a high yield of 60%. The synthesis to achieve the micro-scaled gold nanoplates can be finished in less than 1 h under proper reaction conditions. Therefore, the reported synthesis is time- and cost-effective. The gold nanoplates were further employed as the SERS substrates and investigated individually. Interestingly, only those adsorbed with gold nanoparticles exhibit pronounced Raman signals of probe molecules, where a maximum enhancement factor of 1.7 × 107 was obtained. Our work demonstrated that a designed nanostructure consisting of a nanoplate adsorbed with a nanoparticle on its upper surface can be used as an efficient SERS substrate for reproducible enhancement.

 

Acknowledgment

 

This work is supported by the National Natural Science Foundation of China (NSFC) (grants 21271181 and 21473240), Ministry of Science and Technology of China (Inter-governmental S&T Cooperation Project, grant no. 6-10), and Thousand Youth Talents Program of China.

 

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[44] J. Tang, F.S. Ou, H.P. Kuo, M. Hu, W.F. Stickle, Z. Li, R.S. Williams, Silver-coated si nanograss as highly sensitive surface-enhanced raman spectroscopy substrates. Appl. Phy. A 96(4), 793-797 (2009). doi:10.1007/s00339-009-5305-0

[45] S. Nie, Probing single molecules and single nanoparticles by surface-enhanced raman scattering. Science 275(5303), 1102-1106 (1997). doi:10.1126/science.275.5303.1102

[46] J. Jiang, K. Bosnick, M. Maillard, L. Brus, Single molecule raman spectroscopy at the junctions of large ag nanocrystals. J. Phys. Chem. B 107(37), 9964-9972 (2003). doi:10.1021/jp034632u

[47] W. Ji, X. Xue, W. Ruan, C. Wang, N. Ji, L. Chen, Z. Li, W. Song, B. Zhao, J.R. Lombardi, Scanned chemical enhancement of surface-enhanced raman scattering using a charge-transfer complex. Chem.Commun.47(8), 2426-2428 (2011). doi:10.1039/c0cc03697h

[48] J. Cabalo, J.A. Guicheteau, S. Christesen, Toward understanding the influence of intermolecular interactions and molecular orientation on the chemical enhancement of sers.J. Phys. Chem. A 117(37), 9028-9038 (2013). doi:10.1021/jp403458k

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Citation Information

 

Sheng Chen, Pengyu Xu, Yue Li, Junfei Xue, Song Han, Weihui Ou, Yaping Ding, Weihai Ni. Rapid Seedless Synthesis of Gold Nanoplates with Microscaled Edge Length in a High Yield and Their Application in SERS. Nano-Micro Lett. 8(4), 328-335 (2016). http://dx.doi.org//10.1007/s40820-016-0092-6

History

Received: 15 February 2016 / Accepted: 8 April 2016

 

 


Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    Rapid Seedless Synthesis of Gold Nanoplates with Micro-scaled Edge Length in a High Yield and TheirApplication in SERS

  • Author: Sheng Chen, Pengyu Xu, Yue Li, Junfei Xue, Song Han2, Weihui Ou2, Yaping Ding, Weihai Ni
  • Year: 2016
  • Volume: 8
  • Issue: 3
  • Journal Name: Nano-Micro Letters
  • Publisher: OPEN ACCESS HOUSE SCIENCE & TECHNOLOGY
  • ISSN: 2150-5551
  • URL: http://dx.doi.org/10.1007/s40820-016-0092-6
  • Abstract:

    We report a facile and reproducible approach towards rapid seedless synthesis of single crystalline gold nanoplates with edge length on the order of microns. The reaction is carried out by reducing gold ions with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB). Reaction temperature and molar ratio of CTAB/Au are critical for the formation of gold nanoplates in a high yield, which are respectively optimized to be 85°Cand 6. The highest yield that can be achieved is 60% at the optimized condition. The synthesis to achieve the micro-scaled gold nanoplates can be finished in less than 1 h under proper reaction conditions. Therefore, the reported synthesis approach is time- and cost-effective. The gold nanoplates were further employed as the surface-enhanced Raman scattering (SERS) substrates and investigated individually. Interestingly, only those adsorbed with gold nanoparticles exhibit pronounced Raman signals of probe molecules, where a maximum enhancement factor of 1.7 × 107 was obtained. The obtained Raman enhancement can be ascribed to the plasmon coupling between the gold nanoplate and nanoparticle adsorbed on it.

  • DOI: 10.1007/s40820-016-0092-6