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Abstract
In this study, we experimentally prepared composite structures of 170 nm gold nanocube, poly(methyl methacrylate) (PMMA) spacer layers with different thicknesses, and 50 nm gold film as substrates for surface plasmons excitation and surface-enhanced Raman scattering (SERS). The SERS spectra of gold nanocube and the composite structure were studied by using a 633 nm laser as an excitation source and rhodamine 6G (R6G) as the Raman probe molecule with 5.625 µg/mL gold nanocube aqueous solution. It was found that the composite structures produced much stronger SERS signals than that with the single gold nanocube structure. The relationship between the intensities of SERS Raman peaks and the PMMA spacer layer thicknessess of the composite structures was found to have the same trend as that between the Raman enhancement factor and PMMA spacer layer thickness obtained by the finite element method simulation. Furthermore, a R6G concentration as low as 10−12 M could be detected by the composite structure.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Free electrons in nanoparticles oscillate when metal micro- and nanostructures are irradiated by light. As the frequency of incident light is close to the resonance frequency of the free electrons, surface plasmons resonance (SPR) occurs, which generates a strong local electromagnetic field on the surface of the metal structures [1–3]. Therefore, various metal micro- and nanostructures are widely used in absorption enhancement [4–8], Fano resonance [9–11], sensors [12–17], photocatalysis [18–25], lithography [26–28], terahertz oscillation [29–31], magnetic field enhancement [32–34], and other physical and chemical fields. In particular, surface-enhanced Raman scattering (SERS) [35–41] based on SPR is one of the hot topics for researchers. Since the material, shape, and size of a SERS substrate affect the intensity of the SERS spectrum, researchers have prioritized the fabrication of a variety of micro- and nanostructures. Nowadays, SERS substrates include single structures formed by metal nanoparticles with different shapes and sizes, such as triangles, cubes, nanorods, and nanoporous, and composite structures formed by metal nanoparticles assembled on different substrates. In the latter case, owing to the excitation of local surface plasmons (LSP) on the metal nanoparticles, the propagating surface plasmons (PSP) of the metal and dielectric interfaces are simultaneously excited [42,43]. The strong coupling between the LSP and the PSP forms a “hot gap” on the composite structure SERS substrate. SERS benefits from the large electromagnetic field enhancement at the metal nanostructure hot spot, which further enhances the SERS signal of the probe molecule [43,44]. Sadrieyeh et al. [45] prepared Au/Ag-TiO2 mesoporous nanocomposite aerogels as SERS substrates and achieved the lowest detectable concentration of 10−10 M for a crystal violate. Zhang et al. [46] developed a carbon nanotube and Ag nanoparticles composite SERS substrate for quantification analysis, which can calibrate the SERS intensity of analyte molecules. Zhu et al. [47] studied the SERS signal of 4-aminobenzenethiol (4-ABT) adsorbed on a triangular Au-AuAg hierarchical multishell nanostructure. The SERS signal of a 4-ABT molecule at a peak of 1140 cm−1 was three times that of a single gold nanostar structure, and the detection limit was 10−8 M. Singha et al. [48] used gold nanoparticle-decorated MoS2 nanoflowers (Au-MoS2 NFs) as an excellent SERS substrate, which was capable of sensing a bilirubin concentration as low as 10−12 M. Zhang et al. [49] combined graphene oxide, silver nanoparticles, and pyramidal silicon (PSi) arrays to form a new SERS substrate, and the SERS signal of rhodamine 6G (R6G) molecules was found to be 2.5 times stronger than that obtained with graphene oxide/PSi substrate. Li et al. [50] used Ag nanowire and nanoparticle structures to decorate a MoS2 monolayer by the spin coating method, and they fabricated a hybrid SERS substrate that afforded strong Raman signals at a low R6G concentration of 10−11 M.
The above-mentioned fabrication processes for composite SERS substrates are complex and expensive. On the other hand, the chemical method for the fabrication of gold nanocube, and the magnetron sputtering of metal films are simple and mature processes. In this study, gold nanocube with an edge length of 170 nm were composited on gold thin films with poly(methyl methacrylate) (PMMA) spacer layers of different thicknesses to prepare composite structures, which were used for SERS substrates. The strong resonance coupling between the LSP excited on the gold nanocube and the PSP excited on the PMMA-gold interface in the composite structure resulted in strong Raman signals. We investigated the effects of different thicknesses of PMMA spacer layers on the SERS spectra of the composite structures.
2. Experimental sample preparation
The preparation process of the proposed composite structure is shown in Fig. 1. SiO2 glass substrates were subjected to 1800 s of ultrasonic treatment with acetone, hydrogen peroxide, absolute ethanol, and deionized water, each. Then, a 5 nm Ti film and a 50 nm gold film was deposited on the SiO2 substrate using ion beam magnetron sputtering. Next, PMMA films of different thicknesses were spin coated at different speeds onto the gold thin films using PMMA anisole solutions of different concentrations. A drop of the mixed aqueous solution of gold nanocube and R6G was dropped onto the PMMA films of different thicknesses. Finally, the composite structures were stored at room temperature (25 °C) to allow the solvent (water) to evaporate completely. The obtained SERS substrates with a composite structure were used for Raman spectrum measurement.
Fig. 1. Schematic of the preparation process of the proposed composite structure: I) clean glass substrate; II) sputtering of 5 nm Ti film; III) sputtering of 50 nm Au film; IV) spin-coating of PMMA film; V) dropping of mixed aqueous solution of gold nanocube and R6G; and VI) measurement of SERS spectrum with 633 nm laser.
The gold nanocube aqueous solution was obtained from Zhongke Leiming Daojin Technology Co., LTD., Beijing. The average edge length of the gold nanocube is 170 nm and the concentration is 50 µg/mL. Laser-grade R6G (99%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD. The PMMA anisole solution (Labspin 6) was obtained from SUSS. Deionized water was used in the experiments.
After the gold nanocube aqueous solution was ultrasonically treated for 60 s, 1 mL of gold nanocube aqueous solution was diluted with water to 8 mL and further ultrasonicated for 60 s to obtain a 6.25 µg/mL gold nanocube aqueous solution. The R6G aqueous solutions with concentrations of 10−3, 10−5, 10−7, 10−9, 10−10, 10−11, and 10−12 M were prepared as follows: The 10−3 M R6G aqueous solution was prepared by adding 4.8 mg of R6G solid powder into 10 mL water and ultrasonicating this solution for 600 s. Then, 4.95 mL water was added into 50 µL of 10−3 M R6G aqueous solution using a microinjector, and it was ultrasonicated for 30 s to obtain 10−5 M R6G aqueous solution. R6G aqueous solutions with concentrations of 10−7, 10−9, and 10−11 M were prepared by repeating the above process. Finally, 450 µL of water was added into 50 µL of 10−9 M R6G aqueous solution and ultrasonicated for 60 s to prepare 10−10 M R6G aqueous solution; the 10−12 R6G aqueous solution was prepared by repeating the same process.
The mixed aqueous solution of R6G and gold nanocube was prepared by adding 30 µL of 10−3 M R6G aqueous solution into 270 µL of the gold nanocube aqueous solution mentioned above. After 60 s of ultrasonic treatment, the mixed aqueous solution of gold nanocube with a concentration of 5.625 µg/mL and R6G with a concentration of 10−4 M was obtained. Here, the gold nanocube aqueous solution was used as the solvent to prepare the R6G solution. Next, 30 µL of 10−5, 10−7, 10−9, 10−10, 10−11 and 10−12 M R6G aqueous solutions were respectively added to 270 µL of gold nanocube aqueous solution and ultrasonicated for 60 s to obtain mixed aqueous solutions of gold nanocube with a concentration of 5.625 µg/mL and R6G with concentrations of 10−6, 10−8, 10−10, 10−11, 10−12, and 10−13 M.
Magnetron sputtering was used to sequentially deposit 5 nm Ti films (used to increase the adhesion of gold) and 50 nm gold films on clean glass substrates. Then, PMMA spacer layers with different thicknesses were spin coated on the gold film. We prepared PMMA spacer layers with six different thicknesses: 14, 25, 30, 36, 50, and 70 nm. On this basis, we dropped 5 µL of the mixed solution of R6G and gold nanocube onto glass substrates, and gold film/glass substrates coated with different thicknesses of PMMA spacer layer, respectively. After naturally drying in air, single structure of gold nanocube and composite structure of gold nanocube-PMMA-gold films with various PMMA thicknesses were obtained. The Raman spectrum were measured using Renishaw’s Invia micro confocal Raman spectrometer with a 633 nm He-Ne laser and a 50× long focal lens. The SERS substrates were irradiated with a 0.5 mW detection light with a spot diameter of 2 µm, and in each measurement, the integral time was selected as 10 s.
3. Results and discussion
The synthetic gold nanocube aqueous solution was brick red. The SEM image of the synthetic gold nanocube is shown in Fig. 2(a), which indicates that the gold nanocube have an edge length of about 170 nm and are monodisperse. The ultraviolet-visible absorption spectrum of the 50 µg/mL gold nanocube aqueous solution in Fig. 2(b) shows that the 170 nm gold nanocube have a wide absorption range, with the maximum absorption peak appearing near 590 nm and a strong absorption intensity near the wavelength of 633 nm.
Fig. 2. (a) SEM image of gold nanocube. (b) Ultraviolet-visible absorption spectrum of gold nanocube aqueous solution.
The Raman spectra were collected at three different random positions on SERS substrates, and their average values were obtained. Figure 3 shows the SERS spectra of the single gold nanocube structure and composite structure substrate with a 30-nm-thick PMMA spacer under the condition of the mixed aqueous solution of 5.625 µg/mL gold nanocube and 10−4 M R6G. The eight most prominent Raman peaks of R6G appeared at 612, 773, 1182, 1307, 1361, 1502, 1575, and 1648 cm−1, which are the same as those for the R6G powder. The three high-intensity Raman peaks are 1307, 1361, and 1502 cm−1, respectively. The intensities of the three Raman peaks for the single gold nanocube structure are 864, 2078, and 3246 a.u., respectively, while those for the three peaks for the composite substrate are 10261, 14974, and 17579 a.u., respectively. The SERS signal intensity of the composite structure is much higher than that of the single gold nanocube structure.
Fig. 3. SERS spectra of single gold nanocube structure and composite structure with 30 nm PMMA in the condition of the mixed aqueous solution of 5.625 µg/mL gold nanocube and 10−4 M R6G.
Furthermore, we investigated the effect of PMMA spacer thickness on the SERS spectra of the composite structures. To compare the Raman signals of R6G on composite structures with different PMMA thicknesses (0, 14, 25, 30, 36, 50, and 70 nm) clearly, the Raman spectra of the all composite structures with the mixed aqueous solution of 10−4 M R6G and 5.625 µg/mL gold nanocube are plotted in Fig. 4, where the fluorescence background are deducted. Figure 4 indicates that the SERS intensities of Raman peaks of all composite SERS substrates are much higher than those of the single gold nanocube structure, which is due to the excitation of the LSP of the gold nanocube and the excitation of the PSP in the interface between PMMA and gold film. Meanwhile, the PSP interact with the LSP to produce a strong resonance coupling, which leads to higher electric field enhancement of the composite structure.
Fig. 4. SERS spectra of composite structures in the condition of 5.625 µg/mL gold nanocube and 10−4 M R6G with different PMMA thicknesses of (a) 0 nm; (b) 14 nm; (c) 25 nm; (d) 30 nm; (e) 36 nm; (f) 50 nm; and (g) 70 nm.
In addition, the intensity of the SERS signal is affected by the thickness of the PMMA spacer. To quantitatively explain the effect of PMMA thickness on the SERS intensity of composite structures, we calculated the SERS intensities of SERS substrates with different PMMA thicknesses for three Raman peaks:1307, 1361, and 1502 cm−1; the calculated results are listed in Table 1. The SERS intensity of the composite structure is much higher than that of the single structure. Figure 5 shows the relationship between the intensity of the 1502 cm−1 Raman peak and the thickness of the PMMA spacer. The results show that the intensity of the Raman peak increases with an increase in the thickness of the spacer layer and reaches a maximum at a thickness of 30 nm. When the PMMA thickness is increased beyond 30 nm, the intensity of the Raman peak decreases. Furthermore, when the thickness of the PMMA spacer is 30 nm, the SERS signal of the composite structure is five times stronger than that of the single gold nanocube structure at a Raman peak of 1502 cm−1 and three times that of the gold nanocube-gold film composite structure.
Table 1. Raman peak intensities of composite structures with different PMMA thicknesses.
SERS is caused by the electric field enhancement of the excited surface plasmons on metal micro- and nanostructures. We use a strong coupling resonance model between the LSP excited by gold nanocube and the PSP at the interface between the PMMA and the gold film, the coupling model of LSP and PSP, to explain the experimental results. The dispersion relation of the PSP on the interface between the metal and the dielectric is expressed as:
Fig. 6. Electric field distributions of single nanocube structure on glass(a) and composite structures with PMMA = 0 nm (b) and PMMA = 35 nm (c), the relationship between Raman enhancement factor and the PMMA thickness of composite structures(d).
In order to determine the lowest measurable concentration of R6G in the composite structure, we measured the Raman spectra of R6G solutions with different concentrations of 10−6, 10−8, 10−10, 10−11, 10−12, and 10−13 M. The results are shown in Fig. 7(a). With a decrease in the concentration of R6G, the number of R6G molecules adsorbed on the surface of the composite structure decreases, the peak intensity of the SERS signal decreases,and the Raman peak cannot be measured when the concentration of R6G is 10−13 M. However, we obtained a strong Raman signal for a R6G concentration of 10−12 M, which is due to the electric field enhancement caused by the composite structure. For clearer observation, Fig. 7(b) shows the Raman spectrum of R6G solution with 10−12 M concentration. As can be seen, the Raman peak of R6G is clearly visible in the spectrum. Therefore, the measurable concentration of R6G in the composite structure is as low as 10−12 M.
Fig. 7. SERS spectra of composite structures: (a) Different concentrations of R6G, and (b) R6G with 10−12 M concentration.
Finally, the stability and uniformity of SERS substrates are very important. The gold nanocube cannot be easily oxidized in air and has good stability. The SERS spectrum was measured once again for the composite substrate with a 30-nm-thick PMMA spacer exposed to air for 112 days. Figure 8 shows the SERS spectra of the composite substrate. The red line is the Raman spectrum after preparation, while the black line is the Raman spectrum of the same substrate 112 days later. After 112 days, the intensity of Raman peak at 1502 cm−1 is 90% to that of the initial result, which shows that our SERS substrate has good stability. For the uniformity of SERS substrate, we randomly select five measuring points in the Raman measurement of one SERS substrate; the obtained intensities of the SERS spectra are the same, which means that the SERS substrates has good uniformity.
Fig. 8. SERS spectra of composite structure with 30-nm PMMA spacer after preparation and 112 days later.
4. Conclusion
A SERS substrate with a composite structure of 170 nm gold nanocube-PMMA-50 nm gold thin film was prepared, and the SERS spectra of the composite structures with different PMMA spacer thicknesses and R6G concentrations were studied. A strong SERS signal was obtained for the composite structure compared with the single gold nanocube structure because the composite structure can excite the LSP of the gold nanocube and the PSP at the interface between the PMMA and gold film. Besides, strong coupling resonance occurs between the LSP and PSP, resulting in a high electric field enhancement effect and a strong Raman signal. With an increase in the thickness of the PMMA spacer layer, the SERS signal intensity of the composite structure first increased and then decreased. When the spacer layer thickness was 30 nm, the SERS signal of the composite structure was the maximum. This trend is consistent with the relationship between the Raman enhancement factor and PMMA thickness in the composite structure, as deduced by numerical simulation using the finite element method. A R6G concentration as low as 10−12 M could be detected using the composite structure substrate. The SERS substrate proposed in this paper has the advantages of simple operation, low cost, high sensitivity, good stability, and uniformity, which is expected to be widely used in low-concentration detection.
Funding
National Natural Science Foundation of China (NSFC) (61505074, 61865008).
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