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Abstract
Recently, hybrid plasmonic metal/semiconductor-based surface-enhanced Raman scattering (SERS) has attracted ever-increasing attention due to its combined characteristics of electromagnetic (EM) enhancement and chemical (CM) enhancement, holding great potential for trace molecular detection. Herein, we demonstrate an interesting heterostructure by linking Cu2O nano-octahedrons with intertwined Ag nanovines (NVs). The obtained Ag NVs/Cu2O heterostructures exhibit excellent SERS activity, which is about 2.7 and 7.0 times higher than that of monodispersed Ag or Au nanoparticles (NPs) modified Cu2O. The intertwined Ag NVs among adjacent Cu2O octahedrons serve as efficient electron transport channels, which can obviously promote the separation of electrons and holes, reduce the recombination of photogenerated carriers, and then improve the CM enhancement effect. Meanwhile, the accumulated electrons on plasmonic NVs can effectively optimize the collective oscillation of electrons and further improve the EM enhancement. The optimal SERS substrate possesses fascinating multifunctional SERS properties, including ultra-low detection limit (CV, 10−14 M), excellent anti-interference capability and selectivity. Finally, the established nanosensor can be effectively applied for the quantitative detection of pesticide thiram molecules in soil and biological samples, with low detection limits of 0.48 ng g−1 and 10−7 M, respectively. The proposed work demonstrates a high-performance SERS heterostructure with both improved CM enhancement and enhanced EM effect by linking adjacent Cu2O nano-octahedrons with Ag NVs, which is particularly suitable for ultrasensitive residual pesticide detection in real-world environment.
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1. Introduction
In the past decades, surface-enhanced Raman scattering (SERS) spectroscopy has attracted considerable attention in the fields of food safety, environmental monitoring, biological and medical applications due to its distinguished features of ultrasensitivity, no-label, fingerprint recognition ability, and nondestructive property to samples [1–5]. Till now, two enhancement mechanisms of SERS have been widely accepted: (1) Electromagnetic (EM) enhancement mechanism is associated with the localized surface plasmon resonance (LSPR) of plasmonic metal nanostructures such as Au, Ag and Cu. (2) Chemical (CM) enhancement mechanism originates from the charge transfer (CT) between the band-gap semiconductor materials and adsorbed molecules [6,7]. The primary SERS signal is mainly attributed to EM enhancement with a maximum contribution of ∼106−107, while the contribution factor of CM enhancement to the total signal enhancement is ∼10−100 [7]. The SERS substrates that possess both CM enhancement and EM enhancement are made up of plasmonic noble metal and semiconductor. Compared to pure noble metal or semiconductor nanomaterials, metal-semiconductor heterostructures have the ability to further improve SERS activity because of synergistic properties generated by the interaction among different components [8–10].
Over the past few decades, plasmonic metal-semiconductor heterostructures have stimulated great interest for being used as SERS substrates due to the formation of suitable CT path at the interface of metal/semiconductor, thereby generating strong SERS signals. For instance, Zhou et al. designed Au-ZnO heterogeneous nanorod with a good epitaxial interface, and it demonstrated a stronger SERS signal than bare Au nanoscale seeds for the detection of dopamine molecules, which was attributed to the enhanced CT effect of ZnO [11]. Rajkumar et al. fabricated Ag/ZnO heterostructures as a remarkable SERS substrate for sensitive detection of hydroxyapatite nanoparticles (NPs) [12]. Ji et al. synthesized hierarchical Ag/Cu2O/ITO substrate as a self-cleaning SERS substrate, realizing high SERS activity for the detection of RhB molecules owing to the high-efficiency interfacial CT process [13]. Plasmonic metal in hybrid nanostructures plays an important role for enhancing Raman scattering effect: (1) The hybridization of semiconductor with plasmonic metal nanostructures can promote light-matter interactions due to the effective LSPR effect, boost the performance of light absorption and conversion, and trigger the generation of plasmon-induced hot carriers. (2) Plasmonic metal nanocrystal in heterostructures can effectively suppress the recombination of electron-hole pairs and facilitate charge separation, thereby enhancing the CT contribution to SERS. Due to the inequivalent Fermi level between semiconductor and noble metal, the schottky barrier can be built at interface of semiconductor and plasmonic metal, facilitating electrons transfer until the energy reaches thermodynamic equilibrium [14]. The redistribution of charge will induce a stronger electromagnetic field, which makes plasmonic metal excite a stronger LSPR under visible light irradiation, thus greatly enhancing the SERS signals of adsorbed molecules [15]. Therefore, the modification of semiconductor material using plasmonic metal nanostructure has been widely studied in previous works. It has become one of the most popular methods for improving SERS activity. As a kind of semiconductor material, Cu2O has been widely explored due to a narrow bandgap (2.17 eV), wide spectral absorption range, good electrical conductivity, and has become an excellent candidate for being used as SERS substrate [16–20]. Although numerous great achievements have been obtained by constructing Cu2O-based heterostructures SERS substrates, it should be noted that most of previous works have mainly concentrated on two kinds of hybrid Cu2O-metal heterogeneous systems: hybrid metal NPs/Cu2O nanostructures [13,21,22] and hybrid core-shelled metal@Cu2O nanostructures [23–25]. If semiconductor Cu2O are linked by one-dimensional (1D) plasmonic metal nanowires (NWs), better SERS results may be obtained: (1) Due to “lightning rod effect” and “interstitial electromagnetic coupling”, wire shaped plasmonic structures demonstrate higher SERS activity than spherical NPs. The external sharp nanoedges can exhibit higher electromagnetic field, which is conducive to the enhancement of Raman signal [26,27]. (2) This 1D nanostructure connecting semiconductor can be used as an efficient electronic transmission channel for improving separation efficiency of electrons and holes, and more effectively inhibiting the recombination of photogenerated carriers, thus improving CM enhancement. Moreover, the electrons gathered and trapped in plasmonic NWs can effectively optimize the collective oscillation of electrons and improve their LSPR, then further promote EM enhancement. In this way, the novel system can not only increase CM enhancement, but also improve EM enhancement, thereby further improving the SERS activity and achieving ultra-sensitive detection of target molecules.
Herein, we connect Cu2O nano-octahedrons with high-density Ag NWs, forming novel nanocomposites (NCs) as high active SERS substrate. In this hybrid nanostructures, adjacent Cu2O are connected by intertwined Ag nanovines (NVs). The obtained nanoproducts are characterized by both improved CM enhancement derived from Ag NV modified the band-gap of Cu2O nano-octahedrons and enhanced EM effect due to additional trapped energetic electron transferred from Ag NVs to semiconductor. The obtained unique Ag NVs/Cu2O heterostructures demonstrate excellent SERS activity, and the detection limit of CV molecules could reach 10−14 M. Furthermore, in order to demonstrate the feasibility of the developed nanosensor, the quantitative analysis of thiram residues in soil and urine were realized, and the limits of detection (LODs) were as low as 0.48 ng g−1 and 10−7 M, respectively. Therefore, it is believed that the obtained appealing nanosensor with excellent SERS activity, anti-interference ability and selectivity holds great potential for ultrasensitive detection of pesticide residues in practical application.
2. Experimental
2.1 Chemicals
Copper chloride (CuCl2), thiram, tricyclazole, benzimidazole and carbaryl were obtained from Aladdin Chemistry Co., Ltd. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), polyvinyl pyrrolidone (PVP, K30) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethanol was obtained from Tianjin Fuyu Fine Chemical Co., Ltd. Ascorbic acid (AA) and crystal violet (CV) were purchased from Macklin Chemistry Co., Ltd. All chemical reagents were of analytical grade and used as received without further purification in all experiments.
2.2 Preparation of octahedral Cu2O and Ag NVs/Cu2O heterostructures
In a typical synthesis process of octahedral Cu2O, 4.5 g PVP was added into 50 mL CuCl2 aqueous solution (0.0 l M). Then, 5 mL NaOH aqueous solution (2.0 M) was added dropwise into the above solution. After stirring for 30 min, an AA aqueous solution (0.6 M, 5 mL) was added dropwise into the dark brown solution. The mixture was aged for 3 h in a 55°C water bath under constant stirring and then cooled down to room temperature. The brick-red precipitates were collected by centrifugation, then washed with deionized (DI) water and ethanol for several times. Finally, the precipitates were dried in vacuum at 60°C for 6 h. In the case of synthesizing Ag NVs/Cu2O heterostructures, 1 mL ethanol and 1 mL AgNO3 solution (0.02 M) were added to the as-prepared Cu2O octahedral colloidal solution (5 mL). The mixed solution was irradiated by a continuous 405 nm laser beam with power of ∼1000 mW. Then, the obtained Ag NVs/Cu2O heterostructures were centrifuged by 5000 rpm for 10 min in an ultracentrifuge. Moreover, the concentration of AgNO3 (5–50 mM) and the excitation wavelength (375 and 532 nm) of the laser were also changed to explore the effect on the synthesis of nanoproducts.
2.3 Preparation of thiram standard solution and soil samples containing thiram
As for the preparation of thiram standard solution, 0.0024 g thiram powder was dissolved in 10 mL H2O to form 10−3 M thiram solution. Then, the original solution was diluted with H2O to prepare thiram standard solutions with different concentrations (10−4 M–10−10 M). Moreover, in order to prepare spiked soil samples, we collected the soil from the garden of Shandong University. Firstly, thiram powder (0.048 g) was added to 100 g soil, ground thoroughly to mix evenly. Then, 1 g thiram-spiked soil sample was added to centrifuge tube containing 2 mL H2O to form 0.48 mg g−1 thiram solution. In this way, a series of soil samples containing thiram solutions with different concentrations were prepared well. Before SERS test, 10 μL supernate was dropped onto the SERS substrates and dried naturally.
2.4 Preparation of urine samples containing thiram
Urine samples were taken from healthy Sprague-Dawley in Shiyanjia Lab. To prepare thiram-spiked urine samples, different concentrations of thiram were added to the urine samples (the ratio of urine to thiram standard solution was 1:1), and mixed thoroughly. Then, 10 μL different concentrations of urine samples containing thiram were dropped on silicon wafers, and then dried in air at room temperature.
2.5 SERS measurements
The SERS measurements were performed on the InVia spectrometers with 633 nm laser at room temperature. The laser power was set at 0.85 mW with exposure time 5 s. The spectra were processed (smoothed and baseline corrected) using Wire 5.4 software (Renishaw). In a typical Raman spectroscopic analysis, the SERS substrates were prepared by dropping the obtained nanoproducts on clean Si wafers and drying naturally at room temperature.
2.6 Material characterization
The morphological structures and chemical compositions of the obtained nanoproducts were performed by focused ion beam electron microscope (FIB, Helios G4 UC) and transmission electron microscopy (TEM, JEOL-JEM-2100F) equipped with an energy dispersive X-ray (EDX) spectroscopy via a scanning transmission electron microscopy (STEM) unit. The crystallographic measurements of the as-prepared products were characterized by X-ray diffraction (XRD) using Cu Kα radiation (λ=0.15406 nm). The absorption spectra were carried out via UV-Vis-NIR spectrometer (UV-1800, Shimadzu) and UV–Vis–NIR microspectrophotometry (CRAIC 20/30 PVTM). The detailed surface composition and element valence analysis were further investigated by X-ray photoelectron spectra (XPS, Thermo ESCALAB 250XI).
3. Results and discussion
3.1 Characterizations of Ag NVs/Cu2O heterostructures
The morphologies of the as-prepared samples were characterized by scanning electron microscopy (SEM) in Fig. 1. Figure 1(a) demonstrates that original Cu2O samples are made up of numerous regular and well-defined octahedral nanostructures with a smooth surface and an edge length of 600 nm. After 405 nm laser irradiation of Cu2O precursor in AgNO3 solution, the morphologies of the nanoproducts were examined by SEM in Figs. 1(b)–1(c). As illustrated in Fig. 1(b), it can be clearly seen that plenty of vine-like Ag nanostructures are interconnected among adjacent octahedral Cu2O, forming a plasma interlaced network, which is beneficial for accelerating electronic transmission. Additionally, some Ag NPs as well as short Ag nanochains are deposited on the surface of Cu2O nano-octahedrons, and the octahedral structure of Cu2O can be well maintained after the overgrowth reaction. The chemical composition of the obtained Ag NVs/Cu2O heterostructures was measured by EDS spectroscopy (Supplement 1, Fig. S1). According to the EDS result, the atomic ratio of Cu/O/Ag can be calculated about 46.8:29.7:23.5, further revealing the successful formation of metal/semiconductor heterostructures in this work. Then, X-ray diffraction (XRD) analysis were used to determine the crystal structures of the obtained nanoproducts. As shown in Fig. 1(d), a series of diffraction peaks with 2θ values of 29.64°, 36.54°, 42.37°, 61.52° and 73.66° correspond to the (110), (111), (200), (220), (311) and (222) crystal planes of Cu2O (JCPDS, no. 35–1091). As for the obtained Ag NVs/Cu2O heterostructures, new diffraction peaks at 38.10°, 44.29° and 64.55° can be assigned to the (111), (200) and (220) planes of face-centered cubic (fcc) silver, indicating the formation of Ag nanostructure in NCs by laser-induced photochemical method.
Fig. 1. (a) SEM image of original octahedral Cu2O. (b) The typical SEM image of Ag NVs/Cu2O heterostructures. (c) The enlarged SEM image of Ag NVs interlinked among Cu2O. (d) XRD patterns of pristine Cu2O and Ag NVs/Cu2O heterostructures.
On the other hand, in order to further investigate the microscopic structure of the obtained nanoproducts, the morphologies were examined by TEM analysis in Figs. 2(a)–2(b). It can be found that numerous short Ag nanochains are deposited on the surface of Cu2O nano-octahedrons, resulting in rugged surface. In addition, the adjacent Cu2O are connected by intertwined vine-like Ag nanostructure, forming plasma-semiconductor interconnected NCs. The inset in Fig. 2(a) depicts the selected area electron diffraction (SAED) pattern of Ag NVs/Cu2O heterostructures, the presence of a ring pattern in SAED confirms its polycrystalline nature. The high resolution transmission electron microscopy (HRTEM) of Ag NVs/Cu2O NCs inset in Fig. 2(b) shows noticeable lattice fringe of 0.237 nm, which could be well indexed as (111) plane of Ag nanocrystal. Moreover, the element distributions of the as-synthesized Ag NVs/Cu2O NCs were determined by energy dispersive X-ray (EDX) technique, as shown in Figs. 2(c)–2(f). The EDX elemental mapping images show that the composites are indeed composed of Cu, O and Ag elements. Clearly, numerous vine-like Ag nanostructures are interconnected between adjacent Cu2O nano-octahedrons, as well as some short Ag nanochains are attached on the surface of octahedral Cu2O, which indicates the successful overgrowth of Ag NVs on precursor.
Fig. 2. (a–b) Different magnified TEM images of the obtained Ag NVs/Cu2O heterostructures, the insets correspond to the SAED pattern and HRTEM image, respectively. (c–f) HAADF-STEM image of Ag NVs/Cu2O and the corresponding elemental mapping images.
Furthermore, the chemical composition and valence state of pristine Cu2O and Ag NVs/Cu2O heterostructures were carried out by X-ray photoelectron spectroscopy (XPS) measurements. As shown in Fig. 3(a), the main peaks of C1s (C1s signal came from organic contamination), Cu2p, O1s and Ag3d can be detected in the XPS pattern of Ag NVs/Cu2O NCs. Then, high-resolution elemental spectra of Cu, O and Ag were examined in Figs. 3(b)–3(d), respectively. Figure 3(b) reveals the high-resolution XPS spectrum of copper, the double peaks of Cu2p located at 932.0 eV and 951.9 eV are assigned to Cu2p3/2 and Cu2p1/2 of Cu2O [28,29]. As compared to original Cu2O, the intensities of the two peaks originated from Ag NVs/Cu2O NCs decrease as the deposition of Ag NVs. Additionally, it can be clearly observed that Cu2p3/2 and Cu2p1/2 binding energies of Ag NVs/Cu2O NCs have positive shifts compared to pristine Cu2O, which can be attributed to the electron interaction between metallic Ag and semiconductor Cu2O. Figure 3(c) shows the O1s region of the XPS spectra, which can be divided into two peaks by using a Gaussian-Lorentzian fitting method. The characteristic line at 530.3 eV (peak 1) is assigned to surface lattice oxygen species, whereas the peak located at 531.7 eV (peak 2) can be ascribed to O adsorbed on the surface of octahedral Cu2O [29–31]. With regards to Ag3d spectra of Ag NPs in Fig. 3(d), two distinct spectra are detected at 367.8 and 373.8 eV, which is attributed to Ag3d5/2 and Ag3d3/2, respectively. Obviously, as for Ag NVs/Cu2O heterostructures, the two peaks of Ag3d shift (0.20 eV) to higher binding energies after the deposition of Ag, attributing to the interaction between Ag and semiconductor Cu2O. And the spin-orbit energy segregation is 6.0 eV, suggesting that the metallic states of Ag° formed in Ag NVs/Cu2O NCs [32].
Fig. 3. XPS spectra of the as-prepared Cu2O and Ag NVs/Cu2O heterostructures. (a) Survey spectra, (b–c) Cu2p and O1s, respectively. (d) Ag3d spectra of Ag NPs and Ag NVs/Cu2O heterostructures.
Then, in order to investigate the optical properties of the as-prepared nanoproducts, the UV-Vis-NIR absorption spectroscopy were performed, as shown in Fig. 4. It is noticed that original Cu2O nano-octahedrons have an absorption band centered at 528 nm, which can be attributed to the intrinsic absorption of semiconductor Cu2O. During the overgrowth process of Ag, a broad absorption band appear, which is mainly due to the LSPR originated from the dipole resonance absorption of Ag nanostructure. The increase of LSPR peak intensity is also an evidence for obtaining higher Ag yield in Ag NVs/Cu2O NCs. The overgrowth process can be further clearly visualized by the change of solution color (from brick red to dark gray). Moreover, it can be observed that the absorption peak of Ag NVs/Cu2O NCs at 518 nm has an obvious blue shift compared to original Cu2O (528 nm), which can be attributed to electron interaction between Cu2O and Ag in heterostructures, the results are consistent with XPS spectra. In addition, since the substrates used in SERS tests were film materials made of the nanoproducts (Cu2O and Ag NVs/Cu2O), the optical properties of Cu2O nano-octahedrons film and Ag NVs/Cu2O film were also investigated to illustrate the corresponding LSPR, as shown in Fig. S2 (Supplement 1). It can be found that Ag NVs/Cu2O film exhibits enhanced absorption capacity in the visible light range compared with the original Cu2O film, which is beneficial for providing higher visible light resonance response. Since the Fermi level of Cu2O (4.84 eV) is higher than that of Ag (4.26 eV), electrons will transfer from Ag to Cu2O until thermodynamic equilibrium is established. The redistribution of charge will induce a stronger electromagnetic field, thus greatly improving the SERS signal [13]. Therefore, the unique structure of Cu2O octahedrons grafted with high-density intertwined plasmonic Ag NVs is expected to exhibit excellent SERS activity.
Fig. 4. UV-Vis-NIR absorption spectra of Ag NVs/Cu2O heterostructures via ranging laser irradiation time (0–45 min).
3.2 Optimal SERS substrate exploration
On the other hand, it is well known that adjusting the metal composition in metal/semiconductor heterostructures is of great significance for optimizing SERS activity. In this work, different metal composition of Ag in Ag NVs/Cu2O hybrid nanostructures could be achieved by changing laser irradiation time, and the corresponding SERS results of CV (10−7 M) molecules adsorbed on different nanosubstrates are shown in Fig. S3 (Supplement 1). It can be revealed that several dominating characteristic bands at 729, 759, 799, 914, 1175, 1297, 1378, 1435, 1585 and 1616 cm−1 originated from CV molecules can be clearly identifiable in SERS spectra, and meanwhile, the optimal SERS signal of Ag NVs/Cu2O heterostructures can be obtained with the irradiation time of 45 min. However, the SERS signal decreases with further prolonging irradiation time to 60 min, which is related to excessive aggregations of large-sized Ag NPs during the overgrowth process. Moreover, the effects of Ag precursor concentration and excitation wavelength on the growth of metal/semiconductor heterostructures were also studied in this paper. Fig. S4 (Supplement 1) shows the SEM images obtained with different concentrations of AgNO3. When the concentration of AgNO3 is 5 mM, numerous Ag NPs are attached on the surface of Cu2O, and some short Ag NVs begin to form. Further increasing AgNO3 concentration to 10 mM, vine-like Ag nanostructures with longer length are formed, so that adjacent Cu2O can be connected through Ag NVs. Additionally, denser Ag NPs and Ag NVs are deposited on Cu2O surface, which makes the originally smooth surface of Cu2O become rough. When the AgNO3 concentration increases to 50 mM, it can be found that the amount of Ag NPs and Ag NVs is significantly reduced. This is caused by strong oxidation ability of high concentration AgNO3, which greatly inhibits the reduction of Ag ions. The corresponding SERS activities are shown in Fig. S5 (Supplement 1), it can be observed that the highest SERS activity can be obtained with the AgNO3 concentration of 0.02 M, which benefits from the unique structure of octahedral Cu2O grafted with intertwined plasmonic Ag NVs. As a result, this plasmonic Ag NVs-linked Cu2O is beneficial for directional electron transport, then induces a stronger electromagnetic field, and greatly improves the Raman intensity. In addition, we also compared the morphologies and corresponding SERS activities of Ag NVs/Cu2O heterostructures generated by different excitation wavelengths (Fig. S6, Supplement 1). It should be noted that the morphology and corresponding SERS activity of Ag NVs/Cu2O heterostructures are different with different excitation wavelengths. When the excitation wavelength is 375 nm, it can be observed that a large number of spherical Ag NPs and short Ag nanochains are deposited on Cu2O surface, causing serious agglomeration. Therefore, the SERS activity is not as high as that excited by 405 nm laser. When the precursor solution is irradiated with 532 nm laser, dense Ag nanosheets and irregular Ag nanochains are formed, the overgrowth of Ag reduces the number of hot spots, thereby decreasing the SERS activity. To sum up, the optimal SERS activity of Ag NVs/Cu2O heterostructures can be achieved with 405 nm continuous laser irradiation for 45 min.
3.3 Sensitivity of Ag NVs/Cu2O heterostructures for the detection of CV
On the basis of the above discussions, the obtained Ag NVs/Cu2O NCs with unique plasma-connected semiconductor structure are expected to possess excellent SERS performance. For comparison, the SERS spectra of 10−8 M CV molecules adsorbed on Ag NVs/Cu2O, Ag NPs/Cu2O as well as Au NPs/Cu2O heterostructures were illustrated in Fig. 5(a). It can be clearly found that the SERS signals originated from Ag NVs/Cu2O heterostructures are much higher than that of Ag NPs/Cu2O and Au NPs/Cu2O heterostructures. For instance, as shown in Fig. 5(b), the characteristic band of CV molecules at 1616 cm−1 is measured ∼22054 a.u in the presence of Ag NVs/Cu2O heterostructures, which is about 2.7 and 7.0 times higher than that of Ag NPs/Cu2O (8211 a.u) and Au NPs/Cu2O (3165 a.u), respectively. In addition, the CM enhancement originated from CT between Cu2O nano-octahedrons and probe molecules was verified in Fig. S7 (Supplement 1). It can be found that the characteristic bands originated from CV molecules can be clearly detected even the concentration is reduced to 10−6 M, which is much better than that of original CV molecules in the absence of any nanosubstrates. Therefore, the enhanced SERS activity should be attributed to the synergistic effect of improved EM enhancement originated from the LSPR effect of plasmonic Ag nanostructure as well as profound CM enhancement derived from efficient CT. In addition, the intertwined Ag NVs among adjacent Cu2O could further improve the efficiency of electron-hole separation, facilitate the directional movement of photo-excited electrons on NVs, and generate stronger electromagnetic field. Therefore, the SERS signals of probe molecules adsorbed on Ag NVs will be further enhanced through a long-range EM enhancement. To sum up, the unique Ag NVs/Cu2O heterostructures realize the combination of both improved EM enhancement and profound CM enhancement, which can be used as an excellent SERS substrate for ultrasensitive detection of target molecules in practical scenes. Then, with the aim to evaluate the SERS sensitivity of Ag NVs/Cu2O heterostructures, we implemented SERS measurement with different concentrations of CV molecules from 10−7–10−14 M, as illustrated in Fig. 5(c). It can be seen that the SERS intensity gradually decreases with the decrease of CV concentration, but with distinguishable characteristic Raman peaks even the concentration decreases to 10−14 M. Moreover, the Raman peak at 1616 cm−1 was selected to further investigate the relationship between SERS intensities and CV concentrations (Fig. 5(d)). It can be clearly illustrated that a well-defined linear relationship was established between SERS signal intensities and CV concentrations (10−10–10−14 M) (Inset in Fig. 5(d)). The above results indicate that the as-prepared Ag NVs/Cu2O NCs possess enhanced SERS activity due to its unique structure, which is suitable for the detection of target molecules in practical applications.
Fig. 5. (a–b) The SERS spectra of 10−8 M CV molecules adsorbed on Au NPs/Cu2O, Ag NPs/Cu2O and Ag NVs/Cu2O heterostructures, respectively. The insets correspond to the SEM images of Au NPs/Cu2O and Ag NPs/Cu2O heterostructures, respectively. (c) SERS spectra of different concentrations of CV molecules adsorbed on Ag NVs/Cu2O heterostructures. (d) The plot of peak intensity at 1616 cm−1 versus the logarithm of CV concentration. Insert: the linear calibration curve of the peak intensities at 1616 cm−1 against logarithm of CV concentrations.
3.4 Quantitative detection of thiram in standard solution
Furthermore, in order to verify the ultrasensitive detection capability of the established nanosensor, thiram molecule was selected to evaluate the feasibility of quantitative SERS measurement. Thiram is often used as a fungicide to prevent many kinds of crop diseases in agriculture. Additionally, thiram is highly toxic to human, it can cause nausea, vomiting, diarrhea and other symptoms [33]. Therefore, ultrasensitive monitoring of thiram residues on diverse samples is of great importance. Fig. S8(a) (Supplement 1) shows the Raman characteristic peaks of pure thiram at 441, 560, 866, 930, 1145, 1382, 1442 and 1510 cm−1, and the band assignments are presented in Table S1 (Supplement 1). It can be seen that the intensities of these characteristic peaks gradually decrease with the concentration of thiram molecules decreases from 10−5 M to 10−10 M, and the characteristic Raman peaks of thiram can still be observed when the concentration is as low as 10−10 M. The detection limit of Ag NVs/Cu2O NCs in this work is superior to many previous studies, as illustrated in Table 1, which also demonstrates the excellent performance of the established nanosensor. Moreover, the intensities of SERS peaks at 1145, 1382 and 1510 cm−1 versus logarithm of thiram concentrations are plotted in Fig. S8(b) (Supplement 1). All these plots of SERS intensities exhibit good linear relationship at a wide concentration range of thiram molecules, demonstrating excellent quantitative analysis ability of SERS based on the obtained Ag NVs/Cu2O nanosensor.
Table 1. Comparison of SERS detection of pure thiram solution based on different substrate.
3.5 Anti-interference ability and selectivity
As is well known, in practical application, it is of vital importance for the SERS-based sensor to immune the interference from other substances that coexist in the analyzing system. To demonstrate the anti-interference ability of Ag NVs/Cu2O SERS sensor, some potential interfering substances were separately added into the system for interference experiment, including metal ions (K+, Na+, Ca2+, Mg2+, 0.01 M) and biological organics (Glucose, urea, sucrose, 0.01 M). As shown in Figs. 6(a)–6(b), it can be seen that the interfering substances have no obvious influence on SERS signals, even though their concentration is much higher than that of thiram, confirming good anti-interference capability of the as-prepared Ag NVs/Cu2O NCs. Furthermore, the selectivity of the developed nanosensor was investigated by adding other three pesticides (tricyclazole, benzimidazole, carbaryl, 10 μM for each) into thiram solution, as illustrated in Figs. 6(c)–6(d). It can be clearly observed that there is no significant change in SERS signal of thiram in the presence of other interfering pesticides. The above results indicate that the developed SERS nanosensor possesses excellent anti-interference capability and selectivity, which is particularly suitable for the detection of thiram molecules in the real-world scenarios.
Fig. 6. (a–b) Effect of interfering reagents on SERS intensity of thiram (10−6 M), with each of their concentrations being 10 mM. (c–d) Selectivity investigation of the obtained nanosensor toward pesticides detection. I0 represents the SERS intensity of control sample, and I is the SERS intensity after the addition of different interfering reagents.
3.6 Quantitative detection of thiram in soil
More importantly, in order to evaluate the potential of the developed Ag NVs/Cu2O nanosensor in real-world applications, the optimal SERS substrate was applied to analysis of thiram residues in soil. Fig. S9 (Supplement 1) shows the SERS spectrum obtained in original soil without adding additional thiram, it is found that no SERS signal can be detected except the silicon peak at 520 cm−1, indicating that the soil is not contaminated by pesticides. Then, the soil samples contaminated by thiram are further analyzed, and the corresponding SERS spectra of thiram molecules with different concentrations (4.8 μg g−1–0.48 ng g−1) are shown in Fig. 7(a). It can be seen that the SERS spectra of thiram in soil exhibit similar characteristic peaks in comparison with standard thiram solution, and the LOD of thiram in soil can be achieved at 0.48 ng g−1. Figure 7(b) further illustrates the calibration curve for SERS intensities at 1145, 1382 and 1510 cm−1 as a function of logarithm of thiram concentration. It is clear that three well-defined linear relationships can be established, suggesting that the as-prepared SERS nanosensor holds great application prospect for quantitative analysis of pesticide residues in the real environment.
Fig. 7. (a) SERS spectra of different concentrations of thiram in soil and (b) the linear calibration curve of the peak intensities at 1145 cm−1, 1382 cm−1 and 1510 cm−1 against logarithm of thiram concentration in soil.
3.7 Quantitative detection of thiram in biological samples
Finally, to further evaluate the practicability of the established SERS-based nanosensor in biological samples, the urine samples taken from Sprague-Dawley were spiked with different concentrations of thiram using standard addition method. As shown in Fig. 8(a), there is no interfering signal in original urine. After adding thiram standard solution to urine, the SERS spectra of thiram with different concentrations are shown in Fig. 8(b). It can be found that the Raman signal of thiram molecules increase with the increase of analyte concentrations ranging from 10−7 M to 10−3 M, and the LOD of Ag NVs/Cu2O heterostructures toward thiram could reach 10−7 M. In addition, it should be noticed that the LOD of thiram in urine is much lower than that of standard thiram solution, which is mainly due to the presence of proteins and other interfering substance in urine. But the LOD of thiram in urine is still lower than many previous works [40–42]. Figure 8(c) shows the linear correlation curves between SERS intensities of peaks and the logarithm of thiram concentrations. Obviously, the intensities of characteristic bands are positively correlated with the logarithm of thiram concentrations. For example, the linear relationship of the peak at 1382 cm−1 can be described by the following linear function: y = 2493.4x+17635 (R2 = 0.9927, where y and x represent the SERS intensity and the logarithm of thiram concentration, respectively). The above results confirm the excellent SERS performance and potential application prospects of the developed Ag NVs/Cu2O heterostructures as SERS active material for quantitative analysis of biological samples.
Fig. 8. (a) SERS spectrum of no thiram molecules added to the original urine. (b) The Raman spectra of thiram molecules in urine with different concentrations ranging from 10−3 M−10−7 M. (c) The linear relationship of SERS intensities at 1145 cm−1, 1382 cm−1 and 1510 cm−1 for thiram in urine against the logarithm of concentration.
4. Conclusion
In summary, a novel SERS active substrate based on the construction of Ag NVs/Cu2O nano-octahedrons has been developed via photochemical reaction. The intertwined Ag NVs among adjacent Cu2O act as excellent channels for electronic transmission, which is beneficial to promote the separation efficiency of electrons and holes, and further improve the enhancement effect of CM. Meanwhile, the directional movement of photo-triggered electrons from plasmonic Ag NVs to semiconductor can generate stronger electromagnetic field, further improving the EM enhancement. Therefore, the unique nanostructures combined with both improved EM enhancement and enhanced CM effect exhibit a remarkable higher SERS activity, which is ∼ 7.0 and 2.7 times higher than that of monodispersed Au or Ag NPs modified Cu2O for the detection of CV molecules. The obtained Ag NVs/Cu2O heterostructures are characterized by excellent SERS sensitivity, anti-interference capability and selectivity, which is particularly suitable for ultrasensitive detection of pesticide residues in real environment. Furthermore, based on the developed nanosensor, the quantitative detections of thiram residues in soil and urine are realized. The results show that the novel SERS nanosensor has strong anti-interference ability and good linear response with low LODs of 0.48 ng g−1 and 10−7 M toward thiram detection, respectively, which is superior to many previous works. Thus, it is hoped that the established nanosensor could also be employed to detect other pesticide residues in the near future.
Funding
National Natural Science Foundation of China (11375108, 11575102, 11775134, 11905115, 12175126); Shandong Jianzhu University XNBS Foundation (1608); Fundamental Research Fund of Shandong University (2018JC022).
Acknowledgments
The authors thank Shiyanjia Lab (www.shiyanjia.com) for XPS analysis and biological sample (urine) extraction.
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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