Preparation and application of multifunctional coumarin-polyethyleneimine fluorescent carbon dots

Abstract

The application discloses fluorescent carbon dots and a preparation method thereof. The raw material coumarin and polyethylene imine adopted in the method are simple and easy to obtain and non-toxic, the target product can be obtained through a simple one-step hydrothermal method, the preparation condition requirement is low, and the obtained fluorescent carbon dots can be applied to multiple fields such as ion detection, fingerprint identification and information encryption.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of fluorescent carbon nanomaterials, and relates to preparation and application of fluorescent carbon dots, in particular to multifunctional coumarin-polyethyleneimine fluorescent carbon dots and application thereof in ion detection, fingerprint identification and information encryption. BACKGROUND

[0002] Carbon dots (CDs) are a novel spherical luminescent nanomaterial with a size of less than 10 nm, and the main component element is C. In 2004, Xu et al. first accidentally discovered the existence of carbon dots in the process of electrophoretic purification of single-walled carbon nanotubes. Due to its excellent photoluminescence properties, chemical stability, easy chemical modification, light bleaching resistance and good biocompatibility, it has attracted great attention in the fields of biological imaging, optoelectronic devices, sensing, catalysis, environmental detection and the like.

[0003] Fe 3+ Iron is an essential trace element for the human body, which assists in human metabolism, promotes biochemical reactions as a catalyst and participates in oxygen transport processes. However, lack or excessive Fe 3+ can also harm human health. Traditional ion detection methods such as X-ray fluorescence spectroscopy, atomic absorption spectroscopy, electrochemical method, atomic emission spectroscopy, ultraviolet-visible spectrophotometry and the like have expensive instrument and equipment costs, complex operation and are not convenient to carry. Carbon dots are often used as high-sensitivity and high-selectivity fluorescent probes by taking advantage of the change in fluorescence after the interaction of carbon dots with metal ions. Therefore, it is of great significance to establish a method for detecting iron ions in aqueous solution which is rapid, selective, sensitive and easy to operate.

[0004] Fingerprint, as a unique feature of each person, almost never changes. Therefore, it is often used as a person's "identity card" and "information bank", and plays a very important role in identity proofing and criminal investigation. When the finger contacts an object, even if it is completely wiped off, latent fingerprints will remain on the contact place. The currently commonly used methods such as optical appearance method, powder appearance method, fumigation appearance method and dyeing method are not only complex in operation, expensive in price, easy to pollute and destroy the object and harmful to the collector. Therefore, the present fluorescent carbon material-based fingerprint appearance method has attracted more attention and research. At present, most of the carbon dot materials used for latent fingerprint appearance adopt powder method. The powder method not only consumes a large amount of product, but also can cause certain harm to the respiratory tract if not handled properly.

[0005] With the progress of science and technology, new means of counterfeiting are constantly emerging, and methods to prevent counterfeiting are constantly innovating. Many anti-counterfeiting technologies such as nuclear track, radio frequency identification and fluorescent printing have been widely used. The use of the fluorescence of materials can encrypt information multiple times, effectively preventing important documents, currency and brands from being counterfeited or copied. A large number of fluorescent materials such as semiconductor quantum dots, organic fluorescent dyes and rare earth doped fluorescent dyes have been used as fluorescent anti-counterfeiting ink. Due to the advantages of controllable structure, adjustable luminescence, biocompatibility and good water solubility, carbon dots have good application prospects in fluorescent anti-counterfeiting. SUMMARY

[0006] To solve the above problems, the application provides a kind of fluorescent carbon dots and its preparation method, the raw material coumarin and polyethylene imine used in the method are simple and easy to obtain, non-toxic, and the target product can be obtained by simple one-step hydrothermal method, the preparation condition is low, and the obtained fluorescent carbon dots can be applied to ion detection, fingerprint identification and information encryption and other fields.

[0007] To achieve the above purpose, the technical scheme provided by the application is:

[0008] A preparation method of multifunctional coumarin-polyethylene imine fluorescent carbon dots, comprising the following steps:

[0009] S1: hydrothermal reaction: polyethylene imine and coumarin are dissolved in a solvent to obtain a homogeneous solution, which is transferred to a hydrothermal kettle and reacted at 90-240 DEG C for 3-24 h;

[0010] S2: centrifugal filtration: after centrifugation, the supernatant is collected and filtered through a water-based filter membrane with a pore size of 0.4-0.5 μm to remove large particles;

[0011] S3: dialysis: dialysis for 24-48 h with a dialysis bag with a molecular weight cut-off of 3000-4000 Da;

[0012] S4: freeze-drying: freeze-drying to obtain fluorescent carbon dots.

[0013] As a preferred scheme of the application, in the S1 step, the mass fraction ratio of coumarin to polyethylene imine is 0.1-4:1.

[0014] As a preferred scheme of the application, in the S1 step, the mass fraction ratio of coumarin to polyethylene imine is 0.5-4:1.

[0015] As a preferred scheme of the application, in the S1 step, the solvent includes methanol and / or ethanol.

[0016] As a preferred scheme of the application, the reaction temperature is 90-120 DEG C, and the reaction time is 3-9 h.

[0017] As a preferred embodiment of the present invention, the molecular weight M of the polyethyleneimine is... w It ranges from 10,000 Da to 25,000 Da.

[0018] More preferably, the polyethyleneimine is a dendritic polyethyleneimine with a molecular weight M. w It is 10000Da.

[0019] The yield and quantum yield of the multifunctional coumarin-polyethyleneimine fluorescent carbon dots are not directly proportional to the mixing ratio of the raw materials. A higher yield is achieved when the mass ratio of coumarin to polyethyleneimine is 3:1. Higher quantum yields are observed when the mass ratio of coumarin to polyethyleneimine is 1–4:1.

[0020] The second objective of this invention is to provide a method for preparing multifunctional coumarin-polyethyleneimine fluorescent carbon dots.

[0021] The third objective of this invention is to provide a multifunctional coumarin-polyethyleneimine fluorescent carbon dot in Fe 3+ Applications in ion detection.

[0022] Multifunctional coumarin-polyethyleneimine fluorescent carbon dots in the preparation of Fe 3+ Applications in ion detection probes.

[0023] The multifunctional coumarin-polyethyleneimine fluorescent carbon dots are applied to Fe 3+ The steps for ion detection are as follows:

[0024] Multifunctional coumarin-polyethyleneimine fluorescent carbon dots were prepared into a fluorescent carbon dot solution, and the fluorescence emission intensity was measured at an excitation wavelength of 380 nm, which was F0.

[0025] The sample was added to a fluorescent carbon dot solution, and the maximum fluorescence emission intensity was measured at an excitation wavelength of 380 nm. The sample was then tested and measured as F. A solution with only UP water added was used as a blank sample.

[0026] The sample concentration was calculated using a standard curve.

[0027] The fourth objective of this invention is to provide an application of multifunctional coumarin-polyethyleneimine fluorescent carbon dots in the development of latent fingerprints.

[0028] Multifunctional coumarin-polyethyleneimine fluorescent carbon dots were formulated into a low-concentration aqueous solution for the development of oily latent fingerprints.

[0029] The low-concentration aqueous solution is between 0.5 mg / mL and 1.5 mg / mL. The most preferred concentration is 1 mg / mL.

[0030] The steps for applying the multifunctional coumarin-polyethyleneimine fluorescent carbon dots in the development of latent fingerprints are as follows:

[0031] Prepare a low-concentration aqueous solution of fluorescent carbon dots;

[0032] Immersion method: Immerse the fingerprint carrier in an aqueous solution, let it stand for 1 to 5 minutes, and then remove it;

[0033] Spray method: Pour the prepared aqueous solution into a spray bottle and spray it repeatedly from a distance of 3cm to 10cm from the fingerprint carrier, then let it stand for 1min to 2min.

[0034] Rinse the carrier surface with ultrapure water, dry the carrier with a hair dryer, and irradiate it with a 365nm ultraviolet lamp.

[0035] The fifth objective of this invention is to provide an application of multifunctional coumarin-polyethyleneimine fluorescent carbon dots in optical encryption and fluorescent anti-counterfeiting.

[0036] Prepare a dilute solution of carbon dots to a certain concentration. Dip the tip of an empty pen into the carbon dot solution and write the content or drawing to be encrypted on a prepared white (non-fluorescent) sheet of paper. After writing, let it sit for a while to allow the solution to dry, then observe it under a white light and finally irradiate the paper with a 365nm ultraviolet light to observe the content.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] 1. The method for preparing coumarin-polyethyleneimine fluorescent carbon dots in this invention is simple; neither of the two synthetic raw materials contains toxic metallic elements, is easy to obtain, and is environmentally friendly; the obtained product has high yield, good stability, and adjustable wavelength.

[0039] 2. The fluorescent carbon dots prepared by this invention have uniform morphology and small size, and can be dispersed in water to form a colorless and transparent solution; under ultraviolet light (380nm) excitation, the aqueous solution can emit green fluorescence and the luminescence is stable.

[0040] 3. The coumarin-polyethyleneimine fluorescent carbon dots prepared in this invention are formulated into an aqueous solution and reacted with Fe... 3+ Fluorescence is rapidly quenched upon solution binding. In Fe solutions of 100–1000 μM... 3+ Within the concentration range, the fluorescence intensity of carbon dots is the same as that of Fe. 3+ The concentration shows a linear relationship, with a detection limit of 29.14 μM, exhibiting good selectivity, strong anti-interference ability, and high sensitivity. This carbon dot material can be used for Fe in aqueous solutions. 3+ Ion detection has broad application prospects in the field of environmental monitoring;

[0041] 4. The fluorescent carbon dots provided by this invention employ a green, environmentally friendly, and simple immersion and spraying method to reveal the first to third level structural features of latent fingerprints in a short time. The material usage is small, and it effectively avoids the damage to the human respiratory tract caused by powder methods.

[0042] 5. The fluorescent carbon dots in this invention can also be used for information encryption and fluorescent anti-counterfeiting. By utilizing the characteristic that the fluorescence of the carbon dot solution is quenched under acidic conditions and can be restored by adjusting the solution to alkaline conditions, the encryption and decryption of the designed pattern can be quickly achieved by adjusting the acidity or alkalinity of the substrate. It has broad application prospects in the fields of anti-counterfeiting and information encryption. Attached Figure Description

[0043] Figure 1 This is a high-resolution transmission electron microscope image of the fluorescent carbon dots of this invention.

[0044] Figure 2 This is a particle size distribution diagram of the fluorescent carbon dots of the present invention;

[0045] Figure 3 The XRD pattern of the fluorescent carbon dots of this invention;

[0046] Figure 4 Fourier transform infrared (FTIR) images of fluorescent carbon dots, coumarin, and polyethyleneimine in this invention.

[0047] Figure 5 This is the proton NMR spectrum of the fluorescent carbon dots of this invention;

[0048] Figure 6 The UV-Vis absorption spectrum of the fluorescent carbon dots of this invention;

[0049] Figure 7 The fluorescence emission spectrum of the fluorescent carbon dots of this invention;

[0050] Figure 8 The fluorescence emission spectra of the fluorescent carbon dots of this invention at 380 nm under different pH conditions are shown.

[0051] Figure 9 This is a pH response diagram of the fluorescent carbon dots of the present invention;

[0052] Figure 10 The quenching ability of different metal ions and anions on the fluorescent carbon dots of this invention;

[0053] Figure 11 To assess the anti-interference ability of the fluorescent carbon dots of this invention for iron ion recognition in the presence of different ions;

[0054] Figure 12 The fluorescence emission spectra of the fluorescent carbon dots of the present invention in the presence of different concentrations of iron ions;

[0055] Figure 13The relationship between the fluorescence intensity ratio of the fluorescent carbon dots in this invention and the concentration of iron ions;

[0056] Figure 14 Fluorescent images of latent fingerprints loaded on different substrates at different immersion times;

[0057] Figure 15 Fluorescent images of latent fingerprints on different substrates (spray method);

[0058] Figure 16 A detailed feature map of latent fingerprints on glass (spray method);

[0059] Figure 17 Images showing the encrypted information of the fluorescent carbon dots of this invention under different acid and alkaline conditions. Detailed Implementation

[0060] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0061] Example 1

[0062] A method for preparing multifunctional coumarin-polyethyleneimine fluorescent carbon dots includes the following steps:

[0063] S1: Hydrothermal reaction: 1 part by mass of polyethyleneimine with a molecular weight of 10000 Da and 3 parts by mass of coumarin were dissolved in 20 mL of methanol to obtain a homogeneous solution, which was then transferred to a hydrothermal reactor and reacted at 120 °C for 9 h.

[0064] S2: Centrifugal filtration: After centrifugation, collect the supernatant and filter it through a 0.45μm aqueous filter membrane to remove large particles;

[0065] S3: Dialysis: Dialyze for 24 hours using a dialysis bag with a molecular weight cutoff of 3500 Da;

[0066] S4: Freeze-drying: Fluorescent carbon dots are obtained by freeze-drying.

[0067] The yield was 58.6%, and the fluorescence quantum yield was 5.59%.

[0068] Structural characterization:

[0069] The fluorescent carbon dots of this invention have a graphene-like structure. From... Figure 1The high-resolution transmission electron microscopy (HRTEM) images of the fluorescent carbon dots show that the carbon dots have a spherical morphology, a particle size of approximately 3 nm, a lattice spacing of 0.19 nm, and are uniformly dispersed, corresponding to the (100) crystal plane of graphene. The XRD pattern of the fluorescent carbon dots (…) Figure 3 The fluorescence showed a strong peak near 20°, similar to the diffraction peak observed in graphene at 26.3°. The particle size distribution of the fluorescent carbon dots ranged from 2 nm to 4 nm. Figure 2 The infrared spectrum of the fluorescent carbon dots is shown in [reference needed]. Figure 4 Located at 3305cm -1 The peak at 2960 cm⁻¹ is due to the stretching vibration of the amine and hydroxyl groups, while the peak at 2960 cm⁻¹ is due to the stretching vibration of the amine and hydroxyl groups. -1 and 2885cm -1 The double peak at 1627 cm⁻¹ is likely due to the symmetrical and asymmetrical stretching vibrations of the CH bond. -1 C=O stretching vibration at 1589 cm -1 The C=N stretching vibration peak at 1410 cm⁻¹ jointly proves the formation of the amide bond. -1 The peak at 1104 cm⁻¹ is due to the stretching vibration of -COO⁻. Additionally, the peak at 1104 cm⁻¹... -1 The peak at 767 cm⁻¹ may be related to the stretching vibration of CO, while the peak at 767 cm⁻¹ is related to the stretching vibration of CO. -1 The peak is caused by the bending vibration of the adjacent disubstituted planes of the benzene ring. This result shows that the carbon dots have a significantly different structural composition from the raw materials coumarin and polyethyleneimine, indicating a marked change in their internal structure. The proton NMR spectra of the fluorescent carbon dots are shown below. Figure 5 The broad chemical shift peaks appearing at 7 ppm–8 ppm and 1 ppm–3 ppm are due to the presence of coumarin benzene ring hydrogen and methylene hydrogen in polyethyleneimine in the raw materials. The results indicate that the fluorescent carbon dots have a functional group structure similar to that of the raw materials.

[0070] Fluorescence characteristics:

[0071] The UV-Vis absorption spectrum of fluorescent carbon dots Figure 6 As can be seen, the carbon dot material exhibits ultraviolet absorption peaks at both 278 nm and 380 nm, with the 278 nm absorption peak attributed to the π-π transition of the C=C bond. The fluorescent carbon dots show the strongest fluorescence emission at an excitation wavelength of 380 nm. Figure 7 .

[0072] The fluorescence emission spectra at 380 nm at different pH values ​​are shown below. Figure 8The results showed that the fluorescence emission intensity gradually increased with increasing pH value, reaching its peak under strongly alkaline conditions. Adding NaOH solution enhanced the fluorescence intensity of the carbon dot solution under strongly alkaline conditions. Adding HCl solution to make the solution strongly acidic caused the fluorescence intensity to drop sharply to approximately 60 times its previous level. Further addition of NaOH solution to make the solution strongly alkaline restored the fluorescence intensity to more than 20 times its previous level. Similar results were obtained with repeated experiments. Figure 9 Therefore, it can be seen that after the solution is adjusted to a strongly acidic state, the fluorescence intensity of the carbon dots is significantly weakened; after further adjustment to a strongly alkaline state, the fluorescence intensity of the carbon dots is significantly enhanced, but it cannot be restored to the initial fluorescence intensity.

[0073] Example 2

[0074] A method for preparing multifunctional coumarin-polyethyleneimine fluorescent carbon dots includes the following steps:

[0075] S1: Hydrothermal reaction: 1 part by mass of polyethyleneimine (molecular weight of 25000 Da) and 4 parts by mass of coumarin were dissolved in 20 mL of ethanol to obtain a homogeneous solution, which was then transferred to a hydrothermal reactor and reacted at 90 °C for 24 h.

[0076] S2: Centrifugal filtration: After centrifugation, collect the supernatant and filter it through a 0.4μm aqueous filter membrane to remove large particles;

[0077] S3: Dialysis: Dialyze for 48 hours using a dialysis bag with a molecular weight cutoff of 3000 Da;

[0078] S4: Lyophilization: Lyophilization yielded fluorescent carbon dots. The yield was 21.46%, and the fluorescence quantum yield was 15.22%.

[0079] Example 3

[0080] A method for preparing multifunctional coumarin-polyethyleneimine fluorescent carbon dots includes the following steps:

[0081] S1: Hydrothermal reaction: 1 part by mass of dendritic polyethyleneimine (molecular weight of 10000 Da) and 0.5 parts by mass of coumarin were dissolved in a solvent (10 mL methanol + 10 mL ethanol) to obtain a homogeneous solution, which was then transferred to a hydrothermal reactor and reacted at 240 °C for 3 h.

[0082] S2: Centrifugal filtration: After centrifugation, collect the supernatant and filter it through a 0.5μm aqueous filter membrane to remove large particles;

[0083] S3: Dialysis: Dialyze for 24 hours using a dialysis bag with a molecular weight cutoff of 4000 Da;

[0084] S4: Lyophilization: Lyophilization yielded fluorescent carbon dots. The yield was 4.2%, and the fluorescence quantum yield was 7.32%.

[0085] Example 4

[0086] Applied to Fe 3+ The steps for ion detection are as follows:

[0087] (1) The fluorescent carbon dots prepared in Example 1 were dissolved in water to prepare a 1 mg / mL carbon dot solution. 0.1 mL of the carbon dot solution (1 mg / mL) was placed in a cuvette and added to 2900 μL of ultrapure water as a blank control group. The test was performed at an excitation wavelength of 380 nm, and the maximum fluorescence emission intensity was measured as F0.

[0088] (2) The quenching abilities of different types of metal ions and different anions on carbon dots were studied. 0.1 mL of K+ was added to 0.1 mL of carbon dot solution (1 mg / mL). + Hg 2+ Na + Cr 3+ Ag + Cu 2+ Zn 2+ Ca 2+ Mn 2+ Mg 2+ Fe 3+ Al 3+ SCN - I - F - Cl - NO 3- CH3COO - S 2- SO3 2- The sample consisted of 10 mM GSH aqueous solution and 2800 μL of ultrapure water. Tests were conducted at an excitation wavelength of 380 nm, and the maximum fluorescence emission intensity was measured as F0. x .

[0089] (3) The anti-interference ability of carbon dots to recognize iron ions in the presence of different ions was studied. 0.1 mL of Fe was added to solutions of different ions containing 0.1 mL of carbon dots (1 mg / mL). 3+ Solution (10 mM). Tests were performed at an excitation wavelength of 380 nm, with the maximum fluorescence emission intensity measured as F. A solution containing only UP water was used as a blank sample.

[0090] Test results are as follows Figure 10 As shown, when Fe was added 3+Subsequently, the fluorescence intensity ratio F / F0 changed significantly, with the fluorescence intensity in different ions decreasing markedly and to essentially the same degree. This indicates that the carbon dots exhibit specificity and good selectivity in response to iron ions, and still possess good anti-interference properties in the presence of other ions.

[0091] (4) Add 0.1 mL of Fe with a concentration ranging from 100 to 1000 μM to 0.1 mL of carbon dot solution (1 mg / mL). 3+ The solution and 2800 μL of ultrapure water were tested at an excitation wavelength of 380 nm.

[0092] Test results are as follows Figure 11 As shown, Fe 3+ Cu 2+ Cr 3+ The three metal ions exhibited significant fluorescence quenching effects on carbon dots. Among them, Fe... 3+ The effect of ions is most pronounced, resulting in a relative fluorescence emission intensity ratio below 0.1, demonstrating the influence of carbon dots on Fe. 3+ The ion response exhibits specificity and good selectivity. Figure 12 and Figure 13 It can be seen that with Fe 3+ With increasing ion concentration, the fluorescence intensity of carbon dots gradually decreases. In Fe... 3+ When the concentration range is 100 μM to 650 μM, Fe 3+ The concentration-to-fluorescence-intensity ratio is linearly related, R 2 =0.9936, and the lowest detectable concentration was calculated to be 29.14 μM.

[0093] Example 5

[0094] The fluorescent carbon dots prepared in Example 1 were applied to the development of latent fingerprints, including the following steps:

[0095] (1) Preparation of latent fingerprints: After washing your hands, rub them thoroughly on the oily areas of your forehead to ensure that your fingers have sufficient oil, and then imprint them on a cover glass, coin, plastic bottle cap, aluminum foil, or other carriers.

[0096] (2) Immersion method: Immerse the fingerprint-printed carrier in carbon dot solution, let it stand for 1 to 5 minutes, and then take out the object; Spray method: Put the prepared carbon dot solution into a spray bottle, spray the solvent continuously multiple times from a distance of 3 to 10 cm from the fingerprint-printed carrier, and let it stand for 1 to 2 minutes.

[0097] (3) Rinse the carrier surface with ultrapure water to remove excess carbon dots, and finally dry the carrier with a hair dryer. Irradiate it with a 365nm ultraviolet lamp to clearly see the latent fingerprints on the carrier.

[0098] Figure 14These are fluorescence images of latent fingerprints mounted on different substrates after different immersion times. Under normal illumination, the fingerprint veins are visible to the naked eye in latent fingerprint samples prepared on glass and aluminum foil substrates, while the image is not visible to the naked eye in coins due to their uneven surface. Under 365nm UV light irradiation, the tertiary latent fingerprint image obtained after 1 min of immersion is the clearest. With prolonged immersion time, image clarity decreases, possibly due to the stacking of carbon dot products, resulting in less clear images. The results indicate that latent fingerprints on glass substrates can achieve good imaging results even with a relatively short immersion time (1 min); although the surface of the coin has a large amount of relief, high-quality and clear LFP fluorescence images can still be obtained.

[0099] Figure 15 These are fluorescent images of latent fingerprints on different substrates obtained using the spray method. For easier routine detection, the spray method can produce clearer images of latent fingerprints on glass, aluminum foil, coins, and wood substrates, with results comparable to those obtained using the immersion method. Wood, in particular, is the most common material for fingerprint residue in criminal investigations. Using carbon dots and a simple spray method, latent fingerprints can be visualized, providing a new direction for crime investigation and evidence collection.

[0100] Figure 16 This image shows the detailed features of a latent fingerprint on glass obtained using a spray method. The image clearly displays the primary (spiral), secondary, and tertiary features of the fingerprint (concentric rings, deltas, bifurcations, ridges, islands, and pores). The presentation of these details is crucial for fingerprint identification. Because each person's fingerprint possesses specific tertiary structural features, the carbon dot material we prepared can serve as a safe and harmless ideal material for rapidly revealing fingerprints using the spray method.

[0101] Example 6

[0102] The fluorescent carbon dots prepared in Example 1 were applied to optical encryption and fluorescent anti-counterfeiting.

[0103] The application includes the following steps:

[0104] (1) Prepare a dilute solution of carbon dots with a certain concentration. Dip the tip of a pen that has run out of ink into an appropriate amount of carbon dot solution and write the content or drawing to be encrypted on a pre-prepared white (non-fluorescent) sheet of paper. After writing, let it sit for a while to allow the solution to dry, observe it under a white light, and then observe the content on the paper under a 365nm ultraviolet light.

[0105] (2) To explore the shielding and imaging effects of acid and alkaline solutions on fluorescence, based on the previous step, dip the pen tip into an appropriate amount of a strongly acidic solution prepared with hydrochloric acid and trace the content just written under ultraviolet light. After letting it stand for a period of time, observe it again under white light and ultraviolet light. Finally, under acidic conditions, dip the pen tip into an appropriate amount of a strongly alkaline solution prepared with sodium hydroxide and trace the previous traces while the solution is still not completely dry. After completion, observe the content on the paper under white light and ultraviolet light.

[0106] Figure 17 This section describes the images of encrypted information under different acidic and alkaline conditions, under normal white light and UV 365nm light. We used a prepared carbon dot solution instead of traditional writing ink, drawing ink with a fountain pen and gently designing the encrypted pattern on non-fluorescent white paper. After the water evaporated, the pattern was indistinguishable under natural light, but became clearly visible under 365nm ultraviolet light. Applying a strong acid solution effectively masked the information, and the image no longer appeared under 365nm ultraviolet light. Further application of a strong alkaline solution caused the encrypted information to reappear under 365nm ultraviolet light, thus achieving encryption and decryption of the information.

[0107] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing multifunctional coumarin-polyethyleneimine fluorescent carbon dots, characterized by comprising the following steps: 1) preparing a coumarin-polyethyleneimine fluorescent carbon dot precursor; 2) preparing a multifunctional coumarin-polyethyleneimine fluorescent carbon dot. The method comprises the following steps: S1: hydrothermal reaction: polyethyleneimine and coumarin are dissolved in a solvent to obtain a homogeneous solution, which is transferred to a hydrothermal kettle, the reaction temperature is 90-120℃, and the reaction time is 3-9h, wherein the solvent comprises methanol and / or ethanol; S2: centrifugal filtration: after centrifugation, the supernatant is collected and filtered through a water-based filter membrane with a pore size of 0.4-0.5μm to remove large particles; S3: dialysis: dialysis is performed for 24-48h using a dialysis bag with a molecular weight cut-off of 3000-4000Da; S4: freeze-drying: freeze-drying is performed to obtain fluorescent carbon dots.

2. The method according to claim 1, wherein the method comprises the following steps: 1) synthesizing a multifunctional coumarin-polyethyleneimine fluorescent carbon dot by a hydrothermal method. In the S1 step, the mass ratio of coumarin to polyethyleneimine is 0.1-4:

1. 3.The method according to claim 2, characterized in that: In the S1 step, the mass ratio of coumarin to polyethyleneimine is 0.5-4:

1.

4. The method according to claim 1, wherein the method comprises the following steps: 1) synthesizing a multifunctional coumarin-polyethyleneimine fluorescent carbon dot by a hydrothermal method. The polyethyleneimine is dendritic polyethyleneimine with a molecular weight of 10000-25000Da.

5. The multifunctional coumarin-polyethyleneimine fluorescent carbon dots are prepared by the method of claim 1.

6. The multifunctional coumarin-polyethyleneimine fluorescent carbon dots of claim 5 for application in Fe 3+ ion detection.

7. The multifunctional coumarin-polyethyleneimine fluorescent carbon dots are used for developing latent fingerprints.

8. The multifunctional coumarin-polyethyleneimine fluorescent carbon dots are used for optical encryption and fluorescent anti-counterfeiting.

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