CN112521931B

CN112521931B - Room temperature phosphorescence test strip based on carbon dots and preparation method and application thereof

Info

Publication number: CN112521931B
Application number: CN202011478664.0A
Authority: CN
Inventors: 郑保战; 杜娟; 郭勇; 肖丹; 王冰; 袁晓英; 吕旭
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2020-12-15
Filing date: 2020-12-15
Publication date: 2022-10-28
Anticipated expiration: 2040-12-15
Also published as: CN112521931A

Abstract

The invention provides a carbon dot-based room temperature phosphorescent test strip, a preparation method and application thereof. In the room temperature phosphorescent test strip, the effective substance with high room temperature phosphorescent emission performance is N-CDs/PAA, and the N-CDs/PAA is a room temperature phosphorescent material obtained after embedding N-CDs in a polyacrylic acid matrix. The room temperature phosphorescence test strip provided by the invention has the advantages that the RTP emission is bright green under the excitation of ultraviolet light (365 nm), the service life is 769ms, and the macroscopic discernment time of the phosphorescence emission is 15s; meanwhile, under the action of water, the room temperature phosphorescence of the N-CDs/PAA is quenched due to the destruction of hydrogen bonds in the N-CDs/PAA system, so that the room temperature phosphorescence response to the water is realized. Therefore, the room temperature phosphorescence test strip provided by the invention has the advantages of high sensitivity, good stability, accuracy, reliability and the like, and has a wide application prospect in the field of water detection.

Description

Room temperature phosphorescence test strip based on carbon dots and preparation method and application thereof

Technical Field

The invention belongs to the field of analysis and detection, and particularly relates to a room temperature phosphorescence test strip based on carbon dots, and a preparation method and application thereof.

Background

Water is a source of life of all things and plays an important role in human survival and production. However, the presence of water in organic solvents is generally considered to be the most common impurity. For example, trace amounts of water in organic solvents can determine reactivity, reduce yield, or reduce reaction products. In addition, because water is ubiquitous in the sample, it is difficult for the experimenter to detect water.

The karl fischer titration method is widely used as a conventional method and is considered to be the most reliable method for monitoring trace water in a solvent. But the further application is severely limited due to the defects of lack of continuous monitoring, requirement of special instruments, use of toxic chemicals, complex operation, long time and the like. Therefore, the development of simple, sensitive analytical methods to accurately quantify water in organic solvents is a very important and widespread analytical problem in laboratory and industrial applications.

In recent years, various electrochemical methods, photometric methods and luminescence methods have been developed and widely applied to water detection in organic solvents, and optical strategies based on fluorescence are considered as a powerful and reliable detection method due to their high sensitivity and low detection limit, shorter response time, real-time monitoring, etc. Compared with fluorescence, room-temperature phosphorescence (RTP), which is abbreviated as RTP, has become a very useful light sensing detection mode in recent years due to its advantages of long emission lifetime, large Stokes shift, high selectivity, and the like. The long lifetime of RTP avoids any interference of the short lifetime fluorescence emission and scattered light, thereby greatly improving the sensitivity and selectivity of the sensor. Thus, RTP sensors have advantages over fluorescence methods, and RTP materials play an extremely important role in sensor construction. Based on this, various RTP sensor materials having excellent phosphorescent properties, such as organic compounds, semiconductor quantum dots, metal Organic Frameworks (MOFs), and the like, have been developed. However, these RTP materials usually have the disadvantages of complex synthesis process, long time consumption, high cost, high toxicity, poor light stability, etc., and are not suitable for practical application.

Therefore, the RTP optical sensor material which is simple in design, environment-friendly and reliable has very important significance and urgency. In addition, the RTP optical sensor in the prior art also has the problems of high preparation cost, complex use process, slow acquisition of detection results and the like. Therefore, in the prior art, both the RTP photosensor material and the RTP photosensor used need to be improved.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a room temperature phosphorescent test strip based on carbon dots, and a preparation method and application thereof, so as to solve the problems of complex synthesis process, long time consumption, high cost, high toxicity, poor light stability and the like of room temperature phosphorescent materials in the prior art, and solve the problems of high preparation cost, complex use process, slower detection result acquisition and the like of an RTP (real time processing) optical sensor in the prior art.

The invention provides a room temperature phosphorescence test strip based on carbon dots, and a preparation method and application thereof. The specific contents are as follows:

in a first aspect, the invention provides a carbon dot-based room temperature phosphorescent test strip, which is a filter paper strip containing N-CDs/PAA; wherein the N-CDs are nitrogen-doped carbon dots; the PAA is a polyacrylic acid matrix;

the N-CDs/PAA is a room temperature phosphorescent material obtained by embedding N-CDs in a polyacrylic acid matrix.

Alternatively, the N-CDs comprise a heteroatom-containing group comprising an oxygen-containing group and a nitrogen-containing group;

the room temperature phosphorescent emission of the N-CDs/PAA is realized by enhancing the phosphorescent intensity of the N-CDs through hydrogen bond formation of polyacrylic acid and the heteroatom-containing group;

wherein the enhancement of the phosphorescence intensity of the N-CDs is achieved by suppressing non-radiative transitions of the N-CDs through the hydrogen bond.

Optionally, the phosphorescence of the N-CDs/PAA is quenched by water.

Optionally, the quenching under the action of water is realized by the following processes:

the hydrogen bonds between the N-CDs and the polyacrylic acid are broken by the binding of water molecules to the polyacrylic acid, so that phosphorescence enhanced based on the hydrogen bonds is quenched.

In a second aspect, the present invention provides a method for preparing a room temperature phosphorescent test strip as described in the first aspect, the method comprising:

coating the N-CDs/PAA solution on filter paper to obtain N-CDs/PAA wet filter paper;

drying the N-CDs/PAA wet filter paper for 30-40 min at 60-80 ℃ to obtain room temperature phosphorescence test paper;

and cutting the room temperature phosphorescence test paper into room temperature phosphorescence test strips according to preset conditions.

Optionally, the concentration of the N-CDs/PAA solution is 0.1-0.6 g/ml;

the preparation steps of the N-CDs/PAA comprise: adding N-CDs into polyacrylic acid solution under the condition of water bath at 50-60 ℃, and stirring for 50-60 min to obtain N-CDs/PAA.

Optionally, the mass concentration of the polyacrylic acid solution is 0.1-0.5 g/ml;

the mass ratio of the N-CDs to the polyacrylic acid is 1:2 to 6.

Optionally, the step of preparing the N-CDs comprises:

dissolving isophthalic acid in deionized water, and stirring to obtain an isophthalic acid solution;

adding ethylenediamine into the isophthalic acid solution under stirring to obtain a colorless mixed solution;

transferring the colorless mixed solution into a high-pressure reaction kettle, and carrying out hydrothermal reaction to obtain a mixed system after the reaction is finished;

and carrying out post-treatment on the mixed system to obtain N-CDs powder.

Optionally, the molar ratio of the isophthalic acid to the ethylenediamine is 1:4-8;

the reaction temperature of the hydrothermal reaction is 160-200 ℃, and the reaction time of the hydrothermal reaction is 6-10 h;

the post-processing comprises: cooling the mixed system to room temperature, and centrifuging to obtain a light yellow solution; purifying the light yellow solution in a dialysis bag for 12-24 hours; and (3) drying the purified solution in vacuum at 40-60 ℃ to obtain N-CDs powder.

In a third aspect, the present invention provides a use of a carbon dot-based phosphorescent room temperature test strip, wherein the phosphorescent room temperature test strip of the first aspect is used for detecting moisture in an organic solvent.

The embodiment of the invention provides a carbon dot-based room temperature phosphorescent test strip, a preparation method and application thereof, wherein the room temperature phosphorescent test strip is a filter paper strip containing N-CDs/PAA; wherein the N-CDs are nitrogen-doped carbon dots; the PAA is a polyacrylic acid matrix; the N-CDs/PAA is a room temperature phosphorescent material obtained by embedding N-CDs in a polyacrylic acid matrix. Compared with the prior art, the invention at least comprises the following advantages:

(1) According to the invention, the room temperature phosphorescent test strip for detecting moisture can be obtained by coating the room temperature phosphorescent material on the filter paper, so that the room temperature phosphorescent test strip provided by the invention has the advantages of simple preparation method, low preparation cost and the like, and can be used for industrial large-scale production.

(2) When the room temperature phosphorescence test strip based on the carbon dots is used, only the organic reagent to be tested needs to be dripped on the room temperature phosphorescence test strip, and then whether room temperature phosphorescence quenching occurs can be observed directly by naked eyes. Therefore, the room temperature phosphorescence test strip provided by the invention has the advantages of convenient and rapid use, high sensitivity, accuracy, reliability, good specificity, low price and the like, and has wide application prospect in the field of water detection.

(3) The room temperature phosphorescent material in the room temperature phosphorescent test strip based on the carbon dots is N-CDs/PAA, the material is obtained by embedding N-CDs in a polyacrylic acid matrix, wherein the N-CDs are connected with polyacrylic Acid (APP) through hydrogen bonds, the RTP emission of bright green is presented under the excitation of ultraviolet light (365 nm), the service life is 769ms, and the green phosphorescence can be distinguished for 15s by naked eyes, so that the material has the advantage of stable phosphorescence performance; meanwhile, under the action of water, the hydrogen bond in the N-CDs/PAA is destroyed, so that the room-temperature phosphorescence of the N-CDs/PAA is quenched, and therefore, the material also has the advantages of high sensitivity, accuracy, reliability and the like.

(4) In the room temperature phosphorescent material (N-CDs/PAA) based on carbon dots, polyacrylic acid (PAA) is introduced as a matrix, so that hydrogen bonds are formed between the PAA and the N-CDs, and the polyacrylic acid (PAA) is used for forming hydrogen bondsHydrogen bond inhibition of T in N-CDs ₁ To S ₀ The non-radiative decay process of (2) is used for enhancing the phosphorescence intensity of the N-CDs, so that the room temperature phosphorescence material with high phosphorescence intensity is obtained. The PAA has the characteristics of low price and simplicity and easiness in obtaining, so that the phosphorescent intensity of the N-CDs can be enhanced by embedding the N-CDs into the PAA, and the aim of successfully preparing the material by using the characteristics of low economic investment and simple and feasible preparation process is fulfilled. (5) The room temperature phosphorescent material (N-CDs/PAA) based on carbon dots provided by the invention is prepared by only adding N-CDs powder into a polyacrylic acid solution and stirring, so that the material has the advantages of simple preparation method, mild reaction conditions (stirring at 50-60 ℃) and the like.

(6) The carbon dots provided by the invention are nitrogen-doped carbon dots, and the introduction of heteroatoms (such as N and O) enhances N → pi transition and effective intersystem crossing (ISC), so that the carbon dots generate effective RTP emission.

Drawings

FIG. 1 is a schematic diagram illustrating a synthetic route of RTP-TS in an embodiment of the present invention; wherein FIG. 1 (a) shows a schematic synthesis of N-CDs and N-CDs/PAA in an embodiment of the present invention; FIG. 1 (b) shows phosphorescence images of solid N-CDs (I) and N-CDs/PAA (II) after turning off UV irradiation (365 nm) in an embodiment of the present invention; FIG. 1 (c) shows the construction of RTP-TS at room temperature and its phosphorescent response to water in this example;

FIG. 2 shows a characteristic diagram of N-CDs synthesized in the present example, wherein FIG. 2 (a) shows a TEM image of N-CDs in the present example, and an inset in FIG. 2 (a) shows a particle size distribution of N-CDs; FIG. 2 (b) shows the IR spectra of N-CDs and IPA in this example; FIG. 2 (c) shows an X-ray diffraction pattern of N-CDs in this example; FIG. 2 (d) shows a high resolution XPS spectrum of C1s for N-CDs in this example; FIG. 2 (e) shows a high resolution XPS spectrum of N1s for N-CDs in this example; FIG. 2 (f) shows a high resolution XPS spectrum for O1s of N-CDs in this example;

FIG. 3 shows X-ray diffraction (XRD) patterns of N-CDs and IPA in an example of the present invention;

FIG. 4 shows a graph representing optical properties of N-CDs synthesized in the present example, wherein FIG. 4 (a) shows an ultraviolet-visible absorption spectrum, a fluorescence excitation spectrum and a fluorescence emission spectrum of an aqueous N-CDs solution (from left to right, three lines respectively represent the ultraviolet-visible absorption spectrum, the fluorescence excitation spectrum and the fluorescence emission spectrum), and an inset in FIG. 4 (a) shows a luminescence diagram of the aqueous N-CDs solution under irradiation of sunlight (I) and ultraviolet rays (II); FIG. 4 (b) shows phosphorescence emission and excitation spectra of the N-CDs powder, and the inset in FIG. 4 (b) shows fluorescence (I) and phosphorescence (II) luminescence spectra of solid N-CDs under UV irradiation (365 nm); FIG. 4 (c) shows fluorescence emission and excitation spectra of N-CDs/PAA powder, and an inset in FIG. 4 (c) shows a phosphorescence image of solid N-CDs/PAA under 365nm ultraviolet light; FIG. 4 (d) shows time-resolved photoluminescence decay curves at 510nm under 350nm excitation for solid N-CDs/PAA and N-CDs powders; FIG. 4 (e) is a graph showing the luminescence of N-CDs/PAA powder under UV (365 nm) illumination as a function of time;

FIG. 5 shows a fluorescence spectrum of an N-CDs solid in an example of the present invention;

FIG. 6 shows a schematic representation of the RTP mechanism for N-CDs and N-CDs/PAA in an embodiment of the present invention; wherein, FIG. 6 (a) shows the energy level diagram of the N-CDs related photophysical process and the response diagram thereof to water, and FIG. 6 (b) shows the RTP water quenching diagram of N-CDs/PAA;

FIG. 7 shows FT-IR spectra for PAA, N-CDs and N-CDs/PAA in an embodiment of the invention; wherein FIG. 7 (a) shows FT-IR spectra for PAA, N-CDs and N-CDs/PAA in an embodiment of the present invention; FIG. 7 (b) shows respective FT-IR spectra at 2020-1450cm in the embodiments of the invention for PAA, N-CDs and N-CDs/PAA ^-1 An enlarged view of the interval;

FIG. 8 shows N-CDs/PAA at room temperature in the presence of H in an example of the present invention ₂ Phosphorescence decay curve in O ethanol;

FIG. 9 shows fluorescence and phosphorescence characterization for RTP-TS in organic solvents of varying water content in an embodiment of the invention; wherein, FIG. 9 (a) shows the fluorescence and phosphorescence images of RTP-TS in 5 ethanol samples (ethanol with water content of 0-20%, v/v) under UV lamp (365 nm); FIG. 9 (b) shows RTP-TS at different water contentsPhosphorescence emission spectrum in a quantitative ethanol sample (ethanol 1-30%, v/v); FIG. 9 (c) shows the quenching efficiency (P/P) of RTP-TS plotted according to FIG. 9 (b) ₀ ) A graph relating to different water contents;

FIG. 9 (d) shows fluorescence and phosphorescence pictures of RTP-TS in 5 acetone samples (acetone with water content of 0-8%, v/v) under UV lamp (365 nm); FIG. 9 (e) shows the phosphorescence emission spectra of RTP-TS in acetone samples of different water content (water content of acetone 0.2-13%, v/v); FIG. 9 (f) shows the quenching efficiency (P/P) of RTP-TS plotted according to FIG. 9 (e) ₀ ) A graph relating to different water contents;

FIG. 10 shows the optical performance test chart of RTP-TS in acetonitrile and tetrahydrofuran with different water content in the embodiment of the invention; wherein, FIG. 10 (a) shows the phosphorescence emission spectra of RTP-TS in acetonitrile samples of different water contents (acetonitrile 0.2-10%, v/v); FIG. 10 (b) shows the quenching efficiency (P/P) of RTP-TS plotted according to FIG. 10 (a) ₀ ) A graph relating to different water contents; FIG. 10 (b) is an inset showing fluorescence and phosphorescence pictures of RTP-TS in 5 acetonitrile samples (acetonitrile water content 0-8%, v/v) under UV lamp (365 nm);

FIG. 10 (c) shows the phosphorescence emission spectra of RTP-TS in tetrahydrofuran samples of different water content (water content of tetrahydrofuran is 0.5-6%, v/v); FIG. 10 (d) shows the quenching efficiency (P/P) of RTP-TS plotted according to FIG. 10 (c) ₀ ) A graph relating to different water contents; FIG. 10 (d) is an inset showing fluorescence and phosphorescence pictures of RTP-TS in 5 tetrahydrofuran samples (water content of tetrahydrofuran 0-4%, v/v) under UV lamp (365 nm);

FIG. 11 shows the phosphorescence intensity of RTP-TS in anhydrous organic solvent as a function of time in an embodiment of the present invention;

FIG. 12 shows the room temperature phosphorescent response of 10 RTP-TS in absolute ethanol and ethanol with 12% water content in an embodiment of the invention.

Detailed Description

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below. The following examples are given for the detailed implementation and specific operation of the present invention, but the scope of the present invention is not limited to the following examples.

In the present invention, carbon dots (CDs for short) can be widely used for preparing fluorescent materials due to their advantages of excellent luminescence property, good biocompatibility, easy synthesis, adjustable components, etc. In the prior art, efforts have been made to develop CDs as a fluorescent material for detecting organic solvents. However, these fluorescent materials are mainly based on fluorescence of CDs rather than phosphorescence.

In fact, CDs, due to its unique optical properties and easy functionalization, can also be used as an ideal RTP material. However, synthesis of phosphorescent CDs presents a significant challenge due to factors such as spin suppression properties of triplet exciton transitions, nonradiative decay processes, or oxygen quenching under ambient conditions, which can easily hinder the synthesis of phosphorescence.

In embodiments of the present invention, heteroatoms (e.g., N, O) are introduced into CDs to enhance N → pi transition and effective intersystem crossing, thereby enabling CDs to generate effective RTP emission; meanwhile, in the implementation of the invention, CDs are embedded into a polyacrylic acid matrix to limit the vibration of the CDs, so that the RTP emission is enhanced.

Based on the above, the embodiment of the invention provides a carbon dot-based room temperature phosphorescence test strip and a preparation method and application thereof. The specific contents are as follows:

in a first aspect, the embodiments of the present invention provide a carbon dot-based room temperature phosphorescent test strip, which is a filter paper strip containing N-CDs/PAA; wherein, N-CDs is nitrogen-doped carbon dots, PAA is a polyacrylic acid matrix, and N-CDs/PAA is a room temperature phosphorescent material obtained by embedding N-CDs in the polyacrylic acid matrix.

The N-CDs/PAA in the embodiment of the invention has the advantages of long luminescence life, avoidance of background fluorescence interference and the like as phosphorescent CDs, so that the N-CDs/PAA can be used as a room temperature phosphorescent material and used in the field of phosphorescence detection.

In this embodiment, optionally, the N-CDs includes a heteroatom-containing group including an oxygen-containing group and a nitrogen-containing group, e.g., -C = N, -C = O, -C-N-C/C = N, -N- (C) ₃ -N-H, etc.; in the N-CDs/PAA, polyacrylic acid forms hydrogen bonds with the heteroatom-containing groups, and the hydrogen bonds can inhibit non-radiative transition of the N-CDs, so that the phosphorescence intensity of the N-CDs is enhanced, and the N-CDs/PAA can perform strong room-temperature phosphorescence emission.

In this example, optionally, the phosphorescence of N-CDs/PAA is quenched by water. The quenching under the action of water is realized by the following processes:

the hydrogen bond between the N-CDs and polyacrylic acid is broken by the binding of water molecules to polyacrylic acid, so that phosphorescence enhanced based on the hydrogen bond is quenched.

In a second aspect, embodiments of the present invention provide a method for preparing a room temperature phosphorescent test strip as described in the first aspect, the method comprising:

coating the N-CDs/PAA solution on filter paper to obtain filter paper loaded with the N-CDs/PAA; drying the N-CDs/PAA filter paper for 30-40 min at 60-80 ℃ to obtain room temperature phosphorescence test paper; and cutting the room temperature phosphorescence test paper into room temperature phosphorescence test strips according to preset conditions.

In specific implementation, optionally, the concentration of the N-CDs/PAA solution is 0.1-0.6 g/ml;

the preparation steps of the N-CDs/PAA comprise: adding N-CDs into polyacrylic acid solution under the condition of water bath at 50-60 ℃, and stirring for 50-60 min to obtain N-CDs/PAA. Wherein, in specific implementation, optionally, the mass concentration of the polyacrylic acid solution is 0.1-0.5 g/ml; the mass ratio of the N-CDs to the polyacrylic acid is 1:2 to 6.

It should be noted that, although the N-CDs/PAA solution contains deionized water, when the N-CDs/PAA solution is drop-coated on the filter paper and the filter paper is dried (baked), the deionized water is removed, and the N-CDs/PAA will remain on the filter paper and show the characteristic of strong room temperature phosphorescence emission, and finally the N-CDs/PAA room temperature phosphorescence test strip is obtained, which has a good response to water in organic solvent.

In another embodiment, optionally, the step of preparing the N-CDs comprises: dissolving isophthalic acid in deionized water, and stirring to obtain an isophthalic acid solution; adding ethylenediamine into an isophthalic acid solution under stirring to obtain a colorless mixed solution; transferring the colorless mixed solution into a reaction kettle, and carrying out hydrothermal reaction to obtain a mixed system after the reaction is finished; and carrying out post-treatment on the mixed system to obtain N-CDs powder.

Wherein, in specific implementation, the molar ratio of the isophthalic acid to the ethylenediamine is 1:4-8; the reaction temperature of the hydrothermal reaction is 160-200 ℃, and the reaction time of the hydrothermal reaction is 6-10 h; the post-processing comprises the following steps: cooling the mixed system to room temperature, and centrifuging to obtain a light yellow solution; purifying the solution in a dialysis bag for 12-24 h; and (4) carrying out vacuum drying on the purified solution at the temperature of 40-60 ℃ to obtain the N-CDs.

The embodiment of the present invention provides a method for preparing N-CDs/PAA (as shown in fig. 1 (a)), and the prepared N-CDs/PAA is drop-coated on filter paper and cut into carbon dot-based room temperature phosphorescent test strips (RTP strip, abbreviated as RTP-TS) (as shown in fig. 1 (c)), and the prepared room temperature phosphorescent test strips are used for visible water detection in organic solvent (as shown in fig. 1 (c)). Referring to fig. 1, the specific implementation contents are as follows:

first, as shown in FIG. 1 (a), N-doped CDs (i.e., N-CDs) were synthesized by hydrothermal treatment with isophthalic acid (IPA) and Ethylenediamine (EDA), and the obtained N-CDs emitted bright blue fluorescence in an aqueous solution and weak RTP emission in a solid state (I in FIG. 1 (b)), and then, after N-CDs were embedded in a polyacrylic acid (PAA) matrix, hydrogen bonds were formed between PAA and N-CDs to obtain N-CDs/PAA, and RTP of the obtained N-CDs/PAA was greatly increased (II in FIG. 1 (b)). Then, as shown in fig. 1 (c), RTP-TS is simply prepared by coating N-CDs/PAA on filter paper, RTP-TS can respectively emit bright blue Fluorescence (FL) and green Phosphorescence (PL) under the excitation of ultraviolet light, and the PL of RTP-TS can be significantly quenched by water, so that RTP-TS prepared by the embodiment of the present invention can be used for water detection in organic solvents, and has great application potential in RTP sensors.

In order to provide a better understanding of the present invention to those skilled in the art, the carbon dot-based room temperature phosphorescent test strip provided by the present invention and its performance characteristics and applications are illustrated by the following specific examples.

Example 1

First, the materials used in this embodiment include: ethylenediamine (EDA), isophthalic acid (IPA), acetonitrile (ACN), anhydrous calcium chloride (CaCl) ₂ ) And molecular sieves were purchased from chengdong chemical agents limited (chinese achievements). Polyacrylic acid (PAA) was produced by Sigma-Aldrich Co., ltd., shanghai, china. Anhydrous ethanol, acetone, tetrahydrofuran (THF), magnesium sulfate (MgSO) ₄ ) And potassium permanganate (KMnO) ₄ ) Purchased from chengdu jinshan chemicals, china. Phosphorus pentoxide (P) ₂ O ₅ ) And calcium oxide (CaO) was obtained from tianjin koku mikou chemical agents ltd (tianjin). The organic solvent is dehydrated before use, other chemicals used in the experiment are analytically pure, and further purification is not needed when the organic solvent is used. Deionized water was used throughout the experiment.

Then, the implementation process is as follows:

step 1, synthesis of Nitrogen-doped carbon dots (N-CDs)

Isophthalic acid (IPA) and Ethylenediamine (EDA) are used as precursors, and a hydrothermal method is adopted to synthesize N-CDs. The method specifically comprises the following steps: IPA (4.15 g) was dissolved in 20mL of deionized water to give an IPA solution, and then 7mL of EDA was added to the IPA solution under continuous stirring to give a colorless mixed solution; transferring the obtained colorless mixed solution into a 100ml high-pressure reaction kettle with a polytetrafluoroethylene lining, heating at 180 ℃ for 8h, and naturally cooling to room temperature; subsequently, the reacted system was centrifuged at 10000rpm for 10min, the centrifuged solution was purified in a 1000da dialysis bag for 12h, and the purified solution was vacuum-dried at 40 ℃ to finally obtain a pale yellow solid (i.e., N-CDs).

In the implementation step, N-CDs are synthesized by a simple hydrothermal method, and in the process, the cross-linking polymerization, dehydration and carbonization reactions of IPA and EDA occur to finally form the N-CDs.

Step 2,N-preparation of CDs-based RTP test strips (RTP-TS)

Dissolving 2.0g polyacrylic acid (PAA) in 20ml deionized water, and continuously stirring at 55 ℃ for 30min to obtain a PAA solution; then 0.5g of N-CDs synthesized in the step 1 is added into the PAA solution, and the mixture is stirred for 1 hour to obtain the N-CDs/PAA solution. The N-CDs/PAA solution was coated on a filter paper by a dropping method, and then dried at 80 ℃ for 30min to prepare an RTP test paper, and finally the RTP test paper was cut into strips of 2cm by 1cm for further use.

Step 3,N-CDs and N-CDs/PAA Performance characterization

3.1, the instruments used in the characterization are as follows:

the transmission electron microscope used was an H-800 electron microscope (Hitachi, japan). Using a TD-3500X-ray powder diffractometer (china dada) (Cu ka ray,

) X-ray diffraction (XRD) measurements were performed. Ultraviolet-visible absorption spectrum used was Hitachi U-2900 spectrophotometer (Hitachi, japan). The FT-IR spectra were recorded using a Nicolet 6700 fourier transform infrared (FT-IR) spectrometer (sugar land, texas, usa). X-ray photoelectron spectroscopy (XPS) was obtained by Thermo ESCALB 250-XI X-ray photoelectron spectroscopy (Seimer Feishel, USA) with an excitation source of Al-Ka (1486.6 ev) and a working voltage of 15kv. XPS was processed with Casa-XPS software. The fluorescence spectrum and phosphorescence spectrum were measured by Hitachi F-7000 spectrophotometer (Hitachi, japan). Phosphorescence lifetimes were measured by a Horiba-3 fluorescence spectrometer using a Spectra LED (280nm, S-280) as the excitation source and a picosecond photon counting detector (PPD-850) as the detector.

3.2, the characterization contents are as follows:

3.2.1 characterization of N-CDs (as shown in FIG. 2)

The Transmission Electron Microscope (TEM) image shown in FIG. 2 (a) shows that N-CDs are spherical, well dispersed in aqueous solution, and have an average particle size of 2.2nm and a narrow particle size distribution of 1.0 to 3.5nm.

The X-ray diffraction (XRD) patterns of N-CDs and IPA, respectively, are shown in FIG. 3. As can be seen from FIG. 3, disappearance of the diffraction peak of IPA indicates carbonization during the synthesis process, and the typical diffraction peak of N-CDs at 22.3 ℃ reflects the amorphous formation of N-CDs.

FIG. 2 (b) shows the IR spectra of N-CDs and IPA in this example. As shown in FIG. 2 (b), 3500-3000cm ^-1 The wide absorption peak between the two is due to the stretching vibration of-OH/-NH, which makes N-CDs have good dispersibility in aqueous solution. In addition, IPA is 3000-2500cm ^-1 The characteristic absorption band between the two disappears, indicating that dehydration and carbonization of IPA occurred during the hydrothermal process. N-CDs at 1643cm ^-1 The peak at (D) is due to the C = O stretching vibration of N-CDs, and IPA (1688 cm) ^-1 ) In contrast, the wavenumber shift is lower. N-CDs at 1544cm ^-1 The absorption peak at (a) should be from the tensile vibration of-C = N or the bending vibration of N-H. N-CDs at 1303cm ^-1 The absorption peak at (a) is due to the tensile vibration of C-N. The results in FIG. 2 (b) demonstrate that not only IPA and EDA form N-CDs, but also that the N-CDs contain abundant oxygen-containing and nitrogen-containing groups on their surface, such as-OH, -NH ₂ -C = O, -C = N, etc.

FIG. 2 (C) shows the X-ray diffraction pattern of N-CDs in this example, and it can be seen from FIG. 2 (C) that the characteristic peaks of C1s (284.6 eV), N1s (400.78 eV) and O1s (530.69 eV) indicate that N-CDs mainly contain C, N, O element, and the higher N (12.7%) and O (17.0%) elements indicate that rich O-and N-containing groups are present on N-CDs.

FIG. 2 (d) shows the high resolution XPS spectrum of C1s for N-CDs in this example, FIG. 2 (e) shows the high resolution XPS spectrum of N1s for N-CDs in this example, and FIG. 2 (f) shows the high resolution XPS spectrum of O1s for N-CDs in this example.

Fig. 2 (d) shows a high resolution spectrum of C1s, with the four peaks at 284.34ev, 284.79ev, 286.20ev, and 287.92ev assigned to-C = C, -C-N, and-C = N/C = O, respectively, demonstrating the presence of-C = N, -C = O groups on the surface of N-CDs.

In the high-resolution spectrum of N1s shown in FIG. 2 (e), at 399.28eV, 400.68eV and 401.28eV, three peaks are assigned, respectively fitted to-C-N-C/C = N, -N- (C) ₃ and-N-H.

In the high resolution spectrum of O1s shown in fig. 2 (f), there are two peaks at 530.48ev and 531.38ev, corresponding to the-C = O and-C-O-C/O-H groups.

As can be seen from the above, the XPS results in FIG. 2 (d-f) also confirmed the presence of-C = N/C = O, -OH, -NH on N-CDs ₂ Group, which is consistent with the results for FT-IR. These heteroatom-containing groups will play an important role in the phosphorescence of N-CDs.

3.2.2 optical Properties of N-CDs (as shown in FIG. 4)

As shown in fig. 4 (a), two distinct adsorption peaks are clearly observed in the uv-vis spectrum, at 275nm and 350nm, respectively, and referring to the uv-vis spectra of IPA and EDA (in this example, the uv-vis spectra of IPA and EDA, referring to the spectra in the prior art, which is not shown repeatedly in this example), the newly generated adsorption at 350nm is caused by the N → pi transition of-C = N/C = O of N-CDs.

As can be seen from the inset in fig. 4 (a), bright blue fluorescence emission (fig. 4 (a) inset ii) shows its excellent fluorescence characteristics. The maximum emission at 462nm is attributable to radiation excited by singlet states of N-CDs, with an optimal excitation peak of 380nm (FIG. 4 (a)). In this characterization, no phosphorescence emission of N-CDs was observed and detected, probably due to quenching of phosphorescence by dissolved oxygen and water in solution. To investigate these effects, nitrogen was bubbled through the solution to remove dissolved oxygen, and also no phosphorescence was detected, indicating that dissolved oxygen is not a critical point for phosphorescence quenching in this system.

The influence of water on the luminescence property of the N-CDs is researched by measuring the optical property of the solid N-CDs powder. As shown in the insert of fig. 4 (b), the solid N-CDs powder not only exhibits bright blue fluorescence emission under the irradiation of ultraviolet light (365 nm, which is the excitation wavelength of a handheld ultraviolet lamp in this embodiment) (as shown in the insert i of fig. 4 (b)), but also emits weak green phosphorescence after the ultraviolet light is turned off (as shown in the insert ii of fig. 4 (b)). It is known that the RTP of N-CDs is mainly hindered by water, which is one of the most efficient triplet quenchers in the nonradiative decay process.

As shown in FIG. 4 (b), the N-CDs powder had a weak PL emission at 527nm, with an excitation wavelength of 360nm. However, this weak PL intensity of N-CDs is detrimental to its use in high visual sensitivity sensingApplication is carried out. To overcome this problem, polyacrylic acid (PAA) was introduced as a matrix by suppressing T in N-CDs ₁ To S ₀ To enhance the phosphorescence intensity of N-CDs and thereby contribute to phosphorescence (abbreviated as PL) emission at room temperature.

As shown in FIG. 4 (c), the phosphorescence intensity of N-CDs/PAA is greatly enhanced compared to that of N-CDs, and the bright green phosphorescence emission of N-CDs/PAA can also be clearly observed (as shown in the inset of FIG. 4 (c)). However, N-CDs/PAA showed a blue-shifted phosphorescent emission (510 nm) compared to N-CDs (527 nm), which is probably due to the modification of the excited state of N-CDs by the introduction of PAA.

As shown in FIG. 4 (d), the average phosphorescence lifetime of N-CDs/PAA is 769ms, which is longer than that of N-CDs (309 ms), further demonstrating that N-CDs/PAA has better luminescence properties than N-CDs.

As shown in fig. 4 (e), the N-CDs/PAA powder can emit strong fluorescence emission under the irradiation of ultraviolet light, and emit bright green phosphorescence, which is visible to the naked eye within 15s after the irradiation source is turned off, and only 10s of phosphorescence is shown in this embodiment. Therefore, the N-CDs/PAA prepared by the embodiment of the invention has good phosphorescent performance at room temperature, and simultaneously shows the potential application of the N-CDs/PAA in the construction of the vision sensor based on the RTP.

3.2.3 RTP mechanism of N-CDs and N-CDs/PAA (as shown in FIG. 6)

As can be seen from the above, N-CDs are rich in-C = O, -C = N, -OH/-NH ₂ Groups that facilitate the generation of triplet excitons by facilitating efficient intersystem crossing (ISC) and producing room temperature phosphorescence; meanwhile, the broad room temperature phosphorescence excitation band between 320-430nm overlaps with the absorption band of 300-400nm (as shown in fig. 4 (a) and 4 (b)), indicating that the-C = N/C = O group, resulting in generation of RTP by increasing the number of triplet excitons by N → pi transition.

In addition, the lowest singlet state (S) was quantitatively calculated from the Fluorescence (FL) and phosphorescence PL emission wavelengths of N-CDs ₁ ) And triplet (T) ₁ ) Energy gap between excitations (Δ E) _ST ). The results show that there is a difference between the fluorescent (484 nm, FIG. 5) and phosphorescent (527 nm, FIG. 4 (b)) emissions of N-CDsThere is a Stokes shift of 43nm, which is consistent with the energy gap of 0.21eV shown in FIG. 6 (a). Such a small energy gap Δ E _ST Making the spin-orbit coupling efficient further promotes RTP generation of N-CDs, despite the relatively weak phosphorescence.

By embedding N-CDs in the PAA matrix, the phosphorescence intensity of N-CDs/PAA is greatly improved (as shown in FIG. 7) due to the hydrogen bond formed between the PAA chain and the N-CDs (as shown in FIG. 6 (b)). As shown in FIG. 7, after introduction of PAA, N-CDs/PAA ranged from 3500 to 3000cm ^-1 The absorption peak between (-OH/-NH) was significantly reduced (as shown in FIG. 7 (a)), with N-CDs at 1643cm ^-1 The vibration of (a) moves to 1615cm ^-1 (as shown in FIG. 7 (b)), these changes are due to the formation of hydrogen bonds (H-bonds) between PAA and N-CDs. The formation of hydrogen bonds can not only increase the rigidity of the N-CDs system, but also limit the vibration of the N-CDs, thereby effectively reducing the nonradiative transition between PAA and the N-CDs caused by vibration, and promoting the S-transition ₁ To T ₁ I.e., cross-linking enhanced emission effect (CEE). Thus, the introduction of PAA greatly facilitates RTP enhancement of N-CDs, with N-CDs/PAA exhibiting longer RTP lifetime and higher RTP strength than N-CDs.

Based on this, in the embodiment of the present invention, the RTP mechanism of N-CDs and the RTP mechanism of N-CDs/PAA are shown. As shown in fig. 6: i) As shown in FIG. 6 (a), the presence of N and O elements increases the singlet and triplet excited levels of N-CDs and decreases S ₁ And T ₁ The energy gap between the two elements realizes effective spin orbit coupling and ISC, so that part of excited electrons can be separated from S ₁ Transfer to T ₁ And from T ₁ To S ₀ Will emit strong phosphorescence; ii) as shown in FIG. 6 (b), -C = O, -OH and-NH present on the surface of N-CDs ₂ The group induces the formation of H-bond between N-CDs and PAA matrix, and the formation of the hydrogen bond can promote the stability of triplet excited state, limit the vibration and rotation of the N-CDs and finally realize high-efficiency room temperature phosphorescence.

Note that, as shown in FIG. 6 (b), since H is present ₂ O can disrupt the hydrogen bonds between N-CDs and PAA, thus, if H is present in the N-CDs/PAA system ₂ O, will be weakenedThe rigidity of the N-CDs/PAA system enhances the vibration of-C = O, -C = N and other groups on the N-CDs, thereby increasing the nonradiative transition, which in turn leads to quenching of the RTP emission.

In this example, the presence of H at room temperature was also measured ₂ As shown in FIG. 8, the room temperature phosphorescence lifetime of N-CDs/PAA at O (FIG. 8) is greatly shortened (291 ms), i.e., H, from FIG. 8 ₂ The destruction of O to the H bond between N-CDs and PAA significantly reduces the stability of the N-CDs excited triplet state, ultimately increasing the probability of its non-radiative transition.

Step 4, detecting water in the organic solvent

4.1 dehydration of organic solvents

All organic solvents were analytically pure and were dehydrated to remove traces of water prior to testing.

Preparing absolute ethyl alcohol: 1g CaO was added to 100ml ethanol and refluxed for 2h. Then distilling the solvent and storing it in a container containing the activated

Molecular sieves in sealed glass containers.

Preparation of anhydrous acetone: 100mL of acetone and KMnO ₄ Mix and reflux until purple color no longer disappears. Then, the solvent was distilled off and stored in a sealed glass container containing anhydrous calcium chloride.

Preparation of anhydrous ACN (acetonitrile): mixing 1g P ₂ O ₅ Added to 100ml of ACN solvent and refluxed for 12 hours. After separation by distillation and filtration, the anhydrous ACN obtained was stored in a sealed glass container for further use.

Preparation of anhydrous THF: 100mL of a refluxed solution containing 1g of MgSO ₄ 12h. The solvent was then distilled off and stored in a sealed glass bottle.

4.2, detection of Water in organic solvent Using RTP-TS prepared in step 2 above (as shown in FIGS. 9 and 10)

In the implementation step, four organic solvents, namely ethanol, acetone, ACN and THF, are used as samples to research the application of RTP-TS in water detection.

4.2.1 sample preparation

4 kinds of anhydrous organic solvents prepared in the step 4.1 are adopted to respectively prepare organic solvent samples with different water contents. The method specifically comprises the following steps: preparing 34 ethanol samples with different water contents based on absolute ethanol, wherein the water content is 0-30%, and v/v; preparing 25 acetone samples with different water contents based on anhydrous acetone, wherein the water contents are respectively 0-13% and v/v; based on anhydrous acetonitrile, 22 acetonitrile samples with different water contents are prepared, wherein the water content is 0-10 percent, and v/v; preparing 18 tetrahydrofuran samples with different water contents based on anhydrous tetrahydrofuran, wherein the water content is 0-6%, and v/v;

4.2.2 measurement of luminescence intensity

The luminescence intensity of RTP-TS in different organic solvents was measured.

4.2.3, data processing

According to quenching efficiency (P/P) ₀ ) Calculating the water content, wherein P and P ₀ Respectively represents the phosphorescence intensity of RTP-TS in organic solvents with water and without water.

It is noted that in this example, all characterization experiments were performed at room temperature.

4.2.4, the detection result is as follows:

FIG. 9 (a) shows the visible photoluminescence change of RTP-TS in ethanol solvent with water content of 0-20%. As shown in FIG. 9 (a), the phosphorescence emitted by RTP-TS is visible to the naked eye, so that RTP-TS can perform semi-quantitative water detection more conveniently and rapidly (semi-quantitative water detection means that the water content can be preliminarily judged, or water is preliminarily judged, but the specific water content is uncertain).

FIG. 9 (b) shows the phosphorescence spectra of RTP-TS in ethanol samples of different water content. As shown in fig. 9 (b), as the moisture content was increased from 1.0% to 30.0% (v/v), the photoluminescence intensity was gradually decreased. From this, RTP-TS was found to be responsible for H in ethanol ₂ The O reaction is sensitive.

Quenching efficiency (P/P) as shown in 9 (c) ₀ ) Linearly related to the water content (R) in the range of 1-24% (v/v) ² = 0.9932), the linear regression equation is expressed as P/P ₀ =1.073-0.0435C, wherein C is the water content (v/v) in ethanol, P and P ₀ Respectively, the phosphorescence intensity of RTP-TS (510 nm) in ethanol is shown.

FIG. 9 (d-f) shows the response of RTP-TS to water in acetone solvent. RTP-TS also showed a clear visual response to water in acetone solvent compared to ethanol as shown in fig. 9 (d). The RTP emission in acetone (515 nm) appears red-shifted due to the influence of the solvent compared to ethanol (510 nm). As shown in FIG. 9 (e), the RTP strength also follows H ₂ The increase in O decreases. As shown in FIG. 9 (f), the linear regression equation may be expressed as P/P ₀ Linear correlation R of =1.035-0.1237C, between 0.2% and 8.0% (v/v) ² ＝0.9998。

In addition, RTP-TS based visualization and quantitative determination of water in ACN and THF was also performed using a procedure similar to ethanol and acetone, and the results are shown in FIG. 10. As shown in FIG. 10 (a-b), the response of RTP-TS to water in ACN with water content of 0.2% -6.0% is specifically: P/P ₀ ＝1.024-0.1497C，R ² =0.9984; as can be seen from FIGS. 10 (c-d), the response of RTP-TS to water in THF with a water content of 0.5% to 4.0% is specifically: P/P ₀ ＝1.138-0.2768C，R ² ＝0.9939。

As can be seen from the above, the RTP-TS provided by the embodiment of the invention has good response to water within a certain water content range, so that the RTP-TS provided by the embodiment of the invention has a wide application prospect in the detection of water in an organic solvent as a room temperature phosphorescence sensor.

In addition, in this example, RTP-TS used for water detection stability and reproducibility was also studied, to demonstrate its potential application in organic solvents. The specific contents are as follows:

FIG. 11 shows phosphorescence spectra of RTP-TS in anhydrous organic solvent in an embodiment of the present invention; wherein, the anhydrous organic solvents are ethanol, acetone, ACN and THF, and the research shows that the phosphorescence intensity of RTP-TS at 510nm changes with the irradiation of ultraviolet light for 1 hour in the solvents, and the RTP-TS has good stability in the organic solvents as shown in 11.

FIG. 12 shows the quenching efficiency of RTP-TS in anhydrous ethanol and ethanol with 12% water content in the examples of the invention. Specifically, RTP-TS in anhydrous and water in ethanol solvent, 10 pieces of RTP-TS to water response. As can be seen from FIG. 12, the Relative Standard Deviation (RSD) is 3.15%, and the smaller coefficient of standard deviation proves that RTP-TS has good reproducibility in practical application.

In addition, to further verify the accuracy of RTP-TS in conventional analysis, the recovery of water detection in organic solvents was determined by testing two different samples of water content in each of the samples prepared at 4.2.1 above and comparing the test results, which are shown in table 1. As can be seen from Table 1, the recovery values are 98.5-110% and the relative standard deviation is 1.02-4.99%, demonstrating the good accuracy and reliability of RTP-TS in water detection in these organic solvents.

TABLE 1 determination of water in RTP-TS samples in organic solvents

a mean of the results of three tests. b the difference between the two water contents in the same solvent.

In the embodiment of the invention, the test strip with excellent RTP performance and stability is constructed on the basis of N-doped CDs. Wherein, the N-CDs synthesized by a simple hydrothermal method contain abundant oxygen-containing groups and nitrogen-containing groups (such as-C = O, -C = N, -OH and-NH) in the structure ₂ ) These groups, which promote the n → pi transition, increase the ISC probability and thus increase the ratio of triplet excitons (which are important factors in RTP).

In the embodiment of the invention, by embedding N-CDs in the PAA matrix, the triplet excitons can be stabilized due to the introduction of the PAA, so that the non-radiative quenching process of the N-CDs is inhibited, and the obtained N-CDs/PAA has the RTP emission of bright green under the excitation of 350nm and the average life of 769ms. However, due to H ₂ The presence of O can break the H bonds in the N-CDs/PAA, thereby quenching the RTP of the N-CDs/PAA. Thus, RTP-T constructed with N-CDs/PAAAnd S, the method can be used for visual quantitative detection of water in various organic solvents. And the test result shows that the RTP-TS is sensitive to the response of water and has good stability, accuracy and reliability.

In a word, the RTP-TS based on the N-CD/PAA provided by the embodiment of the invention has the advantages of simple preparation, low price, environmental friendliness, good stability, no background interference and the like, and has a wide application prospect in an organic solvent. Meanwhile, the embodiment of the invention provides a green, simple and convenient construction method of the visual RTP-TS with good phosphorescent performance, and opens up a new way for developing a simple, convenient, sensitive and economic visual detection device for analyzing the water in the organic solvent.

Example 2

In the embodiment of the present invention, experimental materials and experimental facilities used are the same as those in embodiment 1, and are not described herein again.

The preparation and procedure of this example, similar to example 2, differs as follows:

in the step 1, the dosage of EDA is 10ml; the reaction temperature in the polytetrafluoroethylene lining high-pressure reaction kettle is 160 ℃, and the reaction time is 8 hours; the purification time in dialysis bag was 12h, the temperature of vacuum drying was 40 ℃ and the final product was also a pale yellow powder.

In step 2, dissolving 4.0g of polyacrylic acid (PAA) in 20ml of deionized water, and continuously stirring for 30min at 55 ℃ to obtain a PAA solution; then 0.5g of N-CDs synthesized in the step 1 is added into the PAA solution, and the mixture is stirred for 1 hour to obtain the N-CDs/PAA solution. The N-CDs/PAA solution was coated on a filter paper by a dropping method, then annealed at 80 ℃ for 30min to prepare an RTP test paper, and finally the RTP test paper was cut into strips of 2cm by 1cm for further use.

The same operation as that of step 3 to step 4 in example 1 above was carried out for characterization and examination, and the data results obtained were the same as or similar to those in example 1, and are not repeated in this example.

Example 3

In this embodiment, the experimental materials and devices used are the same as those in embodiment 1, and are not described herein again. The preparation and procedure of this example, similar to example 1, differs as follows:

in the step 1, the dosage of EDA is 14ml; the reaction temperature in the polytetrafluoroethylene-lined high-pressure reaction kettle is 180 ℃, and the reaction time is 6 hours; purifying in dialysis bag for 20h, vacuum drying at 50 deg.C to obtain light yellow powder; in step 2, the concentration of the PAA solution was 0.3g/ml.

Example 4

In this embodiment, experimental raw materials and experimental facilities such as equipment used are the same as those in embodiment 1, and are not described herein again. The preparation and procedure of this example, similar to example 1, differs as follows:

in the step 1, the reaction temperature in a polytetrafluoroethylene-lined high-pressure reaction kettle is 200 ℃, and the reaction time is 10 hours; purifying in dialysis bag for 24h, vacuum drying at 60 deg.C to obtain light yellow powder; in step 2, stirring was continued at 60 ℃ for 40min to obtain a PAA solution having a concentration of 0.5 g/ml.

In a third aspect, embodiments of the present invention provide an application of a carbon dot-based room temperature phosphorescent test strip, and a specific application may be to apply the room temperature phosphorescent test strip described in the first aspect to detection of moisture in an organic solvent.

For simplicity of explanation, the method embodiments are described as a series of acts or combinations, but those skilled in the art will appreciate that the present invention is not limited by the order of acts, as some steps may occur in other orders or concurrently in accordance with the invention. Further, those skilled in the art will appreciate that the embodiments described in the specification are preferred embodiments and that the acts and elements referred to are not necessarily required to practice the invention.

The present invention provides a carbon dot-based room temperature phosphorescent test strip, a method for preparing the same and an application of the same, wherein the principle and the implementation mode of the present invention are explained by using specific examples, and the description of the examples is only used for helping to understand the method of the present invention and the core concept thereof; meanwhile, for a person skilled in the art, according to the idea of the present invention, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present invention.

Claims

1. A preparation method of a room temperature phosphorescence test strip based on carbon dots is characterized by comprising the following steps:

cutting the room temperature phosphorescence test paper into room temperature phosphorescence test strips according to preset conditions;

the preparation steps of the N-CDs/PAA comprise: adding N-CDs into polyacrylic acid solution under the condition of water bath at 50-60 ℃, and stirring for 50-60 min to obtain N-CDs/PAA;

the concentration of the N-CDs/PAA solution is 0.1-0.6 g/ml;

the preparation steps of the N-CDs comprise:

and carrying out post-treatment on the mixed system to obtain N-CDs powder.

2. The method according to claim 1, wherein the mass concentration of the polyacrylic acid solution is 0.1 to 0.5g/ml;

the mass ratio of the N-CDs to the polyacrylic acid is 1: 2-6.

3. The method of claim 1, wherein the molar ratio of isophthalic acid to ethylene diamine is 1:4 to 8;