Preparation of Ag/Ag 2 W 2 O 7 nanoribbons and their SERS detection for dye molecules

1D nanomaterials including nanorods, nanowires, nanoribbons, etc. have attracted research interests [1], and quickly became research frontier in the nanoscience and nanotechnology due to their optical, electrical, and magnetic properties. Therefore many methods have been developed to fabricate 1D nanomaterials, such as chemical vapour deposition [2], template [3], and hydrothermal and solvothermal [4] methods etc. However, up to now, there is no literature to report synthesis of ultrathin 1D Ag2W2O7 NRs except Ag2W2O7 powder [5] and nanowires [6]. Non-metallic [7] and triclinic structure phases Ag2W2O7 belongs to the direct bandgap semiconductor material [8] with a band gap width 3.18 eV [9]. It is difficult to prepare Ag2W2O7 thin NRs because it is not stable. In this work, we prepared Ag2W2O7 NRs by hydrothermal method via strictly controlling pH value and temperature and then reduced them to Ag/ Ag2W2O7 NRs by NaBH4. MB and Rh B were used as target molecules for evaluating SERS performance. It revealed that the Ag/ Ag2W2O7 NRs possess high enhancement factor (EF) (2.78× 107, 4.34× 106) and low LOD (8.91 × 10−13 M, 8.71 × 10−12 M) for MB and Rh B. Compared with other substrates [10], our Ag/Ag2W2O7 NRs have supersensitivity to inorganic dyes. Furthermore, according to the literature investigation, there is no report on SERS performance of 1D Ag2W2O7 NRs. Therefore, it can be extend to detect food additives and carcinogens.


Synthesis of Ag 2 W 2 O 7 NRs
The synthesis of Ag 2 W 2 O 7 NRs can be divided into several steps: 200.8 mg dehydrated tungstate sodium (Na 2 WO 4 ⋅2H 2 O) and 101 mg silver nitrate (AgNO 3 ) were dissolved in 30 mL deionized water respectively, and then stirred by magnetic stirrers for 10 min. The completed AgNO 3 solution was dropwise added to the Na 2 WO 4 ⋅2H 2 O solution. Then, HNO 3 solution was dropped in the mixture to adjust its pH value to 2.8, and stir the suspension again for 2 h. Transfer the prepared reagent to a stainless steel autoclave lined with TEflon (100 mL) and keep it at 200 • C for 24 h. Take out and cool naturally to room temperature. After centrifugation, wash with alcohol and ionized water several times. Finally, Ag 2 W 2 O 7 NRs were obtained by vacuum drying at 60°C for 5 h.

Fabrication of Ag/Ag 2 W 2 O 7 NRs powder
Ag/Ag 2 W 2 O 7 NRs were prepared by a simple chemical reduction. 1 mg Ag 2 W 2 O 7 NRs were dispersed into 10 mL deionized water under sonic oscillation to make them homogeneous. Then, 50 mg NaBH 4 was dissolved in 10 mL of deionized water. Then, 20 μL of the obtained NaBH 4 solution were dissolved in the Ag 2 W 2 O 7 solution under magnetic stirring for 30 min to get uniform Ag/Ag 2 W 2 O 7 NRs. In the end, the obtained black precipitate was centrifuged and washed several times with deionized water, and dried in vacuum at 60 • C for 5 h.

Characterization
The morphology of the samples were observed by a scanning electron microscope (SEM), which was operated at an accelerating voltage of 20 kV. The microstructures of the samples were further characterized by transmission electron microscopy (TEM) and high-resolution electron transmission microscopy (HRTEM). The phase composition of the sample was examined by x-ray diffraction (XRD). UV-visible absorption spectroscopy (UV-vis) was recorded on a UV-vis spectrophotometer (U-4100, Hitachi, Japan) in the range of 200-800 nm. Raman

SERS examination
The SERS performance of the Ag 2 W 2 O 7 and Ag/Ag 2 W 2 O 7 NRs were investigated by selecting MB and Rh B as probe molecules. Before the SERS activity measurement, some single crystal silicon wafers (0.7 cm × 0.7 cm) were rinsed several times by acetone and alcohol, drying it at atmosphere. 20 μL of Ag 2 W 2 O 7 NRs (180 mg/L) solution were dropped onto the treated silicon wafer and dried naturally. Then, 10 μL of probe molecule solution were dropped on the prepared substrate. The SERS measurement was performed under a confocal Raman system with an excitation wavelength of 532 nm, a spot size of 0.785 μm 2 and a 50× objective lens. Raman grating with a 1200 L/mm, the acquisition time was 10 s and the accumulation number is three for every test.

XRD of the samples
In order to reveal the crystal structures of Ag 2 W 2 O 7 NRs and Ag/Ag 2 W 2 O 7 NRs, XRD patterns of the samples were denoted in Figure 1(a,b  The UV-vis spectrum was used to characterize the absorption property of Ag 2 W 2 O 7 NRs, as shown in Figure 3(a). It is observed that the wide absorption peaks of Ag 2 W 2 O 7 NRs and Ag/Ag 2 W 2 O 7 NRs were in the range of from 250 to 650 nm. The optical band gap of the Ag 2 W 2 O 7 and Ag/Ag 2 W 2 O 7 can be calculated according to the Kubelka and Munk method [11], which can be expressed by the following equation [12]: where n = 2, h is Planck's constant, hν stands for the energy of photons, α is the absorbance, and C is a constant. The energy bandgaps can be determined from the intercept of the tangent by plotting (αhν) 2 to hν. The energy bandgaps of the Ag 2 W 2 O 7 NRs and Ag/Ag 2 W 2 O 7 NRs were estimated as 3.11 and 2.94 eV respectively, as described in Figure 3(b). Ag/Ag 2 W 2 O 7 has a smaller band gap than Ag 2 W 2 O 7 , which conclude that the silver NPs cause the bandgap to shrink [13].

SERS measurements
To further explore the applications of Ag 2 W 2 O 7 and Ag/Ag 2 W 2 O 7 NRs in the sensing field, the as-prepared SERS substrates were used to detect MB molecule in ethanol solution. MB is a cationic dye, which is very sensitive to change in the polarity of its surrounding environment [14]. Cationic MB and anionic WO 4 2− interact with each other, and the high conductivity between MB-Ag can amplify the signal [15]. In addition, MB can reach a reversible balance between oxidation and  reduction, so it is widely used as an electron transfer medium [16]. Therefore, in most similar sensors, MB has been studied in most of the similar sensors. Figure 4 and Ag/Ag 2 W 2 O 7 NRs are presented in Figure 4(b,c), respectively. Some intrinsic Raman peaks appeared at 772, 862, 952, 1041, 1075, 1181, 1300, 1398, and 1625 cm −1 and their intensities are almost as strong as those of solid MB [17]. It is pointed out that the minimum detected concentration of MB on the Ag 2 W 2 O 7 NRs is only 1.0 × 10 −6 M, whereas its counterpart is 1.0 × 10 −12 M on the Ag/Ag 2 W 2 O 7 NRs. The SERS intensity of the Ag/Ag 2 W 2 O 7 NRs at 1625 cm −1 is 28.8 times  Figure 4(c). It is obvious that the characteristic Raman bands of MB are too weak at MB concentration of 1.0 × 10 −12 M, because its concentration is so low [17] that they are buried by the Raman scattering arisen from the Ag/Ag 2 W 2 O 7 NRs. But the vibration frequency of MB at 1625 cm −1 is still observable.
The detection of limit is a significant important parameter for evaluating the overall performance of SERS substrates. We choose the peak at the 1625 cm −1 to assess the LOD. Figure 4(d) shows the calibration curve of average peak intensities at the 1625 cm −1 against the logarithmic concentration of MB for Ag/Ag 2 W 2 O 7 NRs with a correlation coefficient (R 2 ) of 0.993 and I = 1517.15 + 106.44log|C|. According to the definition of LOD, the detection limit is calculated to be about 8.91 × 10 −13 M. Meanwhile, the LOD on the Ag 2 W 2 O 7 NRs is 8.4 × 10 −7 M, which is six orders larger than that on the Ag/Ag 2 W 2 O 7 NRs.
The MB molecule contains 108 normal modes [18]. Here, we give a preliminary assignment for only a small part of the Raman frequency. The most intense peaks are the ring stretch (ν(C C)) at 1625 cm −1 and the symmetric C-H stretches at 1398 cm −l . The peak at 1041 cm −1 originates from C-H inplane bending mode [18]. The peaks at 1075 cm −1 is assigned to C-H stretching mode. The peaks at 1181 cm −1 is assigned to C-N stretching mode.
To extend the SERS application of the Ag/Ag 2 W 2 O 7 NRs, Raman spectra of Rh B on the Ag 2 W 2 O 7 and Ag/Ag 2 W 2 O 7 NRs substrates are also examined, as displayed in Figure 5(a,b). The peak at 1284 cm −1 is attributed to the C O C stretching mode, while the peaks at 1363, 1435, 1509, 1533 and 1650 cm −1 are assigned to the aromatic C C stretching mode [19]. Their corresponding minimum detected concentrations are 1.0 × 10 −6 and 1.0 × 10 −11 M. Similarly, there is a good linear relationship (I = 3595.68 + 312.54log|C|, R 2 = 0.92) in Figure 5(c). The SERS intensity of the vibration frequency at 1650 cm −1 for Ag 2 W 2 O 7 NRs, the corresponding LOD is 8.71 × 10 −12 M. And the LOD of pure Ag 2 W 2 O 7 NRs is 8.12 × 10 −7 M.
For practical application, the enhancement factor is another important parameter, its definition is as follows: where N SERS is the number of molecules covering the array within the laser spot area, and N NR the number of molecules excited in the reference sample. I SERS and I NR are the integrated intensities of the same band for the absorbed probe molecule in the SERS spectra and unabsorbed probe molecules in the SERS spectra of Ag 2 W 2 O 7 NRs, respectively [20]. The EF values of our samples are listed in Table 1. In order to compare them with  other substrate, we also list them in Table 1. It is seen that our experimental results are even higher than those obtained from other more complicated experiments. We further evaluate the SERS stability of MB at a concentration of 1.0 × 10 −6 M on the Ag/Ag 2 W 2 O 7 NRs substrate. 30 points are randomly selected on the substrate to capture the Raman spectra as shown in Figure 6(a), with the intensities of peaks 1300, 1398 and 1624 cm −1 are extracted for a more intuitive comparison in Figure 6(b-d)

MECHANISM
The generally accepted SERS enhancement mechanism includes electromagnetic mechanism (EM) and the chemical mechanism (CM) [26]. The former mainly involves the enhancement of the local electric field at the metal surface, which is the surface plasmon resonance (SPR). The latter originates from the polarizability due to chemical interactions between metals and molecules [27,28]. For the Ag/Ag 2 W 2 O 7 NRs, the SERS enhancement is obtained from two aspects, one of which is the localized surface plasmon resonance (LSPR) can form very large field enhancements at the junctions between adjacent Ag NPs. The other is the charge transfers between Ag NPs and Ag 2 W 2 O 7 NRs under the induction of incident light, resulting in a resonance-like phenomenon, which greatly increases the polarization rate of the adsorbed molecules, resulting in signal enhancement.

CONCLUSION
In summary, we prepared 1D NRs. Although the fluorescence method has high sensitivity, it has problems such as fluorescence quenching effect and scattered light interference [29]. Compared with the fluorescence method, the detection of SERS has a narrower spectral band, which can provide more useful information, which helps to reduce signal overlap. It shows that SERS has wide applicability [30]. This is the first observation of Ag 2 W 2 O 7 nanoribbon in SERS detection. Our Ag 2 W 2 O 7 NRs provide a potential application for the determination of MB and Rh B in food colorants, disease treatment and environmental pollution, etc.