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Construction of pure worm-like AuAg nanochains for ultrasensitive SERS detection of pesticide residues on apple surfaces
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 209 (2019) 241–247
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Construction of pure worm-like AuAg nanochains for ultrasensitive SERS detection of pesticide residues on apple surfaces Anxin Jiao, Xuejian Dong, Hua Zhang, Linlin Xu, Yue Tian, Xiangdong Liu, Ming Chen ⁎ School of Physics, State Key laboratory of Crystal Materials, Shandong University, Shandong, Jinan 250100, PR China
a r t i c l e
i n f o
Article history: Received 24 July 2018 Received in revised form 23 October 2018 Accepted 28 October 2018 Available online 29 October 2018 Keywords: Surface-enhanced Raman scattering Nanomaterials Laser materials processing
a b s t r a c t Ultrasensitive detection of pesticide residues on agricultural products using surface-enhanced Raman spectroscopy (SERS) is of significant interest in food security. Herein, worm-like AuAg nanochains with highly interconnected ultrafine (~6.2 nm) bimetallic particles were developed as an excellent SERS nanosensor via laser-assisted strategy. The SERS detection limit of thiram molecules on apple surfaces is about 10−7 M (0.03 ppm), which is about 200 times lower than the maximal residue limit (MRL, 7 ppm) in fruit prescribed by the U.S. Environmental Protection Agency (EPA). Importantly, the established excellent linear relationships between the SERS intensities and thiram concentrations can sensitively monitor the slight variation of pesticide residues in agriculture. © 2018 Elsevier B.V. All rights reserved.
1. Introduction At present, pesticide residues originated from overusing pesticides for protecting agricultural products from insects and diseases give rise to unprecedented toxic or lethal effects on human body. Recently, the application of surface enhanced Raman scattering spectroscopy (SERS) for simple, rapid, nondestructive and accurate pesticide detection has gained extensive attention in ultrasensitive monitoring of food (fruits, vegetables, crops, etc.) security [1–6]. Superior to traditional detection technologies such as enzyme-linked immunosorbent assay (ELISA) [7], gas chromatography mass spectroscopy (GC–MS) [8], high performance liquid chromatography (HPLC) [9], etc., the SERS analysis represents a double benefit: excellent “molecular fingerprint” information derived from specific vibrational energy of chemical bonds, and ultralow detection limit due to the enhanced optical properties of plasmonic metallic (Ag, Au) nanostructures. The localized surface plasmon resonance (LSPR) of Ag, Au-based nanomaterials can be effectively excited by laser irradiation during Raman scattering process, which will provide enhanced electromagnetic (EM) field and then significantly magnify the Raman spectral intensity of adsorbed molecules on nanosubstrates [10–13]. Most recently, an interesting work illustrated the fabrication of a versatile dual-functional SERS substrate by incubating polydimethysiloxane (PDMS) sponge in Au nanoparticles solution, which enables the lowest detected concentration of pesticide triazophos and methyl parathion in fruit juice to be achieved at 100 ppb and 1 ppm, respectively [2]. Moreover, it has been well recognized that the bimetallic or more complex Ag or Au-based nanocomposites will result in much higher SERS activity in comparison with ⁎ Corresponding author. E-mail address: [email protected] (M. Chen).
https://doi.org/10.1016/j.saa.2018.10.051 1386-1425/© 2018 Elsevier B.V. All rights reserved.
monometallic nanostructures, owing to the unique intermetallic synergies among different metals [13–15]. So, numerous Ag or Au-based nanomaterials with various structures such as silica nanocore/Ag nanoshell [3], Au/silver core-shell nanorods [5], Au@Ag nanocubes [6], etc. have been prepared as SERS-active nanosubstrates for sensitive detection of pesticides. While, there are two urgent issues should be considered in this area as follows. Firstly, most of previous works merely focused on the SERS detection limit of pesticide residues by optimizing the size, composition or shape of Au and Ag-based nanosubstrates. It is really true that the ultralow SERS detectable concentration of multiple pesticides on agricultural products is attracting more public attention and merit further consideration. However, very few reports have mainly focused on the linear relationships between SERS spectral lines and pesticide residual concentrations. The established linear relationships will provide important information for exact and quantitative analyzing the pesticide residual on agricultural products. Compared with ultralow SERS detection, the sensitive monitoring the pesticide variation by establishing the linear relationships between SERS signals and concentrations is more important for practical assessing the food security. However, the corresponding research has not been explored extensively up to now. Secondly, numerous Au or Ag-based nanocomposites have been fabricated for the SERS analysis of pesticide molecules by various growth methods with the aid of complex polymer stabilizers/capping agents/structure directing additives. For example, hexadecyltrimethylammonium bromide (CTAB) [4–6], polyvinylpyrrolidone (PVP) [3,16], polyethylenimine (PEI) [17], etc. have been widely used for sculpting anisotropic-nanostructures. Meanwhile, the organic contaminants adsorbed on nanosubstrates will lead to the formation of unprecedented SERS background noise or undesired spectral signals. So, these complex additives should be
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carefully removed from the SERS nanosubstrates by extra purification process before being processed toward functional applications. However, especially for hollow, porous or branched nanocomposites, the completely removal of these additives is rather difficult. Unfortunately, even with few residual additives, the corresponding signal interferences will be significantly enhanced during SERS analysis of ultralow pesticide molecules. It is an obstacle to establish well-defined relationships between SERS results and pesticide concentrations. In order to avoid the use of polymer additives, some novel SERS substrates such as Ag aligned Ag nanorods arrays generated by thermal evaporation of Ag powder [1], Si nanowire paper (SiNWP) performed in a home-built vertical high-frequency induction furnace (VHFIR) [18], polydimethylsiloxane (PDMS) sponge formed by heated in the vacuum oven for 2 h [2], etc. have been developed in recent years. However, such harsh and expensive experimental setups will also seriously hinder their potentials in practical applications with the consideration of economic benefits. To our knowledge, it remains a great challenge to develop a rapid, simple, reliable, economic and green strategy to convenient synthesis of clean and pure Au or Ag-based nanocomposites, which is particular favorable for establishing well-defined linear relationship between SERS signals and pesticide concentrations. Herein, we report on the successful fabrication of clean and pure worm-like AuAg nanochains by laser-assisted strategy. The asprepared Au nanotwins generated by laser ablation of Au plate in distilled water are characterized by abundant hydroxyl groups (–OH) formed on the surfaces. Taking advantage of these –OH groups, the moderate reduction of Ag ions and then overgrowth of Ag species enable the AuAg nanochains with highly interconnected ultrafine (~6.2 nm) bimetallic particles to be simply obtained by adding AgNO3 to the initial Au nanotwins solution. Firstly, as for SERS tests, the crystal violet (CV) molecules were selected as probe molecules. The CV molecules served as stable organic contaminants have been validated as a mitotic poisoning agent. The ultrasensitive SERS detection of CV molecules in liquid is very important for human health. The SERS intensities of CV molecules adsorbed on the bimetallic AuAg nanochains is about 4.5 times higher than that of monometallic Au nanotwins. So, the obtained AuAg nanochains provide enhanced SERS activity in this paper. Based on the obtained AuAg nanochains, the SERS analyses demonstrate that the dominating characteristic bands of pesticide thiram molecules on apple surface can be clearly distinguished even with the concentration decreased to as low as 10−7 M (0.03 ppm), which is about 200 times lower than the maximal residue limit (MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency (EPA). The ultrasensitive SERS detection limit of pesticide molecules in this paper already downs to the ultralow-level obtained in many previous works [1–6,16–19]. More interestingly, the corresponding SERS results illustrate several well-defined linear relationships between SERS peak intensities and thiram concentrations in a wide range of 10−3–10−7 M. Therefore, the obtained results in this paper provide a reliable strategy for precise identification and exact quantitative analysis of agricultural products, which can unambiguously possess high applicability in foodsupervise applications.
2 mm) in distilled water. In a typical experiment, the Au target was placed on the bottom of a rotating glass dish with a speed of ~500 rpm that was filled with 3 mm depth of distilled water. A Qswitched Nd-YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra Physics) beam operating at a wavelength of 1064 nm with pulse duration of 10 ns, 10 Hz repetition rate was focused on the Au target. The energy density of laser beam was about 280 mJ/pulse, and the average spot size on the Au target was about 1 mm in diameter. After 20 min laser ablation, the claret-red colored colloidal solution will be obtained. Then, under magnetic stirring condition, different amounts (0–165 μL, 0.05 M) of AgNO3 water solution were added to the Au nano-products solution. The mixed solution will be rapidly changed into red-violet color within ~2 min, implying the chemical reaction occurs in this experiment. The obtained products were centrifuged by 5000 rpm for 10 min in an ultracentrifuge. 2.3. Materials Characterization The morphological structures of the sediments were investigated by transmission electron microscopy (JEOL-JEM-2100F). The chemical compositions were analyzed by elemental mapping images via energy dispersive X-ray spectroscopy (EDS) using a JEOL-2100F electron microscope equipped with a STEM unit. The X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kαradiation (λ = 0.15406 nm) were used to illustrate the crystallographic structure of the products. The Fourier transforms infrared spectrums (FTIR) of the nanomaterials were measured by a UV–Vis-NIR spectrometer (370–7800 cm−1, ALPHA-T, Bruker). The absorption spectra were recorded by a UV–Vis-IR spectrometer (UV-1800, Shimadzu). 2.4. SERS Measurement In a typical Raman spectroscopic analysis of CV molecules, the AuAg nanochains-based SERS substrates were performed by dropping 0.5 M/10 μL sediments on clean silicon wafers. Then, the SERS nanosubstrates were totally immersed into certain concentration of CV molecules ethanol solution under lucifugal circumstance for 12 h and then dried spontaneously. The experimental section of SERS analysis of pesticide thiram molecules on apple surface was illustrated in Fig. 1, which is similar to that described by previous works [2,3,5,19]. Pesticide thiram powder was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Briefly, 0.024 g thiram powder was dissolved in 10 mL ethanol to form 0.01 M solution, which will be separately diluted with distilled water to prepare pesticide solutions with different concentrations of 10−3–10−7 M. The cleaned apple peels with a uniform squares size of ~2 cm were obtained by carefully peeling of a whole apple that was bought from a supermarket. Then, the 8 μL of the as-prepared thiram solution with different
2. Experimental 2.1. Chemicals All reagents were used as received without further purification and de-ionized water was used in all experiments. All the regents were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). 2.2. Fabrication of Worm-like AuAg Nanochains The initial Au nanotwins were firstly fabricated by pulsed laser ablation of pure (99.99%) Au plate (diameter of 3.5 cm and thickness of
Fig. 1. Schematic illustration of SERS detection of pesticide thiram molecules on apple surface based on AuAg nanochains.
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concentrations of 10−3–10−7 M were separately dropped on to a series of apple peel samples and dried at room temperature. Once complete evaporation, the obtained AuAg nanochains with the dose of 0.5 M/1 μL were separately dropped on the same spot of thiram-contaminated apple peels, and dried at room temperature again. Then the SERS analyses of thiram molecules on apple surfaces were carried out by removing background signals from original apple surface. The background signals were obtained from the uncontaminated apple surface treated with the same amount of ethanol and AuAg nanochains. In this way, the pure SERS signals of thiram molecules with different concentrations can be recorded in this paper. The Raman spectra were carried out by using a LabRAM HR 800 spectrograph with 633 nm laser beam at room temperature. The exciting laser power on all samples was located at about 1 mW, and the acquisition time used for each spectrum was 10s. 3. Results and Discussion After cumulative pulse laser ablation of Au metal target in distilled water, Au nanotwins were fabricated in water solution. The morphologies of Au nanostructures were illustrated by transmission electron microscopy (TEM), as shown in Fig. 2. The low-magnification TEM image in Fig. 2(a) exhibits a typical overall morphology of Au nanostructures, which consists of numerous twin-shaped nanoarchitectures. The enlarged TEM image of a representative nanotwins in Fig. 2 (b) provides a structure detail of the cross region between two nanoparticles. It is evident that the Au nanotwins are indeed interconnected and accreted with each other. The average size of nanoparticles is about 6 nm by measuring the diameters of N200 nanoparticles in sight on the TEM images. The interconnected lattice fringes at cross-region with a periodicity corresponding to a d-spacing of 0.236 nm could be indexed with reference to the Au (111) plane structure. Meanwhile, the HRTEM image in Fig. 2(c) reveals the crystal structures of Au nanotwins, which is mainly composed of three different Au orientations. For example, the region marked by red line in Fig. 2(c) with a dspacing of 0.236 nm is indexed as the (111) plane in Au structure, while the areas marked by green line and yellow line with a d-spacing of 0.204 and 0.123 nm can be separately attributed to the (200) and (311) structures. Moreover, in order to get more information of the
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surface composition, the FTIR spectrum of the obtained Au nanotwins is shown in Fig. 2(d), As illustrated in Fig. 2(d), in addition to the Au nanomaterial lattice vibration at ~582 cm−1, the FTIR pattern is dominated by the broad band with peak at about 3440 cm−1, which is attributed to the strongly adsorbing –OH stretching vibrations [20]. The obvious characteristic spectrum of –OH groups at about 3440 cm−1 confirms that abundant –OH groups can be formed on the Au nanotwins. The possible growth mechanism of Au nanotwins with numerous –OH groups formed on the surfaces will be described in the following section. At the moment of pulsed laser beam reaching at Au target in distilled water condition, the high laser energy will be significantly adsorbed via Au target and surrounding water molecules. The adsorbed laser energy enables the superheating, melting, rapid boiling and then vaporization of Au metal to occur simultaneously, resulting in the generation of hotter and denser Au plasma on the irradiated spot region. Meanwhile, the photon-induced splitting of surrounded water molecules will be driven by the adsorbed laser energy in water, giving rise to the formation –OH groups around Au plasma. In this way, the nucleation of Au species surrounded with –OH groups will take place in the stage of rapid condensation of Au plasma, and sharply terminate owing to the expiration of the pulse laser. The formation of unique Au nanotwins covered with –OH groups is highly related the laser fraction of Au target and laser splitting of water molecules. On the other hand, in the absence of any polymer stabilizers or surfactants in this experiment, the Au nanotwins will be interconnected with each other by subsequent laser sintering, which has been well verified in many previous works [21,22]. During the subsequent laser irradiation process, the asprepared Au nanotwins will be significantly heated by the laser energy. The laser sintering of adjacent nanostructures enable the recrystallization of closed regions to occur, resulting in the formation of twinshaped structure instead of mono-separated Au nanospheres. The initial Au nanotwins with numerous –OH groups formed on the surfaces will play a crucial role in the following fabrication of AuAg nanochains. The initial Au nanotwins covered with –OH groups were served as nanoseeds for further fabrication of AuAg nanochains in this paper. After adding 165 μL, 0.05 M AgNO3, the TEM images of the obtained nanoproducts are shown in Fig. 3. The typical low and enlarged magnification TEM images in Fig. 3(a)–(d) obviously demonstrate that the
Fig. 2. (a–c) Typical TEM images of Au nanostructures generated by 1064 nm laser ablation of Au target in distilled water. (d) The FTIR spectrum of the Au nanomaterials.
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Fig. 3. (a) The typical low-magnification and (b–d) enlarged-TEM images of obtained AuAg nanochains by adding 165 μL, 0.05 M AgNO3. (e) The HRTEM image of the interconnected regions.
initial nanotwins were interconnected/accreted with each other and then significantly changed into curvilinear groups with chain-shaped nanostructures, resulting in the formation of worm-like nanochains. The average size of the obtained nanochains is about 6.2 nm, which is slightly larger than the original Au nanotwins. Moreover, the HRTEM image in Fig. 3(e) provides the detailed interconnected regions of the nanochains. The measured lattice-spacing values in Fig. 3(e) is about 0.237 nm. On the other hand, the theoretical values of (111) lattice planes of Ag and Au are 0.238 and 0.235 nm, respectively. So, the lattice fringes with a d- spacing of 0.237 nm in this paper can be attributed to
the (111) plane structure of AgAu alloys. It can be deduced that the Au and Ag plane structures are miscible phase in this paper, since they possess the same face-centered cubic crystal structures. Moreover, the corresponding elemental mapping images of two typical regions of wormlike AuAg nanochains (Fig. 4(a)) are shown in this paper. The left and blow elemental pictures clearly confirm that the obtained nanochains are indeed constituted of Au and Ag elements on the outside regions and core sites. It can be found that the uniform distributions of Ag and Au elements throughout the chained structures, supporting the formation of bimetallic Ag shell and Au core nanostructures in this work.
Fig. 4. (a) The representative image of AuAg nanochains. The left and blow pictures show the corresponding elemental mapping images of two typical regions. (b) The XRD pattern of the obtained AuAg nanochains.
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Fig. 5. The UV–Visible-NIR absorption spectra of the obtained AuAg nanochains (A–K) by adding 0.05 M AgNO3 with amount increasing from 0–165 μL in Au nanotwins solution.
The relative Au/Ag atomic ratio can be calculated about 92:8, confirming the overgrowth of Ag species on Au nanotwins. In addition, the crystallographic structures of the AuAg nanochains were also examined by Xray diffraction (XRD) pattern in Fig. 4(b). The four diffraction peaks with 2θ centered at 38.18°, 44.22°, 64.59° and 77.8° are located between the standard positions from pure Au (JCPDS. No. 65-2870) and Ag (JCPDS. No. 04-0783) crystallites. It significantly further confirms the formation of alloyed AuAg nanocrystals in this work. Since the relative higher peak formed at 38.18°, the preferential alignment of the (111) orientation has been generated in the final AuAg nanochains. The overgrowth of Ag species was also carefully monitored by UV– Vis-NIR absorption spectra, which can sensitively reveal the localized surface plasmon resonance (LSPR) properties of plasmonic AuAg bimetallic nanostructures with different shapes and compositions. As shown in Fig. 5, the initial Au nanotwins solution exhibits an obvious narrow absorption peak at about 520 nm, which can be attributed to the typical LSPR of Au nanotwins. After adding 15 μL, 0.05 M AgNO3 into the seedsolution, in addition to the obvious LSPR of Au nanotwins at 520 nm, another broadened absorption peak located at about 680 nm began to appear in the absorption spectrum (blue line in Fig. 5). It can be deduced that the obtained AuAg nanochains exhibit two different surface plasmon peaks in visible and higher wavelength regions. Considering the curvilinear array of AuAg nanochains (Fig. 4), the LSPR peak at ~520 nm is caused by the transverse band of Au nanotwins in onedimensional (1D) chain-shaped structure. Meanwhile, the appearance of the plasmon resonance band at higher wavelength of ~680 nm should be attributed to the formation of longitudinal surface plasmon band in 1D chain-like nanoarchitectures, which is coincident with previous work [23]. Moreover, increasing the Ag ions content from 30 to 165 μL enables the longitudinal surface plasmon resonance band to be significantly enhanced from about 0.3a.u to 0.62 a.u, and red-shifted from about 680 to 750 nm, implying the higher-yield synthesis of chainshaped AuAg nanostructures. During the overgrowth process, the solution color changed from claret-red to red-violet (inset in Fig. 5). It
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should be noted that the longitudinal resonance of the AuAg nanoproducts will not change any more by further increasing AgNO3 amount (N165 μL), indicating the accomplishment of chemical reaction between Ag ions and Au nanotwins in this paper. The above results and discussions have indeed confirmed that the well-defined worm-like and ultrafine AuAg nanochains can be simply fabricated by adding Ag ions into Au nanotwins solution, without the aid of any polymer stabilizers/capping agents/structure directing additives. The formation of clean and pure AuAg nanochains is highly related to the abundant – OH groups formed on the surfaces of nanotwins, which is severed as unique reducing agent in this paper, resulting in moderate reduction of Ag ions and overgrowth of Ag species. The reduction process of Ag ions via these –OH groups after adding AgNO3 in as-prepared Au nanotwins solution can be expressed as follows [24,25]: ð1Þ In this way, without any surfactant in reactive solution, the overgrowth of Ag species on Au nanotwins gives rise to the formation of interconnected and short curvilinear chain-like nanostructures. Finally, the enhanced SERS performances of the AuAg nanochains were illustrated by using crystal violet (CV) as the probe molecules. For comparison, mono-dispersed Au nanotwins by laser ablation of Au target in distilled water solution were selected as a reference SERS substrate. The SERS spectra of CV molecules of 10−6 M adsorbed on the above two samples were illustrated in Fig. 6(a). The dominating characteristic bands of CV molecules such as 1176, 1375 and 1620 cm−1, etc. that originated from plasmonic Au nanotwins and AuAg nanochains are all clearly detected in SERS spectra. Furthermore, it can be obviously found that the SERS signals originated from the obtained AuAg nanochains are much higher than that of mono-dispersed Au nanotwins. For example, the characteristic band of CV molecules at about 1176 cm−1 is about 12,886 a.u, which is about 4.5 times higher than that (2844 a.u) of Au nanotwins. The enhanced SERS activity should be highly related to the Ag shell-Au core nanochains. Compared with Au species, the Ag shell structure possesses a promising role for further enhancing SERS performance, which has been verified in many previous works [26,27]. The obvious comparative result confirms that the pure bimetallic AuAg nanochains have a strong competitive advantage in the SERS applications. Then, the SERS tests of CV molecules with different concentrations of 10−5–10−9 M were presented in Fig. 6(b). As shown in Fig. 6(b), the dominating characteristic bands are also clearly distinguishable even the concentration decreased to as low as 10−9 M. The ultrasensitive detection limit of AuAg nanochains already approaches the requirement (~nM) for single molecule detection. More importantly, the obtained AuAg nanochains should also be served as an excellent SERS substrate for precise assessment of pesticide residues on agricultural products, which will be applicable to sensitively monitor the food security in a simple, rapid, nondestructive way.
Fig. 6. (a) The SERS spectra of 10−6 M CV molecules adsorbed on Au nanotwins fabricated by 1064 nm laser ablation of Au target in distilled water and AuAg nanochains obtained by adding 165 μL AgNO3 (0.05 M) to the Au nanotwins solution. (b) SERS spectra of CV molecules with different concentrations (10−5–10−9 M) adsorbed on AuAg nanochains.
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Fig. 7. (a) The SERS spectra of different concentration (10−3–10−7 M) of pesticide thiram molecules on apple surface based on AuAg nanochains prepared by adding 0.05 M,165 μL AgNO3 to the initial Au nanotwins solution. (b) The variations of SERS intensities at 560, 1442, 1145, 1518 and 1386 cm−1versus the different concentration of thiram molecules on apple surface.
Therefore, AuAg nanochains-based SERS analyses were carried out to identify pesticide thiram molecules on apple surfaces. Thiram is one of the most widely used as an animal repellent and as a fungicide in agriculture [28,29], and it can also lead to serious skin and eye illness which has a great influence on our health [30]. The SERS signals originated from thiram molecules with different concentrations (10−3– 10−7 M) on cleaned apple peels are shown in Fig. 7(a). It reveals that several dominating characteristic bands in the range of 400–1800 cm−1 can be clearly identified in spectrum. The pure AuAg nanochains-based SERS substrate provides excellent higher signal-to noise (SN) ratio signals, giving much richer “molecular fingerprint” information for thiram detection in comparison with many previous works [1–4,6,19,31]. In detail, based on previous discussions [1,32,33], the prominent characteristic bands of thiram molecules in this paper are identified as follows: peak located at 1518 cm−1 can be attributed to pure υ(C_N) stretching vibration, and the most intense band at 1386 cm−1 is related to δs(CH3) vibrations; another two peaks at 1442 and 1148 cm−1 should be originated from δas(CH3) and ρ(CH3) or υ(N\\CH3); the peak at 928 cm−1 is corresponded to υ(N\\CH3) or υ(S_C); the peak at 864 cm−1 is attributed to υ(CH3\\N); the final peaks at 560 and 441 cm−1 are derived from σ(CH3-NC) and υ(S_S), respectively. Furthermore, the SERS variation of thiram molecules with different concentrations of 10−3–10−7 M were illustrated to evaluate the sensitive detection of the as-prepared AuAg nanochains. As shown in Fig. 7(a), it should be noted that the dominating characteristic bands of thiram molecules are also clearly distinguishable even the concentration decreased to as low as 10−7 M (0.03 ppm), which is about N200 times lower than the maximal residue limit(MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency(EPA). The detection limit of pesticide thiram residues on fruits already downs to the ultralow-levels obtained in previous works [1–6,16–19]. In addition to the excellent SERS activity of AuAg nanochains, the linear relationships between SERS intensities of dominating characteristic bands and thiram concentrations were further illustrated in this paper. The variations of SERS intensities at 560, 1145, 1442, 1386 and 1518 cm−1 as a function of thiram concentration (10−3–10−7 M) is shown in Fig. 7(b). Taking advantage of the pure AuAg nanochains characterized by no polymer contamination on the surfaces, the SERS intensities of thiram molecules monotonically decrease as the concentration decreases from 10−3 to 10−7 M. In a typical detailed analysis, it can be clearly found that the five well-defined linear relationships (for example: R2 = 0.9978 at 560 cm−1) can be separately obtained by plotting the different characteristic peak intensities versus thiram concentration on apple surfaces. It can be deduced that the established excellent linear relationships can sensitively response to the slight variation of pesticide residues concentration on fruit surface, providing unprecedented opportunity for simple, rapid and precise assessment of agricultural products.
4. Conclusions In summary, an excellent SERS nanosubstrate has been constructed by fabricating pure/clean worm-like AuAg nanochains with highly interconnected ultrafine (~6.2 nm) bimetallic particles (Au/Ag: 92/8). The Au nanotwins with abundant –OH groups formed on surfaces were initially produced by pulsed laser ablation of Au target in distilled water. Without the aid of any polymer additives served as surfactant or stabilizer agents, the overgrowth of Ag species can be driven by reduction of Ag ions via these –OH groups after adding AgNO3 in asprepared Au nanotwins solution. The obtained AuAg nanochains exhibit much higher SERS activity in comparison with monometallic Au nanotwins, owing to the intermetallic synergies between two different metals. Furthermore, the clean AuAg nanochains-based SERS substrate provides abundant “molecular fingerprint” information for detection of pesticide thiram molecules on apple surfaces. More importantly, the corresponding SERS analyses reveal several well-defined linear relationships between the Raman intensities of dominating characteristic bands and thiram concentrations range over a wide region (10−3– 10−7 M). Therefore, the established excellent linear responses in this paper will give rise to the exact quantitative assessment of food security in a simple, rapid, nondestructive way. Acknowledgements National Natural Science Foundation of China (Nos. 11575102, 11105085 and 11775134). Natural Science Foundation of Shandong Province (No. ZR2016CM02). References [1] S. Kumar, P. Goel, J.P. Singh, Flexible and robust SERS active substrates for conformal rapid detection of pesticide residues fruits, Sensors Actuators B 241 (2017) 577–583. [2] J. Sun, L. Gong, Y.T. Lu, D.M. Wang, Z.J. Gong, M.K. Fan, Dual functional PDMS sponge SERS substrate for the on-site detection of pesticides both on fruit surfaces and in juice, Analyst 143 (2018) 2689–2695. [3] J.K. Yang, H. Kang, H. Lee, A. Jo, S. Jeong, S.J. Jeon, H.I. Kim, H.Y. Lee, D.H. Jeong, J.H. Kim, Y.S. Lee, Single-step and rapid growth of silver nanoshells as SERS-active nanostructures for label-free detection of pesticides, ACS Appl. Mater. Interfaces 6 (2014) 12541–12549. [4] J. Zhu, M.J. Liu, J.J. Li, X. Li, J.W. Zhao, Multi-branched gold nanostars with fractal structure for SERS detection of the pesticide thiram, Spectrochim. Acta A Mol. Biomol. Spectrosc. 189 (2018) 586–593. [5] Y.Z. Zhang, Z.Y. Wang, L. Wu, Y.W. Pei, P. Chen, Y.P. Cui, Rapid simultaneous detection of multi-pesticide residues on apple using SERS technique, Analyst 139 (2014) 5148–5154. [6] P.Z. Guo, D. Sikdar, X.Q. Huang, K.J. Si, W. Xiong, S. Gong, L.W. Yap, M. Premaratne, W.L. Cheng, Plasmonic core-shell nanoparticles for SERS detection of the pesticide thiram: size-and shape-dependent Raman enhancement, Nanoscale 7 (2015) 2862–2868. [7] E. Watanabe, S. Miyake, Y. Yogo, Review of enzyme-linked immunosorbent assays (ELISAs) for analyses of neonicotinoid insecticides in agro-environments, J. Agric. Food Chem. 61 (2013) 12459–12472.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Construction of pure worm-like AuAg nanochains for ultrasensitive SERS detection of pesticide residues on apple surfaces Anxin Jiao, Xuejian Dong, Hua Zhang, Linlin Xu, Yue Tian, Xiangdong Liu, Ming Chen ⁎ School of Physics, State Key laboratory of Crystal Materials, Shandong University, Shandong, Jinan 250100, PR China
a r t i c l e
i n f o
Article history: Received 24 July 2018 Received in revised form 23 October 2018 Accepted 28 October 2018 Available online 29 October 2018 Keywords: Surface-enhanced Raman scattering Nanomaterials Laser materials processing
a b s t r a c t Ultrasensitive detection of pesticide residues on agricultural products using surface-enhanced Raman spectroscopy (SERS) is of significant interest in food security. Herein, worm-like AuAg nanochains with highly interconnected ultrafine (~6.2 nm) bimetallic particles were developed as an excellent SERS nanosensor via laser-assisted strategy. The SERS detection limit of thiram molecules on apple surfaces is about 10−7 M (0.03 ppm), which is about 200 times lower than the maximal residue limit (MRL, 7 ppm) in fruit prescribed by the U.S. Environmental Protection Agency (EPA). Importantly, the established excellent linear relationships between the SERS intensities and thiram concentrations can sensitively monitor the slight variation of pesticide residues in agriculture. © 2018 Elsevier B.V. All rights reserved.
1. Introduction At present, pesticide residues originated from overusing pesticides for protecting agricultural products from insects and diseases give rise to unprecedented toxic or lethal effects on human body. Recently, the application of surface enhanced Raman scattering spectroscopy (SERS) for simple, rapid, nondestructive and accurate pesticide detection has gained extensive attention in ultrasensitive monitoring of food (fruits, vegetables, crops, etc.) security [1–6]. Superior to traditional detection technologies such as enzyme-linked immunosorbent assay (ELISA) [7], gas chromatography mass spectroscopy (GC–MS) [8], high performance liquid chromatography (HPLC) [9], etc., the SERS analysis represents a double benefit: excellent “molecular fingerprint” information derived from specific vibrational energy of chemical bonds, and ultralow detection limit due to the enhanced optical properties of plasmonic metallic (Ag, Au) nanostructures. The localized surface plasmon resonance (LSPR) of Ag, Au-based nanomaterials can be effectively excited by laser irradiation during Raman scattering process, which will provide enhanced electromagnetic (EM) field and then significantly magnify the Raman spectral intensity of adsorbed molecules on nanosubstrates [10–13]. Most recently, an interesting work illustrated the fabrication of a versatile dual-functional SERS substrate by incubating polydimethysiloxane (PDMS) sponge in Au nanoparticles solution, which enables the lowest detected concentration of pesticide triazophos and methyl parathion in fruit juice to be achieved at 100 ppb and 1 ppm, respectively [2]. Moreover, it has been well recognized that the bimetallic or more complex Ag or Au-based nanocomposites will result in much higher SERS activity in comparison with ⁎ Corresponding author. E-mail address: [email protected] (M. Chen).
https://doi.org/10.1016/j.saa.2018.10.051 1386-1425/© 2018 Elsevier B.V. All rights reserved.
monometallic nanostructures, owing to the unique intermetallic synergies among different metals [13–15]. So, numerous Ag or Au-based nanomaterials with various structures such as silica nanocore/Ag nanoshell [3], Au/silver core-shell nanorods [5], Au@Ag nanocubes [6], etc. have been prepared as SERS-active nanosubstrates for sensitive detection of pesticides. While, there are two urgent issues should be considered in this area as follows. Firstly, most of previous works merely focused on the SERS detection limit of pesticide residues by optimizing the size, composition or shape of Au and Ag-based nanosubstrates. It is really true that the ultralow SERS detectable concentration of multiple pesticides on agricultural products is attracting more public attention and merit further consideration. However, very few reports have mainly focused on the linear relationships between SERS spectral lines and pesticide residual concentrations. The established linear relationships will provide important information for exact and quantitative analyzing the pesticide residual on agricultural products. Compared with ultralow SERS detection, the sensitive monitoring the pesticide variation by establishing the linear relationships between SERS signals and concentrations is more important for practical assessing the food security. However, the corresponding research has not been explored extensively up to now. Secondly, numerous Au or Ag-based nanocomposites have been fabricated for the SERS analysis of pesticide molecules by various growth methods with the aid of complex polymer stabilizers/capping agents/structure directing additives. For example, hexadecyltrimethylammonium bromide (CTAB) [4–6], polyvinylpyrrolidone (PVP) [3,16], polyethylenimine (PEI) [17], etc. have been widely used for sculpting anisotropic-nanostructures. Meanwhile, the organic contaminants adsorbed on nanosubstrates will lead to the formation of unprecedented SERS background noise or undesired spectral signals. So, these complex additives should be
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carefully removed from the SERS nanosubstrates by extra purification process before being processed toward functional applications. However, especially for hollow, porous or branched nanocomposites, the completely removal of these additives is rather difficult. Unfortunately, even with few residual additives, the corresponding signal interferences will be significantly enhanced during SERS analysis of ultralow pesticide molecules. It is an obstacle to establish well-defined relationships between SERS results and pesticide concentrations. In order to avoid the use of polymer additives, some novel SERS substrates such as Ag aligned Ag nanorods arrays generated by thermal evaporation of Ag powder [1], Si nanowire paper (SiNWP) performed in a home-built vertical high-frequency induction furnace (VHFIR) [18], polydimethylsiloxane (PDMS) sponge formed by heated in the vacuum oven for 2 h [2], etc. have been developed in recent years. However, such harsh and expensive experimental setups will also seriously hinder their potentials in practical applications with the consideration of economic benefits. To our knowledge, it remains a great challenge to develop a rapid, simple, reliable, economic and green strategy to convenient synthesis of clean and pure Au or Ag-based nanocomposites, which is particular favorable for establishing well-defined linear relationship between SERS signals and pesticide concentrations. Herein, we report on the successful fabrication of clean and pure worm-like AuAg nanochains by laser-assisted strategy. The asprepared Au nanotwins generated by laser ablation of Au plate in distilled water are characterized by abundant hydroxyl groups (–OH) formed on the surfaces. Taking advantage of these –OH groups, the moderate reduction of Ag ions and then overgrowth of Ag species enable the AuAg nanochains with highly interconnected ultrafine (~6.2 nm) bimetallic particles to be simply obtained by adding AgNO3 to the initial Au nanotwins solution. Firstly, as for SERS tests, the crystal violet (CV) molecules were selected as probe molecules. The CV molecules served as stable organic contaminants have been validated as a mitotic poisoning agent. The ultrasensitive SERS detection of CV molecules in liquid is very important for human health. The SERS intensities of CV molecules adsorbed on the bimetallic AuAg nanochains is about 4.5 times higher than that of monometallic Au nanotwins. So, the obtained AuAg nanochains provide enhanced SERS activity in this paper. Based on the obtained AuAg nanochains, the SERS analyses demonstrate that the dominating characteristic bands of pesticide thiram molecules on apple surface can be clearly distinguished even with the concentration decreased to as low as 10−7 M (0.03 ppm), which is about 200 times lower than the maximal residue limit (MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency (EPA). The ultrasensitive SERS detection limit of pesticide molecules in this paper already downs to the ultralow-level obtained in many previous works [1–6,16–19]. More interestingly, the corresponding SERS results illustrate several well-defined linear relationships between SERS peak intensities and thiram concentrations in a wide range of 10−3–10−7 M. Therefore, the obtained results in this paper provide a reliable strategy for precise identification and exact quantitative analysis of agricultural products, which can unambiguously possess high applicability in foodsupervise applications.
2 mm) in distilled water. In a typical experiment, the Au target was placed on the bottom of a rotating glass dish with a speed of ~500 rpm that was filled with 3 mm depth of distilled water. A Qswitched Nd-YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra Physics) beam operating at a wavelength of 1064 nm with pulse duration of 10 ns, 10 Hz repetition rate was focused on the Au target. The energy density of laser beam was about 280 mJ/pulse, and the average spot size on the Au target was about 1 mm in diameter. After 20 min laser ablation, the claret-red colored colloidal solution will be obtained. Then, under magnetic stirring condition, different amounts (0–165 μL, 0.05 M) of AgNO3 water solution were added to the Au nano-products solution. The mixed solution will be rapidly changed into red-violet color within ~2 min, implying the chemical reaction occurs in this experiment. The obtained products were centrifuged by 5000 rpm for 10 min in an ultracentrifuge. 2.3. Materials Characterization The morphological structures of the sediments were investigated by transmission electron microscopy (JEOL-JEM-2100F). The chemical compositions were analyzed by elemental mapping images via energy dispersive X-ray spectroscopy (EDS) using a JEOL-2100F electron microscope equipped with a STEM unit. The X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kαradiation (λ = 0.15406 nm) were used to illustrate the crystallographic structure of the products. The Fourier transforms infrared spectrums (FTIR) of the nanomaterials were measured by a UV–Vis-NIR spectrometer (370–7800 cm−1, ALPHA-T, Bruker). The absorption spectra were recorded by a UV–Vis-IR spectrometer (UV-1800, Shimadzu). 2.4. SERS Measurement In a typical Raman spectroscopic analysis of CV molecules, the AuAg nanochains-based SERS substrates were performed by dropping 0.5 M/10 μL sediments on clean silicon wafers. Then, the SERS nanosubstrates were totally immersed into certain concentration of CV molecules ethanol solution under lucifugal circumstance for 12 h and then dried spontaneously. The experimental section of SERS analysis of pesticide thiram molecules on apple surface was illustrated in Fig. 1, which is similar to that described by previous works [2,3,5,19]. Pesticide thiram powder was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Briefly, 0.024 g thiram powder was dissolved in 10 mL ethanol to form 0.01 M solution, which will be separately diluted with distilled water to prepare pesticide solutions with different concentrations of 10−3–10−7 M. The cleaned apple peels with a uniform squares size of ~2 cm were obtained by carefully peeling of a whole apple that was bought from a supermarket. Then, the 8 μL of the as-prepared thiram solution with different
2. Experimental 2.1. Chemicals All reagents were used as received without further purification and de-ionized water was used in all experiments. All the regents were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). 2.2. Fabrication of Worm-like AuAg Nanochains The initial Au nanotwins were firstly fabricated by pulsed laser ablation of pure (99.99%) Au plate (diameter of 3.5 cm and thickness of
Fig. 1. Schematic illustration of SERS detection of pesticide thiram molecules on apple surface based on AuAg nanochains.
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concentrations of 10−3–10−7 M were separately dropped on to a series of apple peel samples and dried at room temperature. Once complete evaporation, the obtained AuAg nanochains with the dose of 0.5 M/1 μL were separately dropped on the same spot of thiram-contaminated apple peels, and dried at room temperature again. Then the SERS analyses of thiram molecules on apple surfaces were carried out by removing background signals from original apple surface. The background signals were obtained from the uncontaminated apple surface treated with the same amount of ethanol and AuAg nanochains. In this way, the pure SERS signals of thiram molecules with different concentrations can be recorded in this paper. The Raman spectra were carried out by using a LabRAM HR 800 spectrograph with 633 nm laser beam at room temperature. The exciting laser power on all samples was located at about 1 mW, and the acquisition time used for each spectrum was 10s. 3. Results and Discussion After cumulative pulse laser ablation of Au metal target in distilled water, Au nanotwins were fabricated in water solution. The morphologies of Au nanostructures were illustrated by transmission electron microscopy (TEM), as shown in Fig. 2. The low-magnification TEM image in Fig. 2(a) exhibits a typical overall morphology of Au nanostructures, which consists of numerous twin-shaped nanoarchitectures. The enlarged TEM image of a representative nanotwins in Fig. 2 (b) provides a structure detail of the cross region between two nanoparticles. It is evident that the Au nanotwins are indeed interconnected and accreted with each other. The average size of nanoparticles is about 6 nm by measuring the diameters of N200 nanoparticles in sight on the TEM images. The interconnected lattice fringes at cross-region with a periodicity corresponding to a d-spacing of 0.236 nm could be indexed with reference to the Au (111) plane structure. Meanwhile, the HRTEM image in Fig. 2(c) reveals the crystal structures of Au nanotwins, which is mainly composed of three different Au orientations. For example, the region marked by red line in Fig. 2(c) with a dspacing of 0.236 nm is indexed as the (111) plane in Au structure, while the areas marked by green line and yellow line with a d-spacing of 0.204 and 0.123 nm can be separately attributed to the (200) and (311) structures. Moreover, in order to get more information of the
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surface composition, the FTIR spectrum of the obtained Au nanotwins is shown in Fig. 2(d), As illustrated in Fig. 2(d), in addition to the Au nanomaterial lattice vibration at ~582 cm−1, the FTIR pattern is dominated by the broad band with peak at about 3440 cm−1, which is attributed to the strongly adsorbing –OH stretching vibrations [20]. The obvious characteristic spectrum of –OH groups at about 3440 cm−1 confirms that abundant –OH groups can be formed on the Au nanotwins. The possible growth mechanism of Au nanotwins with numerous –OH groups formed on the surfaces will be described in the following section. At the moment of pulsed laser beam reaching at Au target in distilled water condition, the high laser energy will be significantly adsorbed via Au target and surrounding water molecules. The adsorbed laser energy enables the superheating, melting, rapid boiling and then vaporization of Au metal to occur simultaneously, resulting in the generation of hotter and denser Au plasma on the irradiated spot region. Meanwhile, the photon-induced splitting of surrounded water molecules will be driven by the adsorbed laser energy in water, giving rise to the formation –OH groups around Au plasma. In this way, the nucleation of Au species surrounded with –OH groups will take place in the stage of rapid condensation of Au plasma, and sharply terminate owing to the expiration of the pulse laser. The formation of unique Au nanotwins covered with –OH groups is highly related the laser fraction of Au target and laser splitting of water molecules. On the other hand, in the absence of any polymer stabilizers or surfactants in this experiment, the Au nanotwins will be interconnected with each other by subsequent laser sintering, which has been well verified in many previous works [21,22]. During the subsequent laser irradiation process, the asprepared Au nanotwins will be significantly heated by the laser energy. The laser sintering of adjacent nanostructures enable the recrystallization of closed regions to occur, resulting in the formation of twinshaped structure instead of mono-separated Au nanospheres. The initial Au nanotwins with numerous –OH groups formed on the surfaces will play a crucial role in the following fabrication of AuAg nanochains. The initial Au nanotwins covered with –OH groups were served as nanoseeds for further fabrication of AuAg nanochains in this paper. After adding 165 μL, 0.05 M AgNO3, the TEM images of the obtained nanoproducts are shown in Fig. 3. The typical low and enlarged magnification TEM images in Fig. 3(a)–(d) obviously demonstrate that the
Fig. 2. (a–c) Typical TEM images of Au nanostructures generated by 1064 nm laser ablation of Au target in distilled water. (d) The FTIR spectrum of the Au nanomaterials.
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Fig. 3. (a) The typical low-magnification and (b–d) enlarged-TEM images of obtained AuAg nanochains by adding 165 μL, 0.05 M AgNO3. (e) The HRTEM image of the interconnected regions.
initial nanotwins were interconnected/accreted with each other and then significantly changed into curvilinear groups with chain-shaped nanostructures, resulting in the formation of worm-like nanochains. The average size of the obtained nanochains is about 6.2 nm, which is slightly larger than the original Au nanotwins. Moreover, the HRTEM image in Fig. 3(e) provides the detailed interconnected regions of the nanochains. The measured lattice-spacing values in Fig. 3(e) is about 0.237 nm. On the other hand, the theoretical values of (111) lattice planes of Ag and Au are 0.238 and 0.235 nm, respectively. So, the lattice fringes with a d- spacing of 0.237 nm in this paper can be attributed to
the (111) plane structure of AgAu alloys. It can be deduced that the Au and Ag plane structures are miscible phase in this paper, since they possess the same face-centered cubic crystal structures. Moreover, the corresponding elemental mapping images of two typical regions of wormlike AuAg nanochains (Fig. 4(a)) are shown in this paper. The left and blow elemental pictures clearly confirm that the obtained nanochains are indeed constituted of Au and Ag elements on the outside regions and core sites. It can be found that the uniform distributions of Ag and Au elements throughout the chained structures, supporting the formation of bimetallic Ag shell and Au core nanostructures in this work.
Fig. 4. (a) The representative image of AuAg nanochains. The left and blow pictures show the corresponding elemental mapping images of two typical regions. (b) The XRD pattern of the obtained AuAg nanochains.
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Fig. 5. The UV–Visible-NIR absorption spectra of the obtained AuAg nanochains (A–K) by adding 0.05 M AgNO3 with amount increasing from 0–165 μL in Au nanotwins solution.
The relative Au/Ag atomic ratio can be calculated about 92:8, confirming the overgrowth of Ag species on Au nanotwins. In addition, the crystallographic structures of the AuAg nanochains were also examined by Xray diffraction (XRD) pattern in Fig. 4(b). The four diffraction peaks with 2θ centered at 38.18°, 44.22°, 64.59° and 77.8° are located between the standard positions from pure Au (JCPDS. No. 65-2870) and Ag (JCPDS. No. 04-0783) crystallites. It significantly further confirms the formation of alloyed AuAg nanocrystals in this work. Since the relative higher peak formed at 38.18°, the preferential alignment of the (111) orientation has been generated in the final AuAg nanochains. The overgrowth of Ag species was also carefully monitored by UV– Vis-NIR absorption spectra, which can sensitively reveal the localized surface plasmon resonance (LSPR) properties of plasmonic AuAg bimetallic nanostructures with different shapes and compositions. As shown in Fig. 5, the initial Au nanotwins solution exhibits an obvious narrow absorption peak at about 520 nm, which can be attributed to the typical LSPR of Au nanotwins. After adding 15 μL, 0.05 M AgNO3 into the seedsolution, in addition to the obvious LSPR of Au nanotwins at 520 nm, another broadened absorption peak located at about 680 nm began to appear in the absorption spectrum (blue line in Fig. 5). It can be deduced that the obtained AuAg nanochains exhibit two different surface plasmon peaks in visible and higher wavelength regions. Considering the curvilinear array of AuAg nanochains (Fig. 4), the LSPR peak at ~520 nm is caused by the transverse band of Au nanotwins in onedimensional (1D) chain-shaped structure. Meanwhile, the appearance of the plasmon resonance band at higher wavelength of ~680 nm should be attributed to the formation of longitudinal surface plasmon band in 1D chain-like nanoarchitectures, which is coincident with previous work [23]. Moreover, increasing the Ag ions content from 30 to 165 μL enables the longitudinal surface plasmon resonance band to be significantly enhanced from about 0.3a.u to 0.62 a.u, and red-shifted from about 680 to 750 nm, implying the higher-yield synthesis of chainshaped AuAg nanostructures. During the overgrowth process, the solution color changed from claret-red to red-violet (inset in Fig. 5). It
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should be noted that the longitudinal resonance of the AuAg nanoproducts will not change any more by further increasing AgNO3 amount (N165 μL), indicating the accomplishment of chemical reaction between Ag ions and Au nanotwins in this paper. The above results and discussions have indeed confirmed that the well-defined worm-like and ultrafine AuAg nanochains can be simply fabricated by adding Ag ions into Au nanotwins solution, without the aid of any polymer stabilizers/capping agents/structure directing additives. The formation of clean and pure AuAg nanochains is highly related to the abundant – OH groups formed on the surfaces of nanotwins, which is severed as unique reducing agent in this paper, resulting in moderate reduction of Ag ions and overgrowth of Ag species. The reduction process of Ag ions via these –OH groups after adding AgNO3 in as-prepared Au nanotwins solution can be expressed as follows [24,25]: ð1Þ In this way, without any surfactant in reactive solution, the overgrowth of Ag species on Au nanotwins gives rise to the formation of interconnected and short curvilinear chain-like nanostructures. Finally, the enhanced SERS performances of the AuAg nanochains were illustrated by using crystal violet (CV) as the probe molecules. For comparison, mono-dispersed Au nanotwins by laser ablation of Au target in distilled water solution were selected as a reference SERS substrate. The SERS spectra of CV molecules of 10−6 M adsorbed on the above two samples were illustrated in Fig. 6(a). The dominating characteristic bands of CV molecules such as 1176, 1375 and 1620 cm−1, etc. that originated from plasmonic Au nanotwins and AuAg nanochains are all clearly detected in SERS spectra. Furthermore, it can be obviously found that the SERS signals originated from the obtained AuAg nanochains are much higher than that of mono-dispersed Au nanotwins. For example, the characteristic band of CV molecules at about 1176 cm−1 is about 12,886 a.u, which is about 4.5 times higher than that (2844 a.u) of Au nanotwins. The enhanced SERS activity should be highly related to the Ag shell-Au core nanochains. Compared with Au species, the Ag shell structure possesses a promising role for further enhancing SERS performance, which has been verified in many previous works [26,27]. The obvious comparative result confirms that the pure bimetallic AuAg nanochains have a strong competitive advantage in the SERS applications. Then, the SERS tests of CV molecules with different concentrations of 10−5–10−9 M were presented in Fig. 6(b). As shown in Fig. 6(b), the dominating characteristic bands are also clearly distinguishable even the concentration decreased to as low as 10−9 M. The ultrasensitive detection limit of AuAg nanochains already approaches the requirement (~nM) for single molecule detection. More importantly, the obtained AuAg nanochains should also be served as an excellent SERS substrate for precise assessment of pesticide residues on agricultural products, which will be applicable to sensitively monitor the food security in a simple, rapid, nondestructive way.
Fig. 6. (a) The SERS spectra of 10−6 M CV molecules adsorbed on Au nanotwins fabricated by 1064 nm laser ablation of Au target in distilled water and AuAg nanochains obtained by adding 165 μL AgNO3 (0.05 M) to the Au nanotwins solution. (b) SERS spectra of CV molecules with different concentrations (10−5–10−9 M) adsorbed on AuAg nanochains.
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Fig. 7. (a) The SERS spectra of different concentration (10−3–10−7 M) of pesticide thiram molecules on apple surface based on AuAg nanochains prepared by adding 0.05 M,165 μL AgNO3 to the initial Au nanotwins solution. (b) The variations of SERS intensities at 560, 1442, 1145, 1518 and 1386 cm−1versus the different concentration of thiram molecules on apple surface.
Therefore, AuAg nanochains-based SERS analyses were carried out to identify pesticide thiram molecules on apple surfaces. Thiram is one of the most widely used as an animal repellent and as a fungicide in agriculture [28,29], and it can also lead to serious skin and eye illness which has a great influence on our health [30]. The SERS signals originated from thiram molecules with different concentrations (10−3– 10−7 M) on cleaned apple peels are shown in Fig. 7(a). It reveals that several dominating characteristic bands in the range of 400–1800 cm−1 can be clearly identified in spectrum. The pure AuAg nanochains-based SERS substrate provides excellent higher signal-to noise (SN) ratio signals, giving much richer “molecular fingerprint” information for thiram detection in comparison with many previous works [1–4,6,19,31]. In detail, based on previous discussions [1,32,33], the prominent characteristic bands of thiram molecules in this paper are identified as follows: peak located at 1518 cm−1 can be attributed to pure υ(C_N) stretching vibration, and the most intense band at 1386 cm−1 is related to δs(CH3) vibrations; another two peaks at 1442 and 1148 cm−1 should be originated from δas(CH3) and ρ(CH3) or υ(N\\CH3); the peak at 928 cm−1 is corresponded to υ(N\\CH3) or υ(S_C); the peak at 864 cm−1 is attributed to υ(CH3\\N); the final peaks at 560 and 441 cm−1 are derived from σ(CH3-NC) and υ(S_S), respectively. Furthermore, the SERS variation of thiram molecules with different concentrations of 10−3–10−7 M were illustrated to evaluate the sensitive detection of the as-prepared AuAg nanochains. As shown in Fig. 7(a), it should be noted that the dominating characteristic bands of thiram molecules are also clearly distinguishable even the concentration decreased to as low as 10−7 M (0.03 ppm), which is about N200 times lower than the maximal residue limit(MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency(EPA). The detection limit of pesticide thiram residues on fruits already downs to the ultralow-levels obtained in previous works [1–6,16–19]. In addition to the excellent SERS activity of AuAg nanochains, the linear relationships between SERS intensities of dominating characteristic bands and thiram concentrations were further illustrated in this paper. The variations of SERS intensities at 560, 1145, 1442, 1386 and 1518 cm−1 as a function of thiram concentration (10−3–10−7 M) is shown in Fig. 7(b). Taking advantage of the pure AuAg nanochains characterized by no polymer contamination on the surfaces, the SERS intensities of thiram molecules monotonically decrease as the concentration decreases from 10−3 to 10−7 M. In a typical detailed analysis, it can be clearly found that the five well-defined linear relationships (for example: R2 = 0.9978 at 560 cm−1) can be separately obtained by plotting the different characteristic peak intensities versus thiram concentration on apple surfaces. It can be deduced that the established excellent linear relationships can sensitively response to the slight variation of pesticide residues concentration on fruit surface, providing unprecedented opportunity for simple, rapid and precise assessment of agricultural products.
4. Conclusions In summary, an excellent SERS nanosubstrate has been constructed by fabricating pure/clean worm-like AuAg nanochains with highly interconnected ultrafine (~6.2 nm) bimetallic particles (Au/Ag: 92/8). The Au nanotwins with abundant –OH groups formed on surfaces were initially produced by pulsed laser ablation of Au target in distilled water. Without the aid of any polymer additives served as surfactant or stabilizer agents, the overgrowth of Ag species can be driven by reduction of Ag ions via these –OH groups after adding AgNO3 in asprepared Au nanotwins solution. The obtained AuAg nanochains exhibit much higher SERS activity in comparison with monometallic Au nanotwins, owing to the intermetallic synergies between two different metals. Furthermore, the clean AuAg nanochains-based SERS substrate provides abundant “molecular fingerprint” information for detection of pesticide thiram molecules on apple surfaces. More importantly, the corresponding SERS analyses reveal several well-defined linear relationships between the Raman intensities of dominating characteristic bands and thiram concentrations range over a wide region (10−3– 10−7 M). Therefore, the established excellent linear responses in this paper will give rise to the exact quantitative assessment of food security in a simple, rapid, nondestructive way. Acknowledgements National Natural Science Foundation of China (Nos. 11575102, 11105085 and 11775134). Natural Science Foundation of Shandong Province (No. ZR2016CM02). References [1] S. Kumar, P. Goel, J.P. Singh, Flexible and robust SERS active substrates for conformal rapid detection of pesticide residues fruits, Sensors Actuators B 241 (2017) 577–583. [2] J. Sun, L. Gong, Y.T. Lu, D.M. Wang, Z.J. Gong, M.K. Fan, Dual functional PDMS sponge SERS substrate for the on-site detection of pesticides both on fruit surfaces and in juice, Analyst 143 (2018) 2689–2695. [3] J.K. Yang, H. Kang, H. Lee, A. Jo, S. Jeong, S.J. Jeon, H.I. Kim, H.Y. Lee, D.H. Jeong, J.H. Kim, Y.S. Lee, Single-step and rapid growth of silver nanoshells as SERS-active nanostructures for label-free detection of pesticides, ACS Appl. Mater. Interfaces 6 (2014) 12541–12549. [4] J. Zhu, M.J. Liu, J.J. Li, X. Li, J.W. Zhao, Multi-branched gold nanostars with fractal structure for SERS detection of the pesticide thiram, Spectrochim. Acta A Mol. Biomol. Spectrosc. 189 (2018) 586–593. [5] Y.Z. Zhang, Z.Y. Wang, L. Wu, Y.W. Pei, P. Chen, Y.P. Cui, Rapid simultaneous detection of multi-pesticide residues on apple using SERS technique, Analyst 139 (2014) 5148–5154. [6] P.Z. Guo, D. Sikdar, X.Q. Huang, K.J. Si, W. Xiong, S. Gong, L.W. Yap, M. Premaratne, W.L. Cheng, Plasmonic core-shell nanoparticles for SERS detection of the pesticide thiram: size-and shape-dependent Raman enhancement, Nanoscale 7 (2015) 2862–2868. [7] E. Watanabe, S. Miyake, Y. Yogo, Review of enzyme-linked immunosorbent assays (ELISAs) for analyses of neonicotinoid insecticides in agro-environments, J. Agric. Food Chem. 61 (2013) 12459–12472.
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