In situ analysis of arsenic speciation based on surface enhanced Raman and gradient diffusion thin films

Abstract

The present invention relates to a method for in-situ analysis of arsenic forms based on surface enhanced Raman and gradient diffusion film. The present invention adopts a slow in-situ growth method to load silver nanoparticles on a polyacrylamide hydrogel film to prepare a DGT fixed film, and forms a complete DGT device with an agar diffusion gel and a filter membrane to monitor arsenic As(III) and As(V) in different valence states in the environment. The DGT fixed film of the present invention has the ability to adsorb arsenic in a complex matrix to the surface and enhance the Raman signal of arsenic molecules, thereby achieving qualitative and quantitative analysis of trace arsenic in water bodies and sediments, wherein the detection limit As(III) is 5.0μg / L, and As(V) is 2.4μg / L. Through the combination of DGT-SERS, the unification of arsenic in-situ passive sampling and on-site rapid analysis is achieved.

Description

Technical Field

[0001] The invention relates to a method for in-situ analysis of arsenic forms based on surface enhanced Raman and gradient diffusion film, and belongs to the field of environmental analysis. Technical Background

[0002] Arsenic is a carcinogen. Arsenic pollution is widespread in my country's water bodies. Drinking and contacting arsenic-containing water bodies affect the health of residents. It is crucial to establish a method for rapid on-site screening of arsenic content. Arsenic exists in two forms: trivalent As(III) and pentavalent As(V). The toxicity of As(III) is much greater than that of As(V), so it is necessary to conduct qualitative and quantitative detection of the forms of arsenic.

[0003] Existing arsenic detection methods include colorimetry, X-ray fluorescence, voltammetry, atomic absorption spectrometry (AAS), atomic emission spectrometry (AES), atomic fluorescence spectrometry (AFS), and inductively coupled plasma mass spectrometry (ICP-MS). However, traditional methods will inevitably cause certain physical and chemical changes in samples during storage, resulting in the collected samples not being able to truly reflect the actual existing form. For example, in water samples, the kinetic equilibrium between particles, colloids, and solutes is likely to be destroyed during storage, resulting in changes in the form and concentration of arsenic, causing researchers to make incorrect judgments on the health risks of water bodies / sediments and the geochemical behavior of arsenic.

[0004] Gradient diffusion thin film technology (DGT) is a passive sampling technology that can more realistically reflect the effective state of pollutants in water in situ. The DGT measurement device consists of a filter membrane, a diffusion membrane and a fixed membrane ( Figure 1 ), where the diffusion membrane is a hydrogel film with a high water content, and the fixed membrane is mostly a hydrogel / resin material with adsorption capacity. Ions diffuse through the diffusion membrane and are then adsorbed by the fixed membrane, forming a linear gradient distribution on the diffusion membrane. The ion concentration near the fixed membrane is maintained at zero, and the concentration (C DGT ):

[0005]

[0006] Where: t is the placement time of the DGT device, s; M is the accumulation of the target ion on the adsorption membrane during the placement time of the DGT device, ng; A is the exposure window area of ​​the DGT device, cm 2 ; Δg is the diffusion membrane thickness, cm; D is the diffusion coefficient of the target ion in the diffusion layer, cm 2 ·s -1 , C DGT is the concentration at the interface between the diffusion layer and the ambient medium, μg L -1 .

[0007] However, DGT technology usually transports samples fixed to the adsorption film to the laboratory at low temperature, elutes with concentrated acid, and then uses mass spectrometry or atomic absorption / fluorescence spectroscopy for determination. It is expensive, time-consuming and labor-intensive to determine; and the instruments used are large and it is impossible to obtain results online on site. These shortcomings limit the application of DGT technology in on-site rapid testing, environmental management, and immediate decision-making.

[0008] In order to overcome the drawback that traditional analytical methods cannot be used to obtain results on-site after DGT sampling, domestic and international efforts are being made to develop new DGT combined with portable and fast detection technologies, such as DGT combined with colorimetry, portable X-ray fluorescence and fluorescence spectrometers. However, the current combined methods are still not sensitive enough or cannot perform multi-morphological analysis of the same element.

[0009] Surface enhanced Raman spectroscopy (SERS) is a portable molecular spectroscopy technology suitable for on-site analysis. It has the advantages of high sensitivity and fingerprint recognition, and is suitable for the detection of trace pollutants. However, SERS technology has inherent disadvantages of substrate contamination and poor repeatability when detecting complex medium samples. DGT technology has the advantage of resisting medium interference due to the barrier of the diffusion membrane.

[0010] If surface enhanced Raman spectroscopy (SERS) is combined with DGT technology, the advantages of the two technologies can complement each other. However, there are currently technical difficulties that prevent the two from being combined. Traditional DGT technology fixed membranes are mostly hydrogel / resin materials with adsorption capacity, which cannot be directly used for SERS without Raman effect, and the current SERS substrate cannot in situ adsorb arsenic in the environment. Therefore, surface enhanced Raman spectroscopy (SERS) cannot be combined with DGT technology. There is no precedent for combining DGT and SERS technology at home and abroad. Summary of the invention

[0011] In view of the shortcomings of the prior art, the present invention provides a method for in situ analysis of arsenic morphology based on surface enhanced Raman and gradient diffusion film. First, the present invention adopts an in situ growth method to in situ grow silver nanoparticles on a hydrogel skeleton to obtain a DGT fixed film. The DGT fixed film can adsorb target pollutants in situ and can also serve as a surface enhanced Raman substrate to enhance the Raman signal. The surface enhanced Raman and gradient diffusion film technologies are successfully combined to meet the in situ sampling of trivalent and pentavalent arsenic and the in situ analysis of morphology.

[0012] The present invention is achieved through the following technical solutions:

[0013] A method for in-situ analysis of arsenic speciation based on surface enhanced Raman and gradient diffusion thin film, comprising the following steps:

[0014] 1) The DGT fixed membrane, diffusion membrane, and filter membrane are pressed into the base with caps in sequence to assemble into a piston-type DGT device;

[0015] 2) Immerse the DGT device in step 1) into the test environment to enrich the target for 4 to 8 hours;

[0016] 3) After the enrichment of the DGT device, tear off the filter membrane to expose the diffusion membrane, use a portable Raman spectrometer or laser confocal Raman spectroscopy to focus the laser spot on the DGT fixed membrane under the diffusion membrane to obtain a SERS spectrum and perform in situ analysis of arsenic forms.

[0017] Preferably, according to the present invention, in step 1), the DGT fixed membrane is prepared by the following method:

[0018] (1) mixing acrylamide:N,N′-methylenebisacrylamide solution, ammonium persulfate solution and tetramethylethylenediamine (TEMED) and pouring the mixture onto a mold to obtain a hydrogel sheet;

[0019] (2) Add KOH solution to AgNO3 solution and stir magnetically to form a fine brown precipitate. Then slowly add NH3·H2O to make the solution gradually clear. Add AgNO3 solution and NH3·H2O again, and the solution turns brown and clear. Finally, add methanol, Na5P3O 10 solution and Na2HPO4 solution to obtain a precursor solution, immediately immerse the hydrogel sheet in the precursor solution, then add glucose solution, oscillate and react to obtain a DGT fixed film.

[0020] Preferably, according to the present invention, in step (1), the mass concentration of the acrylamide:N,N′-methylenebisacrylamide solution is 25-35%, and the mass concentration of the ammonium persulfate solution is 5-15%.

[0021] Preferably, in step (1), the volume ratio of acrylamide:N,N′-methylenebisacrylamide solution to ammonium persulfate solution is 1000:(5-10), and the volume ratio of ammonium persulfate solution to tetramethylethylenediamine (TEMED) is (5-10):(1-5).

[0022] Preferably according to the present invention, in step (2), the mass concentration of KOH solution is 3-4%, the mass concentration of AgNO3 solution added for the first time is 1-5%, the mass concentration of NH3·H2O added for the first time is 28-32%, the volume ratio of KOH solution to AgNO3 solution is (1-5):(1500-2000), and the volume ratio of AgNO3 solution to NH3·H2O is (1500-2000):(40-60).

[0023] Preferably, according to the present invention, in step (2), the mass concentration of the AgNO3 solution added again is 4-10%, the mass concentration of the NH3·H2O added again is 3-8%, the volume ratio of the KOH solution to the AgNO3 solution is (1-5): (80-120), and the volume ratio of the AgNO3 solution to the NH3·H2O is (80-120): (10-30).

[0024] According to the preferred embodiment of the present invention, in step (2), Na5P3O 10 The mass concentration of the solution is 5-15%, the mass concentration of the Na2HPO4·12H2O solution is 3-8%, and the mass concentration of the Na5P3O 10 The volume ratio of the solution to the AgNO3 solution with a mass concentration of 1-5% is (0.5-2.0): (1500-2000), and the volume ratio of the Na2HPO4·12H2O solution to the AgNO3 solution with a mass concentration of 1-5% is (10-30): (1500-2000).

[0025] Preferably, according to the present invention, in step (2), the volume ratio of methanol to Na2HPO4 solution is (20-40): (1-5).

[0026] Preferably, according to the present invention, in step (2), the mass concentration of the glucose solution is 30-40%, and the volume ratio of the glucose solution to the AgNO3 solution with a mass concentration of 1-5% is (3-8):(15-20).

[0027] Preferably, in step (2), the oscillation reaction is carried out at 25-35° C. for 0.5 to 4 h, and the oscillation rate is 100-200 r / min.

[0028] The DGT fixed film prepared by the present invention has a thickness of 0.4 to 1 mm and an arbitrary shape; the area is 15 to 50 cm 2 ; The polyacrylamide content is 8-30%.

[0029] In situ analysis of arsenic speciation methods:

[0030] Preferably according to the present invention, in step 2), the environment to be tested is a water body.

[0031] Preferably according to the present invention, in step 3), the surface enhanced Raman laser wavelength is 780-790 nm, the power is 1-5 mW, and the integration time is 2-6 s.

[0032] In the preparation process of the DGT fixed film of the present invention, a precursor solution is first prepared, and then a hydrogel sheet is immersed in the precursor solution, and a glucose solution is added. Since the reaction process is relatively slow and the pore size on the hydrogel sheet is relatively small, less than 5nm, it is conducive to the formation of a silver film on the surface of the hydrogel sheet. Therefore, a part of the silver ions forms a dense silver film on the surface of the hydrogel sheet, and then another part of the silver ions slowly and evenly grow in situ on the dense silver film to generate silver nanoparticles, so that the obtained silver nanoparticles have uniform particle size and high loading amount, Na2HPO4 plays a reducing role, P3O 10 5- Arsenic can be adsorbed, and the DGT fixed film obtained by the invention can be used as a surface enhanced Raman substrate with a strong Raman signal.

[0033] In summary, the present invention grows silver nanoparticles in situ on the hydrogel skeleton, and the obtained DGT fixed film successfully combines surface enhanced Raman and gradient diffusion thin film technology. The DGT fixed film of the present invention has the following functions: 1. In situ enrichment of trivalent arsenic and pentavalent arsenic, and efficient adsorption of pollutants in complex matrices to the substrate surface; 2. As a SERS active substrate, the silver nanoparticles on the substrate surface can enhance the Raman signal, thereby simultaneously obtaining Raman spectra of arsenic in different valence states; 3. It can be combined into a DGT device to achieve the unification of in situ and on-site analysis. 4. Perform two-dimensional high-resolution imaging.

[0034] The method of the present invention has the following technical characteristics and advantages:

[0035] 1. The method of the present invention is based on DGT fixed film. The DGT fixed film can adsorb target pollutants in situ and can also serve as a surface enhanced Raman substrate to enhance Raman signals. It successfully combines surface enhanced Raman and gradient diffusion thin film technology to meet the in situ sampling and in situ analysis of trivalent and pentavalent arsenic.

[0036] 2. The method of the present invention is fast and efficient in analysis. It only takes a few seconds to obtain the results using a Raman spectrometer, which is very fast. In addition, there is no need to disassemble the DGT device before testing, which is very convenient.

[0037] 3. The method of the present invention simultaneously measures the contents of As(III) and As(V) in one spectrum data, thereby realizing in-situ morphological analysis of arsenic.

[0038] 4. The method of the present invention is applicable to a wider pH range, a wider ionic strength range, and has strong anti-interference ability.

[0039] 5. The method of the present invention has high sensitivity. The detection limit of As(III) tested by DGT fixed membrane is 5.0 μg / L, and that of As(V) is 2.4 μg / L, which are lower than the national drinking water limit. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of the DGT principle and a physical picture of the DGT device, wherein the fixed membrane is the DGT fixed membrane of the present invention. From the appearance of the DGT fixed membrane, it can be seen that the silver nanoparticles are evenly loaded and shiny.

[0041] Figure 2 This is a scanning electron microscope top view of the DGT fixed film. It can be seen that silver particles with a particle size of about 100 nm are attached to the dense silver film.

[0042] Figure 3 This is a scanning electron microscopy longitudinal section of the DGT fixed membrane. It can be seen that a porous silver layer has grown on both sides of the hydrogel.

[0043] Figure 4 For the attachment Figure 3 The energy spectrum scanning diagram in the direction of the middle vertical line confirmed the presence of silver element and the thickness of the grown silver layer was about 45μm.

[0044] Figure 5 The SERS spectra (a) and the corresponding linear relationship diagram (b) were obtained after the DGT fixed film was directly immersed in different concentrations of As(III) (0-500 μg / L) for 5 min. The peak at 721 cm -1 The main peak is the As-O vibration of As(III), and its intensity increases with increasing concentration. The linear fitting R 2 Reaching 0.994.

[0045] Figure 6 The SERS spectra (a) and the corresponding linear relationship diagram (b) were obtained by immersing the DGT fixed film directly in different concentrations of As(V) (0-500μg / L) for 5 min. The peak at 780cm -1 The main peak is the As-O vibration of As(V), and its intensity increases with increasing concentration. The linear fitting R 2 Reaching 0.985.

[0046] Figure 7 The DGT fixed film is assembled into a DGT device. The relationship between the mass of As(III) / As(V) accumulated on the fixed film and the SERS peak intensity shows that the two are highly correlated. Through this correlation, the SERS intensity can be converted into the M required for calculation in formula (1). DGT parameter.

[0047] Figure 8 The DGT device was thrown into 2L simulated groundwater with different proportions of As(III) and As(V) added for 4 hours. The SERS spectrum (a) measured by a portable Raman spectrometer and the C calculated by formula (1) were DGTThe corresponding relationship between the concentration and the bulk concentration of the solution (b). The spectrum shows that the method can distinguish As(III) from As(V). DGT With C Sol It is directly proportional to the concentration of active arsenic in the solution.

[0048] Fig. 9 The effect of solution pH on the concentration of As(III) and As(V) in DGT-SERS test. The applicable pH range of As(III) is 4.8-9.9, while the applicable pH range of As(V) is 4.8-9.0.

[0049] Fig.10 The effect of solution ionic strength on the concentration of As(III) and As(V) measured by DGT-SERS. III and As V The error is less than 10% in the ionic strength range of 1-50 mM and less than 15% in the ionic strength range of 0.1-100 mM, which indicates that the DGT-SERS is applicable to a wider range of ionic strength. DETAILED DESCRIPTION

[0050] The present invention is further described below by way of examples.

[0051] Example 1

[0052] Applicable to the preparation of piston type DGT fixed film:

[0053] (1) 10 mL of 30% acrylamide:N,N′-methylenebisacrylamide solution (29:1) and 70 μL of 10% ammonium persulfate solution were mixed evenly, 20 μL of TEMED catalyst was added to initiate polymerization reaction, and the mixture was immediately poured onto a mold and kept in an incubator at 30°C for 4 h to obtain a hydrogel sheet.

[0054] (2) In a 100 mL polypropylene plastic container, 20 μL KOH (3.2% wt.) was added to 18 mL AgNO3 (2% wt.) solution and stirred magnetically to form a fine brown precipitate;

[0055] (3) Slowly add 460 μL NH3·H2O (29.5% wt.) to step (2), and the solution gradually becomes clear. After adding 1 mL 6% AgNO3 and 150 μL NH3·H2O (5% wt.), the solution becomes brown and clear.

[0056] (4) Add 3 mL of methanol and 10 μL of Na5P3O to step (3) in sequence. 10(10% wt.) and 200 μL Na2HPO4·12H2O (5% wt.), mixed evenly to obtain a precursor solution.

[0057] (5) The hydrogel sheet of step (1) was cut into a hydrogel disc with a diameter of 2.5 cm and a thickness of 0.4 mm. The hydrogel disc was immediately immersed in the precursor solution, and then 6 mL of a 35% glucose solution was added. The hydrogel disc was shaken (150 r / min) at 30° C. for 2 h. The hydrogel disc was taken out and washed with a large amount of water, placed in ultrapure water, and stored at 4° C. for later use.

[0058] The appearance of the DGT fixed film prepared in this embodiment is shown in Figure 1 ,From the appearance of the DGT fixed film, it can be seen that the silver nanoparticles are loaded evenly and are shiny.

[0059] The scanning electron microscope top view of the fixed membrane prepared in this embodiment is shown in Figure 2 ,from Figure 2 It can be seen that silver particles with a particle size of about 100 nm are attached to the dense silver film.

[0060] The scanning electron microscope longitudinal section of the fixed membrane prepared in this embodiment is shown in FIG. Figure 3 ,from Figure 3 It can be seen that a porous silver layer has grown on both sides of the hydrogel.

[0061] Figure 4 It is attached Figure 3 The energy spectrum scan in the direction of the middle vertical line confirms the presence of silver elements and the thickness of the grown silver layer is about 45μm.

[0062] Example 2

[0063] Applicable to the preparation of flat-plate DGT fixed membrane:

[0064] Step (1) is the same as in Example 1;

[0065] Step (2) is the same as in Example 1;

[0066] Step (3) is the same as in Example 1;

[0067] Step (4) is the same as in Example 1;

[0068] Step (5), cut the hydrogel sheet of step (1) into a hydrogel strip with a length of 16 cm, a width of 2.2 cm, and a thickness of 0.4 mm, immerse a piece of hydrogel strip in the precursor solution by spiraling it against the wall, and then add 6 mL of 35% glucose solution, shake at 30°C (150 r / min) to react for 3 hours, take out the gel disc, wash it with a large amount of water, place it in ultrapure water, and store it at 4°C for use.

[0069] Example 3

[0070] The method for in-situ analysis of arsenic speciation based on surface enhanced Raman and gradient diffusion thin film includes the following steps:

[0071] 1) The DGT fixed membrane, diffusion membrane, and filter membrane prepared in Example 1 are sequentially pressed into the base with caps to assemble into a piston-type DGT device;

[0072] 2) The DGT device of step 1) is completely immersed in the water to be tested to enrich the target for 6 hours;

[0073] 3) After enrichment, the DGT device was taken out, and the filter membrane was torn off from the sampling window with sharp tweezers to expose the transparent diffusion membrane. The SERS spectrum was obtained by using a portable Raman spectrometer, and the laser spot was focused on the fixed membrane below the diffusion membrane. The laser wavelength was 785nm, the power was 3mW, and the integration time was 3s. The concentrations of As(III) and As(V) in the water were calculated by formula (1). The SERS intensity of the characteristic peak was obtained by Raman spectroscopy. Figure 7 Convert the intensity to mass (M). A is the exposure window area of ​​the DGT device, 3.14 cm 2 ; Δg is the diffusion membrane thickness, 0.08 cm; D is the diffusion coefficient of the target ion in the diffusion layer, therefore, As(III) is (5.7±0.2)×10 -6 cm 2 ·s -1 , As(V) is (5.0±0.2)×10 -6 cm 2 ·s -1 .

[0074]

[0075] Example 4

[0076] Two-dimensional imaging of available arsenic in sediments:

[0077] The DGT fixed membrane, diffusion membrane, and filter membrane prepared in Example 2 are stacked in sequence and pressed against the lower plate with the upper plate to assemble into a flat-plate DGT device. The flat-plate DGT is inserted into the sediment to be tested for 8 hours and then pulled out, the soil is rinsed with ultrapure water, the filter membrane is torn off, and the target area is scanned or scanned by a laser confocal Raman spectrometer with a laser wavelength of 785nm, a power of 3mW, and an integration time of 1s. In addition, due to the protection of the diffusion membrane, the fixed membrane can be maintained in room temperature air for 12 hours without shrinkage.

[0078] Experimental example:

[0079] 1. The DGT fixed film prepared in Example 1 was directly immersed in simulated water samples with different concentrations of As(III) (0-500 μg / L) for 5 min, and the SERS spectrum was tested according to the method of step 3) of Example 3. The measured SERS spectrum and the corresponding linear relationship diagram are shown in FIG. Figure 5 As shown, through Figure 5 It can be seen that at 721cm -1 The main peak is the As-O vibration of As(III), and its intensity increases with increasing concentration. The linear fitting R 2 Reaching 0.994.

[0080] 2. The DGT fixed film prepared in Example 1 was directly immersed in simulated water samples with different concentrations of As(V) (0-500 μg / L) for 5 min, and the SERS spectrum was tested according to the method of step 3) of Example 3. The measured SERS spectrum and the corresponding linear relationship diagram were as follows: Figure 6 As shown, through Figure 6 It can be seen that at 780cm -1 The main peak is the As-O vibration of As(V), and its intensity increases with increasing concentration. The linear fitting R 2 Reaching 0.985.

[0081] 3. The DGT fixed membrane prepared in Example 1 was assembled into a DGT device according to the method of Example 3. After being placed in 4 L of a simulated solution containing 100 μg / L As(III) or As(V) for enrichment for 0, 1, 2, 4, 6, 8, and 12 h, the relationship between the mass of As(III) / As(V) accumulated on the fixed membrane and the SERS peak intensity was measured, as shown in FIG. Figure 7 As shown, through Figure 7 It can be seen that the two are highly correlated, through which the SERS intensity can be converted into the M required for calculation in formula (1). DGT parameter.

[0082] 4. The DGT fixed membrane prepared in Example 1 was assembled into a DGT device according to the method of Example 3, and thrown into 2L simulated groundwater with different proportions of As(III) and As(V) added for enrichment for 4 hours. The SERS spectrum (8a) measured by a portable Raman spectrometer and formula (1) were used to calculate C DGT The corresponding relationship between the concentration and the bulk concentration of the solution (8b) shows that the method can distinguish As(III) from As(V) through the spectrum. DGT With C SolIt can be seen from the positive proportional correlation that this method can effectively reflect the active arsenic concentration in the solution. (Lines 1 to 5 are: 37.5ppb As(III)+150ppb As(V), 75ppb As(III)+225ppb As(V), 150ppb As(III)+150ppb As(V), 300ppb As(III)+100ppb As(V), 450ppb As(III)+112.5ppb As(V)).

[0083] 5. Prepare 2L of As(III)+As(V) (50 μg L each) at different pH values. -1 ) and 10mM NaNO3 mixed solution. The pH value was measured by a SevenCompact pH meter (Mettler-Toledo, Switzerland), and the pH value was adjusted to 4.2-9.9 using 0.1M nitric acid or sodium hydroxide solution. Three DGT probes were placed in each pH solution for 4 hours, and the SERS spectrum of As on the fixed membrane was measured and converted into As concentration values. The test results are shown in Fig. 9 ,pass Fig. 9 It can be seen that the applicable pH range for As(III) is 4.8-9.9, while the applicable pH range for As(V) is 4.8-9.0.

[0084] 6. Add As(III)+As(V) (50 μg L each) -1 ) solution, 0.1-100 mM sodium nitrate was added to prepare 2 L solutions with different ionic strengths. 0.1 M nitric acid or sodium hydroxide solution was used to adjust the pH to 7.0. Three DGT probes were placed in each ionic strength solution for enrichment for 4 hours, and the SERS spectrum of As on the fixed membrane was measured and converted into As concentration values. The effect of solution ionic strength on the concentration of As(III) and As(V) in DGT-SERS test is shown in Fig.10 , DGT-SERS determination of As III and As V The error is less than 10% in the ionic strength range of 1-50 mM, and less than 15% in the range of 0.1-100 mM, indicating that the method of the present invention is applicable to a wider range of ionic strength.

[0085] Example 5

[0086] The method for preparing a piston-type DGT fixed membrane is the same as that described in Example 1, except that:

[0087] In step (3), the amount of 29.5% wt. NH3·H2O added is 520 μL, and the amount of 5% wt. NH3·H2O added is 200 μL. The rest is carried out as in Example 1.

[0088] Example 6

[0089] The method for preparing a piston-type DGT fixed membrane is the same as that described in Example 1, except that:

[0090] In step (4), 3.5 mL of methanol and 15 μL of Na5P3O were added to step (3) in sequence. 10 (10% wt.) and 180 μL Na2HPO4·12H2O (5% wt.) were mixed evenly to obtain a precursor solution.

Claims

1. A method for in-situ analysis of arsenic speciation based on surface enhanced Raman and gradient diffusion thin film, comprising the following steps: 1) Press the DGT fixed membrane, diffusion membrane and filter membrane into the base with caps in sequence to assemble into a piston-type DGT device; The DGT fixed membrane is prepared as follows: (1) acrylamide: N,N′-methylenebisacrylamide solution, ammonium persulfate solution and tetramethylethylenediamine (TEMED) are mixed and poured onto a mold to obtain a hydrogel sheet; In step (1), the volume ratio of acrylamide:N,N′-methylenebisacrylamide solution to ammonium persulfate solution is 1000:(5-10), and the volume ratio of ammonium persulfate solution to tetramethylethylenediamine (TEMED) is (5-10):(1-5); (2) Add KOH solution to AgNO3 solution and stir magnetically to form a fine brown precipitate. Then slowly add NH3·H2O to make the solution gradually clear. Add AgNO3 solution and NH3·H2O again, and the solution turns brown and clear. Finally, add methanol, Na5P3O 10 solution and Na2HPO4 solution to obtain a precursor solution, immediately immerse the hydrogel sheet in the precursor solution, then add glucose solution, oscillate and react to obtain a DGT fixed film; In step (2), the volume ratio of methanol to Na2HPO4 solution is (20-40): (1-5), the mass concentration of glucose solution is 30-40%, the volume ratio of glucose solution to AgNO3 solution with a mass concentration of 1-5% is (3-8): (15-20), and the oscillation reaction is carried out at 25-35°C for 0.5-4 h, and the oscillation rate is 100-200 r / min; The mass concentration of KOH solution is 3-4%, the mass concentration of AgNO3 solution added for the first time is 1-5%, the mass concentration of NH3·H2O added for the first time is 28-32%, the volume ratio of KOH solution to AgNO3 solution is (1-5): (1500-2000), the volume ratio of AgNO3 solution to NH3·H2O is (1500-2000): (40-60); The mass concentration of the AgNO3 solution added again is 4-10%, the mass concentration of the NH3·H2O added again is 3-8%, the volume ratio of the KOH solution to the AgNO3 solution is (1-5): (80-120), and the volume ratio of the AgNO3 solution to the NH3·H2O is (80-120): (10-30); Na5P3O 10 The mass concentration of the solution is 5-15%, the mass concentration of the Na2HPO4·12H2O solution is 3-8%, and the mass concentration of the Na5P3O 10 The volume ratio of the solution to the AgNO3 solution with a mass concentration of 1-5% is (0.5-2.0): (1500-2000), and the volume ratio of the Na2HPO4·12H2O solution to the AgNO3 solution with a mass concentration of 1-5% is (10-30): (1500-2000); 2) Immerse the DGT device in step 1) into the test environment to enrich the target for 4 to 8 hours; 3) After enrichment, tear off the filter membrane of the DGT device to expose the diffusion membrane, and use a portable Raman spectrometer or laser confocal Raman spectroscopy to focus the laser spot on the DGT fixed membrane under the diffusion membrane to obtain the SERS spectrum and perform in situ analysis of arsenic form.

2. The method according to claim 1, characterized in that In step (1), the mass concentration of acrylamide:N,N′-methylenebisacrylamide solution is 25-35%, and the mass concentration of ammonium persulfate solution is 5-15%.

3. The method according to claim 1, characterized in that In step 2), the environment to be tested is a water body.

4. The method according to claim 1, characterized in that: In step 3), the laser wavelength of surface enhanced Raman is 780-790 nm, the power is 1-5 mW, and the integration time is 2-6 s.

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