CN109097025B

CN109097025B - Preparation method of adjustable fluorescent nitrogen dots

Info

Publication number: CN109097025B
Application number: CN201810890999.XA
Authority: CN
Inventors: 李攻科; 霍志铭; 肖小华
Original assignee: Sun Yat Sen University
Current assignee: Sun Yat Sen University
Priority date: 2018-08-07
Filing date: 2018-08-07
Publication date: 2021-07-27
Anticipated expiration: 2038-08-07
Also published as: CN109097025A

Abstract

The invention relates to a preparation method of an adjustable fluorescent nitrogen dot. The invention discloses a novel polymer stripping method, which can effectively and quickly prepare nitrogen-rich carbon quantum dots with different surface groups, firstly synthesize a heterocyclic azide precursor with electron-deficient carbon-carbon double bonds and azide groups, then prepare the corresponding heterocyclic azide into a polymer type nano precursor with high activity at a micron level by thermal polymerization, and finally carry out nucleophilic reaction on the polymer type nano precursor by different nucleophilic reagents to strip the precursor into adjustable fluorescent nitrogen dots with corresponding surface groups at a nano level. The method is simple and convenient, easy to control and high in repeatability; the prepared adjustable fluorescent nitrogen dot has the characteristics of excellent performance, simple and adjustable surface groups, high quantum yield, strong light stability and good biocompatibility.

Description

Preparation method of adjustable fluorescent nitrogen dots

Technical Field

The invention relates to the field of preparation of quantum dots, in particular to a preparation method of an adjustable fluorescent nitrogen dot.

Background

Fluorescent nitrogen-rich carbon quantum dots (fluorescent nitrogen dots for short) have excellent luminescence property, biocompatibility and photo-thermal stability, and have been widely concerned by the fields of photoelectricity, analytical chemistry, life science and the like since the discovery. The efficient preparation of the novel fluorescent nitrogen dots, the surface modification and the subsequent research on the relationship between the structure and the optical performance are important development directions of material science and applied chemistry.

At present, the common preparation and surface modification process of the fluorescent nitrogen dots mainly comprises the steps of synthesizing carbon dot background by a top-down method or a bottom-up method, and then obtaining the required surface functional groups by subsequent surface modification. However, the top-down method requires harsh experimental conditions, the macroscopic carbon-based material is crushed into smaller carbon nanoparticles by certain physical and chemical methods, the process generally involves harsh conditions such as high-voltage arc, high-temperature etching, strong acid and strong base, and the like, the reaction system is very complex and difficult to operate and perform subsequent effective structural analysis; on the contrary, the method from bottom to top is that small molecules or polymers are subjected to established reaction or physical agglomeration to obtain carbon nanoparticles with relatively higher molecular weight, the reaction is mild, but the effective regulation and control of the fluorescent nitrogen dot structure and the surface groups are difficult. And the common precursor for preparing the fluorescent nitrogen dots has various components and complex reaction crosslinking, so that the reaction process cannot be effectively controlled, and great obstruction is brought to the subsequent performance research based on the structure. Therefore, the development of a preparation method of the adjustable fluorescent nitrogen spot, which has the advantages of single raw material, simple mechanism and easy modification of the surface group, is imperative.

Disclosure of Invention

Based on the above, the invention aims to provide a preparation method of a tunable fluorescent nitrogen dot, which is used for preparing a surface group tunable fluorescent nitrogen dot by a novel polymer stripping method, and has the characteristics of simple and convenient preparation method, rich nitrogen content of the prepared tunable fluorescent nitrogen dot, simple and tunable surface group, high quantum yield, strong light stability and good biocompatibility.

The technical scheme adopted by the invention is as follows:

a preparation method of a tunable fluorescent nitrogen dot is characterized by comprising the following steps:

s1: preparing a precursor heterocyclic azide;

s2: dissolving the heterocyclic azide compound prepared in the step S1 in an aprotic solvent, heating for reaction, and then carrying out vacuum filtration and drying to obtain a polymer type nano precursor;

s3: dispersing the polymer type nano precursor prepared in the step S2 in an aprotic solvent, and adding a nucleophilic reagent corresponding to a target surface group for heating reaction; and centrifuging after the reaction is finished, collecting supernatant, carrying out rotary evaporation on an organic phase to obtain solid residue, dissolving the solid residue in water again to obtain filtrate, and carrying out freeze drying on the filtrate to obtain the adjustable fluorescent nitrogen spot with the target surface group.

Compared with the prior art, the preparation method of the adjustable fluorescent nitrogen dot combines the advantages of a top-down method and a bottom-up method, and the fluorescent nitrogen dot with the adjustable surface groups is prepared by adopting a novel polymer stripping method. Using heterocyclic azide as a precursor, firstly synthesizing heterocyclic azide (such as azidoimidazole, azidothiazole and the like) which simultaneously has electron-deficient carbon-carbon double bonds and azido, and preparing the corresponding heterocyclic azide into a polymer type nano precursor at a micron level by thermal polymerization; finally, a suitable nucleophilic reagent (such as ethylenediamine, methanol and the like) is adopted to carry out nucleophilic addition functionalization and nucleophilic substitution stripping on the polymer type nano precursor, and the precursor is stripped into a nano-scale adjustable fluorescent nitrogen spot with a corresponding surface group. The method is simple and convenient, easy to control and high in repeatability; the prepared adjustable fluorescent nitrogen dot has the characteristics of excellent performance, simple and adjustable surface groups, high quantum yield, strong light stability and good biocompatibility.

Further, the precursor heterocyclic azide compound in S1 is prepared by an azide reaction using a heterocyclic amino compound.

Further, the heterocyclic amino compound is a heterocyclic amino compound having an electron-deficient carbon-carbon double bond and an amino group.

Further, the heterocyclic amino compound is one of 2-aminoimidazole, 2-aminothiazole, 6-aminoindole and 3-aminopyrazole; the heterocyclic azide compound has electron-deficient carbon-carbon double bonds and azide groups and is one of 2-azidoimidazole, 2-azidothiazole, 6-azidoindole and 3-azidopyrazole; the polymeric nano-precursor is one of an imidazole-polymeric nano-precursor, a thiazole-polymeric nano-precursor, an indole-polymeric nano-precursor, and a pyrazole-polymeric nano-precursor.

The preparation method of the 2-azidoimidazole comprises the following steps: adding hydrochloric acid into 2-aminoimidazole, stirring and adding a sodium nitrite aqueous solution under the condition of ice-water bath, adding a sodium azide aqueous solution, and reacting for 4 hours; sodium bicarbonate was added to adjust the pH to neutral and the aqueous mixture was extracted with ethyl acetate and repeated three times. And combining and washing the organic phases, and removing the organic solvent to obtain the 2-azidoimidazole. Wherein the concentration of the hydrochloric acid is 5mol/L, and the molar ratio of the sodium nitrite to the sodium azide to the 2-aminoimidazole is 1: 1: 0.95.

the preparation method of the 2-azidothiazole comprises the following steps: adding hydrochloric acid into 2-aminothiazole, stirring and adding a sodium nitrite aqueous solution under the condition of ice water bath, then adding a sodium azide aqueous solution, and reacting for 4 hours; sodium bicarbonate was added to adjust the pH to neutral and the aqueous mixture was extracted with ethyl acetate and repeated three times. And combining and washing the organic phases, and removing the organic solvent to obtain the 2-azidoimidazole. Wherein the concentration of the hydrochloric acid is 5mol/L, and the molar ratio of the sodium nitrite to the sodium azide to the 2-aminoimidazole is 1: 1: 0.95.

the preparation method of the 6-azidoindole comprises the following steps: adding hydrochloric acid into 6-aminoindole, stirring and adding a sodium nitrite aqueous solution under the condition of ice-water bath, adding a sodium azide aqueous solution, and reacting for 4 hours; sodium bicarbonate was added to adjust the pH to neutral and the aqueous mixture was extracted with ethyl acetate and repeated three times. And combining and washing the organic phases, and removing the organic solvent to obtain the 2-azidoimidazole. Wherein the concentration of the hydrochloric acid is 5mol/L, and the molar ratio of the sodium nitrite to the sodium azide to the 2-aminoimidazole is 1: 1: 0.95.

the preparation method of the 3-azido pyrazole comprises the following steps: adding hydrochloric acid into 3-azido aminopyrazole, stirring and adding a sodium nitrite aqueous solution under the condition of ice water bath, then adding a sodium azide aqueous solution, and reacting for 4 hours; sodium bicarbonate was added to adjust the pH to neutral and the aqueous mixture was extracted with ethyl acetate and repeated three times. And combining and washing the organic phases, and removing the organic solvent to obtain the 2-azidoimidazole. Wherein the concentration of the hydrochloric acid is 5mol/L, and the molar ratio of the sodium nitrite to the sodium azide to the 2-aminoimidazole is 1: 1: 0.95.

further, in steps S2 and S3, the aprotic solvent is tetrahydrofuran, chloroform, toluene, acetonitrile, dioxane, ethyl acetate, butanone, or acetone. Wherein the polymer type nanometer precursor prepared by taking tetrahydrofuran as a solvent has the best structure and stability.

Further, in step S2, the concentration of the heterocyclic azide compound in the solvent is 10 to 60mg/mL, the temperature of the heating reaction is 70 ℃, and the time of the heating reaction is 2 d. When the reaction time is too short, incomplete reaction may result; if the reaction time is too long, the product is agglomerated and difficult to use.

Further, in step S2, the temperature of the vacuum drying is 50 ℃, and the time of the vacuum drying is 24 h.

Further, in step S3, the nucleophilic reagent is one of ethylenediamine, propylenediamine, butylenediamine, monomethylethylenediamine, dimethylethylenediamine, propylamine, formamide, aniline, water, ammonia water, hydrazine hydrate, methanol, ethanol, butanol, ethylene glycol, ethanolamine, formic acid, acetic acid, hydrochloric acid, phosphoric acid, cysteamine, and ethanedithiol.

Further, in step S3, the reactant and solvent ratio is 1g of polymeric nano-precursor: 10mL of nucleophile: 20-60mL of solvent.

Further, in step S4, the freeze-drying time is 36 h.

Compared with the prior art, the invention has the following beneficial effects:

1. the precursor heterocyclic azide adopted in the preparation method is heated and polymerized to form the aziridine polymer, the polymer can further undergo nucleophilic addition under the action of a nucleophilic reagent, and the chain is broken to form a nano-scale adjustable fluorescent nitrogen dot, and the nitrogen dot has a surface group corresponding to the nucleophilic reagent. The adjustable fluorescent nitrogen dots prepared by the method have the advantages that the nitrogen doping amount is up to 35.78%, the quantum yield is up to 35%, the nitrogen content is rich, and the quantum yield is high; the surface group can be conveniently adjusted according to a nucleophilic reagent, and the optical stability and the biocompatibility are strong;

2. by combining the advantages of the top-down method and the bottom-up method, the method has the advantages of mild reaction conditions, definite target direction and the like, and the product has higher purity, narrow particle size distribution and uniform morphology;

3. compared with the raw materials, the polymer type nano precursor prepared by the invention has higher stability and can be stably stored for a long time in a dry environment. The corresponding adjustable fluorescent nitrogen spot can be further prepared by selecting a proper nucleophilic reagent according to the requirement under a mild condition, and the convenience is higher.

For a better understanding and practice, the invention is described in detail below with reference to the accompanying drawings.

Drawings

FIG. 1 is a schematic diagram of the synthesis of the polymer-based nano-precursor and the adjustable fluorescent nitrogen dots according to the present invention;

FIG. 2 is a scanning electron microscope image of the imidazole-polymer type nano precursor prepared by the present invention;

FIG. 3 is a scanning electron microscope image of thiazole-polymer type nano-precursors prepared by the present invention;

FIG. 4 is a solid state nuclear magnetic diagram of imidazole-polymeric nano-precursors and thiazole-polymeric nano-precursors prepared according to the present invention;

FIG. 5 is a photo-electron energy spectrum of a water-imidazole fluorescent nitrogen spot, an ethylenediamine-imidazole fluorescent nitrogen spot, and an ethylenediamine-thiazole fluorescent nitrogen spot prepared according to the present invention;

FIG. 6 shows nuclear magnetic hydrogen spectra of water-imidazole fluorescent nitrogen dots, ethylenediamine-imidazole fluorescent nitrogen dots, and ethylenediamine-thiazole fluorescent nitrogen dots prepared according to the present invention;

FIG. 7 is a transmission electron microscope image of the fluorescent nitrogen spot of ethylenediamine-imidazole prepared by the present invention;

FIG. 8 is a transmission electron microscope image of the fluorescent nitrogen spot of water-imidazole prepared by the present invention.

FIG. 9 is a transmission electron microscope image of the fluorescent nitrogen spots of ethylenediamine-thiazole prepared by the present invention.

Detailed Description

The invention discloses a preparation method of an adjustable fluorescent nitrogen dot, which is prepared by a novel polymer stripping method to enable a surface group of the dot to be adjustable, and comprises the following steps:

s1: preparing a precursor heterocyclic azide;

s2: dissolving the heterocyclic azide prepared in the step S1 in a proper aprotic solvent, and preparing a polymer type nano precursor through thermal polymerization, vacuum filtration and drying;

s3: dispersing the polymer prepared in the step S2 in a proper aprotic solvent, and adding a nucleophilic reagent corresponding to a target surface group for heating reaction; and centrifuging after the reaction is finished, collecting supernatant, carrying out rotary evaporation on an organic phase to obtain solid residues, re-dissolving the solid residues in water to obtain filtrate, washing the filtrate for multiple times by using ethyl acetate, and carrying out freeze drying to obtain the adjustable fluorescent nitrogen dots with the target surface groups.

Example 1

Please refer to fig. 1, which is a schematic diagram illustrating the synthesis of the tunable fluorescent nitrogen dot according to the present invention. The preparation method of the adjustable fluorescent nitrogen dot comprises the following steps:

step S1: preparation of 2-azidoimidazole: 8.4g of 2-aminoimidazole are weighed into a round bottom flask, 50mL of distilled water and 30mL of concentrated hydrochloric acid are added, and the mixture is stirred well until the solution is completely dissolved. Placing the flask in an ice-water bath for 10min, and slowly dropwise adding 80mL of sodium nitrite aqueous solution under rapid stirring, wherein the mass of the sodium nitrite is 7.0 g; after the reaction is carried out for 20min, slowly dropwise adding 40mL of sodium azide aqueous solution, wherein the mass of the sodium azide is 6.6 g; the temperature was slowly increased and the reaction was guaranteed to be at least 2h at room temperature. And after the reaction is finished, extracting the mixture for three times by using ethyl acetate, collecting an organic layer, drying the organic layer by using anhydrous sodium sulfate, and carrying out reduced pressure rotary evaporation to obtain the 2-azidoimidazole.

Step S2: weighing 1.09g of 2-azidoimidazole in a test tube, adding 30mL of tetrahydrofuran, and adopting argon protection; the reaction was then heated to reflux at 70 ℃ for 2 d. After the reaction is finished and cooled, removing the solvent by vacuum filtration, and cleaning filter residue by ethyl acetate for many times to remove unreacted raw materials; vacuum drying (50 deg.C) the filter residue to obtain dried imidazole-polymer type nanometer precursor. The scanning electron microscope and solid state nuclear magnetic field of the resulting imidazole-polymer type nanoprecursors are shown in fig. 2 and 4, respectively. As can be seen from FIG. 2, the obtained polymer particles have good uniformity and a diameter of about 1 μm; from fig. 4, it can be seen that the main skeleton is composed of three groups of carbon elements with large differences in chemical environments.

Step S3: 1g of the polymer obtained in step S2 was weighed, dispersed in 30mL of tetrahydrofuran, sonicated for 5min, 10mL of ethylenediamine was added, and the mixture was refluxed at 70 ℃ for 1 day. And centrifuging after the reaction is finished, collecting supernatant, carrying out rotary evaporation on an organic phase, dissolving the solid residue in water again to obtain filtrate, and filtering the filtrate through a filter membrane of 0.2 mu m to obtain the ethylenediamine modified fluorescent nitrogen dot concentrated solution. Freeze-drying in a centrifuge tube for 36h to obtain solid loose ethylenediamine modified fluorescent nitrogen dots, wherein the photoelectron spectroscopy, nuclear magnetic hydrogen spectroscopy and transmission electron microscopy are shown in FIGS. 5-7. It can be seen from fig. 5 and 6 that the resulting nitrogen spots are composed mainly of C, H, O and the N element; from FIG. 6, it can be seen that the obtained multiplex peak signal of ethylenediamine group on the surface of nitrogen spot appears at 2.5-3ppm (dotted line frame); as can be seen from FIG. 7, the particle size of the obtained nitrogen dots is approximately 15nm, and the distribution is relatively uniform.

Example 2

This example is substantially the same as example 1, and differs mainly in that the nucleophile used in the synthesis of the tunable fluorescent nitrogen spot is different, and specifically comprises the following steps:

Step S2: weighing 1.09g of 2-azidoimidazole in a test tube, adding 30mL of tetrahydrofuran, and adopting argon protection; the reaction was then heated to reflux at 70 ℃ for 2 d. After the reaction is finished and cooled, removing the solvent by vacuum filtration, and cleaning filter residue by ethyl acetate for many times to remove unreacted raw materials; the filter residue was vacuum dried (50 ℃) to obtain a dry polymer type nano precursor. The scanning electron microscope and solid state nuclear magnetic field of the resulting imidazole-polymer type nanoprecursors are shown in fig. 2 and 4, respectively. As can be seen from FIG. 2, the obtained polymer particles have good uniformity and a diameter of about 1 μm; from fig. 4, it can be seen that the main skeleton is composed of three groups of carbon elements with large differences in chemical environments.

Step S3: 1g of the polymer obtained in step S2 was weighed, dispersed in 30mL of tetrahydrofuran, sonicated for 5min, and 10mL of water was added and refluxed at 70 ℃ for 1 day. And (3) directly performing rotary evaporation on the organic phase after the reaction is finished, adding 10mL of distilled water into the solid residue (or containing a small amount of water), performing ultrasonic treatment for 10min, centrifuging to obtain a supernatant, and filtering the filtrate through a filter membrane of 0.2 mu m to obtain the hydroxyl fluorescent nitrogen dot concentrated solution. Freeze drying in a centrifuge tube for 36h to obtain solid loose water-imidazole fluorescent nitrogen dots, wherein the photoelectron spectrum, nuclear magnetic hydrogen spectrum and transmission electron microscope images are shown in FIGS. 5, 6 and 8. It can be seen from fig. 5 and 6 that the resulting nitrogen spots are composed mainly of C, H, O and the N element; as can be seen from FIG. 8, the obtained nitrogen dots are strip-shaped nanoparticles with the size of about 20nm, and the distribution is relatively uniform.

Example 3

This example is substantially the same as example 1, and differs mainly in the heterocyclic azide used in the synthesis of the polymer, and specifically comprises the following steps:

step S1: preparation of 2-azidothiazole: 10.1g of 2-aminothiazole were weighed into a round-bottomed flask, 50mL of distilled water and 30mL of concentrated hydrochloric acid were added, and the mixture was sufficiently stirred to be completely dissolved. Placing the flask in an ice-water bath for 10min, and slowly dropwise adding 80mL of sodium nitrite aqueous solution under rapid stirring, wherein the mass of the sodium nitrite is 10.0 g; after the reaction is carried out for 20min, slowly dropwise adding 40mL of sodium azide aqueous solution, wherein the mass of the sodium azide is 9.6 g; the temperature was slowly increased and the reaction was guaranteed to be at least 2h at room temperature. And after the reaction is finished, extracting the mixture for three times by using ethyl acetate, collecting an organic layer, drying the organic layer by using anhydrous sodium sulfate, and carrying out reduced pressure rotary evaporation to obtain the 2-azidoimidazole.

Step S2: weighing 1.26g of 2-azidothiazole into a test tube, adding 30mL of tetrahydrofuran, and adopting argon protection; the reaction was then heated to reflux at 70 ℃ for 2 d. After the reaction is finished and cooled, removing the solvent by vacuum filtration, and cleaning filter residue by ethyl acetate for many times to remove unreacted raw materials; vacuum drying (50 ℃) to filter residue to obtain the dried thiazole-polymer type nano precursor. Scanning electron microscopy and solid state nuclear magnetic microscopy of thiazole-polymer type nanoprecursors are shown in figures 3 and 4, respectively. As can be seen from FIG. 3, the obtained polymer particles have good uniformity and a diameter of about 0.6 μm; from fig. 4, it can be seen that the main skeleton is composed of five groups of carbon elements with large differences in chemical environments.

Step S3: 1g of the polymer obtained in step S2 was weighed, dispersed in 30mL of tetrahydrofuran, sonicated for 5min, and 10mL of water was added and refluxed at 70 ℃ for 1 day. And (3) directly performing rotary evaporation on the organic phase after the reaction is finished, adding 10mL of distilled water into the solid residue (or containing a small amount of water), performing ultrasonic treatment for 10min, centrifuging to obtain a supernatant, and filtering the filtrate through a filter membrane of 0.2 mu m to obtain the hydroxyl fluorescent nitrogen dot concentrated solution. And (3) placing the mixture in a centrifugal tube for freeze drying for 36h to obtain solid loose hydroxyl fluorescent nitrogen dots, wherein photoelectron spectroscopy, nuclear magnetic hydrogen spectroscopy and transmission electron microscopy are shown in figures 5, 6 and 9. It can be seen from fig. 5 and 6 that the resulting nitrogen spots are composed mainly of C, H, O and the N element; from FIG. 6, it can be seen that the obtained multiplex peak signal of ethylenediamine group on the surface of nitrogen spot appears at 2.5-3ppm (dotted line frame); as can be seen from FIG. 9, the particle size of the obtained nitrogen dots is about 25nm, and the distribution is relatively uniform.

The data of nuclear magnetic hydrogen spectrum, transmission electron microscope, photoelectron spectrum and the like show that the synthesized adjustable fluorescent nitrogen dot has the characteristics of simple and adjustable surface groups, high uniformity, rich nitrogen content and high quantum yield. The method is simple and convenient, easy to control and high in repeatability; the prepared adjustable fluorescent nitrogen dot has excellent performance.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.

Claims

1. A preparation method of a tunable fluorescent nitrogen dot is characterized by comprising the following steps:

s1: preparing a precursor heterocyclic azide compound, wherein the heterocyclic azide compound is a heterocyclic azide compound with electron-deficient carbon-carbon double bonds and azide groups and is one of 2-azidoimidazole, 2-azidothiazole, 6-azidoindole and 3-azidopyrazole;

s3: dispersing the polymer type nano precursor prepared in the step S2 in an aprotic solvent, adding a nucleophilic reagent corresponding to a target surface group for heating reaction, wherein the nucleophilic reagent is one of ethylenediamine, propylenediamine, butylenediamine, monomethylethylenediamine, dimethylethylenediamine, propylamine, formamide, aniline, water, ammonia water, hydrazine hydrate, methanol, ethanol, butanol, ethylene glycol, ethanolamine, formic acid, acetic acid, hydrochloric acid, phosphoric acid, cysteamine and ethanedithiol; and centrifuging after the reaction is finished, collecting supernatant, carrying out rotary evaporation on an organic phase to obtain solid residue, dissolving the solid residue in water again to obtain filtrate, and carrying out freeze drying on the filtrate to obtain the adjustable fluorescent nitrogen spot with the target surface group.

2. The method for preparing a tunable fluorescent nitrogen spot according to claim 1, wherein: the precursor heterocyclic azide compound in the S1 is prepared by adopting a heterocyclic amino compound through an azide reaction.

3. The method for preparing a tunable fluorescent nitrogen spot according to claim 2, wherein: the heterocyclic amino compound is a heterocyclic amino compound having an electron-deficient carbon-carbon double bond and an amino group.

4. The method for preparing a tunable fluorescent nitrogen spot according to claim 3, wherein: the heterocyclic amino compound is one of 2-aminoimidazole, 2-aminothiazole, 6-aminoindole and 3-aminopyrazole; the polymeric nano-precursor is one of an imidazole-polymeric nano-precursor, a thiazole-polymeric nano-precursor, an indole-polymeric nano-precursor, and a pyrazole-polymeric nano-precursor.

5. The method for preparing a tunable fluorescent nitrogen spot according to claim 1, wherein: in steps S2 and S3, the aprotic solvent is tetrahydrofuran, chloroform, toluene, acetonitrile, dioxane, ethyl acetate, butanone, or acetone.

6. The method for preparing a tunable fluorescent nitrogen spot according to claim 1, wherein: in step S2, the concentration of the heterocyclic azide compound in the solvent is 10-60mg/mL, the heating reaction temperature is 70 ℃, and the heating reaction time is 2 d.

7. The method for preparing a tunable fluorescent nitrogen spot according to claim 1, wherein: in step S2, the temperature of the vacuum drying is 50 ℃, and the time of the vacuum drying is 24 hours.

8. The method for preparing a tunable fluorescent nitrogen spot according to claim 1, wherein: in step S3, the reactant and solvent ratios were 1g polymeric nanoprecursor: 10mL of nucleophile: (20-60) mL of solvent.

9. The method for preparing a tunable fluorescent nitrogen spot according to claim 1, wherein: in step S4, the freeze-drying time was 36 hours.