Oxidized fluorescent carbon dots as well as preparation method and application thereof

Abstract

The invention provides an oxidized fluorescent carbon dot as well as a preparation method and application thereof, belongs to the technical field of carbon nanomaterials, and is used for solving the technical problems that Co < 2 + > detection probes are few, and iron ions and cobalt ions cannot be continuously detected. The carbon dots provided by the invention can specifically detect Fe < 2 + > and Fe < 3 + > without other treatments, and show a good linear relationship in a concentration range of 10-50 [mu] M and a concentration range of 2-40 [mu] M respectively. The H2O2 solution with the mass fraction of 10% is used for treating the carbon dot solution, and the treated carbon dot solution shows a better detection effect on Co < 2 + > within the concentration range of 0-40 [mu] M. Therefore, the carbon dot can realize detection of Fe < 2 + > and Fe < 3 + >, and can realize detection of Co < 2 + > after simple treatment, so that the application range of the carbon dot in the field of ion detection is further expanded, and the carbon dot has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon nanomaterials, and particularly relates to a fluorescent carbon dot. Background Art

[0002] Carbon dots are a new type of carbon nanomaterial. Compared with traditional quantum dots, they have the characteristics of small size, low cost, wide raw materials, good biocompatibility, and excellent optical properties. Since their discovery, they have immediately attracted extensive attention, attracting many scholars to explore them. After in-depth exploration and excavation, they have been widely used in many fields such as bioimaging, information transmission, catalysis, and sensing. The preparation methods of carbon dots are also diverse, mainly divided into two approaches: top-down and bottom-up. It is precisely because of the wide range of carbon dot raw materials and the diversity of preparation methods that their luminescence mechanism has not been clarified and further research is still needed.

[0003] Iron is one of the most abundant metals on the earth and plays a crucial role in biological, environmental, and industrial processes. In the human body, iron ions participate in the synthesis of hemoglobin and the transportation of oxygen. In addition, iron ions are widely present in various environments and have a profound impact on human health and the ecosystem. Therefore, for the health of humans and the safety of the ecological environment, the detection of iron ions is of great significance. And carbon dots have many advantages such as high sensitivity and selectivity, convenience, and environmental friendliness for the detection of iron ions, and play an important role in the development of life safety.

[0004] Cobalt ions, as an important metal ion, play a key role in many aspects. First of all, cobalt ion materials have characteristics such as high electrochemical activity and high energy storage density, and can be used as electrode materials to improve the performance and service life of batteries; secondly, cobalt is one of the essential trace elements in the human body, mainly existing in the form of vitamin B12, and is involved in many biological processes such as hematopoiesis and metabolism, but excessive intake will cause symptoms such as dizziness, nausea, and vomiting; in addition, if cobalt ions are discharged into the environment, they are likely to pollute the soil and even contaminate groundwater. Therefore, due to the importance of cobalt, the quantitative detection of cobalt ions is also very important, and today's carbon dots play a major role in this regard.

[0005] Patent Publication No. CN117903792A discloses a biomass fluorescent carbon dot, its preparation method, and its application in detecting iron ions. The preparation method of the biomass fluorescent carbon dot includes the following steps: Take corn starch and citric acid, add them to pure water, stir evenly, heat and then cool to obtain a yellow solid; dissolve the yellow solid in ultrapure water, and centrifuge to take the supernatant; put the solution into a dialysis bag and dialyze to remove impurities to obtain a yellow solution, that is, the biomass fluorescent carbon dot solution. The highly fluorescent carbon quantum dots prepared by this invention can be used as a fluorescent probe for quantitative measurement of iron ions. Patent Publication No. CN118083958A discloses an orange fluorescent carbon dot for ratio detection of ciprofloxacin and cobalt ions, its preparation method, and application, belonging to the technical field of fluorescent carbon dots. The preparation method of the carbon dot: Weigh methylene blue and dissolve it in water, and ultrasonically obtain a uniformly mixed solution; transfer the above solution to a hydrothermal reaction kettle, react at 150 - 200 °C for 2 - 6 h, wait for the reaction to stop, then let it stand and cool to room temperature, centrifuge to remove insoluble substances and take the supernatant, pass through a dialysis bag with a molecular weight cut-off of 500 - 1000 Da, and dialyze in a glass container for at least three days to obtain a pure carbon dot aqueous solution; freeze-dry the above carbon dot aqueous solution to obtain carbon dots with orange fluorescence emission. The prepared orange fluorescent carbon dots can be used for ratio detection of ciprofloxacin and cobalt ions. Although the prior art discloses fluorescent carbon dots that can detect iron ions and cobalt ions respectively, the probes for detecting Co 2+ are still relatively rare. Therefore, further development is needed. In addition, in some application scenarios, when multiple metal ions need to be detected, multiple fluorescent carbon dots need to be used in combination, which is difficult to detect continuously and the operation is relatively complex. Summary of the Invention

[0006] Aiming at the technical problems of few types of detection probes for Co 2+ and the inability to continuously detect iron ions and cobalt ions, the present invention proposes an oxidized fluorescent carbon dot, its preparation method, and application. After treating the carbon dot solution with an oxidant, the carbon dot retains its original detection ability for Fe 2+ and Fe 3+ , and can detect Co 2+ in a smaller range.

[0007] To achieve the above object, the technical solution of the present invention is realized as follows:

[0008] A preparation method of an oxidized fluorescent carbon dot, which is prepared by reacting a carbon dot in an oxidant solution.

[0009] The carbon dot is prepared by the ultrasonic method.

[0010] The steps of preparing the carbon dot by the ultrasonic method are: adding a carbon source to an alkaline solution and reacting ultrasonically.

[0011] The carbon source can be selected from glucose, sucrose, citric acid, etc.; the alkaline solution can be selected from sodium hydroxide solution, potassium hydroxide solution, barium hydroxide solution, etc.

[0012] The oxidant solution is hydrogen peroxide, potassium permanganate, or a mixed solution of sulfuric acid and nitric acid.

[0013] The concentration of the oxidant solution is 0.1-15 wt%.

[0014] Preferably, the concentration of the prepared hydrogen peroxide is 5-15 wt%; the concentration of the prepared KMnO4 solution is 0.01-0.02 mol / L; the concentration of the prepared concentrated sulfuric acid / nitric acid solution (the volume ratio of sulfuric acid to nitric acid is 3:1 or 4:1) is 0.15-0.2 mol / L.

[0015] The mass ratio of the carbon dots to the oxidant in the oxidant solution is 1-10:1;

[0016] The reaction temperature is 60-70 °C and the time is 1-2 h.

[0017] A method for detecting Co 2+ ions, comprising the following steps:

[0018] (1) Prepare an oxidized fluorescent carbon dots solution;

[0019] (2) Add cobalt ions to the oxidized fluorescent carbon dots solution, measure the fluorescence spectra of the oxidized fluorescent carbon dots solutions containing different concentrations of cobalt ions, and obtain the change curve of the maximum fluorescence intensity of the oxidized fluorescent carbon dots solution with the cobalt ion concentration through fitting; prepare cobalt ion standard solutions with different concentrations respectively, add the cobalt ion standard solutions to the oxidized fluorescent carbon dots solution, measure the fluorescence spectra of the oxidized fluorescent carbon dots solution before and after adding the cobalt ion standard solutions, and obtain the change curve of the maximum fluorescence intensity of the oxidized fluorescent carbon dots solution with the added cobalt ion concentration through fitting;

[0020] (3) Add the test solution to the oxidized fluorescent carbon dots solution and test the fluorescence spectrum, and substitute the maximum fluorescence intensity into the fitting curve in step (2) to obtain the cobalt ion concentration in the test solution.

[0021] A method for continuously detecting Fe 2+ 、Fe 3+ and Co 2+ , comprising the following steps:

[0022] (1) Prepare iron ion standard solutions with different concentrations of Fe 2+ and Fe 3+ respectively, add the iron ion standard solutions to the carbon dots solution, measure the fluorescence spectra of the carbon dots solution before and after adding the iron ion standard solutions, and obtain the change curve of the maximum fluorescence intensity of the carbon dots solution with the added Fe 2+ and Fe3+ Change curve of ion concentration;

[0023] (2) Prepare an oxidized fluorescent carbon dot solution; prepare cobalt ion standard solutions with different concentrations respectively, add the cobalt ion standard solutions to the oxidized fluorescent carbon dot solution, measure the fluorescence spectra of the oxidized fluorescent carbon dot solution before and after adding the iron ion standard solution, and obtain the change curve of the maximum fluorescence intensity of the oxidized fluorescent carbon dot solution with the added cobalt ion concentration by fitting;

[0024] (3) Add the test solution to the carbon dot solution and measure the fluorescence spectrum, and substitute the maximum fluorescence intensity into the fitting curve in step (1) to obtain the concentrations of Fe 2+ and Fe 3+ in the test solution;

[0025] (4) Add an oxidant to the carbon dot solution containing the test solution in step (3), continue to measure the fluorescence spectrum, and substitute the maximum fluorescence intensity into the fitting curve in step (2) to obtain the cobalt ion concentration in the test solution.

[0026] Advantages of the present invention: The present invention discovers that carbon dots have the ability to detect Fe 2+ and Fe 3+ before being untreated, and further expands the detection of Co 2+ after oxidation treatment. By deeply exploring its internal mechanism, it is very likely that due to the intervention of the oxidant, significant changes have occurred in the surface groups and microstructure of the carbon dots. The oxidant reacts chemically with the carbon dots, modifies the functional groups on its surface or triggers the reconstruction of the structure, thereby changing the interaction mode and affinity between the carbon dots and different ions.

[0027] The present invention greatly expands the application scope of carbon dots in the field of ion detection. In the past, in complex ion detection scenarios, often multiple different detection reagents and methods were required to separately determine different ions, with cumbersome operations and high costs. However, the ultrasonic carbon dots in the present invention, after being treated with an oxidant, can continuously detect multiple important metal ions, such as Co2+, Fe 2+ and Fe 3+ , providing a more convenient, efficient and economical solution for ion detection. In the field of environmental monitoring, it can be used to quickly detect the content of these metal ions in water or soil, and timely discover pollution situations; in the field of biomedicine, it can help determine the concentration of relevant metal ions in organisms, providing key data support for the diagnosis and treatment of diseases. In summary, the ultrasonic carbon dots in the present invention show good application prospects, are expected to trigger innovative application changes in many fields, and promote the ion detection technology to a new stage of development. Description of the Drawings

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0029] Figure 1 TEM image (A) and particle size statistical distribution histogram (B) of the carbon dots described in the present invention.

[0030] Figure 2 FTIR image of the carbon dots described in the present invention.

[0031] Figure 3 Ultraviolet-visible absorption spectrum, fluorescence excitation spectrum and emission spectrum of the carbon dots described in the present invention.

[0032] Figure 4 Fluorescence spectra of the carbon dots described in the present invention shortly after preparation (A) and after being placed for a period of time (B).

[0033] Figure 5 For Fe 2+ and Fe 3+ Selectivity and anti-interference detection.

[0034] Figure 6 For detecting Fe 2+ Concentration and linear relationship diagram.

[0035] Figure 7 For detecting Fe 3+ Concentration and linear relationship diagram.

[0036] Figure 8 TEM image (A) and particle size statistical distribution histogram (B) of the carbon dots treated with H2O2 described in the present invention.

[0037] Figure 9 Fluorescence spectrum of the carbon dots treated with H2O2 described in the present invention.

[0038] Figure 10 For Co 2+ Selectivity and anti-interference detection.

[0039] Figure 11 For detecting Co 2+ Concentration and linear relationship diagram. Detailed implementation manners

[0040] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] Example 1

[0042] Weigh 1.98 g of glucose and 0.6 g of NaOH, add deionized water to 10 ml respectively, shake evenly, mix well, use a 400 W ultrasonic oscillator to ultrasonically treat in a water bath for 4 h. After the ultrasonic treatment, adjust the pH to 7 with 6% hydrochloric acid by mass fraction, then use a piston burette to dropwise add 100 ml of absolute ethanol. During the dropping process, use a magnetic stirrer to stir at a speed of 2000 r / min. After the titration, add 1.9 g of magnesium sulfate and continue to stir at the same speed for 30 min. After the stirring is completed, place the solution in an environment of about 6 °C and let it stand for 30 h. Then take out the supernatant and store it in an environment of about 6 °C. The concentration of the carbon dot solution is 8.7 mg / mL.

[0043] The characterization of the carbon dots prepared in this example is as Figures 1-4 shown.

[0044] From Figure 1 it can be seen that the carbon dots have good dispersibility and an average particle size of 4.014 nm; Figure 2 shows the rich functional group structure of the carbon dots; Figure 3 shows the emission peak of the carbon dots under excitation at a wavelength of 375 nm and its corresponding excitation peak; Figure 4 shows the fluorescence spectra of the carbon dots when they are freshly prepared and after being placed for some time.

[0045] From Figure 4 it can be seen that the carbon dots prepared in the early stage have dual-peak emission ( Figure 4 (A)), but the intensity is low. However, after being placed for some time, the fluorescence intensity of the carbon dots increases and the emission peak moves closer to about 440 nm to form a single emission peak ( Figure 4 (B)). Therefore, the carbon dots after being placed for some time are used for the subsequent detection of Fe 2+ and Fe 3+ .

[0046] To prove the stability of the carbon dots. First, the change in the intensity of the carbon dots within 60 days was explored, and it was found that as time extended, the degree of oxidation of the carbon dots increased, resulting in a slow and stable increase in the fluorescence intensity of the carbon dots. Figure 4It can be used as a reference, so the carbon dots are stable over time. Secondly, when measuring with a spectrophotometer, a temperature control device was used for measurements at 10 - 70 °C, and it was found that the fluorescence intensity of the carbon dots was stable without significant fluctuations. Finally, the salt tolerance test of the carbon dots was also carried out. The carbon dots were mixed with NaCl solutions at concentrations of 0.01 - 3 mol / L in a 1:1 ratio, shaken, and the measured fluorescence intensity did not show large fluctuations, indicating that the carbon dots have good salt tolerance.

[0047] Fe 2+ and Fe 3+ Selectivity test:

[0048] Weigh CuSO4·5H2O, ZnSO4·7H2O, CoSO4·7H2O, MnSO4·H2O, MgSO4·7H2O, FeSO4·7H2O, KCl, CaCl2, NaCl, FeCl3·6H2O, and AlCl3·6H2O respectively and dissolve them in 5 ml of deionized water, shake to make various ionic solutions with a concentration of 4 mmol / L. Take 200 μL of each ionic solution and place it in a sample bottle, then add 1.4 ml of the carbon dot solution to each bottle, mix and shake to make them evenly distributed. After standing for 3 min, use a fluorescence spectrophotometer to perform fluorescence tests on each sample. The blank control group should add the same amount of deionized water as the ionic solution. Set the excitation wavelength to 375 nm, and draw an image by taking the maximum fluorescence intensity of each test result as shown in Figure 5 (A). It can be seen that at a concentration of 500 μM, the carbon dots only have an obvious fluorescence response to Fe 2+ and Fe 3+ and show a fluorescence quenching phenomenon.

[0049] Fe 2+ and Fe 3+ Interference resistance test: Weigh equal amounts of CuSO4·5H2O, ZnSO4·7H2O, CoSO4·7H2O, MnSO4·H2O, MgSO4·7H2O, KCl, CaCl2, NaCl, and AlCl3·6H2O and place them in sample bottles, and weigh two portions of each. Then weigh 9 portions of 5.56 mg of FeSO4·7H2O and 9 portions of 5.41 mg of FeCl3·6H2O and mix them with one portion of other ions bottle by bottle, add 5 ml of deionized water, shake to mix evenly to make various mixed ionic solutions with a concentration of 4 mmol / L. Take 200 μL of each mixed ionic solution and place it in a sample bottle, then add 1.4 ml of the carbon dot solution to each bottle, mix and shake to make them evenly distributed. After standing for 3 min, use a fluorescence spectrophotometer to perform fluorescence tests on each sample. The blank control group should add the same amount of deionized water as the ionic solution. Set the excitation wavelength to 375 nm, and draw an image by taking the maximum fluorescence intensity of each test result as shown inFigure 5 As shown in (B), it can be seen that at a concentration of 500 μM, the presence of other ions does not affect the fluorescence quenching effect of carbon dots on Fe 2+ and Fe 3+ .

[0050] Fe 2+ and Fe 3+ Concentration trends and linear ranges

[0051] Take certain amounts of Fe 2+ and Fe 3+ solutions respectively. Then take 2 portions of 4 ml carbon dot solutions. Based on the ionic concentration after adding ions, gradually add a certain amount of ionic solution to the carbon dot solutions in gradients to prepare test solutions with ionic concentrations of 10 - 50 μM. After each addition, the mixed solution should be shaken and left to stand for 2 - 5 min to make the solution mix evenly and react. Before and after each addition, use a fluorescence spectrophotometer to detect the fluorescence spectra of the carbon dot solutions to ensure that the fluorescence intensities at each concentration gradient are detected. Select the maximum fluorescence intensity according to the detection results, plot the curve of the fluorescence intensity of the carbon dot solution changing with the concentration of the test ions, and use origin software to perform linear fitting on the data. The results are as Figures 6-7 shown. The detection limits of Fe 2+ and Fe 3+ are 6.47 μM and 5.83 μM respectively.

[0052] Example 2

[0053] A kind of oxidized fluorescent carbon dots, the preparation method includes the following steps: Take 15 ml of 30% H2O2 solution by mass fraction and 30 ml of deionized water, mix them, and shake for 5 min to make them mix evenly. Subsequently, mix the carbon dot solution (8.7 mg / mL) and 10 wt% H2O2 solution in a volume ratio of 50:1, and shake for 5 min. After the carbon dot solution treated with H2O2 is left to stand at room temperature for 2 h, seal the container and place it in a water bath at 70 °C for water bath heating for 2 h to obtain oxidized fluorescent carbon dots.

[0054] The characterization of the carbon dots prepared in this example is as Figures 8-9 shown.

[0055] From Figure 8 it can be seen that the carbon dots are evenly distributed, and the average particle size is 2.547 nm; Figure 9 is the fluorescence spectrum diagram of the carbon dots after treatment. It can be seen from the figure that the fluorescence emission peak of the carbon dots is concentrated near 440 nm. Its trend is similar to that of the carbon dot solution after long-term natural oxidation, but the fluorescence intensity has been greatly improved in a very short time.

[0056] Co 2+Selectivity: Take 40 μL of each ionic solution and place it in a sample vial. Then add 1.56 mL of the carbon dot solution to each vial, mix, shake to make it evenly distributed, and let it stand for 3 min. Then use a fluorescence spectrophotometer to perform fluorescence tests on each sample, with other steps remaining unchanged. The results are as shown in Figure 10 (A). It can be seen that the original carbon dots could not detect Co at an ionic concentration of 500 μM. 2+ Surprisingly, after treatment, an ionic concentration of only 100 μM can quench the fluorescence intensity of the carbon dots by nearly 90%.

[0057] Co 2+ Anti-interference of Co: Weigh equal amounts of CuSO4·5H2O, ZnSO4·7H2O, MnSO4·H2O, MgSO4·7H2O, KCl, CaCl2, NaCl, AlCl3·6H2O, FeSO4·7H2O, and FeCl3·6H2O and place them in sample vials. Then weigh 10 portions of 5.62 mg of CoSO4·7H2O and mix them with one portion of other ions in each vial one by one; take 40 μL of each mixed ionic solution and place it in a sample vial, and then add 1.56 mL of the carbon dot solution to each vial. The results are as shown in Figure 10 (B). At a concentration of 100 μM, the presence of other ions will not have a significant impact on the fluorescence quenching effect of the carbon dots on Co. 2+

[0058] Co 2+ Concentration trend and linear range of Co

[0059] Take a certain amount of Co 2+ solution, and then take 4 mL of the carbon dot solution. According to the detection method in Example 1, add a certain amount of Co 2+ solution to the carbon dot solution successively, with other steps remaining unchanged. The results are as shown in Figure 11 . A good linear relationship is shown in the range of 0 - 40 μM, and the detection limit is measured to be 1.13 μM. Compared with the detection effects of Fe 2+ and Fe 3+ , there has been a great improvement.

[0060] Example 3

[0061] An oxidized fluorescent carbon dot, the preparation method includes the following steps: Take 15 mL of a 30% H2O2 solution by mass fraction and 30 mL of deionized water, mix them, shake for 5 min to make them evenly mixed. Then mix the carbon dot solution (8.7 mg / mL) and the 10 wt% H2O2 solution in a volume ratio of 25:1, shake for 5 min. After the carbon dot solution treated with H2O2 is allowed to stand at room temperature for 2 h, seal the container and place it in a water bath at 70 °C and heat it in the water bath for 2 h to obtain the oxidized fluorescent carbon dot.

[0062] Example 4

[0063] An oxidized fluorescent carbon dot, and the preparation method includes the following steps: Take 15 ml of H2O2 solution with a mass fraction of 30% and 30 ml of deionized water, mix them, and oscillate for 5 min to make the mixture uniform. Subsequently, mix the carbon dot solution (8.7 mg / mL) and the 10 wt% H2O2 solution at a volume ratio of 100:1, oscillate for 5 min. After the carbon dot solution treated with H2O2 is left standing at room temperature for 2 h, seal the container and place it in a water bath, and heat it in a water bath at 70 °C for 2 h to obtain the oxidized fluorescent carbon dot.

[0064] Example 5

[0065] An oxidized fluorescent carbon dot, and the preparation method includes the following steps: Take 15 ml of H2O2 solution with a mass fraction of 30% and 30 ml of deionized water, mix them, and oscillate for 5 min to make the mixture uniform. Subsequently, mix the carbon dot solution (8.7 mg / mL) and the 10 wt% H2O2 solution at a volume ratio of 50:1, oscillate for 5 min. After the carbon dot solution treated with H2O2 is left standing at room temperature for 2 h, seal the container and place it in a water bath, and heat it in a water bath at 60 °C for 1 h to obtain the oxidized fluorescent carbon dot.

[0066] Example 6

[0067] An oxidized fluorescent carbon dot, and the preparation method includes the following steps: Prepare a KMnO4 solution with a concentration of 0.015 mol / L. Subsequently, mix the carbon dot solution (8.7 mg / mL) and the KMnO4 solution at a volume ratio of 1:1, oscillate for 5 min. After the carbon dot solution treated with H2O2 is left standing at room temperature for 2 h, seal the container and place it in a water bath, and heat it in a water bath at 70 °C for 2 h to obtain the oxidized fluorescent carbon dot.

[0068] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of oxidized fluorescent carbon dots, characterized in that, It is prepared by reacting carbon dots in an oxidant solution.

2. The preparation method of the oxidized fluorescent carbon dots according to claim 1, wherein, The carbon dots are prepared by an ultrasonic method.

3. The preparation method of the oxidized fluorescent carbon dots according to claim 2, characterized in that, The steps for preparing carbon dots by the ultrasonic method are as follows: adding a carbon source into an alkaline solution for ultrasonic reaction.

4. The preparation method of the oxidized fluorescent carbon dots according to claim 3, wherein, The oxidant solution is hydrogen peroxide, potassium permanganate, or a mixed solution of sulfuric acid and nitric acid.

5. The preparation method of the oxidized fluorescent carbon dots according to claim 4, wherein, The concentration of the oxidant solution is 0.1-15 wt%.

6. The preparation method of the oxidized fluorescent carbon dots according to claim 5, wherein The mass ratio of the carbon dots to the oxidant in the oxidant solution is 1-10:

1.

7. The preparation method of the oxidized fluorescent carbon dots according to claim 6, characterized in that, The reaction temperature is 60-70 °C and the time is 1-2 h.

8. Oxidized fluorescent carbon dots prepared by the method according to any one of claims 1-7.

9. A method for detecting Co 2+ ions, characterized in that It includes the following steps: (1) Prepare an oxidized fluorescent carbon dot solution; (2) Add cobalt ions into the oxidized fluorescent carbon dot solution, measure the fluorescence spectra of the oxidized fluorescent carbon dot solutions containing different concentrations of cobalt ions, and obtain the change curve of the maximum fluorescence intensity of the oxidized fluorescent carbon dot solution with the cobalt ion concentration by fitting; (3) Add the test solution into the oxidized fluorescent carbon dot solution and test the fluorescence spectrum, and substitute the maximum fluorescence intensity into the fitting curve in step (2) to obtain the cobalt ion concentration in the test solution.

10. A method for continuously detecting Fe 2+ , Fe 3+ and Co 2+ , characterized in that It includes the following steps: (1) Add Fe 2+ and Fe 3+ into the carbon dot solution respectively, measure the fluorescence spectra of the oxidized fluorescent carbon dot solution containing different concentrations of Fe 2+ and Fe 3+ , and obtain the change curves of the maximum fluorescence intensity of the oxidized fluorescent carbon dot solution with the concentrations of Fe 2+ and Fe 3+ through fitting; (2) Prepare an oxidized fluorescent carbon dot solution; add cobalt ions into the oxidized fluorescent carbon dot solution, measure the fluorescence spectra of the oxidized fluorescent carbon dot solutions containing different concentrations of cobalt ions, and obtain the change curve of the maximum fluorescence intensity of the oxidized fluorescent carbon dot solution with the cobalt ion concentration by fitting; (3) Add the test solution to the carbon dot solution and measure the fluorescence spectrum. Substitute the maximum fluorescence intensity into the fitting curve in step (1) to obtain the concentrations of Fe 2+ and Fe 3+ in the test solution; (4) Add the oxidant into the carbon dot solution containing the test solution in step (3), continue to measure the fluorescence spectrum, and substitute the maximum fluorescence intensity into the fitting curve in step (2) to obtain the cobalt ion concentration in the test solution.

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