System and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals

Abstract

The present invention discloses a system and method for detecting heavy metal elements in aqueous solutions by enhancing LIBS signals. The system and method relate to the fields of spectral analysis technology and laser spectral detection technology. The system and method use an ordered nanoarray substrate for liquid-solid conversion to improve signal stability, and construct a spatial constraint condition to cooperate with the ordered nanoarray to enhance the LIBS signal. The system and method for detecting heavy metal elements in aqueous solutions by enhancing LIBS signals prepare an Au-Si cone-sphere ordered nanoarray enhanced substrate by nanosphere etching and magnetron sputtering deposition. PVC tape with circular holes is attached to the surface of the ordered nanoarray substrate as a liquid-limiting groove. The sample to be tested is drop-coated into the groove on the surface of the enhanced substrate to perform liquid-solid conversion sampling. Finally, a quartz glass sheet with a through hole is covered on the substrate surface after liquid-solid conversion as a spatial constraint cavity, so that the spatial constraint condition cooperates with the ordered nanoarray to enhance the laser-induced breakdown spectroscopy signal.

Description

Technical Field

[0001] The present invention relates to the fields of spectral analysis technology and laser spectral detection technology, and specifically to a system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals. Background Art

[0002] Heavy metals are common pollutants. Exceeding standards in water can pose significant risks to human health and the ecological environment. For example, excessive levels of heavy metals in natural water can directly or indirectly harm aquatic populations and ecosystems, impacting the overall ecological balance. Long-term consumption of water containing excessive levels of heavy metals can lead to various illnesses and pose a direct threat to human health. Therefore, rapid testing for heavy metal levels in water is crucial for early detection of problems and the implementation of effective control and protection measures.

[0003] Traditional methods for detecting heavy metal content in aqueous solutions mainly include spectrophotometry, inductively coupled plasma optical emission spectrometry, inductively coupled plasma mass spectrometry, and atomic fluorescence spectrometry. Although these methods can all achieve quantitative detection of heavy metals in aqueous solutions, they often require complex sample preparation or long analysis times. In practical applications, rapid measurement of heavy metal content in liquids is often required, and these methods are not well suited to meet these requirements. Therefore, there is a significant need to develop rapid methods for detecting heavy metals in aqueous solutions.

[0004] Laser induced breakdown spectroscopy (LIBS) is an emerging atomic emission spectroscopy technique. It utilizes a laser pulse to ablate a sample to generate a plasma, and then uses a spectrometer to obtain the atomic emission spectrum of the plasma. LIBS offers numerous advantages, including rapid detection and simultaneous analysis of multiple elements. However, the weak signal and insufficient sensitivity in LIBS detection of heavy metals in liquid samples are major bottlenecks restricting its widespread application. Generally speaking, the sensitivity of LIBS is directly determined by the element being analyzed, and its limit of detection (LOD) typically ranges from a few to several hundred μg / mL, which is insufficient for detecting excessive heavy metals in water. Compared to other spectral analysis techniques, the sensitivity of LIBS is not competitive.

[0005] Furthermore, when testing liquid samples, direct liquid-phase detection of samples by LIBS is prone to problems such as liquid splashing, liquid surface fluctuations, and plasma quenching, resulting in poor spectral signal stability. To address these issues, researchers have proposed various improved methods, including jet detection, droplet detection, laminar flow detection, atomization detection, and capillary-assisted detection. However, direct liquid-phase detection and its various improved methods fail to fundamentally eliminate the energy loss from solvent evaporation, plasma quenching, and liquid splashing. Many researchers, both domestically and internationally, have proposed various methods for converting liquid-phase samples into solid-phase samples for LIBS analysis, including freezing, adsorption, and substrate liquid-solid conversion. The adsorption method requires complex sample pretreatment, prone to adsorption saturation, and is time-consuming. The freezing method requires experimental conditions such as liquid nitrogen to rapidly convert the liquid-phase sample into a solid phase. The substrate liquid-solid conversion method offers simpler sample pretreatment and is easy to store after liquid-solid conversion.

[0006] To address the problem of weak spectral signal intensity, researchers have developed various methods to enhance the signal intensity of LIBS and improve detection sensitivity to meet the application requirements of heavy metal detection in water bodies. Commonly used enhancement methods include dual-pulse enhancement technology, laser-induced fluorescence combined technology, electric spark enhancement technology, microwave-assisted enhancement technology, spatial confinement enhancement technology, nanoparticle enhancement technology, spatial confinement enhancement technology, etc. Dual-pulse enhancement technology, laser-induced fluorescence combined technology, microwave-assisted enhancement technology (CN105067572), and electric spark enhancement technology all have good signal enhancement effects, but will significantly increase the cost and complexity of the system. Nanoparticle enhancement technology and spatial confinement enhancement technology can both achieve simple and effective LIBS signal enhancement without significantly increasing experimental costs and requiring too many additional devices. Among them, the confinement cavity of the spatial confinement enhancement method is reusable and low-cost, but the enhancement effect is relatively limited. The nanoparticle enhancement technology has a better enhancement effect and can achieve a signal enhancement of 1-3 orders of magnitude. However, the current nanoparticle enhancement substrates are mostly prepared by drop coating of metal nanoparticle sols. During the drying process of the nanosol, the nanoparticles are prone to clustering and coffee ring effects. The particle size of the nanoparticles is relatively uniform, but the spacing and uniformity are uncontrollable, resulting in unstable signal enhancement effects and poor reproducibility. Summary of the Invention

[0007] In response to the deficiencies of the prior art, the present invention provides a system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals, which solves the problems raised in the above-mentioned background technology.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solutions: a system and method for detecting heavy metal elements in aqueous solutions by enhancing LIBS signals, using an ordered nanoarray substrate for liquid-solid conversion to improve signal stability, and constructing spatial constraints to coordinate the ordered nanoarray to enhance the LIBS signal;

[0009] The method for preparing the ordered nanoarray substrate is as follows:

[0010] Step A1: A single-side polished N-type silicon wafer is ultrasonically cleaned in acetone and ultrapure water, followed by further cleaning in a mixture of concentrated sulfuric acid and hydrogen peroxide. The wafer is then ultrasonically cleaned again in ultrapure water to obtain a clean silicon wafer. Finally, the wafer is irradiated and cleaned in a UV-ozone cleaning machine to obtain a hydrophilic silicon wafer substrate.

[0011] Step A2: A suspension of 100 nm-diameter polystyrene nanospheres is diluted with ethanol at a 1:1 ratio and ultrasonically dispersed. A hydrophilic silicon wafer is placed in the diluent and then slowly tilted out of the liquid. This allows the formation of a large, densely packed monolayer array of PS nanospheres on the silicon substrate by utilizing the air-liquid interface self-assembly effect at the PS diluent surface.

[0012] Step A3: Using the densely packed PS nanospheres as an etching template, a reactive ion etcher is used with SF6 gas as a reaction gas to etch the silicon wafer covered with a single layer of PS nanospheres; ultimately, a nanocone array is formed on the silicon wafer;

[0013] Step A4: Since some PS balls remain on the top of the silicon cones due to incomplete etching, the etched silicon wafer is further placed in a muffle furnace and calcined at 550°C for 2 hours to remove the remaining PS balls, ultimately obtaining a silicon nanocone array substrate free of PS residues.

[0014] Step A5: Place the obtained silicon nanocone array substrate into a magnetron sputtering coating system and deposit an Au film at a sputtering rate of 20 nm / min. The sharp tips of the silicon cone array will preferentially nucleate and condense, growing into nano-spherical structures. Simultaneously, a layer of Au film will also grow on the sides of the silicon cones.

[0015] A method for preparing a sample of a heavy metal sample in aqueous solution by liquid-solid conversion comprises the following steps:

[0016] Step B1: using a dicing knife to cut the large-area ordered nanoarray substrate into small pieces of ordered nanoarray substrates of appropriate sizes;

[0017] Step B2: To ensure uniformity in the size of the solute spots formed by drop coating the liquid sample, an ordered nanoarray substrate with grooves was fabricated for liquid-solid conversion. A circular hole punch of uniform pore size was used to punch a number of holes of the same size on a PVC tape. The tape was then attached to the ordered nanoarray substrate to fabricate an ordered nanoarray substrate with grooves.

[0018] Step B3: Use a pipette to drop a certain amount of the sample solution onto the substrate groove. Use a heater at 70°C to quickly dry the aqueous solvent in the solution, forming a solute layer on the surface of the groove, thereby completing the liquid-solid conversion of the liquid sample.

[0019] The method of constructing spatial constraints and coordinated ordered nanoarrays to enhance LIBS signals is as follows:

[0020] Step C1: Using a quartz glass plate of a certain thickness with a circular through hole to construct a spatial constraint condition; and strictly aligning the through hole of the quartz glass plate with the through hole with the substrate groove to form a spatial constraint cavity;

[0021] Step C2: placing the groove substrate sample fitted with the spatial confinement cavity obtained in step C1 on the sample stage of a laser induced breakdown spectrometer system for testing and analyzing the heavy metal content in the sample.

[0022] Optionally, the size of the N-type silicon wafer in step A1 is set to 6 cm*6 cm.

[0023] Optionally, the mixing ratio of the concentrated sulfuric acid and hydrogen peroxide mixed solution in step A1 is as follows: the concentration of hydrogen peroxide is 30%, the concentration of concentrated sulfuric acid is greater than 98%, and the volume ratio of hydrogen peroxide to concentrated sulfuric acid is 1:3.

[0024] Optionally, the SF6 gas flow parameter in step A3 is 50 sccm / min, the etcher chamber pressure is maintained at 20 Pa, the etching power is set to 300 W, and the etching time is 1 min.

[0025] A system for detecting heavy metal elements in an aqueous solution by enhancing LIBS signals, comprising an Nd:YAG pulse laser, a right-angle prism reflector provided at the output end of the Nd:YAG pulse laser, a beam splitter provided below the right-angle prism reflector, a horizontal focusing lens provided on one side of the beam splitter, a vertical focusing lens provided below the beam splitter, an ordered nanoarray substrate sample with a spatially constrained cavity provided parallel to the vertical focusing lens, and the ordered nanoarray substrate sample with a spatially constrained cavity placed on a three-dimensional translation sample stage;

[0026] The input end of the Nd:YAG pulse laser is connected to a delay device, the input end of the delay device is connected to a mid-step spectrometer, the output end of the mid-step spectrometer is connected to one end of an optical fiber, the other end of the optical fiber is connected to a fiber optic probe, and the input end of the mid-step spectrometer is connected to a computer terminal.

[0027] The present invention provides a system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals, which has the following beneficial effects:

[0028] The method proposed in this project can effectively enhance the LIBS signal, and the sample preparation method only requires minimal, low-cost, simple processing of the sample. After batch liquid-solid conversion, the detection of a single sample can be completed in seconds. The nanosphere etching method used to make the ordered nanoarray is a large-area ordered nanoarray preparation method known for its low cost, and the cost of a single liquid-solid conversion substrate is relatively low. The spatial constraint conditions only require the use of a plate with holes, which is low-cost and simple in structure. The use of ordered nanoarray substrates for sample preparation, combined with a spatially constrained sample chamber, can be directly applied to existing commercial LIBS instruments. The method proposed in this invention is low-cost and has good scalability.

[0029] Currently, metal nanoparticles are mostly used to enhance LIBS signals by drop-coating nanoparticle sols. This is prone to clustering and the coffee ring effect. The spacing and distribution uniformity between nanoparticles are uncontrollable, resulting in unstable signal enhancement and poor repeatability. The method proposed in this paper uses ordered nanoarrays to enhance LIBS signals. The nanoparticles in the ordered nanoarrays have uniform spacing and spatial distribution, which can effectively improve the stability and repeatability of signal enhancement.

[0030] The perforated tape is attached to the ordered nanoarray substrate to produce an ordered nanoarray substrate with liquid confinement grooves. The sample droplets to be tested are confined in the substrate grooves and dried to achieve liquid-solid conversion of the droplets, which can effectively avoid the problems of uneven droplet diffusion and inconsistent diffusion size.

[0031] Converting liquid samples into solid samples. Liquid-solid conversion of liquid samples can effectively avoid the problems of weak signal and poor signal stability caused by energy loss of solvent evaporation, plasma quenching, liquid splashing, etc. when the laser pulse interacts with the liquid sample in laser induced breakdown spectroscopy detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A schematic diagram of a method for preparing an ordered nanoarray substrate according to an embodiment of the system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals;

[0033] Figure 2Schematic diagram of the liquid-solid conversion method of a liquid sample on a grooved ordered nano-substrate according to an embodiment of the system and method for detecting heavy metal elements in an aqueous solution by enhancing LIBS signals;

[0034] Figure 3 A schematic diagram of a method for constructing spatial constraints involved in an embodiment of the system and method for detecting heavy metal elements in an aqueous solution by enhancing LIBS signals;

[0035] Figure 4 This is a schematic diagram of the structure of a laser-induced breakdown spectroscopy system used in an embodiment of the system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals;

[0036] Figure 5 This is a SEM (scanning electron microscope) image of an ordered nanoarray produced in an embodiment of the system and method for detecting heavy metal elements in an aqueous solution by enhancing LIBS signals;

[0037] Figure 6 Comparison of Cu element spectral intensity obtained by liquid-solid conversion of ordered nanoarray substrate + spatial constraint conditions and Si substrate drop coating in the embodiment of the system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signal;

[0038] Figure 7 This is a single variable calibration curve diagram of the Cu element obtained by the ordered nanoarray substrate + spatial constraint condition and Si substrate drop coating liquid-solid conversion in the embodiment of the system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals.

[0039] In the figure: 1. Nd:YAG pulse laser; 2. Time delay; 3. Medium-step spectrometer; 4. Optical fiber; 5. Optical fiber probe; 6. Horizontal focusing lens; 7. Right-angle prism reflector; 8. Beam splitter; 9. Vertical focusing lens; 10. Ordered nanoarray substrate sample with spatial confinement cavity; 11. Three-dimensional translation sample stage; 12. Computer terminal. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0041] In the description of the present invention, unless otherwise specified, "plurality" means two or more; terms such as "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," and "tail" indicate positions or relationships based on those shown in the accompanying drawings. These terms are intended solely to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present invention. Furthermore, terms such as "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0042] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "connected" and "connection" should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integral connection; mechanical connection, electrical connection; direct connection, or indirect connection through an intermediary. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0043] Example 1

[0044] See also Figures 1 to 7 The present invention provides a technical solution: a system and method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals, using an ordered nanoarray substrate for liquid-solid conversion to improve signal stability, and constructing spatial constraints to coordinate the ordered nanoarray to enhance the LIBS signal.

[0045] In this embodiment, Figure 1 As shown, the method for preparing the ordered nanoarray substrate is as follows:

[0046] Step 1: A single-side polished N-type silicon wafer is ultrasonically cleaned in acetone and ultrapure water, followed by further cleaning in a mixture of concentrated sulfuric acid and hydrogen peroxide. Ultrasonic cleaning is then performed again with ultrapure water to obtain a clean silicon wafer. Finally, the wafer is irradiated and cleaned in a UV-ozone cleaning machine to obtain a hydrophilic silicon wafer substrate.

[0047] Step 2: A 100nm diameter polystyrene nanosphere suspension is then diluted with ethanol in a 1:1 ratio and ultrasonically dispersed. A hydrophilic silicon wafer is placed in the diluent and then slowly tilted out of the liquid. This allows the formation of a large-area, single-layer, densely packed PS nanosphere array on the silicon substrate by utilizing the air-liquid interface self-assembly effect at the PS diluent surface.

[0048] Step 3: Using the densely packed PS nanospheres as an etching template, a reactive ion etcher is used with SF6 gas as the reaction gas to etch the silicon wafer covered with a single layer of PS nanospheres; ultimately, a nanocone array is formed on the silicon wafer;

[0049] Step 4: Since some PS balls remain on the top of the silicon cones that have not been completely etched, the etched silicon wafer is further placed in a muffle furnace and calcined at 550°C for 2 hours to remove the remaining PS balls, ultimately obtaining a silicon nanocone array substrate free of PS residues.

[0050] Step 5: Place the obtained silicon nanocone array substrate into a magnetron sputtering coating system and deposit an Au film at a sputtering rate of 20 nm / min. The sharp tops of the silicon cone array will preferentially nucleate and condense, growing into nano-spherical structures. At the same time, a thinner layer of Au film will also grow on the sides of the silicon cones.

[0051] In this embodiment, Figure 1 As shown, the liquid-solid conversion sample preparation method for aqueous heavy metal samples comprises the following steps:

[0052] Step 1: Use a dicing knife to cut the large-area ordered nanoarray substrate into small pieces of ordered nanoarray substrates of appropriate sizes;

[0053] Step 2: To ensure the uniformity of the size of the solute spots formed by the drop coating of the liquid sample, an ordered nanoarray substrate with grooves is prepared for liquid-solid conversion. Figure 2 As shown, a PVC tape is punched with a circular hole puncher of the same aperture size to form a number of small holes of the same size, and then the PVC tape is attached to the ordered nanoarray substrate to produce an ordered nanoarray substrate with grooves;

[0054] Step 3: Use a pipette to take a certain amount of the sample solution to be tested and drop it on the groove of the substrate. Use a heater at a temperature of 70°C to quickly dry the water solvent of the aqueous solution to form a solute layer on the surface of the groove, thereby completing the liquid-solid conversion of the liquid sample.

[0055] In this embodiment, Figure 1 As shown in FIG, the method for constructing a spatial constraint condition to coordinate an ordered nanoarray to enhance LIBS signals comprises the following steps:

[0056] Step 1: Use a quartz glass plate with a certain thickness and a circular through hole to construct a space constraint condition; and strictly align the through hole of the quartz glass plate with the substrate groove to form a space constraint cavity;

[0057] Step 2: Place the groove substrate sample fitted with the spatial confinement cavity obtained in step 1 on the sample stage of a laser induced breakdown spectrometer system for testing and analyzing the heavy metal content in the sample.

[0058] In this embodiment, Figure 1 As shown, the size of the N-type silicon wafer in step 1 is set to 6cm*6cm.

[0059] In this embodiment, Figure 1 As shown, the mixing ratio of the concentrated sulfuric acid and hydrogen peroxide mixed solution in step 1 is as follows: the concentration of hydrogen peroxide is 30%, the concentration of concentrated sulfuric acid is greater than 98%, and the volume ratio of hydrogen peroxide to concentrated sulfuric acid is 1:3.

[0060] In this embodiment, Figure 1 As shown, the SF6 gas flow parameter in step three is 50 sccm / min, the etcher chamber pressure is maintained at 20 Pa, the etching power is set to 300 W, and the etching time is 1 min.

[0061] In this embodiment, Figure 1 As shown, a right-angle prism reflector 7 is provided at the output end of the Nd:YAG pulse laser 1, a beam splitter 8 is provided below the right-angle prism reflector 7, a horizontal focusing lens 6 is provided on one side of the beam splitter 8, a vertical focusing lens 9 is provided below the beam splitter 8, and an ordered nanoarray substrate sample 10 with a spatial confinement cavity is provided parallel to the vertical focusing lens 9. The ordered nanoarray substrate sample 10 with a spatial confinement cavity is placed on a three-dimensional translation sample stage 11;

[0062] The input end of the Nd:YAG pulse laser 1 is connected to a delay device 2, the input end of the delay device 2 is connected to a mid-step spectrometer 3, the output end of the mid-step spectrometer 3 is connected to one end of an optical fiber 4, the other end of the optical fiber 4 is connected to a fiber optic probe 5, and the input end of the mid-step spectrometer 3 is connected to a computer terminal 12.

[0063] Example 2

[0064] (1) Preparation of ordered nanoarray substrates: single-side polished N-type silicon wafers (6 cm × 6 cm) were placed in acetone and ultrapure water for ultrasonic cleaning, and then placed in a mixed solution of concentrated sulfuric acid and hydrogen peroxide (hydrogen peroxide concentration of 30%, concentrated sulfuric acid concentration greater than 98%, and the volume ratio of hydrogen peroxide to concentrated sulfuric acid was 1:3) for further cleaning. Subsequently, the wafers were ultrasonically cleaned again with ultrapure water to obtain clean silicon wafers; finally, the wafers were placed in an ultraviolet ozone cleaning machine for irradiation cleaning to obtain a hydrophilic silicon wafer substrate;

[0065] Subsequently, a polystyrene (PS) nanosphere suspension (5 wt.%) with a diameter of 100 nm was diluted with ethanol at a ratio of 1:1 and ultrasonically dispersed; a hydrophilic silicon wafer substrate was placed in the dilution solution, and then the silicon wafer was slowly tilted out of the liquid surface. At this time, the air-liquid interface self-assembly effect on the surface of the PS dilution solution was used to prepare a large-area single-layer densely packed PS nanosphere array on the silicon substrate surface, as shown in FIG. Figure 1 As shown in a;

[0066] Using densely packed PS nanospheres as an etching template, a reactive ion etcher with SF6 gas as the reaction gas was used to etch a silicon wafer covered with a single layer of PS nanospheres. The SF6 gas flow rate parameter was 50 sccm / min, the etcher chamber pressure was maintained at 20 Pa, the etching power was set to 300 W, and the etching time was 1 minute. Finally, a nanocone array was formed on the silicon wafer.

[0067] Since some PS balls are not completely etched on the top of the silicon cone, such as Figure 1 As shown in b; therefore, the silicon wafer after the etching process is further placed in a muffle furnace and calcined at 550 ° C for 2 hours to remove the PS ball residue, and finally a silicon nanocone array substrate without PS residue is obtained, as shown in FIG. Figure 1 As shown in c;

[0068] The obtained silicon nanocone array substrate is placed in a magnetron sputtering coating system and an Au film is deposited at a sputtering rate of 20 nm / min. The sharp tops of the silicon cone array will preferentially nucleate and condense to grow into nano-spherical structures. At the same time, a thin layer of Au film will also grow on the sides of the silicon cones, such as Figure 1 d; SEM image of the finally prepared ordered nanoarray, as shown in Figure 5 As shown;

[0069] Use a dicing knife to cut the large-area ordered nanoarray substrate into small pieces of ordered nanoarray substrates of appropriate size (10 mm × 30 mm);

[0070] (2) Preparation of liquid-solid conversion samples: To ensure the uniformity of the size of the solute spots formed by drop coating of the liquid sample, an ordered nanoarray substrate with grooves is prepared for liquid-solid conversion; the method is as follows: Figure 2 As shown, a PVC tape that does not contain the heavy metal elements to be measured and does not absorb aqueous solutions is selected, and a hole punch with a 5 mm aperture is used to punch several holes on the surface of the tape to obtain a tape with several holes. The tape is then attached to the ordered nanoarray substrate to produce an ordered nanoarray substrate with grooves.

[0071] Using a pipette, 20 μL of standard solutions with copper concentrations of 0.1 μg / ml, 0.2 μg / ml, 0.4 μg / ml, 0.8 μg / ml, and 1 μg / ml were dispensed into the grooves of the ordered nanoarray substrate. The substrate with the sample droplets was then placed on a hot plate heater and heated at 70°C to quickly dry out the water, forming a uniformly distributed solute layer in the groove area.

[0072] (3) Construction of spatial constraint conditions: a circular through hole with a diameter of 5 mm is made on the surface of a 6 mm thick quartz glass plate (metal plates such as stainless steel and Al plates can also be used) as a spatial constraint cavity; when constructing the spatial constraint condition, the through hole is aligned with the groove hole of the liquid limiting groove substrate and placed on the three-dimensional sample stage;

[0073] Placing the substrate sample fitted with the spatial confinement cavity on a sample stage for laser induced breakdown spectroscopy detection to analyze the heavy metal content in the aqueous solution;

[0074] (4) Optimize experimental parameters and perform data acquisition. Refer to the NIST database for the characteristic spectral lines of Cu element commonly used in relevant literature, and select CuI324.75nm for analysis; optimize experimental conditions and obtain the best detection parameters. The specific experimental conditions used are: laser energy of 80mJ, pulse laser frequency of 2Hz, spectrum acquisition delay and gate width are both set to 4μs; perform spectral detection on the sample under the optimal experimental conditions, collect a characteristic spectrum for every 50 accumulated laser pulses, and repeatedly collect 6 spectra for each sample (collecting the spectral line intensity of Cu324.7nm), and obtain LIBS spectral data of Cu element at different concentrations;

[0075] For comparison, 20 μL of standard solutions with copper concentrations of 0.1 μg / ml, 0.2 μg / ml, 0.4 μg / ml, 0.8 μg / ml, and 1 μg / ml were measured using a pipette and directly drop-coated onto the surface of a silicon substrate with only a liquid-limiting groove (the preparation method is to use a 5 mm aperture puncher to punch several holes on the surface of a tape to obtain a tape with several holes, and then stick it on the Si substrate). The tape was then placed on a 70°C hot plate heater to quickly dry the water to form a solute layer in the groove to prepare a Si substrate sample to be tested; the sample was directly placed on the LIBS three-dimensional translation stage, and LIBS detection was performed directly using the same experimental conditions (i.e., a Si wafer was used as the substrate and no spatial confinement cavity was used).

[0076] The original spectral data is preprocessed, and the spectral intensity obtained by the spectrum enhancement method disclosed in the present invention is compared with that obtained by direct silicon substrate solid-liquid conversion, as shown in FIG. Figure 6 As shown in the figure, the intensity of the CuI324.75nm spectral line is compared. Compared with the direct use of the Si substrate liquid-solid conversion method, the spectral intensity of the sample of the method disclosed in the present invention is improved by 124.7 times by using the spatial constraint conditions and the ordered nanoarray signal enhancement method disclosed in the present invention. The enhancement effect is very significant.

[0077] The single variable calibration method was used to quantitatively analyze the Cu element and a calibration curve was established, such as Figure 7As shown in the figure; it can be seen from the figure that the fitting coefficients of the calibration curves obtained by the spatial constraint conditions coordinated ordered nanoarray enhanced LIBS signal method disclosed in the present invention and the direct liquid-solid conversion method on the Si substrate both reach above 0.99, but the detection limits obtained by the method disclosed in the present invention and the direct Si substrate liquid-solid conversion method are 0.02μg / ml and 2.4μg / ml, respectively; according to the "National Environmental Protection Standard Urban Wastewater Discharge Standard" (GB8978-1996), the discharge standard for copper in wastewater is 2.0μg / ml; therefore, the use of the substrate solid-liquid conversion method LIBS detection alone cannot meet the limit standard, while the use of the enhancement method disclosed in the present invention can meet the limit standard.

[0078] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals, characterized in that: Use ordered nanoarray substrates for liquid-solid conversion to improve signal stability, and construct spatial constraints to coordinate ordered nanoarrays to enhance LIBS signals; The method for preparing the ordered nanoarray substrate is as follows: Step A1: A single-side polished N-type silicon wafer is ultrasonically cleaned in acetone and ultrapure water, followed by further cleaning in a mixture of concentrated sulfuric acid and hydrogen peroxide. The wafer is then ultrasonically cleaned again in ultrapure water to obtain a clean silicon wafer. Finally, the wafer is irradiated and cleaned in a UV-ozone cleaning machine to obtain a hydrophilic silicon wafer substrate. Step A2: A suspension of 100 nm-diameter polystyrene nanospheres is diluted with ethanol at a 1:1 ratio and ultrasonically dispersed. A hydrophilic silicon wafer is placed in the diluent and then slowly tilted out of the liquid. This allows the formation of a large, densely packed monolayer array of PS nanospheres on the silicon substrate by utilizing the air-liquid interface self-assembly effect at the PS diluent surface. Step A3: Using the densely packed PS nanospheres as an etching template, a reactive ion etcher is used with SF6 gas as a reaction gas to etch the silicon wafer covered with a single layer of PS nanospheres; ultimately, a nanocone array is formed on the silicon wafer; Step A4: Since some PS balls remain on the top of the silicon cones due to incomplete etching, the etched silicon wafer is further placed in a muffle furnace and calcined at 550°C for 2 hours to remove the remaining PS balls, ultimately obtaining a silicon nanocone array substrate free of PS residues. Step A5: Place the obtained silicon nanocone array substrate into a magnetron sputtering coating system and deposit an Au film at a sputtering rate of 20 nm / min. The sharp tips of the silicon cone array will preferentially nucleate and condense, growing into nano-spherical structures. Simultaneously, a layer of Au film will also grow on the sides of the silicon cones. A method for preparing a sample of a heavy metal sample in aqueous solution by liquid-solid conversion comprises the following steps: Step B1: using a dicing knife to cut the large-area ordered nanoarray substrate into small pieces of ordered nanoarray substrates of appropriate sizes; Step B2: To ensure uniformity in the size of the solute spots formed by drop coating the liquid sample, an ordered nanoarray substrate with grooves was fabricated for liquid-solid conversion. A circular hole punch of uniform pore size was used to punch a number of holes of the same size on a PVC tape. The tape was then attached to the ordered nanoarray substrate to fabricate an ordered nanoarray substrate with grooves. Step B3: Use a pipette to drop a certain amount of the sample solution onto the substrate groove. Use a heater at 70°C to quickly dry the aqueous solvent in the solution, forming a solute layer on the surface of the groove, thereby completing the liquid-solid conversion of the liquid sample. The method of constructing spatial constraints and coordinated ordered nanoarrays to enhance LIBS signals is as follows: Step C1: Using a quartz glass plate of a certain thickness with a circular through hole to construct a spatial constraint condition; and strictly aligning the through hole of the quartz glass plate with the through hole with the substrate groove to form a spatial constraint cavity; Step C2: placing the groove substrate sample fitted with the spatial confinement cavity obtained in step C1 on the sample stage of a laser induced breakdown spectrometer system for testing and analyzing the heavy metal content in the sample.

2. The method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals according to claim 1, characterized in that: The size of the N-type silicon wafer in step A1 is set to 6 cm*6 cm.

3. The method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals according to claim 1, characterized in that: The mixing ratio of the concentrated sulfuric acid and hydrogen peroxide mixed solution in step A1 is as follows: the concentration of hydrogen peroxide is 30%, the concentration of concentrated sulfuric acid is greater than 98%, and the volume ratio of hydrogen peroxide to concentrated sulfuric acid is 1:

3.

4. The method for detecting heavy metal elements in aqueous solution by enhancing LIBS signals according to claim 1, characterized in that: The SF6 gas flow parameter in step A3 is 50 sccm / min, the etcher chamber pressure is maintained at 20 Pa, the etching power is set to 300 W, and the etching time is 1 min.

5. A system for detecting heavy metal elements in aqueous solution by enhancing LIBS signals according to any one of claims 1 to 4, comprising a Nd:YAG pulsed laser (1), characterized in that: The output end of the Nd:YAG pulse laser (1) is provided with a right-angle prism reflector (7), a beam splitter (8) is provided below the right-angle prism reflector (7), a horizontal focusing lens (6) is provided on one side of the beam splitter (8), a vertical focusing lens (9) is provided below the beam splitter (8), an ordered nano-array substrate sample (10) with a spatial confinement cavity is provided in parallel below the vertical focusing lens (9), and the ordered nano-array substrate sample (10) with a spatial confinement cavity is placed on a three-dimensional translation sample stage (11); The input end of the Nd:YAG pulse laser (1) is connected to a time delay device (2), the input end of the time delay device (2) is connected to a mid-step spectrometer (3), the output end of the mid-step spectrometer (3) is connected to one end of an optical fiber (4), the other end of the optical fiber (4) is connected to an optical fiber probe (5), and the input end of the mid-step spectrometer (3) is connected to a computer terminal (6).

Images