Preparation method and application of sorghum red carbon quantum dots

Abstract

The present application belongs to the technical field of preparation and application of carbon nanomaterials, and particularly relates to a preparation method and application of sorghum red carbon quantum dots. The present application mainly solves the problems of existing chromium (VI) ion detection methods, such as environmental pollution and poor safety, and effectively detects Cr (VI) in tap water and wastewater by synthesizing carbon quantum dots from sorghum red and anhydrous p-aminobenzenesulfonic acid through a hydrothermal method. The present application has the advantages of convenient raw material source, low cost, abundant supply, simple processing technology, safety, non-toxicity, natural color, and simple preparation operation steps. The sorghum red carbon quantum dots prepared by the present application have water solubility, can be used for detecting the concentration of chromium (VI) ions in water, have a specific recognition effect on potassium dichromate, can be used for detecting potassium dichromate, have good selectivity, and have high sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of carbon nanomaterial preparation and application technology, specifically relating to a method for preparing and applying sorghum red carbon quantum dots. Background Technology

[0002] With the continuous development of industrial production, the unreasonable discharge of waste gas, wastewater, and solid waste has become increasingly serious, leading to increasingly severe heavy metal pollution. This has become an environmental problem threatening human health and survival. Chromium (Cr), as a relatively typical heavy metal pollutant, poses a significant threat to the natural environment. Cr exists in nature in two valence states: Cr(III) and Cr(VI). Cr(III) has low toxicity, while Cr(VI) is highly toxic, causing not only diseases such as cirrhosis and hemolytic anemia, but also exhibiting strong carcinogenicity, posing a serious threat to human health. In its natural state, Cr(VI) mainly exists as CrO4. 2- CrO7 2- Cr(VI) exists in anionic form and possesses strong oxidizing and mobility, causing significant pollution to soil and water sources. Therefore, exploring the prevention and control of Cr(VI) pollution is of great importance to maintaining ecological balance.

[0003] Since the discovery of fluorescent carbon quantum dots, they have gradually become a research hotspot as a novel fluorescent probe material. Compared with other fluorescent probes, carbon quantum dots have the advantages of being easy to prepare, low cost, optically stable, biocompatible, and low toxicity.

[0004] Carbon quantum dots can be applied to the detection of various substances under different conditions by different surface modifications. However, existing methods for detecting chromium(VI) ions have problems such as environmental pollution and poor safety. For example, a method for detecting chromium(VI) ions using an oxide nanozyme-based test strip prepared by patent number CN115876756A requires a stable pH of 4 and the precursor contains ammonia, which is volatile and can create an explosive atmosphere, resulting in poor safety. The phosphorus-doped carbon dot precursor prepared by patent number CN116891739A contains phosphoric acid. Concentrated solutions of phosphoric acid are highly irritating to the skin and eyes, and can corrode the skin and cause dermatitis.

[0005] Fluorescent ink is one of the most widely used inks in ticket printing, and its main component is fluorescent pigment. Fluorescent pigments are functional luminescent pigments, differing from ordinary pigments in that when exposed to external light (including ultraviolet light), they absorb a certain form of energy, exciting photons and releasing the absorbed energy in the form of low-visible light, thus producing fluorescence of different hues. This ink, invisible to the naked eye, only emits light under ultraviolet or infrared light. Its convertible and concealable characteristics, as well as its inability to be precisely copied on color copiers, make it widely used in the specialty printing industry. The invisible patterns or text printed with this ink will appear as clear, bright, and vividly colored fluorescent patterns under ultraviolet light. This anti-counterfeiting measure is convenient, requiring only an ultraviolet light source or a banknote detector. Therefore, developing a preparation method based on sorghum red carbon quantum dots (CDs) and its applications is of great significance. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing sorghum red carbon quantum dots, as well as the detection of Cr(VI) in actual water samples and the application of fluorescent ink.

[0007] Sorghum red is a natural pigment extracted from sorghum husks. Its main components are apigenin and quercetin, exhibiting a natural and soft hue, strong temperature and light resistance, and good solubility. This invention synthesizes carbon quantum dots using a hydrothermal method with sorghum red and anhydrous p-aminobenzenesulfonic acid. These quantum dots can be used for low-cost, rapid, and convenient qualitative and quantitative detection of Cr(VI). Furthermore, the sulfur-doped carbon quantum dots prepared by this invention have less environmental impact, a detection limit of 0.212 μM for Cr(VI), higher sensitivity, and can effectively detect Cr(VI) in tap water and wastewater. Moreover, the modified sorghum red carbon quantum dots are paler in color under sunlight but exhibit significant fluorescence under 365 nm ultraviolet excitation, making them suitable for anti-counterfeiting color development.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0009] A method for preparing sorghum red carbon quantum dots, the specific steps of which are as follows:

[0010] (1) Mix sorghum red dye and anhydrous p-aminobenzenesulfonic acid evenly, add ultrapure water and stir, and use ultrasound to aid dissolution to obtain a mixed solution.

[0011] (2) The mixed solution is subjected to a hydrothermal reaction;

[0012] (3) After the reaction is complete, the mixed solution is centrifuged to obtain the supernatant, then filtered through a microporous membrane and freeze-dried to obtain the target sorghum red carbon quantum dot powder.

[0013] Furthermore, the volume of ultrapure water added in step (1) is 20 ml.

[0014] Furthermore, the ultrasonic dissolution time in step (1) is 20 min.

[0015] Furthermore, the hydrothermal reaction heating temperature in step (2) is 170–220°C.

[0016] Furthermore, the hydrothermal reaction heating time in step (2) is 4 to 8 hours.

[0017] Furthermore, the pore size of the microporous filter membrane in step (3) is 0.22 μm.

[0018] Application of sorghum red carbon quantum dots prepared by any of the above methods in detecting the concentration of chromium(VI) ions in water.

[0019] An application of sorghum red carbon quantum dots as described above in the detection of potassium dichromate concentration in water.

[0020] Application of sorghum red carbon quantum dots prepared by any of the above methods in the preparation of fluorescent inks.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. The operation steps of this invention are simple, and sorghum red carbon quantum dots can be obtained without surface passivation agent treatment or modification. They are water-soluble and have a specific recognition effect on potassium dichromate.

[0023] 2. The N,S-CDs prepared by this invention have excellent photostability and chemical stability, and the fluorescent inks prepared based on N,S-CDs have good application potential in the field of anti-counterfeiting.

[0024] 3. The sorghum red used in this invention is extracted from sorghum husks. It is readily available, inexpensive, abundant, easy to process, safe, non-toxic, and has a natural color. Attached Figure Description

[0025] Figure 1 TEM images of N,S-CDs.

[0026] Figure 2 This is a particle size distribution diagram of N,S-CDs.

[0027] Figure 3 The image shows the FTIR plot of N,S-CDs.

[0028] Figure 4 XPS full spectrum of N,S-CDs.

[0029] Figure 5 The fluorescence excitation, emission, and absorption spectra of N,S-CDs are shown.

[0030] Figure 6 The fluorescence emission spectrum of sorghum red carbon quantum dots under excitation wavelengths of 300 nm to 400 nm is shown.

[0031] Figure 7 The fluorescence intensity diagrams of N,S-CDs at different KCl concentrations are shown.

[0032] Figure 8 The graph shows the fluctuation of fluorescence intensity of N,S-CDs in different pH environments.

[0033] Figure 9 The image shows the fluorescence intensity of N,S-CDs after a period of xenon lamp irradiation.

[0034] Figure 10 This is a graph showing the change in fluorescence intensity of N,S-CDs after a period of refrigeration.

[0035] Figure 11 This is a selective detection graph for N,S-CDs solutions.

[0036] Figure 12 The fluorescence spectra of carbon dots after adding different concentrations of potassium dichromate are shown.

[0037] Figure 13 , 14 The graph shows the linear relationship between ΔF and Cr(VI) concentration.

[0038] Figure 15 , 17 Images 19 and 21 show the characters "Shanxi", "ABCDE", and the anti-counterfeiting seal of "Shanxi University" written and drawn on stone paper using two different types of carbon dots under sunlight.

[0039] Figure 16 , 18 Images 20 and 22 show the writing and drawing of the characters "Shanxi", "ABCDE", and the anti-counterfeiting seal of "Shanxi University" on stone paper using two types of carbon dots under 365nm ultraviolet light. Detailed Implementation

[0040] To facilitate understanding of the present invention, a more comprehensive description will be given below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0041] Example 1

[0042] A method for preparing and applying red carbon dots based on sorghum includes the following steps:

[0043] (1) Weigh 20 mg of anhydrous p-aminobenzenesulfonic acid and 20 mg of sorghum red dye into a beaker, add 20 ml of ultrapure water and stir, then use ultrasound to help dissolve for 20 min.

[0044] (2) After the mixture is completely dissolved, transfer it into a 50ml stainless steel reactor with a polytetrafluoroethylene liner, place it in an oven, and hydrothermally react at 180℃ for 6 hours.

[0045] (3) After the reaction is complete, the mixture is filtered through a 0.22 μm microporous membrane, centrifuged at 10,000 rpm for 10 min, and the supernatant is freeze-dried to obtain carbon dot powder for later use.

[0046] The transmission electron microscopy characterization results of the N,S-CDs prepared in Example 1 are as follows: Figure 1 As shown, from Figure 1 The N,S-CDs were found to have good dispersibility, which indicates that N,S-CDs have a crystal structure of 0.21 nm. Such a small particle size is conducive to obtaining a large specific surface area and more Cr(VI) reactive sites. Figure 2 The figure shows the particle size distribution of N,S-CDs. It can be seen from the figure that the particle size follows a normal distribution with an average particle size of 2.5 nm, and is concentrated between 2.0 and 3.3 nm.

[0047] The FTIR spectral characterization results of the N,S-CDs prepared in Example 1 are as follows: Figure 3 As shown in the figure, 3058.22cm -1 The absorption peak is for the stretching vibration of -C=CH, at 1408.41 cm⁻¹. -1 The absorption peak for the bending vibration of CH is 1121.30 cm⁻¹. -1 The absorption peak is the COC stretching vibration peak, at 1032.74 cm⁻¹. -1 The absorption peak is due to the bending vibration of OH, at 685.63 cm⁻¹. -1 The out-of-plane bending vibration absorption peak of CH indicates that the CD surface has a large number of hydrophilic functional groups, such as -C=C- and -OH. Water solubility means that substitution reactions can easily occur, thereby changing its properties.

[0048] The XPS full spectrum of the N,S-CDs prepared in Example 1 is shown below. Figure 4 As shown, the characteristic peaks of CDs at 167eV, 286eV, 401eV and 548eV correspond to S2p, C1s, O1s and N1s peaks respectively, proving that CDs are composed of four elements: C, O, N and S.

[0049] The fluorescence excitation and emission spectra of the N,S-CDs prepared in Example 1 are as follows: Figure 5As shown, CDS emits blue fluorescence at a wavelength of 467 nm at 357 nm. This is mainly because the high crystallinity of CDS facilitates electron transfer to the surface and generates sp. 2 π-π* transition.

[0050] The fluorescence emission spectra of the N,S-CDs prepared in Example 1 at excitation wavelengths of 316 nm to 396 nm are as follows: Figure 6 As shown. By Figure 6 It can be seen that as the excitation wavelength changes, the emission wavelength does not exhibit a significant redshift, indicating that the CDs do not have an excitation wavelength dependence. This confirms the uniformity of the sorghum red carbon quantum dots surface and that their fluorescence mainly originates from the carbon nucleus or molecular fluorophore of the carbon quantum dots.

[0051] Example 2

[0052] Determination of N,S-CDs in Salt Solutions

[0053] (1) Mix 2 ml of KCl solutions with concentrations of 0.01 M, 0.025 M, 0.05 M, 0.075 M, 0.1 M, 0.25 M, 0.5 M, 0.75 M, 1 M and 2 M with 120 μl of N,S-CDs solution (1 mg / ml) thoroughly.

[0054] (2) Measure the fluorescence intensity of the mixed solution using a fluorescence spectrophotometer.

[0055] Experimental results are as follows Figure 7 As shown, the fluorescence intensity of N,S-CDs varies very little in mixed solutions of 0.01–2 M KCl, and is almost unaffected by the concentration of KCl solution. This indicates that the fluorescence intensity of N,S-CDs can remain stable under the influence of different salt concentrations, making it suitable for the determination of pollutants in environments with different salt concentrations.

[0056] pH stability determination of N,S-CDs

[0057] (1) KCl solutions with pH values ​​of 3, 4, 5, 6, 7, 8, 9 and 10 were thoroughly mixed with 120 μL of N,S-CDs solution (1 mg / mL);

[0058] (2) Measure the fluorescence intensity of the mixed solution using a fluorescence spectrophotometer.

[0059] like Figure 8 As shown, the fluorescence intensity of N,S-CDs changes very little under both acidic and alkaline conditions, proving that the fluorescence intensity of N,S-CDs is not affected by pH changes and that N,S-CDs have good stability.

[0060] Example 3

[0061] Xenon lamp stability determination of N,S-CDs

[0062] (1) Mix 2 ml of PBS buffer solution with 120 μl of carbon dot solution (1 mg / ml) thoroughly;

[0063] (2) Measure a group of samples every 2 minutes in a fluorescence spectrophotometer and observe the change in intensity.

[0064] The results are as follows Figure 9 As shown, the fluorescence intensity of N,S-CDs changed very little within 1 hour, remaining basically at a stable level.

[0065] Time stability determination of N,S-CDs

[0066] (1) The N,S-CDs carbon dot solutions were refrigerated for 30, 60 and 90 days respectively;

[0067] (2) Take 120 μL of carbon dot solution (1 mg / mL) and mix it thoroughly with 2 mL of PBS buffer solution, and measure the change in intensity.

[0068] The results are as follows Figure 10 As shown, the fluorescence intensity of N,S-CDs remained very small 90 days after preparation, indicating that the prepared carbon dots can be preserved for a considerable period of time without any change in their properties.

[0069] Example 4

[0070] Effect of potassium dichromate concentration on the fluorescence intensity of N,S-CDs.

[0071] (1) Mix 2 ml of PBS buffer solution with 120 μl of N,S-CDs solution (1 mg / ml) thoroughly;

[0072] (2) Continuously add 0.01M potassium dichromate solution to it and mix thoroughly;

[0073] (3) Fluorescence intensity was measured using a fluorescence spectrophotometer.

[0074] The results are as follows Figure 11 and 12 As shown, when the concentration of potassium dichromate in the mixed solution was increased to 350 μM, the fluorescence intensity decreased from 597 to 107, indicating that N,S-CDs have excellent selectivity for potassium dichromate; the detection of potassium dichromate by N,S-CDs is as follows. Figure 13 , 14 As shown, the detection limit is 0.212 μM, and the detection range is 2.5-50 μM; 75-250 μM.

[0075] Example 5

[0076] Detection of potassium dichromate by N,S-CDs in actual samples

[0077] (1) Mix 2 ml of tap water with N,S-CDs (1 mg / ml) thoroughly and measure the chromium (VI) ion content in the tap water;

[0078] (2) Add 10, 20 and 30 μM potassium dichromate to the mixed solution respectively, measure the fluorescence intensity, and calculate the recovery rate and RSD value.

[0079] (3) The treatment method for acid mineral water is the same as in steps (1) and (2), except that tap water is replaced with acid mineral water.

[0080] The results are shown in Table 1. In tap water, the recovery rate ranged from 93.94% to 102%, and the RSD value ranged from 0.43% to 1.56%. The N,S-CDs method for detecting potassium dichromate in acidic mineral water is shown in Table 2. The recovery rate ranged from 98% to 102%, and the RSD value ranged from 0.54% to 3.37%. This indicates that the method has good detection performance in both tap water and acidic mineral water, meaning it can be applied to the detection of practical samples.

[0081] Table 1. Spiking recoveries of N,S-CDs in tap water

[0082]

[0083] Table 2. Spiking recoveries of N,S-CDs in acidic mineral water

[0084]

[0085] Example 6

[0086] Modification and preparation methods of N,S-CDs

[0087] (1) Weigh 20mg of sorghum red dye, 20mg of anhydrous p-aminobenzenesulfonic acid and 92.5mg of boric acid and place them in a beaker. Add 20ml of ultrapure water and stir. Use ultrasound to help dissolve for 20min.

[0088] (2) After it is completely dissolved, transfer it into a 50ml stainless steel reactor with a polytetrafluoroethylene liner, place it in an oven, and hydrothermally react at 180℃ for 6 hours.

[0089] (3) After the reaction is complete, the mixture is filtered through a 0.22 μm microporous membrane, centrifuged at 10,000 rpm for 10 min, and the supernatant is freeze-dried to obtain carbon dot powder for later use.

[0090] Example 7

[0091] Application of NS-CDs in anti-counterfeiting

[0092] (1) N,S-CDs and the modified CDs obtained in Example 6 were respectively prepared into CDs solutions of 1 mg / ml to prepare fluorescent inks;

[0093] (2) Encapsulate fluorescent ink into the ink sac of a fountain pen for writing and drawing on paper.

[0094] N,S-CDs writing performance under sunlight is as follows Figure 15 As shown, Figure 16 N,S-CDs emit a pale blue fluorescence when irradiated with 365nm ultraviolet light.

[0095] Modified CDs writing effect Figures 17-22 As shown, the patterns "Shanxi" and "ABCDE" written on stone paper by modified CDs are almost invisible under sunlight, but the blue fluorescence emitted by modified CDs under 365nm ultraviolet light is brighter than that of N,S-CDs. This also proves that N,S-CDs can be used as materials for preparing fluorescent inks, and its derivatives can be used as anti-counterfeiting stamps.

[0096] The embodiments described above are merely specific examples of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing sorghum red carbon quantum dots, characterized in that, The specific steps are as follows: (1) Mix 20 mg of sorghum red dye and 20 mg of anhydrous p-aminobenzenesulfonic acid evenly, add 20 mL of ultrapure water and stir, then use ultrasound to aid dissolution to obtain a mixed solution; (2) The mixed solution is subjected to a hydrothermal reaction; (3) After the reaction is complete, the mixed solution is centrifuged to obtain the supernatant, then filtered through a microporous membrane and freeze-dried to obtain the target sorghum red carbon quantum dot powder.

2. The method for preparing sorghum red carbon quantum dots according to claim 1, characterized in that, The ultrasonic dissolution time in step (1) is 20 min.

3. The method for preparing sorghum red carbon quantum dots according to claim 1, characterized in that, The hydrothermal reaction heating temperature in step (2) is 170~220℃.

4. The method for preparing sorghum red carbon quantum dots according to claim 1, characterized in that, The hydrothermal reaction heating time in step (2) is 4~8h.

5. The method for preparing sorghum red carbon quantum dots according to claim 1, characterized in that, The microporous filter membrane mentioned in step (3) has a pore size of 0.22 μm.

6. An application of sorghum red carbon quantum dots prepared according to any one of claims 1 to 5 in detecting the concentration of hexavalent chromium ions in water.

7. An application of sorghum red carbon quantum dots prepared according to any one of claims 1 to 5 in detecting potassium dichromate concentration in water.

8. The application of sorghum red carbon quantum dots prepared according to any one of claims 1 to 5 in the preparation of fluorescent ink.

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