Detection method for in-situ detection of heavy metals in soil through combination of DGT and portable Raman spectrometer

Abstract

The invention relates to a detection method for in-situ detection of heavy metals in soil by combining DGT and a portable Raman spectrometer, which comprises the following steps: digesting a DGT binding film after heavy metal detection to obtain a solution to be detected; comprising the following steps: removing acid from a to-be-detected solution; comprising the steps of preparing a reinforced substrate; comprising the following steps: mixing a to-be-detected solution with a reinforced substrate; comprising a detection step using a Raman spectrometer. According to the scheme, the preparation method of the DGT binding membrane is simple and easy to implement, the obtained binding membrane material has the characteristics that the uniformity is good, the particle size of binding membrane particles is at the nanometer (nm) level, the adsorption performance on heavy metal ions such as As, Cd and Cr is good, and the like, and a technical support is provided for in-situ extraction of heavy metal. Volatile acid is used in the elution process of the binding membrane, the influence of the acid on SERS detection is eliminated through the pretreatment process, the defect that in-situ detection cannot be achieved due to the fact that the DGT technology needs to be combined with large scientific research facilities is overcome, and unique performance and potential application value are achieved.

Description

Technical Field

[0001] The invention relates to a detection method for in-situ detection of heavy metals in soil by combining DGT with a portable Raman spectrometer, and in particular to a method for in-situ treatment of DGT eluate and in-situ detection. Background Art

[0002] The total amount of heavy metals in soil cannot completely determine their environmental behavior and ecological effects. The environmental behavior of heavy metals in soil is mainly affected by their existing forms and the proportion of forms. Different forms have different mobility, bioavailability and toxicity. Therefore, understanding these forms and their proportions is crucial for predicting the behavior of heavy metals. Cadmium (Cd) is a metal element widely present in nature. The stability of Cd in soil makes it easy to accumulate, leading to the decline of soil quality. It will reduce the fertility of soil, affect the community structure and function of soil microorganisms, and thus destroy the balance of soil ecosystems. And it can be released into the environment through natural processes and human activities. Long-term exposure to high Cd environment poses a serious threat to ecosystems and human health. Cd has various chemical forms. Cd exists in the environment mainly in the form of ions (such as Cd2+), combined with organic matter (such as combined with humic acid), and complexes formed with inorganic matter (such as cadmium sulfide, carbonate, etc.). Given the potential harm of Cd to the environment and health, it is particularly important to develop a tool that can realize in situ dynamic monitoring of Cd content in soil.

[0003] Diffusive gradients in thin-films technique (DGT), based on Fick's first diffusion law, is an innovative in-situ dynamic monitoring method specifically used to assess the effective content of target pollutants (including nutrients, heavy metals, metalloids and radioactive elements) in nature. The DGT device includes: the outermost protective film, which is used to isolate external physical and chemical interference; the middle diffusion membrane, which allows the target molecules to diffuse through at a specific rate according to Fick's law; the internal binding membrane, which is responsible for capturing and stabilizing the diffused target molecules; and the device shell that supports and fixes these membrane layers. The advantage of DGT technology is that it can directly reflect the biologically effective state of the target pollutant in the environmental medium, that is, those parts that have potential effects on organisms, rather than the total concentration. This feature makes DGT irreplaceable in assessing the ecological risks of pollutants and formulating corresponding management strategies. In addition, DGT technology has also demonstrated a wide range of applicability and has been successfully applied to the monitoring of various environmental media such as water, soil, and sediment, providing strong technical support for environmental science research and protection practice.

[0004] The application of DGT technology in two-dimensional high-resolution imaging analysis has become a current research feature. In terms of technical methods, circular DGT devices are deployed in environmental media or directly above microscopic interfaces and recovered after a preset exposure period. Subsequently, a series of high-precision analysis methods, such as two-dimensional slicing-colorimetry, laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), and computer / colorimetric density imaging metrology (CID), etc., are used to analyze the spatial distribution of the target analytes enriched on the binding membrane. However, due to the need to mail samples, the time for the entire detection process will increase significantly compared to in-situ detection locally. Various unforeseen situations may occur during the mailing process, such as weather reasons, transportation failures, etc., resulting in the samples not arriving at the detection laboratory on time. For some detection projects with high timeliness requirements, delays may render the detection results meaningless and prevent timely adoption of effective countermeasures.

[0005] Surface-enhanced Raman spectroscopy is a technique that utilizes the surface plasmon resonance (SPR) of nanostructured metals (such as gold, silver, etc.) to enhance Raman signals. When an excitation light with an appropriate wavelength irradiates these metal nanostructures, a strong electromagnetic field is generated, which is called a "hot spot". If the molecules near this hot spot have Raman activity, then the interaction between these molecules and the electromagnetic field will lead to a significant enhancement of the Raman signal, usually by several orders of magnitude, even up to 10 6 to 10 8 times. Gold nanoparticles have become one of the most commonly used substrate materials in SERS research due to their unique physical and chemical properties, such as good biocompatibility, easy modification and manipulation, etc. The surface plasmon resonance of gold nanoparticles can generate a strong local electromagnetic field within a specific wavelength range, thereby effectively enhancing the Raman signals of nearby molecules. In addition, the small size and large specific surface area of gold nanoparticles are also beneficial to improving the SERS effect. Summary of the Invention

[0006] In order to overcome the deficiencies of existing methods, the present invention provides a detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer. Compared with several commonly used laboratory detection and measurement methods, this method has the advantages of low cost, avoiding sample alteration, conforming to the actual environment, and obtaining detection results quickly, and has broad development potential and application prospects in practical application fields such as heavy metal pollution monitoring and environmental risk assessment.

[0007] The object of the present invention is to achieve the above object, and the technical solution adopted by the present invention is:

[0008] A detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer, comprising the step of digesting the DGT binding membrane after detecting heavy metals to obtain a solution to be measured;

[0009] It includes the step of removing acid from the test solution until the pH of the test solution is 7.0 ± 0.5;

[0010] It includes the step of preparing an enhanced substrate;

[0011] It includes the step of mixing the test solution with the enhanced substrate;

[0012] It includes the detection step using a Raman spectrometer.

[0013] Moreover, the DGT binding membrane digestion method is: putting the DGT binding membrane into 5 ml of 0.5 M nitric acid and soaking for 30 min to obtain the test solution.

[0014] Moreover, the acid removal method is: evaporating the test solution to dryness according to the national standard method, adding ultrapure water and ultrasonically treating, and repeating the operation until the pH of the solution is 7.0 ± 0.5.

[0015] Moreover, the preparation method of the enhanced substrate is: adding dopamine solution, chloroauric acid solution and NaOH solution into water in sequence, preparing a solution and standing; centrifuging to obtain a substrate with upper and lower layers, and removing the supernatant to obtain the enhanced substrate;

[0016] The molar ratio of dopamine: chloroauric acid: NaOH is: 2 - 5: 5 - 10: 3 - 6.

[0017] Moreover, the volume ratio of the test solution to the substrate is 1: 1 - 5, and the mixing time is 1 - 10 minutes.

[0018] Moreover, the laser wavelength of the Raman spectrometer is 785 nm, and the scanning time is 1 - 5 seconds.

[0019] Moreover, the preparation method of the DGT binding membrane is as follows:

[0020] Using strain XZ - 1 to synthesize TiO2 and in - situ precipitate it on the diffusion membrane to obtain a Bio - TiO2 - DGT binding membrane with uniform particle size. The specific steps are: using metatitanic acid as the raw material, carrying out a biosynthesis process with strain XZ - 1. During this process, the pre - prepared diffusion membrane is immersed to make it fully contact with the reaction system. Through the biological action of strain XZ - 1, the conversion of TiO(OH)2 is promoted, and the in - situ precipitation of TiO2 is realized, and then the Bio - TiO2 - DGT binding membrane is prepared; the strain XZ - 1, the strain name is: XZ - 1, the taxonomic name is: Pantoea sp., the preservation number is: CGMCC No.32330, the preservation date is: October 24, 2024, and the preservation unit is: China General Microbiological Culture Collection Center, and the preservation address is: Institute of Microbiology, Chinese Academy of Sciences, No. 3, Beichen West Road, Chaoyang District, Beijing.

[0021] Moreover, the preparation method of the Bio-TiO2-DGT binding membrane is as follows:

[0022] S1. Biosynthesis step: Mix 10 mL of bacterial strain XZ-1 suspension, 3 diffusion membranes, and 500 mL of 0.5 mol / L TiO(OH)2 solution in a constant temperature shaker at 30 °C for 3 h.

[0023] S2. Chemical precipitation: After mixing, rinse 1 - 2 times with deionized water to wash away the bacteria, and then immerse in 100 mL of 0.05 mol / L 2-(N-morpholino)ethanesulfonic acid (MES) buffer solution, which is pre-adjusted to pH 6.7 with approximately 4 mL of 1 mol / L NaOH solution. White precipitate appears after soaking for a certain period of time.

[0024] S3. Change water and preservation: After gently shaking for about 40 minutes to fully develop the gel, take out the gel from the buffer solution, soak it in MQ water for 24 hours, change water 5 - 6 times during this period, and then soak it in 0.01 mol·L -1 NaNO3 and store it in a refrigerator at 4 °C for standby.

[0025] Moreover, the preparation method of the bacterial strain XZ-1 suspension: Take 100 μL of the activated strain XZ-1 and incubate it in a shaker at 30 °C in 50 mL of LB medium for 8 h. Among them, the LB medium is prepared as follows: Dissolve 1 g of tryptophan, 0.5 g of yeast extract, and 1 g of NaCl in 100 mL of deionized water, and adjust the pH of the medium to 7.0 with 1 mol / L NaOH.

[0026] The advantages of the present invention are as follows:

[0027] 1. The detection method of the present invention has high simplicity and easy operation. At the same time, the binding membrane used has significant optimization in terms of uniformity and particle size compared with traditional methods, and can fully meet the requirements of two-dimensional high-resolution analysis technology. After simple pretreatment of the binding membrane used in the present invention, the obtained eluate can be pretreated by acid removal and then enhanced by a Raman enhancement substrate to detect heavy metals in soil in situ.

[0028] 2. The binding membrane of the present invention combines the dual advantages of DGT in-situ extraction and portable Raman spectroscopy. The binding membrane of the present invention uses modified materials for synthesis, ensuring the uniformity of the membrane layer structure and the accuracy of analytical performance. More importantly, the eluate of this binding membrane can obtain a solution that can be detected by portable Raman spectroscopy after simple pretreatment, that is, after the eluate is mixed with a nano-gold substrate and detected by Raman spectroscopy. This characteristic not only effectively simplifies the detection process and ensures the detection effect, but also can continuously detect and monitor environmental changes in real time for a long time. Description of the Drawings

[0029] Figure 1 This is the schematic diagram for the detection of the present invention. Among them, Figure 1 a is the schematic diagram of the DGT device described in the present invention, Figure 1 b is the schematic diagram of the detection method flow for in-situ detection of heavy metals in soil by combining DGT with a portable Raman spectrometer;

[0030] Figure 2 is the ultraviolet spectrum for synthesizing gold nanoparticles;

[0031] Figure 3 is the pH of the eluate after the pretreatment of Cd by the national standard method

[0032] Figure 4 is the Raman enhancement effect diagram of the eluates with different concentrations of cadmium obtained by the pretreatment in Example 5;

[0033] Figure 5 is the Raman spectrum detection effect diagram of Example 6.

[0034] Figure 6 is the Raman spectrum detection effect diagram of Example 7.

[0035] Figure 7 is the Raman spectrum detection effect diagram of Example 8.

[0036] In Table 1 is the retention rate of Cd in the eluate of the binding membrane by different pretreatment methods. Detailed implementation manners

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present application. The following further elaborates on the embodiments of the present invention with reference to the accompanying drawings.

[0038] The present invention relates to a detection method for in-situ detection of heavy metals in soil by combining DGT with a portable Raman spectrometer, covering a DGT device and its preparation method, a pretreatment method for the DGT binding membrane, and a preparation method for the SERS substrate. The DGT device consists of: a heavy metal available state binding membrane made of EDTA-modified material, a diffusion membrane, a nitrocellulose membrane, etc. The method synthesizes a binding membrane using EDTA-modified material and modified agarose as raw materials, and nano-gold as a Raman enhancement substrate. The preparation method of the DGT binding membrane in the present invention is simple and easy to implement. The obtained binding membrane material has good uniformity, the particle size of the binding membrane particles is at the nanometer (nm) level, and it has good adsorption performance for heavy metal ions such as As, Cd, Cr, etc., providing technical support for the in-situ extraction of heavy metals. Surface-enhanced Raman scattering (SERS) is a technique that can significantly increase the intensity of Raman scattering signals. When cadmium is adsorbed onto the surface of rough metals (such as silver, copper, gold, etc.), their Raman scattering intensity increases by 10 6 ~10 7 times. However, the elution process of this binding membrane uses volatile acids, and it is necessary to exclude the influence of acids on SERS detection through a pretreatment process, overcoming the drawback that the DGT technology needs to be combined with large-scale scientific research facilities and cannot be detected in-situ, and has unique properties and potential application values.

[0039] The present invention provides a preparation method for a diffusion membrane for DGT: Prepare a gel solution. Weigh 0.6 g of allyl-modified low-temperature active agarose (public literature: 201400085993, CN103752285A), dissolve it in 95 mL of hot water, and ultrasonically dissolve it until completely dissolved. Weigh 30 g of acrylamide and dissolve it in 75 mL of deionized water. Add the above acrylamide solution to the crosslinking agent solution and ultrasonically mix it evenly to obtain a gel solution. Mix 10 mL of the gel solution ultrasonically with 25 μL of freshly prepared 10% ammonium persulfate and 70 μL of N,N,N`,N`-tetramethylethylenediamine (TEMED). Pump it into the space between two glass plates separated by a spacer of a certain thickness through a peristaltic pump, seal it, and cure it at 42 °C for 1 h to obtain a diffusion membrane. The volume ratio of the gel solution: 10% ammonium persulfate: TEMED is 400:1:2.8.

[0040] The thicknesses of the diffusion membrane and the binding membrane used are set according to requirements. Different thicknesses of the diffusion membrane result in different diffusion coefficients. The thicknesses of the diffusion membrane and the binding membrane used in the following examples are both 0.04 cm, and the corresponding diffusion coefficients of Cd are 0.631×10 -6 cm 2 ·s -1 -1.

[0041] Cut the adsorption film described in the embodiments of the present invention into circular pieces with a diameter of 1.9 cm and place them in a DGT device. The DGT device includes an outermost circular plastic housing with an opening at one end, and the opening is covered with a layer of filter membrane to prevent mechanical damage and biological contamination to the gel. Behind the filter membrane is a diffusion phase, which is used to control the diffusion flux of the measured components. Heavy metal ions reach the adsorption film through the filter membrane and the diffusion phase and are fixed.

[0042] DGT binding membrane digestion method: Put the DGT binding membrane into 5 ml of 0.5 M nitric acid and soak for 30 min to obtain an acidic eluate. Since the acidic solution will damage the portable Raman spectrometer and the national standard method cannot exclude the acid in the eluate, the acidic eluate is evaporated to dryness according to the national standard method and then ultrasonically treated with ultrapure water to obtain an amount of cadmium in the reconstituted aqueous solution equal to that in the DGT binding membrane eluate. This method can not affect the results of DGT binding membrane extraction during the acid removal process.

[0043] The preparation method of the enhanced substrate suitable for surface-enhanced Raman spectroscopy includes the following steps: 50 μL of 4 mM DA solution, 100 μL of 8 mM chloroauric acid solution, and 5 μL of 0.1 M NaOH solution are successively added to 845 μL of water, and formulated into 1 mL of solution and left standing for 90 min. Use a centrifuge to centrifuge for 30 min at 8000 rpm to obtain an upper and lower layered substrate. Use a pipette to take out 990 μL of the supernatant, leaving 10 μL of the substrate (containing about 5 μL of clear liquid and 5 μL of substrate), which can generate a strong local electromagnetic field using surface plasmon resonance and effectively enhance the Raman signal.

[0044] Example 1

[0045] The present invention recommends a DGT of a heavy metal available state binding membrane, including: a ZnAl-EDTA heavy metal available state adsorption membrane (CN107790104 A), a Bio-TiO2-DGT binding membrane, and other common DGT binding membranes. For the recommended detection device, please refer to Figure 1 a Cut the binding membrane into circular pieces and assemble them into a circular DGT device. The assembly steps are as follows: Place the protective film (5), the diffusion film (4), and the binding membrane (3) on the base (1, 2) of the circular DGT device from top to bottom in sequence, and then fasten the DGT device housing (6) and fasten and tighten it.

[0046] The present invention also provides a Bio-TiO2-DGT (can it adsorb cadmium) binding membrane with the best effect. The preparation method is as follows:

[0047] The preparation method of the Bio-TiO2-DGT binding membrane: uses strain XZ-1 to synthesize TiO2 and in-situ precipitate it on the diffusion membrane to obtain a Bio-TiO2-DGT binding membrane with uniform particle size. The specific steps are as follows: Using metatitanic acid as the raw material, through the biosynthesis process with the help of strain XZ-1. During this process, the pre-prepared diffusion membrane is immersed to make it fully contact the reaction system. Through the biological action of strain XZ-1, the transformation of TiO(OH)2 is promoted, realizing the in-situ precipitation of TiO2, and then the Bio-TiO2-DGT binding membrane is prepared. The diffusion membrane is a commonly used diffusion membrane for DGT detection. The strain XZ-1, the strain name is: XZ-1, the taxonomic name is: Pantoea sp., the preservation number is: CGMCC No. 32330, the preservation date is: October 24, 2024, and the preservation unit is: China General Microbiological Culture Collection Center, and the preservation address is: Institute of Microbiology, Chinese Academy of Sciences, No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing.

[0048] The preparation method of the Bio-TiO2-DGT binding membrane is as follows:

[0049] S1. Biosynthesis step: Mix 10 mL of bacterial suspension, 3 diffusion membranes and 500 mL of 0.5 mol / L TiO(OH)2 solution in a constant temperature shaker at 30 °C for 3 h.

[0050] S2. Chemical precipitation: After mixing, rinse 1-2 times with deionized water to wash away the bacteria, and then immerse in 100 mL of 0.05 mol / L 2-(N-morpholino)ethanesulfonic acid (MES) buffer solution, which is pre-adjusted to pH 6.7 with about 4 mL of 1 mol / L NaOH solution. White precipitate appears after soaking for a certain time.

[0051] S3. Change water and preservation: After gently shaking for about 40 minutes to fully develop the gel, take out the gel from the buffer solution, soak it in MQ water for 24 hours, change water 5-6 times during this period, and then soak it in 0.01 mol·L -1 NaNO3 and store it in a 4 °C refrigerator for later use.

[0052] Preparation of bacterial suspension: Take 100 μL of the activated strain XZ-1 and incubate it in a shaker at 30 °C in 50 mL of LB medium for 8 h. Among them, the preparation of LB medium: 1 g of tryptophan, 0.5 g of yeast extract and 1 g of NaCl are dissolved in 100 mL of deionized water, and the pH of the medium is adjusted to 7.0 with 1 mol / L NaOH.

[0053] Example 2

[0054] A nano-gold Raman enhancement substrate suitable for enhancing Cd signals and its preparation method

[0055] Synthesis of gold nanoparticles substrate: 50 μL of 4 mM dopamine (DA) solution, 100 μL of 8 mM chloroauric acid solution and 5 μL of 0.1 M NaOH solution were successively added to 845 μL of water to prepare a 1 mL solution, which was allowed to stand for 90 min. The substrate with upper and lower layers was obtained by centrifuging at 8000 rpm for 30 min using a centrifuge. 990 μL of the supernatant was taken out with a pipette, leaving 10 μL of the substrate (containing about 5 μL of clear liquid and 5 μL of substrate), which can generate a strong local electromagnetic field using surface plasmon resonance and effectively enhance the Raman signal.

[0056] To explore the chemical properties of the gold nanoparticles Raman enhancement substrate, the ultraviolet spectrum (UV) characterization method of the present invention was used to analyze the gold nanoparticles substrate obtained in Example 2. Please refer to Figure 2 , Figure 2 is the ultraviolet spectrum (UV) of the gold nanoparticles substrate, in which a characteristic peak appears at 528 nm, indicating that the obtained material conforms to the characteristics of 44 nm gold nanoparticles.

[0057] Example 3

[0058] Pretreatment of deacidification of the combined membrane eluate

[0059] Prepare a Cd solution with a concentration of 10 mg·L -1 . The total volume of the solution is 5 mL, and the pH is adjusted to 7.0 ± 0.5. The specific operation is as follows: The cut circular Bio-TiO2-DGT combined membranes were respectively placed in 5 mL of 10 mg·L -1 Cd(II) solution, in triplicate. The samples were shaken at 25 °C and 150 rpm on a shaker. After 24 h, the combined membranes were taken out and eluted with 5 mL of 0.5 M nitric acid for 30 min to obtain an acid-washed eluate containing cadmium. The cadmium pretreatment technology mainly includes differential pressure digestion and microwave digestion. According to the microwave digestion method of GB / T42175-2022, it was initially determined that the sample was placed on a hot plate and heated from low temperature to 180 °C until almost dry, and a small amount of water was repeatedly added to drive out all the acid until no more fumes were emitted as the deacidification scheme for removing the DGT eluate. The results are as Figure 3 . After the remaining eluate was titrated back to 5 mL with water, it was repeated 8 times. The eluates in these eight times were diluted 100 times, and the pH results are as Figure 3 . Repeated acid expulsion did not make the eluate neutral.

[0060] Since the acidic environment may affect the accuracy of Raman spectroscopy. On the one hand, the acidic environment may change the chemical structure of the sample, resulting in changes in the Raman spectrum. On the other hand, the acidic environment may damage the optical components of the Raman spectrometer, affecting the performance of the instrument. The eluate sample is liquid and volatile, and it can be poured into a colorless and transparent glass bottle and sealed before testing. This can not only prevent the sample from volatilizing, but also ensure that it is not contaminated by the outside world. However, for surface-enhanced Raman spectroscopy, the sample needs to be mixed with the gold nanoparticle substrate, and the gold nanoparticle substrate is used as the target substance for detection. Considering the cost, it is impossible to pour the gold nanoparticle substrate into a colorless and transparent glass bottle and seal it before testing. Therefore, excluding volatile acids from the eluate through pretreatment is the key point for the combination of DGT technology and surface-enhanced Raman spectroscopy.

[0061] Example 4

[0062] Effect of deacidification pretreatment of combined membrane eluate

[0063] Prepare Cd solutions with different concentrations (10, 50, 100 mg·L -1 ), the total volume of the solution is 5 mL, and the pH is adjusted to 7.0 ± 0.5. The specific operation is as follows: Place the cut circular combined membranes in 5 mL of Cd solutions with different concentrations, in triplicate. After the samples are shaken at 25 °C and 150 rpm for 24 h in a shaker, take out the combined membranes and elute them with 5 mL of 0.5 M nitric acid for 30 min to obtain the acid eluate containing cadmium. Calculate the recovery rates according to different digestion temperatures (120 °C, 150 °C, 180 °C, 200 °C) and different dilution conditions. The results are shown in Table 1. Among them, the evaporation temperature and the concentration of Cd have little effect on the retention rate. The highest recovery rate is obtained when 5 mL of ultrapure water is added under the rehydration condition and ultrasonicated for 1 hour. Moreover, such a pretreatment method will make the pH of the eluate become 6 - 7, meeting the pH requirements of the samples for Raman detection.

[0064] Example 5

[0065] Prepare Cd solutions with different concentrations (100, 200, 500, 1000 μg·L -1), the total volume of the solution is 5 mL, and the pH is adjusted to 7.0 ± 0.5. The cut circular binding membranes are respectively placed in 5 mL of Cd solutions with different concentrations, in triplicate. After the samples are shaken at 25 °C and 150 rpm for 24 h in a shaker, the binding membranes are taken out, eluted with 5 ml of 0.5 M nitric acid for 30 min to obtain an acid eluate containing cadmium. The acid eluate is evaporated to dryness at a digestion temperature of 150 °C, and the eluate is rehydrated by adding 5 ml of ultrapure water and ultrasonically treating for 1 hour. Such a pretreatment method can recover cadmium almost 100% and make the pH of the eluate become 6 - 7, meeting the sample pH requirements for Raman detection. Take 10 μL of the deacidified eluate and mix it with 10 μL of the nanogold substrate from which the supernatant has been removed for 7 min. Take 5 μL of the mixture and place it in the small hole of an aluminum sheet with a small hole having a diameter of 0.05 cm and a depth of 0.1 cm, and start detection at 785 nm, 30% power, and 1 - second integration time. For the detection effect, please refer to Figure 4 , after pretreatment, the peaks of Cd eluates with different concentrations at 1600 cm -1 increase linearly with the increase in concentration.

[0066] Example 6

[0067] The prepared Cd solution (1000 μg·L -1 ), the total volume of the solution is 5 mL, and the pH is adjusted to 7.0 ± 0.5. Pretreat according to Example 4. Take 10 μL of the deacidified eluate and mix it with 10 μL of the nanogold substrate from which the supernatant has been removed for different times (1, 3, 5, 7, 8, 10 min). Take 5 μL of the mixture and place it in the small hole of an aluminum sheet with a small hole having a diameter of 0.05 cm and a depth of 0.1 cm, and start detection at 785 nm, 30% power, and 1 - second integration time. For the detection effect, please refer to Figure 5 , the signal of Cd with different mixing times of cadmium and the enhanced substrate will increase with time from 1 to 7 minutes, which may be because cadmium reacts fully with the enhanced substrate. After 7 minutes, the signal of Cd decreases a little, which is because the density of the nanogold enhanced substrate is relatively large, and too long standing time will cause the aggregation of nanogold, resulting in the decrease of Raman spectral signal. Therefore, the mixing time of 7 min is the optimal method.

[0068] Example 7

[0069] Prepare a Cd solution with a concentration of 1000 μg·L -1 , the total volume of the solution is 5 mL, and the pH is adjusted to 7.0 ± 0.5. Pretreat according to Example 4. Take 10 μL of the deacidified eluate and mix it with the nanogold substrate with different NaOH addition amounts (5, 10, 20, 50 μL) from which the supernatant has been removed for 7 min. Take 5 μL of the mixture and place it in the small hole of an aluminum sheet with a small hole having a diameter of 0.05 cm and a depth of 0.1 cm, and start detection at 785 nm, 30% power, and 1 - second integration time. For the detection effect, please refer toFigure 6 The intensity of the signal decreases with the addition of NaOH. The addition of more than 20 μL of NaOH solution will cause the peak of Cd to disappear. Therefore, 5 μL of NaOH solution is added during the synthesis of nanogold.

[0070] Example 8

[0071] Prepare a Cd solution with a concentration of 1000 μg·L -1 The total volume of the solution is 5 mL, and the pH is adjusted to 7.0 ± 0.5. Pretreatment is carried out according to Example 4. Take 10 μL of the deacidified eluate and mix it with the nanogold substrate for 7 min. The synthesized nanogold substrate will have different enhancement effects at different centrifugation times (15, 20, 25, 30 min). Take 5 μL of the mixture and place it in the small hole of an aluminum sheet with a small hole of 0.05 cm in diameter and 0.1 cm in depth, and start detection at 785 nm, 30% power, and 1 s integration time. For the detection effect, please refer to Figure 7 The intensity of the signal will increase with the increase of the centrifugation time, but the centrifugation time exceeding 30 minutes will cause the aggregation of nanogold. Therefore, the centrifugation time of 30 min is the most suitable centrifugation time for nanogold.

[0072] Table 1

[0073]

[0074] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer, characterized in that: Including the following steps Including the step of digesting the DGT binding membrane after detecting heavy metals to obtain a solution to be measured; Including the step of removing acid from the solution to be measured until the pH of the solution to be measured is 7.0 ± 0.5; Including the step of preparing an enhanced substrate; Including the step of mixing the solution to be measured with the enhanced substrate; Including the detection step using a Raman spectrometer.

2. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 1, wherein: The method for digesting the DGT binding membrane is as follows: Immerse the DGT binding membrane in 5 ml of 0.5 M nitric acid for 30 min to obtain a solution to be measured.

3. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 1, wherein: The method for removing acid is as follows: Evaporate the solution to be measured to dryness according to the national standard method, then add ultrapure water and sonicate, and repeat the operation until the pH of the solution is 7.0 ± 0.

5.

4. The detection method for in-situ detection of heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 1, characterized in that: The method for preparing the enhanced substrate is as follows: Add dopamine solution, chloroauric acid solution and NaOH solution to water in sequence, mix them into a solution and let it stand; Centrifuge to obtain a substrate with upper and lower layers, and remove the supernatant to obtain the enhanced substrate; The molar ratio of dopamine: chloroauric acid: NaOH is: 2-5: 5-10: 3-6.

5. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 1, wherein: The volume ratio of the solution to be measured to the substrate is 1:1-5, and the mixing time is 1-10 minutes.

6. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 1, wherein: The laser wavelength of the Raman spectrometer is 785 nm, and the scanning time is 1-5 seconds.

7. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 1, characterized in that: The preparation method of the DGT binding membrane is as follows: Use strain XZ-1 to synthesize TiO2 and in-situ precipitate it on the diffusion membrane to obtain a Bio-TiO2-DGT binding membrane with uniform particle size. The specific steps are as follows: Using metatitanic acid as the raw material, carry out the biosynthesis process with strain XZ-1. During this process, immerse the pre-prepared diffusion membrane to make it fully contact with the reaction system. Through the biological action of strain XZ-1, promote the transformation of TiO(OH)2 and realize the in-situ precipitation of TiO2, and then prepare the Bio-TiO2-DGT binding membrane; The strain XZ-1, the strain name is: XZ-1, the taxonomic name is: Pantoea sp., the preservation number is: CGMCC No. 32330, the preservation date is: October 24, 2024, and the preservation unit is: China General Microbiological Culture Collection Center, and the preservation address is: Institute of Microbiology, Chinese Academy of Sciences, No. 3, Beichen West Road, Chaoyang District, Beijing.

8. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 7, wherein: The preparation method of the Bio-TiO2-DGT binding membrane is as follows: S1. Biosynthesis step: Mix 10 mL of bacterial strain XZ-1 suspension, 3 diffusion membranes and 500 mL of 0.5 mol / L TiO(OH)2 solution in a constant temperature shaker at 30 °C for 3 h. S2. Chemical precipitation: After mixing, rinse with deionized water 1-2 times to wash away the bacteria, then immerse it in 100 mL of 0.05 mol / L 2-(N-morpholino)ethanesulfonic acid (MES) buffer solution, and the buffer solution is pre-adjusted to pH 6.7 with about 4 mL of 1 mol / L NaOH solution. After soaking for a certain time, a white precipitate appears. S3, Water replacement and storage: After gently shaking for about 40 minutes to fully develop the gel, take the gel out of the buffer solution, soak it in MQ water for 24 hours, change the water 5 - 6 times during this period, and then soak it in 0.01 mol·L -1 NaNO3, and store it in a refrigerator at 4°C for future use.

9. The detection method for in-situ detecting heavy metals in soil by combining DGT with a portable Raman spectrometer according to claim 8, characterized in that: Method for preparing suspension of bacterial strain XZ-1: Take 100 μL of the activated strain XZ-1 and incubate it in a shaker at 30 °C in 50 mL of LB medium for 8 h. Among them, the preparation of LB medium: Dissolve 1 g of tryptophan, 0.5 g of yeast extract and 1 g of NaCl in 100 mL of deionized water, and adjust the pH of the medium to 7.0 with 1 mol / L NaOH.

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