A preparation method of carbon quantum dots based on biochar

Abstract

The application discloses a preparation method of carbon quantum dots based on biochar, and comprises the following steps: step S1, plant-based biochar is put into water for ultrasonic cleaning to remove impurities and incompletely carbonized plant residues; step S2, the biochar in step S1 is added into a hydrogen peroxide solution with a concentration of 20-40%, and after stirring, a black suspension is formed; step S3, the suspension is subjected to ultrasonic treatment, and then the suspension is vacuum filtered by using a 0.22 mu m mixed cellulose ester membrane to remove non-fluorescent precipitates, and then dialysis is performed, so that a biochar carbon quantum dot solution is prepared. The carbon quantum dots are prepared by using an ultrasonic method in one step, no subsequent chemical reagent is added, and the prepared carbon quantum dots have good fluorescence performance.

Description

Technical Field

[0001] This invention relates to the field of biochar carbon quantum dot preparation technology, specifically to a biochar carbon quantum dot preparation method and a metal ion analysis and detection technology. Background Technology

[0002] Carbon quantum dots (CQDs) are fluorescent nanoparticles typically smaller than 10 nm, representing a type of carbon-based fluorescent nanomaterial. Compared to traditional semiconductor quantum dots and common fluorescent dyes, CQDs possess advantages such as high solubility, excellent photostability, good biocompatibility, and high chemical inertness, leading to their successful applications in numerous fields, including sensing, drug delivery, gene therapy, bioimaging, photocatalysis, and electrocatalysis. The synthesis methods for CQDs can be categorized into two types: top-down and bottom-up methods. Top-down methods decompose macroscopic carbon materials into nanoscale particles under conditions such as arc discharge, laser ablation, and acid oxidation exfoliation. Bottom-up methods use small molecules as carbon sources and can synthesize CQDs through microwave, hydrothermal, or solvothermal assisted treatments. Compared to bottom-up methods, top-down methods offer significant advantages such as controllable synthesis size and clear core structure and composition. Currently, the commonly used carbon precursors for carbon quantum dots mainly include graphite, carbon nanoparticles, graphite, carbon nanotubes, carbon black, and activated carbon. However, the high price of these carbon materials greatly limits the preparation and application of carbon quantum dots.

[0003] The industries that produce and process agricultural raw materials generate millions of tons of biomass byproducts and waste, much of which remains untapped. These biomass wastes can be converted into syngas, bio-oil, and biochar through gasification or pyrolysis, serving as alternative resources. Syngas and bio-oil, as biofuels, can replace fossil fuels and alleviate energy depletion. However, the large amount of biochar generated in this process remains largely unutilized, hindering the industrialization of this technology. Therefore, developing clean, efficient, and high-value-added products from biochar to unlock the economic potential of this process is crucial.

[0004] Heavy metal ion pollution has become a significant issue in environmental sanitation. Excessive heavy metal ions in drinking water pose a serious threat to human health. Commonly used methods for detecting heavy metal ions, such as mass spectrometry, electrochemical detection, and X-ray fluorescence spectroscopy, are largely limited in application due to high costs and complex equipment. However, there are currently no reports on using biochar to replace commonly used and expensive carbon precursors in the preparation of carbon quantum dots and applying them to the detection of heavy metal ions in water. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing carbon quantum dots based on biochar, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing carbon quantum dots based on biochar includes the following steps:

[0008] Step S1: Place the plant-based biochar in water and ultrasonically clean it to remove impurities and incompletely carbonized plant residues.

[0009] Step S2: Add the biochar from S1 to a 20-40% hydrogen peroxide solution and stir to form a black suspension.

[0010] Step S3: The suspension is sonicated, then the suspension is vacuum filtered through a 0.22μm mixed cellulose ester membrane to remove non-fluorescent precipitates, and then dialyzed to obtain a brownish-yellow biochar quantum dot solution.

[0011] Preferably, the ultrasonic treatment in step S1 has a power of 480±60W, a frequency of 40±20kHz, and a duration of 2±1h.

[0012] Preferably, the ultrasonic cleaning time in step S1 is 30±10 min.

[0013] Preferably, the dialysis in step S3 involves purifying the quantum dot solution using a dialysis bag with a cutoff of 3500±500 Da for 48±12 hours.

[0014] Preferably, the plant-based biochar is pine wood biochar (PWB), corn stalk biochar (CSB), and rice husk biochar (RHB).

[0015] The average particle size of pine biochar quantum dots (PWB-CQDs) prepared by the above method is 2.16±0.50 nm, the average particle size of corn biochar quantum dots (CSB-CQDs) is 3.28±0.81 nm, and the distribution is relatively dispersed. The average particle size of rice husk biochar quantum dots (RHB-CQDs) is 1.37±0.30 nm, and the distribution is relatively concentrated.

[0016] The application of biochar-based carbon quantum dots in the analysis and detection of metal ions.

[0017] Preferably, the metal ion is Cu. 2+ Fe 2+ Pb 2+ Ni 2+ Co 2+ Mn 2+ Al 3+ Ba2+ Ag 2+ Cd 2+ .

[0018] The heavy metal ion quenching analysis includes the following steps;

[0019] A series of metal salts were prepared into 25 mM·L⁻¹ stock solutions using ultrapure water for later use.

[0020] Using 1.5 μM·L⁻¹ heavy metal ions (Cu) 2+ Fe 2+ Pb 2+ Ni 2+ Co 2+ Mn 2+ Al 3+ Ba 2+ Ag 2+ Cd 2+ Quenching experiments were conducted with three different types of biochar quantum dots;

[0021] First, the fluorescence intensity (F0) of the three biochar carbon quantum dot solutions was measured respectively;

[0022] Then, 10 heavy metal ions (Cu) 2+ Fe 2+ Pb 2+ Ni 2+ Co 2+ Mn 2+ Al 3+ Ba 2+ Ag 2+ Cd 2+ The heavy metal ions were mixed with 2 ml of each of the three biochar carbon quantum dot solutions in a cuvette to achieve a heavy metal ion concentration of 1.5 μM·L⁻¹. Then, the fluorescence intensity (F) of the metal ion / biochar carbon quantum dot solution was measured.

[0023] The degree of fluorescence quenching is expressed as a percentage reduction in fluorescence (%), where percentage reduction in fluorescence (%) = (F0 - F) / F0;

[0024] Using Cu, a metal ion with the highest influence in the fluorescence of three biochar carbon quantum dots. 2+ To determine the quenching of metal ions, Cu solutions of 0.01–50 μM·L⁻¹ were used. 2+ To assess the quenching ability of three biochar carbon quantum dots, the fluorescence intensity (F0) of the three biochar carbon quantum dot solutions was first measured.

[0025] Then a series of concentrations of Cu 2+ The solution was added to a cuvette containing biochar carbon quantum dots, and then Cu was measured. 2+ Fluorescence intensity (F) of biochar carbon quantum dot solution.

[0026] Preferably, the ultrasonic cleaning time in step S1 is 30 minutes.

[0027] Preferably, the refrigeration temperature in step S6 is below 4°C.

[0028] Preferably, the series of metal salts includes CuCl2, FeCl2, Pb(NO3)2, NiCl2, CoCl2, MnCl2, AlCl3, BaCl2, AgSO4, and CdCl2.

[0029] Preferably, the processing time in step S4 is 2 hours.

[0030] This invention provides a method for preparing carbon quantum dots (CQDs) from biochar using an ultrasonic method. The ultrasonic method requires simple equipment, consumes little energy, is easy and safe to operate, and requires no additional organic reagents, making it environmentally friendly and clean. Its application in detecting heavy metal ions in water systems paves the way for the use of biochar-derived carbon quantum dots as novel fluorescent probes. Specifically, biochar-derived carbon quantum dots (CQDs) were prepared using an ultrasonic method from three types of biochar produced by biomass thermal conversion (pine biochar (PWB), corn straw biochar (CSB), and rice husk biochar (RHB)). The PWB-CQDs, CSB-CQDs, and RHB-CQDs were characterized using spectral and microscopic techniques. They were used as fluorescent detection probes for 10 heavy metal ions, showing different responses to each ion. The effects of the three CQDs on the metal ion Cu were investigated. 2+ The relationship between fluorescence intensity and metal ion concentration during detection.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] (1) Using biochar as raw material, carbon quantum dots are prepared in one step by ultrasonic method, which is different from the conventional hydrothermal synthesis method. No subsequent chemical reagents are added, and the prepared carbon quantum dots have good fluorescence properties.

[0033] (2) The carbon quantum dot preparation technology is green and pollution-free, opening up a new path for the comprehensive utilization of biochar. The preparation process will not generate huge energy consumption, nor will it make the preparation method too complicated and dangerous. Attached Figure Description

[0034] Figure 1Scanning electron microscope (SEM) images of the prepared carbon quantum dots (A: pine biochar (PWB), B: corn stalk biochar (CSB), and C: rice husk biochar (RHB)), transmission electron microscope (TEM) images and particle size distribution diagrams (D: pine biochar carbon quantum dots (PWB-CQDs), E: corn stalk biochar carbon quantum dots (CSB-CQDs), and F: rice husk biochar carbon quantum dots (RHB-CQDs)), and TEM lattice fringe images (G: pine biochar carbon quantum dots (PWB-CQDs), H: corn stalk biochar carbon quantum dots (CSB-CQDs), and I: rice husk biochar carbon quantum dots (RHB-CQDs)).

[0035] Figure 2 XRD patterns of biochar and prepared carbon quantum dots are shown; A represents pine biochar (PWB) and pine biochar carbon quantum dots (PWB-CQDs), B represents corn stalk biochar (CSB) and corn stalk biochar carbon quantum dots (CSB-CQDs), and C represents rice husk biochar (RHB) and rice husk biochar carbon quantum dots (RHB-CQDs).

[0036] Figure 3 Infrared spectra of biochar and the prepared carbon quantum dots.

[0037] Figure 4 Raman spectra of biochar and prepared carbon quantum dots; A is pine charcoal biochar (PWB), B is corn stalk biochar (CSB) and C is rice husk biochar (RHB).

[0038] Figure 5 The images show the emission spectra, UV absorption spectra, and excitation wavelength versus emission wavelength relationships of the prepared carbon quantum dots under different excitation wavelengths. A represents pine biochar carbon quantum dots (PWB-CQDs), B represents corn stalk biochar carbon quantum dots (CSB-CQDs), and C represents rice husk biochar carbon quantum dots (RHB-CQDs). The middle image is a dark-field image under a 365nm UV lamp, and the right image is an FL image. D represents the maximum emission wavelength and fluorescence intensity of pine biochar carbon quantum dots (PWB-CQDs), corn stalk biochar carbon quantum dots (CSB-CQDs), and rice husk biochar carbon quantum dots (RHB-CQDs) under different excitation wavelengths.

[0039] Figure 6 The fluorescence intensity of the prepared carbon quantum dots under different pH conditions is shown in the figure.

[0040] Figure 7 The graph shows the change in fluorescence intensity of the prepared carbon quantum dots over time under optimal excitation light irradiation.

[0041] Figure 8The addition of cations (concentration of 1.5 μM·L⁻¹, namely Cu) 2+ Fe 2+ Pb 2+ Ni 2+ Co 2+ Mn 2+ Al 3+ Ba 2+ Ag 2+ Cd 2+ The graph shows the change in the fluorescence intensity ratio F / F0 of the carbon quantum dot dispersion before and after.

[0042] Figure 9 Add 0–50 μM·L⁻¹ Cu 2+ The fluorescence spectrum changes and F0 / F vs Cu 2+ Fitted curves of concentration relationship. Detailed Implementation

[0043] The plant-based biochar of this invention was purchased from Guangzhou Cuilin Biotechnology Co., Ltd. (item number CLSW00001).

[0044] Example 1

[0045] A method for preparing carbon quantum dots based on biochar includes the synthesis and purification of biochar carbon quantum dots and the analysis of heavy metal ion quenching.

[0046] The synthesis of biochar carbon quantum dots includes the following steps:

[0047] Step S1: Place pine charcoal biochar (PWB), corn stalk biochar (CSB), and rice husk biochar (RHB) into ultrapure water for ultrasonic cleaning to remove impurities and incompletely carbonized plant residues.

[0048] Step S2: After cleaning, place the pine charcoal biochar (PWB), corn stalk biochar (CSB), and rice husk biochar (RHB) into a drying oven to dry for later use.

[0049] Step S3: Take out 4.0g of the cleaned and dried biochar, add it to a 30% hydrogen peroxide solution (70ml), stir to form a black suspension;

[0050] Step S4: The suspension was sonicated at room temperature (power 480W, frequency 40kHz, time 2h), and then the suspension was vacuum filtered through a 0.22μm mixed cellulose ester membrane to remove the non-fluorescent precipitate. The filtered solution was a brownish-yellow biochar carbon quantum dot solution.

[0051] Step S5: Purify the solution using a dialysis bag with a 3500 Da cutoff for 48 hours to remove hydrogen peroxide and impurities;

[0052] Step S6: Finally, refrigerate the dialyzed solution.

[0053] Furthermore, the prepared biochar carbon quantum dots were characterized by... Figure 1 SEM observation of the original morphology and structure of biochar revealed that pine biochar exhibited the distinct vascular structure inherent in pine wood, along with nanoparticles formed from the collapse and fragmentation of plant tissue during pyrolysis and carbonization. Corn biochar was more porous and porous than pine biochar, possessing a more complete microporous structure. Rice husk biochar exhibited a blocky, amorphous carbon structure without pores. SEM results and quantum dot yield indicate that biochar structures prepared from different biomass vary significantly. The more porous the initial biochar, the greater its contact area with the solution during the reaction, making it easier to exfoliate into nanoparticles and resulting in higher yields.

[0054] Furthermore, Figure 1 Transmission electron microscopy revealed that the average particle size of PWB-CQDs was 2.16 ± 0.50 nm, the average particle size of CSB-CQDs was 3.28 ± 0.81 nm, and their distribution was relatively dispersed. The average particle size of RHB-CQDs was 1.37 ± 0.30 nm, and their distribution was relatively concentrated. All biochar carbon quantum dots possessed a (101) diffraction plane of graphitic carbon with a lattice spacing of 0.21 nm.

[0055] Figure 2 The XRD patterns of the biochar show a distinct broad diffraction peak at 23° (2θ) for graphite microcrystals (002). The XRD patterns of PWB-CQDs and CSB-CQDs show a distinct broad diffraction peak for graphite microcrystals (002) at approximately 26° (2θ). The raw material characteristics of RHB-CQDs result in its main component being CaC2O4·H2O.

[0056] Figure 3 As shown in the infrared spectra, the three types of unultrasonicated biochar exhibit similar spectral bands. The -OH groups (3408 cm⁻¹) of the carbon quantum dots in the biochar can be clearly observed. -1 ), C=O and C=C (1654cm) -1 The intensity and sharpness of the group peaks were significantly higher than those of the biochar raw material, indicating that hydrogen peroxide modified the surface of the quantum dots with a large number of oxygen-containing functional groups during the ultrasonic process.

[0057] Figure 4 Raman spectroscopy revealed that both pine and corn biochar, as well as biochar quantum dots prepared from them, are primarily composed of sp2 graphitic carbon and sp3 amorphous carbon. Rice husk biochar and rice husk biochar quantum dots exhibit less of both carbon structures. At 1460 cm⁻¹... -1 The peak of CO stretching vibration in CaC2O4·H2O is present.

[0058] At the microscopic level, the crystalline domains in the biochar structure are linked by methylene chains. These crystalline domains represent the degree of carbonization and contain many small sp2 carbon microcrystals that are easily modified by surface treatment and oxidative exfoliation.

[0059] Table 1. Data on fluorescence decay lifetime of carbon quantum dots prepared in this invention.

[0060]

[0061] As shown in Table 1, the fluorescence lifetime of the synthesized carbon quantum dots comes from two parts: the emission of intrinsic states distributed in the range of 3.76-4.08 ns and the emission of defect states distributed in the range of 3.76-4.08 ns.

[0062] The carbon quantum dots of this invention exhibit wavelength-dependent emission peaks at different excitation wavelengths. Figure 5 ); Figure 6 The fluorescence properties of biochar quantum dots are not sensitive to solution pH; their fluorescence spectra do not change significantly between pH 5 and 9. Figure 7 The carbon quantum dots of this patent exhibit good photostability, with minimal change in their emission spectrum under continuous optimal excitation light irradiation. Further, the heavy metal ion quenching analysis includes the following steps:

[0063] A series of metal salts were prepared into 25 mM·L⁻¹ stock solutions using ultrapure water for later use.

[0064] Using 1.5 μM·L⁻¹ heavy metal ions (Cu) 2+ Fe 2+ Pb 2+ Ni 2+ Co 2+ Mn 2+ Al 3+ Ba 2+ Ag 2+ Cd 2+ Quenching experiments were conducted with three different types of biochar quantum dots;

[0065] First, the fluorescence intensity (F0) of three biochar carbon quantum dot solutions was measured;

[0066] Then, 10 heavy metal ions (Cu) 2+ Fe 2+ Pb 2+ Ni 2+ Co 2+ Mn 2+ Al 3+ Ba 2+ Ag 2+ Cd 2+The heavy metal ions were mixed with 2 ml of each of the three biochar carbon quantum dot solutions in a cuvette to achieve a heavy metal ion concentration of 1.5 μM·L⁻¹. Then, the fluorescence intensity (F) of the metal ion / biochar carbon quantum dot solution was measured.

[0067] Figure 8 The fluorescence quenching degree of different metal ions on biochar quantum dots is expressed as the fluorescence reduction percentage (%), (fluorescence reduction percentage (%) = (F0-F) / F0);

[0068] Using Cu, a metal ion with the highest influence in the fluorescence of three biochar carbon quantum dots. 2+ To determine the quenching of metal ions, Cu solutions of 0.01–50 μM·L⁻¹ were used. 2+ To assess the quenching ability of three biochar carbon quantum dots, the fluorescence intensity (F0) of the three biochar carbon quantum dot solutions was first measured.

[0069] Then a series of concentrations of Cu 2+ The solution was added to a cuvette containing biochar carbon quantum dots, and then Cu was measured. 2 + Fluorescence intensity (F) of biochar carbon quantum dot solution Figure 9 To add 0–50 μM·L⁻¹ Cu 2+ Post-fluorescence intensity (F0) / Post-quenched biochar carbon quantum dot fluorescence intensity (F) and Cu 2+ The relationship between solution concentration and the model was studied and fitted. Cu in PWB-CQDs 2 + The detection limit (LOD) is 0.002 μM. The quenching fitting curve is y = -0.24 + 1.01 * ln(x + 3.47), R0 2 =0.999; CSB-CQDs for Cu 2+ The limit of detection (LOD) was 0.045 μM, and its fitted curve was y = -0.64 + 0.97ln(x + 5.56), R0 2 =0.999; RHB-CQDs for Cu 2+ The limit of detection (LOD) was 0.007 μM, and its fitted curve was y = -0.92 + 1.29 * ln(x + 4.57), R0. 2 =0.998. As shown in Table 2, compared with the CQDs reported as heavy metal sensors, the CQDs prepared by ultrasonic method using biochar as raw material not only have a lower detection limit, but also have low cost, green and environmentally friendly preparation process, and do not require the use of any surface passivating agents or dopants.

[0070] Table 2 Comparison of CQDs prepared by different methods as heavy metal sensors

[0071]

[0072] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. An application of biochar-based carbon quantum dots in the analysis and detection of metal ions, characterized in that, The metal ion is Cu. 2+ A method for preparing carbon quantum dots based on biochar includes the following steps: Step S1: Place the plant-based biochar in water and ultrasonically clean it to remove impurities and incompletely carbonized plant residues. Step S2: Add the biochar from S1 to a 20-40% hydrogen peroxide solution and stir to form a black suspension. Step S3: The suspension is ultrasonically treated, and then the suspension is vacuum filtered through a 0.22μm mixed cellulose ester membrane to remove non-fluorescent precipitates. After dialysis, the biochar carbon quantum dot solution is obtained.

2. The application according to claim 1, characterized in that, In step S3, the ultrasonic treatment power is 480±60W, the frequency is 40±20kHz, and the time is 2±1h.

3. The application according to claim 2, characterized in that, The ultrasonic cleaning time in step S1 is 30±10 min.

4. The application according to claim 3, characterized in that, The dialysis described in step S3 involves purifying the quantum dot solution using a dialysis bag with a cutoff of 3500±500 Da for 48±12 hours.

5. The application according to any one of claims 1 to 4, characterized in that, The plant-based biochar is pine wood biochar, corn stalk biochar, and rice husk biochar.

6. The application according to claim 5, characterized in that, The average particle size of quantum dots in pine biochar was 2.16±0.50 nm, that in corn biochar was 3.28±0.81 nm, and that in rice husk biochar was 1.37±0.30 nm.

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