A Fluorescent Probe for Detecting Permanganate Ions and Its Preparation Method

Abstract

The present invention relates to a preparation method of a fluorescent probe for detecting permanganate ions, comprising the following steps: dissolving purslane powder and o-phenylenediamine (OPD) in ethanol, mixing evenly, carrying out a heating reaction, and then cooling to room temperature to obtain a suspension; filtering and dialyzing the suspension to obtain a purified solution; rotary evaporating the purified solution in a water bath to obtain a solute; dissolving the solute in ultrapure water and freeze-drying to obtain a carbon quantum dot fluorescent probe powder. In the present invention, purslane is used as a precursor and o-phenylenediamine is used as a nitrogen doping agent to synthesize nitrogen-doped carbon nanodots (N-CDs) by a solvothermal reaction in ethanol. The obtained N-CDs exhibit excellent antioxidant properties and show excellent fluorescence stability under extreme pH, salt concentration, and continuous ultraviolet irradiation conditions. At the same time, N-CDs have good selectivity for MnO4 ‑ ions, showing a linear relationship with MnO4 ‑ ions in the concentration range of 0.5-168 μM, and the lowest detection limit reaches 41.8 nM.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluorescent probes, and particularly relates to a fluorescent probe for detecting permanganate ions and a preparation method thereof. Background Art

[0002] Permanganate ions (MnO4 - or Mn(VII) ions) are widely used in environmental water treatment due to their strong oxidizing property, mainly for oxidizing organic matters and metal ions such as iron and manganese in water. However, excessive residues of MnO4 - ions in environmental water bodies will pose a serious threat to the ecosystem and may even lead to the death of aquatic organisms. Therefore, the residue amount of MnO4 - ions is an important indicator for water quality monitoring. According to the "National Drinking Water Standard" (GB 5749-2006), the residue limit of MnO4 - ions in Chinese drinking water is 0.05 mg / L; according to the "Industrial Wastewater Discharge Standard" (GB 8978-1996), the discharge limit of MnO4 - ions in industrial wastewater is 5 mg / L; according to the "Surface Water Environmental Quality Standard" (GB 3838-2002), the specific residue limits of MnO4 - ions in natural water bodies vary according to water body categories. For example, it is 2 mg / L for Class I water bodies and 5 mg / L for Class II water bodies.

[0003] Currently, there are mainly four common methods for detecting the residue amount of MnO4 - ions in water bodies. Among them, the spectrophotometric method is to generate a purple product through the reaction of xylene with MnO4 - ions, and measure the absorbance to determine its concentration. The titration method uses a reducing agent such as sodium sulfite to titrate MnO4 - ions, and the color change (from purple to colorless) is used to indicate the end point to determine its concentration. The electrochemical method analyzes the behavior of MnO4 - ions on the electrode surface, while the colorimetric method determines its concentration by the color change generated by the reaction of a specific reagent with MnO4 - However, these methods all have certain limitations. First, the spectrophotometric method and the titration method have weak anti-interference ability and are easily affected by other oxidizing substances in water, thus affecting the accuracy of the detection results. Second, the electrochemical method requires complex sample pretreatment, expensive equipment, and the operation process requires professional personnel to have a high technical level. Finally, the colorimetric method has low sensitivity for detecting MnO4 - ions, and the accuracy of the test results is easily interfered by the pH value, salt concentration, and other oxidizing substances in the water sample. Therefore, for detecting MnO4 -An ideal fluorescent probe for ions should have the following characteristics: (a) excellent selectivity to effectively distinguish MnO4 - ions from other common highly oxidative ions; (b) inherent antioxidant properties to reduce interference from other oxidizing substances; (c) a low detection limit, preferably lower than the 0.05 mg / L residual limit specified in the Chinese drinking water standard; (d) stable fluorescence performance in complex environments such as different pH values and high salt concentrations; (e) simple operation, high cost-effectiveness, easy popularization, and good environmental sustainability.

[0004] Zero-dimensional carbon dots (CDs) are a class of emerging functional nanomaterials that have been widely used in the field of environmental monitoring due to their simple preparation process, tunable photoluminescence properties, and excellent anti-fluorescence photobleaching performance. Existing studies have shown that CDs as fluorescent probes have made significant progress in detecting MnO4 - ion residues in environmental water samples. First, Jayaweera S, Yin K, Hu X, et al (2019) Fluorescent N / Al co-doped carbon dots from cellulose biomass for sensitive detection of manganese(VII). J Fluoresc 29:1291 - 1300. synthesized CDs using durian shells, urea, and aluminum nitrate, achieving a low detection limit (46.8 nM) for MnO4 - ions. However, this study did not explore how to distinguish MnO4 - ions from other common strong oxidative ions, nor did it study the antioxidant properties of CDs themselves. Second, Mohammed A, Gugulothu Y, Bandi R, et al (2021) Ultraspeed synthesis of highly fluorescent N-doped carbon dots for the label-free detection of manganese(VII). J Chin Chem Soc(Weinheim, Ger.) 68:1514 - 1521. synthesized CDs using threonine and guanidine hydrochloride, but this study also lacked an exploration of the specific distinction between MnO4 - ions and other strong oxidative ions, and its MnO4 -The detection limit of the ions is 0.66 μM, exceeding the Chinese drinking water standard (0.361 μM, based on KMnO4). Finally, Newar R, Sultana N, Das S, et al (2024) Development of FRET-based optical sensors using N-doped carbon dots for detection of chromium(VI) and manganese(VII) in water for a sustainable future. J Environ Chem Eng 12:111721. CDs were synthesized using Alstonia scholaris leaves and ethylenediamine. These CDs showed high selectivity in differentiating MnO4 - ions from other common oxidizing ions. However, the fluorescence stability of these CDs was poor in solutions with different pH values. Their antioxidant properties themselves were not explored, and the detection limit of MnO4 - ions was 0.394 μM, higher than the Chinese drinking water standard. Therefore, there is an urgent need to develop a new type of CDs that can efficiently and sensitively detect MnO4 - ions in environmental water bodies and have higher selectivity, stability, and antioxidant properties to meet the needs of environmental water body monitoring. Summary of the Invention

[0005] The purpose of the present invention is to provide a fluorescence probe for detecting permanganate ions and its preparation method, which can efficiently and sensitively detect MnO4 - ions in environmental water bodies;

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A preparation method of a fluorescence probe for detecting permanganate ions, comprising the following steps:

[0008] Dissolve purslane powder and o-phenylenediamine in ethanol, mix evenly, then carry out a heating reaction, and after cooling to room temperature, obtain a suspension; filter and dialyze the suspension to obtain a purified solution; rotary evaporate the purified solution in a water bath to obtain a solute; dissolve the solute in ultrapure water and freeze-dry to obtain carbon quantum dot fluorescence probe powder.

[0009] Preferably, the mass-volume ratio of the purslane powder, o-phenylenediamine, and ethanol is: 0.15 - 0.25 g: 0.08 g: 8 - 12 mL.

[0010] Preferably, the heating reaction temperature is 190 - 210 °C, and the reaction time is 9.5 - 10.5 h.

[0011] On the other hand, the present invention provides a fluorescent probe for detecting permanganate ions prepared by the preparation method described above.

[0012] On the other hand, the present invention provides a method for detecting permanganate ions using the fluorescent probe described above, comprising the following steps:

[0013] (1) Add the fluorescent probe powder to ultrapure water and ultrasonically dissolve it to obtain an N-CDs stock solution;

[0014] (2) Add the N-CDs stock solution to the sample to be measured. By measuring the change in the fluorescence intensity of N-CDs at 486 nm in the sample to be measured and substituting it into the linear equation, the content of MnO4 - ions in the sample can be obtained;

[0015] Preferably, in step (1), the concentration of the N-CDs stock solution is 5 mg / mL.

[0016] Preferably, the method for obtaining the linear equation is as follows: Dilute the N-CDs stock solution with ultrapure water. Under the excitation at 358 nm, record the fluorescence value of N-CDs at 486 nm as F0; then add different volumes of MnO4 - ions and record the fluorescence value of the mixed solution of N-CDs and MnO4 - ions, which is F; through linear fitting, obtain the linear relationship between the MnO4 - ion concentration and the fluorescence intensity of N-CDs.

[0017] Preferably, within the linear range of 0.5 - 168 uM, the linear equation is: F0 - F = 3.2039Log c (MnO4-) + 30.6872, R 2 = 0.9916.

[0018] On the other hand, the present invention provides the application of the fluorescent probe for detecting permanganate ions described above in detecting permanganate ions in environmental water bodies.

[0019] In the present invention, purslane is used as a precursor and o-phenylenediamine (OPD) is used as a nitrogen doping agent to synthesize nitrogen-doped carbon dots (N-CDs) by solvothermal reaction in ethanol. The obtained N-CDs exhibit excellent antioxidant properties and show excellent stability under extreme pH, salt concentration, and continuous ultraviolet irradiation conditions. At the same time, N-CDs have good selectivity for MnO4 - ions and show a linear relationship with MnO4 - ions within the concentration range of 0.5 - 168 μM, and the lowest detection limit reaches 41.8 nM.

[0020] In the present invention, the precursor purslane is widely distributed, inexpensive and easily available, and o-phenylenediamine (OPD) and ethanol are both common reagents and are easy to purchase. The preparation method of the fluorescent probe is simple and does not require expensive instruments, and can quickly and efficiently detect the content of MnO4 - ions. The prepared fluorescent probe has stable performance, strong anti-interference ability, high accuracy and sensitivity.

[0021] In summary, compared with other methods for detecting MnO4 - ions in environmental water bodies, this method has multiple advantages: (a) excellent selectivity, capable of effectively distinguishing MnO4 - ions from other common highly oxidative ions; (b) inherent antioxidant properties, capable of reducing the interference of other oxidizing substances; (c) stable fluorescence performance in complex environments such as different pH and high salt concentrations; (d) simple operation, cost-effective, easy to popularize and having good environmental sustainability. Description of the Drawings

[0022] Figure 1 In, a is the ultraviolet spectrum and fluorescence spectrum of the N-CDs prepared by the present invention, and b is the excitation wavelength-dependent spectrum.

[0023] Figure 2 For 15 typical anions (Br - , I - , F - , Cl - , CO3 2- , C2O4 2- , S2O3 2- , SO3 2- , SO4 2- , SCN - , NO2 - , H2PO4 2- , NO3 - , ClO4 - , MnO4 - ) The selectivity and anti-interference test results of N-CDs for detecting MnO4 - ion graph.

[0024] Figure 3 In, a is the fluorescence stability of the N-CDs prepared by the present invention under different pH, salt concentration (KCl), H2O2 solution and continuous UV irradiation.

[0025] Figure 4 For titrating the N-CDs solution with MnO4 - ions, the change curve of the fluorescence intensity of N-CDs and the linear graph of the MnO4 - ion concentration and the fluorescence intensity of N-CDs. Detailed implementation manners

[0026] The following further describes the present invention in conjunction with the accompanying drawings and specific embodiments.

[0027] I. Preparation and characterization of N-CDs

[0028] (1) Hydrothermal reaction: Weigh 0.2 g of purslane powder and 0.08 g of o-phenylenediamine (OPD), and place them in another 10 mL of ethanol. Then, thoroughly mix them evenly in a 50 mL beaker. Subsequently, transfer the mixture to a high-pressure reaction kettle with a polytetrafluoroethylene inner liner and place it in a constant temperature incubator for heating reaction at 200 °C for 10 h.

[0029] (2) Purification: After the reaction kettle is naturally cooled to room temperature (about 25 °C), a dark brown suspension (crude product) is obtained. Then, filter the suspension with a 0.45 μm filter paper to remove insoluble substances and larger particles. Next, continue to perform dialysis with a 500 Da dialysis bag (Spectra / Por 6, Spectral Laboratory, USA) to eliminate unreacted molecules and ions, and obtain a purified solution. Finally, rotate and evaporate the above-obtained purified solution in a water bath at 50 °C to remove ethanol, and obtain a refined product.

[0030] (3) Precipitation: Dissolve the prepared N-CDs in ultrapure water, and obtain a carbon quantum dot fluorescent probe, namely N-CDs solid powder, through freeze-drying.

[0031] Obtaining the above-mentioned purslane powder: First, purchase air-dried purslane from China Taiyuan Tongrentang Co., Ltd., then crush it, and finally obtain purslane powder by sieving through a 200-mesh sieve.

[0032] The above-mentioned o-phenylenediamine (OPD) is purchased from Aladdin Reagent Co., Ltd. (Shanghai, China), reagent grade, with a purity of 99.5%.

[0033] The above-mentioned ethanol is commercially available analytical grade ethanol with a purity of 100%.

[0034] N-CDs stock solution: Weigh 50 mg of N-CDs solid powder and add it to 10 mL of ultrapure water, and ultrasonicate for 15 min to completely dissolve it, obtaining an N-CDs stock solution with a concentration of 5 mg / mL.

[0035] For property characterization, see Figure 1 . The N-CDs aqueous solution exhibits bright light blue fluorescence under ultraviolet light irradiation at 358 nm and transparent light pink under visible light (see Figure 1a). The UV-Vis absorption of N-CDs shows a strong peak at 245 nm. The peak that appears here may be caused by the π-π* transition of aromatic C-C bonds and C=C bonds (see Figure 1 a). The maximum emission wavelength of N-CDs is 486 nm, and the excitation wavelength is 358 nm (see Figure 1 a). When the excitation wavelength changes from 308 nm to 408 nm, the emission band shifts to the red side (from 473 nm to 497 nm), showing a typical excitation-dependent fluorescence phenomenon (see Figure 1 b).

[0036] II. MnO4 - Selectivity and anti-interference experiments for ion detection

[0037] Step 1: Weigh 50 mg of N-CDs solid powder, add 10 mL of ultrapure water, and ultrasonicate for 15 min to completely dissolve it, obtaining an N-CDs stock solution with a concentration of 5 mg / mL.

[0038] Step 2: Prepare 15 typical anions (Br - , I - , F - , Cl - , CO3 2- , C2O4 2- , S2O3 2- , SO3 2- , SO4 2- , SCN - , NO2 - , H2PO4 2- , NO3 - , ClO4 - , MnO4 - ), with a concentration of 100 mM each.

[0039] Step 3: Take 20 μL of the above N-CDs stock solution and each test solution into a quartz cuvette, then dilute to a final volume of 2 L with ultrapure water, mix well, incubate for 1 min, and record the fluorescence spectrum value at an excitation wavelength of 358 nm.

[0040] Step 4: Add MnO4 - ions and other test solutions (both at 1 mM) to the N-CDs solution. After incubating for 1 min, monitor the fluorescence spectrum of each sample at an excitation wavelength of 358 nm to check whether the presence of other test solutions has a significant impact on the quenching effect of MnO4 - ions.

[0041] MnO4 - The results of the selectivity and anti-interference experiments for ion detection show that when MnO4- After the addition of iodide ions, the fluorescence decreased sharply, while 14 typical anions did not or hardly caused any fluorescence change ( Figure 2 a). Compared with the quenching effect of MnO4 - ions, the effect of coexisting substances was almost negligible ( Figure 2 b), indicating that N-CDs have good selectivity and anti-interference ability for the detection of MnO4 - ions.

[0042] III. Fluorescence Stability of N-CDs in Different Environmental Solutions

[0043] Step 1: Prepare pH solutions with different strengths: Gradually dilute concentrated hydrochloric acid (12 M) directly with deionized water, measure the pH of the solution with a pH meter, determine the dilution end point, and obtain acidic solutions with pH equal to 1-6. Then mix solutions of sodium dihydrogen phosphate (NaH2PO4) and disodium hydrogen phosphate (Na2HPO4) with a concentration of 0.1 M, measure and adjust with a pH meter to prepare a neutral solution with pH equal to 7. Finally, gradually dilute NaOH (1 M) directly with deionized water, measure and adjust with a pH meter to prepare alkaline solutions with pH equal to 8-12.

[0044] Step 2: Prepare KCl solutions with different concentrations: Weigh 745.5 mg of KCl powder into 10 mL of deionized water to obtain a KCl solution with a concentration of 1 M. Then perform serial dilutions to gradually obtain KCl solutions with concentrations of 0.8-0.2 M.

[0045] Step 3: Prepare H2O2 solutions with different concentrations: Purchase 100 mL of commercially available 30% (w / w) H2O2 stock solution, pipette 1.133 mL of the H2O2 stock solution into a volumetric flask, and then make up the volume to 10 mL with deionized water to obtain an H2O2 solution with a concentration of 1 M. Then perform serial dilutions to gradually obtain H2O2 solutions with concentrations of 0.8-0.2 M.

[0046] Step 4: Take 20 μL of the above N-CDs stock solution in a quartz cup, then add pH, KCl solutions and H2O2 solutions with different concentrations to a final volume of 2 L, mix well, incubate for 1 min, and record the fluorescence spectrum values at an excitation wavelength of 358 nm.

[0047] Step 5: Take 20 μL of the above N-CDs stock solution in a quartz cup, then dilute the solution with deionized water to a final volume of 2 L, mix well, place it under a 365 nm ultraviolet lamp for irradiation, and then record the fluorescence spectrum values at an excitation wavelength of 358 nm, recording once every 15 min for a total of 120 min. The experimental results are shown in Figure 3 d.

[0048] The results showed that in acidic solutions (pH values from 2 to 6), the fluorescence intensity of N-CDs remained almost unchanged. In alkaline solutions (pH values from 7 to 10), the fluorescence intensity of N-CDs gradually decreased, with a decrease amplitude of less than 10%, which might be due to the protonation of carboxyl groups on the surface of N-CDs( Figure 3 a). In addition, N-CDs showed stable fluorescence intensity under higher ionic strength (0.2 - 1.0 M NaCl) and high concentration of H2O2, indicating its strong salt tolerance and antioxidant ability( Figure 3 b - c). Finally, under continuous irradiation of ultraviolet light, the fluorescence of N-CDs gradually decreased, and the fluorescence intensity only decreased by 15% after 120 minutes, indicating that N-CDs had strong anti-photobleaching ability( Figure 3 d). In summary, N-CDs had strong acid - base resistance, salt tolerance, antioxidant ability and anti-photobleaching ability, and were suitable for the detection of MnO4 - in complex environmental conditions.

[0049] IV. Linear Equation between the Concentration of MnO4 - Ions and the Fluorescence Intensity of N-CDs

[0050] Step 1: Adopt the fluorescence titration method, that is, dilute 200 μL of the N-CDs stock solution to 2 L with ultrapure water. Under the excitation at 358 nm, record the fluorescence value of N-CDs at 486 nm as F0.

[0051] Step 2: Add different volumes of MnO4 - ions (1 mM) to the above solution, and record the fluorescence value of the mixture of N-CDs and MnO4 - ions as F. The change in fluorescence intensity is shown in Figure 4 a.

[0052] Step 3: Through linear fitting with Origin software, obtain the linear relationship between the concentration of MnO4 - ions and the fluorescence intensity of N-CDs. The results are shown in Figure 4 b.

[0053] The results showed that as the concentration of MnO4 - ions increased from 0 - 249 μM, the fluorescence intensity at the emission peak position of N-CDs (about 486 nm) decreased( Figure 4 a). In the linear range of 0.5 - 168 μM, the linear equation was: R 2 = 0.9916, and the lowest detection limit was 41.8 nM( Figure 4 b).

[0054] V. MnO4 in Actual Samples -Ion content detection

[0055] The actual samples selected were tap water (from the tap water in Laboratory 302 of the Institute of Biomedicine of Shanxi University) and local river water (the Fen River in Taiyuan, China).

[0056] Step 1: Add 20 μL of the N-CDs stock solution to 2 mL of ultrapure water. At this time, the concentration of N-CDs is 5 μg·mL -1 , and measure the fluorescence intensity at 486 nm, denoted as F0.

[0057] Step 2: First, centrifuge the water sample at 8000 rpm for 5 min, then filter it with a 0.22 μm filter membrane for standby; finally, add 20 μL of the N-CDs stock solution to 2 mL of tap water and river water respectively. At this time, the concentration of N-CDs is 5 μg·mL -1 , measure the fluorescence intensity at 486 nm, denoted as F, calculate F0 - F, and substitute it into the linear equation: Calculate the concentration of MnO4 - ions in the actual sample.

[0058] The results are shown in Table 1. It can be seen that N-CDs can be used to detect the content of MnO4 - ions in the actual sample, and the relative standard deviation is less than 2.15%, showing good reproducibility.

[0059] Table 1. Content of MnO4 - ions in two actual samples

[0060]

[0061] VI. Spike recovery experiment of MnO4 - ions in the actual sample

[0062] Step 1: Weigh 50 mg of N-CDs solid powder into a beaker, add 10 mL of ultrapure water to it, and ultrasonically dissolve it to obtain an N-CDs mother liquor with a concentration of 5 mg / mL.

[0063] Step 2: Weigh 1.5804 mg of potassium permanganate powder, add it to 10 mL of distilled water, and ultrasonically dissolve it to obtain an MnO4 - ion stock solution with a concentration of 1 mM.

[0064] Step 3: Add 20 μL of the N-CDs mother liquor to 2 mL of ultrapure water. At this time, the concentration of N-CDs is 5 μg·mL -1 , and measure the fluorescence intensity at 486 nm, denoted as F0.

[0065] Step 4: Add 20 μL of the N-CDs mother liquor to 2 mL of tap water. At this time, the concentration of N-CDs is 5 μg·mL -1 , and measure the fluorescence intensity at 486 nm, denoted as F.

[0066] Step 5: Add 20 μL of the N-CDs mother liquor to 2 mL of tap water, and then add 5 μM, 10 μM, and 20 μM of MnO4 - ion spike stock solutions. At this time, the concentration of N-CDs is 5 μg·mL -1 , and measure the fluorescence intensity at 486 nm, denoted as F.

[0067] Step 6: Add 20 μL of the N-CDs mother liquor to 2 mL of river water. At this time, the concentration of N-CDs is 5 μg·mL -1 , and measure the fluorescence intensity at 486 nm, denoted as F.

[0068] Step 7: Add 20 μL of the N-CDs mother liquor to 2 mL of river water, and then add 5 μM, 10 μM, and 20 μM of MnO4 - ion spike stock solutions. At this time, the concentration of N-CDs is 5 μg·mL -1 , and measure the fluorescence intensity at 486 nm, denoted as F.

[0069] Step 8: Substitute F0 - F into the linear equation to calculate the spike recovery rates of MnO4 - ions in the three actual samples.

[0070] The results are shown in Table 2. The spike recovery rates of MnO4 - ions in the two actual samples are between 95.67% and 103.42%, and the relative standard deviation is less than 5.23% (see Table 2), indicating that N-CDs can be used for the spike detection of MnO4 - ions in actual samples, are not interfered by other substances in the actual samples, and have good recovery results and reproducibility.

[0071] Table 2. Results of the spike recovery experiment of MnO4 - ions in two actual samples

[0072]

Claims

1. A method for detecting permanganate ions using a fluorescent probe, characterized in that: The steps include: (1) Add the fluorescent probe powder to ultrapure water and dissolve it completely by ultrasonication to obtain N-CDs stock solution; (2) Add the N-CDs stock solution to the sample to be tested, measure the change in the fluorescence intensity of N-CDs at 486 nm in the sample to be tested, and substitute it into the linear equation to obtain the MnO4 - Ion content; The method for preparing the fluorescent probe powder in step (1) comprises the following steps: The purslane powder and o-phenylenediamine are dissolved in ethanol, mixed evenly, heated for reaction, and then cooled to room temperature to obtain a suspension; the suspension is filtered and dialyzed to obtain a purified solution; the purified solution is rotary evaporated in a water bath to obtain a solute; the solute is dissolved in ultrapure water, and freeze-dried to obtain a carbon quantum dot fluorescent probe powder; the heating reaction temperature is 190-210°C, and the reaction time is 9.5-10.5h.

2. The method according to claim 1, characterized in that The mass volume ratio of the purslane powder, o-phenylenediamine and ethanol is: 0.15-0.25 g: 0.08 g: 8-12 mL.

3. The method according to claim 1, characterized in that In step (1), the concentration of the N-CDs stock solution is 5 mg / mL.

4. The method according to claim 1, characterized in that: The linear equation is obtained by diluting the N-CDs stock solution with ultrapure water, recording the fluorescence value of N-CDs at 486 nm as F0 under the excitation of 358 nm; then adding different volumes of MnO4 - ions, recording N-CDs and MnO4 - The fluorescence value of the ion mixture is F; through linear fitting, we get MnO4 - Linear relationship between ion concentration and fluorescence intensity of N-CDs.

5. The method according to claim 1 or 4, characterized in that: In the linear range of 0.5-168 uM, the linear equation is: F0- F = 3.2039 Log c (MnO4 - ) +30.6872, R 2 = 0.9916.

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