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Facile synthesis of Fe3O4@Au core–shell nanocomposite as a recyclable magnetic surface enhanced Raman scattering substrate for thiram detection To cite this article: Donglai Han et al 2019 Nanotechnology 30 465703
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Nanotechnology Nanotechnology 30 (2019) 465703 (11pp)
https://doi.org/10.1088/1361-6528/ab3a84
Facile synthesis of Fe3O4@Au core–shell nanocomposite as a recyclable magnetic surface enhanced Raman scattering substrate for thiram detection Donglai Han1,2, Boxun Li1,2, Yue Chen3,4, Tong Wu3,4, Yichuan Kou3,4, Xiaojing Xue3,4, Lei Chen3,4, Yang Liu3,4,5 and Qian Duan1,2,5 1
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, People’s Republic of China 2 Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun 130022, People’s Republic of China 3 College of Physics, Jilin Normal University, Siping 136000, People’s Republic of China 4 Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, People’s Republic of China E-mail: [email protected] and [email protected] Received 14 May 2019, revised 20 July 2019 Accepted for publication 12 August 2019 Published 2 September 2019 Abstract
The Fe3O4@Au core–shell nanocomposites, as the multifunctional magnetic surface enhanced Raman scattering (SERS) substrates, were fabricated successfully by the seeds growth method based on the Fe3O4–Au core-satellite nanocomposites. The SERS properties of the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites were compared using 4-aminothiophenol (4-ATP) as the probe molecule. It was found that Fe3O4@Au core–shell nanocomposites showed better SERS performance than Fe3O4–Au core-satellite nanocomposites. The Au shell provided an effectively large surface area for forming sufficient plasmonic hot spots and capturing target molecules. The integration of magnetic core and plasmonic Au nanocrystals endowed the Fe3O4@Au core–shell nanocomposites with highly efficient magnetic separation and enrichment ability and abundant interparticle hot spots. The Fe3O4@Au core–shell nanocomposites could be easily recycled because of the intrinsic magnetism of the Fe3O4 cores and had good reproducibility of the SERS signals. For practical application, the Fe3O4@Au core–shell nanocomposites were also used to detect thiram. There was a good linear relationship between the SERS signal intensity and the concentration of thiram from 1×10−3 to 1×10−8 M and the limit of detection was 7.69×10−9 M. Moreover, residual thiram on apple peel was extracted and detected with a recovery rate range of 99.3%. The resulting substrate with high SERS activity, stability and strong magnetic responsivity makes the Fe3O4@Au core–shell nanocomposites a perfect choice for practical SERS detection applications. Supplementary material for this article is available online Keywords: Fe3O4@Au core–shell nanocomposites, SERS substrate, reproducibility, thiram, apple peel (Some figures may appear in colour only in the online journal)
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1. Introduction Pesticides play crucial roles in the modern agriculture, not just because they can protect plants from weeds, fungi, injurious insects and other organisms, but most importantly, because they can regulate the plant growth and improve the crop yields and qualities [1, 2]. However, pesticides overdose, which is always inevitable in pesticides usage, will cause a series of adverse effects, such as pesticide residues and environmental pollution, hence posing a huge threat to living health [3–5]. For example, a widely used fungicide, thiram—an oxidized dimer of dimethyl dithiocarbamate—is toxic by ingestion and deadly toxic by inhalation [6] despite its capability in preventing fungal diseases in seed and crops [7–9]. Even a trace of thiram could lead to headache, dizziness, tiredness, diarrhea and nausea [10]. Therefore, establishing a convenient and accurate detection method for thiram is urgently needed but still a great challenge. So far, several approaches including electrochemistry method, resonance Rayleigh scattering method and fluorescence analysis method have been developed for the trace detection of thiram [11–13]. But none of them meets the timestringency and detection limit request, and hence hindered their practical applications. The enhancement of Raman scattering with the analyte molecules adsorbing on rough metal surfaces made surface-enhanced Raman scattering (SERS) technique a convenient way to detect single molecules [14–16], and further a powerful testing method for the detection of thiram due to its high sensitivity, operation convenience and non-destructive data acquisition And due to the unique localized surface plasmon resonance (LSPR) characteristics and the local field enhancement effect, the noble metal nanocrystals (mainly Au and Ag) have attracted enormous attention for the SERS detection of trace thiram [17–19]. For instance, the silver-coated gold nanoparticles (Au@Ag NPs) were synthesized by seed growth method and were applied for the detection of thiram residues at various fruit peels by Han et al [20]. Saute et al reported the use of the dog-bone shaped gold NPs as SERS substrates based identification of ultra-low levels of thiram [21]. Guo et al prepared Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) with different Ag shell thinness, and those NPs were used as SERS substrates for thiram detection [22]. Although these SERS substrates can greatly help with thiram detection, little attention has been devoted to the recyclability of the SERS substrates, which could be realized by the magnetic separation technology. Among all the magnetic nanomaterials, the ferrimagnetic magnetite Fe3O4 NPs are most welcomed due to their high saturation magnetization, simple synthesis, good chemical stability and biocompatibility [23–25]. The cubelike Fe3O4@SiO2@Ag nanocomposites synthesized by a layer-by-layer procedure have recently been used as the SERS substrates, but the detection limit is only 1×10−6 M [26]. The magnetic properties of the Fe3O4 NPs enabled a high concentration of nanocomposites into a small area, which can effectively create a high density of interparticle hot spots by magnetism-induced aggregation. Thus, the magnetic-embedded
Scheme 1. Scheme of the synthesis protocols and the recyclable SERS detection process for 4-ATP of Fe3O4@Au core–shell nanocomposites.
SERS-active materials can significantly improve SERS detection sensitivity. In this work, we reported an efficient and reproducible preparation method of Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. We applied Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites as the SERS substrates, compared their SERS activities with 4-aminothiophenol (4-ATP) as the probe molecule, and then studied the enhancement mechanism of SERS. Scheme 1 presented the synthesis protocols and the recyclable SERS detection process for 4-ATP of Fe3O4@Au core–shell nanocomposites. For the practical application, Fe3O4@Au core–shell nanocomposites are also used to detect thiram on apple peel, and the experimental results show that Fe3O4@Au core–shell nanocomposites exhibit excellent sensitivity for the SERS detection of thiram.
2. Experimental section 2.1. Materials
The source materials included iron (III) acetylacetonate (Fe(acac)3), benzyl ether (C14H14O), oleic acid (C18H34O2), polyethyleneimine (PEI, branched, MW≈& 25 000 g mol−1), potassium hydroxide (KOH), carbon disulfide (CS2), gold(III) chloride hydrate (HAuCl4·3H2O), sodium citrate dihydrate 2
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(Na3C6H5O7·2H2O), sodium borohydride (NaBH4), potassium carbonate (K2CO3), hydroxylamine hydrochloride (NH2OH·HCl), 4-aminothiophenol (C6H7NS), and thiram (C6H12N2S4). All the aforementioned chemical reagents were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used without further purification.
2.6. Synthesis of Fe3O4@Au core–shell nanocomposites
The growth solution was prepared by adding 166 mg K2CO3 and 1.182 ml HAuCl4·4H2O to 100 ml deionized water [28]. After that, we thoroughly mixed 80 ml growth solution and 20 ml Fe3O4–Au core-satellite nanocomposites, and then added 100 ml of 0.04 M NH2OH·HCl. The prepared Fe3O4@Au core–shell nanocomposites were washed several times with deionized water and stored in 20 ml deionized water.
2.2. Synthesis of Fe3O4 NPs
The magnetite Fe3O4 NPs were prepared according to the report by Liu et al [27]. Typically, 1.268 ml of C18H34O2 (4 mM) was added into a two-necked round-bottomed flask with mixture of 706 mg Fe(acac)3 and 15 ml C14H14O. Then, the mixed solution passed through argon at room temperature for 5 min to expel the air in the flask. The solution was heated to reflux at 300 °C for 2 h under argon. Later, the solution was cooled to room temperature naturally, the product was washed several times with the mixed solution of toluene and hexane (ratio 4:1, 30 ml). The black precipitate was separated and collected from the solution by a magnet.
2.7. SERS detection for 4-ATP and thiram
1 mg Fe3O4–Au core-satellite nanocomposites and 1 mg Fe3O4@Au core–shell nanocomposites were dispersed in 20 ml of 1×10−3 M 4-ATP solution, respectively. The SERS spectra were collected using a Renishaw Invia Raman microscope (New Mills, UK; 633 nm excitation wavelength, 50×objective, 0.1 mW laser power and 10 s exposure time. The SERS measurement of thiram was conducted in the same way. 1 mg Fe3O4@Au core–shell nanocomposites was added into the different amounts of the thiram (1×10−3–1×10−8 M), then the Raman signals of the thiram were measured by the Raman microscope. Besides, the suspension liquid of Fe3O4@Au core–shell nanocomposites was smeared on apple peel, then transferred from contaminated apple peel to specially cleaned glass slide with the aid of the external magnetic field and finally, Raman spectrum of the thiram was collected.
2.3. Synthesis of PEI-DTC
250 mg PEI was dissolved in 25 ml methanol. Then 325 mg KOH was added and stirred magnetically until fully dissolved. Right after that, 347.5 μl CS2 was added dropwise to the solution. Finally, the mixture was stirred for another 10 min and the yellowish PEI-DTC solution could be obtained.
2.8. Characterizations
2.4. Synthesis of gold colloids
The x-ray diffraction patterns (XRD) of the samples were measured on a D/max-2500 copper rotating-anode x-ray diffractometer (Rigaku Corporation, Tokyo, Japan; Cu Kα radiation of wavelength λ=1.5406 Å (40 kV, 200 mA)). The morphologies of the samples were characterized by a field emission scanning electron microscope (FESEM, JEOL 7800F, Tokyo, Japan). The elemental composition was estimated by the energydispersive x-ray spectroscopy (EDX) (JEOL Ltd, Tokyo, Japan). Transmission electron micrographs (TEM) images were taken on a FEI Tenai G2 F20 electron microscope (JEOL Ltd, Tokyo, Japan) equipped with an x-ray energy dispersive spectrometer (EDS) (JEOL Ltd, Tokyo, Japan). Chemical components and the binding energies of Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites were analyzed by x-ray photoelectron spectroscopy (XPS) (Thermo Scientific ESCALAB 250Xi A1440 system, Thermo Fisher Scientific, Waltham, USA). The ultraviolet-visible (UV–vis) spectra of the samples were measured by UV–vis spectrophotometer (UV5800PC, Shanghai Metash Instruments Co., Ltd, Tokyo, Japan). SERS spectra were collected using a Renishaw Invia Raman microscope (Renishaw, London, UK). Magnetism was analyzed by a Quantum Design MPMS3 superconducting quantum interference device (SQUID) magnetometer (MicroSense, Lowell, USA).
3 ml of 0.025 M HAuCl4 solution was added to 300 ml deionized water and stirred magnetically for 5 min 9 ml of 0.034 M sodium citrate solution was added and stirred for 2 min. Then 3 ml of 0.02 M NaBH4 solution was added and stirred vigorously. Finally, the mixed solution was continuously stirred in the dark for 12 h to obtain gold colloids. 2.5. Synthesis of Fe3O4–Au core-satellite nanocomposites
The synthesis consists of two steps: the functionalization of Fe3O4 NPs surfaces and the deposition of Au nanocrystals on the Fe3O4 NPs surfaces. 10 mg Fe3O4 NPs was dispersed in 20 ml methanol, and then the solution was added dropwise to the prepared PEI-DTC solution under vortex mixing conditions. The mixture was stored at room temperature for 1 h, then the Fe3O4@PEI-DTC NPs were collected through a magnet and washed three times with deionized water, they were re-dispersed in 4 ml of deionized water. The prepared 80 ml gold colloids was then added dropwise to 4 ml of the Fe3O4@PEI-DTC NPs mixture under vortex mixing, followed by 1 h sonication. The resulting nanocomposites were washed several times with deionized water to remove the residual Au NPs. Finally, the Fe3O4–Au core-satellite nanocomposites were dispersed in 20 ml deionized water. 3
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provide a large number of nucleation sites for the subsequent growth of the Au shells. A shown in figure 2(b), the Au seeds appear blacker than Fe3O4 NPs, owing to Au’s higher electron density than Fe3O4 [31]. With the continuous reduction of Au3+ ions in the growth solution, Au nanocrystals grow randomly around the nucleation sites. Ultimately, the dense Au shells are formed on the surfaces of the Fe3O4 NPs, as shown in figure S1(c) and figure 2(c). Due to the formation of Au shells, the average particle size of the Fe3O4@Au core– shell nanocomposites increases to 82.4 nm. Figure S2(a) and S2b exhibit the high-resolution TEM (HRTEM) image of Fe3O4–Au core-satellite and Fe3O4@Au core–shell nanocomposites, respectively. It can be clearly observed that the interplanar spacings are 0.25 and 0.20 nm, which match the (311) plane of Fe3O4 and the (200) plane of Au, respectively. It can be seen that after the dense Au shell is coated on the Fe3O4-core, the Fe3O4@Au core–shell nanocomposites still maintain the microscopic morphology of the truncated hexahedron. As shown in figures S1(b) and S1(c), the presence of Fe, O and Au in EDX spectra confirms that the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites are composed of Fe3O4 and Au. Compared with Fe3O4–Au core-satellite nanocomposites, the Au peaks of the Fe3O4@Au core–shell nanocomposites increase remarkably and the Fe peaks decrease, indirectly suggesting the growth of Au seeds on the Fe3O4 NPs surfaces. In addition, EDS-mappings are used to investigate the elemental distributions of the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. As shown in figures 2(b) and (c), EDS elemental mapping images of Fe, Au and O correspond to the red line square drawn. It is clearly observed that the Au seeds are uniformly attached on the surfaces of Fe3O4 NPs. Moreover, compared with Fe3O4–Au core-satellite nanocomposites, the amount of Au seeds on the surfaces of the Fe3O4 NPs for the Fe3O4@Au core–shell nanocomposites increase significantly. XPS was employed to characterize the elemental chemical states of the Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. The C 1s peak located at 284.8 eV is considered to be a reference for charge correction. The XPS survey spectra of the three samples in figure 3(a) show the indexed peaks of C, Fe, O and Au, no impurities are observed within the detection limit. The high resolution XPS spectra of Fe 2p for Fe3O4 NPs in figure 3(b) show two contributions, Fe 2p3/2 and Fe 2p1/2 resulted from the spin–orbit splitting, located respectively at 712.1 and 726.1 eV, which can be assigned to magnetite Fe3O4 [32]. As shown in figure 3(c), the Au 4f7/2 and Au 4f5/2 peaks for the Fe3O4–Au core-satellite nanocomposites are observed at 83.2 and 86.9 eV with a spin–orbit splitting of 3.7 eV, which is in good agreement with the standard reference XPS spectrum of metallic Au [33]. An interesting finding is that the intensity of the Au 4f peak become stronger and the intensity of the Fe 2p peak become weaker with the formation of the Au shells, indicating an increase of Au amount on the surfaces of the Fe3O4 NPs because the intensity of the XPS spectrum is proportional to the atomic concentration. The Fe 2p peak shifts to the low
Figure 1. XRD patterns of the prepared Fe3O4 NPs, Fe3O4–Au coresatellite nanocomposites and Fe3O4@Au core–shell nanocomposites.
3. Results and discussion Figure 1 reveals the XRD patterns of the prepared Fe3O4 NPs, the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites. The diffraction peaks of Fe3O4 NPs located at about 30.1, 35.4, 43.1, 53.4, 56.9 and 62.5° are assigned to the (220), (311), (400), (422), (511) and (440) of the magnetite Fe3O4 (Joint Committee on Powder Diffraction Standards, JCPDS card No. 19-0629), respectively [29]. No hints of other impurities are found within the XRD detection limit. As for the Fe3O4–Au core-satellite nanocomposites, apart from the diffraction peaks of magnetite Fe3O4, the new diffraction peaks are located at 2θ=38.2, 44.4, 64.5 and 77.5°, which correspond to the crystal planes (111), (200), (220), (311) of Au (JCPDS card No. 04-0784), respectively [30]. From the results we observed, it can be preliminarily concluded that the Au seeds have adsorbed on the surfaces of Fe3O4 NPs. When Au colloids were further deposited onto the Fe3O4–Au core-satellite nanocomposites surfaces, the peaks intensity of Au increased distinctly. Meanwhile, the peaks intensity of Fe3O4 significantly decreased and almost vanished, which indicated that Fe3O4 NPs were fully covered with Au seeds and Au shells have formed on the Fe3O4 NPs surfaces for the Fe3O4@Au core– shell nanocomposites. The SEM and TEM images of the Fe3O4 NPs, the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites are shown in figure S1 (available online at stacks.iop.org/NANO/30/465703/mmedia) and figure 2. Figure S1(a) and figure 2(a) present that the Fe3O4 NPs are truncated as hexahedron with an average diameter of about 69.2 nm. As shown in figure S1(b) and figure 2(b), well-dispersed Au seeds are attached to the surfaces of the Fe3O4 NPs, which further verifies the formation of Fe3O4–Au core-satellite nanocomposites. The deposited Au seeds 4
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Figure 2. TEM images of the Fe3O4 NPs (a), Fe3O4–Au core-satellite nanocomposites (b), Fe3O4@Au core–shell nanocomposites (c) and EDS elemental mapping images (Fe, Au and O).
Figure 3. XPS survey spectra (a), high resolution XPS scans of Fe 2p (b) and Au 4f (c) of the Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites.
resonance peak of Au nanocrystals [35]. Compared with the absorption peak of the gold colloids, that of Fe3O4–Au coresatellite nanocomposites red-shifts from 513 to 656 nm owing to the electrons transfer from metallic Au to Fe3O4 [36]. However, when the Au shells form, the absorption peak of Fe3O4@Au core–shell nanocomposites at 608 nm shows an abnormal blue-shift. A possible explanation is that once the Fe3O4 NPs are fully coated with Au shells, the dielectric effects between metallic Au and Fe3O4 may be suppressed [37]. In addition, theoretical calculations based on the discrete
binding energy side and the Au 4f peak shifts to the high binding energy side with the increase of Au seeds on the surfaces of the Fe3O4 NPs, which demonstrates the interaction between Fe3O4 and Au [34]. UV–vis absorption spectra of the Fe3O4 NPs, gold colloids, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites are presented in figure 4. Fe3O4 NPs have no significant absorption peak in the visible region while the gold colloids show an absorption peak at 513 nm, which is the characteristic surface plasmon 5
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Figure 6. SERS spectra of 4-ATP (1×10−3 M) adsorbed on the
Fe3O4@Au core–shell nanocomposites and Fe3O4–Au core-satellite nanocomposites. Figure 4. UV–vis spectra of the Fe3O4 NPs, Gold colloids, Fe3O4– Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites.
To evaluate the SERS performance of the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites, 4-ATP was chosen as the probe molecule. The SERS spectra of 4-ATP adsorbed on the Fe3O4–Au coresatellite nanocomposites and the Fe3O4@Au core–shell nanocomposites are compared in figure 6. The SERS spectra of 4-ATP were in good agreement with the previous studies [39]. The peak at 1073 cm−1 owes to the C–S stretching vibration. The bands located at 1140 and 1388 cm−1 are attributed to C–H formation vibration. The bands at 1435 and 1570 cm−1 are due to C–C stretching vibration. To better estimate the performance of the SERS substrates, the SERS enhancement factor (EF) is calculated as follows [40]: EF =
[ISERS / CSERS] , [IRaman / CRaman ]
(1 )
where CRaman and CSERS represent the molar concentration of 4-ATP probed by the regular Raman spectroscopy and SERS, respectively, and IRaman and ISERS are their corresponding peak intensity. The EF of Fe3O4@Au core–shell nanocomposites is evaluated to be 3.76×105, which is nearly fifteen times that of the Fe3O4–Au core-satellite nanocomposites (EF=2.56×104). (For details of the EF calculation see the supporting information.) The calculation results show that Fe3O4@Au core–shell nanocomposites have higher SERS efficiency. To further investigate the enhancement effect of the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites, a theoretical simulation was employed to visualize the electric field distribution using the finite-difference time domain (FDTD) method. The FDTD simulation results show that, in agreement with our SERS observations, the formation of the Aunanostructure from satellite to shell was accompanied by elevation of the localized electromagnetic field (figures S3(a) and S3(b)) due to effectively confined light in the Au cavities. If the wavelength of incident light matches the localized surface plasmon resonance of the metal nanostructure, a
Figure 5. M–H loops of Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. Inset is the photograph of Fe3O4@Au core–shell nanocomposites in deionized water before and after magnet separation.
dipole approximation (DDA) indicated that the increase of the Au shell thickness could lead to the blue-shift of the surface plasmon peak of Au [38]. Figure 5 shows the magnetic hysteresis (M–H) loops of pure Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites, respectively. The saturation magnetization (Ms) of Fe3O4 NPs value is 68 emu g−1. With the increase of the deposited Au seeds, the Ms value decreases significantly due to the diamagnetism of Au nanocrystals. However, although the coating of Au shell results in a decrease in the Ms value, the Fe3O4@Au core– shell nanocomposites still exhibit good magnetic response, as shown in the inset of figure 5. 6
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Figure 7. SERS spectra of 4-ATP adsorbed on the Fe3O4@Au core–shell nanocomposites for 50 cycles (a); SERS intensity distribution histograms around 1073 cm−1 (b), 1388 cm−1 (c), and 1570 cm−1 (d).
strong localized electromagnetic field will be excited in the vicinity of the nanogaps. So this well design Fe3O4@Au core–shell nanocomposites with the dense Au shell nanostructures significantly increases the SERS enhancement ability compared with the Fe3O4–Au core-satellite nanocomposites. The greatly improved performance observed on the Fe3O4@Au core–shell nanocomposites can be ascribed to the following factors. Firstly, based on the UV–vis spectra, the laser excitation wavelength of 633 nm is closer to the SPR absorption of the Fe3O4@Au core–shell nanocomposites than to that of the Fe3O4–Au core-satellite nanocomposites, which may lead to the enhancement of LSPR effect. Secondly, it is widely believed that the SERS enhancement depends on the numbers of the hot spots at the rough surfaces of the nanocomposites [41]. A large enhancement could occur when a SERS active molecule is positioned in the gap between two closely spaced metallic nanostructures [42]. Compared with the Fe3O4–Au core-satellite nanocomposites, the Fe3O4@Au core–shell nanocomposites have more hot spots formed between the two adjacent Au nanocrystals or between the Au nanocrystals and Fe3O4 NPs. Thirdly, EF of the Raman signals is also closely related to the particle diameter of SERS substrates [43]. The particle size of the Fe3O4@Au core–shell nanocomposites is bigger than that of the Fe3O4–Au coresatellite nanocomposites, which may be the other possible reason for the increase of EF. In addition, the magnetic-
embedded SERS-active materials have the enrichment ability, which is a major advantage compared with other SERS-active substrates [44]. The magnetic properties of the Fe3O4 cores can effectively create a high density of interparticle hot spots by magnetism-induced aggregation. The target analytes can be quickly enriched with the Fe3O4@Au core–shell nanocomposites from the solution by an external magnetic field. A main advantage of the Fe3O4@Au core–shell nanocomposites over traditional SERS substrates is their recyclability. The Fe3O4@Au core–shell nanocomposites can be easily separated and recycled from the medium with an external magnet. After washing with the deionized water and ethanol, the target molecules are removed and reusable SERS substrates can be obtained. Figure S4 shows Raman spectrum of Fe3O4@Au core–shell nanocomposites after cleaning by the deionized water and ethanol, which shows that the adsorbed target molecules (4-ATP) on the SERS substrates have been successfully washed off. As can be seen from figure 7(a), the Fe3O4@Au core–shell nanocomposites can be reused 50 times without obvious changes in the characteristic peaks of SERS spectra of 4-ATP. The relative standard deviation (RSD) of major peak is used to evaluate the reproducibility and stability of SERS signals [45]. As shown in figures 7(b)–(d), the RSD at 1073, 1388, and 1570 cm−1 characteristic bands of 4-ATP is about 10.83%, 10.97% and 10.90%, respectively. With RSD values less than 16%, we can conclude that the SERS 7
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Figure 8. SERS spectra of different concentration thiram absorbed on Fe3O4@Au core–shell nanocomposites (a); SERS intensities of thiram
at 930 cm−1 versus the logarithms of the thiram concentrations (b).
substrates have good spatial uniformity, which further demonstrate that the Fe3O4@Au core–shell nanocomposites as SERS substrates have good reproducibility and high detection sensitivity [46]. The Fe3O4@Au core–shell nanocomposites were further applied for the detection of trace thiram. Thiram molecules easily form the resonated radical structures by the cleavage of the S–S bond when interacting with the noble metal surfaces, inducing two dimethyl residues strongly adsorbed on the noble metal surfaces [47]. The adsorption of the thiram on the surfaces of the Fe3O4@Au core–shell nanocomposites facilitates identification and detection using SERS techniques. Figure 8(a) shows the SERS spectra of different concentrations of thiram adsorbed on the Fe3O4@Au core–shell nanocomposites. Six characteristic peaks can be observed, and the intensity of the SERS peaks decreases with the decrease of the thiram concentration [48, 49]. It can be seen that the SERS detection limit of thiram adsorbed on Fe3O4@Au core–shell nanocomposites is 10−8 M. Furthermore, this direct detection can also be exploited to obtain quantitative information about the concentration of thiram and the SERS intensity in the standard sample. Figure 8(b) reflects the relationship between the normalized SERS intensity at 930 cm−1 versus the logarithm of thiram concentration in the range from 1×10−3 M to 1×10−8 M. Quantitative analysis of thiram shows the linear relationship of I=999.343 log C+8108.886 with a squared correlation coefficient with R2=0.992. Based on the above linear equation, the theoretically calculated value for the limit of detection of thiram is 7.69×10−9 M. Comparison of several SERS substrates for the detection limit of the thiram is presented in figure S5, indicating that Fe3O4@Au core–shell nanocomposites exhibit better sensitivity to the trace thiram due to the synergy among component materials [46, 50, 51]. For the realistic application, the Fe3O4@Au core–shell nanocomposites were applied to detect the thiram on the apple peels, as shown in figure 9. In a typical experiment, 5× 10−5 M thiram solution was sprayed on apple peels and dried at room temperature (figure 9(a)). Then the diluted suspension
of the Fe3O4@Au core–shell nanocomposites was spread on contaminated apple peels by thiram (figure 9(b)). The thiram molecules would interact with the Au nanocrystals and locate at the surfaces of the Fe3O4@Au core–shell nanocomposites. After the complete evaporation at room temperature, the Fe3O4@Au core–shell nanocomposites were transferred from contaminated apple peels to specially cleaned glass slides with the aid of the external magnetic field (figure 9(c)). Therefore, in this work, after dried, after the Fe3O4@Au core– shell nanocomposites were assembled on the glass slides, Raman spectrum of the thiram was recorded by Raman spectrometer with 633 nm laser excitation (figure 9(d)). Figure 9(e) shows the Raman spectrum of the thiram on contaminated apple peels. The characteristic bands of the thiram are observed. The concentrations of thiram in the pretreated apple peels are calculated using the linear regression equation in figure 8(b). The recovery of the thiram onto the surfaces of the apple peels is 99.3% for the addition of 5×10−5 M of thiram, which implies that the Fe3O4@Au core–shell nanocomposites have great potential as SERS substrates to detect thiram on apple peels.
4. Conclusions In summary, the magnetic-embedded SERS-active materials were synthesized and used to detect thiram residues over the apple peel. With 4-ATP as the probe molecule, the SERS properties of the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites were studied. Fe3O4@Au core–shell nanocomposites proved to have better SERS activity and higher sensitivity than Fe3O4@Au coresatellite nanocomposites. The Fe3O4 core provided good magnetism and a dense Au shell provided sufficient plasmonic hot spots to capture target molecules. The highly efficient substrate achieved quantitative analysis of thiram solution with excellent signal reproducibility, good correlation coefficient and low limit of detection. Besides, the 8
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Figure 9. 5×10−5 M thiram solution was sprayed on apple peel (a); Fe3O4@Au core–shell nanocomposites suspension liquid was spread on apple peel (b); Fe3O4@Au core–shell nanocomposites were transferred from contaminated apple peel to glass slide with the aid of the external magnetic field (c); the spectrum of thiram was collected by Raman spectrometer with an excitation wavelength of 633 nm (d); SERS spectrum of thiram (e).
[3] Lin X H, Aik S X L, Angkasa J, Le Q, Chooi K S and Li S F Y 2018 Selective and sensitive sensors based on molecularly imprinted poly(vinylidene fluoride) for determination of pesticides and chemical threat agent simulants Sensors Actuators B 258 228–37 [4] Ao M, Zhu Y, He S, Li D, Li P, Li J and Cao Y 2013 Preparation and characterization of 1-naphthylacetic acidsilica conjugated nanospheres for enhancement of controlled-release performance Nanotechnology 24 035601 [5] Rawtani D, Khatri N, Tyagi S and Pandey G 2018 Nanotechnology-based recent approaches for sensing and remediation of pesticides J. Environ. Manag. 206 749–62 [6] Xiong Z, Lin M, Lin H and Huang M 2018 Facile synthesis of cellulose nanofiber nanocomposite as a SERS substrate for detection of thiram in juice Carbohyd. Polym. 189 79–86 [7] Li P, Chen W, Liu D, Huang H, Dan K, Hu X, Yu S, Chu P K and Yu X-F 2019 Template growth of Au/Ag nanocomposites on phosphorene for sensitive SERS detection of pesticides Nanotechnology 30 275604 [8] Chen M, Luo W, Liu Q, Hao N, Zhu Y, Liu M, Wang L, Yang H and Chen X 2018 Simultaneous in situ extraction and fabrication of surface-enhanced Raman scattering substrate for reliable detection of thiram residue Anal. Chem. 90 13647–54 [9] Geng F, Zhao H, Fu Q, Mi Y, Miao L, Li W, Dong Y, Wu M and Lei Y 2018 Gold nanochestnut arrays as ultrasensitive SERS substrate for detecting trace pesticide residue Nanotechnology 29 295502 [10] Zahoor M 2010 Removal of thiram from aqueous solutions J. Chin. Chem. Soc. 57 1361–6 [11] Nováková K, Navrátil T, Dytrtová J J and Chýlková J 2013 The use of copper solid amalgam electrodes for determination of the pesticide thiram J. Solid State Electrochem. 17 1517–28 [12] Parham H, Pourreza N and Marahel F 2015 Determination of thiram using gold nanoparticles and Resonance Rayleigh scattering method Talanta 141 143–9
substrate also displayed good recyclability and the recovery of the thiram onto the surfaces of the apple peels is 99.3%. Thus, we believe that the Fe3O4@Au core–shell nanocomposites as highly efficient SERS substrates would have potential applications in food safety monitoring.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Numbers 21676115, 51609100, 61575080, 61675090 and 61705020); and the Thirteenth FiveYear Program for Science and Technology of Education Department of Jilin Province (Grant Numbers JJKH20191018KJ and JJKH20191022KJ).
ORCID iDs Yang Liu
https://orcid.org/0000-0003-1485-8764
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Facile synthesis of Fe3O4@Au core–shell nanocomposite as a recyclable magnetic surface enhanced Raman scattering substrate for thiram detection To cite this article: Donglai Han et al 2019 Nanotechnology 30 465703
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Nanotechnology Nanotechnology 30 (2019) 465703 (11pp)
https://doi.org/10.1088/1361-6528/ab3a84
Facile synthesis of Fe3O4@Au core–shell nanocomposite as a recyclable magnetic surface enhanced Raman scattering substrate for thiram detection Donglai Han1,2, Boxun Li1,2, Yue Chen3,4, Tong Wu3,4, Yichuan Kou3,4, Xiaojing Xue3,4, Lei Chen3,4, Yang Liu3,4,5 and Qian Duan1,2,5 1
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, People’s Republic of China 2 Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun 130022, People’s Republic of China 3 College of Physics, Jilin Normal University, Siping 136000, People’s Republic of China 4 Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, People’s Republic of China E-mail: [email protected] and [email protected] Received 14 May 2019, revised 20 July 2019 Accepted for publication 12 August 2019 Published 2 September 2019 Abstract
The Fe3O4@Au core–shell nanocomposites, as the multifunctional magnetic surface enhanced Raman scattering (SERS) substrates, were fabricated successfully by the seeds growth method based on the Fe3O4–Au core-satellite nanocomposites. The SERS properties of the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites were compared using 4-aminothiophenol (4-ATP) as the probe molecule. It was found that Fe3O4@Au core–shell nanocomposites showed better SERS performance than Fe3O4–Au core-satellite nanocomposites. The Au shell provided an effectively large surface area for forming sufficient plasmonic hot spots and capturing target molecules. The integration of magnetic core and plasmonic Au nanocrystals endowed the Fe3O4@Au core–shell nanocomposites with highly efficient magnetic separation and enrichment ability and abundant interparticle hot spots. The Fe3O4@Au core–shell nanocomposites could be easily recycled because of the intrinsic magnetism of the Fe3O4 cores and had good reproducibility of the SERS signals. For practical application, the Fe3O4@Au core–shell nanocomposites were also used to detect thiram. There was a good linear relationship between the SERS signal intensity and the concentration of thiram from 1×10−3 to 1×10−8 M and the limit of detection was 7.69×10−9 M. Moreover, residual thiram on apple peel was extracted and detected with a recovery rate range of 99.3%. The resulting substrate with high SERS activity, stability and strong magnetic responsivity makes the Fe3O4@Au core–shell nanocomposites a perfect choice for practical SERS detection applications. Supplementary material for this article is available online Keywords: Fe3O4@Au core–shell nanocomposites, SERS substrate, reproducibility, thiram, apple peel (Some figures may appear in colour only in the online journal)
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1. Introduction Pesticides play crucial roles in the modern agriculture, not just because they can protect plants from weeds, fungi, injurious insects and other organisms, but most importantly, because they can regulate the plant growth and improve the crop yields and qualities [1, 2]. However, pesticides overdose, which is always inevitable in pesticides usage, will cause a series of adverse effects, such as pesticide residues and environmental pollution, hence posing a huge threat to living health [3–5]. For example, a widely used fungicide, thiram—an oxidized dimer of dimethyl dithiocarbamate—is toxic by ingestion and deadly toxic by inhalation [6] despite its capability in preventing fungal diseases in seed and crops [7–9]. Even a trace of thiram could lead to headache, dizziness, tiredness, diarrhea and nausea [10]. Therefore, establishing a convenient and accurate detection method for thiram is urgently needed but still a great challenge. So far, several approaches including electrochemistry method, resonance Rayleigh scattering method and fluorescence analysis method have been developed for the trace detection of thiram [11–13]. But none of them meets the timestringency and detection limit request, and hence hindered their practical applications. The enhancement of Raman scattering with the analyte molecules adsorbing on rough metal surfaces made surface-enhanced Raman scattering (SERS) technique a convenient way to detect single molecules [14–16], and further a powerful testing method for the detection of thiram due to its high sensitivity, operation convenience and non-destructive data acquisition And due to the unique localized surface plasmon resonance (LSPR) characteristics and the local field enhancement effect, the noble metal nanocrystals (mainly Au and Ag) have attracted enormous attention for the SERS detection of trace thiram [17–19]. For instance, the silver-coated gold nanoparticles (Au@Ag NPs) were synthesized by seed growth method and were applied for the detection of thiram residues at various fruit peels by Han et al [20]. Saute et al reported the use of the dog-bone shaped gold NPs as SERS substrates based identification of ultra-low levels of thiram [21]. Guo et al prepared Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) with different Ag shell thinness, and those NPs were used as SERS substrates for thiram detection [22]. Although these SERS substrates can greatly help with thiram detection, little attention has been devoted to the recyclability of the SERS substrates, which could be realized by the magnetic separation technology. Among all the magnetic nanomaterials, the ferrimagnetic magnetite Fe3O4 NPs are most welcomed due to their high saturation magnetization, simple synthesis, good chemical stability and biocompatibility [23–25]. The cubelike Fe3O4@SiO2@Ag nanocomposites synthesized by a layer-by-layer procedure have recently been used as the SERS substrates, but the detection limit is only 1×10−6 M [26]. The magnetic properties of the Fe3O4 NPs enabled a high concentration of nanocomposites into a small area, which can effectively create a high density of interparticle hot spots by magnetism-induced aggregation. Thus, the magnetic-embedded
Scheme 1. Scheme of the synthesis protocols and the recyclable SERS detection process for 4-ATP of Fe3O4@Au core–shell nanocomposites.
SERS-active materials can significantly improve SERS detection sensitivity. In this work, we reported an efficient and reproducible preparation method of Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. We applied Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites as the SERS substrates, compared their SERS activities with 4-aminothiophenol (4-ATP) as the probe molecule, and then studied the enhancement mechanism of SERS. Scheme 1 presented the synthesis protocols and the recyclable SERS detection process for 4-ATP of Fe3O4@Au core–shell nanocomposites. For the practical application, Fe3O4@Au core–shell nanocomposites are also used to detect thiram on apple peel, and the experimental results show that Fe3O4@Au core–shell nanocomposites exhibit excellent sensitivity for the SERS detection of thiram.
2. Experimental section 2.1. Materials
The source materials included iron (III) acetylacetonate (Fe(acac)3), benzyl ether (C14H14O), oleic acid (C18H34O2), polyethyleneimine (PEI, branched, MW≈& 25 000 g mol−1), potassium hydroxide (KOH), carbon disulfide (CS2), gold(III) chloride hydrate (HAuCl4·3H2O), sodium citrate dihydrate 2
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(Na3C6H5O7·2H2O), sodium borohydride (NaBH4), potassium carbonate (K2CO3), hydroxylamine hydrochloride (NH2OH·HCl), 4-aminothiophenol (C6H7NS), and thiram (C6H12N2S4). All the aforementioned chemical reagents were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used without further purification.
2.6. Synthesis of Fe3O4@Au core–shell nanocomposites
The growth solution was prepared by adding 166 mg K2CO3 and 1.182 ml HAuCl4·4H2O to 100 ml deionized water [28]. After that, we thoroughly mixed 80 ml growth solution and 20 ml Fe3O4–Au core-satellite nanocomposites, and then added 100 ml of 0.04 M NH2OH·HCl. The prepared Fe3O4@Au core–shell nanocomposites were washed several times with deionized water and stored in 20 ml deionized water.
2.2. Synthesis of Fe3O4 NPs
The magnetite Fe3O4 NPs were prepared according to the report by Liu et al [27]. Typically, 1.268 ml of C18H34O2 (4 mM) was added into a two-necked round-bottomed flask with mixture of 706 mg Fe(acac)3 and 15 ml C14H14O. Then, the mixed solution passed through argon at room temperature for 5 min to expel the air in the flask. The solution was heated to reflux at 300 °C for 2 h under argon. Later, the solution was cooled to room temperature naturally, the product was washed several times with the mixed solution of toluene and hexane (ratio 4:1, 30 ml). The black precipitate was separated and collected from the solution by a magnet.
2.7. SERS detection for 4-ATP and thiram
1 mg Fe3O4–Au core-satellite nanocomposites and 1 mg Fe3O4@Au core–shell nanocomposites were dispersed in 20 ml of 1×10−3 M 4-ATP solution, respectively. The SERS spectra were collected using a Renishaw Invia Raman microscope (New Mills, UK; 633 nm excitation wavelength, 50×objective, 0.1 mW laser power and 10 s exposure time. The SERS measurement of thiram was conducted in the same way. 1 mg Fe3O4@Au core–shell nanocomposites was added into the different amounts of the thiram (1×10−3–1×10−8 M), then the Raman signals of the thiram were measured by the Raman microscope. Besides, the suspension liquid of Fe3O4@Au core–shell nanocomposites was smeared on apple peel, then transferred from contaminated apple peel to specially cleaned glass slide with the aid of the external magnetic field and finally, Raman spectrum of the thiram was collected.
2.3. Synthesis of PEI-DTC
250 mg PEI was dissolved in 25 ml methanol. Then 325 mg KOH was added and stirred magnetically until fully dissolved. Right after that, 347.5 μl CS2 was added dropwise to the solution. Finally, the mixture was stirred for another 10 min and the yellowish PEI-DTC solution could be obtained.
2.8. Characterizations
2.4. Synthesis of gold colloids
The x-ray diffraction patterns (XRD) of the samples were measured on a D/max-2500 copper rotating-anode x-ray diffractometer (Rigaku Corporation, Tokyo, Japan; Cu Kα radiation of wavelength λ=1.5406 Å (40 kV, 200 mA)). The morphologies of the samples were characterized by a field emission scanning electron microscope (FESEM, JEOL 7800F, Tokyo, Japan). The elemental composition was estimated by the energydispersive x-ray spectroscopy (EDX) (JEOL Ltd, Tokyo, Japan). Transmission electron micrographs (TEM) images were taken on a FEI Tenai G2 F20 electron microscope (JEOL Ltd, Tokyo, Japan) equipped with an x-ray energy dispersive spectrometer (EDS) (JEOL Ltd, Tokyo, Japan). Chemical components and the binding energies of Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites were analyzed by x-ray photoelectron spectroscopy (XPS) (Thermo Scientific ESCALAB 250Xi A1440 system, Thermo Fisher Scientific, Waltham, USA). The ultraviolet-visible (UV–vis) spectra of the samples were measured by UV–vis spectrophotometer (UV5800PC, Shanghai Metash Instruments Co., Ltd, Tokyo, Japan). SERS spectra were collected using a Renishaw Invia Raman microscope (Renishaw, London, UK). Magnetism was analyzed by a Quantum Design MPMS3 superconducting quantum interference device (SQUID) magnetometer (MicroSense, Lowell, USA).
3 ml of 0.025 M HAuCl4 solution was added to 300 ml deionized water and stirred magnetically for 5 min 9 ml of 0.034 M sodium citrate solution was added and stirred for 2 min. Then 3 ml of 0.02 M NaBH4 solution was added and stirred vigorously. Finally, the mixed solution was continuously stirred in the dark for 12 h to obtain gold colloids. 2.5. Synthesis of Fe3O4–Au core-satellite nanocomposites
The synthesis consists of two steps: the functionalization of Fe3O4 NPs surfaces and the deposition of Au nanocrystals on the Fe3O4 NPs surfaces. 10 mg Fe3O4 NPs was dispersed in 20 ml methanol, and then the solution was added dropwise to the prepared PEI-DTC solution under vortex mixing conditions. The mixture was stored at room temperature for 1 h, then the Fe3O4@PEI-DTC NPs were collected through a magnet and washed three times with deionized water, they were re-dispersed in 4 ml of deionized water. The prepared 80 ml gold colloids was then added dropwise to 4 ml of the Fe3O4@PEI-DTC NPs mixture under vortex mixing, followed by 1 h sonication. The resulting nanocomposites were washed several times with deionized water to remove the residual Au NPs. Finally, the Fe3O4–Au core-satellite nanocomposites were dispersed in 20 ml deionized water. 3
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provide a large number of nucleation sites for the subsequent growth of the Au shells. A shown in figure 2(b), the Au seeds appear blacker than Fe3O4 NPs, owing to Au’s higher electron density than Fe3O4 [31]. With the continuous reduction of Au3+ ions in the growth solution, Au nanocrystals grow randomly around the nucleation sites. Ultimately, the dense Au shells are formed on the surfaces of the Fe3O4 NPs, as shown in figure S1(c) and figure 2(c). Due to the formation of Au shells, the average particle size of the Fe3O4@Au core– shell nanocomposites increases to 82.4 nm. Figure S2(a) and S2b exhibit the high-resolution TEM (HRTEM) image of Fe3O4–Au core-satellite and Fe3O4@Au core–shell nanocomposites, respectively. It can be clearly observed that the interplanar spacings are 0.25 and 0.20 nm, which match the (311) plane of Fe3O4 and the (200) plane of Au, respectively. It can be seen that after the dense Au shell is coated on the Fe3O4-core, the Fe3O4@Au core–shell nanocomposites still maintain the microscopic morphology of the truncated hexahedron. As shown in figures S1(b) and S1(c), the presence of Fe, O and Au in EDX spectra confirms that the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites are composed of Fe3O4 and Au. Compared with Fe3O4–Au core-satellite nanocomposites, the Au peaks of the Fe3O4@Au core–shell nanocomposites increase remarkably and the Fe peaks decrease, indirectly suggesting the growth of Au seeds on the Fe3O4 NPs surfaces. In addition, EDS-mappings are used to investigate the elemental distributions of the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. As shown in figures 2(b) and (c), EDS elemental mapping images of Fe, Au and O correspond to the red line square drawn. It is clearly observed that the Au seeds are uniformly attached on the surfaces of Fe3O4 NPs. Moreover, compared with Fe3O4–Au core-satellite nanocomposites, the amount of Au seeds on the surfaces of the Fe3O4 NPs for the Fe3O4@Au core–shell nanocomposites increase significantly. XPS was employed to characterize the elemental chemical states of the Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. The C 1s peak located at 284.8 eV is considered to be a reference for charge correction. The XPS survey spectra of the three samples in figure 3(a) show the indexed peaks of C, Fe, O and Au, no impurities are observed within the detection limit. The high resolution XPS spectra of Fe 2p for Fe3O4 NPs in figure 3(b) show two contributions, Fe 2p3/2 and Fe 2p1/2 resulted from the spin–orbit splitting, located respectively at 712.1 and 726.1 eV, which can be assigned to magnetite Fe3O4 [32]. As shown in figure 3(c), the Au 4f7/2 and Au 4f5/2 peaks for the Fe3O4–Au core-satellite nanocomposites are observed at 83.2 and 86.9 eV with a spin–orbit splitting of 3.7 eV, which is in good agreement with the standard reference XPS spectrum of metallic Au [33]. An interesting finding is that the intensity of the Au 4f peak become stronger and the intensity of the Fe 2p peak become weaker with the formation of the Au shells, indicating an increase of Au amount on the surfaces of the Fe3O4 NPs because the intensity of the XPS spectrum is proportional to the atomic concentration. The Fe 2p peak shifts to the low
Figure 1. XRD patterns of the prepared Fe3O4 NPs, Fe3O4–Au coresatellite nanocomposites and Fe3O4@Au core–shell nanocomposites.
3. Results and discussion Figure 1 reveals the XRD patterns of the prepared Fe3O4 NPs, the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites. The diffraction peaks of Fe3O4 NPs located at about 30.1, 35.4, 43.1, 53.4, 56.9 and 62.5° are assigned to the (220), (311), (400), (422), (511) and (440) of the magnetite Fe3O4 (Joint Committee on Powder Diffraction Standards, JCPDS card No. 19-0629), respectively [29]. No hints of other impurities are found within the XRD detection limit. As for the Fe3O4–Au core-satellite nanocomposites, apart from the diffraction peaks of magnetite Fe3O4, the new diffraction peaks are located at 2θ=38.2, 44.4, 64.5 and 77.5°, which correspond to the crystal planes (111), (200), (220), (311) of Au (JCPDS card No. 04-0784), respectively [30]. From the results we observed, it can be preliminarily concluded that the Au seeds have adsorbed on the surfaces of Fe3O4 NPs. When Au colloids were further deposited onto the Fe3O4–Au core-satellite nanocomposites surfaces, the peaks intensity of Au increased distinctly. Meanwhile, the peaks intensity of Fe3O4 significantly decreased and almost vanished, which indicated that Fe3O4 NPs were fully covered with Au seeds and Au shells have formed on the Fe3O4 NPs surfaces for the Fe3O4@Au core– shell nanocomposites. The SEM and TEM images of the Fe3O4 NPs, the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites are shown in figure S1 (available online at stacks.iop.org/NANO/30/465703/mmedia) and figure 2. Figure S1(a) and figure 2(a) present that the Fe3O4 NPs are truncated as hexahedron with an average diameter of about 69.2 nm. As shown in figure S1(b) and figure 2(b), well-dispersed Au seeds are attached to the surfaces of the Fe3O4 NPs, which further verifies the formation of Fe3O4–Au core-satellite nanocomposites. The deposited Au seeds 4
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Figure 2. TEM images of the Fe3O4 NPs (a), Fe3O4–Au core-satellite nanocomposites (b), Fe3O4@Au core–shell nanocomposites (c) and EDS elemental mapping images (Fe, Au and O).
Figure 3. XPS survey spectra (a), high resolution XPS scans of Fe 2p (b) and Au 4f (c) of the Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites.
resonance peak of Au nanocrystals [35]. Compared with the absorption peak of the gold colloids, that of Fe3O4–Au coresatellite nanocomposites red-shifts from 513 to 656 nm owing to the electrons transfer from metallic Au to Fe3O4 [36]. However, when the Au shells form, the absorption peak of Fe3O4@Au core–shell nanocomposites at 608 nm shows an abnormal blue-shift. A possible explanation is that once the Fe3O4 NPs are fully coated with Au shells, the dielectric effects between metallic Au and Fe3O4 may be suppressed [37]. In addition, theoretical calculations based on the discrete
binding energy side and the Au 4f peak shifts to the high binding energy side with the increase of Au seeds on the surfaces of the Fe3O4 NPs, which demonstrates the interaction between Fe3O4 and Au [34]. UV–vis absorption spectra of the Fe3O4 NPs, gold colloids, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites are presented in figure 4. Fe3O4 NPs have no significant absorption peak in the visible region while the gold colloids show an absorption peak at 513 nm, which is the characteristic surface plasmon 5
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Figure 6. SERS spectra of 4-ATP (1×10−3 M) adsorbed on the
Fe3O4@Au core–shell nanocomposites and Fe3O4–Au core-satellite nanocomposites. Figure 4. UV–vis spectra of the Fe3O4 NPs, Gold colloids, Fe3O4– Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites.
To evaluate the SERS performance of the Fe3O4–Au core-satellite nanocomposites and the Fe3O4@Au core–shell nanocomposites, 4-ATP was chosen as the probe molecule. The SERS spectra of 4-ATP adsorbed on the Fe3O4–Au coresatellite nanocomposites and the Fe3O4@Au core–shell nanocomposites are compared in figure 6. The SERS spectra of 4-ATP were in good agreement with the previous studies [39]. The peak at 1073 cm−1 owes to the C–S stretching vibration. The bands located at 1140 and 1388 cm−1 are attributed to C–H formation vibration. The bands at 1435 and 1570 cm−1 are due to C–C stretching vibration. To better estimate the performance of the SERS substrates, the SERS enhancement factor (EF) is calculated as follows [40]: EF =
[ISERS / CSERS] , [IRaman / CRaman ]
(1 )
where CRaman and CSERS represent the molar concentration of 4-ATP probed by the regular Raman spectroscopy and SERS, respectively, and IRaman and ISERS are their corresponding peak intensity. The EF of Fe3O4@Au core–shell nanocomposites is evaluated to be 3.76×105, which is nearly fifteen times that of the Fe3O4–Au core-satellite nanocomposites (EF=2.56×104). (For details of the EF calculation see the supporting information.) The calculation results show that Fe3O4@Au core–shell nanocomposites have higher SERS efficiency. To further investigate the enhancement effect of the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites, a theoretical simulation was employed to visualize the electric field distribution using the finite-difference time domain (FDTD) method. The FDTD simulation results show that, in agreement with our SERS observations, the formation of the Aunanostructure from satellite to shell was accompanied by elevation of the localized electromagnetic field (figures S3(a) and S3(b)) due to effectively confined light in the Au cavities. If the wavelength of incident light matches the localized surface plasmon resonance of the metal nanostructure, a
Figure 5. M–H loops of Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites. Inset is the photograph of Fe3O4@Au core–shell nanocomposites in deionized water before and after magnet separation.
dipole approximation (DDA) indicated that the increase of the Au shell thickness could lead to the blue-shift of the surface plasmon peak of Au [38]. Figure 5 shows the magnetic hysteresis (M–H) loops of pure Fe3O4 NPs, Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites, respectively. The saturation magnetization (Ms) of Fe3O4 NPs value is 68 emu g−1. With the increase of the deposited Au seeds, the Ms value decreases significantly due to the diamagnetism of Au nanocrystals. However, although the coating of Au shell results in a decrease in the Ms value, the Fe3O4@Au core– shell nanocomposites still exhibit good magnetic response, as shown in the inset of figure 5. 6
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Figure 7. SERS spectra of 4-ATP adsorbed on the Fe3O4@Au core–shell nanocomposites for 50 cycles (a); SERS intensity distribution histograms around 1073 cm−1 (b), 1388 cm−1 (c), and 1570 cm−1 (d).
strong localized electromagnetic field will be excited in the vicinity of the nanogaps. So this well design Fe3O4@Au core–shell nanocomposites with the dense Au shell nanostructures significantly increases the SERS enhancement ability compared with the Fe3O4–Au core-satellite nanocomposites. The greatly improved performance observed on the Fe3O4@Au core–shell nanocomposites can be ascribed to the following factors. Firstly, based on the UV–vis spectra, the laser excitation wavelength of 633 nm is closer to the SPR absorption of the Fe3O4@Au core–shell nanocomposites than to that of the Fe3O4–Au core-satellite nanocomposites, which may lead to the enhancement of LSPR effect. Secondly, it is widely believed that the SERS enhancement depends on the numbers of the hot spots at the rough surfaces of the nanocomposites [41]. A large enhancement could occur when a SERS active molecule is positioned in the gap between two closely spaced metallic nanostructures [42]. Compared with the Fe3O4–Au core-satellite nanocomposites, the Fe3O4@Au core–shell nanocomposites have more hot spots formed between the two adjacent Au nanocrystals or between the Au nanocrystals and Fe3O4 NPs. Thirdly, EF of the Raman signals is also closely related to the particle diameter of SERS substrates [43]. The particle size of the Fe3O4@Au core–shell nanocomposites is bigger than that of the Fe3O4–Au coresatellite nanocomposites, which may be the other possible reason for the increase of EF. In addition, the magnetic-
embedded SERS-active materials have the enrichment ability, which is a major advantage compared with other SERS-active substrates [44]. The magnetic properties of the Fe3O4 cores can effectively create a high density of interparticle hot spots by magnetism-induced aggregation. The target analytes can be quickly enriched with the Fe3O4@Au core–shell nanocomposites from the solution by an external magnetic field. A main advantage of the Fe3O4@Au core–shell nanocomposites over traditional SERS substrates is their recyclability. The Fe3O4@Au core–shell nanocomposites can be easily separated and recycled from the medium with an external magnet. After washing with the deionized water and ethanol, the target molecules are removed and reusable SERS substrates can be obtained. Figure S4 shows Raman spectrum of Fe3O4@Au core–shell nanocomposites after cleaning by the deionized water and ethanol, which shows that the adsorbed target molecules (4-ATP) on the SERS substrates have been successfully washed off. As can be seen from figure 7(a), the Fe3O4@Au core–shell nanocomposites can be reused 50 times without obvious changes in the characteristic peaks of SERS spectra of 4-ATP. The relative standard deviation (RSD) of major peak is used to evaluate the reproducibility and stability of SERS signals [45]. As shown in figures 7(b)–(d), the RSD at 1073, 1388, and 1570 cm−1 characteristic bands of 4-ATP is about 10.83%, 10.97% and 10.90%, respectively. With RSD values less than 16%, we can conclude that the SERS 7
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Figure 8. SERS spectra of different concentration thiram absorbed on Fe3O4@Au core–shell nanocomposites (a); SERS intensities of thiram
at 930 cm−1 versus the logarithms of the thiram concentrations (b).
substrates have good spatial uniformity, which further demonstrate that the Fe3O4@Au core–shell nanocomposites as SERS substrates have good reproducibility and high detection sensitivity [46]. The Fe3O4@Au core–shell nanocomposites were further applied for the detection of trace thiram. Thiram molecules easily form the resonated radical structures by the cleavage of the S–S bond when interacting with the noble metal surfaces, inducing two dimethyl residues strongly adsorbed on the noble metal surfaces [47]. The adsorption of the thiram on the surfaces of the Fe3O4@Au core–shell nanocomposites facilitates identification and detection using SERS techniques. Figure 8(a) shows the SERS spectra of different concentrations of thiram adsorbed on the Fe3O4@Au core–shell nanocomposites. Six characteristic peaks can be observed, and the intensity of the SERS peaks decreases with the decrease of the thiram concentration [48, 49]. It can be seen that the SERS detection limit of thiram adsorbed on Fe3O4@Au core–shell nanocomposites is 10−8 M. Furthermore, this direct detection can also be exploited to obtain quantitative information about the concentration of thiram and the SERS intensity in the standard sample. Figure 8(b) reflects the relationship between the normalized SERS intensity at 930 cm−1 versus the logarithm of thiram concentration in the range from 1×10−3 M to 1×10−8 M. Quantitative analysis of thiram shows the linear relationship of I=999.343 log C+8108.886 with a squared correlation coefficient with R2=0.992. Based on the above linear equation, the theoretically calculated value for the limit of detection of thiram is 7.69×10−9 M. Comparison of several SERS substrates for the detection limit of the thiram is presented in figure S5, indicating that Fe3O4@Au core–shell nanocomposites exhibit better sensitivity to the trace thiram due to the synergy among component materials [46, 50, 51]. For the realistic application, the Fe3O4@Au core–shell nanocomposites were applied to detect the thiram on the apple peels, as shown in figure 9. In a typical experiment, 5× 10−5 M thiram solution was sprayed on apple peels and dried at room temperature (figure 9(a)). Then the diluted suspension
of the Fe3O4@Au core–shell nanocomposites was spread on contaminated apple peels by thiram (figure 9(b)). The thiram molecules would interact with the Au nanocrystals and locate at the surfaces of the Fe3O4@Au core–shell nanocomposites. After the complete evaporation at room temperature, the Fe3O4@Au core–shell nanocomposites were transferred from contaminated apple peels to specially cleaned glass slides with the aid of the external magnetic field (figure 9(c)). Therefore, in this work, after dried, after the Fe3O4@Au core– shell nanocomposites were assembled on the glass slides, Raman spectrum of the thiram was recorded by Raman spectrometer with 633 nm laser excitation (figure 9(d)). Figure 9(e) shows the Raman spectrum of the thiram on contaminated apple peels. The characteristic bands of the thiram are observed. The concentrations of thiram in the pretreated apple peels are calculated using the linear regression equation in figure 8(b). The recovery of the thiram onto the surfaces of the apple peels is 99.3% for the addition of 5×10−5 M of thiram, which implies that the Fe3O4@Au core–shell nanocomposites have great potential as SERS substrates to detect thiram on apple peels.
4. Conclusions In summary, the magnetic-embedded SERS-active materials were synthesized and used to detect thiram residues over the apple peel. With 4-ATP as the probe molecule, the SERS properties of the Fe3O4–Au core-satellite nanocomposites and Fe3O4@Au core–shell nanocomposites were studied. Fe3O4@Au core–shell nanocomposites proved to have better SERS activity and higher sensitivity than Fe3O4@Au coresatellite nanocomposites. The Fe3O4 core provided good magnetism and a dense Au shell provided sufficient plasmonic hot spots to capture target molecules. The highly efficient substrate achieved quantitative analysis of thiram solution with excellent signal reproducibility, good correlation coefficient and low limit of detection. Besides, the 8
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Figure 9. 5×10−5 M thiram solution was sprayed on apple peel (a); Fe3O4@Au core–shell nanocomposites suspension liquid was spread on apple peel (b); Fe3O4@Au core–shell nanocomposites were transferred from contaminated apple peel to glass slide with the aid of the external magnetic field (c); the spectrum of thiram was collected by Raman spectrometer with an excitation wavelength of 633 nm (d); SERS spectrum of thiram (e).
[3] Lin X H, Aik S X L, Angkasa J, Le Q, Chooi K S and Li S F Y 2018 Selective and sensitive sensors based on molecularly imprinted poly(vinylidene fluoride) for determination of pesticides and chemical threat agent simulants Sensors Actuators B 258 228–37 [4] Ao M, Zhu Y, He S, Li D, Li P, Li J and Cao Y 2013 Preparation and characterization of 1-naphthylacetic acidsilica conjugated nanospheres for enhancement of controlled-release performance Nanotechnology 24 035601 [5] Rawtani D, Khatri N, Tyagi S and Pandey G 2018 Nanotechnology-based recent approaches for sensing and remediation of pesticides J. Environ. Manag. 206 749–62 [6] Xiong Z, Lin M, Lin H and Huang M 2018 Facile synthesis of cellulose nanofiber nanocomposite as a SERS substrate for detection of thiram in juice Carbohyd. Polym. 189 79–86 [7] Li P, Chen W, Liu D, Huang H, Dan K, Hu X, Yu S, Chu P K and Yu X-F 2019 Template growth of Au/Ag nanocomposites on phosphorene for sensitive SERS detection of pesticides Nanotechnology 30 275604 [8] Chen M, Luo W, Liu Q, Hao N, Zhu Y, Liu M, Wang L, Yang H and Chen X 2018 Simultaneous in situ extraction and fabrication of surface-enhanced Raman scattering substrate for reliable detection of thiram residue Anal. Chem. 90 13647–54 [9] Geng F, Zhao H, Fu Q, Mi Y, Miao L, Li W, Dong Y, Wu M and Lei Y 2018 Gold nanochestnut arrays as ultrasensitive SERS substrate for detecting trace pesticide residue Nanotechnology 29 295502 [10] Zahoor M 2010 Removal of thiram from aqueous solutions J. Chin. Chem. Soc. 57 1361–6 [11] Nováková K, Navrátil T, Dytrtová J J and Chýlková J 2013 The use of copper solid amalgam electrodes for determination of the pesticide thiram J. Solid State Electrochem. 17 1517–28 [12] Parham H, Pourreza N and Marahel F 2015 Determination of thiram using gold nanoparticles and Resonance Rayleigh scattering method Talanta 141 143–9
substrate also displayed good recyclability and the recovery of the thiram onto the surfaces of the apple peels is 99.3%. Thus, we believe that the Fe3O4@Au core–shell nanocomposites as highly efficient SERS substrates would have potential applications in food safety monitoring.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Numbers 21676115, 51609100, 61575080, 61675090 and 61705020); and the Thirteenth FiveYear Program for Science and Technology of Education Department of Jilin Province (Grant Numbers JJKH20191018KJ and JJKH20191022KJ).
ORCID iDs Yang Liu
https://orcid.org/0000-0003-1485-8764
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