Research Article www.acsami.org
Engineering Gold Nanoparticles in Compass Shape with Broadly Tunable Plasmon Resonances and High-Performance SERS Youju Huang,*,†,§ Liwei Dai,†,§ Liping Song,† Lei Zhang,† Yun Rong,† Jiawei Zhang,† Zhihong Nie,*,‡ and Tao Chen*,† †
Division of Polymer and Composite Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China ‡ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-4454, United States S Supporting Information *
ABSTRACT: We present the uniform and high-yield synthesis of a novel gold nanostructure of compass shape composed of a Au sphere at the central and two gradually thinning conical tips at the opposed poles. The Au compass shapes were synthesized through a seed-mediated growth approach employing a binary mixture of cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) as the structure-directing agents. Under the condition of single surfactant (CTAB), the spherical seeds tend to grow into larger spherical Au nanoparticles (NPs); while the spherical seeds favor the formation of Au compass shaped NPs using two mixed surfactants (CTAB/NaOL). The reaction kinetics clearly shows a growth mechanism of Au compass shaped NPs. Interestingly, due to their anisotropic structure, Au compass shaped NPs show two distinctive plasmonic resonances, similar to those from Au nanorods. Particularly, the longitudinal surface plasmon resonances of Au compass shaped NPs exhibit a broadly tunable range from 600 to 865 nm. In addition, the obtained Au compass shaped NPs can be self-assembled into a two-dimensional monolayer with closely packed and highly aligned NPs, which results in periodic arrays of overlapped Au tips, generating hot spots for high-performance surface-enhanced Raman scattering. KEYWORDS: gold nanoparticles, compass shape, self-assemble, plasmon resonances, surface-enhanced Raman scattering, seed-mediated growth
1. INTRODUCTION Gold nanoparticles (AuNPs) have drawn enormous attention due to their richness of fascinating optical, electrical, and catalytic properties, as well as their vast potential applications in selective catalysis,1,2 biosensors,3 photothermal heating4−6 and enhanced imaging,7,8 information storage,9 and nanomedicine.10,11 The unique properties of AuNPs, which is essential to their practical performance, are greatly dependent on their size, shape, crystal facets and surface chemistry. Thus, the large-scale synthesis of AuNPs with well controlled structures is challenging, and is also of paramount importance in the advancement of nanoscience and nanotechnology. Various Au nanostructures have been synthesized, such as spheres,12,13 cubes,14 rods,15−17 wires,18−20 cages,21 cones,22 octahedrons,23 and branched multipods.24,25 In particular, AuNPs with anisotropic shape exhibit shape-dependent localized surface plasmon resonance (LSPR), facilitating their applications in such as biomedical sensing and optoelectronics. For instance, anisotropic Au nanorods26−32 display the splitting of LSPR into two modes corresponding to transverse and longitudinal axis in a widely tunable range from the visible to infrared regions (600 to 1000 nm). Previous studies33−35 showed that anisotropic triangular Au nanoplates exhibit LSPR © XXXX American Chemical Society
peaks with a longer wavelength in the near-infrared region from 800 to1346 nm. Recently, we reported that anisotropic concave Au nanocuboids36 exhibit three distinctive plasmonic bands compared with two peaks from nonconcave equivalents. The plasmonic hotspots, arising from drastically enhanced localized near-field, of Au NPs have shown great potential in surface enhanced Raman scattering (SERS) for molecular detection. The SERS enhancement factor is strongly dependent on the arrangement of a collection of Au NPs within aggregates or the presence of sharp tips or valleys on individual NPs. Previous studies37−39 have shown that the organization of Au spheres in dimers and trimers increased SERS signal by 2−3 orders of magnitude compared with individual NPs. This enhancement can be further maximized by replacing spheres with NPs with sharp tips.40 It is, therefore, of significant value to pursue an anisotropic Au shape with sharp tips, which can simultaneously display tunable plasmonic properties and high SERS performance. Received: May 3, 2016 Accepted: September 26, 2016
A
DOI: 10.1021/acsami.6b05258 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration (a) of two different growth models for the formation of AuNPs in the presence of single (left) and mixed surfactants in the growth solution. SEM images of large-sized AuNPs (b), Au sphere seeds (c), and Au nancompasses (d). The scale bars in image b−d represent 100 nm.
show two strong peaks at 534 and 718 nm, corresponding to the transverse and longitudinal LSPR modes of the NPs (Figure S1 in the Supporting Information). Figures 2 and S2 show
Herein we report the synthesis of a novel Au nanostructure (namely anisotropic Au nanocompass) comprising an Au sphere at the central and two gradually thinning conical tips at opposed poles. The anisotropic Au nanocompasses were synthesized using a mixture of binary surfactants. The growth of Au nanocompasses involves isotropic growth of seeds to bigger spherical NPs, followed by protrusion of two gradually thinning tips symmetrically at two poles. The size and length of conical tips can be readily tuned by adjusting the ratio of [Au seeds]/[growth solution], which results in a widely tunable plasmonic wavelength from 600 to 865 nm. Moreover, the selfassembly of Au nanocompass into monolayers with overlapped tips of neighboring NPs generate high density hot spots with superior SERS performance.
2. RESULTS AND DISCUSSION In this work, a mixture of binary surfactants (cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL)) have been used to control the growth kinetics and hence tailors the shape and the size of Au nanocompasses. In contrast to the single surfactant,15,16,24,25,34,36 it has been shown that binary surfactant mixtures28−30,41−43 provide better control over the structural geometry, dimensional tunability, and monodispersity of the AuNPs. The schematic in Figure 1a illustrates the different growth models of Au seeds in the presence of single surfactant and binary surfactant mixtures. The sodium citratecapped Au NPs have a size around 12 nm (Figure 1c) and were further used as seeds.35 When only CTAB was used in the growth solution, Au seeds grew isotropically into larger spherical Au NPs (Figure 1b), which is in good agreement with previous literature.44,45 Interestingly, the Au seeds prefer anisotropic growth to form highly monodispersed compass-like nanostructures in growth solution containing binary surfactants (Figure 1d). The Au nanocompasses are composed of a Au sphere at the center and two gradually thinning tips at the two poles. The length of compass shaped NPs is around 100 nm, and the average transverse size of central sphere and end tip are 55 and 5 nm, respectively. The striking feature is that the conical tips gradually become thinner and smaller, thus differing them from Au nanorods (AuNRs). The Au nanocompasses
Figure 2. TEM images (a and b) of Au nanocompasses prepared by using 12 nm seeds. The scale bars in images a and b represent 100 nm.
TEM images of Au nanocompasses, further confirming the compass structure composed of an Au sphere at the central and two thin tips at the opposed poles, which was consistent with the structure in SEM (Figure 1). To gain an insight into the synthetic mechanism, we first explored the effect of binary surfactants on the anisotropic growth of Au nanocompasses. The concentration of binary surfactants plays a crucial role in controlling the shape of AuNPs, as indicated by SEM images of AuNPs obtained at different concentrations of binary surfactants (Figure 3). The mass ratio (R) of binary surfactants was first optimized in Figure S3, and found that a mass ratio of 5:1 was the best option to prepare Au nanocompasses. At high concentration (C0, C0 represents growth solution containing 56 mg/mL CTAB and 9.6 mg/mL NaOL), the mole ratio between CTAB and NaOL was initially optimized to 1:11, and the same ratio was kept while diluting the mixture), Au seeds (12 nm) tend to form Au nancompasses with sharp and thin tips (Figure 3a). When the concentration of binary surfactants is reduced to 0.5 × C0, the tips of Au nanocompasses become bigger and thicker (Figure 3b). At 0.2 × C0, the formation nanobipyramids with B
DOI: 10.1021/acsami.6b05258 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
were used (Figure 4b). With the increase in the size of Au seeds, the yield of anisotropic AuNPs decreased, and many isotropic Au spheres appeared (Figure 4c). Much larger sized Au seeds (30 nm) liked to grow into irregular Au spheres (Figure. 4d). To eliminate different types of capping surfactants on seeds, 12 nm seeds with different surfactants such as glucose, CTAB, and sodium citrate were prepared and further used in the synthesis of Au compasses. As shown in Figure S4A, the three spectra were basically the same, and the slight shift could be negligible. SEM images in Figure S4B−D clearly show that all particles were compasses in shape, indicating that the different surfactants coated on the surfaces of seeds have little effect on the growth. Different sized particles with the same surfactant (CTAB) were also explored to study the effect. As shown in Figure S5, CTAB-capped 4 nm seeds grew into Au NRs, which was consistent with the previous report.30 The CTAB-capped 12 seeds tend to grow into a compass shape (Figure S5B). The CTAB-capped Au seeds (18 nm) lead to the formation of Au compasses, as well as isotropic Au spheres appeared (Figure S5C). The CTAB-capped 30 nm seeds liked to grow into irregular Au spheres (Figure S5D). Therefore, compared with surface properties of Au seeds, the size of Au seeds play the crucial role in formation of Au compasses. It is obvious that both the Au seed size and the concentration of binary surfactants play very important roles in the anisotropic growth mechanism of Au compass shaped NPs. In our current study, it was found that only at an appropriate size of Au seeds do the optimized molar ratio and concentration of binary surfactants (CTAB/NaOL) lead to preferable anisotropic growth at two poles of seeds, probably due to the facet-specific binary surfactants on the surfaces of seeds. This further results in the formation of the compass shapes. Figure S6 showed a diffraction pattern with diffuse diffraction spots, which could be easily indexed by the (111) structure, and the same lattice distance from different part of Au compass was obtained, demonstrating that the growth direction of nanocompasses was along the [111] direction. This is similar to the growth of Au bipyramids in previous report.46 To further elucidate the growth mechanism of the Au nanocompasses, we monitored the evolution of NP shape by imaging samples obtained in different amounts of growth solution. Tuning the amount of growth solution offers an easy and precise means to investigate different growth stages at various reaction times, while not disturbing the growth of AuNPs. Figure 5 shows SEM images of AuNPs synthesized in different amounts of growth solution. In a small amount of growth solution that resembles conditions at the early stage of growth, Au seeds tend to isotropically grow into larger NPs with diameters of around 30 nm. While interestingly, two small sharp antennas are germinated at two poles of Au sphere to form peach-like AuNPs (Figure 5a). The UV−vis absorption spectrum of the peach-like AuNPs shows a strong peak at 530 nm and a weak shoulder peak at 570 nm (Figure. 6a). When the amount of growth solution slightly increased, analogous to the consecutive growth of AuNPs in Figure 5a, the deposition of Au atoms occurs preferentially on the two germinated antennas to form tips, while the size of the central sphere remains constant. The formation of tips is confirmed by the appearance of a broad and strong plasmonic peak shoulder located at 600 nm in UV−vis absorption spectrum (Figure 6b). The further gradual increase in the amount of growth solution
Figure 3. SEM images of AuNPs obtained using CTAB/NaOL mixed surfactants with different concentrations (a: C0, C0 represents growth solution containing 56 mg/mL CTAB and 4.8 mg/mL NaOL), (b: 0.5 × C0), (c: 0.2 × C0), and (d: 0.05 × C0) using 12 nm Au seeds. The scale bars in images a−d represent 100 nm.
distinct facets were produced (Figure 3c). Further lowering the concentration of binary surfactants to 0.05 × C0 led to the formation of a mixture of irregular spheres and long rods (Figure 3d). Previous work has shown that the size and uniformity of Au seeds have an important impact on the growth kinetics and hence the monodispersity and purity of resultant AuNPs. We systematically varied the size of Au seeds from 4 to 30 nm and found that the size of Au seeds is crucial to the shape of the obtained Au NPs. Small Au seeds favor the anisotropic growth, while big Au seeds tend to form isotropic Au spheres. Au-seeds with 4 nm in diameter grew into monodispersed AuNRs in the mixed surfactants (CTAB/NaOL) (Figure 4a), in good agreement with previous report.30 The Au compass shaped NPs can be obtained when seeds with moderate size of 12 nm
Figure 4. SEM images of AuNPs prepared using different gold seeds (a: 4 nm), (b: 12 nm), (c: 18 nm), and (d: 30 nm) in CTAB/NaOL mixed surfactants based growth solution. The scale bars in images a−d represent 100 nm. C
DOI: 10.1021/acsami.6b05258 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. SEM images of AuNPs obtained using the same amount (0.5 mL) of Au seeds (12 nm) grown in different amounts ((a: 5 mL), (b: 7 mL), (c: 10 mL), (d: 20 mL), (e: 50 mL), and (f: 100 mL)) of CTAB/NaOL mixed surfactants based growth solution. The scale bars in a−f represent 100 nm.
Figure 6. UV−vis spectra of AuNPs obtained using the same amount (0.5 mL) of Au seeds (12 nm) grown in different amounts (a: 5 mL; b: 7 mL; c: 10 mL; d: 20 mL; e: 50 mL; and f: 100 mL) of CTAB/NaOL mixed surfactants based growth solution.
(20, 50, and 100 mL) produces monodispesed Au nanocompasses with longer and bigger tips (Figure 5c−f). The increase in the length of tips leads to the transformation of a plasmonic shoulder into a strong sharp peak corresponding to the longitudinal mode of the anisotropic Au NPs (Figure 6c). It is worth pointing out that the intensity of the longitudinal plasmonic peak is similar to the transverse one at 534 nm, which is relatively smaller and lower in AuNRs. This can be ascribed to the predominant contribution of the central sphere in the Au nanocompasses. The length of the Au compass shaped NPs can be finely tuned from 35 to 190 nm by varying the amount of growth solution (Figure 6c−f). The longitudinal LSPR peak can be tuned from 600 to 865 nm (Figure 6c−f), thus facilitating their applications in such as orientation sensors, photonic devices, therapeutics, and medical diagnostics. The sharp tips of the Au nanocompasses make them highly attractive for SERS detection. Close-packed aligned Au nanocompasses are expected to show significantly enhanced SERS signal, thanks to the sharp tips and hotspots between closely placed neighboring tips. The 2D monolayer with highly aligned Au compass shaped NPs (Figure 7a) was assembled by using conventional drop-dry method.13 It is striking that Au nanocompasses assemble into closely packed monolayers with
Figure 7. Representative SEM image (a) of assembled 2D monolayer with highly aligned Au compass shaped NPs; schematic illustration (b) of orientation and hot spots of Au compass shaped NPs in 2D monolayer structure; the chemical structure of SERS probe molecules (c) and Raman spectrum (d) from Au compass shaped NPs and 50 nm Au spheres covered substrates.
the NPs aligned in the same orientation in one line and the opposite orientation in the neighbored line (see arrows in Figure 7b). The antiparallel packing model of AuNPs has rarely been observed previously in the self-assembly of NP arrays. Fourier transform pattern (Figure S7) of the SEM image of Au compass array shows clear orientation fringe patterns with good quality, indicating the high crystallinity of the array structures. This unique packing of NPs maximizes the overlap of Au tips in the periodic arrays, generating hot spots for high-performance SERS. Rhodamine 6G (R6G) was selected as the probe molecule in SERS measurement (Figure 7c). As a demonstration, we compared the SERS of two representative samples, spherical AuNPs and Au compass shaped NPs. The SERS intensity on monolayer of Au nanocompasses (Figure 7d) shows remarkably larger enhancement than that on the AuNPs D
DOI: 10.1021/acsami.6b05258 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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concentrated Au NPs solution onto the silica wafer (dimension: 0.5 × 0.5 cm2), and dried naturally in air. Then, a drop of R6G solution in ethanol (1 mM) was deposited on the silica wafer and kept undisturbed for 30 min. After drying at room temperature, substrates were washed with deionized water and absolute ethanol for several times to remove the free R6G molecules. Raman spectra were collected on a R-3000HR Spectrometer (Raman Systems, Inc., R-3000 series) using a red LED laser (532 nm). The acquisition time is 10 s. Laser power was 10%, and the focal diameter was around 850 nm for all spectra. Apparent Enhancement Factors (AEF) = ISERS [CNRS]/ INRS [CSERS], where CNRS is the concentration of R6G deposited on bare silicon substrate. CSERS is the concentration of the adsorbed molecules on the surface of Au nanostructures, which was calculated using the formula (C = N·S/V). The monolayer coverage8 of R6G (0.71 nmol cm−2) was used to calculate the number (N) of adsorbed R6G. The size (S) of silica wafer was 0.5 × 0.5 cm2. V is the volume of R6G solution dropped on the silica wafer. INRS and ISERS are the peak intensities at 1510 cm−1 in normal Raman spectra and SERS spectra measured from baseline, respectively.
(50 nm) substrates (Figure 7d)). Particularly, the SERS peaks at 612, 1089, 1364, 1509, and 1574 cm−1 can be assigned to CH, COC, and the aromatic CC vibrations of R6G.8 The absolute value of the enhancement factor (EF) was calculated by using Si substrates at 520 cm−1 as reference for calibration. The EF of Au nanocompasses and AuNPs (40 nm) was approximately 4.4 × 106 and 8 × 104, respectively. The drastic increase in the EF (around 2 orders) from Au compass shaped NPs are considered to be due to the sharp tips and rich hot spots in 2D self-assembled pattern of compass shaped NPs. In our previous work,47 the apparent enhancement factors for the array structures of highly branched hierarchical nanowires and flower-like 3D hierarchical nanostructure were calculated to be 4.8 × 106 and 5.7 × 106 respectively. In our present work, although Au nanocompasses is not hierarchical structure, the apparent enhancement factor is around 4.5 × 106, showing comparable SERS performance, due to the sharp tips and rich hot spots for Au compass monolayer. In our future studies, we plan to implement it in fabricating flexible and adhesive SERS active tapes for rapid detection of pesticide residues in fruits and vegetables.48
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05258. UV−vis spectra and TEM images of synthesized AuNPs, HRTEM images and ED pattern of Au compass, SEM image of Au compass array and its corresponding Fourier transform pattern (PDF)
3. CONCLUSIONS We have demonstrated that a novel anisotropic Au nanostructure, compass shaped NPs could be successfully synthesized using binary surfactants. SEM was used to monitor reaction kinetics and investigate the evolution of their shape for revealing the growth mechanism of Au compass shaped NPs. The growth of Au nanocompasses involves isotropic growth of seeds to bigger spherical NPs, followed by protrusion of two gradually thinning tips symmetrically at two poles. The thinning and sharp tips endow Au compass shaped NPs with broadly tunable plasmonic resonances and highly improved SERS signals.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.H.). *E-mail:
[email protected] (Z.N.). *E-mail:
[email protected] (T.C.). Author Contributions §
These authors contribute equally to this work.
4. EXPERIMENTAL SECTION
Notes
The authors declare no competing financial interest.
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4.1. Chemicals and Solutions. Cetyltrimethylammonium bromide (CTAB), hydrogen tetrachloroaurate (III) trihydrate (HAuCl4.3H2O), ascorbic acid (AA), and sodium citrate were purchased from Sigma−Aldrich. Sodium oleate (NaOL) was bought from Tokyo Chemical Industry (TCI). Ultraviolet−visible (UV−vis) absorption spectra were recorded with Shimadzu UV-2450 spectrophotometer in transmission mode. Field-emission scanning electron microscopy (FE-SEM) was performed on the JEOL instrument (JSM6700F) at an acceleration voltage of 5 kV and a working distance between 7 and 8 mm. 4.2. Preparation of Gold Seeds. Four nm seed solution was prepared according to previous work.15 Gold seeds with an average diameter of 12 nm were synthesized via the procedure of Frens.12 Au NPs with the size of 18 and 30 nm were synthesized according to our previous work.13 4.3. Preparation of Au Compass Shaped NPs. Seven grams of CTAB and 1.2 g of NaOL were dissolved in 150 mL of distilled water, and further mixed with HAuCl4 solution (6.25 × 10−4 M, 100 mL). Then, a freshly prepared aqueous solution of ascorbic acid (0.064 M, 6.25 mL) was added and mixed thoroughly by inversion. Next, 12 nm seed solution with a predetermined amount was added. The solution was mixed by gentle inversion for 10 s and then left undisturbed for 6 h. Au compasses were purified according to previous report.49 As shown in Figure S4A, there were sphere-like and irregular shaped nanoparticles before purification. After purification, it is hard to find other shaped Au particles, and the number percentage of nanocompasses is approaching 100% (Figure S4B). 4.4. Raman Measurements. The surface-enhanced Raman scattering (SERS) substrates were prepared by dropping the
ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (21404110, 51473179, 51303195), Excellent Youth Foundation of Zhejiang Province of China (LR14B040001), the Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (2015), Ningbo Science and Technology Bureau (2014B82010, 2015C110031), Youth Innovation Promotion Association of Chinese Academy of Sciences (2016268), and the Bureau of Frontier Science and Education of Chinese Academy of Sciences (QYZDB-SSW-SLH036).
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