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Abstract
Film wrap nanoparticle system (FWPS) is proposed and fabricated to perform SERS effect, where the Ag nanoparticle was completely wrapped by Au film and the double-layered graphene was selected as the sub-nano spacer. In this system, the designed nanostructure can be fully rather than partly used to generate hotspots and absorb probe molecules, compared to the nanoparticle to nanoparticle system (PTPS) or nanoparticle to film system (PTFS). The optimal fabricating condition and performance of this system were studied by the COMSOL Multiphysics. The simulation results show that the strongly large-scale localized electromagnetic field appears in the whole space between the Ag nanoparticle and Au film. The experimental results show that the FWPS presents excellent sensitivity (crystal violet (CV): 10−11 M), uniformity, stability and high enhancement factor (EF: 2.23×108). Malachite green (MG; 10−10 M) on the surface of fish and DNA strands with different base sequence (A, T, C) were successfully detected. These advanced results indicate that FWPS is highly promising to be applied for the detection of environmental pollution and biomolecules.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface enhanced Raman scattering (SERS) technology can label-free distinguish and analyze the detected molecules by the fingerprint information of molecular vibration, which is widely applied in food safety, environmental monitoring, and medical diagnosis [1–6]. As we all know, due to its’ electromagnetic mechanism (EM) and chemical enhancement mechanism (CM), the Raman signal of probe molecules can be amplified 108 and 102 times, respectively [7–9]. Therefore, EM is the main concern mechanism for SERS substrate design. Under the irradiation of the laser, the free electrons on the surface of the metal nanostructures oscillate collectively, forming a highly concentrated electric field enhancement, that is, hotspots [10–12]. Recently, researchers have made great efforts to design and prepare various efficient SERS substrates with abundant hotspots, such as pentacle structure, dendritic structure, sea urchin structure, etc, where a great contribution to SERS through EM was achieved [13–17]. However, the electric field around the plasmonic nanostructures is not uniformly distributed but highly confined to a point or a narrow space, which still needs further optimization for the ultrasensitive detection of biomolecules.
Coupled nanostructures with controllable nanogaps (hotspots) between nanostructures have been employed as SERS substrates [18–20]. Due to the uniform nanoscale thickness of two-dimensional (2D) materials, multi-layer structures with 2D materials as spacer has been achieved. For example, J. Baumberg et al. proposed graphene as a spacer to prevent the fusion of the upper layer gold particles and the lower layer gold film, which confirms that graphene can create a robust, repeatable, and stable sub-nanometer gap for a large number of plasma field enhancements [21]. Choy et al. also demonstrated the strong coupling between metal particles and metal films using graphene as nano-spacers [22]. To simplify the steps of manufacturing the substrate, Wei et al. directly deposited Cu nanoparticles onto graphene sheets that had been grown on Cu foil by chemical vapor deposition (CVD) method, and also obtained a high enhancement factor [23]. These plasmonic structures greatly facilitate the studies of SERS substrates, which investigated hotspots including lateral hotpots in adjacent nanoparticles in one plane and the vertical hotspots in graphene nanospacer. However, these hotspots in a point-to-point form exist in a very small space, restricting the entry of molecules to be detected and limiting the application of the effective area of the substrate. Therefore, to expand the hotspot space, the perfect plasmonic nanostructures coupling is a significant topic and is a goal that all researchers pursue.
Here, we proposed the novel film wrap nanoparticle system (FWPS) to perform SERS effect. The designed nanostructure can be fully rather than partly used to generate hotspots and absorb probe molecules, which reverses the previous approach of point-to-point hotspot generation for the nanoparticle to nanoparticle system (PTPS) or nanoparticle to film system (PTFS) with graphene as a spacer. In FWPS, Ag nanoparticle was tightly and completely wrapped by graphene and Au film. Double-layer graphene was used to form sub-nanospace between Ag nanoparticle and Au film, which can effectively avoid quantum tunneling effect and provide CM [24,25]. COMSOL Multiphysics simulations results show that the whole space between Ag nanoparticle and Au film can generate high intensity localized electromagnetic field, which dramatically improves the utilization of the designed structure and allows easier access of molecules to the hotspots region. Large-scale and high-intensity hotspots allow FWPS to display excellent SERS capabilities. High sensitivity (CV: 10−11 M), uniformity, stability, and high enhancement factor (EF: 2.23×108) were realized. FWPS also shows high potential in practical application. Residual of MG on the surface of fish was successfully detected and all the main fingerprint peaks of DNA strands with three type base sequence (A, T, C; 10−6 M) were successfully obtained. FWPS is expected to further advance the application of SERS substrate in practical detection.
2. Experimental section
2.1 Theoretical simulation of the FWPS, PTPS, and PTFS
To obtain the optimal fabricating condition and compare the SERS performance of the PTPS, PTFS, and FWPS, the localized electromagnetic field distribution was simulated by using commercial COMSOL Multiphysics software, and the simulation set-up of the three systems were shown in Fig. 1. The Au film thickness is designated as 1 nm, 5 nm, 9 nm, 15 nm and 30 nm, and the nanogaps on Au film were set as 30 nm, 8 nm, 5 nm, 1.5 nm, and 0 nm, respectively, which are consistent with the experimental results by scanning electron microscopy (SEM) characterizations. To control the uniqueness variables, the diameter of all the Ag nanoparticles in three systems is set as 140 nm. Besides, the sub-nanospace is set as 0.68 nm which is corresponding to the thickness of double-layer graphene. The wavelength of incident light is set as 532 nm which is incident along the Z axis.
2.2 Preparation of the FWPS
The synthesis process of FWPS is illustrated in Fig. 2. Firstly, 15 nm Ag film is evaporated onto SiO2 substrate with an evaporating rate of 0.5 A/s. Secondly, the Ag film turns into Ag nanoparticles after 40 minutes annealing process at 500 °C with the flowing Ar (40 sccm) in a tube furnace. Thirdly, graphene that is obtained by the chemical vapor deposition method is transferred to the surface of Ag nanoparticles, later, the Ag nanoparticles will be tightly and completely wrapped by graphene film. Finally, Au film with 9 nm is evaporated onto the graphene surface. Here graphene works as the sub-nanospacer between Ag nanoparticles and Au film.
2.3 Characterization
The appearance and elements of FWPS were characterized by scanning electron microscopy (SEM, ZEISS Sigma 500) with an energy dispersive spectrometer (EDS). The thickness and light transmittance of Au film were characterized by an Atomic force microscope (AFM, Park XE 100) and Double beam UV-VI spectrophotometer (UV, TU-1900), respectively. Raman spectrometer (Horiba HR Evolution 800) is used to characterized fingerprint peaks of detected molecules. The 532 nm laser wavelength, 600 gr/mm diffraction grating, 50× objective lens, 4s integral time, and 0.48 mW laser energy were selected.
3. Results and discussion
The optimal condition of fabricating the FWPS was theoretically studied by COMSOL Multiphysics and all the detailed simulation parameters were shown in Fig. 1. As shown in Figs. 3(a) and 3(b), for the PTPS or PTFS, the high-intensity localized electric fields were generated only around the sub-nanospace between the nanoparticle to nanoparticle, or between the nanoparticle to film. The structure of the localized electric field in PTPS or PTFS generated low-intensity hotspots and the positions of these hotspots are unfriendly for the attachment of probe molecule to detect. Here, we proposed a FWPS where the Ag nanoparticle was completely wrapped by the Au film and the double-layer graphene was used as sub-nanospacer to avoid quantum tunneling effect. In FWPS, large-scale sub-nanospace and high-intensity electric fields almost appear at the whole space between Au film and Ag nanoparticle, as shown in Fig. 3(c). Designed nanostructure can be fully rather than partly utilized, compared to the PTPS or PTFS. The FWPS with thick Au film (15 nm, 30 nm) has more sub-nanospace but low light transmittance, while the FWPS with thin Au film (1 nm, 5 nm) has less sub-nanospace but high light transmittance. The morphologies and transmittance of Au films with different thickness are shown in Figs. 4(a)–(e) and Fig. 4(f), respectively. Due to the size of the sub-nano space and the light transmittance (Au film) jointly determine the intensity of the localized electric field, so the FWPS with different thicknesses Au films and fixed Ag nanoparticles were studied by COMSOL Multiphysics to obtain the optimal SERS performance. As shown in Fig. 3(d), simulation results show that the FWPS with 9 nm Au film performs the highest intensity localized electric field, compared to the FWPS with others Au film. Appropriate scale of sub-nanospace and light transmittance makes Au film with thickness 9 nm be the optimal condition to fabricate the FWPS.
Fig. 3. COMSOL simulation of electromagnetic field distribution of (a) PTPS, (b) PTFS, (c) FWPS system with 9 nm Au film. (d) FWPS composed of Au films with a thickness of 1 nm, 5 nm, 9 nm,15 nm, and 30 nm from right to left. (e) Schematic diagram of localized electric field distribution of FWPS.
Fig. 4. (a)–(e) are respectively the SEM characterizations of Au films with thickness that 1nm, 5nm, 9nm, 15nm and 30nm. The statistical graphs of gaps spacing on 1nm, 5nm, 9nm thickness Au films are respectively shown the inset of (b)–(d). (f) The transmittance spectra of Au films with thickness of 1 nm, 5 nm, 9 nm, 15 nm and 30 nm.
Based on the simulation results, FWPS with the optimal SERS performance was successfully prepared in the experiment. As shown in Fig. 5(a), all of the Ag nanoparticles were successfully wrapped by graphene and the average diameter of Ag nanoparticles was counted by the image processing software Nano Measurer of 60 samples (the inset of Fig. 5(a)). As shown in Fig. 5(b), the wrinkles at the surface of the substrate and the bottom of silver nanoparticles suggest that graphene with excellent flexibility can tightly wrap the silver nanoparticles by an annealing process. Raman spectra of these wrinkles double-layer graphene are characterized as shown in the inset of Fig. 5(b). As shown in the top inset of Fig. 5(c), the image of the height characterized by the AFM demonstrates the thickness of Au film is ∼9 nm. And, Au film with a uniform gap of ∼5 nm (the inset in Fig. 4(c)) was evaporated onto the graphene-coated Ag nanoparticles to form the FWPS. The existence of nanogaps in Au film allows the light and probe molecules easier to enter the sub-nanospace as shown in the top insert of Fig. 5(d). Comparing Fig. 5(d) with Fig. 5(b), it is clear that there are some different phenomena as shown in follows: (1) SEM image of Fig. 5(d) becomes clearer and brighter than that of Fig. 5(b) at the same voltage, because Au film increase the electrical conductivity of the structure that graphene-coated Ag nanoparticles; (2) The surface of the structure in Fig. 5(d) looks rougher than that in Fig. 5(b), due to smooth surface of graphene was covered by Au film with a rough surface; (3) There are more wrinkles appear and these wrinkles become stronger. These phenomena indicate that Au film can successfully and completely wrap the structure that graphene-coated Ag nanoparticles. Energy dispersive spectrum was used to characterize the element composition of FWPS. Uniform elements distributions of C, Au, and Ag confirm the co-presence of Au film, graphene and Ag nanoparticles on the substrate, as shown in the top inset of Fig. 5(e). The sharp peaks of Si, C, Ag, and Au further confirmed that the compositions of FWPS include SiO2 substrate, Ag nanoparticles, graphene, and Au film, as shown in Fig. 5(e). Thus, we could make the conclusion that the FWPS was successfully fabricated in the experiment, according to the simulated parameters.
Fig. 5. (a) SEM morphology characterization of Ag nanoparticles wrapped with graphene. The inset is the statistical graph of AgNPs diameter. (b) The detailed SEM of Ag nanoparticles wrapped with graphene. The inset is the Raman spectra of double-layer graphene. (c) SEM morphology characterization of large-area 9 nm Au film, the top inset shows the thickness of 9 nm Au film being characterized using AFM. (d) The SEM characterization of FWPS. (e) Elemental spectrum on the FWPS and EDS elemental maps for the C, Au, Ag. (f) The Raman spectrum for R6G molecules (10−6 M) on different FWPS. (g) The average intensity of the SERS signal of R6G at 613 cm−1 and 774 cm−1.
Besides, FWPS with 1 nm, 5 nm, 15 nm, and 30 nm Au film were also fabricated and studied. R6G at a concentration of 10−6 M was used as a probe molecule to study their SERS performance. Figure 5(f) reveals that all the fingerprint peaks of the Rhodamine 6G (R6G) can be easily detected and that the relative intensities of R6G fingerprint peaks that detected on the FWPS with 9 nm Au film were much higher than those detected on the other four FWPS. In addition, as a comparison, the Au film wrapping Ag nanoparticle has also been fabricated for investigating SERS properties for the case of 9 nm without graphene. As shown in Fig. 5(f), the SERS spectrum of FWPS without graphene is approximately three times lower than FWPS with graphene. The phenomenon can be attributed to three factors: (1) without graphene as a spacer, there is the direct contact between Au film and AgNPs, and thus no gaps are created, suppressing the generation of hotspots [26]. (2) The graphene spacer can also provide CM that enables the enhanced Raman signals of the target molecules adsorbed to be detected from transferring electrons. (3) Graphene possesses the high affinity of aromatic molecule compared to that of metal surface. To intuitively and clearly compare the SERS enhancement performance of Au films with different thicknesses, the relative intensity of the characteristic peaks at 613 cm−1 and 774 cm−1 were collected as presented in Fig. 5(g). It is obvious that the FWPS with 9 nm Au film shows much better SERS behaviors than the other four systems, which is highly consistent with the simulation results. Therefore, we select the FWPS systems with 9 nm Au film as the optimal FWPS and perform further research throughout the following experiments.
CV with concentration from 10−6 M to 10−11 M were successfully detected, which verify the FWPS has a good detection capability for an ultra-low concentration. As shown in Fig. 6(a), all the main fingerprint peaks can be clearly distinguished with concentrations ranging from 10−6 M to 10−11 M and the intensity of Raman signals decreases as the concentration decrease. The peak at 223 cm−1 applies to the breath of central bond, the peak at 915 cm−1 applies to the Radial aromatic ring skeleton vibrates, the peak at 1178 cm−1 applies to the ring C-H bend, the peak at 1372 cm−1 applies to the N-ph stretching and the peak at 1588 cm−1 applies to the ring C-C stretching [27]. Herein, peaks at 223 cm−1 and 1588 cm−1 were selected to study the relationship between the intensity of Raman signals and CV concentration. Figure 6(b) shows the reasonable linear response of the FWPS, and the value of R2 respectively equal to 0.988 and 0.996 at 223 cm−1 and 1588cm−1. The intensity of Raman signals changes in a liner relation with the decreasing of CV concentration. The enhancement factor (EF) of CV molecule at 915 cm−1 peak on the FWPS was calculated according to the following standard equation [28]:
ISERS and IRS are the peak strengths of SERS and Raman respectively. NSERS and NRS are the number of probe molecules, respectively, on SERS substrate and on normal Raman scattering substrate. Here, the ISERS and NSERS of the 10−10 M CV was considered to calculate the EF, due to the Raman signal of the 10−11 M CV is too weak to completely present all the fingerprint peaks. 3 µL CV with the concentration 10−2 and 10−10 M were dropped onto the SiO2 substrate and FWPS, respectively. After drying, the average diameters of evaporation traces that left on SiO2 and FWPS separately are 3.5 mm and 3 mm. The density of probe molecules in the evaporation trace can be calculated according to the formula: D = NCV/S. (N is Avogadro constant, C and V respectively are the concentration and volume of the dropped probe molecule, and S is the area covered by the probe molecule). The same-size incident spot makes:
Fig. 6. (a) SERS spectra of the CV molecules with concentrations from 10−11 M to 10−6 M. (b) The Raman intensities of CV at 1588 cm−1 and 223 cm−1 as a function of the molecular concentration on the FWPS in log scale. (c) The Raman spectra of CV with concentrations of 10−10 M and 10−2 M collected from the FWPS and SiO2 substrate, respectively.
The volume of probe molecules that dropped on the FWPS and SiO2 substrates is the same, making formula (2) can be simplified to
In FWPS, according to the Fig. 6(c), the SERS intensity of CV with the concentration 10−10 M is 260.27 (peak: 915 cm−1), and in SiO2 substrate, the Raman intensity of CV with the concentration 10−2 M is 100.34 (peak: 915 cm−1). Thus, we can obtain the EF = 2.23×108. The strong EF can be attributed to that (1) Graphene acted as an assisted enhancer, which can provide additional CM enhancement; (2) The whole space between Ag particle and Au film provided large-scale high intensity localized electromagnetic field, which caused there are vast hotspots region was generated; (3) The existence of nanogaps in Au film can also generate hotspots; (4) Good biocompatibility of graphene and Au film is facilitate for the hotspots to interact with more probe molecules.
Uniformity and stability of the Raman signal are other important indicators for the FWPS to the practical application except for a good detection capability for an ultra-low concentration. The FWPS obtained on the SiO2 substrate exhibits excellent homogeneity and the double-layer graphene plays an important role to form the homogeneous sub-nanospace and homogeneous localized electromagnetic fields. The graphene and Au film with excellent antioxidant properties can isolate the Ag particle from the air, thus slowing down the oxidation rate and make the FWPS has good stability. 10−6 M CV and 10−6 M R6G were respectively selected as the probe molecule to characterize the uniformity and stability. As shown in Fig. 7(a), there was almost no significant difference among the CV Raman signals that were randomly collected from 10 positions in a same FWPS, which indicates the high homogeneity of the FWPS. To more directly investigate the uniformity, the Raman signals of CV at 915 cm−1, 1178 cm−1, 1588 cm−1, 1621 cm−1 were collected and compared. Figure 7(b) shows that the intensity of these peaks is almost on a horizontal line and their relative standard deviations (RSD) respectively are 10.09%, 8.21%, 7.19% and 6.37%. These RSD values are much lower than the scientific standards (20%) reported by Natan on account of the designed nanostructure [29], which prove that the intensity of these selected characteristic peaks keeps the same order magnitude although there are some fluctuation and that the FWP SERS systems perform good uniformity. As shown in Fig. 7(c), seven groups of data that were collected once every two days show that the intensity of R6G Raman signals decreases slightly in two weeks, which indicate that the FWPS perform good stability.
Fig. 7. (a) SERS spectra of the CV molecules with a concentration of 10−6 M detected on 10 random spots in the same FWPS. (b) The intensity distributions of the 915, 1178, 1588, and 1621 cm−1 peaks for CV. (c) SERS spectra of CV detected by FWPS once every two days.
The numerical model for detecting MG is established to make FWPS can contribute a reliable and convenient role in practical application. In the actual production and life, people can directly determine the MG content of aquatic products through this model, avoiding other complex and expensive testing processes. MG that acts as a bactericidal and parasitic chemical agent is widely decomposed in aquaculture to solve water mold [30]. However, it is very carcinogenic to the human body and it has been banned by many countries [31]. China listed MG as a banned drug in the agricultural industry standard “NY5071-2002 Guidelines for the use of Pollution-Free Food and Fish Drugs”; US Food and Drug Administration banned edible the aquatic products including MG or recessive MG; EU Act 2002/675/EC stipulates that the total amount of MG and colorless MG residues in animal-derived foods is limited to 2 µg/kg (Equivalent to 10−9 M) [32]. To establish this numerical model, six groups MG solution with concentration ranging from 10−5 M to 10−10 M were added to FWPS respectively and all the main fingerprint peaks of MG with different concentrations were obtained, as shown in Fig. 8(b). The intensity of these Raman signals decreases as the concentration decrease and the linear relationship Log I=0.31LogC+5.66 between the intensity of Raman signals (peak at 1588 cm-1) and MG concentration was obtained, as shown in Fig. 8(c). Hereto, this numerical model was obtained and we performed this numerical model to the following practical detection of MG that exists in aquatic products. The fish that submerged in 10−6 M MG solution for 24 hours was set as polluted fish in the pollution source and the polluted fish that was washed by 1 L deionized water work as the fish that people buy from the market in their daily life, as shown in Fig. 8(a). In our experiment, ten group Raman signals of the extracted sample was collected from the washing water. Take the value of the average intensity of the peak at 1588 cm−1 (the inset of Fig. 8(d)) into the numerical model Log I=0.31LogC+5.66, the concentration of MG in washed solution can be easily obtained and the C is 3.59×10−8 M. In practical life, quality inspection personnel can extract some sample from the aquatic pond in aquatic market and use the numerical model to quantitatively determine the residue of MG in aquatic products.
Fig. 8. (a) The optical picture of detecting of MG by detection the wash water of the contaminated fish. (b) SERS spectra of the MG molecules with concentrations from 10−10 M to 10−6 M on the FWPS. (c) The Raman intensities of MG at 1588 cm−1 collected as a function of the molecular concentration on the FWPS in log scale. (d) SERS spectra of the MG molecules detected on 10 random spots in the FWPS.
SERS with the advantages that label-free, easy to operate and repaid were viewed as the high potential method for DNA detection. Large-scale high-intensity hotspots and good biocompatibility of FWPS make it successfully detect the single-stranded DNA with three types base sequences (adenine: A, cytosine: C, and thymine: T). As shown in Fig. 9(a), all the main fingerprint peaks of DNA strand can be effectively detected and distinguished. The peaks at 668 cm−1 and 1540 cm−1 of base sequences A, the peaks at 650 cm−1 and 1624 cm−1 of base sequences T and the peaks at 698 cm−1 and 1582 cm−1 of base sequences C can be used as the characteristic peaks for qualitative and quantitative analysis of A, T and C. The peaks marked with pentacle indicates the ring breathing vibration of molecules on the internal plane of A, T and C bases. Figure 9(b) shows the Raman peaks that collect from five different locations on the FWPS of base A and the height of these peaks is almost on the same horizontal line, which prove that DNA is evenly distributed on the substrate of FWPS. Some differences in the relative size of the peaks may be caused by the different contact mode between DNA and the FWPS [33–35]. Therefore, it can be concluded that the FWPS can work as an effective and precise sensor to detect and analyze DNA molecules.
Fig. 9. (a) The Raman spectra of DNA with different base (adenine, thymine, cytosine), and the inset of Fig. 9(a) is molecular outline diagram of DNA with a different base (adenine, thymine, cytosine). (b) The Raman spectra of DNA with base (adenine) in five different locations on the FWPS.
4. Conclusion
In this paper, we studied the FWPS experimentally and theoretically. 9 nm Au film and double-layer graphene can completely wrap the Ag nanoparticle to form the optimal FWPS. The utilization rate of the designed structure in FWPS has been greatly improved, compared to that in traditional PTPS or PTFS. Large-scale high-intensity hotspots make FWPS present an excellent SERS effect. High sensitivity (CV: 10−11 M) and good uniformity were realized in our experiment. Besides, large-area graphene and Au film make FWPS obtain good biocompatibility and provided more area for absorbing probe molecules. The ultra-low concentration detection of MG molecules and the three type DNA strands were successfully realized except for the common probe molecules. These results suggest that FWPS is reliable and high promising to be applied in the areas of food security and biotechnology.
Funding
Key Research and Development Plan of Shandong Province (2019GGX102048); National Natural Science Foundation of China (11874244, 11974222).
Disclosures
The authors declare no conflicts of interest.
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