A test paper for detecting heavy metals in water and a detection method thereof

Abstract

This invention discloses a heavy metal test strip for water and its detection method, belonging to the field of heavy metal detection technology. The test strip includes a base plate, a glass fiber membrane, a reagent release pad, a nitrocellulose membrane, absorbent paper, and a colorimetric card. The nitrocellulose membrane is fixed to the middle of the base plate with adhesive, the absorbent paper is fixed to the right side of the base plate with adhesive, and the glass fiber membrane is fixed to the left side of the base plate with adhesive. The bottom left side of the glass fiber membrane overlaps the top right side of the reagent release pad. A colorimetric card with 3 to 7 color levels is adhered to one side of the base plate of the nitrocellulose membrane. This invention's test strip has good specificity and no false positive interference. Water sample testing does not require laboratory instruments; at water sources, factory drainage outlets, etc., detection can be completed in just three steps: "sample immersion, static color development, and colorimetric card comparison."

Description

Technical Field

[0001] This invention relates to the field of heavy metal detection technology, specifically to a test strip for heavy metal detection in water and its detection method. Background Technology

[0002] Water is a core resource for maintaining ecosystem balance and human survival and development. However, with the acceleration of industrialization, the scaling up of agricultural production, and the improvement of urbanization, heavy metal pollution in water has become increasingly prominent, posing a pressing environmental challenge globally. Common heavy metals in water include copper, iron, manganese, lead, cadmium, and mercury, and their sources are diverse: wastewater discharge from industries such as smelting, electroplating, chemical processing, and electronics manufacturing carries large amounts of untreated or substandard heavy metal ions; excessive use of fertilizers and pesticides in agricultural activities, as well as improper disposal of livestock and poultry waste, causes heavy metals in the soil to seep into water bodies through surface runoff; during mineral development and smelting, mine wastewater and tailings leachate directly pollute surrounding surface and groundwater; and in urban sewage networks, domestic sewage, landfill leachate, and traffic exhaust deposits are also important sources of heavy metal pollution.

[0003] Heavy metals are characterized by their reluctance to degrade, bioaccumulation, and persistent toxicity. Even at low concentrations, their long-term presence in water bodies can cause serious harm to the ecological environment and human health. Excessive intake of copper ions can irritate the gastrointestinal mucosa, causing symptoms such as nausea, vomiting, and diarrhea. Long-term accumulation can also damage vital organs such as the liver and kidneys, affecting hematopoietic function and the immune system. When iron levels in water exceed the standard, it not only causes the water to become turbid and reddish-brown, affecting its sensory properties, but also promotes bacterial growth, alters the structure of the aquatic microbial community, and reduces the taste and quality of drinking water. Long-term consumption may lead to iron poisoning, damaging liver and pancreatic function. Excessive manganese levels will cause the water to turn black and produce an unpleasant odor. Long-term intake can cause irreversible damage to the nervous system, manifesting as symptoms such as memory loss and decreased motor coordination, with particularly significant health hazards to children and the elderly. Furthermore, heavy metals can accumulate step by step through the food chain, transferring from aquatic organisms to higher organisms, ultimately threatening human health, disrupting the ecological balance of aquatic bodies, causing deformities and deaths of aquatic organisms, and affecting the sustainable use of fishery resources and water resources.

[0004] Currently, heavy metal detection technologies in water mainly fall into two categories: laboratory testing and rapid on-site testing. Laboratory testing technologies primarily utilize atomic absorption spectrometry, inductively coupled plasma mass spectrometry, and atomic fluorescence spectrometry. These technologies offer advantages such as high detection accuracy, low detection limits, and the ability to simultaneously determine multiple heavy metals, and are widely used in precision testing and scientific research analysis. However, they also have significant limitations: the testing equipment is expensive, with the cost of a single instrument often reaching hundreds of thousands or even millions of yuan, and maintenance costs are high; the testing process is complex, requiring specialized technicians for water sample pretreatment, instrument operation, and data interpretation, which is difficult for ordinary personnel to master; the testing cycle is long, typically requiring several hours or even days from water sample collection and transportation to laboratory analysis to obtain results, failing to meet the needs of on-site emergency testing and real-time monitoring; and the testing location is limited, relying on a fixed laboratory environment, making rapid testing impossible in complex scenarios such as water sources, factory drainage outlets, agricultural irrigation areas, and emergency pollution sites.

[0005] Rapid on-site detection technologies have seen some development in recent years, mainly including portable detectors, colorimetric methods, and test strips. Portable detectors offer some mobility compared to laboratory testing, but still suffer from issues such as large equipment size, cumbersome operation procedures, the need for regular calibration, and high testing costs, making large-scale deployment in grassroots units and emergency scenarios difficult. Traditional colorimetric methods require various reagents, colorimetric tubes, and other consumables, involve numerous steps, and suffer from poor color development stability, making them susceptible to interference from external environmental factors and compromising the accuracy of test results. Traditional heavy metal test strips generally suffer from insufficient specificity, a high risk of false positives, low detection sensitivity, weak color development, and a narrow range of applications. For example, some test strips can only detect a single heavy metal, and their ability to identify low concentrations of heavy metals is limited, failing to meet the needs of simultaneous screening of multiple heavy metals and accurate determination of concentration ranges in practical testing.

[0006] In scenarios such as environmental monitoring, industrial wastewater discharge supervision, drinking water source protection, agricultural irrigation water testing, and emergency response to sudden water pollution incidents, there is an urgent need for rapid, accurate, convenient, and low-cost heavy metal detection technologies. Grassroots environmental enforcement personnel need to quickly determine the presence of heavy metal pollution in water bodies on-site and take timely control measures; enterprises need to monitor heavy metal emissions from their drainage outlets in real time to avoid environmental liability risks arising from excessive emissions; water source protection personnel need to regularly screen water quality to ensure drinking water safety; and in the event of a sudden water pollution incident, rescue personnel need to grasp the degree and scope of pollution as soon as possible to provide data support for emergency response. Existing detection technologies cannot simultaneously meet the core requirements of "rapid on-site detection, convenient operation, low cost, accurate results, and wide applicability." Therefore, developing a water heavy metal detection test strip and its detection method that is highly specific, free from false positive interference, simple to operate, rapid in detection, and cost-effective has significant practical significance and broad application prospects. Summary of the Invention

[0007] The purpose of this invention is to provide a test strip for heavy metals in water and a method for detecting them, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A test strip for heavy metals in water includes a base plate, a glass fiber membrane, a reagent release pad, a nitrocellulose membrane, absorbent paper, and a contrast color scale card. The nitrocellulose membrane is fixed to the middle of the base plate with an adhesive, the absorbent paper is fixed to the right side of the base plate with an adhesive, the glass fiber membrane is fixed to the left side of the base plate with an adhesive, the bottom left side of the glass fiber membrane overlaps the top right side of the reagent release pad, and a contrast color scale card is adhered to one side of the base plate of the nitrocellulose membrane. The contrast color scale card has 3 to 7 color levels.

[0010] The nitrocellulose membrane is formed by blending 70-80 parts by weight of nitrocellulose, 10-15 parts by weight of plasticizer, 5-10 parts by weight of surfactant, 3-8 parts by weight of pore-forming agent and 1-3 parts by weight of stabilizer, and is then dissolved, degassed, cast and cured. The average pore size of the nitrocellulose membrane is 2.0-4.0 μm and the porosity is 70-80%.

[0011] Furthermore, the base plate is a white PVC base plate.

[0012] Furthermore, the left end of the nitrocellulose membrane overlaps the top right side of the glass cellulose membrane, and the right end of the nitrocellulose membrane overlaps the bottom left side of the absorbent paper.

[0013] Furthermore, the base plate is provided with positioning grooves for attaching contrast color scale cards.

[0014] Furthermore, the nitrocellulose membrane is treated using the following method:

[0015] Place 75 parts of nitrocellulose in a sealed mixing tank, add an appropriate amount of mixed solvent, and stir at 250 rpm for 2 hours at a constant temperature of 30°C until the nitrocellulose is completely dissolved to form a transparent and viscous matrix solution. Then, add 12 parts of plasticizer, 8 parts of surfactant, 4 parts of pore-forming agent and 1 part of stabilizer to the matrix solution at once, and continue stirring at 200 rpm for 1.5 hours at 30°C to make the components evenly dispersed in the polymer chain and form a uniform casting solution.

[0016] The above-mentioned uniformly mixed casting solution was transferred to a vacuum degassing tank and allowed to stand for 60 minutes under a vacuum of -0.07 MPa and a temperature of 25°C until there were no visible bubbles on the surface of the casting solution and the viscosity returned to stability.

[0017] The degassed casting solution is uniformly cast onto a corona-treated polyester film carrier through a slit-type die of a casting machine. The doctor blade gap is set to 250 μm, the casting speed is controlled at 1.2 m / min, the ambient temperature is maintained at 28℃, and the relative humidity is controlled at 55%.

[0018] The cast wet film, along with the polyester film carrier, is sent into the drying tunnel for gradient drying and curing. After the film is completely dry, it is peeled off from the polyester film carrier to obtain the finished nitrocellulose film.

[0019] Furthermore, the mixed solvent consists of acetone and ethyl acetate, with an acetone:ethyl acetate ratio of 7:3.

[0020] Furthermore, the drying and curing process is divided into a pre-drying section and a curing section, wherein:

[0021] The pre-drying section is at 40°C for 10 minutes, allowing most of the acetone to evaporate.

[0022] Curing stage: Temperature 55℃, time 20 minutes, to remove residual solvent, allow plasticizer and nitrocellulose to fully blend, and complete the curing of the membrane structure.

[0023] A method for detecting heavy metals in water using a test strip, comprising the following steps:

[0024] S1. Prepare water sample: Take the water sample to be tested. The water sample temperature should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0025] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad of the test paper. The water sample will automatically flow towards the absorbent paper under capillary action.

[0026] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The water sample flows through the reagent release pad, glass cellulose membrane, and nitrocellulose membrane in sequence. Heavy metal ions and colorimetric reagents complex and develop color on the nitrocellulose membrane.

[0027] S4. Compare and obtain the results: Observe the color on the nitrocellulose membrane and visually compare it with the contrast color scale card (6). Determine the concentration range of heavy metals in the water sample based on the color depth.

[0028] Furthermore, in S3, the reagent release pad may be any one of Hach4315-30 / 70, Hach2603649, CytivaRapid24 / 27 and MDIPT-R6 / R7 that has been loaded with heavy metal colorimetric reagent.

[0029] Furthermore, in S3, the colorimetric reagent is dissolved in water on the reagent release pad and then flows with the water to the nitrocellulose membrane, where it reacts with the heavy metals in the water to produce color.

[0030] Compared with the prior art, the present invention has the following technical advantages:

[0031] 1. Excellent on-site rapid detection capabilities, meeting both emergency and real-time monitoring needs.

[0032] This invention completely breaks away from the reliance of traditional detection technologies on laboratory instruments, enabling rapid on-site screening of heavy metals in water and providing an efficient solution for water quality monitoring in various scenarios. The test strip requires only three core steps: preparing the water sample, adding the sample for reaction, and comparing with the color chart. It eliminates the need for complex water sample pretreatment procedures and specialized testing equipment. The test strip can be used normally within a wide temperature range of 15–35°C, requiring no additional temperature control equipment and adapting to different environmental conditions. After adding the sample, the color reaction is completed within 3–5 minutes. Compared to laboratory testing cycles of several hours or even days, and traditional on-site testing methods with reaction times exceeding 30 minutes, the detection efficiency is significantly improved. In complex scenarios such as water sources, factory drainage outlets, farmland irrigation areas, and sudden water pollution sites, staff do not need to carry heavy instruments; they only need a small amount of test strips and droppers to quickly complete water quality testing and obtain timely information on heavy metal concentration ranges, providing real-time data support for pollution control, emergency response, and compliance assessment.

[0033] 2. High specificity and no false positive interference, accurate and reliable test results.

[0034] This invention achieves extremely high detection specificity through synergistic optimization of test strip structure design, core material preparation, and colorimetric system selection, completely solving the industry pain point of false positive results in traditional test strips. The core reaction area of ​​the test strip uses a custom-prepared nitrocellulose membrane with an average pore size controlled between 2.0 and 4.0 μm and a porosity of 70% to 80%. This structure ensures rapid diffusion and contact between the water sample and the colorimetric reagent while effectively retaining reaction products, avoiding interference caused by non-specific adsorption. The reagent release pad uses specialized materials such as Hach4315-30 / 70 and Hach2603649, which are loaded with specific heavy metal colorimetric agents. For different heavy metal ions such as copper, iron, and manganese, corresponding specific colorimetric agents are selected, such as sodium diethyldithiocarbamate for copper ions, o-phenanthroline for iron ions, and formaldehyde oxime complex for manganese ions. These colorimetric agents have high specificity in complexation reaction with the target heavy metal ions and will not react with common macro ions such as calcium, magnesium, potassium, and sodium in water.

[0035] Experimental data fully validated its excellent specificity: when testing 0.5 mg / L copper, iron, and manganese standard solutions, the test strip exhibited distinct characteristic colors, perfectly matching the concentration colors on the colorimetric chart; however, when testing blank water samples such as distilled water, the test strip showed no significant color change, remaining consistent with the blank area on the colorimetric chart, with no false positive results. Furthermore, during the preparation of the nitrocellulose membrane, surfactants were added to optimize the membrane's hydrophilicity and reactivity, and stabilizers were added to ensure the stability of the colorimetric system, further reducing the interference of external environmental factors (such as temperature and humidity) on the reaction and ensuring the accuracy and repeatability of the test results. Whether in complex industrial wastewater and domestic sewage, or in relatively simple surface water and drinking water, this test strip can accurately identify target heavy metal ions, providing reliable data support for water quality assessment and pollution control, and avoiding misjudgments and unnecessary disposal costs caused by false positive results.

[0036] 3. Extremely easy to operate, reducing the barrier to entry and labor costs.

[0037] This invention is designed with the operational capabilities of different user groups in mind. By simplifying the process and optimizing the structure, it achieves a user experience of "no professional training required, operable by everyone," significantly lowering the barrier to entry for testing technology. Unlike traditional testing methods that require professional technicians to master complex operations such as water sample pretreatment, instrument calibration, and reagent mixing, the usage process of this test strip is extremely simple: just use a dropper to draw 1-2 drops of the water sample to be tested and add them to the reagent release pad. There is no need to control the precise value of the added amount, nor is there any need for water sample filtration, acidification, or other pretreatment. After adding the sample, no additional operation is required. The water sample will automatically flow through the reagent release pad, glass cellulose membrane, and nitrocellulose membrane in sequence under capillary action to complete the colorimetric reaction. Finally, by visually comparing the color of the nitrocellulose membrane with the colorimetric chart, the heavy metal concentration range can be quickly determined without the need for any auxiliary equipment for reading.

[0038] The test strip's structural design further enhances ease of use: the base plate is made of white PVC material, providing a clear background for color observation and avoiding interference from background color on the test results; a dedicated positioning groove is set on the base plate to ensure accurate placement of the contrast color scale card, facilitating quick comparison by users; the overlap length between each membrane layer is controlled between 1.0mm and 2.5mm, ensuring smooth flow of water samples while preventing membrane layer detachment or displacement, thus ensuring the structural stability of the test strip during use.

[0039] 4. It has a wide detection range and strong adaptability, meeting the needs of multiple scenarios and multiple metal detection.

[0040] This invention achieves comprehensive detection coverage of multiple heavy metals through a modular design of core materials and a colorimetric system, while also possessing strong environmental adaptability to meet detection needs in various scenarios. Regarding the detection targets, the test strip can accurately detect the most common heavy metal ions in water, such as copper, iron, and manganese. By changing different reagent release pads (loaded with specific colorimetric reagents corresponding to the heavy metals) and contrast colorimetric cards, it can be extended to detect other heavy metals such as lead and cadmium, demonstrating good scalability and versatility. The contrast colorimetric card has 3 to 7 color levels, and the concentration range can be adjusted according to actual needs (e.g., 0 mg / L, 0.5 mg / L, 1.0 mg / L, 1.5 mg / L, etc.), enabling rapid determination of the concentration range of heavy metals in water samples. This not only meets the need for rapid screening to determine whether levels exceed standards but also provides a reference for assessing the degree of pollution.

[0041] In terms of environmental adaptability, the test strip can operate normally within a temperature range of 15–35℃, covering the temperature conditions of most natural environments and industrial scenarios. Strict humidity control is not required during the detection process, and it exhibits stable color development in both typical indoor and outdoor environments. The core component of the test strip, the nitrocellulose membrane, is prepared using a gradient drying and curing process. This results in a stable membrane structure, good flexibility, and resistance to damage, making it easy to store and carry. It can be stored for extended periods under sealed conditions at room temperature, and requires no special protection during transportation, further enhancing its applicability in complex scenarios such as fieldwork and emergency situations. Whether in the hot and rainy southern regions or the dry and cold northern regions; whether in high-concentration pollution scenarios such as factory drainage outlets and mine perimeters, or in low-concentration heavy metal detection scenarios such as water sources and drinking water, this test strip consistently performs its detection function, enabling rapid screening and concentration determination of multiple heavy metals. It solves the problem of traditional detection technologies being "single-function and limited in application scenarios," providing an integrated solution for water quality monitoring in various fields.

[0042] 5. The preparation process is controllable and inexpensive, facilitating large-scale production and promotion.

[0043] The test strip preparation process of this invention has high controllability and repeatability, and the raw materials are readily available and low in cost, laying a solid foundation for large-scale production and widespread application. The nitrocellulose membrane, as the core component of the test strip, has undergone precise optimization in its preparation process: the mixed solvent uses a 7:3 ratio of acetone to ethyl acetate, which can fully dissolve the nitrocellulose while controlling the solvent evaporation rate, ensuring the stability of the casting solution; the weight ratios of each component (70-80 parts nitrocellulose, 10-15 parts plasticizer, etc.) have been screened extensively through experiments to ensure the stability of the membrane's structure and performance; all process parameters, such as stirring, degassing, casting, and drying (e.g., stirring temperature 30℃, casting speed 1.2m / min, gradient drying temperature, etc.), are clearly defined and controllable, enabling precise replication in industrial production and ensuring consistent performance of mass-produced test strips.

[0044] In terms of cost, the main raw materials of the test strip include nitrocellulose, plasticizers, surfactants, pore-forming agents, and stabilizers, all of which are commonly used materials in the chemical industry, resulting in low procurement costs. The preparation process does not require complex high-end equipment; casting machines, vacuum degassing tanks, and drying tunnels are all mature industrial equipment, making equipment investment and operating costs controllable. The detection process does not consume expensive reagents and consumables, and the cost per test strip is far lower than the cost per test in laboratory testing and portable instruments. This low-cost advantage enables the test strip to meet the needs of large-scale, routine monitoring, such as the regular screening of numerous water sources and sewage outlets by environmental protection departments, and the daily monitoring of industrial wastewater by enterprises, significantly reducing the overall cost of water quality monitoring. At the same time, large-scale production can further reduce costs, promoting the widespread application of the test strip in grassroots units, small and medium-sized enterprises, and the agricultural sector, allowing more users to enjoy convenient and accurate heavy metal detection services and contributing to the overall improvement of water environmental quality. Attached Figure Description

[0045] Fig. 1 This is a schematic diagram of the structure of the water heavy metal detection test strip of the present invention;

[0046] Fig. 2 This is a schematic diagram of the contrast color scale card structure in this invention.

[0047] In the diagram: 1. Base plate; 2. Glass cellulose membrane; 3. Reagent release pad; 4. Nitrocellulose membrane; 5. Absorbent paper; 6. Comparison color scale card. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Please see Figs. 1-2A water heavy metal test strip includes a base plate 1, a glass fiber membrane 2, a reagent release pad 3, a nitrocellulose membrane 4, absorbent paper 5, and a contrast color scale card 6. The base plate 1 is a white PVC base plate. The nitrocellulose membrane 4 is fixed to the middle of the base plate 1 with adhesive. The absorbent paper 5 is fixed to the right side of the base plate 1 with adhesive. The left end of the nitrocellulose membrane 4 overlaps the top right side of the glass fiber membrane 2, and the right end of the nitrocellulose membrane 4 overlaps the bottom left side of the absorbent paper 5. The glass fiber membrane 2 is fixed to the left side of the base plate 1 with adhesive, which can be 3M adhesive. The bottom left side of the glass fiber membrane 2 overlaps the top right side of the reagent release pad 3, with each overlap length being 1.0mm to 2.5mm. A contrast color scale card 6 is adhered to one side of the base plate 1 of the nitrocellulose membrane 4. The base plate 1 has positioning grooves for attaching the contrast color scale card 6. The contrast color scale card 6 has 3 to 7 color levels, and the corresponding heavy metal contrast color scale card can be replaced according to the usage requirements.

[0050] The nitrocellulose membrane 4 is made by blending 70-80 parts by weight of nitrocellulose, 10-15 parts by weight of plasticizer, 5-10 parts by weight of surfactant, 3-8 parts by weight of pore-forming agent and 1-3 parts by weight of stabilizer, and then dissolving, degassing, casting and curing. The average pore size of the nitrocellulose membrane 4 is 2.0-4.0 μm and the porosity is 70-80%.

[0051] Nitrocellulose membrane 4 was treated using the following method:

[0052] 75 parts of nitrocellulose were placed in a sealed stirring tank, and an appropriate amount of mixed solvent was added. The mixed solvent consisted of acetone and ethyl acetate in a ratio of 7:3. The mixture was stirred at 250 rpm for 2 hours at a constant temperature of 30°C until the nitrocellulose was completely dissolved, forming a transparent and viscous matrix solution. Then, 12 parts of plasticizer, 8 parts of surfactant, 4 parts of pore-forming agent and 1 part of stabilizer were added to the matrix solution at once. The mixture was stirred at 200 rpm for 1.5 hours at 30°C to ensure that the components were evenly dispersed in the polymer chain and to form a uniform casting solution.

[0053] The above-mentioned uniformly mixed casting solution was transferred to a vacuum degassing tank and allowed to stand for 60 minutes under a vacuum of -0.07 MPa and a temperature of 25°C until there were no visible bubbles on the surface of the casting solution and the viscosity returned to stability.

[0054] The degassed casting solution is uniformly cast onto a corona-treated polyester film carrier through a slit-type die of a casting machine. The doctor blade gap is set to 250 μm, the casting speed is controlled at 1.2 m / min, the ambient temperature is maintained at 28℃, and the relative humidity is controlled at 55%.

[0055] The cast wet film, along with the polyester film carrier, is sent into the drying tunnel for gradient drying and curing. The drying and curing process is divided into a pre-drying section and a curing section. The pre-drying section is at a temperature of 40°C for 10 minutes to allow most of the acetone to evaporate. The curing section is at a temperature of 55°C for 20 minutes to remove residual solvents and allow the plasticizer to fully integrate with the nitrocellulose, thus completing the curing of the film structure. After the film layer is completely dry, it is peeled off from the polyester film carrier to obtain the finished nitrocellulose film.

[0056] A method for detecting heavy metals in water using a test strip, comprising the following steps:

[0057] S1. Prepare water sample: Take the water sample to be tested. The water sample temperature should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0058] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0059] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The colorimetric reagent will dissolve in water on the reagent release pad 3 and then flow with the water to the nitrocellulose membrane 4. The reagent will then react with the heavy metals in the water on the nitrocellulose membrane 4 to produce a color. The reagent release pad 3 can be any one of the following: Hach4315-30 / 70, Hach2603649, Cytiva Rapid24 / 27 and MDIPT-R6 / R7, which is a release pad loaded with heavy metal colorimetric reagent.

[0060] S4. Compare and obtain the results: Observe the color on the nitrocellulose membrane 4 and visually compare it with the contrast color scale card 6. Determine the concentration range of heavy metals in the water sample based on the color depth.

[0061] Copper, iron, and manganese are the three most common heavy metals exceeding the standard in environmental water samples (domestic sewage, industrial wastewater, and surface water). Therefore, copper, iron, and manganese solutions were selected as the water samples to be tested in this example.

[0062] The color scale chart 6 used in the comparison uses a color scale chart containing three metals: copper, iron, and manganese, with concentrations of 0 mg / L, 0.5 mg / L, 1.0 mg / L, and 1.50 mg / L, respectively. The color scale for copper is: white → light brownish-orange → reddish-brown / brick red → dark reddish-brown; the color scale for iron is: white → light pink → true red → dark red; the color scale for manganese is: white → light yellow → earthy yellow → coffee. As the concentration increases from 0 mg / L to 1.5 mg / L, the color gradually deepens from white.

[0063] Technical principle of the invention:

[0064] (a) The role of each raw material

[0065] 1. Nitrocellulose: As the matrix material of nitrocellulose membranes, it accounts for 70-80 parts of the membrane composition and is the core raw material that determines the membrane structure and performance. Nitrocellulose has good film-forming properties, mechanical strength, and chemical stability, providing a stable support for colorimetric reactions. The ester groups in its molecular structure have a certain polarity, which can interact with plasticizers, surfactants, and other components, ensuring the structural integrity of the membrane. At the same time, the porous nature of nitrocellulose provides channels for the diffusion and contact of water samples and colorimetric agents, ensuring that heavy metal ions can fully react with the colorimetric agents.

[0066] 2. Plasticizer: Dibutyl phthalate (DBP), added at a ratio of 10-15 parts, mainly to improve the flexibility and plasticity of nitrocellulose membranes. Nitrocellulose itself is brittle and easily breaks or breaks after direct film formation. By adding plasticizers, its molecules can insert between the polymer chains of nitrocellulose, weakening the intermolecular forces and significantly improving the flexibility of the membrane, making it easier to cut, store, and use the test strips. At the same time, the plasticizer can also fully integrate with nitrocellulose, optimizing the microstructure of the membrane and preventing cracking or deformation during drying or use, thus ensuring the stability of the membrane.

[0067] 3. Surfactant: Sodium dodecyl sulfate (SDS), added at a ratio of 5-10 parts. Its core function is to reduce the surface tension of the nitrocellulose membrane and improve its hydrophilicity. Nitrocellulose itself has a certain degree of hydrophobicity, which affects the diffusion rate and uniformity of water samples within the membrane. The surfactant can form hydrophilic groups on the membrane surface, improving the membrane's wettability to the water sample, allowing the water sample to diffuse quickly and uniformly within the membrane, ensuring sufficient contact between heavy metal ions and the colorimetric reagent. In addition, the surfactant can also promote the dissolution and dispersion of the colorimetric reagent in the water sample, improving reaction efficiency and avoiding problems such as uneven color development and decreased sensitivity caused by excessively high or low local concentrations of the colorimetric reagent.

[0068] 4. Pore-forming agent: Polyethylene glycol (PEG-400) is added at a ratio of 3-8 parts to control the pore size and porosity of the nitrocellulose membrane. The pore-forming agent forms a temporary pore template during membrane preparation and precipitates out during the drying and curing stage as the solvent evaporates, ultimately forming a uniformly distributed microporous structure within the membrane. This invention controls the average pore size of the membrane to 2.0-4.0 μm and the porosity to 70-80%. This structure ensures rapid passage of water samples and chromogenic agents while effectively retaining complexation products of heavy metal ions and chromogenic agents, concentrating the color on the membrane surface for easy observation and comparison. Simultaneously, the optimized pore structure reduces non-specific adsorption and improves the specificity of the detection.

[0069] 5. Stabilizer: Ascorbic acid (vitamin C), added at a ratio of 1-3 parts. Its main function is to ensure the long-term stability of the nitrocellulose membrane and the colorimetric system. The stabilizer can inhibit the degradation of nitrocellulose during storage and use, preventing changes in the membrane's structure and properties. At the same time, it can synergistically work with the colorimetric agent to prevent oxidation and decomposition of the colorimetric agent, ensuring the stability and repeatability of the colorimetric reaction and avoiding inaccurate test results due to colorimetric agent failure.

[0070] 6. Mixed Solvent: Composed of acetone and ethyl acetate in a 7:3 ratio, serving as the dissolving medium for nitrocellulose. Acetone has extremely strong dissolving power, rapidly dissolving nitrocellulose, while ethyl acetate has a relatively slow evaporation rate, allowing for adjustment of the solvent system's evaporation rate and preventing defects such as cracks and bubbles on the film surface caused by excessively rapid solvent evaporation. The synergistic effect of both ensures complete dissolution of nitrocellulose, forming a uniform matrix solution, and provides a stable environment for subsequent casting and drying processes, guaranteeing the quality of the formed film.

[0071] 7. Reagent Release Pad: This pad is made of a specialized material pre-loaded with a heavy metal-specific chromogenic agent (such as Hach4315-30 / 70). Its core function is to store the chromogenic agent and rapidly release it after the water sample is added. The material of the reagent release pad has excellent water absorption and release properties, enabling it to quickly dissolve the chromogenic agent upon contact with the water sample. Through capillary action, the mixture of chromogenic agent and water sample is transferred to the nitrocellulose membrane, providing sufficient reactants for the colorimetric reaction.

[0072] (ii) Synergistic effect of each raw material

[0073] The core advantage of this invention stems from the synergistic effect between the raw materials. Through the interaction of multiple components, the detection efficiency, specificity, and stability are simultaneously improved. First, nitrocellulose, as the matrix, provides a stable loading platform for other components. The fusion of plasticizer and nitrocellulose, along with the synergistic effect of surfactant, optimizes the membrane's flexibility, preventing damage after water sample immersion. The surfactant improves the membrane's hydrophilicity, ensuring rapid diffusion of the water sample. Together, they ensure smooth transport of the water sample within the membrane, creating favorable conditions for the reaction. Second, the pore-forming agent and surfactant synergistically regulate the membrane's microstructure and surface properties: the 2.0–4.0 μm microporous structure formed by the pore-forming agent provides ample space for contact between heavy metal ions and the chromogenic agent, while the surfactant promotes the dissolution and dispersion of the chromogenic agent, ensuring uniform distribution of both within the membrane and significantly improving reaction efficiency and color uniformity. Simultaneously, the combination of the pore structure and the hydrophilicity of the surfactant reduces water sample residue within the membrane, preventing cross-contamination.

[0074] The addition of stabilizers forms a synergistic stabilizing system with nitrocellulose and the chromogenic agent: the stabilizer inhibits the degradation of nitrocellulose, extends the membrane's lifespan, and protects the chromogenic agent from oxidation, ensuring the specificity of the colorimetric reaction. This, combined with the specific chromogenic agent on the reagent release pad, further improves the accuracy of the detection and avoids false positive results. The synergistic effect of the mixed solvent and each membrane component is equally crucial: the 7:3 ratio of acetone to ethyl acetate not only fully dissolves nitrocellulose but also ensures that plasticizers, surfactants, and other components are uniformly dispersed in the matrix solution, laying the foundation for subsequent casting and drying curing, and avoiding differences in membrane performance due to uneven component dispersion. Furthermore, the synergistic effect of the reagent release pad and the nitrocellulose membrane: the reagent release pad rapidly releases the chromogenic agent, while the nitrocellulose membrane provides the reaction site and retains the reaction products. The overlapping design between the two achieves efficient transfer of the chromogenic agent and the water sample, ensuring the reaction is completed rapidly within 3–5 minutes, while also guaranteeing the stability and intuitiveness of the colorimetric effect. The synergistic effect of the raw materials is not a simple superposition of properties, but rather forms a complete and efficient system of "dissolution-film formation-transfer-reaction-color development", which ultimately achieves technical effects of high specificity, rapid detection and convenient operation.

[0075] (III) Necessity and Importance of Process Parameter Screening

[0076] All process parameters in the nitrocellulose membrane preparation and testing process of this invention have been determined through extensive experimental screening. Precise control of each parameter is crucial to the performance of the test strip and directly affects the membrane structure, reaction efficiency, and accuracy of the test results.

[0077] 1. Process parameters for the preparation of nitrocellulose membranes:

[0078] Stirring parameters: When dissolving nitrocellulose, control the temperature at 30℃, the stirring speed at 250 rpm, and the time at 2 hours. After adding other components, maintain the stirring temperature at 30℃, the stirring speed at 200 rpm, and the time at 1.5 hours. The necessity of these parameters is as follows: the constant temperature environment of 30℃ ensures rapid dissolution of nitrocellulose while avoiding degradation of the raw material due to high temperature; the high stirring speed of 250 rpm ensures complete dissolution of nitrocellulose, forming a homogeneous matrix solution; and the 200 rpm speed ensures uniform dispersion of each component while avoiding the generation of bubbles caused by excessive speed, thus ensuring the homogeneity of the casting solution. If the stirring temperature is too high, it may lead to degradation of nitrocellulose and a decrease in the mechanical strength of the membrane; if the stirring speed is too low or the time is insufficient, it will result in uneven dispersion of components and unstable pore structure and performance of the membrane.

[0079] Degassing parameters: vacuum degree -0.07MPa, temperature 25℃, time 60 minutes. The core purpose of the degassing process is to remove air bubbles from the casting solution. If the air bubbles are not completely removed, pores and defects will form on the surface of the cast film, affecting the water sample transfer and reaction effect. The vacuum degree of -0.07MPa and the time of 60 minutes can completely remove visible air bubbles from the casting solution, while the temperature of 25℃ avoids the viscosity change of the casting solution caused by excessive solvent evaporation, thus ensuring the quality of the film formation.

[0080] Casting parameters: doctor blade gap 250μm, casting speed 1.2m / min, ambient temperature 28℃, relative humidity 55%. The doctor blade gap directly determines the film thickness. A gap of 250μm can produce a film layer with uniform thickness and suitable mechanical strength. If it is too thick, the diffusion rate of the water sample will be slowed down, and if it is too thin, the stability of the film will be insufficient. The casting speed of 1.2m / min, combined with the ambient temperature of 28℃ and the relative humidity of 55%, ensures that the casting solution is evenly spread on the polyester film carrier, avoiding uneven film thickness caused by casting too fast, or excessive solvent evaporation caused by unsuitable temperature and humidity, which can lead to cracks.

[0081] Gradient drying parameters: Pre-drying section 40℃, 10 minutes; curing section 55℃, 20 minutes. The low temperature and short time setting of the pre-drying section allows most of the acetone to evaporate slowly, avoiding membrane surface shrinkage and cracking caused by rapid solvent evaporation. The high temperature and long time setting of the curing section thoroughly removes residual solvent and promotes the full integration of plasticizer and nitrocellulose, completing the curing of the membrane structure and ensuring the membrane's flexibility and stability. If the drying temperature is too high or the time is too long, the membrane porosity will decrease, affecting the reaction efficiency; if the temperature is too low or the time is insufficient, residual solvent will interfere with the colorimetric reaction, reducing the accuracy of detection.

[0082] 2. Testing method and process parameters:

[0083] Water sample temperature 15~35℃: This temperature range is the suitable temperature for the colorimetric reaction. Too high a temperature will cause the colorimetric agent to decompose, while too low a temperature will inhibit the complexation reaction, both of which will affect the colorimetric effect and the detection results. Controlling this temperature range ensures the applicability of the test strip in different environments.

[0084] Adding 1 to 2 drops: This amount of sample ensures sufficient water sample for the reaction to proceed fully, while avoiding water overflow or concentration imbalance in the reaction system due to excessive sample addition; at the same time, it eliminates the need for precise control of the sample addition amount, thus lowering the operational threshold.

[0085] Reaction time 3-5 minutes: This time is the optimal time for heavy metal ions to fully complex with the colorimetric reagent. If the time is too short, the reaction will be insufficient, the color development will be unclear, and the concentration cannot be accurately determined. If the time is too long, it may lead to the decomposition of the colorimetric reagent or non-specific reactions, affecting the detection results.

[0086] The selection of various process parameters is not arbitrary, but rather based on the characteristics of raw materials and the reaction mechanism, determined through numerous orthogonal experiments to achieve the optimal combination. Precise control of each parameter is crucial to ensuring stable test strip performance and accurate detection results. Deviation from any parameter can lead to structural defects in the membrane, decreased reaction efficiency, and insufficient specificity. Therefore, the selection and control of process parameters are of irreplaceable importance to the realization of this invention.

[0087] (iv) Unexpected technical effects

[0088] Through the synergistic effect of various raw materials and precise control of process parameters, this invention achieves several unexpected technical effects, significantly outperforming existing technologies:

[0089] 1. Breakthrough in Low-Concentration Detection Sensitivity: The test strip of this invention can accurately detect copper, iron, and manganese standard solutions at concentrations of 0.5 mg / L, with a clear color development effect that is completely consistent with the color of the corresponding concentration on the colorimetric card. Existing traditional test strips typically lack sufficient sensitivity for detecting low concentrations of heavy metals below 1.0 mg / L, with unclear color development and difficulty in accurate judgment. However, this invention significantly improves the ability to identify low concentrations of heavy metal ions by optimizing the pore structure of the nitrocellulose membrane (2.0–4.0 μm pore size, 70–80% porosity), adjusting the hydrophilicity of the surfactant, and synergistically using a specific colorimetric agent. This meets the needs of low-concentration heavy metal detection scenarios such as drinking water sources and surface water. This improvement in sensitivity is difficult to achieve with existing technologies.

[0090] 2. Stable Color Development over a Wide Temperature Range: The test strips of this invention exhibit stable color development over a wide temperature range of 15–35°C, eliminating the need for additional temperature control equipment. Most existing test strips have a narrow suitable temperature range (typically 20–25°C). Temperatures that are too high or too low can lead to lighter color development, prolonged development time, or even no color development at all. This invention, through the addition of stabilizers and a gradient drying process using nitrocellulose membranes, enhances the temperature adaptability of the color development system. Even at low temperatures of 15°C or high temperatures of 35°C, color development can be completed within 3–5 minutes, and the correlation between color depth and concentration remains stable. This adapts to environmental conditions in different regions and seasons, expanding its applicability.

[0091] 3. Cross-interference-free detection of multiple metals: This invention enables the detection of multiple heavy metals such as copper, iron, and manganese by replacing the reagent release pad and the contrast colorimetric card, with no cross-interference between different heavy metal ions. Some existing test strips tend to exhibit color overlap or mutual inhibition when detecting multiple heavy metals, leading to inaccurate results. However, the synergistic effect of the specific colorimetric reagent and nitrocellulose membrane used in this invention ensures that each colorimetric reagent reacts only with the target heavy metal ion and does not cross-react with other heavy metal ions, achieving precise screening of multiple metals. This effect surpasses the single-detection function of traditional test strips, providing a convenient solution for the detection of multi-component heavy metal pollution.

[0092] 4. Excellent long-term storage stability: After 12 months of storage at room temperature and under sealed conditions, the test strips of this invention show no significant decrease in detection performance. They still accurately develop color in 0.5 mg / L heavy metal standard solutions, with results consistent with freshly prepared test strips. Existing traditional test strips typically have a short shelf life (less than 6 months), and long-term storage can lead to membrane aging and colorimetric agent failure, resulting in decreased detection sensitivity. This invention, through the addition of stabilizers, optimization of the membrane structure via gradient drying, and a dedicated packaging design for the reagent release pad, significantly improves the storage stability of the test strips, reduces storage and transportation costs, and facilitates large-scale promotion and application.

[0093] These unexpected technical effects stem from the precise proportions and synergistic effects of the raw materials, as well as the strict control of process parameters. This not only solves many pain points of existing technologies, but also expands the application scenarios and value of test strips, demonstrating the innovation and practicality of this invention.

[0094] To make the present invention more fully disclosed, more specific embodiments are described below.

[0095] Example 1

[0096] A method for detecting heavy metals in water using a test strip, comprising the following steps:

[0097] S1. Prepare water sample: Take a copper standard solution with a concentration of 0.5 mg / L as the water sample to be tested. The water sample temperature should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0098] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0099] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The color reagent is dissolved in water on the reagent release pad 3 and then flows to the nitrocellulose membrane 4 with the water. Then, it reacts with the heavy metal in the water on the nitrocellulose membrane 4 and develops color. The reagent release pad 3 can be a release pad of Hach2603649 that has been loaded with a color reagent based on sodium diethyldithiocarbamate.

[0100] S4. Comparison results: Observe the color on nitrocellulose membrane 4 and visually compare it with the contrast color scale card 6. Nitrocellulose membrane 4 shows a distinct light brownish-orange color, which is consistent with the color at 0.5 mg / L on the contrast color scale card 6.

[0101] Example 2

[0102] A method for detecting heavy metals in water using a test strip, comprising the following steps:

[0103] S1. Prepare water sample: Take a 0.5 mg / L iron standard solution as the water sample to be tested. The water sample temperature should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0104] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0105] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The color reagent is dissolved in water on the reagent release pad 3 and then flows to the nitrocellulose membrane 4 with the water. Then, it reacts with the heavy metal in the water on the nitrocellulose membrane 4 and develops color. The reagent release pad 3 can be a Hach 4315-70 release pad loaded with a color reagent based on o-phenanthroline.

[0106] S4. Comparison results: Observe the color on nitrocellulose membrane 4 and visually compare it with the color scale card 6. Nitrocellulose membrane 4 shows a distinct light pink color, which is consistent with the color at 0.5 mg / L on the color scale card.

[0107] Example 3

[0108] A method for detecting heavy metals in water using a test strip, comprising the following steps:

[0109] S1. Prepare water sample: Take a 0.5 mg / L manganese standard solution as the water sample to be tested. The water sample temperature should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0110] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0111] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The color reagent is dissolved in water on the reagent release pad 3 and then flows to the nitrocellulose membrane 4 with the water. Then, it reacts with the heavy metals in the water on the nitrocellulose membrane 4 and develops color. The reagent release pad 3 can be an MDI PT-R6 release pad loaded with a formaldehyde oxime-based composite color reagent.

[0112] S4. Comparison results: Observe the color on nitrocellulose membrane 4 and visually compare it with the color scale card 6. Nitrocellulose membrane 4 shows a distinct light yellow color, which is consistent with the color at 0.5 mg / L on the color scale card.

[0113] Comparative Example 1

[0114] S1. Prepare water sample: Take distilled water as the water sample to be tested. The temperature of the water sample should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0115] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0116] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The color reagent is dissolved in water on the reagent release pad 3 and then flows to the nitrocellulose membrane 4 with the water. Then, it reacts with the heavy metal in the water on the nitrocellulose membrane 4 and develops color. The reagent release pad 3 can be a release pad of Hach2603649 that has been loaded with a color reagent based on sodium diethyldithiocarbamate.

[0117] S4. Comparison results: Observe the color on the nitrocellulose membrane 4 and visually compare it with the contrast color scale card 6. The nitrocellulose membrane 4 shows no obvious color change and is consistent with the blank area of ​​the color scale card.

[0118] Comparative Example 2

[0119] S1. Prepare water sample: Take distilled water as the water sample to be tested. The temperature of the water sample should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0120] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0121] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The color reagent is dissolved in water on the reagent release pad 3 and then flows to the nitrocellulose membrane 4 with the water. Then, it reacts with the heavy metal in the water on the nitrocellulose membrane 4 and develops color. The reagent release pad 3 can be a Hach 4315-70 release pad loaded with a color reagent based on o-phenanthroline.

[0122] S4. Comparison results: Observe the color on the nitrocellulose membrane 4 and visually compare it with the contrast color scale card 6. The nitrocellulose membrane 4 shows no obvious color change and is consistent with the blank area of ​​the color scale card.

[0123] Comparative Example 3

[0124] S1. Prepare water sample: Take distilled water as the water sample to be tested. The temperature of the water sample should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction.

[0125] S2. Sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad 3 of the test paper. The water sample will automatically flow towards the absorbent paper 5 under capillary action.

[0126] S3. Wait for the reaction: After adding the sample, let it stand for 3-5 minutes. The color reagent is dissolved in water on the reagent release pad 3 and then flows to the nitrocellulose membrane 4 with the water. Then, it reacts with the heavy metals in the water on the nitrocellulose membrane 4 and develops color. The reagent release pad 3 can be an MDI PT-R6 release pad loaded with a formaldehyde oxime-based composite color reagent.

[0127] S4. Comparison results: Observe the color on the nitrocellulose membrane 4 and visually compare it with the contrast color scale card 6. The nitrocellulose membrane 4 shows no obvious color change and is consistent with the blank area of ​​the color scale card.

[0128] As shown in Table 1, the test strip of this invention can detect common heavy metals in water, such as copper, iron, and manganese, with colorimetric reactions. The test strip for 0.5 mg / L copper standard solution produces a purplish-red color, for 0.5 mg / L iron standard solution it produces an orange-red color, and for 0.5 mg / L manganese standard solution it produces a reddish-purple color. All these colors are consistent with the colorimetric results of the corresponding heavy metal detection comparison charts. Furthermore, three groups of distilled water were selected as blank controls, and the test strips showed no significant color change, indicating that the test strips of this invention have good specificity and no false positive interference. Water sample testing does not require laboratory instruments; it can be completed on-site at water sources, factory drainage outlets, etc., requiring only three steps: "sample immersion, static color development, and comparison with the color chart."

[0129] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A test strip for heavy metals in water, characterized in that: The device includes a base plate (1), a glass cellulose membrane (2), a reagent release pad (3), a nitrocellulose membrane (4), absorbent paper (5), and a color scale card (6). The nitrocellulose membrane (4) is fixed to the middle of the base plate (1) with adhesive. The absorbent paper (5) is fixed to the right side of the base plate (1) with adhesive. The glass cellulose membrane (2) is fixed to the left side of the base plate (1) with adhesive. The bottom left side of the glass cellulose membrane (2) overlaps the top right side of the reagent release pad (3). A color scale card (6) is attached to one side of the base plate (1) of the nitrocellulose membrane (4). The color scale card (6) has 3 to 7 color levels. Wherein: the nitrocellulose membrane (4) is made by mixing 70-80 parts by weight of nitrocellulose, 10-15 parts by weight of plasticizer, 5-10 parts by weight of surfactant, 3-8 parts by weight of pore-forming agent and 1-3 parts by weight of stabilizer, and is formed by dissolving, degassing, casting and curing. The average pore size of the nitrocellulose membrane (4) is 2.0-4.0 μm and the porosity is 70-80%.

2. The water heavy metal detection test strip according to claim 1, characterized in that: The base plate (1) is a white PVC base plate.

3. The water heavy metal detection test strip according to claim 2, characterized in that: The left end of the nitrocellulose membrane (4) overlaps the top right side of the glass cellulose membrane (2), and the right end of the nitrocellulose membrane (4) overlaps the bottom left side of the absorbent paper (5).

4. The water heavy metal detection test strip according to claim 1, characterized in that: The base plate (1) is provided with a positioning groove for pasting the contrast color scale card (6).

5. The water heavy metal detection test strip according to claim 1, characterized in that: The nitrocellulose membrane (4) is treated using the following method: Place 75 parts of nitrocellulose in a sealed mixing tank, add an appropriate amount of mixed solvent, and stir at 250 rpm for 2 hours at a constant temperature of 30°C until the nitrocellulose is completely dissolved to form a transparent and viscous matrix solution. Then, add 12 parts of plasticizer, 8 parts of surfactant, 4 parts of pore-forming agent and 1 part of stabilizer to the matrix solution at once, and continue stirring at 200 rpm for 1.5 hours at 30°C to make the components evenly dispersed in the polymer chain and form a uniform casting solution. The above-mentioned uniformly mixed casting solution was transferred to a vacuum degassing tank and allowed to stand for 60 minutes under a vacuum of -0.07 MPa and a temperature of 25°C until there were no visible bubbles on the surface of the casting solution and the viscosity returned to stability. The degassed casting solution is uniformly cast onto a corona-treated polyester film carrier through a slit-type die of a casting machine. The doctor blade gap is set to 250 μm, the casting speed is controlled at 1.2 m / min, the ambient temperature is maintained at 28℃, and the relative humidity is controlled at 55%. The cast wet film, along with the polyester film carrier, is sent into the drying tunnel for gradient drying and curing. After the film is completely dry, it is peeled off from the polyester film carrier to obtain the finished nitrocellulose film.

6. The water heavy metal detection test strip according to claim 5, characterized in that: The mixed solvent consists of acetone and ethyl acetate in a ratio of 7:

3.

7. The water heavy metal detection test strip according to claim 1, characterized in that: The drying and curing process is divided into a pre-drying section and a curing section, wherein: The pre-drying section is at 40°C for 10 minutes, allowing most of the acetone to evaporate. Curing stage: Temperature 55℃, time 20 minutes, to remove residual solvent, allow plasticizer and nitrocellulose to fully blend, and complete the curing of the membrane structure.

8. A method for detecting heavy metals in water based on the test strip according to any one of claims 1-7, characterized in that, Includes the following steps: S1. Prepare water sample: Take the water sample to be tested. The water sample temperature should be between 15 and 35℃ to avoid high or low temperature affecting the colorimetric reaction. S2, sample addition: Use a dropper to draw up the water sample to be tested, and add 1 to 2 drops to the reagent release pad (3) of the test paper. The water sample will automatically flow towards the absorbent paper (5) under capillary action. S3. Wait for the reaction: After adding the sample, let it stand for 3 to 5 minutes. The water sample flows through the reagent release pad (3), glass cellulose membrane (2), and nitrocellulose membrane (4) in sequence. Heavy metal ions and colorimetric reagents complex and develop color on the nitrocellulose membrane (4). S4. Compare and obtain the results: Observe the color on the nitrocellulose membrane (4) and compare it visually with the contrast color scale card (6). Determine the concentration range of heavy metals in the water sample based on the color depth.

9. The detection method of a heavy metal test strip in water according to claim 8, characterized in that: In S3, the reagent release pad (3) is selected from any one of Hach4315-30 / 70, Hach2603649, Cytiva Rapid24 / 27 and MDIPT-R6 / R7, which is a release pad loaded with heavy metal colorimetric reagent.

10. The detection method of a heavy metal test strip in water according to claim 8, characterized in that: In S3, the color reagent is dissolved in water on the reagent release pad (3) and then flows with the water to the nitrocellulose membrane (4), where it reacts with the heavy metals in the water to produce color.

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