The physical phenomenon, surface-enhanced Raman scattering (SERS), is mainly based on the local electromagnetic fields enhancement located at the nano-gaps between metal nanostructures attributed to localized surface plasmon resonance. Therefore, nano-gaps are very important for obtaining high-density hot spots and optimal and uniform SERS signals. However, it remains a challenge to form the three-dimensional ultra-narrow nano-gaps. Here, a gyrus-inspired Gyrus-SERS substrate was fabricated with the nanostructure of Ag gyrus/graphene/Au film using an extremely simple method. The lateral and vertical hot spots respectively were obtained from the dense nano-gaps (~3 nm) between gyrus and the coupling of Ag gyrus and Au film in bilayer graphene nano-gaps (0.68 nm), which were demonstrated in experiment and theory. The proposed Gyrus-SERS platform performs an excellent SERS activity (EF~5 × 109), high sensitivity (the minimum detected concentration of R6G and CV respectively is 10−13 and 10−12 M), and outstanding reproducibility (RSD~7.11%). For practical application, the in situ detection of Malachite green (MG) residue on prawn skin was executed using the prepared flexible Gyrus-SERS substrate, which shows the wide potential in food safety field.
© 2016 Optical Society of America
Since Raman signals of pyridine molecules were sensitively detected on the surface of rough silver electrode, surface enhancement Raman scattering (SERS) has been widely investigated and becomes an extremely powerful spectral tool [1–3]. The SERS mechanisms mainly include electromagnetic mechanism (EM) that can make the enhancement reach to 1014 and chemical mechanism (CM) that can make enhancement reach to 10-100 . With a favorable test bed, one can achieve the ultrasensitive and reproductive detection of probe molecules [3,5]. To obtain the expected SERS substrate, variant substrates have been broadly designed and researched from the first or second generation (single or coupled nanostructures) to the third generation (shell-isolated nanoparticle or some three-dimensional hierarchical structures), etc [6–8]. For the third generation, the materials serving as the coating shell or nanogap are vitally important for the enhancement based on EM, where the thickness of these materials must be thin enough . What’s more, the third generation is limited due to the high fabrication costs and complicated process. Therefore, low-cost and alternative strategies are urgently desirable to fully exploit the third generation SERS substrate.
Recently, the promising SERS substrates introducing graphene as nanogap have been reported, which can skillfully be assembled into nanoparticles/graphene gap/film systems or nanoparticles/graphene gap nanoparticles structures [10–14]. These platforms can greatly facilitate the SERS behaviors by the virtues of the combination of the EM of the lateral hot spots in adjacent NPs in one plane and the vertical hot spots in the vertical nanogaps and the CM introduced by graphene . As we known, the free electrons on the metal surface will collectively oscillate under the laser irradiation, which will further interact with laser and motivate the surface plasmons. On this occasion, the electromagnetic field will be localized on the very narrow region of the metal surface and be greatly enhanced, which is regarded as a near field interaction and can be introduced SERS based on EM. Therefore, this enhancement is remarkably sensitive to the size, shape and duty cycle of the metal nanostructures. Compared with the spherical structure, the nanostructures with relatively complex topography are believed as one ideal candidate for the SERS due to the more hot spots induced by the higher duty cycle and ultra-narrow nano-gaps.
The human’s cerebral cortex including many gyrus and sulcus can lead all body activities, which is because that the existence of gyrus and sulcus can greatly increase the surface area of cerebral cortex and transmit nerve signals more effectively. Here, inspired by this, Ag nanostructure with gyrus-like construction was decorated on graphene/Au film as an ideal Gyrus-SERS platform with an extremely simple physical vapor deposition (PVD) method. Unlike spherical particles, Ag gyrus with long and narrow sulcus as ultra-narrow nano-gap can strongly magnify lateral near-field and increase the density of the lateral hot-spots. It is extremely significant for the virtue of such a Gyrus-SERS substrate that is ultra-sensitive and can meet the practical application to food safety filed. Our Gyrus-SERS platform will contribute to the deeper realization of a notable SERS effect.
2. Experimental setup
2.1 Fabrications of the 1#, 2#, 3#, 4# and 5# substrates
5 pieces of SiO2 substrates were prepared according to our previous report . Then, uniform Au films [38 nm as shown in Fig. 5(a) in Appendices] were evaporated on these SiO2 substrates by PVD. In detail, 0.0080 g Au wire was put into a molybdenum boat (length: 100 mm, width: 10 mm, thickness: 0.3 mm). And the SiO2 substrate was placed on the top of molybdenum boat (the distance is 15 cm). A quartz cover was covered, followed by the vacuum-pumping process where the pressure was pumped to 5 × 10−3 Pa by mechanical pumps and molecular pumps successively. The current was set as 130 A and Au wire was rapidly evaporated on the surface of SiO2 substrates. Homogeneous high-quality bilayer graphene [as shown in Figs. 5(b)-5(d)] was transferred on Au films [3,17,18]: a polymethyl methacrylate (PMMA), which is a support and protective layer, was coated on graphene on Cu foil. And then the derived sample was immersed into FeCl3 solution (~1 M) to absolutely etch Cu foil. After that the remaining PMMA/graphene layer was carefully transferred into DI water for 5 times to remove residual the etching solution. Next, PMMA/graphene layer in DI water was gently fished up by Au film/SiO2 samples and blown dry under nitrogen atmosphere and baked under 130 °C in order to make PMMA/graphene layer closely paste on the surface of Au films. After that these samples were put into 100 mL acetone solution that was heated to 80 °C for 10 times to completely remove the coating layer (PMMA). Finally, Ag wires of 0.0005, 0.0010, 0.0015, 0.0020 and 0.0025 g were respectively evaporated on these prepared 5 pieces samples by PVD following the steps above (the current was set as 90 A), which were labeled as 1#, 2#, 3#, 4# and 5# substrate.
2.2 Apparatus and characterization
To clearly observe the surface morphology of the above mentioned samples, these substrates were characterized by atomic force microscope (AFM Bruker Multimode 8), scanning electron microscope (SEM ZEISS Gemini SUPRA55).
2.3 SERS spectra measurements
SERS spectra were detected by Raman spectrometer (Horiba HR Evolution 800) with laser wavelength of 532 nm. The laser excitation energy and spot respectively were 0.48 mW and 1 μm. The diffraction grid was 600 gr/mm. A 50 × objective (N.A. = 0.50) and the integration time of 8 s were used throughout the experiment.
3. Results and discussions
We synthesized Ag-gyrus/graphene/Au film Gyrus-SERS platform with a simple and low-cost method as shown in Fig. 1(a). Here, the bilayer graphene is supposed to protect the quantum tunneling that will occur when a single layer graphene is chosen as the nano-spacer. Because the quantum tunneling will suppress the surface plasma resonance and lead to the weaker SERS signals . But, the plasmonic coupling effect will decrease exponentially with the more number of graphene layer (>2). The Ag-gyrus/graphene/Au film Gyrus-SERS platform was characterized by SEM in Fig. 1(b) (labeled as 3# substrate). Dense bulges (Ag gyrus) with the width of ~22 nm summarized with the Nano measurer software as shown in the inset of Fig. 1(b), and narrow sulcus (ultra-narrow nano-gap is ~3 nm) were clearly observed, which are similar with the structure of the human’s brain. In addition, the AFM topography of Gyrus-SERS substrate in Fig. 5(e) further presents the alternative distribution between the raised gyrus and cupped sulcus, which is greatly agreed with SEM observations and demonstrates the height of gyrus is ~6 nm. To illustrate Ag gyrus in detail, different substrates [1#, 2#, 4# and 5# respectively presented in Figs. 6(a)-6(d)] with various Ag size and shape were fabricated as control experiments. From 1# to 3# substrate, the transform process from Ag nano-spheres into Ag gyrus nanostructures with changing the quantity of Ag is vividly illustrated and bigger nanostructures are obtained. In addition, with the increase of the size, the nano-gaps between the adjacent structures are also reduced. However, 4# and 5# substrates show not only the Ag nanostructures change bigger but also most of nano-gaps have been obviously disappeared and have formed Ag film. This microprocess of the formation of Ag gyrus can be interpreted as the macroscopic process of rain (light rain and heavy rain), when the light rain falls to the ground, the small particles of rain are formed. With increasing rainfall, the bigger particles of rain in sky turn into the patches with cracks in the ground due to the force of collision between the particles and ground. But, when the heavy rain falls to the ground, a thick water layer is uniformly distributed in the ground.
After that, we systematically investigated the SERS performance on the above mentioned different substrates. The rhodamine 6G (R6G) molecules with concentration of 10−5 M as a reporter was chosen, and the corresponding Raman spectra were detected on these substrates as shown in Fig. 2(a). The relative SERS intensities of the peaks at 613 and 774 cm−1 from R6G molecule changing with different substrates were collected in Fig. 2(b). The intensities initially increase respectively from 1# to 3# substrate, suggesting the process of optimizing SERS effect, but a decreased trend was observed for 4# and 5# substrates. From 1# to 3# substrate, according to Mie theory (a classical electromagnetic theory of metal particles, especially Ag and Au), the size dependence of localized surface plasmon resonances is described, where as increasing particle size, higher resonance contributions are reached. However, for 4# and 5# substrates, with reducing the lateral hot spots and blocking the probe molecules enter into the vertical hot spots, these changes maybe weaken the Raman signals. Therefore, the highest SERS intensity was obtained on 3# substrate, i.e., the extremely praised Gyrus-SERS substrate, which is extremely agreed with mentioned SEM results. Based on this Gyrus-SERS platform, the excellent SERS behaviors of the 3# substrate can be attributed to the localized surface plasmons resonance (LSPR) induced by adjacent Ag gyrus nanoparticles and graphene nano-gaps under laser irradiation which results in the tremendous local field enhancement (EM). And then, the coupling of the LSPR and the surface plasmon polaritons (SPPs) that travel in waves in prolonged Au film interface makes the localized and extremely spatially structured fields enhancement possible . To understand the contribution of more near-field hot spots distributing on Gyrus-SERS substrate to vastly enhance SERS intensity more deeply, we performed the local electric field properties simulation using COMSOL (RF module) Multiphysics software. A theoretical model was structured based on the inset in Figs. 2(d) and (e). The width of Ag gyrus in the Gyrus-SERS substrate was set as 22 nm with a lateral gap of 3 nm and the height of Ag gyrus was set as 6 nm. In addition, the graphene nano-gap between the Ag gyrus and Au film was set as 0.68 nm corresponding to the bilayer graphene, and the thickness of Au film was set as 38 nm. The wavelength of the incident light was selected to be 532 nm, which is consistent with the laser wavelength of Raman spectrometer in experiment. Just as expected, on the proposed Gyrus-SERS platform, the local electric field is greatly enhanced and denser hot spots obviously distribute in the ultra-narrow nano-gaps between the Ag gyrus along the x and y directions respectively when incident light polarizes along the x [in Fig. 2(d)] and y directions [in Fig. 2(e)]. It is noted that dense hot spots also attribute to the other element, i.e., the vertical hot spots induced by the coupling of Ag gyrus and Au film as clearly shown in Fig. 2(c) [10,11]. Thus, the lateral and vertical hot spots together contribute the powerful electromagnetic fields. As a contrast, the other theoretical model was structured on the spherical Ag nanoparticles supported by graphene/Au film as shown in Fig. 7, which exhibits the relative sparser hot spots. Thus, compared with Ag gyrus, the spherical Ag nanoparticles produce the weaker local electric field that precisely is the reason of the obtained weak SERS signals from R6G on 1# substrate. The above results provide convictive evidence, which demonstrate our proposed Ag gyrus/graphene/Au film Gyrus-SERS platform is an ideal SERS substrate.
To further evaluate the SERS activity of the proposed Gyrus-SERS substrate, the R6G was chosen as probe molecule. The Raman characteristic peaks of R6G (613, 774, 1185, 1360 and 1650 cm−1) for the concentration respectively from 10−5 to 10−13 M can be easily detected as presented in Fig. 3(a). The typical Raman spectrum can still be observed even with lower concentration of 10−13 M for R6G. These results demonstrate the high sensitivity of the proposed Gyrus-SERS substrate, which can be attributed to the coupling of the high-density hot spots from both the lateral and vertical nano-gaps. In this case, the probe molecules can adequately locate into hot spots in both lateral and vertical directions respectively induced by the proposed Ag gyrus nanostructure and the tunable graphene nano-gap, although there is trace molecules even single molecule. The relative Raman intensity of the peak of 613 cm−1 for R6G with lower concentration of 10−13 M in Fig. 3(a) is ~48.87, while the relative intensity of the same peak on SiO2 with concentration of 10−3 M is ~98.12 based on the our previous report . Therefore, the enhancement factors (EF) for Gyrus-SERS substrate were calculated according to the standard equation [11,16,21]:15], 5 times larger than that with Ag nanoparticles/graphene/Ag film , 2.6 × 102 times larger than that Cu nanoparticles/graphene/Cu film , 50 times larger than that wafer-scale leaning silver-capped silicon nanopillar  and slightly lower (0.05 times) than that the similar nanostructure (gold dimers/dielectric SiO2/Au mirror) . To investigate the ability of quantitative detection, the linear fit curve of the peaks of R6G located at 613 cm−1 versus the concentrations is illustrated in Fig. 3(b). It is thoroughly obvious that reasonable linear response with the high coefficient of determination for R6G (R2: 0.997) molecule in log scale is achieved between the intensity of SERS signal and the concentration. Similarly, crystal violet (CV) was chosen as probe molecule to further verify the high sensitivity (the minimum detected concentration is 10−12 M) and the ability (R2: 0.972) of quantitative detection of the proposed Gyrus-SERS platform as respectively shown in Figs. 8(a) and (b). Besides the SERS activity and the ability of the quantitative detection, the homogeneity and reproducibility of the SERS signal are the additional indispensable parameters for the practical application. The SERS spectra and the relative intensities for the peak at 613 cm−1 of the R6G with a concentration of 10−8 M respectively were collected on Gyrus-SERS bed from random fifty points as presented in Figs. 3(c) and (d). All the data exhibit a minor fluctuation around the average intensity and the relative standard deviation (RSD) of the peak (613 cm−1) intensities was about 7.11% (less than 20%), which demonstrates that the obtained Gyrus-SERS sensor can provide homogeneous SERS signal and possess excellent reproducibility .
For practical application, this structure was directly synthesized on ultrathin flexible mica sheet and then used as the flexible SERS sensor to detect Malachite green (MG) residual on the surface of prawn skin bought from local seafood supermarket. MG is widely used by fishermen as an anti-microbial in order to prevent branchiomycosic and ichthyophthiriasis of fish. But it will cause high toxicity, high residue, cancer, malformation and mutagenic side effects. Numerous countries have set the safe detection standard of prohibiting MG. For example, the US Food and Drug Administration (US FDA), European Commission and People's Republic of China, respectively require the a minimum required performance limit (MRPL) of 0.001 (Collette, 2006), 0.002 mg kg−1 (Commission Decision, 2004/25/EC) and 0.002 (GB/T 20361-2006), for the sum of MG and leucomalachite green (LMG) . To verify that the prepared flexible Gyrus-SERS sensor can be satisfied with these requirements, the prepared MG solution (10−11 M) was sprayed on prawn skin followed by the flexible substrate tightly attached to the prawn skin as shown in Fig. 4(a). The SERS spectra of MG for in situ detection with the support of the flexible substrate (black curve) and without the flexible substrate (red curve) were presented in Fig. 4(b). Obviously, the characteristic peaks at 236, 1360 and 1614 cm−1 of MG were easily detected with the help of the flexible Gyrus-SERS substrate , which is attributed to the dense hot spots on the obtained substrate surface that adequately contact with molecules and magnify the Raman signals. In addition, the characteristic peaks of graphene (G and 2D) were also detected by using the flexible substrate. It is notable that the peaks at 1009, 1156 and 1519 cm−1 respectively attributed to phenylalanine, protein (C-C, C-N stretching) and porphyrin [v(C = C)] were also detected . The above mentioned results illustrate our prepared flexible Gyrus-SERS sensor has potential to be applied in food safety filed.
In summary, we fabricated Ag gyrus-nanostructure supported on graphene/Au film Gyrus-SERS platform with an extremely method. The three-dimensional ultra-narrow nano-gaps were obtained between Ag gyrus and also the coupling of Ag gyrus and Au film. Hence, the density of the lateral and vertical hot spots were remarkably improved by our proposed Ag gyrus-nanostructure, which were demonstrated in experiment and theory. Indeed, the Gyrus-SERS platform performs high enhanced factor, sensitivity, and outstanding reproducibility. Specially, in the practical application, the flexible Gyrus-SERS substrate was successfully used to the in situ detection for MG residue on prawn skin and demonstrated that it can satisfy the requirement in food safety filed.
National Natural Science Foundation of China (NSFC) (11474187, 11674199, 11604040); Shandong Province Natural Science Foundation (ZR2014FQ032, ZR2016AM19); China Postdoctoral Science Foundation (2016M600550, 2016M602716); A Project of Shandong Province Higher Educational Science and Technology Program (J15LJ01).
References and links
1. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, “Single molecule detection using surface-enhanced Raman scattering (SERS),” Phys. Rev. Lett. 78(9), 1667–1670 (1997). [CrossRef]
2. C. Zhang, S. Z. Jiang, Y. Y. Huo, A. H. Liu, S. C. Xu, X. Y. Liu, Z. C. Sun, Y. Y. Xu, Z. Li, and B. Y. Man, “SERS detection of R6G based on a novel graphene oxide/silver nanoparticles/silicon pyramid arrays structure,” Opt. Express 23(19), 24811–24821 (2015). [CrossRef] [PubMed]
3. S. Xu, B. Man, S. Jiang, J. Wang, J. Wei, S. Xu, H. Liu, S. Gao, H. Liu, Z. Li, H. Li, and H. Qiu, “Graphene/Cu nanoparticle hybrids fabricated by chemical vapor deposition as surface-enhanced Raman scattering substrate for label-free detection of adenosine,” ACS Appl. Mater. Interfaces 7(20), 10977–10987 (2015). [CrossRef] [PubMed]
6. M. D. Sonntag, J. M. Klingsporn, A. B. Zrimsek, B. Sharma, L. K. Ruvuna, and R. P. Van Duyne, “Molecular plasmonics for nanoscale spectroscopy,” Chem. Soc. Rev. 43(4), 1230–1247 (2014). [CrossRef] [PubMed]
8. J. F. Li, Y. F. Huang, Y. Ding, Z. L. Yang, S. B. Li, X. S. Zhou, F. R. Fan, W. Zhang, Z. Y. Zhou, D. Y. Wu, B. Ren, Z. L. Wang, and Z. Q. Tian, “Shell-isolated nanoparticle-enhanced Raman spectroscopy,” Nature 464(7287), 392–395 (2010). [CrossRef] [PubMed]
9. S. Y. Ding, J. Yi, J. F. Li, B. Ren, D. Y. Wu, R. Panneerselvam, and Z. Q. Tian, “Nanostructure-based plasmon-enhanced Raman spectroscopy for surface analysis of materials,” Nature Rev. Mater. 1(6), 16021 (2016). [CrossRef]
10. X. Li, W. C. H. Choy, X. Ren, D. Zhang, and H. F. Lu, “Highly intensified surface enhanced Raman scattering by using monolayer graphene as the nanospacer of metal film–metal nanoparticle coupling system,” Adv. Funct. Mater. 24(21), 3114–3122 (2014). [CrossRef]
11. X. Li, X. Ren, Y. Zhang, W. C. H. Choy, and B. Wei, “An all-copper plasmonic sandwich system obtained through directly depositing copper NPs on a CVD grown graphene/copper film and its application in SERS,” Nanoscale 7(26), 11291–11299 (2015). [CrossRef] [PubMed]
12. Q. Xiang, X. Zhu, Y. Chen, and H. Duan, “Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap,” Nanotechnology 27(7), 075201 (2016). [CrossRef] [PubMed]
13. H. Kim, M. L. Seol, D. I. Lee, J. Lee, I. S. Kang, H. Lee, T. Kang, Y. K. Choi, and B. Kim, “Single nanowire on graphene (SNOG) as an efficient, reproducible, and stable SERS-active platform,” Nanoscale 8(16), 8878–8886 (2016). [CrossRef] [PubMed]
14. Z. Zhan, L. Liu, W. Wang, Z. J. Cao, A. Martinelli, E. Wang, Y. Cao, J. N. Chen, A. Yurgens, and J. Sun, “Ultrahigh Surface-Enhanced Raman Scattering of Graphene from Au/Graphene/Au Sandwiched Structures with Subnanometer Gap,” Adv. Opt. Mater. 4(12), 2021–2027 (2016). [CrossRef]
15. S. C. Xu, J. H. Wang, Y. Zou, H. P. Liu, G. Y. Wang, X. M. Zhang, S. Z. Jiang, Z. Li, D. Y. Cao, and R. X. Tang, “High performance SERS active substrates fabricated by directly growing graphene on Ag nanoparticles,” RSC Advances 5(110), 90457–90465 (2015). [CrossRef]
16. C. H. Li, C. Yang, S. C. Xu, C. Zhang, Z. Li, X. Y. Liu, S. Z. Jiang, Y. Y. Huo, A. H. Liu, and B. Y. Man, “Ag2O@Ag core-shell structure on PMMA as low-cost and ultra-sensitive flexible surface-enhanced Raman scattering substrate,” J. Alloys Compd. 695, 1677–1684 (2017). [CrossRef]
17. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, and A. K. Geim, “Giant intrinsic carrier mobilities in graphene and its bilayer,” Phys. Rev. Lett. 100(1), 016602 (2008). [CrossRef] [PubMed]
18. S. Xu, B. Man, S. Jiang, W. Yue, C. Yang, M. Liu, C. Chen, and C. Zhang, “Direct growth of graphene on quartz substrates for label-free detection of adenosine triphosphate,” Nanotechnology 25(16), 165702 (2014). [CrossRef] [PubMed]
19. K. J. Savage, M. M. Hawkeye, R. Esteban, A. G. Borisov, J. Aizpurua, and J. J. Baumberg, “Revealing the quantum regime in tunnelling plasmonics,” Nature 491(7425), 574–577 (2012). [CrossRef] [PubMed]
20. Z. Zhang, Y. Fang, W. Wang, L. Chen, and M. Sun, “Propagating surface plasmon polaritons: towards applications for remote-excitation surface catalytic reactions,” Adv Sci (Weinh) 3(1), 1500215 (2015). [CrossRef] [PubMed]
21. S. C. Xu, S. Z. Jiang, J. Wang, J. Wei, W. Yue, and Y. Ma, “Graphene isolated Au nanoparticle arrays with high reproducibility for high-performance surface-enhanced Raman scattering,” Sens. Actuat, Biol. Chem. 222, 1175–1183 (2016).
22. K. Y. Wu, T. Rindzevicius, M. S. Schmidt, K. B. Mogensen, A. Hakonen, and A. Boisen, “Wafer-scale leaning silver nanopillars for molecular detection at ultra-low concentrations,” J. Phys. Chem. C 119(4), 2053–2062 (2015). [CrossRef]
23. A. Hakonen, M. Svedendahl, R. Ogier, Z. J. Yang, K. Lodewijks, R. Verre, T. Shegai, P. O. Andersson, and M. Käll, “Dimer-on-mirror SERS substrates with attogram sensitivity fabricated by colloidal lithography,” Nanoscale 7(21), 9405–9410 (2015). [CrossRef] [PubMed]
27. Z. Movasaghi, S. Rehman, and I. U. Rehman, “Raman spectroscopy of biological tissues,” Appl. Spectrosc. Rev. 42(5), 493–541 (2007). [CrossRef]