Preparation Method and Application of a Fluorescent Polymer Dot with Adjustable Chargeability

Abstract

The invention discloses a preparation method and application of a fluorescent polymer dot with adjustable chargeability. Using natural polymer sodium alginate SA as the main carbon source and introducing o-phenylenediamine oPDA as an auxiliary agent, carbonized polymer dots SAPD-CPDs are synthesized by a hydrothermal method. SA and oPDA are mixed in water and sonicated until completely dissolved, then the pH value of the solution is adjusted to 4 with hydrochloric acid and sodium hydroxide solutions, sealed in the polytetrafluoroethylene liner of a reaction kettle, and placed in an oven at 160 °C for reaction for 10 h. It is filtered through a 0.22 μm mixed cellulose filter, dialyzed for 24 h with a dialysis bag having a molecular weight cut-off of 500 Da, vacuum concentrated, and then freeze-dried to obtain a solid powder. The carbon dots retain the characteristics of the sodium alginate polymer chain and can form a stable luminescent film with chitosan due to physical adsorption.

Description

Technical Field

[0001] The present invention belongs to the technical field of luminescent carbon nanomaterials, and particularly relates to a preparation method and application of a fluorescent polymer dot with adjustable charge property. Background Art

[0002] As a novel carbon nanomaterial, fluorescent carbon dots can be classified into graphene quantum dots (GQDs), carbon nanodots (CNDs), and carbonized polymer dots (CPDs) according to the differences in their carbon cores and surface states. The concept and formation mechanism of carbonized polymer dots CPDs were first proposed by Yang et al. Such carbon dots not only inherit the excellent chemical stability and high luminescence performance of traditional carbon dots, but also have good biocompatibility and film-forming properties due to the outer layer structure of the formed polymer chains.

[0003] At present, there is still a lack of direct use of alginate as a carbon source or the addition of auxiliary chemicals to obtain carbonized polymer carbon dots with special luminescence properties and directly apply them to luminescent film-forming research. Carbon dots with adjustable amphoteric charge properties have not been reported yet. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method and performance study of nitrogen-doped carbonized polymer dots.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A preparation method of a fluorescent polymer dot with adjustable charge property, using natural polymer sodium alginate as the main carbon source, introducing o-phenylenediamine as an auxiliary agent, and synthesizing sodium alginate-based carbonized polymer dots SAPD-CPDs by hydrothermal method; specifically including the following steps:

[0007] Mix sodium alginate, o-phenylenediamine and water, place it in an ultrasonic oscillator until completely dissolved, then adjust the pH value of the solution to 4 with hydrochloric acid and sodium hydroxide solution, seal it in the polytetrafluoroethylene inner lining of the reaction kettle, and place it in an oven at 160 °C for reaction for 10 h; after filtration, dialysis, vacuum concentration, freeze-drying to obtain a solid powder, which is the fluorescent polymer dot.

[0008] Further, the filtration is carried out with a 0.22 μm mixed cellulose filter.

[0009] Further, the dialysis is carried out with a dialysis bag with a molecular weight cut-off of 500 Da for 24 h.

[0010] Furthermore, when the fluorescent polymer dot solution is excited at a wavelength of 352 nm, a blue-green fluorescence emission signal appears at 462 nm. The Stokes shift can reach 110 nm, and it shows sensitivity to the pH value. As the solution environment changes from acidic to alkaline, the position of the fluorescence emission peak blue-shifts abruptly from 465 nm to 425 nm. The surface charge state of the carbon dots SAPD-CPDs changes from positive charge to negative charge, and the isoelectric point appears at pH = 6.

[0011] Furthermore, a method for preparing a luminescent polymer dot film using the fluorescent polymer dots with adjustable charge properties is studied. Chitosan is dissolved in an acetic acid aqueous solution and ultrasonically stirred until a homogeneous transparent solution is obtained. The solution is poured into a plastic petri dish to cover the surface of the FTO glass and dried at 50 - 60 °C to form an FTO / CS film; the FTO / CS film is soaked in a sodium hydroxide solution to neutralize acetic acid and washed with pure water until neutral; the dried film is soaked in a SAPD-CPDs solution diluted 10 times, taken out, washed with pure water, and dried to obtain an FTO / CS / CPDs film.

[0012] Application: The application of the fluorescent polymer dots with adjustable charge properties in the detection of quercetin. Accurately weigh the solid powder of SAPD-CPD and prepare a 10 g / L solution with ultrapure water. Pipette 200 μL of this SAPD-CPD solution, add 5 mL of B-R buffer solution with a pH value of 7.96, and respectively add 1 mL of 5×10 -5 mol / L of different types of drug solutions, and make up the volume to 10 mL with ultrapure water. After shaking well, measure the fluorescence intensity I value. Use the solution without adding drugs as the blank control solution and measure the fluorescence intensity I0 value. Perform the same operation. Take the carbon dot system without adding quercetin solution as the initial solution, and then add quercetin solutions with different concentrations to investigate the change in fluorescence intensity under the action of different concentrations of quercetin on CPDs. A parallel solvent control experiment is carried out using an ethanol solvent instead of the drug solution.

[0013] In order to achieve excellent properties of polymer carbon dots in the present invention, the organic small molecule o-phenylenediamine with auxiliary groups is introduced into the polymer as a mixed carbon source. The o-phenylenediamine with rich functional groups forms cross-linked aggregates or hydrophobic cores and outer hydrophilic surface groups through dehydration carbonization. Since the two substituted amino groups on the benzene ring are in the carbonized cross-linked structure, they can play the role of a bridge connecting flexible polymer chains, and a charge transfer interface is formed between the phenyl group and the carbon heteroatom bond, which is beneficial to the generation of new energy levels, thus contributing to the tunable luminescence behavior of CPDs. On the other hand, the presence of hydrophilic groups such as amino and hydroxyl groups makes CPDs have good complexing ability and biocompatibility, and can have a strong interfacial interaction with metal ions, thereby greatly expanding the application fields of CPDs.

[0014] The advantages of the present invention are as follows:

[0015] (1) The carbonized polymer dots obtained by this method have excitation wavelength independence and stable luminescence performance. The Stokes shift can reach 110 nm, and they have stronger luminescence performance than the carbon dots synthesized from a single carbon source. The fluorescence quantum yield is 16.9%.

[0016] (2) For the carbonized polymer dots obtained by this method, with the change of pH in the solution environment, the surface of the particles shows adjustable positive and negative charge conversion. When the fluorescence emission peak of the corresponding solution changes from acidic to alkaline, an obvious blue shift of about 40 nm occurs. Through morphology and composition analysis, it is speculated that a large number of amino groups and carboxyl groups may coexist on the surface of the CPDs, resulting in amphoteric properties similar to those of amino acid molecules and the existence of an isoelectric point.

[0017] (3) The carbonized polymer dots obtained by this method retain the characteristics of the alginate polymer chain. By adjusting the surface charge, a composite film can be formed with chitosan through electrostatic physical adsorption. The performance of the luminescent film is stable, and the carbon dots are not easily detached. This proves the feasibility of directly preparing a fluorescent film from the carbonized polymer dots.

[0018] (4) The fluorescence signal of the carbonized polymer dots obtained by this method has an obvious quenching response to quercetin and can be used as a fluorescent probe to establish a fluorescence detection method for the content of quercetin. In the range of 7.9×10 -6 ~7.7×10 -5 mol / L, the degree of quenching of the fluorescence intensity shows a good linear relationship with the quercetin concentration. The working curve is I0 / I = 0.8788 + 1.6897×10 -6 C Que , the correlation coefficient r 2 = 0.9748, and the detection limit LOD = 1.7×10 -6 mol / L. Description of the Drawings

[0019] Figure 1 is a schematic diagram of the preparation method and performance application of the carbonized polymer dots;

[0020] Figure 2 is the fluorescence spectrum of the carbonized polymer dots synthesized under different reaction temperature conditions;

[0021] Figure 3 is the fluorescence spectrum of the carbonized polymer dots synthesized under different reactant ratio conditions;

[0022] Figure 4 is the fluorescence spectrum of the carbonized polymer dots synthesized under different reaction time conditions;

[0023] Figure 5(a) Fluorescence spectra and (b) solution images of carbonized polymer dots synthesized under different pH conditions; Figure 6 (a) UV spectra of carbonized polymer dots synthesized under different pH conditions, (b) spectral characteristic diagrams of oPD-CDs, SA-CPDs and SAPD-CPDs (the inset is the image of the carbon dot solution under ultraviolet light), (c) FT-IR spectral characterization diagrams of SAPD-CPDs and carbon sources;

[0024] Figure 7 (a) Fluorescence spectra of SAPD-CPDs under different excitation wavelengths and (b) the changing trends of fluorescence intensity and emission peak position, (c) light resistance and (d) stability data;

[0025] Figure 8 (a) XPS full spectra and (b) C 1s spectra, (c) N1s spectra and (d) O 1s spectra of SAPD-CPDs;

[0026] Figure 9 (a) Transmission electron microscope images and (b) corresponding particle size distribution histograms of SAPD-CPDs;

[0027] Figure 10 (a) Fluorescence spectra of SAPD-CPDs in B-R buffer solutions with different pH values, (b) corresponding Zeta potential data, (c) changing trends of fluorescence intensity and emission peak position, and (d) images of a series of solutions under ultraviolet light;

[0028] Figure 11 (a) Photos of solutions formed by mixing different types of polymers with SAPD-CPD and the prepared thin films, (b) fluorescence spectra of the luminescent thin film FTO / CPDs@CS (the inset is the image of the luminescent thin film under ultraviolet light), (c) scanning electron microscope micrographs of FTO / CS, FTO / CPD and FTO / CPDs@CS thin films;

[0029] Figure 12 Under the condition of pH = 7.96, (a) the response of SAPD-CPD to different drug molecules, (b) fluorescence spectra of SAPD-CPD with different concentrations of quercetin drugs, and (c) linear fitting curves of the corresponding data; Figure 13 Fluorescence spectra and images under ultraviolet light of fluorescent carbon dots synthesized using ethylenediamine and o-phenylenediamine carbon sources under the same conditions (SAEDA-CPD and SAPD-CPD solutions are on the left and right respectively);

[0030] Figure 14 Images of SAEDA-CPD solution under ultraviolet light in B-R buffer solutions with different pH values. Specific implementation manners

[0031] To make the above features and advantages of the present invention more obvious and understandable, specific embodiments are given below for detailed description. Unless otherwise specified, the methods of the present invention are conventional methods in the art.

[0032] Example 1

[0033] 1. Preparation and purification of fluorescent carbonized polymer dots

[0034] Accurately weigh 0.1 g of sodium alginate and 0.05 g of o-phenylenediamine and place them in a beaker. Add ultrapure water and stir. Place it in an ultrasonic oscillator until completely dissolved, then adjust the pH value of the solution to 4 with hydrochloric acid and sodium hydroxide solutions, and make up the volume to 15 mL. Transfer it to the inner lining of polytetrafluoroethylene and seal it in a reaction kettle, and place it in an oven at 160 °C for reaction for 10 h. The prepared carbonized polymer dots are labeled as SAPD-CPDs. After the reaction, use a syringe to filter the reaction solution through a 0.22 μm microporous membrane to obtain a brownish carbonized polymer dot solution, which is stored in the refrigerator for later use. The specific synthesis and application processes are as Figure 1 shown.

[0035] 2 Preparation of luminescent polymer dot films

[0036] Dissolve 1.5 g of chitosan in 50 mL of acetic acid aqueous solution (wt% = 3%), and stir ultrasonically until a homogeneous transparent solution is obtained. Pour the solution into a plastic petri dish to cover the surface of the FTO glass, and dry it into an FTO / CS film at 50 - 60 °C. Immerse the FTO / CS film in a sodium hydroxide solution with wt% = 6% to neutralize acetic acid, and wash it with pure water until neutral, and dry it at 50 - 60 °C. Immerse the film in a 1 g / L SAPD-CPDs solution for 3 h, take it out, wash it with pure water and then dry it to obtain an FTO / CS / CPDs film. Orient one side of the film towards the incident light and adjust the angle between the incident light and its surface to about 50°, and measure the fluorescence spectrum of the film.

[0037] 2.1 Optimization of the preparation method and spectral characterization of carbonized polymer dots

[0038] According to the experimental method, explore the optimal hydrothermal reaction synthesis conditions, and the experimental results are as Figures 2 - 5 shown. The raw material ratio, reaction time and temperature conditions also do not change the position of the fluorescence emission peak of the synthesized CPDs. It shows that using the same synthesis raw materials, different ratios and heating conditions only affect the luminescence intensity of the formed carbon dots. However, the fluorescence spectra of CPDs prepared with solutions of different pH values have obvious changes, as Figure 5As shown, a mutation occurs in the emission wavelength position and intensity between pH 4 and 5, and the emission peak undergoes a blue shift of nearly 50 nm. The color of the CPDs solution synthesized under strong acidic to weak acidic conditions changes from dark brown to light yellow. This is significantly different from the previously reported phenomenon of the continuous change of the carbon dot emission peak with pH, indicating that SAPD-CPDs may have a special molecular structure.

[0039] Further investigate the spectral characteristics of the carbonized polymer dots prepared under the condition of pH = 4 reaction solution, and the results are as Figure 6 shown. The UV-Vis spectrum of the SAPD-CPDs solution synthesized under the condition of pH = 4 shows multiple absorption peaks. One is a sharp absorption peak at 235 nm, and the other is a broad absorption peak near 320 nm. These absorption peaks are attributed to the π-π* electronic transition of C═C and the n-π* transition of C═O in the aromatic ring. A significant absorption peak appears at 440 nm, which may be caused by the π-π* transition functional group of the carbonyl / amine group. As the alkalinity of the solution increases, the intensity of this absorption peak decreases significantly due to the deprotonation of the amine group. Infrared spectroscopy characterization was performed on SAPD-CPDs and the raw materials (SA and oPDA). The results are as Figure 6 shown in c. The absorption peak of CPDs near 3270 cm -1 indicates the stretching vibrations of O-H and N-H in the structure, which is similar to the characteristic absorption peak of the hydroxyl group of SA. Another characteristic absorption peak appears at 1630 cm -1 , which may be formed due to the deformation vibration of -NH2. The dense and fine structure peak segment appearing between 1590 cm -1 and 1462 cm -1 corresponds to the characteristic stretching vibration of the aromatic ring C═C of o-phenylenediamine. 1033 cm -1 and 1126 cm -1 are attributed to the stretching vibration of C-O-C. The sharp peak types appearing at 754 cm -1 and 615 cm -1 are caused by the stretching vibration of C-C and the out-of-plane deformation vibration of the C-H bond of the substituted benzene, further indicating the existence of an aniline-like structure in the CPDs formed during the carbonization process. When excited at a wavelength of 352 nm, SAPD-CPDs have the strongest fluorescence emission peak at 462 nm, and the Stokes shift can reach 110 nm. The luminescence intensity is enhanced by about 4 times compared with SA-CPDs synthesized from a single sodium alginate carbon source under the same conditions, and about 10 times compared with single oPD-CDs. Using quinine sulfate as a reference, its fluorescence quantum yield was measured to be 26.9%.

[0040] When the excitation wavelength changes from 302 to 402 nm, the emission spectrum of CPDs shows almost no excitation dependence and stably emits at 455 - 465 nm (Figure 7 In a). The influence of the light source irradiation and the placement time on its fluorescence spectrum was studied. As Figure 7 shown in c) and d) in, the maximum fluorescence peak of the CPDs did not shift, and the emission intensity did not change significantly, indicating that the polymer dots had good luminescence stability.

[0041] 2.2 Structural and Morphological Characterization of Carbonized Polymer Dots

[0042] In Figure 8 the X-ray photoelectron spectroscopy (XPS) data further confirmed the chemical composition of the SAPD-CPDs. There were three obvious peaks at 285.1 eV, 399.7 eV, and 532.5 eV, corresponding to the binding energies of C1s, N 1s, and O1s respectively. After the peak fitting operation, it could be seen from the C1s spectrum resolved in Figure 8 (b) in that the chemical states of the C element were composed of C-C / C-N bonds (284.5 eV), C=O bonds (285.2 eV), and C-O bonds (288.7 eV) respectively. The O1s peak presented three obvious peaks centered at 530.3 eV, 531.5 eV, and 532.1 eV, which were attributed to C=O, C-O-C, and C-O respectively. The peaks at 397.9 eV, 398.7 eV, and 400.0 eV in the N1s spectrum corresponded to O=C-N, C-N, and N-H bonds respectively. The XPS results confirmed that the surface of the SAPD-CPDs was rich in carboxyl, hydroxyl, and amino modifications, which was consistent with the FT-IR analysis results.

[0043] The size and morphology of the SAPD-CPDs were studied by high-resolution transmission electron microscopy (TEM). As Figure 9 shown, the CPDs were easily and uniformly dispersed in water, and the microscopic morphology was spherical without an obvious lattice structure. Using Nano measurer software for analysis, the average particle size of the polymer dots was obtained as 8.23 nm.

[0044] 2.3 Charge Tunability of Carbonized Polymer Dots

[0045] The purified and prepared solid of SAPD-CPDs was dissolved in different pH B-R buffer solutions to obtain an aqueous solution environment with different pH values. Figure 10 (a) in shows the fluorescence spectrum of the CPDs excited at a wavelength of 352 nm, and the change trend is as Figure 10As shown in (c), the fluorescence intensity of CPDs remains stable in the pH range of 2 - 4, drops rapidly at pH = 5, then climbs from pH 6.0 to reach the maximum value at pH 8.0 and remains stable up to pH 12. The position of the emission peak also shows a relatively consistent change with the pH value, with a mutation occurring at pH = 5, shifting from 460 nm to 425 nm blue shift. The fluorescence emission of the color of the carbon dot solution under ultraviolet light changes from blue - green to blue. This property is very different from that of the carbon dots synthesized from single o - phenylenediamine, which can prove that the property of pH - regulated emission wavelength is not attributed to the carbon source o - phenylenediamine, but the result of introducing sodium alginate.

[0046] The measured Zeta (ζ) potential of SAPD - CPDs in the corresponding different pH solutions shows a difference in the charged property near the corresponding mutation points, as Figure 10 shown in b. It shows a positive ζ potential under acidic conditions, a negative ζ potential under alkaline conditions, and in the case of pH 6 - 7, the value is close to zero, showing an isoelectric point similar to that of an amphoteric polymer. Under strong acid (pH < 2) conditions, the absolute value is greater than 30 mV, and in the pH ranges of 3 - 5 and 9 - 12, most of the absolute values are greater than 20 mV, which is usually beneficial to the stable dispersion of nanoparticles. This also verifies that, as described in the above structural inference results, due to the co - existence of a large number of carboxyl groups and amino groups on the surface of the polymer dots, the combined action of the protonation of carboxyl groups and the de - protonation of amino groups leads to the pH - tunable emission characteristics of the charge of SAPD - CPDs.

[0047] 2.4 Preliminary study on the film - forming performance of carbonized polymer dots

[0048] Mix SAPD - CPDs with different types of polyelectrolyte solutions to investigate their stability. The results are as Figure 11 shown. When adding anionic polymers sodium polystyrene sulfonate and sodium alginate to the CPDs solution, the carbon dots are easily electrostatically adsorbed with the negatively charged ones to form visible flocculent precipitates. While with cationic polymers chitosan and poly(diallyldimethylammonium chloride), due to the same - charge repulsion, a relatively uniform and stable solution is formed. It can be seen from the prepared film images that the composite film prepared from SAPD - CPDs and CS is relatively uniform, and the corresponding scanning electron microscope micrograph is as Figure 11 shown in c. The film FTO / CS prepared from single chitosan shows obvious cracking under the test conditions, while the SAPD - CPDs prepared by the hydrothermal method at high temperature retain carbonized particles and fiber structures. The composite film formed by the electrostatic adsorption of the two forms a denser film surface with better strength and will not show cracks under the action of electron beams. The fluorescence spectra and images of the luminescent film are as Figure 11As shown in Figure b, the fluorescence emission wavelength of the thin film in the solution is 425 nm, and there is no fluorescence signal in the aqueous solution after the thin film is taken out. This indicates that the polymer carbon dots have a certain film-forming property and can form a stable luminescent film with CS.

[0049] 3. Application of Quercetin Content Detection

[0050] SAPD-CPDs respond to specific drug molecules and can be used as fluorescence probes to establish a new method for determining drug content. Under the condition of pH = 7.96 where the fluorescence intensity of the probe is the maximum, the response of CPDs to different drugs was investigated. The experimental results are as Figure 12 shown in Figure a. Drugs with reducing components quench the fluorescence signal of carbon dots by more than 20%. Among them, the quenching values of quercetin, rutin, and glutathione with stronger reducing properties are greater than 50%, and the drug response value of quercetin is the largest. The fluorescence emission peak wavelength of SAPD-CPDs remains basically unchanged before and after adding quercetin. In the range of 7.9×10 -6 ~7.7×10 -5 mol / L, the quenching degree of fluorescence intensity shows a good linear relationship with the quercetin concentration. The working curve is I0 / I = 0.8788 + 1.6897×10 -6 C Que , the correlation coefficient r 2 = 0.9748, and the detection limit LOD = 1.7×10 -6 mol / L. A parallel control experiment was carried out by replacing the quercetin solution with the same volume of ethanol solution. The results are as Figure 12 shown in Figure c. As the amount of ethanol used gradually increases, the fluorescence emission peak intensity of SAPD-CPDs hardly changes. This verifies that the fluorescence quenching is not caused by the solvent effect, but by the specific interaction between quercetin and SAPD-CPDs. Thus, a simple and feasible new method for detecting quercetin content based on fluorescence spectroscopy can be established.

[0051] Comparative Example 1 (Changing the Auxiliary Agent to Ethylenediamine)

[0052] Weigh 0.1 g of sodium alginate and 0.05 g of o-phenylenediamine and mix them as the carbon source. Add ultrapure water and stir. After placing it in an ultrasonic oscillator until it is completely dissolved, adjust the pH value of the solution to 4 with hydrochloric acid and sodium hydroxide solutions, make the volume up to 15 mL, seal it in the polytetrafluoroethylene inner lining of the reaction kettle, and place it in an oven at 160 °C for reaction for 10 h to synthesize polymer carbon dots SAPD-CPD. Change o-phenylenediamine to ethylenediamine, and keep the other operating conditions the same to synthesize carbonized polymer carbon dots SAEDA-CPD. The fluorescence spectra of the carbon dots in these two systems are as Figure 13As shown. Compared with SAPD-CPD, the SAEDA-CPD solution has a darker color and exhibits blue fluorescence under ultraviolet light irradiation, but the luminescence effect is very low, and the optimal excitation / emission wavelengths are 307 nm and 398 nm, respectively. This indicates that the system of this study has good luminescence performance (the fluorescence intensity is increased by nearly 10 times or more). This is because the introduction of the benzene ring structure of o-phenylenediamine, and the π electrons formed by the conjugated structure are conducive to the transition of electron energy levels, resulting in a larger fluorescence emission peak wavelength and higher luminescence intensity. Pipette 200 μl of the carbon dot solution respectively to prepare a series of solutions with different pH values, and perform fluorescence spectrum scanning to investigate the change of fluorescence intensity at different pH values. The results show that ( Figure 14 ), with the change of the solution pH environment, the color of the SAEDA-CPD carbon dot solution under ultraviolet light does not show obvious changes. This indicates that the effect of pH on the fluorescence characteristics of carbon dots should be due to the benzene ring structure in o-phenylenediamine, so ethylenediamine cannot be used to replace o-phenylenediamine.

[0053] The above are only the preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. A preparation method of a fluorescent polymer dot with adjustable charge property, characterized in that, Using natural polymer sodium alginate as the main carbon source and introducing o-phenylenediamine as an auxiliary agent, carbonized polymer dots SAPD-CPDs were synthesized by hydrothermal method; Specifically, it includes the following steps: Mix sodium alginate, o-phenylenediamine and water, place it in an ultrasonic oscillator until completely dissolved, then adjust the pH value of the solution to 4 with hydrochloric acid and sodium hydroxide solution, seal it in the polytetrafluoroethylene liner of the reaction kettle, and place it in an oven at 160 °C for reaction for 10 h; After filtration and vacuum concentration, freeze-dry to obtain a solid powder, which is the fluorescent polymer dots; The filtration is carried out with a 0.22 μm mixed cellulose filter.

2. The chargeable fluorescent polymer dots prepared by the preparation method according to claim 1, characterized in that, The fluorescent polymer dot solution has a blue-green fluorescence emission signal at 462 nm when excited at a wavelength of 352 nm. The Stokes shift can reach 110 nm and shows sensitivity to the pH value. As the solution environment changes from acidic to alkaline, the fluorescence emission peak position blue-shifts from 465 nm to 425 nm suddenly, corresponding to the surface charge state of the carbon dots SAPD-CPDs changing from positive charge to negative charge, and the isoelectric point appears at pH = 6.

3. A method for preparing a luminescent polymer dot film by using the fluorescent polymer dots with adjustable chargeability described in claim 2, characterized in that, Dissolve chitosan in acetic acid aqueous solution, stir ultrasonically until a homogeneous transparent solution is obtained, pour the solution into a plastic petri dish and cover the surface of the FTO glass, and dry it into an FTO / CS film at 50-60 °C; Immerse the FTO / CS film in sodium hydroxide solution to neutralize acetic acid, and wash it with pure water until neutral; Immerse the dried film in the SAPD-CPDs solution, take it out, wash it with pure water and then dry it to obtain an FTO / CS / CPDs film.

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