English

• 0.   Introduction

  In recent years, the growing problem of abuse of pesticides is a serious threat to the human health and ecological environment. Therefore, rapid and accurate analysis of pesticide residues in food detection has been developed^[1-3]. The conventional analytic methods for pesticide residues mainly include gas chromatography (GC), high‑performance liquid chromatography (HPLC), chromatography-mass spectrometry, enzyme-linked immunoassay (ELISA), and enzyme inhibition^[4-7]. However, these methods generally need sophisticated instrumentation, time-consuming, and complicated operations, which limit their application for rapid detection.

  Surface-enhanced Raman scattering (SERS) spectroscopy emerging as a non-destructive and sensitive (even at the molecular level) technique can offer the chemical structure and molecular composition fingerprint information^[8-10]. During the past decades, SERS has gained increasing interest in material science because of its tremendous applications in environmental chemistry, medical detection, and food safety^[11-13]. To date, there are mainly two types of enhancement mechanisms including electromagnetic enhancement (EM) and chemical enhancement (CM)^[14-15], where the pronounced SERS effect is generated by local surface plasmon resonance (LSPR) and charge transfer. Generally, LSPR-induced SERS effects largely depend on the geometrical shape of nanomaterials and the gap size between plasmonic nanostructures^[16]. At these "hot spots", several orders of magnitude could amplify the Raman scattering cross-section of adsorbed target molecules^[17]. Therefore, it is desirable and challenging to construct abundant hot spots and improve the adsorption capability of molecules (especially for nonadsorbing organic pollutant molecules) of substrates.

  Among various materials, plasmonic materials with ultrahigh specific surface areas and enrichment ability could provide a great opportunity to rapidly absorb target molecules with weak affinity on metal surfaces and achieve highly effective SERS enhancement and detection^[18-21]. Metal-organic frameworks (MOFs) are expected to emerge as attractive materials with many unique properties, such as high porosity, high specific surface areas, and tunable structure^[22]. MOF-based SERS substrate can show high sensitivity, excellent selectivity, and outstanding stability^[23-26]. Accordingly, MOF-based hybrids provide an opportunity to improve the signal enhancement ability.

  MOFs play crucial roles in MOF-derived metal SERS substrate. (ⅰ) A single-component MOF matrix could be used as an SERS base. Sun et al. demonstrated that the SERS enhancement factors (EFs) of MOF substrate with high tailorability could be greatly improved to 10⁶ and the detection limit is as low as 10^-8 mol·L^{-1 [22]}. (ⅱ) The MOF shell can avoid direct contact between the plasmonic nanoparticles (PNPs) and the external environment, thus improving the stability of the substrate and extending the service life. Sun et al. constructed a highly sensitivity SERS sensor based on magnetic MOF. The Raman signal intensity of the substrate material was stable after 22 d of storage at room temperature^[27]. (ⅲ) By employing the most convenient "seed growth method", Au@MIL-100, Au@MIL-101, Au@Cu₃(BTC)₂, PNPs@ZIF-8, and Au@AgNRs@ZIF-8 substrates have been successfully prepared for various SERS applications^[28-32]. However, the easy in situ methods make it difficult to obtain uniform PNPs with reproductive signals^[33].

  In this work, we have designed a SERS-active substrate by electrostatic self-assembly strategy without complicated surface modification. The composite particles can maintain a stable colloidal form in a water environment. The conjunction of MOF and the highly electromagnetic response of PNPs endow substrates with significantly enhanced Raman sensitivity, high reproducibility, and exceptional molecule harvesting capability. The resultant MIL-101/Au composites reveal sensitive SERS performance with a Rhodamine 6G (R6G) detection limit down to 10^-10 mol·L^-1. Only simple pretreatment is required, and the residue signal can be efficiently obtained even at trace concentrations. Of particular note, the MOF-based substrate could be a dual-functional SERS platform for the simultaneous capture and sensitive detection of pesticide residues.

  1.   Experiment

  1.1   Materials

  Chromium (Ⅲ) nitrate nonahydrate (Cr(NO₃)₃·9H₂O), terephthalic acid, and tetramethylthiuram disulfide (TMTD) were purchased from Aladdin. Sodium borohydride (NaBH₄), sodium citrate, and N, N-dimethylformamide (DMF) were obtained from Tianjin Damao Chemical Reagent. Hydrogen tetrachloroaurate(Ⅲ) trihydrate (HAuCl₄·3H₂O, 99.9%) was purchased from Macklin. Ultrapure water (Millipore, 18.2 MΩ·cm) was used in all experiments.

  1.2   Synthesis of MIL-101

  MIL-101 NPs were synthesized by a typical method without modification. Specifically, 1 g Cr(NO₃)₃·9H₂O and 0.9 g terephthalic acid were introduced into 15 mL deionized water. After 30 min of ultrasonication, the mixture was transferred to a 50 mL polytetrafluoroethylene lining. Then 90 μL HF was added into the mixture. After reaction for 8 h under 220 ℃, the product was collected by centrifugation, washed with ethanol, and dried in a vacuum under 150 ℃. The MIL-101 powder was dispersed in deionized water to obtain 1 mg·mL^-1 MIL-101 dispersion liquid.

  1.3   Synthesis of MIL-101/Au

  The MIL-101/Au NPs were synthesized in a two-step process. The Au NPs were synthesized using a seed-growth method. Briefly, ice-cold NaBH₄ solution (0.1 mol·L^-1, 0.6 mL) was added into an aqueous mixture of HAuCl₄ (5.0×10^-4 mol·L^-1, 10 mL) and sodium citrate (5.0×10^-4 mol·L^-1, 10 mL), followed by rapid stirring for 2 min. The seed solution was kept undisturbed for about 1 h at room temperature. Next, 600 μL Au seed dispersion was added into the growth solution containing HAuCl₄ (5.0×10^-4 mol·L^-1, 50 mL) and sodium citrate (5.0×10^-4 mol·L^-1, 50 mL), respectively, to obtain Au NPs. The resulting solution was gently stirred for 30 min under 80 ℃ and then left undisturbed overnight. After centrifugation and washing with water twice, the Au NPs were re-dispersed in 20 mL water. MIL-101/Au NPs were prepared by mixing different volume ratios of MIL-101 dispersion liquid and Au NPs dispersion liquid solution (1∶1, 1∶2, 1∶3). After gentle vibration for 120 s, the mixture was undisturbed at room temperature for 1 h.

  1.4   Characterization

  The morphologies and structures of the prepared MIL-101/Au NPs were observed by field emission scanning electron microscopy (SEM, Hitachi SU-8010) at an accelerating voltage of 10 kV and by transmission electron microscopy (TEM, JEOL-2100F) at an accelerating voltage of 200 kV. The crystal structure of the synthesized NPs was analyzed by X‑ray diffraction (XRD, Haoyuan 2700B) equipped with Cu Kα (λ=0.154 6 nm), and the diffraction patterns were collected on an X-ray diffractometer in the range of 5°-70° with a scanning rate of 2 (°)·min^-1 at 40 V voltage and 40 mV current. Fourier transform infrared spectra (FTIR) were tested by a Nicolet iS50 spectrometer (Thermo Scientific). The ζ potential of different samples was investigated using an ζ potential analyzer (Zeta sizer NANO, ZS90). The Raman spectra were acquired from a confocal microscopy Raman spectrometer (Renishaw inVia) using a 532 nm laser as the excitation source.

  1.5   SERS measurements

  The prepared MIL-101/Au aqueous suspensions (0.2 mL) were injected into 0.8 mL R6G aqueous solutions with various concentrations (10^-6, 10^-7, 10^-8, 10^-9, 10^-10 mol·L^-1) and stirred for several minutes. Then 0.8 mL of supernatant was removed by centrifugation (4 000 r·min^-1). The NPs were re-dispersed by ultrasound for 30 s. The mixed solutions were spin-coated at 500 r·min^-1 on the silicon wafer and dried under ambient conditions for SRES measurement. The acquisition time was 10 s, and a 532 nm laser with 5 mW power was used as the excitation source.

  1.6   Recycle test of MIL-101/Au substrate

  After the first recording of the SERS signal according to the above method, the substrate was detached from the wafer by sonication and then washed at reflux in 5 mL of ethanol solution for 20 min. After separating the MIL-101/Au particles from the ethanol, they were re-dispersed in 0.2 mL ultrapure water. The washed MIL-101/Au was again mixed with 10^-6 mol·L^-1 R6G solution (0.8 mL) for the next detection to obtain the SERS signal. This detection-washing procedure was repeated to test the recyclability of the MIL-101/Au substrate.

  2.   Results and discussions

  2.1   Fabrication and characterization of MIL-101/Au

  The fabrication process of MIL-101/Au is illustrated in Fig. 1a. Firstly, MIL-101 NPs were synthesized via a hydrothermal method, while Au NPs were prepared using the seed-mediated growth method. The ζ potential on the surface of MIL-101 was measured using a nano-size analyzer, and the potential of MIL-101 in aqueous solution was about 29.7 mV, implying that MIL-101 is surrounded by a layer of positive charge. Meanwhile, due to the surface of Au NPs being modified by citrate anion, Au NPs are negatively charged. This is also evidenced by the value (-31.2 mV) from the ζ potential measurement. Then, the two particles were physically combined through electrostatic attraction, and Au NPs were anchored on the MIL-101 surface.

  Figure 1

  

  Figure 1.  Fabrication process of MIL-101/Au (a); SEM images of MIL-101 (b) and MIL-101/Au (c); TEM images of MIL-101 (d) and MIL-101/Au (e); FTIR spectra (f) and XRD patterns (g) of MIL-101 and MIL-101/Au

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  As seen in Fig. 1b and 1d, the MIL-101 with the uniform size is approximately 300 nm, the particle size distribution is shown in Fig.S1 (Supporting information). Meanwhile, as shown in Fig. 1c and 1e, AuNPs are evenly distributed on the MIL-101 surface. The as-prepared substrate was further characterized using FTIR and XRD. As shown in Fig. 1f, for MIL-101 and MIL-101/Au samples, the two kinds of NPs show the same infrared absorption curve apart from the distinct difference in intensity. This indicates that the combination of MIL-101 and Au does not involve chemical interaction, and the weakening in the combined peak strength is attributed to the reduction of the relative content of MIL-101. The XRD patterns of MIL-101 and MIL-101/Au are shown in Fig. 1g. Diffraction peaks from 5° to 20° are attributed to MIL-101, while the weak peaks of MIL-101/Au at a small angle can be observed. Moreover, diffraction peaks of MIL-101/Au at 38.2°, 44.4°, and 64.6° are attributed to Au(111), (200), and (220), respectively. The formation of tiny peaks at small angles is the weakening effect caused by the high‑intensity Au peak. All the characterization methods above indicate the successful formation of Au-decorated MIL-101 NPs.

  2.2   SERS performance of MIL-101/Au

  It is well known that the plasma coupling effect of metal NPs depends largely on distribution density^[34]. When two or more nanostructures approach each other (< 10 nm), the electric field between the nanostructures could be amplified greatly, generating a prominent "hot spots" effect that enhances the Raman signal. In this paper, the SERS performance of the composite substrate was optimized by changing the density of Au NPs loaded on the surface of MIL-101. Fig. 2a-2c shows the morphologies of MIL-101/Au NPs compounded with volume ratios of 1∶1, 1∶2, and 1∶3, respectively. When the added concentration of Au particles increases to a certain extent (1∶2, 1∶3), there is no significant difference in Au density on MIL-101.

  Figure 2

  

  Figure 2.  SEM images of the composite substrates of MIL-101/Au with volume ratios of 1∶1 (a), 1∶2 (b), and 1∶3 (c)

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  To evaluate the SERS properties of MIL-101/Au, R6G was utilized as a Raman probe molecule. Fig. 3a shows the SERS spectra of 10^-6 mol·L^-1 R6G on the enhanced substrates prepared with three different covering densities of particles. Fig. 3b shows the Raman intensity of peaks at 1 506 cm^-1 in Fig. 3a. After the volume ratio of MIL-101 dispersion liquid and HAuCl₄ solution increased from 1∶1 to 1∶2, the Raman signal was significantly enhanced. However, when the amount of added Au continued to increase, the enhancement of the Raman signal was almost unchanged. Theoretically, increasing the density of Au NPs is beneficial for creating hot spots. However, for electrostatic interactions, potential balance and load saturation are limited. Thermogravimetric (TG) tests were performed to prove the amount of Au particles in the composite particles. It could be known from Fig. 3c that the carbon residue of pure MIL-101 at 700 ℃ is 31.4%. Au NPs remain stable at 700 ℃ and their mass does not change. Thus, the amount of carbon residue increased with the increase of Au particle loading. Moreover, when the load of Au particles reaches saturation turntable, the residual carbon value no longer increases. So the particles of MIL-101 and Au in volume ratio at 1∶2 were used as SERS substrates for subsequent performance tests.

  Figure 3

  

  Figure 3.  SERS spectra of 10^-6 mol·L^-1 R6G molecules at different volume ratios of MIL-101 dispersion liquid and HAuCl 4 solution as SERS substrates (a) and the corresponding Raman intensity at 1 506 cm^-1 (b); TG curves of three composite substrates and pure MIL-101 (c)

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  To explore the Raman enhancement mechanism of the substrate, 10^-6 mol·L^-1 R6G molecules on pure silicon, MIL-101, and MIL-101/Au composite particles were tested. Results are shown in Fig.S2, there was almost no Raman signal of R6G on the pure silicon wafer. For MIL-101, only characteristic peaks attributed to the base itself can be observed, although its high specific surface and enrichment properties, signal peaks of R6G molecule enrichment were hardly observed. Furthermore, the signal of R6G is significantly enhanced after MIL-101 is combined with Au, which combines the characteristics of MOF material and the LSPR of precious metal particles.

  The adsorption capability of MIL-101/Au composite substrates is also crucial for the enhancement of SERS signals. As shown in Fig. 4a and 4b. With the increase of adsorption time, the characteristic signal of R6G was gradually enhanced. After 40 min of adsorption, there was no more significant improvement in signal intensity, implying that the substrate reached adsorption saturation for R6G within 40 min.

  Figure 4

  

  Figure 4.  SERS spectra of 10^-6 mol·L^-1 R6G molecules with MIL-101/Au at different adsorption times (a) and the corresponding Raman intensity at 1 506 cm^-1 (b)

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  To explore the sensitivity of SERS sensing, R6G molecules with different concentrations were selected as target molecules to evaluate the limit of detection (LOD) of prepared substrates. As shown in Fig. 5a, when the concentration of R6G was as low as 10^-10 mol·L^-1, a weak SERS signal could still be observed. The combination of MIL-101 enrichment and LSPR enhancement of Au has a strong synergistic effect for trace detection. Meanwhile, the Raman peak strength and the logarithm of the concentration at the range of 10^-5-10^-10 mol·L^-1 follow a good linear dependence. Such linear dependence enables SERS measurements to be used for quantitative analysis of low-concentration molecules.

  Figure 5

  

  Figure 5.  SERS spectra of R6G with different concentrations on the MIL-101/Au substrates (a) and calibration plot based on Raman intensity at 1 506 cm^-1 (b)

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  Apart from sensitivity, signal reproducibility, which is largely dependent on the uniformity of hotspot distribution, is another prerequisite for enhancing SERS performance and practical application. As shown in Fig. 6a, the Raman spectra of R6G with a concentration of 10^-6 mol·L^-1 from 20 points on the substrate were irregularly selected to evaluate the uniformity of SERS activity of MIL‑101/Au substrates. Fig. 6b reveals that the signal intensity variations are within 7% at the characteristic peak of 1 506 cm^-1, indicating excellent uniform spatial distribution of hot spots and outstanding signal reproducibility.

  Figure 6

  

  Figure 6.  SERS spectra of 10^-6 mol·L^-1 R6G collected from 20 different points on the MIL-101/Au substrates (a) and the corresponding Raman intensity distribution at 1 506 cm^-1 (b)

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  The recyclability of SERS substrates is an effective way to reduce substrate costs and avoid secondary contamination. The recycling of MIL‑101/Au substrates was also investigated in this paper. It can be seen from Fig. 7 that MIL-101/Au substrates can be reused five times without significant attenuation of Raman strength, and there was no obvious signal after the substrate was washed during each cycle.

  Figure 7

  

  Figure 7.  SERS spectra of 10^-6 mol·L^-1 R6G with MIL-101/Au obtained during 1-5 detection-washing cycles

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  To test the stability of synthetic MIL-101-based substrates, time-stable Raman measurements were performed by comparing the signal intensity of 10^-6 mol·L^-1 R6G on freshly prepared substrates and exposure to ambient conditions for 0, 5, 15, and 30 d. As time proceeded, the SERS signal intensity on the MIL-101/Au substrate continued to decay, with a weakening trend after 15 d, and retaining 79% of the initial signal intensity after 30 d (Fig.S3). This may be due to the collapse of part of the MOF structure in the environment, leading to a decrease in adsorption capacity. This phenomenon also indicates the relatively good stability of the substrate.

  2.3   Determination of thiram in acetone

  TMTD is a kind of insecticide widely used in crops. The bioactive site of this kind of compound is the dithiocarbamate group, which can interact with enzymes containing sulfhydryl group (—SH) and coenzymes of fungus cells. It can penetrate human skin and mucous membranes, wherein the derivative has the possibility of causing cancer and human deformity. To investigate the MIL-101/Au-based SERS substrates for practical applications, various amounts of TMTD were spiked into acetone and the Raman spectra are displayed in Fig. 8. The lowest directly detectable concentration of thiram in acetone on the MIL-101/Au-based SERS substrates waw as low as 10^-10 mol·L^-1, further suggesting that this SERS substrate has a wide application prospect in the agricultural products detection field.

  Figure 8

  

  Figure 8.  SERS spectra of thiram with different concentrations on the MIL-101/Au substrates (a) and calibration plot based on Raman intensity at 1 378 cm^-1 (b)

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  3.   Conclusions

  In summary, a new SERS sensor composed of MOFs and Au NPs has been successfully fabricated via an eco-friendly assembly method. The Au NPs are loaded on the surface of MIL-101 at a suitable density via electrostatic effects to generate intense "hot spots" field. The nanomaterial composed of MOFs can concentrate trace target molecules into "hot spots" regions originating from Au NPs. Furthermore, the uniform distribution of Au NPs on the surface of MIL-101 can optimize the uniformity of the substrate. Ultimately, the fabricated MIL-101/Au composites were used as SERS sensors to detect trace TMTD in acetone. The results indicate that the LOD of this substrate can be as low as 10^-10 mol·L^-1 and the linear concentration range is from 10^-5 to 10^-10 mol·L^-1 with a linearly dependent coefficient R²=0.993. This work provides a promising strategy to prepare SERS substrates and has great utilization potential in the fields of food and agricultural safety. Meanwhile, there is a complicated process of residual probe molecules handling during repeated detection for the substrate. Therefore, intervening particles with a photocatalytic effect can be considered to realize a convenient light reaction to remove the probe molecules.

  
  Acknowledgments: This work was supported by the Natural Science Foundation in Shandong Province of China (Grant No.ZR2022MB141) and the National Natural Science Foundation of China (Grant No.51701186). Supporting information is available at http://www.wjhxxb.cn Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Fabrication process of MIL-101/Au (a); SEM images of MIL-101 (b) and MIL-101/Au (c); TEM images of MIL-101 (d) and MIL-101/Au (e); FTIR spectra (f) and XRD patterns (g) of MIL-101 and MIL-101/Au

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  Figure 2  SEM images of the composite substrates of MIL-101/Au with volume ratios of 1∶1 (a), 1∶2 (b), and 1∶3 (c)

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  Figure 3  SERS spectra of 10^-6 mol·L^-1 R6G molecules at different volume ratios of MIL-101 dispersion liquid and HAuCl 4 solution as SERS substrates (a) and the corresponding Raman intensity at 1 506 cm^-1 (b); TG curves of three composite substrates and pure MIL-101 (c)

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  Figure 4  SERS spectra of 10^-6 mol·L^-1 R6G molecules with MIL-101/Au at different adsorption times (a) and the corresponding Raman intensity at 1 506 cm^-1 (b)

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  Figure 5  SERS spectra of R6G with different concentrations on the MIL-101/Au substrates (a) and calibration plot based on Raman intensity at 1 506 cm^-1 (b)

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  Figure 6  SERS spectra of 10^-6 mol·L^-1 R6G collected from 20 different points on the MIL-101/Au substrates (a) and the corresponding Raman intensity distribution at 1 506 cm^-1 (b)

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  Figure 7  SERS spectra of 10^-6 mol·L^-1 R6G with MIL-101/Au obtained during 1-5 detection-washing cycles

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  Figure 8  SERS spectra of thiram with different concentrations on the MIL-101/Au substrates (a) and calibration plot based on Raman intensity at 1 378 cm^-1 (b)

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