Cobalt-based nanozymes with egg yolk shell structure and their application in mercury ion detection

Abstract

This invention belongs to the field of pollutant detection technology, and relates to a cobalt-based nanozyme with an egg yolk shell structure and its application in mercury ion detection. Cobalt nitrate hexahydrate and hexadecyltrimethylammonium bromide are added to water and mixed evenly; then mixed with 2-methylimidazole solution and stirred to prepare ZIF-67 cubes. These cubes are added to ethanol and mixed evenly, then mixed evenly with an aqueous solution of a polybasic oxyacid, and stirred to prepare ZIF-67 / PTA. Finally, these cubes are added to an ethanol solution and mixed evenly, followed by the addition of chloroauric acid solution and sodium borohydride solution, and stirred to prepare ZIF-67 / PTA / Au. The material synthesized by this invention has a mesoporous structure of egg yolk shell nanocubes, the raw materials are inexpensive, the performance is stable, and it is suitable for large-scale synthesis. Visual detection of Hg²⁺ can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of pollutant detection technology, and relates to a cobalt-based nanozyme with an egg yolk shell structure and its application in mercury ion detection. Background Technology

[0002] With the rapid development of modern industry, heavy metal ion pollution has drawn significant attention to its negative impacts on the aquatic environment and organisms. Hg²⁺, a typical toxic heavy metal, originates from natural emissions and anthropogenic pollution and can enter the human body through skin contact, inhalation, and ingestion. The maximum permissible concentration of Hg²⁺ in water is set at 6 μg / L. The toxicity of Hg²⁺ is primarily manifested in its damage to the nervous system, kidneys, and immune system. Long-term exposure to low concentrations of Hg²⁺ may lead to chronic poisoning, manifesting as symptoms such as memory loss, mood instability, and muscle tremors. Furthermore, Hg²⁺ can accumulate in organisms through biomagnification in the food chain, causing severe damage to ecosystems.

[0003] Therefore, significant efforts have been made in developing Hg²⁺ detection techniques, such as atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), electrochemical analysis, and inductively coupled plasma mass spectrometry (ICP-MS). While these traditional methods offer high sensitivity and accuracy, they also have some significant limitations. First, these methods typically require complex sample pretreatment processes, including digestion, extraction, and concentration, which not only increases detection time but can also lead to sample loss and contamination. Second, these techniques rely on large instruments and require specialized technicians for operation and maintenance, making rapid on-site detection difficult. For example, while ICP-MS boasts extremely high sensitivity and the ability to simultaneously detect multiple elements, its equipment is expensive, its operation is complex, and it requires high sample purity. Therefore, in practical applications, especially in environmental monitoring and food safety, there is an urgent need for a rapid, simple, and on-site Hg²⁺ detection method.

[0004] In recent years, with the rapid development of nanotechnology and biosensor technology, some novel Hg²⁺ detection methods have gradually emerged. For example, surface plasmon resonance (SPR)-based sensor technology shows great application potential due to its advantages such as high sensitivity, fast response, and label-free operation. SPR sensors detect target ions by detecting resonance changes in plasmons on metal surfaces, offering real-time monitoring and high-throughput screening capabilities. Furthermore, significant progress has been made in fluorescent probes and electrochemical sensors based on nanomaterials. These novel detection technologies not only improve detection efficiency but also reduce detection costs, enabling rapid on-site detection of Hg²⁺. However, these technologies are still under development, and their application prospects and detection effects still have considerable room for improvement. Summary of the Invention

[0005] This invention addresses the problems existing in traditional Hg²⁺ detection by proposing a novel cobalt-based nanozyme with an egg yolk shell structure and its application in mercury ion detection.

[0006] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0007] A method for preparing a cobalt-based nanozyme with an egg yolk shell structure, comprising the following steps.

[0008] (1) Cobalt nitrate hexahydrate and hexadecyltrimethylammonium bromide were added to water and mixed evenly to obtain mixture A; mixture A was mixed evenly with 2-methylimidazole solution and stirred to react to obtain ZIF-67 cube.

[0009] (2) Add the ZIF-67 cube to ethanol and mix evenly to obtain mixture B; mix mixture B with an aqueous solution of a polyoxoacid and then stir to obtain ZIF-67 / PTA.

[0010] (3) Add ZIF-67 / PTA to an ethanol solution and mix well. Then add chloroauric acid solution and sodium borohydride solution and stir to react to obtain ZIF-67 / PTA / Au.

[0011] Preferably, the polyoxoacid in step (2) is phosphotungstic acid, and the volume fraction of ethanol in the ethanol solution in step (3) is 50%. Chloroauric acid solution and sodium borohydride solution need to be added in sequence.

[0012] Preferably, the stirring reaction time in step (1) is 10-40 min; the stirring reaction time in step (2) is 4-6 h; and the total stirring reaction time in step (3) is 4-6 h.

[0013] This invention proposes the application of cobalt-based nanozymes with egg yolk shell structures prepared by the above method in mercury ion detection.

[0014] The application method is as follows: mix cobalt-based nanozyme TMB and NaAc-HAc buffer to obtain a reaction system, let it stand naturally for at least 5 minutes, then add the test solution, mix well, let it stand naturally for at least 5 minutes, filter, and record the absorbance value at 652 nm using a UV-Vis spectrophotometer.

[0015] Preferably, the pH of the NaAc-Hac buffer is 5.0, and the volume fraction of the NaAc-Hac buffer in the reaction system is 75-80%.

[0016] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0017] 1. The ZIF-67 / PTA / Au material synthesized in this invention possesses a mesoporous structure resembling egg yolk shell nanocubes, effectively increasing the contact area with the substrate and providing more active sites. By adding PTA to compete for coordination with 2-methylimidazole in ZIF-67 to capture Co²⁺, it can also act as a local reducing agent, effectively improving the charge transfer rate. Furthermore, the rapid etching effect of PTA under NaBH4 assistance also benefits the catalytic oxidation reaction of the nanozyme.

[0018] 2. The raw materials are inexpensive and abundant, resulting in low synthesis costs and stable product performance, making it suitable for mass synthesis.

[0019] 3. Based on the principle that Hg²⁺ enhances the activity of ZIF-67 / PTA / Au, it can realize the visual detection of Hg²⁺, with high sensitivity, low detection limit (0.26μM), wide linear response range (1-2000μM) and good selectivity and stability. Attached Figure Description

[0020] Figure 1 X-ray diffraction (XRD) patterns of ZIF-67, ZIF-67 / PTA, and ZIF-67 / PTA / Au.

[0021] Figure 2 A is a scanning electron microscope (SEM) image of the ZIF-67. Figure 2 B is the SEM image of ZIF-67 / PTA. Figure 2 C is the SEM image of ZIF-67 / PTA / Au. Figure 2 D is a transmission electron microscope (TEM) image of the ZIF-67. Figure 2 E is a TEM image of ZIF-67 / PTA. Figure 2 F is a TEM image of ZIF-67 / PTA / Au.

[0022] Figure 3 High-resolution transmission electron microscopy (HRTEM) image of ZIF-67 / PTA / Au.

[0023] Figure 4 A represents the N2 adsorption-desorption isotherm of ZIF-67 / PTA / Au. Figure 4 B represents the corresponding aperture distribution curve.

[0024] Figure 5 This is a comparison diagram of the OXD-like activities of different nanozymes.

[0025] Figure 6 This is a specific detection diagram of ZIF-67 / PTA / Au nanozymes.

[0026] Figure 7 The graph shows the effect of the pH value of the buffer solution on the activity of ZIF-67 / PTA / Au nanozymes. Detailed Implementation

[0027] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below with reference to specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0028] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification.

[0029] Example 1

[0030] 0.6 g of Co(NO3)2·6H2O and 10 mg of hexadecyltrimethylammonium bromide (CTAB) were added to 20 mL of deionized water. Then, a solution containing 140 mL of ultrapure water and 9.03 g of 2-methylimidazole was rapidly added while manually stirring with a glass rod. The mixture was stirred at room temperature for 20 min at a stirring rate of 800 rpm. After the reaction, the mixture was washed three times by alternating centrifugation with ethanol and deionized water, each time using approximately 20 mL of ethanol at a centrifugation rate of 8000 rpm for 8 min. The solid was then scraped off and dried at 60 °C for 12 h to obtain a purple solid, which was designated ZIF-67.

[0031] 50 mg of the prepared ZIF-67 nanocubes were dispersed in 40 mL of ethanol, and phosphotungstic acid (PTA) was dissolved in ultrapure water to prepare 20 mL of a solution with a concentration of 0.125 g / L. The PTA solution was then slowly added to the ZIF-67 dispersion, and the mixture was stirred at 500 rpm for 6 h at room temperature. After the reaction, the mixture was washed three times alternately with ethanol and deionized water, each time using approximately 20 mL of water at a centrifugation rate of 8000 rpm for 8 min. The solid was then scraped off and dried at 60 °C for 12 h to obtain a dark purple solid, denoted as ZIF-67 / PTA.

[0032] 20 mg of the prepared ZIF-67 / PTA was dispersed in 4 mL of 50% ethanol solution and sonicated for 2 min. Then, 0.5 mL of 25 mM chloroauric acid solution was added. After stirring at 1000 rpm and room temperature for 4 h, 2.4 mL of 0.05 M sodium borohydride solution was quickly added, and stirring was continued at 1000 rpm for 1 h to carry out the reaction. After the reaction was completed, the mixture was washed three times alternately by centrifugation with ethanol and deionized water, each time with a volume of about 20 mL. The centrifugation rate was 8000 rpm, and the centrifugation time was 8 min each time. The solid was then scraped off and dried at 60℃ for 12 h to obtain a black solid, which was denoted as ZIF-67 / PTA / Au.

[0033] The prepared ZIF-67, ZIF-67 / PTA, and ZIF-67 / PTA / Au were characterized by XRD, and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that the crystal form of ZIF-67 material did not change significantly after PTA coating; ZIF-67 / PTA / Au can correspond to the standard colorimetric card of Au.

[0034] The prepared ZIF-67, ZIF-67 / PTA, and ZIF-67 / PTA / Au were characterized by SEM and TEM, and the results are as follows: Figure 2 As shown in A-2F. From Figure 2 From A and 2D, we can see that ZIF-67 has a uniform cubic structure with a smooth surface; Figure 2 As can be seen from B and 2E, after PTA coating, a distinct yolk-shell structure was formed on the surface of ZIF-67, with the shell exhibiting a plate-like structure and increased surface roughness; Figure 2 As can be seen from C and 2F, ZIF-67 / PTA / Au successfully introduced gold nanoparticles (Au NPs) on the basis of ZIF-67 / PTA, forming a distinct yolk shell structure, with Au NPs uniformly distributed in the shell.

[0035] The lattice state of ZIF-67 / PTA / Au material is as follows: Figure 3 , Figure 3 Two sets of lattice fringes of approximately 2.36 Å and 2.04 Å are shown, corresponding to the (111) and (200) crystal planes of Au, respectively.

[0036] The N2 adsorption-desorption curves of the ZIF-67 / PTA / Au material are as follows: Figure 4As shown, it exhibits typical Type IV structure with a distinct hysteresis loop, indicating that ZIF-67 / PTA / Au possesses a mesoporous structure. The specific surface area of ​​the material was measured to be 683.33 m² / g, and the total pore volume was 0.85 cm³ / g. The mesoporous nature of the nanozyme facilitates substrate-material contact; the large contact area between the two materials provides more active sites, thereby enhancing the material's catalytic oxidation performance.

[0037] Example 2

[0038] Unless otherwise specified, this embodiment is consistent with Example 1. 0.3 g of Co(NO3)2·6H2O and 10 mg of CTAB were added to 20 mL of deionized water. Then, a solution containing 140 mL of ultrapure water and 9.03 g of 2-methylimidazole was rapidly added while stirring. The mixture was stirred at room temperature for 20 min at a stirring rate of 800 rpm. After the reaction, the mixture was washed three times alternately by centrifugation with ethanol and deionized water at a centrifugation rate of 8000 rpm for 8 min each time. The mixture was then dried at 60 °C for 12 h to obtain a purple solid, ZIF-67 nanocubes.

[0039] 50 mg of the prepared ZIF-67 nanocubes were dispersed in 40 mL of ethanol, and phosphotungstic acid (PTA) was dissolved in ultrapure water to prepare 20 mL of a solution with a concentration of 0.125 g / L. The PTA solution was then slowly added to the ZIF-67 dispersion, and the mixture was stirred at 500 rpm for 10 h at room temperature. After the reaction, the mixture was washed three times alternately by centrifugation with ethanol and deionized water at a speed of 8000 rpm for 8 min each time. The mixture was then dried at 60 °C for 12 h to obtain a dark purple solid, ZIF-67 / PTA.

[0040] 20 mg of the prepared ZIF-67 / PTA was dispersed in 4 mL of ethanol solution and sonicated. Then, 0.5 mL of 25 mM chloroauric acid solution was added. After stirring at room temperature for 4 h, 2.4 mL of freshly prepared 0.05 M sodium borohydride solution was quickly added, and stirring was continued for 1 h at a stirring rate of 1000 rpm. After the reaction was completed, the mixture was washed three times alternately by centrifugation with ethanol and deionized water at a centrifugation rate of 8000 rpm for 8 min each time. The mixture was then dried at 60 °C for 12 h to obtain a black solid, ZIF-67 / PTA / Au.

[0041] Example 3

[0042] Unless otherwise specified, this embodiment is consistent with Example 1. 0.6 g of Co(NO3)2·6H2O and 10 mg of CTAB were added to 20 mL of deionized water. Then, while stirring, a solution containing 50 mL of ultrapure water and 9.03 g of 2-methylimidazole was rapidly added, and the mixture was stirred at room temperature for 40 min at a stirring rate of 800 rpm. After the reaction, the mixture was washed three times alternately by centrifugation with ethanol and deionized water at a centrifugation rate of 8000 rpm for 8 min each time. The mixture was then dried at 60 °C for 12 h to obtain a purple solid, ZIF-67.

[0043] 50 mg of the prepared ZIF-67 nanocubes were dispersed in 40 mL of ethanol, and phosphotungstic acid (PTA) was dissolved in ultrapure water to prepare 20 mL of a solution with a concentration of 0.125 g / L. The PTA solution was then slowly added to the ZIF-67 dispersion, and the reaction was carried out at room temperature for 6 h with a stirring rate of 500 rpm. After the reaction, the mixture was washed three times alternately with ethanol and deionized water by centrifugation at a rate of 8000 rpm for 8 min each time. The mixture was then dried at 60 °C for 12 h to obtain a deep purple solid, ZIF-67 / PTA.

[0044] 20 mg of the prepared ZIF-67 / PTA was dispersed in 4 mL of ethanol and sonicated. Then, 0.5 mL of 25 mM chloroauric acid solution was added. After stirring at room temperature for 2 h, 2.4 mL of freshly prepared 0.05 M sodium borohydride solution was quickly added, and stirring was continued for 1 h at a stirring rate of 1000 rpm. After the reaction was completed, the mixture was washed three times alternately by centrifugation with ethanol and deionized water at a centrifugation rate of 8000 rpm for 8 min each time. The mixture was then dried at 60 °C for 12 h to obtain a black solid, ZIF-67 / PTA / Au.

[0045] The performance of the cobalt-based nanozyme prepared in Example 1 was tested.

[0046] 1. Assay of OXD-like activity of cobalt-based nanozymes

[0047] The ZIF-67 / PTA / Au nanozyme prepared in Example 1 was added to ethanol to prepare a dispersion with a concentration of 500 μg / mL. 50 μL of 3,3',5,5'-tetramethylbenzidine (TMB, purchased from Sangon Biotech (Shanghai) Co., Ltd.) was added to 850 μL of NaAc-HAc buffer solution (pH 5.0). Then, 100 μL of the dispersion was added to the buffer solution, and the mixture was allowed to react naturally at room temperature for 5 min. The mixture was then filtered, and the filtrate was analyzed using a UV-Vis spectrophotometer, recording the UV-Vis absorption spectrum at 652 nm. The results are as follows: Figure 5,Depend on Figure 5 It is known that the ZIF-67 / PTA / Au nanozyme has excellent OXD-like activity.

[0048] 2. Determination of standard curve

[0049] The ZIF-67 / PTA / Au nanozyme was detected by varying the concentration and volume of the Hg²⁺ solution. During detection, 50 μL of TMB was added, and the total volume was brought up to 1 mL with a pH 5.0 NaAc-HAc buffer solution. After reaction and filtration, the UV-Vis absorption spectrum at 652 nm was measured to determine the linear range of detection. The results showed a good linear relationship between the concentration and absorbance of the Hg²⁺ solution within the 1-2000 μM range, with a detection limit of 0.26 μM. Specifically, within the 1-2000 μM concentration range, y = 0.00119x + 0.49326, R... 2 =0.99, where x is the concentration of Hg²⁺ solution and y is the absorbance.

[0050] 3. Specificity test

[0051] Detection of Fe 2+ Sn 2+ Cu 2+ Fe 3+ , Cr 3+ Pb 2+ Ni 2+ Co 2+ Zn 2+ Sn 4+ , Mn 2+ and Cd 2+ Interference with the detection results. (Interfering ions were provided sequentially by the following substances: ferrous chloride, stannous chloride dihydrate, copper chloride, ferric chloride, chromium nitrate nonahydrate, lead nitrate, nickel acetate tetrahydrate, cobalt nitrate hexahydrate, zinc chloride, stannous tetrachloride pentahydrate, manganese chloride, and chromium chloride.) The specific operation was as follows: Weigh the substances providing the interfering ions and add them to 10 mL of deionized water to prepare an interfering solution with a concentration of 1.5 mmol / L for each interfering ion. The ZIF-67 / PTA / Au nanozyme prepared in Example 1 was added to ethanol to prepare a dispersion with a concentration of 500 μg / mL. Then, 100 μL of the dispersion was added to 750 μL of NaAc-HAc buffer solution (pH 5.0) containing 50 μL TMB and 100 μL of the interfering solution. The mixture was allowed to react naturally at room temperature for 5 min, then filtered. The filtrate was detected using a UV-Vis spectrophotometer, and the UV-Vis absorption spectra at 652 nm were recorded. The results are shown below. Figure 6Only the addition of Hg²⁺ solution significantly enhanced the OXD-like activity of ZIF-67 / PTA / Au.

[0052] 4. Real sample testing

[0053] The detection of Hg²⁺ by ZIF-67 / PTA / Au in actual water samples was determined using the standard addition method. The specific procedures were as follows: Tap water from the Chemistry Building of Nanjing University of Technology and water from Zhengxin Lake were used as real samples for testing. Different concentrations of Hg²⁺ solution were added to each water sample using the standard addition method, and 100 μL of each sample was tested. The actual Hg²⁺ content in each test group is shown in Table 1. The ZIF-67 / PTA / Au nanozyme prepared in Example 1 was added to ethanol to prepare a dispersion with a concentration of 500 μg / mL. Then, 100 μL of the dispersion was added to 750 μL of NaAc-HAc buffer solution (pH 5.0) containing 50 μL of TMB and 100 μL of water sample. The mixture was allowed to react naturally at room temperature for 5 min, then filtered. The filtrate was detected using a UV-Vis spectrophotometer, and the UV-Vis absorption spectrum at 652 nm was recorded. This process was repeated three times. The detection limit was calculated by substituting the input curve. The results are shown in Table 1. Table 1 shows that the relative standard deviation of the detected values ​​is ≤3.7% (n = 3), indicating that the colorimetric sensor has good repeatability in the rapid detection of Hg²⁺ content in water. Furthermore, the recovery rate is between 97.0% and 107.5%, demonstrating that this method has good accuracy and reliability for the determination of Hg²⁺ in actual water samples.

[0054] Table 1. Test results of real samples

[0055]

[0056] Comparative Example 1

[0057] Without adding phosphotungstic acid, the remaining preparation process was the same as in Example 1. The obtained nanozyme was designated ZIF-67 / Au. It was added to ethanol to prepare a dispersion with a concentration of 500 μg / mL. Then, 100 μL of the dispersion was added to 800 μL of NaAc-HAc buffer solution (pH 5.0) containing 50 μL of TMB. The mixture was allowed to react naturally at room temperature for 5 min, then filtered. The filtrate was analyzed using a UV-Vis spectrophotometer, and the UV-Vis absorption spectrum at 652 nm was recorded. The results are as follows: Figure 5 (ZIF-67 / Au). By Figure 5 It can be seen that the OXD activity of the ZIF-67 / Au nanozyme prepared in this comparative example is not as high as that of the ZIF-67 / PTA / Au nanozyme prepared in Example 1.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 1 lies in the pH of the buffer solution. NaAc-HAc buffer solutions with pH values ​​of 2, 4, 5, 6, 7, and 8 were used sequentially, while all other conditions remained the same as in Example 1. The samples were treated using the same steps as those used in the cobalt-based nanozyme OXD-like activity assay, and the UV-Vis absorption spectra at 652 nm were recorded. The results are as follows: Figure 7 As shown. By Figure 7 It can be seen that the absorbance value is highest when the pH of the buffer solution is 5.0.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a cobalt-based nanoszyme with a yolk-shell structure, characterized in that, The steps are as follows: (1) adding cobalt nitrate hexahydrate and cetyltrimethylammonium bromide into water and mixing uniformly to obtain a mixed solution A; mixing the mixed solution A with a 2-methylimidazole solution uniformly and stirring to react, thereby preparing a ZIF-67 cube; (2) adding the ZIF-67 cube into ethanol and mixing uniformly to obtain a mixed solution B; mixing the mixed solution B with an aqueous solution of a polybasic oxygen-containing acid uniformly and then stirring to react, thereby preparing a ZIF-67 / PTA; (3) adding the ZIF-67 / PTA into an ethanol solution and mixing uniformly, then adding a chloroauric acid solution and a sodium borohydride solution, and then stirring to react, thereby preparing a ZIF-67 / PTA / Au; The polybasic oxygen-containing acid in step (2) is phosphotungstic acid.

2. The method for preparing cobalt-based nanozymes with an egg yolk shell structure according to claim 1, characterized in that, The volume fraction of ethanol in the ethanol solution in step (3) is 50%.

3. The method of claim 1, wherein the cobalt-based nanoszyme having an egg yolk shell structure is prepared by the steps of: The stirring reaction time in step (1) is 10-40 min; the stirring reaction time in step (2) is 4-6 h; and the total stirring reaction time in step (3) is 4-6 h.

4. Application of the cobalt-based nanoscale enzyme with a yolk-shell structure prepared by the method of any one of claims 1-3 in mercury ion detection.

5. Use according to claim 4, characterized in that, The application method is as follows: mixing the cobalt-based nanoscale enzyme TMB and a NaAc-HAc buffer solution uniformly to obtain a reaction system, naturally standing to react for at least 5 min, then adding a to-be-detected solution, mixing uniformly, naturally standing to react for at least 5 min, filtering, and recording the absorbance value at 652 nm by using a UV-visible spectrophotometer.

6. Use according to claim 5, characterized in that, The pH of the NaAc-HAc buffer solution is 5.0, and the volume fraction of the NaAc-HAc buffer solution in the reaction system is 75-80%.

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