Research on the Raman properties of NiFe/cicada wing composite SERS platform modified by silver nanoparticles
Introduction
Surface enhanced Raman scattering (SERS), as a spectral analysis technique, attracts more and more attention from researchers due to its high sensitivity, rapid response and convenience. When the analytes are placed on the SERS substrates, the intensity of its Raman signal will be greatly enhanced. This method is often used to detect trace amounts of analytes in medical, agricultural and environmental monitoring [[1], [2], [3], [4], [5], [6]]. The enhancement of Raman signal can be explained by electromagnetic enhancement (EM) and chemical enhancement (CM). Most contribution comes from EM, which is mainly caused by the localized surface plasmon resonance (LSPR) effect, i.e. collective electron oscillations in metallic nanoparticles [7]. Therefore, the highly concentrated electric field is crucial [8]. The applications of composite structures are diverse. It not only has excellent catalytic performance [9], but also received close attention from researchers in SERS. Black phosphorus-dye composite films with controllable thickness prepared by Wang and her team exhibited good SERS performance [10]. Han et al. successfully fabricated Fe3O4@Au core-shell nanocomposite SERS substrate by seeds growth method. The substrate was recyclable and the recovery rate of apple peel surface was 99.3% [11]. Similarly, Fe3O4@Au composite nanospheres were synthesized as SERS substrate by Li et al. for detecting malachite green residues in water bodies [12]. A kind of Ag@ZrO2@Ag composite substrate prepared by Zhou et al. exhibited a remarkable performance for the SERS trace detection of Cr (VI) in the environment [13]. Composite SERS substrates are often formed by the combination of different materials. At present, the application of magnetic materials has attracted the attention of researchers. Colloidal magnetic materials have been gradually used in the preparation of SERS substrates [14,15]. However, gaseous and liquid environments were more complex than solid substrates because of their fluidity [16] and colloidal particles prepared by wet chemical method were prone to aggregation when used as SERS substrate, resulting in poor reproducibility of the substrate [17]. The practicability of SERS was commonly questioned by non-SERS group due to its poor reproducibility [18]. Magnetron sputtering technology has the advantages of high speed, low temperature, small damage and easy mass production, which makes the coating effortless to form a repeatable and stable substrate [19,20].
In addition, the size and shape control of NPs also plays an important role in the preparation of SERS substrates [7]. It is necessary to prepare a uniform, convenient and rapid substrate. The SERS substrate with 3D micro/nano structure could effectively increase the “hot spot” density per unit volume, and the coupling effect between micro/nano sized elements in the array structure could also improve the SERS enhanced performance, so this kind of structure system has been explored by many researchers. Li and his group fabricated a 3D SERS substrate with different numbers of Ag NPs layers, which exhibited outstanding detection ability for ultra-low concentration of R6G and CV [21]. Botta et al. fabricated a laser engraved 3D SERS chip decorated with gold NPs for paraquat detection. After calculation, the enhancement factor was up to 5 × 109 [22]. Pal and his team obtained Ag/ZnO/Au 3D hybrid structure substrates by thermal evaporation, hydrothermal growth and sputtering. They confirmed that 3D substrate has higher SERS activity than 2D substrate by experiments [23]. The 3D SERS substrates effectively improved the EF and had relatively high sensitivity [24,25]. However, complex fabrication processes are greatly limited the practical application for high-performance and reliable SERS substrates [26]. In nature, butterfly wings, lotus leaves, rose petals and other biomaterials have 3D periodic microstructures, and they have superhydrophobic properties, which are considered as excellent SERS substrates’ templates. In the previous work, we have prepared SERS substrates with biomaterials as templates, and obtained good enhancement of Raman signal. For instance, coating moth wing with Ag nanoislands [27], sandwich-like Ag@Cu@CW SERS substrate with tunable nanogaps [28] and Ag-coated glasswing butterfly as SERS substrate [29]. The experimental results showed that the structure of these biomaterials had been well applied in SERS.
In this paper, cicada wings were chosen as templates because they are superhydrophobic, anti-reflective and flexible with regular 3D nanoarrays. NiFe alloy nanoparticles were deposited on CWs by magnetron sputtering. NiFe alloy nanoparticles have remarkable anti-air oxidation properties and exhibit low saturation magnetization and coercivity values, which are in line with superparamagnetic characteristic [30]. Then the Ag NPs were decorated on them and the composite SERS substrates we needed were successfully prepared. The substrate we fabricated showed good enhancement effect on Raman signal, and the enhancement factor (EF) was 2.33 × 106. Meanwhile the limit of detection (LOD) of R6G was 10−8 M, which suggested the substrates were sensitive. Furthermore, the substrates also showed superior reproducibility, with relative standard deviations of less than 7%. This kind of composite substrates has enormous potential in sensitive detection.
Section snippets
Materials and instruments
The NiFe alloy sputtering target was purchased from China Material Technology Co., Ltd. The cicada wings were obtained from Beijing Jiaying Grand Life Sciences Co., Ltd. Silver nitrate (AgNO3), sodium citrate, Rhodamine 6G (R6G), CV, acetone and ethanol were all of analytical grade. They were bought from Beijing Chemical Work. Other reagents used in the experiment, unless otherwise mentioned, were also of analytical grade. Deionized water was used for all aqueous solution.
NiFe alloy was
Characterization of materials
The morphology characterization of the fabricated substrates was represented by field emission scanning electron microscopy (FE-SEM, JEOL JSM-2100). As show in Fig. 2(a)-(f), the images exhibit the surface of the CW and the fabricated substrates. Fig. 2(a) shows the sloping view of the CW which comprises numerous evenly distributed protruding nanostructure and these nanostructures have formed regular nanoarrays. The base diameter of the nanostructure was 100 nm and the top diameter was 50 nm.
Conclusion
In summary, the NiFe/CW SERS substrate was prepared by deposition of NiFe alloy layer on regular CW nanoarray by means of magnetron sputtering. The experimental results show that when R6G is used as probe molecule, its Raman signal can be improved. Ag NPs were further decorated on the previous substrate and the Ag/NiFe/CW-30 substrate was selected as optimum substrate, which performed more remarkable enhancement for Raman spectra. The enhancement multiples of the characteristic peaks of R6G
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51771162, No. 21872119& No. 22072127) and Science and Technology Project of Hebei Education Department (ZD2019069).
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