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We present a novel surface-enhanced Raman scattering (SERS) substrate based on graphene oxide/silver nanoparticles/silicon pyramid arrays structure (GO/Ag/PSi). The SERS behaviors are discussed and compared by the detection of R6G. Based on the contrast experiments with PSi, GO/PSi, Ag/PSi and GO/AgA/PSi as SERS substrate, the perfect bio-compatibility, good homogeneity and chemical stability were confirmed. We also calculated the electric field distributions using Finite-difference time-domain (FDTD) analysis to further understand the GO/Ag/PSi structure as a perfect SERS platform. These experimental and theoretical results imply that the GO/Ag/PSi with regular pyramids array is expected to be an effective substrate for label-free sensitive SERS detections in areas of medicine, food safety and biotechnology.
© 2015 Optical Society of America
1. Introduction
In recent years, SERS, as a label-free analytical tool to detect molecular absorbed on the SERS substrate, has drawn greater attention due to the high sensitivity [1–4]. Since the first demonstration of the SERS on the silver electrode in 1974 [5], extensive efforts have been made to explore a SERS substrate with ultra sensitivity, good stability, well homogeneity and perfect affinity. Various SERS substrates based on noble metal (such as Ag, Au and Cu), metallic oxide and semiconductors (such as ZnO, SnO2, ZnS, CuO, TiO2 and CdTe) and two-dimensional materials (such as graphene, graphene derivatives and MoS2) have been designed and researched [6–11]. Despite considerable attempts, it is still a huge challenge to obtain ideal SERS substrates with perfect enhanced effect. Up to now, electromagnetic mechanism (EM) and chemical mechanism (CM) are the widely accepted mechanisms of SERS [12]. With the assist of EM, the SERS enhancement can even reach up to 1014 [13], on the contrary, only10–100 enhancement is obtained in the case of CM [14]. What’s more, the EM and CM are usually concomitant in most cases, where the EM plays the dominate role, and it is very hard to quantitatively attribute the SERS enhancements to any one of these two mechanisms. The surface plasmons can be excited between two particles by the incident light and further contribute to the EM enhancement. To obtain the hot spots, metal nanoparticles based on Au, Ag and Cu have been extensively investigated. Among these various metal nanoparticles, with the virtue of the relatively well stability and high sensitivity, Ag nanoparticles are most chosen and widely used as the SERS substrates [15–19]. Besides the hot spots provided by the metal nanoparticles, the SERS signal is quite sensitive to the imperceptible changes of the SERS substrates’ microstructures. Therefore, searching SERS substrates with suitable microstructure is much beneficial for acquiring the stable, sensitive and reliable SERS signals. It has been demonstrated that porous Si, with regularly nanoporous structure and large specific area, can effectively improve the sensitivity of the SERS signals due to the increase of the hot spots. Multifarious methods including electroless [20], reactive ion etching [21] and metal-assisted wet etching [22] have been carried out to fabricate the porous Si. The preparations of the porous Si using these methods require multiple synthesis steps or expensive. Here, we use a simple and lost-cost method to obtain the pyramidal Si (PSi) SERS substrate possessing nanoporous pyramid arrays structure, different from the traditional pillar arrays.
Graphene, as a two-dimensional material, with distinct optical and electronic properties, was demonstrated as a potential substrate for SERS, which can strongly suppress the fluorescence background of probe molecule [23, 24] and enhance the Raman signal due to the CM existing between graphene and the molecule. Compared with the graphene, graphene oxide (GO) has superior bio-compatibility and chemical stability due to the active oxygen sites [25, 26], which is much significant for selective adsorption of molecules and stable SERS signal with perfect bio-compatibility, homogeneity and chemical stability
Herein, motivated by their advantages, we combine GO, Ag nanoparticles and PSi forming the GO/Ag/PSi substrate. Using rhodamine 6G (R6G) as a probe molecule, we firstly experimentally realize and theoretically demonstrate the excellent enhancement effect of the GO/Ag/PSi substrate.
2. Experimental
Fig. 1 elucidates the procedure of the GO/Ag/PSi SERS substrate fabrication. PSi was fabricated by wet texturing boron-doped single crystal silicon wafer in the NaOH solution with the assist of the anisotropic etching property of single crystal silicon. After the preparation of the PSi, 1M Ag nanoparticles (diameter~15nm) solution was spin-coated on the PSi template. Next, 0.5mg/ml GO dispersion functionalized with COOH groups synthesized according to a Hummers method was immediately deposited on the PSi substrate using a dip-coating method. The prepared GO/Ag/PSi substrate sealed in the nitrogen atmosphere was ready for the SERS measurements. As a comparison, we also prepared PSi, GO/PSi, Ag/PSi and GO/AgA/PSi. The PSi, GO/PSi and Ag/PSi substrates were fabricated with the same method as preparing the GO/Ag/PSi substrate. The GO/AgA/PSi substrate was prepared by dip-coating GO on the AgA/PSi, which is annealed for 30min at 550°C in the atmosphere of H2.
Fig. 1 Schematic representation of the processes for the fabrication of the GO/Ag/Psi SERS substrate.
Scanning electron microscope (SEM, Zeiss Gemini Ultra-55) was used to characterize the surface morphology of the GO/Ag/PSi substrate. SERS spectra were implemented with a Horiba HR Evolution 800 Raman microscope system (laser wavelength 532nm, laser spot ~1μm) on the same conditions. The TEM images of GO and Ag nanoparticles were observed with a transmission electron microscopy system (JEOL, JEM-2100).
3. Results and discussion
After being treated with NaOH solution, the PSi substrate presents a relatively rough surface structure, which can be indicated from the darker color and non-specular appearance compared with the polished one. Figure 2(a) exhibits the SEM image of the PSi, where a good deal of well-separated pyramid arrays can be seen clearly. The size and distribution of the pyramid arrays are consistent with our previous report (average height: ~3μm and average space: ~4μm). The excellent behaviors of these pyramid arrays on the SERS have been well confirmed. Figure 2(b) shows the TEM of the obtained GO. With the virtue of the dip-coating method, GO film is uniformly deposited on the PSi substrate and the GO/PSi still maintains the typical pyramids structure [Fig. 2(c)]. The Raman spectrum shown in Fig. 2(d) respectively collected on the GO/PSi further proves that the PSi is uniformly covered by the GO film. As shown in Fig. 2(e), the Ag nanoparicles are quite uniform in size diameter ~15nm. Figure 2(f) presents the SEM image of the PSi after the coating of Ag nanoparticles. It is very interesting to note that Ag nanoparticles attach relatively uniformly on the surface of the pyramid arrays, where we can also observe that the quantity of the Ag nanoparticles on the valley bottom is slightly larger than that on the surface [insert in Fig. 2(f)]. This phenomenon can be overcome by immediately dip-coating of the GO dispersion on the Ag/PSi substrate. Just as shown in Fig. 2(g), the pyramid arrays are well packaged by the GO films and the distribution of the Ag nanoparticles is much homogeneous on both valley bottom and surface. Meanwhile, similar with that of GO/PSi, the Ag/PSi with the dip-coated GO films still maintains the typical pyramids structure and some wrinkles of GO films are also detected, which is unavoidable in the dip-coating process and has little influence on the SERS signal. The well distribution of the Ag nanoparticles can be attributed to the appearance of the GO films. The GO dispersed in deionized water can interact with Ag nanoparticles aqueous solution and further restrict the Ag nanoparticles uniformly located on both valley bottom and surface of the pyramid arrays. In order to give a strong support of well distribution of the Ag nanoparticles, we further perform the SEM measurement with a large magnification. Just as expected in Fig. 2(h), the Ag nanoparticles uniformly and densely aggregate and form metal clusters on the both valley bottom and surface of the pyramid arrays. From the insert in Fig. 2(h), we can also observe that the Ag nanoparticles are tightly covered by the GO films and form an organic whole, which is beneficial to the sensitivity and stability of the SERS signal. As the rapid decay of electromagnetic enhancement with distance, the organic whole of Ag nanoparticles and GO can make probe molecule closer to the Ag nanoparticles and thus it is much suitable for high SERS sensitivity. Besides the Ag nanoparticles, the narrow space of nanoparticles (hot spots) is also covered by the GO film. Thus the probe molecules can be more effectively absorbed around the hot spots and it is natural to obtain more sensitive Raman signals. The uniform GO film is also obtained in the GO/AgA/PSi sample [Fig. 2(i)]. However, the size and the distribution of the Ag nanoparticles are not the same as that of GO/Ag/PSi. The difference of the size (average size~50nm) may be introduced by the reassociation of the Ag nanoparticles in the annealing process. The distribution of the Ag nanoparticles here is similar with that of Ag/PSi sample. Grounded on these SEM results, it should be realized that the GO film is fairly favorable for the synthesis of well distributed Ag nanoparticles on the PSi substrate.
Fig. 2 (a) SEM image of the PSi sample. (b) TEM of the obtained GO. (c) SEM image of the PSi sample after the GO films coating. (d) Raman spectrum of the GO/PSi. (e) TEM of the obtained Ag nanoparticles. (f) SEM image of the PSi sample after the Ag nanoparticles coating. (g) and (h) are SEM images of the GO/Ag/PSi sample under different magnification. (i) SEM image of the GO/AgA/PSi sample.
We choose the R6G as probe molecule to estimate the SERS behavior of the GO/Ag/PSi. The characteristic Raman peaks of R6G are detected in the region from 600 to 1700 cm−1 for the concentration from 10−4 to 10−7M [Fig. 3(a)]. The peaks located at 613 cm−1 is corresponded to the C-C-C in-plane vibration of the R6G molecular [27]. The peaks located at 774 and 1185 cm−1 can be respectively attributed to the out-of-plane vibration and in-plane vibration of C-H bonds. The peaks observed at 1311, 1360, 1507 and 1645 cm−1 can be assigned to the aromatic C-C stretching vibration mode. The intensities of the SERS signals of the R6G molecular gradually declining with the decrease of the R6G concentration is also revealed in Fig. 3(a). In particular, the SERS spectrum of the R6G molecular with a concentration of 10−5M obtained in our case is comparable with that of 10−3M using Ag@GO core-shell structure as SERS substrate performed in the same condition [28], which exhibits a clear indication that the GO/Ag/PSi substrate is superior to the Ag@GO core–shell structure in terms of the enhancement effect. The enhancement factors of the GO/Ag/PSi substrate will be discussed in detail later. The evolution of the Raman intensities at 613 and 774 cm−1 as a function of the concentrations is plotted in log scale to further investigate the SERS behaviors for R6G in Figs. 3(b) and 3(c). The average value of the intensity based on three spectra randomly collected on the GO/Ag/PSi substrate is chosen to guarantee the dependability of the data. A good linear SERS dependence for the peaks at 613 and 774 cm−1 between the intensity and the R6G concentration from 10−4 to 10−7M is obtained with the high coefficient of determination (R2) (0.975 and 0.983 respectively). In addition to the enhancement effect, the signal homogeneity is another indispensable requirement for practical SERS applications. Figures 3(d) and 3(e) respectively display the SERS spectra of R6G molecules with a concentration of 10−4M and 10−7M from three points randomly chosen on the GO/Ag/PSi substrate. Just as shown in Figs. 3(d) and 3(e), the Raman spectra in both cases are greatly consistent with each other, which indicate the perfect SERS activity and homogeneity of GO/Ag/PSi structure. The perfect homogeneity of the GO/Ag/PSi structures can be ascribed to the well distributed Ag nanoparticles and the existence of GO films. The well distributed Ag nanoparticles on both valley bottom and surface can achieve the well distributed hot spots and the GO films covering both Ag nanoparticles and spaces can make the probe molecule effectively absorbed around the hot spots. Consequently, the perfect SERS activity and homogeneity can be realized in our case.
Fig. 3 (a) The Raman spectra of R6G on the GO/Ag/PSi substrate from 10−4 to 10−7M. (b) and (c) are respectively the Raman intensity of R6G peaks at 613cm−1 and 774cm−1 as a function of the molecular concentration, in log scale. (d) The Raman spectra of R6G with a concentration of 10−4M randomly collected on the GO/Ag/PSi substrate. (e) The Raman spectra of R6G with a concentration of 10−7M randomly collected on the GO/Ag/PSi substrate.
To further evaluate the SERS activity of the GO/Ag/PSi, the SERS spectra of R6G on the PSi, GO/PSi, Ag/PSi and GO/AgA/PSi are also collected. In the case of the PSi substrate, the SERS spectra of R6G can be negligibly detected (the result is not shown here). This phenomenon can be due to the lack of the surface plasmons. As is well known, the PSi substrate with well-separated pyramid arrays can effectively make the incident laser oscillate between the pyramidal valleys, which will further give rise to local enhancement of the incident laser. However, only this local enhancement of the incident laser cannot bring about the SERS based on the EM. It has been widely recognized that the surface plasmons in the metal excited by the incident light is absolutely essential for the SERS based on the EM, where the surface plasmons and the incident light resonate mutually and further contribute to the local field enhancement. For the cases of GO/PSi, Ag/PSi and GO/AgA/PSi, as shown in Figs. 4(a)-4(c), all the typical Raman peaks of the R6G at 613, 774, 1185, 1311, 1360, 1507 and 1642 cm−1 are observed, which agree well with that of GO/Ag/PSi substrate. It is drastically distinct that the intensities of SERS spectra from the GO/Ag/PSi are much stronger than those of GO/PSi, Ag/PSi or GO/AgA/PSi. Figures 4(d) and 4(e) respectively plot the intensity of the R6G SERS signal at 613 cm−1 with a concentration of 10−4M and 774 cm−1 with a concentration of 10−7M collected on different substrates.
Fig. 4 (a)-(c) are respectively the Raman spectra of R6G with different concentrations on the GO/PSi, Ag/PSi and GO/AgA/PSi substrate. (d) and (e) are respectively the intensity of the signal at 613 cm−1 with a concentration of 10−4M and 774cm−1 with a concentration of 10−7M collected using respectively the GO/PSi, Ag/PSi, GO/AgA/PSi and GO/Ag/PSi as substrates. (f)-(h) are respectively the Raman intensity of R6G peaks at 613cm−1 as a function of the molecular concentration on the GO/PSi, Ag/PSi and GO/AgA/PSi substrate, in log scale.
Just as summarized in Table 1, the excellent SERS behavior of the GO/Ag/PSi and the different enhancement versus substrates can be seen clearly. Similar results are also obtained on different peaks with different concentrations. These differences can be attributed to the diverse dominated SERS mechanism for the substrates. Using GO/PSi as SERS substrate, the CM resulting from the charge transfer between GO and the molecules plays a leading role and relatively weaker SERS signal is obtained. Although the enhancement factor is little, the homogeneity is perfect and comparable with that of GO/Ag/PSi, which can be demonstrated by the small standard deviations and the large R2 (0.998) [Fig. 4(f)]. In this case, the GO film also act as the excellent adsorbent towards organic molecules besides introducing the CM enhancement. To quantify the enhancement contributions from GO, we calculated the enhancement factor (EF) based on the following formula
where ISERS and IRaman are the intensity of the same band under SERS and the normal Raman condition without GO, respectively and NRaman and NSERS are the numbers of R6G molecules illuminated by the laser spot under normal Raman and SERS conditions, respectively. The EF for the band at 613 cm−1 is 4.1, which is consistent with the EF based on CM. As a comparison, the EM acts as the major role for the Ag/PSi and the SERS signals are ~2.5 times stronger than that of GO/PSi. However, the large standard deviations and the small R2 (0.962) [Fig. 4(g)] imply that the homogeneity of the Ag/PSi is poor, which may result from the uneven distribution of the Ag nanoparticles. The additional enhancement of SERS signal of R6G on GO/AgA/PSi can be assigned to GO-derived CM and the molecule enrichment from GO. What’s more, the homogeneity of the GO/AgA/PSi is better than that of Ag/PSi, as shown in Fig. 4(h). In a further comparison with GO/AgA/PSi, the better enhancement from GO/Ag/PSi can be ascribed to the well distribution and small size of the Ag nanoparticles. Just as we discussed in Fig. 2, the immediate dip-coating of the GO film on the Ag/PSi is much beneficial for the well distributed Ag nanoparticles and the Ag nanoparticles still maintain their original size (~15nm) as without the annealing process occurred in the GO/AgA/PSi substrate. Besides the well distributed Ag nanoparticles referred in Fig. 3, the size of the Ag nanoparticles is also crucial for the high enhancement factor. As the plasmons only concentrated on the surface of the metal nanoparticles, consequently the enhanced electromagnetic field mainly focuses around the metal nanoparticles [29]. The metal nanoparticles with small size can achieve higher sensitivity SERS signal compared with that of large size. The size effect for the enhanced electromagnetic field will be further demonstrated in the section of theoretical simulation. Furthermore, we measure the stability of the GO/Ag/PSi SERS substrate by subjecting it to aerobic exposure for 10 days.
Table 1. Enhancement on 613 and 774 cm−1 versus substrates
What should be noted here is that, after the expose of oxygen, the intensity of R6G with a concentration of 10−5M on the GO/Ag/PSi substrate is almost unvaried, which provides a strong support to better antioxidant ability of GO/Ag/PSi. On the contrary, the intensity of the R6G with a concentration of 10−5M on the Ag/PSi substrate decreases obviously after being treated with oxidation [Fig. 5] The decrease of the intensity may be due to the oxidation of Ag nanoparticles as it is greatly impressionable to oxidation, which can be confirmed by the EDS result [Fig. 5(c)]. Obviously, the uniform GO film can isolate Ag nanoparticles from surrounding environment and effectively passivate the Ag nanoparticles. Thus, the stability of the SERS signal on the GO/Ag/PSi is perfect in our case.
Fig. 5 (a) and (b) are respectively the SERS spectra of the R6G without and with the oxidation treatment. (c) the EDS result of the Ag/PSi substrate after the expose of oxygen.
In order to give a better understanding for the GO/Ag/PSi structure as a perfect SERS platform, we calculated the electric field distributions of PSi and Ag/Si structure using FDTD analysis. In fact, we also attempted to model the enhancement of the electric field of the GO/Ag/PSi. However, the large difference of the size between PSi and Ag nanoparticles makes it hard to model the PSi and Ag nanoparticles in a sole model. What’s more, GO mainly enhance the Raman signal by CM existing between GO and the molecule and has little influence on the electric field. Consequently, here we model the electric field distributions of PSi and Ag/Si structure respectively to investigate the enhancement behavior of the GO/Ag/PSi. The well-separated PSi geometry (height: ~3μm and space: ~4μm) representative of the actual sample was chosen. Figures 6(a) and 6(b) respectively exhibit the y-z and x-z views of the electric field distribution on the PSi sample with incident light wavelength 532nm. It can be seen clearly from Figs. 6(a) and 6(b) that strong field enhancement exhibits in both the gap and the surface of the silicon pyramid as well as at the valley. Just as we have discussed in Fig. 4, only this local enhancement of the incident laser cannot bring about the SERS based on the EM. However, once attaching the Ag nanoparticles on the surface, it can effectively amplify the incident laser and further bring about the increased electrical field intensity of the plasmonic resonance. As shown in Fig. 6(h) (the x-y view of the electric field distribution), the electric field distribution is relatively uniform, which is the dominant contributor to the homogeneity of SERS signal. To study the size effect for the enhanced electromagnetic field, we set the diameter of Ag nanoparticles respectively to 15 and 50nm, which is corresponding with the experimental parameters. Just as exhibited in Figs. 6(i) and 6(j), the magnitude of electrical field significantly locates at the edge of nanoparticles, especially in the gap region and the magnitude of electrical field with 50nm Ag nanoparticles is approximately one order of magnitude larger than that with 15nm Ag nanoparticles, which is opposite to our experiment results. In our experiment, the GO/Ag/PSi and GO/AgA/PSi are fabricated in the same condition with the same concentration of the Ag nanoparticles. After the annealing process, the actual structure is different from that shown in Figs. 6(i) and 6(j), instead of that exhibited in Figs. 6(c) and 6(d). In these cases, the magnitude of electrical field with 15 nm Ag nanoparticles is comparable with that of 50nm Ag nanoparticles and the latter is about the double of the former. However, the volume of the high field area (gap region) of 15nm Ag nanoparticles is much larger than that of 50 nm Ag nanoparticles, which verify the fact that it is much suitable for the better enhancement than the latter. To further actually demonstrate the perfect SERS behavior of the GO/Ag/PSi, we calculated the electric field distribution of 15nm Ag/Si under different incident power [Figs. 6(e)-6(g)]. With the increase of the incident power, the magnitude of electrical field obviously increases, which indicates that the PSi substrate can play the role as amplifier for incident light and introduce large electrical field intensity of the plasmonic resonance upon attaching the Ag nanoparticles on the surface. These theoretical results suggest a higher SERS sensitivity based on the GO/Ag/PSi may be accomplished by further optimizing the size and gap width.
Fig. 6 (a) and (b) are respectively the y-z and x-z views of the electric field distribution on the PSi sample. (c) x-y view of the electric field distribution on the Ag/Si structure with 15nm Ag nanoparticles and 15nm gap. (d) x-y view of the electric field distribution on the Ag/Si structure with 50nm Ag nanoparticles and 5nm gap. (e)-(f) are respectively the x-y views of the electric field distribution on the Ag/Si structure with 15 nm Ag nanoparticles and 15nm gap with incident power 3.2 × 10−7, 3.2 × 10−6 and 3.2 × 10−5W. (h) x-y view of the electric field distribution on the PSi sample. (i) and (j) are respectively the x-y views of the electric field distribution on the Ag/Si structure respectively with 15 and 50 nm Ag nanoparticles.
4. Conclusion
We fabricated a SERS substrate based on GO/Ag/PSi using a convenient and low-cost method. On the top of the obtained GO/Ag/PSi, the bio-compatibility, homogeneity and chemical stability were demonstrated by the detection of R6G. The contrast experiments using PSi, GO/PSi, Ag/PSi and GO/AgA/PSi further indicated that the GO was fairly favorable for the synthesis of well distributed Ag nanoparticles on the PSi substrate and thus achieved the SERS signal with high sensitivity, perfect bio-compatibility, good homogeneity and chemical stability. With the assist of FDTD, the perfect SERS behaviors obtained in experiment were confirmed by theoretical calculations. Our results suggest the novel GO/Ag/PSi could be a promising and sensitive SERS substrate for molecule detection in areas of medicine, food safety and biotechnology.
Acknowledgments
The authors are grateful for financial support from the National Natural Science Foundation of China 11474187, 11274204, 61205174, 11404193, Shandong Excellent Young Scientist Research Award Fund BS2014CL039, Shandong Province Natural Science Foundation ZR2014FQ032 and Excellent Young Scholars Research Fund of Shandong Normal University.
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