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Abstract
In recent years, biomaterials have increasingly attracted attention on surface-enhanced Raman spectroscopy (SERS) due to their well Raman performance while metal particles are combined with biological substrates. Therefore, we propose an environmentally friendly substrate based on silver-plated cicada wings with seamless graphene layer (Gr-AgNPs-C.w.), which can be prepared with a simple and inexpensive method. Compared with AgNPs-C.w., Gr-AgNPs-C.w. hybrids show better SERS performance with high sensitivity, good uniformity and good stability with R6G detection. The minimum detected concentration can reach 10−15 M, and the value of R2 can reach 0.996, respectively. Theoretical simulation demonstrates the situation of electromagnetic field through COMSOL software. In addition, due to the affinity of graphene for biomolecules, we can successfully detect the DNA biomolecules through a simple process. Therefore, this cheap and efficient natural SERS substrate has great potential for a considerable number of biochemical SERS applications and can broaden the way in which multiple SERS platforms derived from other natural materials are prepared.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman spectroscopy (SERS) is one of the most important analytical technique for molecular detection which could provide high detection sensitivity and has gradually attracted more research in both theoretical and experimental field [1–4]. In recent years, the development of the substrate manufacturing process and the advancement of technology (such as lithography techniques, self-assembly, nanosphere masks, wet etching etc.), the performance of the Raman substrate (for instance sensitivity, stability and trace less detection) has been greatly improved [5–10]. Therefore, SERS is also becoming more mature and shows good practicality in different fields, such as food safety, biomedical diagnostics, chemical molecules and structure identification, environmental monitoring, material science and explosives in military [11–17]. It is popularly accepted that primary electromagnetic mechanism (EM) and chemical mechanism (CM) are mechanisms of SERS enhancement [18,19]. The EM is introduced using noble metals (gold, silver, copper) to form surface plasmon produce the dramatic increase in the electromagnetic field to achieve Raman signals, which can increase pristine Raman signal by 108 times or more [20–24]. The CM is achieved by the electron transfer between the substrate and the target molecule, which can generally enhance the Raman signal by 102 times [25]. In the Raman enhancement mechanism, the EM plays a leading role, but the CM also has great significance.
Traditionally, the clean surfaces and materials with neat arrays of nanostructures are more inclined to researchers, such as nanosilicon substrates, AAO templates, etc. However, the relatively complicated processes make them susceptible to hazards and low success rates during the preparation process. Recently, a series of studies have shown that certain biomaterials can also be used to prepare SERS substrates (such as dragonfly wings, shells, taro leaves, etc) and show good performance [26–28]. Especially, the cicada wings (C.w.), originating from a very common arthropod, possess all the advantages of the traditional SERS substrate. In addition, C.w. as biological materials is flexible, cheap, easily-obtained and environment-friendly. What’s more, the superhydrophobic structure of the C.w. is also beneficial for the detection even with low concentrations [29].
Graphene, an ultra-thin two-dimensional layered material, has attracted wide attention due to its special electrical, optical, and mechanical properties [30,31]. As a classic two-dimensional material, graphene has many advantages in SERS: (1) Graphene has a good affinity for biomolecules that can adsorb molecules and narrow the distance between molecules and SERS “hot spots” [32]; (2) Graphene has high-transmittance which will reduce optical loss to ensure accuracy of detection [33]; (3) Graphene has good chemical inertness and can protect metal nanoparticles from oxidation, making the lifespan of SERS substrates greatly improved; (4) Graphene can separate the metal nanoparticles from the analytes to avoid molecular deformation and signal distortion caused by direct contact between biomolecules and metal nanoparticles.
In this paper, by combining all advantages above, we fabricated the Gr-AgNPs-C.w. SERS substrate with good flexibility and repeatability. Since C.w. cannot withstand high temperatures, a simple chemical reduction method for the fabrication of graphene was introduced, which is commonly used in electrochemistry [34]. Compared with the CVD growth method or the transfer method, this general method cannot only protect the surface topography from damage, but also make the graphene and the “hot spot” tightly attached together [35]. Using R6G as probe molecule, we can prove that Gr-AgNPs-C.w. substrate has better sensitivity (10−15 M), and stability. And the substrate can detect 2 mM DNA molecules. Therefore, the proposed Gr-AgNPs-C.w. substrate also has great potential in biomolecular detection. We further confirm the distribution of electromagnetic fields with COMSOL software. Based on the experimental results, it can be concluded that the perfect combination of the flexibility of the C.w., the good nano-array structure and graphene, makes the substrate have good sensitivity, repeatability and stability.
2. Experimental
2.1 The preparation of AgNPs-C.w. and Gr-AgNPs-C.w
Figure 1 schematically illustrates the preparation processes of the Gr-AgNPs-C.w. substrate. The C.w. were collected from Shandong Shouguang, China. In this experiment, all the wings are cut to 6 mm × 6 mm size firstly. Afterwards, C.w. fragments were cleaned by acetone, alcohol and deionized water in ultrasonic cleaner for 40 min in turn to remove the organic pollutants and impurities and then were dried naturally. To obtain a uniform AgNPs by magnetron sputtering, we must fix the C.w. on the glass slide. The parameters of the magnetron sputtering apparatus were as follows: DC 200 V, pre-sputtering time 5 s, platform rotation speed 30 r/s, sputtering time 200 s (20 second/time with 10 times) in all experiments. Thus, the homogeneous AgNPs was successfully deposited, which will contribute to the reproductive SERS signals. Gr-AgNPs-C.w. substrate can be got by immersing the prepared AgNPs-C.w. substrate into the Poly(allylamine hydrochloride) (PAH) solution (1 g·L−1) solution, the GO dispersion (0.2 g·L−1, diameter: 50-150 nm) for 5 h and in Hydrazine (N2H4, 50%) for 1h at the room temperature in regular sequence. Finally, We need to clean the prepared sample with deionized water slowly and then dry it naturally.
2.2 Characterization
The surface topography of the substrates was observed by scanning electron microscope (SEM, Zeiss Gemini Ultra–55), extra high tension (EHT) is 3 KV, work distance (WD) is 3.8 mm, sample detection temperature is 20 °C. All Raman spectra of the sample were carried out with a Horiba HR Evolution 800 (laser wavelength at 532 nm, laser power 1.2 mW 2.5%, gratings with 600 grooves per 1 mm, objective lens × 50, acquisition time 8 s). Transmission electron microscope (TEM, JEOL JEM-2100) was used to detect graphene synthesized on the surface, acceleration voltage (AV) is 200 KV, magnification power (MP) is 150000 and 300000 respectively, sample detection temperature is 22 °C.
2.3 SERS experiments
By dissolving the R6G in the water, we can obtain the probe R6G to test and compare the SERS performances of the substrate we prepared (AgNPs-C.w. and Gr-AgNPs-C.w.) by just dropping R6G onto the entire sample and drying naturally. Before performing the SERS experiments, we drop probe solution directly on the sample and dry it naturally due to the treated sample are still hydrophobic and the C.w. shape is easily affected by temperature. In addition, by the virtue of the bioavailability of graphene, the substrate can also achieve the biospecifical detection using DNA as a probe molecule. The 1-Pyrenebutanoic acid succinimidyl ester (PBASE), ethanolamine (EA), and N, N-dimethylformamide (DMF) are obtained from Aladdin Co., Ltd. The length of DNA sequences are 15 base pairs and the sequence of the probe DNA is 5′-TGT ACA TCA CAA CTA-3′, the sequence of the fully complementary DNA (FC DNA) is 3′-TAG TTG TGA TGT ACA-5′ DNA were purchased from Sangon Biotech (shanghai) Co., Ltd. We drop the PBASE 10 mM on the substrate, and test it after drying directly. The probe DNA 2 mM was also dropped directly onto the sample and allowed to stand for 12 hours before being tested. FC DNA 2 mM is applied directly to the sample after waiting for drying.
3. Results and discussion
Figure 2(a) shows SERS signal of 10−9 M R6G on AgNPs-C.w. substrate with different sputtering time that from 80 s to 220 s, which we labeled as AgNPs-C.w.-80 and AgNPs-C.w.-220. It can be seen from the Fig. 2(a) that the intensity of the Raman peaks for the R6G gradually increases with the increase of the sputtering time, which maybe introduced by the reduction of the gap between the nanocolumns. Herein, not only the “hot spots” on the surface of the nano-pillars play a role, but also the AgNPs among the nano-pillars will form “hot spots” due to the reduction of the particle gap with the reduce of nano-pillar gap, which will greatly improve the sensitivity of the substrate. This fact can be demonstrated by the following SEM and theoretical results. The Raman spectra of 10−9 M R6G on the Gr-AgNPs-C.w.-200, AgNPs-C.w.-200 and Gr-AgNPs-C.w.-220 substrate were collected in Fig. 2(b). We can see clearly that the Raman peaks of the R6G on the Gr-AgNPs-C.w.-220 substrate is almost ignorable and only the characteristic peaks of the Gr are detected. On the contrary, for the case of Gr-AgNPs-C.w.-200 substrate, the Raman peaks of the R6G can be observed easily and the intensity of these peaks on this substrate is higher than that on the AgNPs-C.w.-200 substrate, which can be attributed to the CM introduced by the Gr. This distinct difference of the enhancement for the Gr-AgNPs-C.w.-200 and Gr-AgNPs-C.w.-220 substrate may be due to the fact that the gap of the nanocolumns on the AgNPs-C.w.-220 substrate is too small and the diameter of the GO is only 50-150nm. Once the Gr layer depositing on the nanocolumns for the case of the AgNPs-C.w.-220 substrate, the small gap of the nanocolumns may be connected by the Gr layer, which will have a negative influence on the SERS enhancement. This fact will be demonstrated by the following SEM results.
Fig. 2 (a) SERS signal of 10−9 M R6G on AgNPs-C.w. substrate with different sputtering time. (b) Raman spectra of 10−9 M R6G on the Gr-AgNPs-C.w.-200, AgNPs-C.w.-200 and Gr-AgNPs-C.w.-220 substrate.
As we mentioned above, many of the nanostructures that exist in biology are evolutionary consequences of adaptation and survival requirements showing amazing optical effects and can be used straightly. Hydrophobicity is often determined by the dense and ordered surface nanoarray structure, and it has been confirmed that the biomaterials with nanostructures is much beneficial for the SERS enhancement. Here we choose the C.w. as pristine Raman substrates. The distance among the top of the nanocolumns is about 130 nm and the bottom is about 90 nm. The average height per nanopillar is approximately 350 nm, the average diameter of the top and bottom columns are approximately 80 and 150 nm, respectively. This top-wide and bottom-narrow structure can effectively trap the light similar with the pyramid structure when the laser irradiates on the surface, which will give a contribution to the SERS enhancement.
Figures 3(a)-3(c) respectively exhibits the SEM front view of the AgNPs-C.w.-80, AgNPs-C.w.-140, AgNPs-C.w.-200, and Fig. 3(d) presents the cross-section of the AgNPs-C.w.-200 sample. As shown in Fig. 3(a), we can obtain a relatively smooth surface with a short sputtering time. With the increase of the sputtering time, the surface of the nanocolumn presents a rough texture as shown in Figs. 3(b)-3(d). What’s more, we can see from the histogram insert in the Fig. 3(d) that the gap of the nanopillars decreases obviously with the sputtering time increasing. This is the original intention of our experiment, which can be used to manufacture a controlled SERS substrate with high performance. Moreover, the gradual decrease of the gap of the nanocolumns not only can cause the plasma oscillation effect on the single nanocolumns, but also can introduce the oscillation between the nanocolumns, which can greatly improve the sensitivity of the substrate and the accuracy of the Raman signal. This can be demonstrated by the following COMSOL software simulation and SERS measurement. It can be clearly seen from the Fig. 3(d) that the surface of the nanocolumn after silver deposition has some rough protrusions, but is relatively smooth. The surface of the Gr-AgNPs-C.w.-200 substrate became rough after chemical treatment due to the strong reducibility of hydrazine hydrate as exhibited in Fig. 3(e), which can create more “hot spots” and further enhance the sensitivity of the signal based on EM. We can also clearly see the existence of wrinkles between the columns from the region marked by the arrow in Fig. 3(e), which indicates that graphene has been coated on the base successfully. On the contrast, just as we discussed above, a thick graphene layer was observed in Fig. 3(f) for the Gr-AgNPs-C.w.-220 substrate due to the small diameter of the GO, which will lead to the gap of the nanocolumns connected. This thick graphene layer will hinder the “hot spot” and make this substrate lose the SERS activity as shown in Fig. 2(b). In order to further characterize the graphene coated nanocolumn structure, TEM characterization was performed in Fig. 3(g). It can be clearly seen that a thin uniform graphene layer (approximately 2.6 nm) was deposited on a single nanocolumn. Besides introducing the enhancement based on the CM, the coated graphene can also isolate the probe molecules from the substrate “hot spots” and can be attached by PBASE through the π-π bond, which is beneficial for the specific detection of the DNA. To give a qualitative characterization of the graphene layer, Raman spectra were carried out on the Gr-AgNPs-C.w.-200 substrate. As shown in Fig. 3(h), the characteristic peaks at 1380, 1580 and 2910 cm−1 confirmed the successful deposition of the graphene on the AgNPs-C.w.-200 substrate. What’s more, the intensity of the characteristic peaks of the graphene on the Gr-AgNPs-C.w.-200 substrate is much higher than that of the initial graphene oxide, which demonstrates the EM enhancement of the AgNPs. The Raman mapping of 2910 cm−1 peak of Gr were collected over an area of 20 × 20 μm2 on the Gr-AgNPs-C.w.-200 substrate as shown in Fig. 3(i), which indicate that the relatively uniform and large-scale distribution of graphene and is much beneficial for the achievement of the uniform SERS signal.
Fig. 3 (a)-(c) are the top-view SEM images of the AgNPs-C.w.-80, AgNPs-C.w.-140 and AgNPs-C.w.-200, the insert are the large-scale SEM images. (d) The cross-sectional SEM image of AgNPs-C.w.-200 substrate. The insert pattern shows the relationship of sputtering time and nanocolumn gap. (e) The cross-sectional SEM image of Gr-AgNPs-C.w.-200 substrate. (f) The cross-sectional SEM image of Gr-AgNPs-C.w.-220 substrate. (g) TEM image of the single nanocolumn of Gr-AgNPs-C.w.-200 substrate, the insert is a large-scale TEM image. (h) shows Raman spectra of graphene oxide and chemically synthesized graphene. (i) Raman mapping of 2910 cm−1 peak of Gr on the Gr-AgNPs-C.w.-200 substrate collected over an area of 20 × 20 μm2.
We chose R6G as the probe molecules to test the SERS performance of our samples under the same conditions. The main peaks of R6G are identified at 610, 772, 1182, 1311, 1362, 1510 and 1647 cm−1 [36,37]. Figures 4(a) and 4(b) respectively exhibit the SERS signals of R6G with different concentrations on AgNPs-C.w.-200 and Gr-AgNPs-C.w.-200 substrates, where we can detect all the characteristic vibrations of R6G, and the intensity of these peaks monotonically decays as the concentration decreases. What’s more, from the higher intensity of the peaks for R6G with the same concentration, we can conclude that the enhancement of the Gr-AgNPs-C.w.-200 is superior to that of the AgNPs-C.w.-200. Importantly, the detection limit of Gr-AgNPs-C.w.-200 can down to 10−15 M, an order of magnitude lower than AgNPs-C.w. The comprehensive factors can be attributed to the following points: First, Gr can provide CM enhancement. Second, the closely distributed on the nanopillars make analyte molecules concentrate more efficiently around the “hot spot”, further improve the SERS sensitivity [38]. Third, the unique hydrophobicity of the C.w. prompts the probe molecules to polymerize, allowing the low-concentration detection. The linear fit curve of the peak at 610 cm−1 shown in Fig. 4(c) demonstrates the ability of the proposed Gr-AgNPs-C.w.-200 for quantitative analysis, where the value of R2 can reach up to 0.996, higher than that of the AgNPs-C.w.-200 (0.959). Figure 4(d) shows the SERS signals of 10−9 M R6G on the Gr-AgNPs-C.w.-200 and AgNPs-C.w.-200 substrates, the Raman signal of 10−3 M R6G on the blank SiO2 substrate. We can use the following formula to calculate the electromagnetic enhancement factors [39]:
Among them, and are the peak intensity of the SERS spectrum and normal Raman spectrum. and represent the number of molecules within the laser spots, respectively [40]. The intensity of peak at 610 cm−1 on Gr-AgNPs-C.w.-200 and AgNPs-C.w.-200 are respectively 13000 and 8000. And the intensity of the same peak on the blank SiO2 substrate is 560. Thus, for the Gr-AgNPs-C.w.-200 substrate, we can calculate that the value of ISERS /IRS is 23.2 and NSERS/NRS is 106. Therefore, we can achieve the EF of Gr-AgNPs-C.w.-200 substrate is 2.32 × 107. In the same way, we can calculate the EF of the AgNPs-C.w. substrate-200 is 1.43 × 107, which is lower than that of the former and can be attributed to the existence of the graphene on the Gr-AgNPs-C.w.-200 substrate. What’s more, the EF of the Gr-AgNPs-C.w.-200 substrate is higher than EF of Ag nanoislands/moth wing (4.16 × 105) and EF of AuNPs/GO/C.w. (1.06 × 106) [41,42]. This phenomenon can be contributed to the combined advantages of the graphene, C.w. and AgNPs during the SERS test and demonstrate the merit of our proposed SERS substrate.Fig. 4 (a) and (b) are SERS signals of different concentrations of R6G molecules on AgNPs-C.w.-200 and Gr-AgNPs-C.w.-200 substrates. (c) The Raman intensity of the 610 cm−1 peak on a logarithmic scaleas a function of different concentrations. (d) SERS signal of 10−9 M R6G on AgNPs-C.w.-200 and Gr-AgNPs-C.w.-200 substrates and 10−3 M R6G on blank SiO2.
High repeatability is another important indicator for testing SERS substrate performance in addition to good sensitivity. Figure 5(a) shows 15 SERS spectra of R6G at a concentration of 10−9 M collected from 15 different Gr-AgNPs-C.w.-200. substrates. It can be seen that there is no significant change for the intensity of each Raman peaks, which indicates that the Gr-AgNPs-C.w.-200 substrate possesses well reproducibility. Figure 5(b) shows the distribution histogram the intensity of 610 cm−1 peak on the Gr-AgNPs-C.w.-200. The black horizontal line indicates the average intensity of the SERS of 610 cm−1 peak from these 15 positions, and the shaded area indicates the fluctuation of the SERS signal. The calculated RSD of the peak intensity of 610 cm−1 is 7.9%, which further confirms the well reproducibility of the Gr-AgNPs-C.w.-200 substrate and should be attributed to the good performance of the natural large-area ordered array of the C.w. and the uniform coverage of the prepared graphene. We also investigate the stability of these substrates using R6G with concentration of 10−9 M as a probe molecule. Figures 5(c) and 5(d) show the SERS signals detected in the same environment for 0 days and 15 days, respectively. The intensity of the 610 cm−1 peak was chosen to compare the effects of aerobic exposure of the two substrates. It is calculated that the intensity reduction on Gr-AgNPs-C.w.-200 substrate is only 13%. However, the intensity on the AgNPs-C.w.-200 substrate drops severely up to 54% due to the oxidation of AgNPs. Therefore, the introduction of graphene to the AgNPs-C.w.-200 substrate can effectively protect the silver particles from oxidation, which endows the substrate with well stability.
Fig. 5 (a) 15 Raman signals of R6G at a concentration of 10−9 M collected from 15 different Gr-AgNPs-C.w.-200 substrates. (b) The intensity of the 610 cm−1 peak on Gr-AgNPs-C.w.-200 substrate. (c) and (d) are respectively the SERS signals of R6G with the concentration of 10−9 M on the AgNPs-C.w.-200 and Gr-AgNPs-C.w.-200 substrates under the exposure to the common air environment for 0 day and 15 days.
Figures 6(a) and 6(b) respectively show the cross-sectional views of the local electric field distribution of Gr-AgNPs-C.w.-200 and AgNPs-C.w.-200 substrate, which can clearly see the place where the electromagnetic oscillation is intense in the gap between the two columns. The electromagnetic enhancement at the Gr-AgNPs-C.w.-200 gap is 49.24 times, and the electromagnetic enhancement at the AgNPs-C.w.-200 gap is 44.70 times. The small differences between the electromagnetic enhancement is due to the fact that graphene mainly enhances the Raman signal through CM, which has little effect on the electric field. Although the effect of electromagnetic enhancement is not outstanding, it has a significant effect on the improvement of SERS activity of the substrate. In addition, Fig. 6(c)-6(e) shows the distribution of the electromagnetic field between the gaps of the AgNPs-C.w. substrate with sputtering time increasing. As we observed in the SEM result, the gap becomes smaller and smaller with the time increasing. Here, we can see that the electromagnetic oscillation is also more and more intense, which is mainly due to the increase of the lateral and longitudinal “hot spots”, thereby greatly improving the sensitivity of the substrate. Figure 6(f) shows the fitting curves of EF variation on the Gr-AgNPs-C.w. and AgNPs-C.w. as the nanocolumns gap changing. As the gap decreasing, the curve trend is exponentially distributed, and the enhancement effect becomes stronger, which can be attributed to the sharp increase in plasmon oscillation caused by the reduction of the gap. What should be noted is the electromagnetic enhancement factor simulated by COMSOL software is smaller than the actual one. The reason for this phenomenon is that the actual nanopillars surface is rough and the surface for COMSOL simulation is smoothed for convenience.
Fig. 6 (a) and (b) are COMSOL simulations of the local electric field distribution of Gr-AgNPs-C.w.-200 and AgNPs-C.w. substrate-200, respectively, with a gap of 10 nm. (c)-(e) shows that the change in the electromagnetic field caused by changing the nanocolumn gap (30 nm, 50 nm and 70 nm), respectively. (f) Fitting curve of EF variation caused by nano-column gap change.
Due to the bio-affinity of graphene, PBASE can be linked by π-π bond on the Gr-AgNPs-C.w. substrate as a target molecule for linking DNA probe molecules, which is achieved by the interaction between the amide group at one end of PBASE and the amino group on DNA. A covalent bond will be formed and the probe DNA can be immobilized on the substrate. The mixing process of Gr-AgNPs-C.w. and DNA that fixed on the Gr-AgNPs-C.w. substrate is shown in Fig. 7(a). Figure 7(b) shows the Raman spectrum of the Gr-AgNPs-C.w. substrate after the addition of PBASE, where the peak at 1410 cm−1 can be attributed to orbital hybridization and inhibition of graphene surface molecules, the peak at 1390 cm−1 peak is assigned to the hybridization of SP3, and the peak at 1617 cm−1 is attributed the resonance group of the sulfonium group. Figures 7(c) and 7(d) present the Raman spectra after the addition of probe DNA and FC DNA, respectively.The peaks at 1245, 1290, 1444, 1470, 1470, 1651 cm−1 can be attributed to the introduction and complementation of DNA molecules, which gives a powerful and credible evidence of the successful detection of DNA [43]. Figure 7(e) shows the Raman spectra for the detection of DNA molecules after 0 and 7 day. It can be seen that the DNA molecule can still be detected after 7 days, where the intensity of the 1470 cm−1 peak is not strong enough, and the change of intensity of other peaks are not obvious. This proves that the stability of the substrate is relatively good, and DNA molecule detection can be realized.
Fig. 7 (a) The mixing process of Gr-AgNPs-C.w.-DNA and effect diagram of DNA detection with 532 nm. (b)-(d) represent Raman spectra for PBASE, probe DNA and FC DNA, the insert are large-scale patterns. (e) shows the detection of DNA molecules after 0 days and 7 days with a large-scale pattern insert.
4. Conclusion
In summary, the surface natural nanoarrays of the C.w. can provide a good platform for “hot spots” gathering. With an inexpensive, controllable magnetron sputtering and simple chemical synthesis method, a close bond between the two-dimensional graphene and the “hot spot” can be achieved to form a Gr-AgNPs-C.w hybrid system. With R6G as a probe molecules, we demonstrated that Gr-AgNPs-C.w. is highly sensitive and stable. The minimum detectable concentration for R6G can reach as low as 10−15 M. By the virtue of nature structure of the C.w., this substrate also possesses good reproducibility, with the RSD as 7.9%. Using the COMSOL software, we theoretically verify the local electric field distribution of Gr-AgNPs-C.w. The successful detection of 2 mM DNA biomolecules demonstrates the good application prospect of the Gr-AgNPs-C.w. substrate. Therefore, Gr-AgNPs-C.w. substrate, as a high-performance SERS platform can realize the value of SERS in various fields such as biochemical materials.
Funding
National Natural Science Foundation of China (11674199, 11804200, 11774208, 11747072); Shandong Province Natural Science Foundation (ZR2017BA004, 2017GGX20120); Shandong Province Higher Educational Science and Technology Program (J18KZ011); China Postdoctoral Science Foundation (2016M602716).
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