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Abstract
The heterogeneous metal nanostructures have attracted great interest in various applications due to the synergistic effects between two noble metals, especially in surface enhanced Raman scattering (SERS) region. Herein, we prepared a 3D SERS active substrate based on heterogeneous and cross-distributed metal structure hybridized with MoS2by in situ synthesizing gold nanoparticles (AuNPs) on MoS2 membrane. The AuNPs-AgNPs/MoS2/P-Si hybrid SERS substrate were characterized by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) to investigate the character and the content of elements. In virtue of the heterogeneous and cross-distributed structure and ultra-narrow interparticle gap generating strong electric fields enhancement, the ultra-low concentration of probe molecule were detected (the LOD of 10−12 M for R6G and CV, 10−11 M for MG), serving the optimal SERS performance. The excellent uniformity and reproducibility were achieved by the proposed substrate. Moreover, the flexible MoS2/AuNPs-AgNPs/PMMA pyramidal SERS substrate was applied to detect melamine molecule in liquid milk (the LOD reached 10−9 M), which revealed great potential to be an outstanding SERS substrate for biological and chemical detection.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to its single-molecule sensitivity, surface enhanced Raman spectroscopy (SERS) has emerged as a nondestructive analytical technique in environmental monitoring, food safety, biomedical detection and so on [1–5]. Because of the localized electromagnetic filed (EM) and charge-transfer complexes (CM), the Raman signal can be typically amplified with an enhancement factor as high as1 × 1014 to 1 × 1015 [6–10]. Particularly, noble metal nanostructures (mainly including Au, Ag and Cu) play the leading role among the various SERS-based materials due to their localized surface plasmon resonance (LSPR) [11]. Au and Ag nano-material are wildly applied for exciting layer generating hot spots, in virtue of plentiful free carrier [12]. What’s more, the Au-generating SERS sensor has advantage of superior sensitivity and stability but poor resolution [13, 14], while Ag-generating SERS sensor possesses high resolution with inferior stability because of oxidation [15].Therefore, the heterogeneous combination of Au and Ag enables to make up their disadvantage and reserve the superiority, which is responsible for SERS enhancement [16, 17].
Recently, Au-Ag bimetallic nanostructure caught much attention in SERS applications. Fan et al. Used ethylene glycol and PVP to fabricate Au:Ag bimetallic alloys NPs, and confirmed a charge transfer from Ag to Au atoms in the alloys [18]. Yang et al. prepared the Ag-Au and Au-Ag core-shell nanostructures on glass substrates for SERS-active substrates indicating that the Au-Ag is more powerful than pure Ag, Au film and Ag-Au as SERS-active substrates [16]. Liu et al. developed a dealloying process for synthesizing highly porous Au−Ag alloy nanoparticles with an ultrathin hollow silica shell as a stable colloid, which achieved high sensitivity and applicability [19]. However, these methods are simply the combination of bimetals, where the probe molecules are in direct contact with metal nanostructures, resulting in high background fluorescence and poor adsorption for the molecules [20].
Molybdenum disulfide (MoS2), the most representative of transition metal dichalcogenides, has become promising candidates in optoelectronics, nanoelectronics, and energy conversion [21–23].It has been found that MoS2 can engender weak SERS activity owing to chemical mechanism (CM) enhancement of both charge transfer and dipole-dipole coupling effect [24]. Furthermore, the inert characteristic of pristine MoS2 as with many inorganic solids endows it with catalytic activity [25]. The MoS2 can be functioned with chemical moieties by directly react with metal precursors, and immobilize metal nanoparticles (NPs) forming the hierarchical composites. To date, various reports have devoted to the MoS2-based hybrid structures with noble metal nanoparticles. T. S. et al. incorporated highly capacitive gold nanoparticles (AuNPs) onto MoS2 utilizing the stable sulfur-metal binding to effectively raise the gate-voltage by an order of magnitude [26]. Su et al. developed a dual-mode electronic biosensor for microRNA-21 detection based on AuNPs-decorated MoS2 nanosheet with high selectivity and stability [27]. Shi et al. reported a controllable wet method for effective decoration of MoS2 layers on the edge sites or defective sites with AuNPs [28]. Whereas, these studies mainly focused on the composition of MoS2 and Au monometallic structure. It is necessary to carry out research on bimetallic material for further improving the sensitivity.
In this work, we proposed a method to construct the AuNPs-AgNPs heterogeneous and cross-distributed metal structure hybridized with MoS2 SERS substrate. The designed hybrid possessing 3D structure was proved with high SERS enhancement by detecting R6G, crystal violet (CV) and malachite green (MG) molecule, which is not only due to the larger specific surface area of three-dimensional pyramid structure [29], but also the AuNPs-AgNPs heterogeneous and cross-distributed metal structure inducing strong electric fields enhancement [30]. The important role of the heterogeneous and cross-distributed metal structure was also proved in theory with a FDTD method. In order to make the practical application more broadly, the MoS2/cross-distributed Au-Ag bimetalic nanoparticles/polymethyl methacrylate (PMMA) was also prepared to detect melamine in liquid milk with the minimum detectable concentration of 10−9 M. The results indicate that this flexible substrate can realize in situ detection on surface of solution.
2. Experimental setup
2.1 Preparation of the heterogeneous and cross-distributed AuNPs-AgNPs/MoS2/P-Si and flexible MoS2/AuNPs-AgNPs/P-PMMA SERS substrates
The P-Si was fabricated by a wet etching method as described in our previous work [31]. Figure 1 schematically illustrates the process for the synthesis of AuNPs-AgNPs/MoS2/P-Si and MoS2/AuNPs-AgNPs/P-PMMA hybrid pyramidal SERS substrate. First of all, MoS2 thin layer was grown on the P-Si substrate through a thermal decomposition method. (NH4)2MoS4 with purity of 99.99% (0.01 g) was added to 1 mL of dimethylformamide (DMF) to form a 1 wt% solution. The solution was ultrasonic with an ultrasonic cleaner for 1 h to ensure all particles dissolved, and then was spined on the P-Si with spin coater at a rotating speed of 3000 rmp for 20 s to from an ultrathin and uniform (NH4)2MoS4 membrane. After that, the P-Si with (NH4)2MoS4 membrane was placed in a quartz tube furnace and annealed twice in 500°C for 30 min (Ar: 80 sccm and H2: 40 sccm) and 800°C for 60 min (Ar: 80 sccm). After the tube was cooled down to room temperature, the prepared 3D MoS2 substrate was dipped in HAuCl4 with concentration of 1mM generating AuNPs, then transferred to deionized water (DI water) to remove residual HAuCl4. The AgNPs was deposited on the AuNPs/MoS2/P-Si substrate using thermal evaporation process with Ag wires of 0.0015 g to obtain AuNPs-AgNPs/MoS2/P-Si nanostructure.
Fig. 1 Schematic illustration of the process for the synthesis of AuNPs-AgNPs/MoS2/P-Si and flexible MoS2/AuNPs-AgNPs/P-PMMA SERS substrates.
Next, to prepare the flexible substrate, the polymethyl methacrylate solution (PMMA/acetone = 0.1500 g/200 mL) was coating on the substrate. After the PMMA dried, the substrate was baked at 130°C for 30 min, making PMMA layer fit tightly with the pyramid structure. Later on, the sample was placed into 33.3% NaOH solution to get rid of the P-Si. Finally, DI water was prepared to wash residual NaOH solution, and the substrate was inverted on glass sheet forming flexible MoS2/AuNPs-AgNPs/PMMA hybrid pyramidal substrate.
2.2 Apparatus and characterization
The morphologies of the prepared samples were characterized by the scanning electron microscope (SEM ZEISS Sigma500) with energy dispersive spectrometer (EDS). Transmission electron microscope (TEM JEM-2100) equipped withselected area electron diffraction (SAED)were also carried out to investigate the characteristic of the sample. We carried out the X-ray photoelectron spectroscopy (XPS Thermo Scientific Escalab 250Xi) measurements to analyze the content of elements quantitatively and qualitatively.
2.3 SERS spectra measurement
To study the SERS behaviors of the proposed AuNPs-AgNPs/MoS2/P-Si andMoS2/AuNPs-AgNPs/PMMA hybrid pyramidal SERS substrate, we chose the Raman spectrometer (Horiba HR Evolution 800) with the excited laser of 532 nm and the excitation of 4.8 mW to collect the SERS spectra. The integration time was 8 s, and the diffraction grid was 600 gr/nm. The laser light was coupled through an objective lens of 50 × . All the parameters maintained invariant throughout all the SERS experiments. The R6G, crystal violet (CV) and malachite green (MG) as probe molecule with different concentration were measured through dropping it on AuNPs-AgNPs/MoS2/P-Si substrate with pipette. The melamine solution with different concentration was detected by floating MoS2/AuNPs-AgNPs/PMMA flexible substrate on it.
3. Results and discussions
The surface morphologies of P-Si and MoS2/P-Si were characterized by SEM, as shown in Fig. 2(a) and 2(b). Compared with pure P-Si, the MoS2/P-Sisubstrate exhibits a darker surface withobvious ripples at the ravine of the P-Si, indicating successful synthesis of MoS2.Fig. 3(a) shows the SEM micrograph of AuNPs decorated on MoS2 framework by chemical reduction with HAuCl4 solution. The size of AuNPs is counted by the image processing software Nano Measurer as revealed in the inset of Fig. 3(a) and the average diameter is about 42 nm. Followed that, dense AgNPs are evaporated on the surface of P-Si/MoS2/AuNPs substrate. It can be clearly seen the darker AgNPs is evenly distributed among the brighter AuNPs forming the heterogeneous and cross-distributed structure and also making the interparticle gap smaller, as illustrated in Fig. 3(b), where the average diameter of AgNPs is ~13 nm (the inset of Fig. 3(b)). Moreover, the SEM image of AgNPs evaporated on P-Si obtained in the same condition is presented in Fig. 2(c), which consisted well with the AgNPs among the AuNPs in Fig. 3(b). The local composition of the AuNPs-AgNPs/MoS2/P-Si sample is also measured with EDS spectra shown in Fig. 4, which clearly revealed the presence of Mo (yellow), S (red), Au (green) and Ag (purple) in the substrate. Hence, it is concluded that the heterogeneous and cross-distributed AuNPs and AgNPs structure is decorated on MoS2 film admirably. Furthermore, the Raman spectra of MoS2 on MoS2/P-Si, AuNPs/MoS2/P-Si and AuNPs-AgNPs/MoS2/P-Si substrate are respectively provided in Fig. 3(c). The characteristic peaks identified as and A1g respectively assigned to the opposite vibrations of two S atoms with its connected Mo atom and the out-of-plane vibration of S atoms in opposite directions can both observed [32]. The Raman intensity of MoS2 decorated with heterogeneous and cross-distributed AuNPs and AgNPs (blue line) has higher enhancement compared with that collected on the MoS2/P-Si (black line) and AuNPs/MoS2/P-Si (red line) substrate, which can be ascribed to the heterogeneous and cross-distributed structure and the ultra-narrow interparticle gap generating a great local electromagnetic field. It should be pointed out that the slight frequency shift of and A1g peaks is observed, which suggests distinct p-doping caused by decoration of AuNPs [28, 33]. In order to further validate the combination of bimetallic nanostructure, Fig. 3(d) reveals the TEM image of the synthesized AuNPs-AgNPs/MoS2. It can be easily distinguished the bigger AuNPs and smaller AgNPs, and their sizes are consistent with that shown in SEM. The inset in Fig. 3(d) shows the corresponding SAED image. The SAED image of a typical AuNPs-AgNPs decorated MoS2 film exhibits four distinguished rings which can be attributable to the MoS2 (100), MoS2 (110), Au (111) and Ag (200) planes with lattice spacing of 0.27, 0.15,0.23 and 0.20 nm respectively. The sharp diffraction rings of (111)Au and (200)Ag reveal the dominating orientation and face-centred-cubic (fcc) lattice of the Au and Ag [34]. It also strongly verifies the successful synthesis of AuNPs-AgNPs/MoS2. In addition, we can notice the cute AgNPs like speckles located around AuNPs in the amplifying TEM image of Fig. 3(e) and the multilayer MoS2 between two AgNPs in the further amplifying image (the inset of Fig. 3(e)). EDS image of this substrate (Fig. 3(f)) is shown to confirm the significant elements of Mo, S, Au and Ag, which is a more potent proof of excellent synthesis the heterostructure.
Fig. 2 SEM morphology characterization respectively from (a) P-Si, (b) P-Si/MoS2, (c) P-Si/AgNPs substrate.
Fig. 3 SEM morphology characterization respectively from (a)AuNPs/MoS2/P-Si, (b) AuNPs-AgNPs/MoS2/P-Si substrate. (c) SERS spectra of MoS2 on MoS2/P-Si, AuNPs/MoS2/P-Si and AuNPs-AgNPs/MoS2/P-Si substrate. (d) TEM image of the synthesized AuNPs-AgNPs/MoS2. The inset shows the SAED pattern from the film. (e) Enlarged TEM image of AuNPs-AgNPs/MoS2 film. (f) the corresponding EDS spectrum.
Fig. 4 EDS elemental maps from (b) Si, (c) Mo, (d) S, (e) Au and (f) Ag on the (a) P-Si/MoS2/AuNPs-AgNPs substrate.
In order to further analyze the chemical composition of synthesized AuNPs-AgNPs/MoS2 substrate, XPS as a kind of surface sensitive technique is applied to measure the content of elements quantitatively and qualitatively. Figure 5(a) shows the XPS survey spectrum obtained fromAuNPs-AgNPs/MoS2/P-Si substrate. The detail XPS spectrum analysis in little dotted box is exhibited in the inset. From them, we can clearly observe the particular elements in prepared substrate. Furthermore, Fig. 5(b)-5(d) display detailed XPS scans for the S, Au and Ag binding energies of the composite material. The separated peaks located at 163.8 and 162.3 eV (Fig. 5(b)) are assigned to be the S 2p1/2 and S 2p3/2 orbital of divalent sulfide ions (S2-) derived from MoS2. It is noteworthy that the peak at 161.2 eV certifies the low binding energy 2nd feature attributed to chemical reaction of MoS2 with HAuCl4. The presence of the peaks at 84.1 and 87.7 eV (Fig. 5(c)) ascribed to Au 4f is attributed to the presence of AuNPs, and the relatively weak peaks at 335.8 and 353.7 eV assigned to Au 4d in survey spectrum (the inset of Fig. 5(a)) also confirm the AuNPs existed in the sample. The strongest peaks at 368.2 and 374.3 eV considered as Ag 3d (Fig. 5(d)) originate from the AgNPs by thermal evaporation. Above all, the XPS in Fig. 5 fully justifies the excellent nanostructure of Au-Ag bimetal detected on MoS2 film.
Fig. 5 (a)XPS survey spectrum obtained from AuNPs-AgNPs/MoS2/P-Si substrate. Inset: Detailed XPS spectrum analysis of that in dotted box. Chemical composition analysis by XPS for (b) S, (C) Au and (D) Ag binding energies of the substrate.
To investigate the best SERS effect of the AuNPs, the R6G (10−5 M), as standard probe molecule, was detected on the AuNPs/MoS2/P-Si substrate with different reaction time (1, 2, 3, 4 and 5min) of MoS2 with HAuCl4 solution. It’s well known that different SERS effects are caused by different size, density and aggregation of AuNPs [35]. As shown in Fig. 6(a), the SERS activity of AuNPs/MoS2/P-Sisubstrate enhances with increasing time from 1 min to 3 min since more AuNPs decorated on MoS2 nanosheets, and decreases with further increasing the time from 3 min to 5 min probably due to aggregation of AuNPs. It is obvious the optimal SERS performance can be obtained when MoS2 reacts with HAuCl4 solution for 3 min. Therefore, we maintained the reaction time as 3 min to further research throughout the following experiments.
Fig. 6 (a) Raman spectra of R6G molecules (10−5 M) detected on the AuNPs/MoS2/P-Si with different reaction time.(b) Raman spectra of R6G molecules (10−5 M) detected on the AuNPs-AgNPs/MoS2/P-Si, AuNPs/MoS2/P-Si and MoS2/P-Si substrates. (c) The corresponding histogram of SERS peak intensities at five typical Raman peaks (610, 773, 1360, 1508, and 1648 cm−1) from the above substrates. (d) and (f) are the schematic of AuNPs-AgNPs and AgNPs-AgNPs structure for FDTD simulation. (e) and (f) are the local electric field distribution of AuNPs-AgNPs and AgNPs-AgNPs structure.
Afterward, in order to demonstrate the superiority of introducing AuNPs and AgNPs, Raman spectra of R6G (10−5 M) absorbed on the AuNPs-AgNPs/MoS2/P-Si, AuNPs/MoS2/P-Si and MoS2/P-Si substrates were measured for comparison, and the typical results are presented in Fig. 6(b). It reveals the SERS signal of R6G on P-Si/MoS2 is weakly enhanced mainly owing to CM, and the AuNPs-AgNPs/MoS2/P-Si substrate gave rise to enormous enhanced Raman signalcompared with the other two samples. In order to compare directly and conveniently, the relative intensity of the characteristic peaks at 610, 773, 1360, 1508 and 1648 cm−1 corresponding to the three substrates was collected to plot the histogram as illustrated in Fig. 6(c). The result shows that the SERS intensity of prepared AuNPs-AgNPs/MoS2/P-Si substrate is much higher than that of AuNPs/MoS2/P-Si and MoS2/P-Si at any characteristic peak, which is attributed to the synergistic effects between AuNPs and AgNPs inducing stronger electric fields enhancement.
Furthermore, the local electric field distribution was calculated by finite-different time-domain simulations to further reveal the advantage of heterogeneousmetal nanostructures. Figure 6(d) shows the schematic of AuNPs-AgNPs substrate, the diameter of AuNPs and AgNPs was respectively set as 42 nm and 13 nm, and all the space between adjacent nanoparticle is 3 nm, which is in accord with the actual size in SEM. As presented in Fig. 6(e), the strong electric field mainly distributes in the nanogaps between the AuNPs and AgNPs where generates dense hot spots and further enhanced SERS intensity. In addition, the AgNPs-AgNPs was applied to compare with AuNPs-AgNPs. The corresponding size is the same with AuNPs-AgNPs structure, shown in Fig. 6(f). Just as expected, it’s clearly observed the intensity of the local electric field on the AgNPs-AgNPs in Fig. 6(g) is much weaker than that on AuNPs-AgNPs in Fig. 6(e), that could be ascribed that the combination of AuNPs and AgNPs leads the useful optical and chemical properties included direct electron transfer and intense plasmonic optical properties [36]. Therefore, the proposed AuNPs-AgNPs substrate could excite strong electric fields enhancement, which will contribute to excellent SERS performance.
To obtain the sensitivity of the AuNPs-AgNPs/MoS2/P-Si structure as a SERS substrate, Raman spectra of R6G molecules with concentrations from 10−5 to 10−12 M on the hybrid substrate are successively measured. As shown in Fig. 7(a), the SERS intensity of the hybrid system gradually weakens as the R6G concentration decreased. Nevertheless, the characteristic Raman shift peaks of R6G molecules centered at 610, 773, 1360, 1508 and 775 cm−1 maintain a sharp intensity at the concentration of R6G molecule from 10−5 to 10−12 M. These characteristic Raman shift peaks is still distinguishable especially when the R6G molecule is diluted to 10−12 M. The limit of detection (LOD) of R6G molecule using AuNPs-AgNPs/MoS2/P-Si structure is 10−12 M. Figure 7(b) indicates the change in intensity of the characteristic peak at 610 cm−1 assigned to the C-C-C deformation in-plane vibration [37] with the concentration of R6G molecules, it reveals a linear dependence of intensity on the concentration of R6G molecules with correlation coefficient (R2) of 0.978. These results mentioned above substantially demonstrate the hybrid nanostructures have the capability to serve as an ultrasensitive SERS substrate, and it can be achieved for quantitative detection utilizing the AuNPs-AgNPs/MoS2/P-Si system.
Fig. 7 (a) Raman spectra of R6G (the concentration from 10−5 M to 10−12 M). (b) Linear relationships (R2 = 0.978): Raman intensities at 613 cm−1 as a function of the concentrations of R6Gmolecules. (c) Raman intensities of R6G molecules at 610, 773, 1360, 1508 and 1648 cm−1 (10−5 M) randomly collected 20 spots on a AuNPs-AgNPs/MoS2/P-Si substrate. (d) The histogramof SERS intensities of the peak at 613 cm−1 (R6G of 10−6 M) respectively collected from 10 different batches AuNPs-AgNPs/MoS2/P-Si substrate.
In order to investigate the enhancement capability of the AuNPs-AgNPs/MoS2/P-Si substrate, the SERS enhancement factor (EF) is calculated according to the formula [38]:
Where ISERSand IRS are the intensities of the same band of SERS spectra and normal Raman, and NSERS and NRS are respectively R6G molecules number excited by SERS laser spot, and the normal Raman condition. The minimum detection concentration of 10−12 M and the peak of R6G at 613 cm−1 are used for EF calculation. The relative Raman intensity of 613 cm−1 for R6G with concentration of 10−12 M in Fig. 7(a) is ~1193, 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 [39].Thus, we can calculatethe EF of AuNPs-AgNPs/MoS2/P-Si substrate is about 1.2 × 1010. The excellent sensitivity of the prepared substrate can be attributed to the ultra-narrow nanogaps supported by the composition of bi-metal generating strong electromagnetic field.For SERS-active substrate, the uniformity and reproducibility play a vital role in practical applications. Hence, we carried out the detection of the SERS signals from randomly selected 20 positions on AuNPs-AgNPs/MoS2/P-Si substrate. In order to directly compare the variation errors, the SERS intensity of R6G (10−5 M) from the 20 site with different characteristic peak on one sample is shown in Fig. 7(c). The result indicated that the variation of the intensity on these main characteristic peaks was fairly uniform, and the relative standard deviation (RSD) of the Raman shifts at 610, 773, 1360, 1508, and 1648 cm−1 are 6.73%, 14.06%, 13.04%, 9.45%, and 10.52% respectively (The detailed standard formula is provided informula(2).), much lower than the scientific standards of 20% in the uniformity of SERS substrates reported by Natan [40]. In addition, to evaluate the reproducibility of SERS signals, the average intensity of R6G (10−6 M) collected from 10 different batches AuNPs-AgNPs/MoS2/P-Si substrates with error bar (from 10 spots on one sample) is shown in Fig. 7(d). The error bar of the 610 cm−1 characteristic peak intensity at every substrate is rather small, and the histogram of SERS intensity exhibits the slight fluctuation from substrate to substrate with RSD of 12.89%, revealing as-prepared AuNPs-AgNPs/MoS2/P-Si nanocomposite could be identified as a promising substrate for SERS detection with high uniformity and reproducibility.
The relative standard deviation (RSD) was calculated by using the standard formula [41]:
where and I are respectively the average intensity and collected relative intensity of the characteristic 612 cm−1 peak. Thus, is the maximum fluctuation surrounding by .
To investigate the feasibility of the AuNPs-AgNPs/MoS2/P-Sias SERS substrate, the CV as probe molecule was detected using the sample with concentration from 10−6 to 10−12 M. As illustrated in Fig. 8(a), the LOD of CV molecule using the hybrid structure is 10−12 M. The linear fit calibration curve (R2 = 0.932) with error bar is shown in Fig. 8(b) to represent the capability of the quantitative detection. Moreover, MG, as an additiveforbidden in fishery aqueous environments, was also detected in Fig. 8(c). The Raman signals of its main characteristic peaks at 1176, 1367 and 1618 cm−1 could be effectively observed when the concentrationwas lower down to 10−11 M. The linear fitted curve of the relativeintensity at the1618 cm−1 peak versus the concentrations is shown in Fig. 8(d) with thehigh coefficient of determination (R2: 0.973) in log scale.Therefore, the sensitive and quantitative detection of CV and MG molecules obtained fromthe AuNPs-AgNPs/MoS2/P-Sisubstrate shows great potential for identifying trace molecules.
Fig. 8 The Raman spectra of (a) CV (concentration from 10−6 to 10−12 M) and (c) MG (concentration from 10−6 to 10−11 M) on the AuNPs-AgNPs/MoS2/P-Si substrate. Raman intensity of (b) CV (at 1620 cm−1) and (d) MG (at 1618 cm−1) on the substrate as a function of the molecule concentration.
In order to improve the demand of practical application, we prepared the flexible MoS2/AuNPs-AgNPs/PMMA pyramidal substrate. Figure 9(a) illustrated the SEM image of the flexible substrate. The PMMA film decorated with AuNPs and AgNPs presented a inverted pyramid structure, and the bimetalic nanoparticles closely arranged on the PMMA membrane. In addition, we detect melamine molecule with different concentrations in liquid milk by using of the flexible MoS2/AuNPs-AgNPs/PMMA substrate. Melamine, as an organic chemical material, has been illegally added to milk products to increase the apparent protein contents due to its low cost and high nitrogen content, and intake of melamine will result in kidney stones and even infant death [42]. Figure 9(b) shows the SERS spectra of melamine with concentration from 10−5 to 10−9 M. The enhanced bands at 1071 cm−1 in the SERS spectrum, assigned to ring stretching, ring in plane deformation as well as NH2 groups deformation vibrations, can be clearly observed [43], and its band intensity increases with the increase of the melamine concentrations. The black flat line presents the detection result in the pure milk without melamine. There is no peak to be observed, showing the add of milk has no influence on the detection of melamine. The LOD of melamine in milk reaches 10−9 M, far below the stipulated minimum limit of 10−5 M in the USA and EU. Moreover, Fig. 9(c) shows an excellent linearity between the melamine band intensity at 1071 cm−1 and its concentration. The correlation coefficient is R2 = 0.964, indicating the decent ability of quantitatively detecting melamine molecules. The excellent results can be ascribed to the dense hot spots and ultra-narrow nanogaps due to synergistic effect of Au-Ag bimetals. Therefore, the proposed flexible MoS2/AuNPs-AgNPs/PMMA pyramidal substrate exhibits the great potential in in situ detection of toxic solution.
Fig. 9 (a) SEM image of flexible MoS2/AuNPs-AgNPs/PMMA hybrid pyramidal SERS substrate. (b) SERS spectra of melamine solution with different concentration. (c) The linear relationships of the peak (1071 cm−1) intensities as a function of the concentrations ranging from 10−5 to 10−9 M.
4. Conclusion
In summary, we demonstrated that the AuNPs-AgNPs/MoS2/P-Si substrate could be facilely prepared through the in situ fabrication of AuNPs on MoS2 film and thermal evaporation of AgNPs. The hybrid pyramidal SERS substrate was proved with high sensitivity, excellent uniformity and reproducibility due to the ultra-narrow nanogaps of Au-Ag bimetal synergistic effect. We also prepared the flexible MoS2/AuNPs-AgNPs/PMMA pyramidal substrate to detect melamine molecule in liquid milk for further application, showing the great potential in in situ detection. The results mentioned above indicate the substrate possesses remarkable ability to detect trace molecule.
Funding
National Natural Science Foundation of China (11774208, 11804200, 11474187, 11747072, 11674199 and 11604040); Shandong Province Natural Science Foundation (ZR2017BA004, ZR2016AM19); China Postdoctoral Science Foundation (2016M602716); Shandong Province Higher Educational Science and Technology Program (J18KZ011)
References
1. D. L. Jeanmaire and R. P. Van Duyne, “Surface Raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode,” J. Electroanal. Chem. Inter. 84(1), 1–20 (1977). [CrossRef]
2. H. Zhao, J. Jin, W. Tian, R. Li, Z. Yu, W. Song, Q. Cong, B. Zhao, and Y. Ozaki, “Three-dimensional superhydrophobic surface-enhanced Raman spectroscopy substrate for sensitive detection of pollutants in real environments,” J. Mater.Chen. A 3(8), 4330–4337 (2015). [CrossRef]
3. M. G. Albrecht and J. A. Creighton, “Anomalously intense Raman spectra of pyridine at a silver electrode,” J. Am. Chem. Soc. 99(15), 5215–5217 (1977). [CrossRef]
4. C. Zhang, C. Li, J. Yu, S. Jiang, S. Xu, C. Yang, Y. J. Liu, X. Gao, A. Liu, and B. Man, “SERS activated platform with three-dimensional hot spots and tunable nanometer gap,” Sens. Actuators B Chem. 258, 163–171 (2018). [CrossRef]
5. S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science 275(5303), 1102–1106 (1997). [CrossRef] [PubMed]
6. P. L. Stiles, J. A. Dieringer, N. C. Shah, and R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu. Rev. Anal. Chem. (Palo Alto, Calif.) 1(1), 601–626 (2008). [CrossRef] [PubMed]
7. Z. Dai, F. Mei, X. Xiao, L. Liao, L. Fu, J. Wang, W. Wu, S. Guo, X. Zhao, W. Li, F. Ren, and C. Jiang, ““Rings of saturn-like” nanoarrays with high number density of hot spots for surface-enhanced Raman scattering,” Appl. Phys. Lett. 105(3), 033515 (2014). [CrossRef]
8. Z. Li, S. Jiang, Y. Huo, T. Ning, A. Liu, C. Zhang, Y. He, M. Wang, C. Li, and B. Man, “3D silver nanoparticles with multilayer graphene oxide as a spacer for surface enhanced Raman spectroscopy analysis,” Nanoscale 10(13), 5897–5905 (2018). [CrossRef] [PubMed]
9. B. Sharma, R. R. Frontiera, A.-I. Henry, E. Ringe, and R. P. Van Duyne, “SERS: Materials, applications, and the future,” Mater. Today 15(1–2), 16–25 (2012). [CrossRef]
10. C. M. Aikens, L. R. Madison, and G. C. Schatz, “Raman spectroscopy: The effect of field gradient on SERS,” Nat. Photonics 7(7), 508–510 (2013). [CrossRef]
11. C. Li, C. Zhang, S. Xu, Y. Huo, S. Jiang, C. Yang, Z. Li, X. Zhao, S. Zhang, and B. Man, “Experimental and theoretical investigation for a hierarchical SERS activated platform with 3D dense hot spots,” Sens. Actuators B Chem. 263, 408–416 (2018). [CrossRef]
12. C. Zhang, Z. Li, S. Z. Jiang, C. H. Li, S. C. Xu, J. Yu, Z. Li, M. H. Wang, A. H. Liu, and B. Y. Man, “U-bent fiber optic SPR sensor based on graphene/AgNPs,” Sens. Actuators B Chem. 251, 127–133 (2017). [CrossRef]
13. J. M. Romo-Herrera, A. L. González, L. Guerrini, F. R. Castiello, G. Alonso-Nuñez, O. E. Contreras, and R. A. Alvarez-Puebla, “A study of the depth and size of concave cube Au nanoparticles as highly sensitive SERS probes,” Nanoscale 8(13), 7326–7333 (2016). [CrossRef] [PubMed]
14. S. K. Vashist, E. M. Schneider, and J. H. Luong, “A rapid sandwich immunoassay for human fetuin A using agarose-3-aminopropyltriethoxysilane modified microtiter plate,” Anal. Chim. Acta 883, 74–80 (2015). [CrossRef] [PubMed]
15. N. Michieli, R. Pilot, V. Russo, C. Scian, F. Todescato, R. Signorini, S. Agnoli, T. Cesca, R. Bozio, and G. Mattei, “Oxidation effects on the SERS response of silver nanoprism arrays,” RSC Advances 7(1), 369–378 (2017). [CrossRef]
16. Y. Yang, J. Shi, G. Kawamura, and M. Nogami, “Preparation of Au–Ag, Ag–Au core–shell bimetallic nanoparticles for surface-enhanced Raman scattering,” Scr. Mater. 58(10), 862–865 (2008). [CrossRef]
17. N. H. T. Tran, B. T. Phan, W. J. Yoon, S. Khym, and H. Ju, “Dielectric Metal-Based Multilayers for Surface Plasmon Resonance with Enhanced Quality Factor of the Plasmonic Waves,” J. Electron. Mater. 46(6), 3654–3659 (2017). [CrossRef]
18. M. Fan, F.-J. Lai, H.-L. Chou, W.-T. Lu, B.-J. Hwang, and A. G. Brolo, “Surface-enhanced Raman scattering (SERS) from Au: Ag bimetallic nanoparticles: the effect of the molecular probe,” Chem. Sci. (Camb.) 4(1), 509–515 (2013). [CrossRef]
19. K. Liu, Y. Bai, L. Zhang, Z. Yang, Q. Fan, H. Zheng, Y. Yin, and C. Gao, “Porous Au–Ag nanospheres with high-density and highly accessible hotspots for SERS analysis,” Nano Lett. 16(6), 3675–3681 (2016). [CrossRef] [PubMed]
20. 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]
21. Z. Li, S. Jiang, S. Xu, C. Zhang, H. Qiu, C. Li, Y. Sheng, Y. Huo, C. Yang, and B. Man, “Few-layer MoS2-encapsulated Cu nanoparticle hybrids fabricated by two-step annealing process for surface enhanced Raman scattering,” Sens. Actuators B Chem. 230, 645–652 (2016). [CrossRef]
22. W. Wu, L. Wang, Y. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X. Zhang, Y. Hao, T. F. Heinz, J. Hone, and Z. L. Wang, “Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics,” Nature 514(7523), 470–474 (2014). [CrossRef] [PubMed]
23. X. Liang, X. J. Zhang, T. T. You, N. Yang, G. S. Wang, and P. G. Yin, “Three-dimensional MoS2-NS@ Au-NPs hybrids as SERS sensor for quantitative and ultrasensitive detection of melamine in milk,” J. Raman Spectrosc. 49(2), 245–255 (2018). [CrossRef]
24. Y. Zhang, M. Wang, E. Zhu, Y. Zheng, Y. Huang, and X. Huang, “Seedless Growth of Palladium Nanocrystals with Tunable Structures: From Tetrahedra to Nanosheets,” Nano Lett. 15(11), 7519–7525 (2015). [CrossRef] [PubMed]
25. H. I. Karunadasa, E. Montalvo, Y. Sun, M. Majda, J. R. Long, and C. J. Chang, “A molecular MoS2 edge site mimic for catalytic hydrogen generation,” Science 335(6069), 698–702 (2012). [CrossRef] [PubMed]
26. T. S. Sreeprasad, P. Nguyen, N. Kim, and V. Berry, “Controlled, defect-guided, metal-nanoparticle incorporation onto MoS2 via chemical and microwave routes: electrical, thermal, and structural properties,” Nano Lett. 13(9), 4434–4441 (2013). [CrossRef] [PubMed]
27. S. Su, W. Cao, W. Liu, Z. Lu, D. Zhu, J. Chao, L. Weng, L. Wang, C. Fan, and L. Wang, “Dual-mode electrochemical analysis of microRNA-21 using gold nanoparticle-decorated MoS2 nanosheet,” Biosens. Bioelectron. 94, 552–559 (2017). [CrossRef] [PubMed]
28. Y. Shi, J.-K. Huang, L. Jin, Y.-T. Hsu, S. F. Yu, L.-J. Li, and H. Y. Yang, “Selective decoration of Au nanoparticles on monolayer MoS2 single crystals,” Sci. Rep. 3(1), 1839 (2013). [CrossRef] [PubMed]
29. C. Zhang, B. Man, S. Jiang, C. Yang, M. Liu, C. Chen, S. Xu, H. Qiu, and Z. Li, “SERS detection of low-concentration adenosine by silver nanoparticles on silicon nanoporous pyramid arrays structure,” Appl. Surf. Sci. 347, 668–672 (2015). [CrossRef]
30. C. Zhang, S. Z. Jiang, C. Yang, C. H. Li, Y. Y. Huo, X. Y. Liu, A. H. Liu, Q. Wei, S. S. Gao, X. G. Gao, and B. Y. Man, “Gold@silver bimetal nanoparticles/pyramidal silicon 3D substrate with high reproducibility for high-performance SERS,” Sci. Rep. 6(1), 25243 (2016). [CrossRef] [PubMed]
31. 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]
32. Y. Deng, M. Chen, J. Zhang, Z. Wang, W. Huang, Y. Zhao, J. P. Nshimiyimana, X. Hu, X. Chi, G. Hou, X. Zhang, Y. Guo, and L. Sun, “Thickness-dependent morphologies of Ag on n-layer MoS2 and its surface-enhanced Raman scattering,” Nano Res. 9(6), 1682–1688 (2016). [CrossRef]
33. B. Chakraborty, A. Bera, D. Muthu, S. Bhowmick, U. V. Waghmare, and A. Sood, “Symmetry-dependent phonon renormalization in monolayer MoS2 transistor,” Phys. Rev. B 85(16), 161403 (2012). [CrossRef]
34. J. Lu, J. H. Lu, H. Liu, B. Liu, L. Gong, E. S. Tok, K. P. Loh, and C. H. Sow, “Microlandscaping of Au nanoparticles on few-layer MoS2 films for chemical sensing,” Small 11(15), 1792–1800 (2015). [CrossRef] [PubMed]
35. S. Su, C. Zhang, L. Yuwen, J. Chao, X. Zuo, X. Liu, C. Song, C. Fan, and L. Wang, “Creating SERS hot spots on MoS2 nanosheets with in situ grown gold nanoparticles,” ACS Appl. Mater. Interfaces 6(21), 18735–18741 (2014). [CrossRef] [PubMed]
36. J.-H. Lee, M.-H. You, G.-H. Kim, and J.-M. Nam, “Plasmonic nanosnowmen with a conductive junction as highly tunable nanoantenna structures and sensitive, quantitative and multiplexable surface-enhanced Raman scattering probes,” Nano Lett. 14(11), 6217–6225 (2014). [CrossRef] [PubMed]
37. C. Li, A. Liu, C. Zhang, M. Wang, Z. Li, S. Xu, S. Jiang, J. Yu, C. Yang, and B. Man, “Ag gyrus-nanostructure supported on graphene/Au film with nanometer gap for ideal surface enhanced Raman scattering,” Opt. Express 25(17), 20631–20641 (2017). [CrossRef] [PubMed]
38. L. Hu, Y. J. Liu, Y. Han, P. Chen, C. Zhang, C. Li, Z. Lu, D. Luo, and S. Jiang, “Graphene oxide-decorated silver dendrites for high-performance surface-enhanced Raman scattering applications,” J. Mater. Chem. C Mater. Opt. Electron. Devices 5(16), 3908–3915 (2017). [CrossRef]
39. C. Li, C. Yang, S. Xu, C. Zhang, Z. Li, X. Liu, S. Jiang, Y. Huo, A. Liu, and B. 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]
40. M. J. Natan, “Surface enhanced Raman scattering,” Faraday Discuss. 132, 321–328 (2006). [CrossRef] [PubMed]
41. Y. Zheng, A. H. Soeriyadi, L. Rosa, S. H. Ng, U. Bach, and J. Justin Gooding, “Reversible gating of smart plasmonic molecular traps using thermoresponsive polymers for single-molecule detection,” Nat. Commun. 6(1), 8797 (2015). [CrossRef] [PubMed]
42. C. Marcott, G. M. Story, A. E. Dowrey, J. T. Grothaus, D. C. Oertel, I. Noda, E. Margalith, and L. Nguyen, “Mining the Information Content Buried in Infrared and Near-Infrared Band Shapes by Temporal, Spatial, and Other Perturbations,” Appl. Spectrosc. 63(12), 346A–354A (2009). [CrossRef] [PubMed]
43. N. E. Mircescu, M. Oltean, V. Chiş, and N. Leopold, “FTIR, FT-Raman, SERS and DFT study on melamine,” Vib. Spectrosc. 62, 165–171 (2012). [CrossRef]
