Rapid Seedless Synthesis of Gold Nanoplates with Microscaled Edge Length in a High Yield and Their Application in SERS
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We report a facile and reproducible approach toward 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 °C and 6. The highest yield that can be achieved is 60 % at the optimized condition. The synthesis to achieve the microscaled gold nanoplates can be finished in less than 1 h under proper reaction conditions. Therefore, the reported synthesis approach is a time- and cost-effective one. The gold nanoplates were further employed as the surface-enhanced Raman scattering 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 the nanoparticle adsorbed onto it.
KeywordsGold nanoplates Seedless synthesis SERS CTAB
Noble-metal nanocrystals have attracted a great amount of attention because of their unique light absorbing and scattering properties due to the localized surface plasmon resonance [1, 2, 3]. Wet chemical approaches have been developed toward the synthesis of a variety of metal nanocrystals [4, 5], such as nanospheres , nanorods [7, 8], nanoplates [9, 10], nanowires [11, 12], etc. Compared to 0D and 1D counterparts, 2D anisotropic nanocrystals, such as gold nanoplates, have large surface areas, sharp corners, and edges which can provide high enhancement of electric field [13, 14, 15, 16, 17], and therefore, achieve extensive applications including bio-imaging , nanodevices , surface-enhanced Raman scattering (SERS) , etc.
The growth of gold nanoplates can be directed by either templates or capping agents. By providing constrictions in a 2D space or dimension, planar substrates  and interfaces in lamellar bilayer membranes  have been used as effective templates for growth of gold nanoplates. Alternatively, capping agents can preferentially adsorb on a specific surface of gold so 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 , polymers [23, 24, 25, 26], biomolecules , 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 onto 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 . Although the seed-mediated synthesis process effectively prohibits secondary nucleation and easily controls the size and shape of the final product, it involves multistep growth of seeds. To solve this problem, Huang group developed a seedless approach to synthesize gold nanoplates in the presence of CTAB via 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 result in higher time consumption for the process and prove to be cost ineffective. Recently, high-yield synthesis of gold nanoplates with submicron edge length was reported where iodide ions were used as both the capping and etching agents . However, rapid synthesis of gold nanoplates in microscaled edge length with high yield, simplicity, and low-cost still remains challenging.
Herein, we report a facile and reproducible approach of rapid, seedless synthesis of single crystalline gold nanoplates with edge lengths in micron orders of magnitude. The reaction was carried 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 the as-synthesized gold nanoplates were also investigated.
Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·3H2O), L-ascorbic acid (AA), cetyltrimethylammonium bromide (CTAB), and 4-Mercaptophenol (Mph) were purchased from Sigma-Aldrich. All chemicals and reagents were used without any further purification. Ultrapure water was obtained from the Milli-Q system (18.2 MΩ cm−1).
2.2 Synthesis of Gold Nanoplates
A typical synthesis procedure is as follows: first, 100 µL of 0.1 M HAuCl4 was added into 3 mL of 0.02 M CTAB aqueous solution in a plastic tube, and the mixture was left undisturbed for several minutes. Then, 100 µL of 0.1 M AA was added to the mixture, followed by rapid inversion for 10 s. The resultant solution was immediately placed in a water bath of 85 °C and kept undisturbed for about 1 h. The products were washed by centrifugation at 4000 rpm for 10 min and finally dispersed in deionized water.
2.3 Preparation of SERS Substrates
The SERS substrate was prepared as follows: gold nanoplate solution was drop-casted onto a clean silicon substrate. The substrate was rinsed and blown dry by nitrogen gas. Afterward, it was immersed into a solution of gold nanoparticles for several minutes, allowing deposition of gold nanoparticles on the gold nanoplates. After thoroughly rinsing with water for several times, it was immersed into a solution of 0.01 M Mph for 3 h. The substrate was carefully rinsed and blown dry by nitrogen gas before the SERS measurement. A cross-bar was finally marked on the substrate for locating the gold nanoplate and investigating them individually under the optical microscope and SEM.
The extinction spectra of the gold nanoplates were recorded using Agilent Cary 60 UV–Vis spectrophotometer using a cuvette having 0.5-cm path length. The morphology 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 spectra were measured on a micro-Raman system (HR evolution 260, Horiba). The sample was excited at 633 nm and 4 mW in the Raman measurement. The Raman scattered light was collected using an Olympus objective (100 X, N.A. = 0.9, W.D. = 1 mm). Raman spectra were recorded using a grating of 600 lines per mm with an integration time of 15 s.
3 Results and Discussion
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 values measured on the SERS substrate and bulk Mph, respectively. NSERS and NBulk are the numbers of Mph molecules adsorbed on the SERS and bulk samples inside of the laser spot, respectively. 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.
In summary, we successfully developed a simple but effective route to the synthesis of single crystalline gold nanoplates with edge length on the order of microns. Optimized reaction temperature and molar ratio of CTAB/Au are found to be, respectively, 85 °C and 6:1 for the formation of gold nanoplates in a high yield of 60 %. The synthesis to achieve the microscaled 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.
This work is supported by the National Natural Science Foundation of China (NSFC) (Grants 21271181 and 21473240), Ministry of Science and Technology of China (Intergovernmental S&T Cooperation Project, Grant No. 6–10), and the Thousand Youth Talents Program of China.
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