MXene-based quantum dot as well as preparation method and application thereof

Abstract

The invention relates to the technical field of quantum dots, in particular to MXene-based quantum dots as well as a preparation method and application thereof. The MXene quantum dots are cut through introduction of OH and nitrogen atoms generated by decomposition of H2O2, the small-size oxidation-state MXene quantum dots and the nitrogen-doped MXene quantum dots are obtained respectively, and the relative quantum yield is increased; a quantum confinement effect, a conjugated structure and electron transition are regulated and controlled through a synthesis method and the like, and the fluorescence performance and the ion detection capacity of the MXene quantum dot are enhanced. MXene quantum dots in a reduction state are obtained through NaBH4 reduction, non-radiative recombination in a defect state is further inhibited by passivating surface defects, more-OH assistant fluorophores are obtained through reduction, and fluorescence enhancement is achieved. As different functional groups are loaded and the size is reduced, more surface functional groups are exposed, and the selectivity and the sensitivity of the MXene quantum dot for ion detection are improved.

Description

Technical Field

[0001] This invention relates to the field of quantum dot technology, specifically to MXene-based quantum dots, their preparation methods, and applications. Background Technology

[0002] Environmental and human health issues have always been a major concern, and metal ions have attracted widespread attention due to their impact on human health. Traditional detection methods suffer from drawbacks such as high precision requirements, high cost, and low selectivity. Fluorescent probes have gained attention for their high sensitivity, good selectivity, and low cost, but traditional fluorescent probes have disadvantages such as complex preparation processes and low fluorescence efficiency.

[0003] MXene is a collective term for 2D hierarchical transition metal carbides, nitrides, or carbonitrides, synthesized by selectively etching the atom element of a 3D ternary compound (called the MAX phase). The molecular formula of MXene is M. n+1 X n Or M n+1 X n T x In this system, T represents surface functional groups such as -O, -F, and -OH, and n ranges from 1 to 3, determining the number of atomic layers in the unit cell. M represents an early transition metal element, such as Sc, Ti, and V; A is one of the elements in IIIA and IVA; and X represents C and / or N.

[0004] MXene quantum dots (MQDs) are novel nanomaterials with sub-nanoscale dimensions, exhibiting low-dimensional, dot-like properties while retaining the characteristic crystal structure of MXene. When the size of MXene materials is reduced to below the exciton Bohr radius of its parent material, the energy levels of its valence and conduction bands change from continuous to discrete, a phenomenon that endows MQDs with unique physicochemical properties. Compared to MXene, MQDs exhibit superior luminescence properties and excitation-dependent photoluminescence characteristics due to their tunable size, quantum confinement effect, and surface defects. These properties give MQDs significant advantages not only in optical performance but also in their unique excitation-dependent luminescence behavior, offering new possibilities for biosensing and imaging. Furthermore, the surface functional groups and abundant edge sites of MQDs enable them to interact with surrounding molecules, making them easily functionalized by natural biomaterials, further enhancing their application potential in biological systems. MQDs prepared by different methods exhibit excellent selectivity for various heavy metal ions and small biomolecules, providing a framework for developing highly sensitive and selective fluorescent probes.

[0005] The related technology discloses the preparation of reduced Ti3C2MQDs (r-MQDs) by hydrothermal treatment with ethylenediamine (EDA) at 120℃ for 8 hours after Ti3AlC2 exfoliation to obtain Ti3C2MXene. Further addition of H2O2 to the r-MQDs followed by hydrothermal treatment at 120℃ for 8 hours yielded oxidized Ti3C2MQDs (o-MQDs). The average particle size of the r-MQDs prepared by this method is approximately 3.62 nm, and the average particle size of the o-MQDs is 3.05 nm. In this method, H2O2 acts as an oxidant rather than a cleaving agent, failing to effectively cleave the quantum dots to further reduce their size. Therefore, it cannot improve intrinsic luminescence performance by enhancing the quantum confinement effect, nor does it expose more active sites and functional groups. Furthermore, the MQDs do not inherit the original structure of the parent MXene. Although this method improves the oxidation degree of the MQDs to some extent and introduces oxygen vacancy defects, these defects actually lead to a decrease in fluorescence performance and low sensitivity for metal ion detection.

[0006] The related technology discloses a method for preparing nitrogen-doped quantum dots (N-MQDs). Using Ti3C2MXene as a raw material, oxidized MQDs are obtained through oxidation with concentrated nitric acid. Then, ethylenediamine is used simultaneously as both a nitrogen source and a reducing agent to achieve nitrogen doping, yielding N-MQDs. The particle size distribution of the N-MQDs ranges from 2 to 9 nm, with an average particle size of approximately 3.4 nm. This method relies on nitric acid oxidation to introduce oxygen vacancies for in-situ cleavage of quantum dots. However, the cleavage effect is limited, making it difficult to effectively control the quantum dot size. The quantum confinement effect is not significant, failing to expose more active sites and functional groups, and resulting in low sensitivity for metal ion detection. Summary of the Invention

[0007] Therefore, the purpose of this invention is to provide MXene-based quantum dots, their preparation methods, and applications. The three types of MXene quantum dots prepared by this invention have small particle sizes, strong quantum confinement effects, and can expose more active sites and functional groups, resulting in high sensitivity for the detection of metal ions.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing oxidized MXene quantum dots, characterized by the following steps: mixing MXene powder, NaOH, H2O2 and water, and carrying out a hydrothermal oxidation reaction to obtain oxidized MXene quantum dots.

[0009] Preferably, the mixing includes: mixing MXene powder, NaOH solution and H2O2 solution to obtain a mixed solution; The mass ratio of MXene powder to NaOH is 1g:61.3~187.2mmol; The mass concentration of the H2O2 solution is 25-35%; The concentration of the NaOH solution is 0.5~1.5 mol / L; The volume of the H2O2 solution is 0.15~1.8% of the total volume of the NaOH solution and the H2O2 solution; The hydrothermal oxidation reaction is carried out at a temperature of 80~120℃ for 4~8 hours.

[0010] The present invention also provides oxidized MXene quantum dots prepared by the preparation method described above, wherein the average particle size of the oxidized MXene quantum dots is 1.3~2.7 nm.

[0011] The present invention also provides a method for preparing nitrogen-doped MXene quantum dots, comprising the following steps: mixing MXene powder, a nitrogen source and water, and carrying out a nitrogen-doped hydrothermal reaction under a protective atmosphere to obtain nitrogen-doped MXene quantum dots; wherein the nitrogen source includes pyrrole.

[0012] Preferably, the mass ratio of the MXene powder to the volume of the nitrogen source is 1g:0.5~1.5mL; The mass ratio of the MXene powder to the volume of water is 1g:100~300mL; The nitrogen-doped hydrothermal reaction is carried out at a temperature of 100-150°C for 6-10 hours.

[0013] The present invention also provides nitrogen-doped MXene quantum dots prepared by the preparation method described above, wherein the particle size of the nitrogen-doped MXene quantum dots is 1~3nm.

[0014] The present invention also provides a method for preparing reduced MXene quantum dots, comprising the following steps: mixing MXene quantum dot solution and NaBH4 solid, and carrying out a reduction reaction to obtain reduced MXene quantum dots; wherein the MXene quantum dots are the oxidized MXene quantum dots described in the above technical solution or the nitrogen-doped MXene quantum dots described in the above technical solution.

[0015] Preferably, the mass ratio of the oxidized MXene quantum dots to NaBH4 is 1:133.3~3333.34; The mass ratio of nitrogen-doped MXene quantum dots to NaBH4 is 1:23.3~233.3; The reduction reaction is carried out at a temperature of 18-30°C for 1-6 days.

[0016] The present invention also provides reduced MXene quantum dots prepared by the preparation method described above, wherein the particle size of the reduced MXene quantum dots is 1~3nm.

[0017] This invention also provides the application of the oxidized MXene quantum dots, nitrogen-doped MXene quantum dots, or reduced MXene quantum dots described in the above-mentioned technical solutions in metal ion detection; the metal ions include Fe. 3+ or Cr 6+ .

[0018] This invention employs a hydrothermal exfoliation strategy synergistic with H2O2 to prepare oxidized MXene quantum dots (o-MXene quantum dots). It fully leverages the in-situ cleavage effect of H2O2 through oxygen vacancies. During the hydrothermal oxidation reaction, H2O2 decomposes to generate hydroxyl radicals (•OH), whose strong oxidizing properties attack the MXene surface and interlayer regions, promoting the breaking of interlayer chemical bonds and achieving the exfoliation of the layered structure. This facilitates the efficient acquisition of smaller MXene quantum dots with more exposed active sites. Based on the mechanism of in-situ quantum dot cleavage through defects using the hydrothermal method, the obtained o-MQDs are smaller and more uniformly distributed, contributing to improved uniformity and stability of their fluorescence performance. Furthermore, this invention achieves green and efficient preparation of MXene quantum dots (o-MQDs) in different oxidation states by controlling the oxidation conditions (H2O2 dosage), thereby adjusting the -OH content and improving the resistance of o-MQDs to Fe. 3+ The response and selection of the surface oxidation state are conducive to the detection of specific metal ions, and the rich surface oxidation state is also more conducive to reduction to further enhance fluorescence performance.

[0019] This invention obtains reduced r-MQDs by reducing o-MQDs with NaBH4. Through an innovative strategy of "in-situ cutting + reduction synergistic regulation," this invention prepares reduced MXene quantum dots, cutting them into smaller sizes while retaining their parent structure and exposing their active sites more fully, thus enhancing their quantum confinement effect. Furthermore, the preparation method provided by this invention effectively reduces surface functional groups while maintaining the small-size structure and strong quantum confinement effect of MXene quantum dots, reducing oxygen-containing defects and non-radiative transition losses, thereby significantly enhancing the intrinsic luminescence performance of MXene quantum dots. The reduction of oxygen defects on the surface of MXene quantum dots also helps to further improve fluorescence performance, thereby increasing the detection sensitivity for metal ions.

[0020] This invention uses monolayer MXene (DL-MXene) as a precursor and pyrrole as a cleaving agent and dopant. MXene is cleaved via a hydrothermal method to prepare nitrogen-doped MXene quantum dots (N-MQDs). While obtaining small-sized quantum dots, nitrogen-containing functional groups (such as amino and pyrrole nitrogen) are introduced to create π-conjugated structures that enhance fluorescence, thereby improving the fluorescence performance of the MXene quantum dots and thus increasing the detection sensitivity for metal ions. Defects in the nitrogen-doped MXene quantum dots are reduced using NaBH4, retaining their small size and stronger quantum confinement effect while reducing non-radiative transition losses caused by defects. This yields reduced-state nitrogen-doped MXene quantum dots with excellent fluorescence performance and excellent detection sensitivity for metal ions as fluorescent probes. When the nitrogen source is pyrrole, the enhanced luminescence of the nitrogen-doped MXene quantum dots mainly originates from the defect energy levels introduced by the pyrrole nitrogen conjugated structure, rather than from the optimization of the intrinsic luminescence mechanism.

[0021] Moreover, the preparation method of each MXene quantum dot provided by the present invention is simple in process, easy to operate, green and environmentally friendly, low in production cost, and suitable for industrial production.

[0022] As shown in the test results of the examples, the average particle size of the r-MQDs prepared in Example 1 was 1.77 nm, which is small; the r-MQDs served as a fluorescent probe for Fe 3+ It exhibits excellent fluorescence quenching performance, with a linear range of 0.8–1 mM and a limit of detection (LOD) of 5 nM. It shows a low detection limit and high sensitivity for metal ions. The N-MQDs prepared in Example 2 have an average particle size of 1.76 nm and a relative quantum yield of 8.30%, demonstrating both small particle size and high quantum yield. The enhanced luminescence of the N-MQDs prepared in this invention mainly originates from the defect energy levels introduced by the pyrrole nitrogen conjugated structure, rather than from the optimization of the intrinsic luminescence mechanism. The relative quantum yield of the rN-MQDs prepared in Example 2 is increased to 12.09%. rN-MQDs can be used as fluorescent probes for Cr in water. 6+ It exhibits excellent fluorescence quenching performance, with a linear range of 0.08–2.7 mM and a detection limit of 0.27 μM. It also demonstrates low detection limits and high sensitivity for metal ions. Compared to po-MQDs obtained through a "post-oxidation method" (hydrothermal preparation followed by H₂O₂ oxidation), the o-MQDs prepared using the "in-situ cleavage" method of this invention show nearly three times higher fluorescence intensity. Compared to e-MQDs and f-MQDs obtained using ethylenediamine and formamide as nitrogen dopant, the N-MQDs prepared using pyrrole as a nitrogen dopant in this invention exhibit the highest fluorescence intensity. Attached Figure Description

[0023] Figure 1The fluorescence spectral characteristics and optical comparison diagrams of o-MQDs solution and r-MQDs solution are shown, where (a)~(b) are fluorescence spectra, and (c)~(d) are physical images under natural light conditions and ultraviolet light irradiation conditions. Figure 2 The fluorescence spectral characteristics and optical comparison diagrams of N-MQDs solution and rN-MQDs solution are shown, where (a)~(b) are fluorescence spectra, and (c)~(d) are physical images under natural light conditions and ultraviolet light irradiation conditions. Figure 3 The fluorescence spectrum of the po-MQDs solution is shown. Figure 4 A comparison of the fluorescence spectra of po-MQDs and o-MQDs; Figure 5 The fluorescence spectrum of e-MQDs; Figure 6 The fluorescence spectrum of f-MQDs is shown below. Figure 7 A comparison of the fluorescence spectra of e-MQDs, N-MQDs, and f-MQDs; Figure 8 Transmission electron microscopy (TEM) images and particle size distributions of o-MQDs and r-MQDs, where (a) is a TEM image of o-MQDs, (b) is a particle size distribution of o-MQDs, (c) is a TEM image of r-MQDs, and (d) is a particle size distribution of r-MQDs. Figure 9 TEM images and particle size distribution maps of N-MQDs and rN-MQDs are shown, where (a) is a TEM image of N-MQDs, (b) is a particle size distribution map of N-MQDs, (c) is a TEM image of rN-MQDs, and (d) is a particle size distribution map of rN-MQDs. Figure 10 FT-IR spectra of o-MQDs and r-MQDs; Figure 11 FT-IR spectra of N-MQDs and rN-MQDs; Figure 12 The ion selectivity (a) of o-MQDs and the ΔI / I0 as a function of Fe 3+ Concentration change curve (b); Figure 13 The ion selectivity (a) of r-MQDs and the ΔI / I0 as a function of Fe 3+ Concentration change curve (b); Figure 14 For rN-MQDs to Cr 6+ Selective response (a) and ΔI / I0 with Cr 6+Concentration change curve. Detailed Implementation

[0024] This invention provides a method for preparing oxidized MXene quantum dots, comprising the following steps: mixing MXene powder, NaOH, H2O2 and water, and carrying out a hydrothermal oxidation reaction to obtain oxidized MXene quantum dots (o-MQDs).

[0025] Unless otherwise specified, the materials and equipment used in this invention are all commercially available products in the field.

[0026] In this invention, the mass ratio of MXene powder to NaOH can be 1g:61.3~187.2mmol, or 1g:70~150mmol, specifically 1g:61.3mmol, 1g:70mmol, 1g:80mmol, 1g:90mmol, 1g:100mmol, 1g:110mmol, 1g:120mmol, 1g:130mmol, 1g:140mmol, 1g:150mmol, 1g:160mmol, 1g:170mmol, 1g:180mmol, or 1g:187.2mmol.

[0027] In this invention, mixing MXene powder, NaOH, H2O2, and water may include mixing MXene powder, NaOH solution, and H2O2 solution. In this invention, the concentration of the NaOH solution may be 0.5~1.5 mol / L (M), or 0.8~1.2 M, specifically 0.5 M, 0.6 M, 0.7 M, 0.8 M, 0.9 M, 1 M, 1.1 M, 1.2 M, 1.3 M, 1.4 M, or 1.5 M. In this invention, the mass concentration of the H2O2 solution may be 25~35%, or 28~32%, specifically 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, or 35%. In this invention, the volume of the H2O2 solution can be 0.15% to 1.8% of the total volume of the NaOH solution and the H2O2 solution, specifically 0.15%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, or 1.8%.

[0028] In this invention, the MXene powder may include Ti3C2T xPowder; the MXene powder can be monolayer MXene (DL-MXene) powder; the present invention does not have a special limitation on the preparation method of the monolayer MXene, and any preparation method of monolayer MXene well known to those skilled in the art can be used.

[0029] In this invention, the mass ratio of MXene powder to H2O2 solution is 1:0.2~2.5, and can also be 1:0.2~1, specifically 1:0.2, 1:0.5, 1:0.8, 1:1, 1:1.5, 1:2 or 1:2.5.

[0030] In this invention, the mass ratio of the MXene powder to the total volume of the H₂O₂ solution and NaOH solution is 1g:100-150mL, or 1g:120-130mL, specifically 1g:100mL, 1g:105mL, 1g:110mL, 1g:115mL, 1g:120mL, 1g:125mL, 1g:130mL, 1g:135mL, 1g:140mL, 1g:145mL, or 1g:150mL. In this invention, the water may include deionized water.

[0031] In this invention, the temperature of the hydrothermal oxidation reaction can be 80~120℃, or 90~110℃, specifically 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃ or 120℃; the time of the hydrothermal oxidation reaction can be 4~8h, or 5~7h, specifically 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h or 8h.

[0032] After completing the hydrothermal oxidation reaction, the present invention may further include a first post-treatment of the reaction solution obtained from the hydrothermal oxidation reaction. The first post-treatment includes: centrifuging the reaction solution, filtering the obtained supernatant and dialyzing it with water to obtain an oxidized MXene quantum dot solution, and drying the oxidized MXene quantum dot solution to obtain oxidized MXene quantum dots. In the present invention, the centrifugation speed can be 6000~10000 rpm, or 7000~9000 rpm, or even 8000 rpm; the centrifugation time can be 3~6 min, or even 4~5 min. In the present invention, the filtration may include filter head filtration or filter membrane filtration, and the pore size of the filter head and filter membrane can be independently 100~300 nm, or 150~250 nm, or even 220 nm. In this invention, the conditions for water dialysis may include: a molecular weight cutoff of 3000-4000 Da for the dialysis bag, which can also be 3200-3800 Da, and further 3500-3600 Da; a water change interval of 2-6 hours for dialysis, which can also be 3-5 hours, and further 4 hours; the dialysis water includes deionized water; and dialysis is stopped when the solution pH=7. In this invention, the drying includes freeze-drying, and this invention does not specifically limit the conditions for freeze-drying; any freeze-drying method will suffice.

[0033] Scheme 1 of this invention employs "in-situ cleavage" to prepare o-MQDs with an average size of 1.3~2.7nm (1.78nm under optimal experimental conditions) and more exposed active sites, fully utilizing the in-situ cleavage effect of H2O2 through oxygen defects. Based on the mechanism of in-situ quantum dot cleavage through defects using hydrothermal methods, the successfully obtained quantum dots are smaller and more uniformly distributed, which helps to improve the uniformity and stability of fluorescence. Compared with po-MQDs obtained by the "post-oxidation method" of preparing MXene quantum dots hydrothermally and then oxidizing them with H2O2, the fluorescence intensity of o-MQDs prepared by "in-situ cleavage" in this invention is nearly three times higher. This invention utilizes the decomposition of H2O2 under high temperature and high pressure hydrothermal conditions to generate hydroxyl radicals (•OH), whose strong oxidizing properties attack the MXene surface and interlayer regions, cleaving to obtain MXene quantum dots. However, the oxidation mechanism in the preparation process of po-MQDs is limited, which is not conducive to enhancing the fluorescence performance of po-MQDs.

[0034] The present invention also provides oxidized MXene quantum dots prepared by the preparation method described above, wherein the average particle size of the oxidized MXene quantum dots is 1.3~2.7nm, or 1.5~2nm, and specifically 1.78nm.

[0035] The oxidized MXene quantum dots provided by this invention enhance the [functionality / quality] by adjusting the C=O conjugated structure. The transition further improves the fluorescence performance of quantum dots.

[0036] The present invention also provides a method for preparing nitrogen-doped MXene quantum dots (N-MQDs), comprising the following steps: mixing MXene powder, a nitrogen source and water, and carrying out a nitrogen-doped hydrothermal reaction under a protective atmosphere to obtain nitrogen-doped MXene quantum dots; wherein the nitrogen source includes pyrrole.

[0037] In this invention, the MXene powder may include Ti3C2T x Powder. In this invention, the mass ratio of the MXene powder to the volume of the nitrogen source can be 1g:0.5~1.5mL, or 1g:0.8~1.2mL, specifically 1g:0.5mL, 1g:0.6mL, 1g:0.7mL, 1g:0.8mL, 1g:0.9mL, 1g:1mL, 1g:1.1mL, 1g:1.2mL, 1g:1.3mL, 1g:1.4mL, or 1g:1.5mL.

[0038] In this invention, the mass ratio of the MXene powder to the volume of water can be 1g:100~300mL, or 1g:150~250mL, or even 1g:200mL. In this invention, the water may include deionized water.

[0039] In this invention, mixing MXene powder, nitrogen source and water may include: dispersing MXene powder in water and then adding nitrogen source to mix.

[0040] In this invention, the temperature of the nitrogen-doped hydrothermal reaction can be 100-150℃, 110-140℃, or 120-130℃; the reaction time can be 6-10 hours, 7-9 hours, or even 8 hours. In this invention, the protective atmosphere can include nitrogen, argon, or helium. In this invention, the protective atmosphere can be obtained by purging the mixed solution with a protective atmosphere for 5-15 minutes (or even 10 minutes). The purpose of purging with a protective atmosphere is to remove excess oxygen from the solution, allowing the reaction to proceed in an oxygen-free environment.

[0041] After completing the nitrogen-doped hydrothermal reaction, the present invention may further include a second post-treatment of the reaction solution obtained from the nitrogen-doped hydrothermal reaction. The second post-treatment may include: centrifuging the reaction solution, filtering the resulting supernatant, and then dialyzing it with water to obtain a nitrogen-doped MXene quantum dot solution; and drying the nitrogen-doped MXene quantum dot solution to obtain nitrogen-doped MXene quantum dots. In the present invention, the molecular weight cutoff of the dialysis bag used for water dialysis can be 50-150 Da, or 80-120 Da, or even 100 Da. In the present invention, other conditions for the second post-treatment can be the same as those for the first post-treatment, and will not be repeated here.

[0042] This invention uses pyrrole as a nitrogen source (dopant) to introduce nitrogen atoms and enhance their in-situ etching effect on MXene quantum dots, successfully preparing nitrogen-doped MXene quantum dots with a particle size of 1-3 nm and an average particle size of 1.76 nm. Pyrrole doping not only significantly increases the relative content of pyrrole nitrogen, but its five-membered ring conjugated structure also effectively enhances the fluorescence performance of the material. Further treatment with NaBH4 as a reducing agent increases the degree of reduction, yielding reduced nitrogen-doped MXene quantum dots with a particle size of 1-3 nm and an average particle size of 2.15 nm. This process effectively suppresses nonradiative recombination by passivating surface defect states; simultaneously, it reduces and generates more hydroxyl (-OH) co-fluorescent groups, thereby achieving a significant enhancement in the fluorescence performance of the reduced nitrogen-doped MXene quantum dots.

[0043] The present invention also provides nitrogen-doped MXene quantum dots prepared by the preparation method described above, wherein the particle size of the nitrogen-doped MXene quantum dots is 1~3nm, and can also be 1.5~2.5nm, and further can be 1.5~2nm; the average particle size of the nitrogen-doped MXene quantum dots can be 1.76nm.

[0044] The nitrogen-doped MXene quantum dots provided by this invention form a pyrrole nitrogen π-conjugated structure through the introduction of nitrogen atoms, thereby enhancing... This transition improves the fluorescence efficiency of quantum dots.

[0045] The present invention also provides a method for preparing reduced MXene quantum dots, comprising the following steps: mixing MXene quantum dots, NaBH4 and water, and carrying out a reduction reaction to obtain reduced MXene quantum dots; wherein the MXene quantum dots are the oxidized MXene quantum dots described in the above technical solution or the nitrogen-doped MXene quantum dots described in the above technical solution.

[0046] In this invention, the mass ratio of the oxidized MXene quantum dots to NaBH4 can be 1:133.3~3333.34, or 1:200~1000, specifically 1:133.3, 1:150, 1:200, 1:300, 1:400, 1:500, 1:600, 1:700, 1:800, 1:900, 1:1000, 1:1500, 1:2000, 1:2500, 1:3000, or 1:3333.3.

[0047] In this invention, the mass ratio of nitrogen-doped MXene quantum dots to NaBH4 can be 1:23.3 to 233.3, or 1:50 to 150, specifically 1:23.3, 1:50, 1:80, 1:100, 1:120, 1:150, 1:180, 1:200, or 1:233.3.

[0048] In this invention, the mixing of MXene quantum dots, NaBH4 and water may include: mixing MXene quantum dot solution and NaBH4; wherein the MXene quantum dot solution is the aforementioned oxidized MXene quantum dot solution or the aforementioned nitrogen-doped MXene quantum dot solution. In this invention, the volume ratio of the oxidized MXene quantum dot solution to the mass ratio of NaBH4 can be 1 mL: 8~200 mg, or 1 mL: 10~100 mg, specifically 1 mL: 8 mg, 1 mL: 10 mg, 1 mL: 20 mg, 1 mL: 30 mg, 1 mL: 40 mg, 1 mL: 50 mg, 1 mL: 60 mg, 1 mL: 70 mg, 1 mL: 80 mg, 1 mL: 90 mg, 1 mL: 100 mg, 1 mL: 110 mg, 1 mL: 120 mg, 1 mL: 130 mg, 1 mL: 140 mg, 1 mL: 150 mg, 1 mL: 160 mg, 1 mL: 170 mg, 1 mL: 180 mg, 1 mL: 190 mg, or 1 mL: 200 mg. In this invention, the volume ratio of the nitrogen-doped MXene quantum dot solution to the mass ratio of NaBH4 can be 1 mL: 1~14 mg, or 1 mL: 5~10 mg, specifically 1 mL: 1 mg, 1 mL: 2 mg, 1 mL: 3 mg, 1 mL: 4 mg, 1 mL: 5 mg, 1 mL: 6 mg, 1 mL: 7 mg, 1 mL: 8 mg, 1 mL: 9 mg, 1 mL: 10 mg, 1 mL: 11 mg, 1 mL: 12 mg, 1 mL: 13 mg, or 1 mL: 14 mg.

[0049] In this invention, the temperature of the reduction reaction can be 18~30℃, or room temperature; the time of the reduction reaction can be 1~6 days, specifically 1 day, 2 days, 3 days, 4 days, 5 days, or 6 days. In this invention, when the MXene quantum dot is a nitrogen-doped MXene quantum dot, the time of the reduction reaction can be further 4~6 days.

[0050] In this invention, when the MXene quantum dots are oxidized MXene quantum dots, the reduction reaction may further include: subjecting the reaction solution obtained from the reduction reaction to a third post-treatment. The third post-treatment may include: centrifuging the reaction solution, filtering the resulting supernatant and dialyzing it with water to obtain a reduced MXene quantum dot solution, and drying the reduced MXene quantum dot solution to obtain reduced MXene quantum dots (denoted as r-MQDs). In this invention, the conditions for the third post-treatment can be the same as those for the first post-treatment, and will not be repeated here.

[0051] In this invention, when the MXene quantum dots are nitrogen-doped MXene quantum dots, the reduction reaction may further include: subjecting the reaction solution obtained from the reduction reaction to a fourth post-treatment. This fourth post-treatment may include: centrifuging the reaction solution, filtering the resulting supernatant and dialyzing it with water to obtain a reduced MXene quantum dot solution, and drying the reduced MXene quantum dot solution to obtain reduced MXene quantum dots (reduced nitrogen-doped MXene quantum dots, denoted as rN-MQDs). In this invention, the conditions for the fourth post-treatment can be the same as those for the first post-treatment, and will not be repeated here.

[0052] The present invention also provides reduced MXene quantum dots prepared by the preparation method described above, wherein the average particle size of the reduced MXene quantum dots is 1-3 nm, and can also be 1.5-2.5 nm, and can further be 1.7-2 nm. In the present invention, the particle size of the reduced MXene quantum dots (r-MQDs) prepared from oxidized MXene quantum dots can be 1-2.4 nm, and can also be 1.5-2 nm, with an average particle size of 1.77 nm; the particle size of the reduced MXene quantum dots (rN-MQDs) prepared from nitrogen-doped MXene quantum dots can be 1-3 nm, and can also be 1.5-2.5 nm, with an average particle size of 2.15 nm.

[0053] This invention also provides the application of the oxidized MXene quantum dots (o-MQDs), nitrogen-doped MXene quantum dots (N-MQDs), or reduced MXene quantum dots (r-MQDs, rN-MQDs) described in the above-mentioned technical solutions in metal ion detection. In this invention, the metal ion may include Fe. 3+ or Cr 6 + In this invention, the o-MQDs and r-MQDs can be used to detect Fe. 3+ The N-MQDs and rN-MQDs mentioned above can be used to detect Cr. 6+ .

[0054] This invention utilizes an "in-situ cleavage-reduction synergistic regulation" strategy, under high-temperature and high-pressure hydrothermal conditions, to efficiently prepare MXene quantum dots with smaller size, stronger quantum confinement effect, and more fully exposed active sites by utilizing the in-situ cleavage effect of hydroxyl radicals (·OH) generated by H2O2 decomposition and nitrogen atoms introduced by pyrrole doping. This method significantly improves the fluorescence quantum yield of the material; simultaneously, pyrrole doping introduces a conjugated structure that is beneficial to luminescence, further enhancing fluorescence performance. Oxidized MXene quantum dots (o-MQDs) are prepared greenly and efficiently using an "oxidation-reduction" method, followed by reduction with NaBH4 to obtain reduced MXene quantum dots (r-MQDs). This process increases the surface hydroxyl (-OH) content, enhancing the resistance to Fe... 3+ Its sensitive recognition and selective response enable the detection of specific metal ions. The reduction of surface oxygen defects also helps to further improve fluorescence performance.

[0055] Nitrogen-doped MXene quantum dots were prepared using pyrrole as the nitrogen source via a nitrogen-doping cleavage mechanism, introducing nitrogen-containing functional groups (such as amino and pyrrole nitrogen) and enhancing the fluorescence performance of the MXene quantum dots through π-conjugation. Further reduction of the nitrogen-doped MXene quantum dots using NaBH4 yielded reduced-state nitrogen-doped MXene quantum dots, significantly improving the sensitivity of MXene quantum dots as fluorescent probes. This method maintains the characteristics of small-sized MXene quantum dots while reducing non-radiative transition losses and enhancing their fluorescence performance.

[0056] This invention reduces o-MQDs and N-MQDs with NaBH4 to obtain reduced MXene quantum dots (e.g., r-MQDs, rN-MQDs), thereby improving the sensitivity of MXene quantum dots as fluorescent probes and restoring their self-reducibility. Furthermore, the reduced r-MQDs exhibit resistance to Fe... 3+ With higher detection sensitivity, the reduced rN-MQDs are more suitable for Cr 6+The detection.

[0057] Structurally, o-MQDs have the smallest size and most uniform distribution, which is beneficial for intrinsic luminescence and the regulation of fluorescence behavior by quantum size effects. r-MQDs further enhance intrinsic fluorescence performance based on o-MQDs. N-MQDs and rN-MQDs are slightly larger than o-MQDs and r-MQDs, but the nitrogen atoms introduced during their preparation alter the band structure of the quantum dots: the defect energy level is located in the middle of the band, enhancing fluorescence by affecting the electronic transition process, resulting in a higher relative quantum yield of 12.09%. In terms of ion detection performance, o-MQDs and r-MQDs are suitable for Fe 3+ Detection, while rN-MQDs for Cr 6+ It exhibits excellent selective response and detection sensitivity.

[0058] To further illustrate the present invention, the following detailed description of the MXene-based quantum dots, their preparation methods, and applications provided by the present invention is provided in conjunction with embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0059] Example 1 Preparation of MXene quantum dots (o-MQDs, r-MQDs) Preparation of oxidized MXene quantum dots (o-MQDs) by H2O2 oxidation synergistic hydrothermal method: 200 mg DL-MXene (Ti3C2T) was added to the oxidized MXene (O-MQDs) and then ... to the oxidized MXene (Ti3C2T) and then to the oxidized MXene (O-MQDs) and then to the oxidized x The powder was placed in a polytetrafluoroethylene (PTFE) liner, and 24.9625 mL of 1M NaOH solution was added to disperse it evenly. Then, 37.5 μL of 30 wt% H₂O₂ solution was added. The liner was placed in a stainless steel reactor and placed in a drying oven. The reactor was reacted at 100°C for 6 hours. The reactor was cooled to room temperature and centrifuged at 8000 rpm for 5 minutes. The supernatant was collected, filtered through a 220 nm filter, and dialyzed with deionized water using a 3500 Da dialysis bag. The dialysis water was changed every 4 hours until the solution pH=7, yielding an o-MQDs solution (concentration 0.06 mg / mL). The solution was freeze-dried to obtain o-MQDs powder solid, which was then sealed for later use.

[0060] Preparation of reduced MXene quantum dots (r-MQDs): r-MQDs were prepared via a simple and green redox reaction using NaBH4 as a strong reducing agent. NaBH4 solid (0.06 g, 0.12 g, 0.24 g, 0.48 g, and 0.96 g, respectively) was added to 5 mL of freshly prepared r-MQDs solution (concentration 0.06 mg / mL) and reacted at room temperature for 1, 2, 3, 4, 5, and 6 days, respectively. The resulting reduction reaction solution was dialyzed against deionized water using a 3500 Da dialysis bag, with the dialysis water changed every 4 hours until the solution pH=7, yielding an r-MQDs solution. This solution was freeze-dried to obtain r-MQDs powder solid, which was then sealed for later use.

[0061] Example 2 Preparation of pyrrole nitrogen-doped MXene quantum dots (N-MQDs): 100 mg of MXene (Ti3C2T) was used to prepare MXene quantum dots. x The powder was dispersed in 20 mL of deionized water, and 100 μL of pyrrole was added. Argon gas was bubbled through the resulting dispersion for 10 min to remove excess oxygen. The reaction was carried out in a high-pressure reactor at 150 °C for 8 h. After cooling to room temperature, the mixture was centrifuged at 8000 rpm for 5 min. The supernatant was filtered through a 220 nm membrane to obtain a crude N-MQDs solution (yellow-brown). The crude N-MQDs solution was dialyzed against deionized water using a 100 Da dialysis bag, with the dialysis water changed every 4 h. Dialysis continued until the solution pH was close to neutral (pH = 6-7) to obtain an N-MQDs solution. This solution was freeze-dried to obtain a solid N-MQDs powder, which was then sealed for later use. The relative quantum yield of N-MQDs was 8.30%.

[0062] Preparation of reduced nitrogen-doped MXene quantum dots (rN-MQDs): 30 mg NaBH4 was added to 5 mL of freshly prepared N-MQDs solution, and the mixture was reacted at room temperature for 5 days. The solution was then dialyzed against deionized water using a 100 Da dialysis bag, with the dialysis water changed every 4 hours. Dialysis continued until the solution pH was close to neutral (pH=7), yielding an rN-MQDs solution. This solution was freeze-dried to obtain rN-MQDs powder solid, which was then sealed for later use. The relative quantum yield of rN-MQDs was 12.09%.

[0063] Comparative Example 1 Post-oxidation MQDs were prepared by hydrothermal preparation of MXene quantum dots followed by oxidation with H2O2. 200 mg of Ti3C2T xMXene powder was placed in a 100 mL polytetrafluoroethylene (PTFE) liner, and 25 mL of pre-prepared 1 M NaOH solution was added to disperse the powder. Argon gas was then introduced for 5 min to remove excess oxygen from the solution. The liner was placed in a stainless steel reactor and then placed in a drying oven. The reaction was carried out at 100 °C for 6 h. After cooling the reactor to room temperature, it was centrifuged at 8000 rpm for 5 min. The supernatant was collected and filtered through a 220 nm filter to obtain a quantum dot solution. 1 mL of 30 wt% H₂O₂ solution was added, and the solution was soaked for 1 day to obtain post-oxidized quantum dots. The solution was then dialyzed with water using a 3500 Da dialysis bag for at least 24 h, with the dialysis water changed every 4 h, until the solution pH reached 7, yielding a po-MQDs solution. This po-MQDs solution was freeze-dried to obtain po-MQDs (yellow powder, sealed for later use).

[0064] Comparative Example 2 Preparation of nitrogen-doped MXene quantum dots (e-MQDs) with ethylenediamine: 100 mg of MXene (Ti3C2T) was used. x The powder was dispersed in 20 mL of deionized water, and 100 μL of ethylenediamine was added. Argon gas was bubbled through the resulting dispersion for 10 min to remove excess oxygen from the solution. The reaction was carried out in a high-pressure reactor at 150 °C for 8 h. After cooling to room temperature, the mixture was centrifuged at 8000 rpm for 5 min. The supernatant was filtered through a 220 nm membrane to obtain a crude e-MQDs solution (yellow-brown). The crude e-MQDs solution was dialyzed against deionized water using a 100 Da dialysis bag, with the dialysis water changed every 4 h. Dialysis continued until the solution pH was close to neutral (pH=7) to obtain an e-MQDs solution. This solution was freeze-dried to obtain a solid e-MQDs powder, which was then sealed for later use.

[0065] Comparative Example 3 Preparation of nitrogen-doped formamide MXene quantum dots (f-MQDs): 100 mg of MXene (Ti3C2T) was used to prepare f-MQDs. x The f-MQDs powder was dispersed in 20 mL of deionized water, and 100 μL of formamide was added. Argon gas was bubbled through the resulting dispersion for 10 min to remove excess oxygen. The mixture was reacted in a high-pressure reactor at 150 °C for 8 h, cooled to room temperature, and centrifuged at 8000 rpm for 5 min. The supernatant was filtered through a 220 nm membrane to obtain a crude f-MQDs solution (yellow-brown). The crude f-MQDs solution was dialyzed against deionized water using a 100 Da dialysis bag, with the dialysis water changed every 4 h. Dialysis continued until the solution pH was close to neutral (pH=7) to obtain the f-MQDs solution. This solution was freeze-dried to obtain a solid f-MQDs powder, which was then sealed for later use.

[0066] Test Example 1 In the preparation conditions of r-MQDs prepared in Example 1 of this test case, the amount of NaBH4 solid used was 0.48 g, and the reaction time was 5 days.

[0067] Structural characterization Photoluminescence fluorescence spectrum (PL) is used to measure the optical properties of fluorescent quantum dots, including excitation / emission wavelengths and relative fluorescence intensity. In addition, it is also used to detect the fluorescence stability of MXene quantum dots as well as the sensitivity and selectivity of metal ions.

[0068] Sample preparation: Each MXene quantum dot solution (concentration 0.06 g / L) was placed in a 10 mm × 10 mm four-sided transparent quartz cuvette and detected using a fluorescence spectrophotometer. The fluorescence of the MXene quantum dots was detected using an F97 fluorescence spectrophotometer. The test parameters were set as follows: excitation / emission bandwidth of 10 nm, scan speed of 1000 nm / min, scan interval of 1 nm, and gain of 8.

[0069] Figure 1 The images show the fluorescence spectra and optical comparisons of o-MQDs and r-MQDs solutions, where (a)-(b) are fluorescence spectra, and (c)-(d) are images of the solutions under natural light and ultraviolet light irradiation. (a)-(b) reveal the significant regulatory effect of the oxidation and reduction processes on the luminescence properties of quantum dots. After reduction treatment with NaBH4, the fluorescence intensity of r-MQDs increased by approximately two times, and a significant blue shift was observed in both excitation and emission wavelengths. This phenomenon may be closely related to changes in the surface ligand structure and the types and quantities of functional groups. (c)-(d) show that both o-MQDs and r-MQDs are clear and transparent under natural light, while emitting bright blue fluorescence under ultraviolet light excitation, indicating that the o-MQDs and r-MQDs prepared in this invention have good dispersibility and fluorescence properties, further confirming the success of the synthesis process.

[0070] Figure 2 The fluorescence spectra of N-MQDs solution and rN-MQDs solution and their optical comparison diagrams are shown. (a)~(b) are fluorescence spectra, and (c)~(d) are physical images under natural light conditions and ultraviolet light irradiation conditions. Figure 2 Images (a) to (b) reveal that the fluorescence intensity of rN-MQDs is nearly twice that of N-MQDs, indicating that pyrrole-type nitrogen doping significantly contributes to the fluorescence enhancement. This is mainly attributed to the fact that this type of doping helps to form a stable π-conjugated structure and promotes the formation of radiative recombination centers. In addition, the reduction treatment also caused a significant blue shift in the fluorescence emission peak, which may be related to changes in the type and number of ligands and functional groups on the material surface. Figure 2Images (c) to (d) show that both N-MQDs and rN-MQDs are clear and transparent solutions under natural light, but emit bright blue fluorescence under ultraviolet light excitation, further confirming the successful preparation of N-MQDs and rN-MQDs.

[0071] Figure 3 The fluorescence spectrum of the po-MQDs solution is shown. Figure 4 This is a comparison of the fluorescence spectra of po-MQDs and o-MQDs. Figure 5 The fluorescence spectrum of e-MQDs is shown below. Figure 6 Here is the fluorescence spectrum of f-MQDs. Figure 7 The comparison diagram of fluorescence spectra of e-MQDs, N-MQDs, and f-MQDs shows that... Figure 1 (a) Figure 3 and Figure 4 The comparison shows that post-oxidation (po-MQDs) significantly weakens fluorescence performance, while H2O2 synergistic hydrothermal method can improve the fluorescence performance of MXene quantum dots. Figure 2 (a) Figure 5 , Figure 6 and Figure 7 The comparison shows that the fluorescence performance of nitrogen-doped MXene quantum dots prepared with pyrrole as the nitrogen source is better than that prepared with ethylenediamine and formamide as nitrogen sources.

[0072] The morphology of o-MQDs and r-MQDs was characterized by transmission electron microscopy (TEM), and the particle size distribution of 100 quantum dot particles of o-MQDs and r-MQDs was statistically analyzed. Figure 8Transmission electron microscopy (TEM) images and particle size distributions of o-MQDs and r-MQDs are shown. (a) is the TEM image of o-MQDs, (b) is the particle size distribution of o-MQDs, (c) is the TEM image of r-MQDs, and (d) is the particle size distribution of r-MQDs. The TEM images show that both o-MQDs and r-MQDs exhibit good dispersion, with no obvious agglomeration observed, indicating that DL-MXene was successfully broken down into MXene quantum dots with uniform particle size distribution. The average particle size distribution shows that the particle size of o-MQDs is mainly distributed in the range of 1.3–2.5 nm, with an average particle size of 1.78 nm; the particle size distribution of r-MQDs is in the range of 1–2.4 nm, with an average particle size of 1.77 nm. Compared with MQDs prepared by conventional hydrothermal methods, the introduction of H2O2 during the preparation of o-MQDs significantly reduced the quantum dot size; while the reduction treatment with NaBH4 had little effect on the size of MXene quantum dots and did not cause aggregation. The results indicate that obtaining small-sized MXene quantum dots through oxidative cleavage can enhance their quantum confinement effect, and further reduction treatment can effectively improve their fluorescence performance while maintaining their structural stability.

[0073] The morphology of N-MQDs and rN-MQDs was characterized by transmission electron microscopy (TEM), and the particle size distribution of 100 quantum dot particles of N-MQDs and rN-MQDs was statistically analyzed. Figure 9 Transmission electron microscopy (TEM) images and particle size distributions of N-MQDs and rN-MQDs are shown. (a) is the TEM image of N-MQDs, (b) is the particle size distribution of N-MQDs, (c) is the TEM image of rN-MQDs, and (d) is the particle size distribution of rN-MQDs. The TEM images show that both N-MQDs and rN-MQDs exhibit a uniform dispersion without significant agglomeration. The average particle size distributions show that the particle sizes of N-MQDs and rN-MQDs are mainly distributed in the range of 1–3 nm, with average particle sizes of 1.76 nm and 2.15 nm, respectively. These results indicate that the reduction treatment with NaBH4 has little effect on the particle size distribution of MXene quantum dots and does not induce significant agglomeration. The results also suggest that N-MQDs prepared with pyrrole nitrogen doping and rN-MQDs obtained after NaBH4 reduction have smaller particle sizes, thus exhibiting a more significant quantum confinement effect.

[0074] Figure 10The FT-IR spectra of o-MQDs and r-MQDs are shown. The spectra reveal that, compared to MXene, both o-MQDs and r-MQDs exhibit significantly enhanced infrared absorption peaks, indicating that the reduced size exposes more functional groups, thus enhancing the absorption signal. The infrared spectra show that o-MQDs and r-MQDs have peaks at 3428 cm⁻¹. -1 1620cm -1 1350cm -1 and 761cm -1 Characteristic absorption peaks were present in the vicinity, attributed to the -OH stretching vibration, C=O stretching vibration, CO stretching vibration, and Ti-O stretching vibration, respectively. The C=O, CO, and Ti-O vibration peaks were prominent in o-MQDs, indicating that the introduction of H2O2 enhanced the oxidation degree of the material and successfully introduced more oxygen vacancies. Furthermore, the -CH stretching vibration peak was more pronounced in o-MQDs, and the CF bond signal almost disappeared, further indicating that the oxidation etching process had a certain destructive effect on the material structure, exposing more active sites. In r-MQDs, the -OH vibration peak was enhanced, indicating that NaBH4 treatment introduced more co-fluorescent groups (-OH) and partially reduced oxygen vacancies, helping to reduce energy loss caused by non-radiative transitions and improve the resistance to Fe. 3+ The sensitivity of the detection.

[0075] Figure 11 The FT-IR spectra of N-MQDs and rN-MQDs were obtained by analyzing... Figure 11 Analysis of the FT-IR spectra allowed for a systematic characterization of the functional group composition on the material surface before and after reduction. Both N-MQDs and rN-MQDs exhibited a series of consistent characteristic absorption peaks: 3481 cm⁻¹. -1 The broad absorption band at 1637 cm⁻¹ is attributed to the stretching vibration of -OH. -1 1395cm -1 773cm -1 and 551cm -1 The characteristic peaks at 1023 cm⁻¹ correspond to the stretching vibrations of C=O, CO, Ti-O, and C-Ti bonds, respectively. Furthermore, at 1023 cm⁻¹... -1The absorption peak observed at 3481 cm⁻¹ corresponds to the CN stretching vibration, confirming successful nitrogen doping in the material. Comparing the intensity of the -OH stretching vibration peak reveals that the signal of rN-MQDs at this location is significantly stronger than that of N-MQDs, indicating that the reduction treatment increases the number of -OH functional groups on the material surface, enhancing hydrophilicity. Simultaneously, the vibrational peak intensities of rN-MQDs at oxygen-containing bonds such as C=O, CO, and Ti-O are significantly weakened, suggesting that the NaBH₄ reduction treatment primarily targets these oxygen-containing functional groups. The reduction process effectively reduces energy losses associated with non-radiative transitions, thus contributing to enhanced fluorescence properties. Furthermore, the newly generated -OH functional groups during the reduction process further enhance the absorption at 3481 cm⁻¹. -1 The absorption peak at that location.

[0076] Test Example 2 MXene quantum dot fluorescent probes (o-MQDs, r-MQDs) for Fe 3+ Detection Good selectivity is a key indicator for evaluating the specificity of fluorescent probes. To assess the effectiveness of o-MQDs and r-MQDs on Fe... 3+ The selectivity was improved by introducing 1M Na into 2mL o-MQDs and r-MQDs systems, respectively. + Mg 2+ Zn 2+ Al 3+ Pb 2+ Ca 2 + Cu 2+ Ni 2+ Co 2+ Ag + Cr 2+ and Fe 3+ A total of 12 metal ions were identified, and their effects on Fe were preliminarily investigated. 3+ The response characteristics. Subsequently, Fe concentrations from low to high were gradually added dropwise to the same system. 3+ The solution was reacted for 30 seconds, and the change in fluorescence intensity was detected.

[0077] Figure 12 The ion selectivity (a) of o-MQDs and the ΔI / I0 as a function of Fe 3+ Concentration change curve (b). Figure 12 As shown in (a), after the addition of high concentrations (1M) of different metal ions, only Fe... 3+ The near-complete quenching of fluorescence caused by o-MQDs, while other ions only resulted in slight changes in fluorescence intensity, indicating that o-MQDs have a significant effect on Fe... 3+ It has excellent selectivity. Figure 12 As shown in (b), at an excitation wavelength of 358 nm, with Fe3+ As the concentration increased from 0 to 1 mM, the fluorescence emission intensity of o-MQDs at 460 nm gradually decreased, eventually dropping by about 90%, and the emission peak position did not shift, indicating that Fe 3+ It induced a fluorescence quenching process that was concentration-dependent, and is suitable for Fe... 3+ Quantitative detection. Further analysis was conducted using the fluorescence intensity change value ΔI (ΔI = I0 - I, where I0 is the initial fluorescence intensity and I is the fluorescence intensity after the reaction) and Fe. 3+ Quantitative analysis was performed to determine the relationship between concentrations. Within the concentration range of 0.2–0.6 mM, ΔI / I0 and Fe... 3+ The concentrations showed a good linear relationship, and the fitted regression equation was y = 0.8673x - 0.088, with a correlation coefficient R0. 2 =0.9974. The effect of o-MQDs on Fe was calculated based on the signal-to-noise ratio (S / N=3). 3+ The limit of detection (LOD) was 0.03 mM. The results indicate that o-MQDs have a high detection limit for Fe... 3+ It possesses high selectivity and high sensitivity.

[0078] Figure 13 The ion selectivity (a) of r-MQDs and the ΔI / I0 as a function of Fe 3+ The concentration change curve (b). For example... Figure 13 As shown in (a), under the same high concentration of metal ion interference, only Fe 3+ This can lead to almost complete quenching of r-MQDs fluorescence, while other ions have little effect on its fluorescence, proving that r-MQDs also affect Fe. 3+ It has a high degree of selectivity. For example... Figure 13 As shown in (b), under 345 nm excitation, Fe 3+ Within the concentration range of 0–2.7 mM, the fluorescence intensity of r-MQDs gradually decreased with increasing concentration, while the emission peak position remained unchanged, further verifying the presence of Fe. 3+ The induced fluorescence quenching behavior can be used for quantitative analysis. This can be achieved by calculating Fe... 3+ The relationship between concentration and (ΔI / I0) was found to be good linear in the concentration range of 0.8–1 mM, with a regression equation of y = 0.0968x + 0.7364 and R0. 2 =0.9936. The calculated value of r-MQDs for Fe... 3+ The detection limit is as low as 5 nM, demonstrating extremely high sensitivity.

[0079] In summary, both o-MQDs and r-MQDs have effects on Fe 3+ It exhibits significant selective fluorescence response, good quantitative detection capability and low detection limit, while r-MQDs have higher sensitivity in Fe...3+ It has potential value in sensing applications.

[0080] Based on the above research, it has been confirmed that reduced nitrogen-doped MXene quantum dots (rN-MQDs) possess the potential to act as Cr... 6+ The potential of fluorescent probes is demonstrated, and they exhibit excellent photoluminescence stability.

[0081] Test Example 3 MXene quantum dot fluorescent probes (rN-MQDs) for Cr 6+ Detection To systematically evaluate the effect of rN-MQDs on Cr 6+ The selective recognition ability was demonstrated by adding 1M Na to 2 mL of rN-MQDs solution. + Mg 2+ Zn 2+ Al 3+ Pb 2+ Ca 2+ Cu 2+ Ni 2+ Co 2+ Ag + Cr 2+ Cr 6+ and Fe 3+ A total of 13 metal ions were identified, and the effects of rN-MQDs on Cr were preliminarily investigated. 6+ The recognition specificity was then assessed. Subsequently, Cr was added dropwise in small amounts from low to high concentrations to a 2 mL rN-MQDs system. 6+ The solution was reacted for 30 seconds, and the change in fluorescence intensity was detected.

[0082] Figure 14 For rN-MQDs to Cr 6+ Selective response (a) and ΔI / I0 with Cr 6+ Concentration change curve. Figure 14 Figure (a) shows that rN-MQDs affect Cr 6+ It exhibits a significant and specific fluorescence quenching effect, while the fluorescence changes caused by other common metal ions are weaker, indicating that this material exhibits a strong fluorescence quenching effect on Cr. 6+ It exhibits excellent metal ion selectivity. Figure 14 As shown in (b), at an excitation wavelength of 314 nm, with Cr 6+ As the concentration gradually increased from 0 to 10 mM, the fluorescence emission intensity of rN-MQDs at 386 nm gradually decreased, eventually decreasing by about 90%, while the emission peak position remained unchanged, indicating that Cr 6+ The introduction of [a specific substance] causes fluorescence quenching in rN-MQDs, and this process is concentration-dependent, which can be used for Cr [a specific application]. 6+Quantitative detection. Further calculations showed that within the concentration range of 0.08–2.7 mM, Cr 6+ The concentration and fluorescence intensity showed a good linear relationship, and the regression equation obtained by fitting was y = 0.1987x + 0.2235 (R²). 2 =0.9802). Based on the signal-to-noise ratio method, rN-MQDs for Cr 6+ The detection limit was 0.27 μM. This indicates that rN-MQDs have a good effect on Cr. 6+ It exhibits high selectivity and good sensitivity, and is suitable for application in real-world Cr samples. 6+ The potential of quantitative detection.

[0083] Existing methods typically suffer from problems such as large quantum dot size, low quantum yield, poor ion selectivity, and low sensitivity, which severely affect the fluorescence performance and practical applications of MXene quantum dots. This invention first utilizes the introduction of •OH and nitrogen atoms generated by the decomposition of H₂O₂ to cleave MXene quantum dots, obtaining smaller MXene quantum dots and improving the relative quantum yield. The oxidized state of MXene quantum dots is further enhanced by adjusting the C=O conjugated structure. The transition further improved the fluorescence performance of the quantum dots. Subsequently, a reduction strategy using NaBH4 was employed to restore the fluorescence performance of MXene quantum dots and increase the -OH content. Then, using pyrrole as a dopant, N atoms were introduced and their in-situ cleavage effect was enhanced, resulting in small-sized N-MQDs. Furthermore, the high pyrrole nitrogen ratio of the pyrrole doping significantly enhanced fluorescence due to its five-membered ring conjugated structure. Using NaBH4 as a reducing agent to improve the reduction process, rN-MQDs were obtained, retaining the small size and structural characteristics of N-MQDs. By passivating surface defects, nonradiative recombination of defect states was further suppressed, leading to the reduction of more -OH co-fluorescent groups and achieving fluorescence enhancement. Due to the reduction process and the reduction in size exposing more surface functional groups, the selectivity and sensitivity of MXene quantum dots for ion detection were improved. Specifically, the reduced r-MQDs, due to the increased -OH content, showed improved sensitivity to Fe. 3+ With higher detection sensitivity, the reduced rN-MQDs are more suitable for Cr due to the increased content of surface reducing groups. 6+ Testing.

[0084] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing oxidized MXene quantum dots, characterized in that, Includes the following steps: MXene powder, NaOH, H2O2 and water were mixed and subjected to a hydrothermal oxidation reaction to obtain oxidized MXene quantum dots.

2. The preparation method according to claim 1, characterized in that, The mixing process includes: mixing MXene powder, NaOH solution, and H2O2 solution to obtain a mixed solution; The mass ratio of MXene powder to NaOH is 1g: 61.3~187.2mmol; The mass concentration of the H2O2 solution is 25-35%; The concentration of the NaOH solution is 0.5~1.5 mol / L; The volume of the H2O2 solution is 0.15~1.8% of the total volume of the NaOH solution and the H2O2 solution; The hydrothermal oxidation reaction is carried out at a temperature of 80~120℃ for 4~8 hours.

3. The oxidized MXene quantum dots prepared by the method according to claim 1 or 2, characterized in that, The average particle size of the oxidized MXene quantum dots is 1.3~2.7 nm.

4. A method for preparing nitrogen-doped MXene quantum dots, characterized in that, Includes the following steps: MXene powder, a nitrogen source, and water are mixed and subjected to a nitrogen-doped hydrothermal reaction under a protective atmosphere to obtain nitrogen-doped MXene quantum dots; the nitrogen source includes pyrrole.

5. The preparation method according to claim 4, characterized in that, The mass ratio of the MXene powder to the volume of the nitrogen source is 1g:0.5~1.5mL; The mass ratio of the MXene powder to the volume of water is 1g:100~300mL; The nitrogen-doped hydrothermal reaction is carried out at a temperature of 100-150°C for 6-10 hours.

6. The nitrogen-doped MXene quantum dots prepared by the method of claim 4 or 5, characterized in that, The nitrogen-doped MXene quantum dots have a particle size of 1~3 nm.

7. A method for preparing reduced-state MXene quantum dots, characterized in that, Includes the following steps: MXene quantum dots, NaBH4, and water are mixed and subjected to a reduction reaction to obtain reduced MXene quantum dots; the MXene quantum dots are either the oxidized MXene quantum dots as described in claim 3 or the nitrogen-doped MXene quantum dots as described in claim 6.

8. The preparation method according to claim 7, characterized in that, The mass ratio of the oxidized MXene quantum dots to NaBH4 is 1:133.3~3333.34; The mass ratio of nitrogen-doped MXene quantum dots to NaBH4 is 1:23.3~233.3; The reduction reaction is carried out at a temperature of 18-30°C for 1-6 days.

9. The reduced MXene quantum dots prepared by the method of claim 7 or 8, characterized in that, The particle size of the reduced MXene quantum dots is 1~3nm.

10. The application of the oxidized MXene quantum dot of claim 3, the nitrogen-doped MXene quantum dot of claim 6, or the reduced MXene quantum dot of claim 9 in metal ion detection; wherein the metal ion includes Fe. 3+ or Cr 6 + .

Images