Visible light excited long lifetime phosphor carbon dot film and preparation method and application thereof

Abstract

The application belongs to the field of phosphorescent materials and information encryption technology, and particularly relates to a visible light excited long lifetime phosphorescent carbon dot film and a preparation method and application thereof. In order to solve the problems that the phosphorescent material cannot be excited by visible light and the phosphorescent lifetime is short in the prior art, polyvinyl alcohol is used as a carbon source precursor, anhydrous ethylenediamine is used as a crosslinking agent, and after a reaction solvent is added, the nitrogen-doped carbon dots (N-CDs) are formed by reaction under high temperature conditions, and the N-CDs are added into a polyvinyl alcohol aqueous solution to prepare the visible light excited long lifetime phosphorescent carbon dot film. The visible light excited long lifetime phosphorescent carbon dot film (N-CDs@PVA film) provided by the application can be excited by visible light and has a tunable lifetime, and compared with organic metal complex materials, has the advantages of simplicity, rapidness, metal-free, long lifetime, economic effectiveness, low toxicity and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of phosphorescent materials and information encryption technology, and particularly relates to a visible light excited long lifetime phosphorescent carbon dot film and a preparation method and application thereof. BACKGROUND

[0002] The information disclosed in this Background section is for the purpose of increasing the understanding of the general background of the application and does not necessarily constitute an admission or a suggestion that the information forms part of the prior art already known in this field.

[0003] Long lifetime room temperature phosphorescence (RTP) is a fascinating optical phenomenon caused by the accumulation of excitation energy and gradual emission through triplet state, which can last for several seconds, minutes or even hours after the excitation light source is stopped. In the early stage, phosphorescence was only visible at low temperatures, which severely limited the practical application of phosphorescence. Subsequently, a large number of RTP materials have been studied, which have attracted people's interest due to their sustained luminescence, considerable Stokes shift and high signal-to-noise ratio, making them have high flexibility in different fields such as decorative jewelry, emergency signs, optoelectronic devices, biological imaging, digital encryption, etc.

[0004] Among various RTP materials, organic RTP systems have become a hot research field and have made rapid progress in just a few years. So far, a number of pioneering methods related to organic RTP materials have been published, some of which exhibit different chromaticity, improved quantum yield and prolonged lifetime. However, due to the limited absorption band, most organic RTP systems require the use of high-energy ultraviolet (UV) excitation illumination with a wavelength lower than 380 nm to excite electrons from the ground state to the excited state. High-energy UV sources can pose a significant risk to humans, and their strict standards limit their widespread use in civilian environments. In contrast, visible light is less toxic to human health, enhances the ability to analyze and image deep into biological samples, and provides greater convenience for practical applications. Although some existing technologies disclose that organic RTP phosphors can be excited by visible light, their decay lifetime is still limited to the millisecond range. In addition, the optical instability of organic RTP materials, the large amount of reagent expenditure and the related drawbacks of strict synthesis requirements inevitably bring difficulties to researchers. SUMMARY

[0005] In order to solve the problems of the prior art, the present application aims to provide a visible light excited long lifetime phosphorescent carbon dot film and a preparation method and application thereof. The present application uses polyvinyl alcohol as a carbon source precursor, anhydrous ethylenediamine as a crosslinking agent, and after adding a reaction solvent, the nitrogen-doped carbon dots (N-CDs) are formed under high temperature conditions. The N-CDs are added to a polyvinyl alcohol aqueous solution to prepare a visible light excited long lifetime phosphorescent carbon dot film. The visible light excited long lifetime phosphorescent carbon dot film (N-CDs@PVA film) provided by the present application can be excited by visible light and has a tunable lifetime. Compared with organic metal complex materials, the visible light excited long lifetime phosphorescent carbon dot film has the advantages of simplicity, rapidness, metal-free, long lifetime, economic efficiency, low toxicity, etc.

[0006] In order to achieve the above-mentioned purpose, the present application is realized by the following technical scheme:

[0007] In a first aspect, the present application provides a preparation method of a visible light excited long lifetime phosphorescent carbon dot film, comprising the following steps:

[0008] S1, polyvinyl alcohol and anhydrous ethylenediamine are dissolved in deionized water, and a hydrothermal reaction is carried out to obtain a brown suspension. After adding acetone to the brown suspension, the precipitate is removed by centrifugation. The transparent solution obtained by centrifugation is further purified with a dialysis membrane in deionized water to obtain a nitrogen-doped carbon dot solution;

[0009] S2, the nitrogen-doped carbon dot solution is added to a polyvinyl alcohol solution, and the mixed solution after ultrasonic treatment is heated to obtain the visible light excited long lifetime phosphorescent carbon dot film.

[0010] In a second aspect, the present application provides a visible light excited long lifetime phosphorescent carbon dot film, which is obtained by the preparation method of the visible light excited long lifetime phosphorescent carbon dot film according to the first aspect.

[0011] In a third aspect, the present application provides an application of the visible light excited long lifetime phosphorescent carbon dot film according to the second aspect in the field of anti-fake security and / or the field of optical information storage.

[0012] The above-mentioned one or more technical solutions of the present application have the following beneficial effects:

[0013] 1. The visible light excited long lifetime phosphorescent carbon dot film (N-CDs@PVA film) provided by the present application shows a fluorescence emission peak at 496 nm (lifetime of 4.59 ns) when excited at 399 nm. The N-CDs@PVA film shows a phosphorescence emission maximum at 558 nm under the best excitation at 417 nm. The phosphorescence lifetime of the N-CDs@PVA film can be adjusted from 0.12 seconds to 2.1 seconds by changing the excitation wavelength. When excited at 420 nm, the phosphorescence lifetime of 2.1 seconds is much longer than the lifetime of the existing visible light excited RTP system.

[0014] 2.The preparation of the visible light excited long-lifetime phosphorescent carbon dot film of the present application uses polyvinyl alcohol as a carbon source precursor, anhydrous ethylenediamine as a crosslinking agent, and water as a solvent.The preparation process is simple, fast, easy to operate, and has high yield;the preparation process does not require complex and expensive equipment, has low cost, and does not produce other harmful substances in the preparation process, and is easy to realize industrialized production. BRIEF DESCRIPTION OF DRAWINGS

[0015] The drawings accompanying the specification of the present application serve to provide further understanding of the present application, and the illustrative embodiments of the present application and the description thereof serve to explain the present application, and do not constitute an improper limitation on the present application.

[0016] Figure 1 Normalized excitation (Ex) and emission (Em) spectra of the PL spectrum of the nitrogen-doped carbon dot solution prepared in Example 1;

[0017] Figure 2 Map spectrum of the nitrogen-doped carbon dot solution prepared in Example 1;

[0018] Figure 3 PL decay curve of the nitrogen-doped carbon dot solution prepared in Example 1 at an excitation wavelength of 394 nm;

[0019] Figure 4 Normalized absorption spectra of CDs@PVA prepared in Comparative Example 1 and N-CDs@PVA prepared in Example 1.

[0020] Figure 5 Normalized excitation (FL (ex)) and emission (FL) spectra of the fluorescence and normalized excitation (RTP (ex)) and emission (RTP) spectra of the phosphorescence of N-CDs@PVA prepared in Example 1 under ambient conditions;

[0021] Figure 6 Fluorescence decay curve of N-CDs@PVA prepared in Example 1 at an excitation wavelength of 399 nm.

[0022] Figure 7 Phosphorescence decay curve of N-CDs@PVA prepared in Example 1 under different excitation wavelengths.

[0023] Figure 8 Normalized absorption (Abs), RTP excitation (RTP (ex)) spectra of N-CDs@PVA prepared in Example 1 and PL spectrum of a WLED lamp (WLED);

[0024] Figure 9 Curve of the RTP intensity of N-CDs@PVA prepared in Example 1 under 420 nm excitation with respect to time and variable aperture.

[0025] Figure 10 RTP intensity of N-CDs@PVA prepared for Example 1 under 420 nm excitation as a function of time and excitation time;

[0026] Figure 11 PL and RTP intensity of N-CDs@PVA prepared for Example 1 under different atmospheres;

[0027] Figure 12 Transmission electron microscope image of N-CDs prepared for Example 1;

[0028] Figure 13 AFM image of N-CDs prepared for Example 1;

[0029] Figure 14 XRD image of N-CDs prepared for Example 1;

[0030] Figure 15 Infrared spectra of N-CDs and N-CDs@PVA prepared for Example 1;

[0031] Figure 16 Full spectra and high resolution C1s, N 1s and O1s spectra of X-ray photoelectron spectroscopy of N-CDs and N-CDs@PVA prepared for Example 1;

[0032] Figure 17 Differential scanning calorimetry (DSC) curves of N-CDs and N-CDs@PVA prepared for Example 1;

[0033] Figure 18 EPR spectra of N-CDs and N-CDs@PVA prepared for Example 1 after irradiation;

[0034] Figure 19 DFT calculated energy levels, NTOs and SOC constants diagram of N-CDs and N-CDs@PVA prepared for Example 1 in simplified system;

[0035] Figure 20 Photos of applications of N-CDs and N-CDs@PVA prepared for Example 1 in the field of anti-counterfeiting security, wherein (a) is the application of N-CDs@PVA composition in the field of anti-counterfeiting security, (b) is the application of N-CDs@PVA dyed yarn in the field of anti-counterfeiting security, (c) is the application of the pattern woven by N-CDs@PVA dyed yarn in the field of anti-counterfeiting security, and (d) is the application of the pattern woven by N-CDs@PVA dyed yarn in the field of anti-counterfeiting security;

[0036] Figure 21The application photos of N-CDs and N-CDs@PVA prepared in Example 1 in the field of optical information storage. DETAILED DESCRIPTION

[0037] In a first typical embodiment of the present application, a preparation method of a visible light excited long lifetime phosphorescent carbon dot film comprises the following steps:

[0038] S1, dissolving polyvinyl alcohol and anhydrous ethylenediamine in deionized water to perform a hydrothermal reaction to obtain a brown suspension, adding acetone to the brown suspension and removing the precipitate by centrifugation, further purifying the transparent solution obtained by centrifugation with a dialysis membrane in deionized water to obtain a nitrogen-doped carbon dot solution;

[0039] S2, adding the nitrogen-doped carbon dot solution to a polyvinyl alcohol solution, heating the mixed solution after ultrasonic treatment to obtain the visible light excited long lifetime phosphorescent carbon dot film.

[0040] In the present application, polyvinyl alcohol is used as a carbon source precursor, anhydrous ethylenediamine is used as a crosslinking agent, and the reaction solvent is added to react under high temperature conditions to form the nitrogen-doped carbon dots (N-CDs). The N-CDs are added to a polyvinyl alcohol aqueous solution to prepare the visible light excited long lifetime phosphorescent carbon dot film. Changing the precursor will affect the generation of the crosslinked network structure, thereby reducing the phosphorescent decay lifetime and the delay time at room temperature. The research of the present application finds that the reaction solvent deionized water is very important for preparing the long lifetime phosphorescent carbon dot film. If the reaction solvent is changed to other solvents such as methanol, ethanol, acetone, dimethyl sulfoxide, N,N-dimethylformamide, etc., and other reaction conditions remain unchanged, the long lifetime phosphorescent carbon dot film cannot be obtained.

[0041] In one or more embodiments of the present embodiment, in step S1, the volume ratio of anhydrous ethylenediamine to water is 1:19-21, and the ratio of anhydrous ethylenediamine to polyvinyl alcohol is 1 mL:1 g.

[0042] In one or more embodiments of the present embodiment, in step S1, the temperature of the hydrothermal reaction is 219-221℃, and the time of the hydrothermal reaction is 5.9-6.1h.

[0043] If the temperature is too high, the carbonization is serious and the phosphorescence is weak. If the temperature is too low, the raw materials are difficult to carbonize, and the yield of nitrogen-doped carbon dots is low. Too short or too long reaction time will affect the generation of the crosslinked network structure, thereby reducing the phosphorescent decay lifetime and the delay time at room temperature.

[0044] In one or more embodiments of the present embodiment, in step S1, the ratio of ethylenediamine to polyvinyl alcohol in the polyvinyl alcohol solution in step S2 is 1 mL:8.9-9.1 g.

[0045] In one or more embodiments of this embodiment, in step S2, the ultrasonic time is 0.9-1.1h.

[0046] In one or more embodiments of this embodiment, in step S2, the heating temperature is 59-61℃, and the heating time is 1.9-2.1h.

[0047] In one or more embodiments of this embodiment, the alcoholysis degree of the polyvinyl alcohol is 78.5-81.5mol%, and the viscosity is 45.0-51.0mPa.s.

[0048] The second typical embodiment of the present application is a visible light excited long-lifetime phosphorescent carbon dot film, which is obtained by the preparation method of the visible light excited long-lifetime phosphorescent carbon dot film as described in the first typical embodiment.

[0049] In one or more embodiments of this embodiment, the visible light excited long-lifetime phosphorescent carbon dot film contains nitrogen-doped carbon dots with a particle size of 2.28-3.16nm.

[0050] The third typical embodiment of the present application is the application of the visible light excited long-lifetime phosphorescent carbon dot film as described in the second typical embodiment in the field of anti-counterfeiting security and / or the field of optical information storage.

[0051] In order to enable those skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below with specific examples and comparative examples.

[0052] Example 1

[0053] 1g of PVA was dissolved in 20mL of deionized water, and then mixed with 1.0mL of EDA. The mixed solution was transferred to a 50mL polytetrafluoroethylene liner, and the liner was loaded into a reaction kettle and heated at 220℃ for 6h. After the reaction was completed, the reaction kettle was naturally cooled to room temperature, the liner was taken out, and a certain amount of acetone was added to the obtained brown suspension to precipitate the excess PVA. The precipitate was removed by centrifugation (15000rpm, 15min), and the obtained transparent solution was further purified with a dialysis membrane in deionized water. The purified solution is a nitrogen-doped carbon dot (N-CDs) solution. 9g of polyvinyl alcohol was dissolved in 300ml of deionized water at 90℃ for 3h. After filtration, a PVA aqueous solution (30mg / ml) was obtained. N-CDs were added to the PVA aqueous solution to prepare an N-CDs@PVA solution. After 1h of ultrasonic treatment, a uniform solution was prepared, and finally a visible light excited long-lifetime phosphorescent carbon dot film was prepared by heating in a vacuum oven at 60℃ for 2h, which is denoted as N-CDs@PVA.

[0054] Comparative Example 1

[0055] A 1 g PVA was dissolved in 20 mL deionized water, and the solution was transferred into a 50 mL polytetrafluoroethylene liner, which was loaded into a reactor and heated at 220 °C for 6 h. After the reaction was completed, the reactor was naturally cooled to room temperature, and the liner was removed. A certain amount of acetone was added to the obtained brown suspension to precipitate the excess PVA. The precipitate was removed by centrifugation (15000 rpm, 15 min), and the obtained transparent solution was further purified with a dialysis membrane in deionized water for 3 days. The purified solution was the carbon dot (CDs) solution. A 9 g polyvinyl alcohol was dissolved in 300 mL deionized water at 90 °C for 3 h. After filtration, a PVA aqueous solution (30 mg / mL) was obtained. The N-CDs were added to the PVA aqueous solution to prepare a CDs@PVA solution. After ultrasonic treatment for 1 h, a uniform solution was prepared, and finally a visible light excited long lifetime phosphorescent carbon dot film was prepared by heating in a vacuum oven at 60 °C for 2 h, which was denoted as CDs@PVA.

[0056] As shown in Figure 1 , the nitrogen-doped carbon dot solution prepared in Example 1 emitted bright blue fluorescence under an ultraviolet lamp (365 nm), and the maximum emission peak was 476 nm at the optimal excitation wavelength of 394 nm. The emission Map spectrum of the nitrogen-doped carbon dot solution at different excitation wavelengths is shown in Figure 2 . As shown in Figure 3 , the fluorescence decay lifetime of the nitrogen-doped carbon dot solution was up to 4.65 ns.

[0057] As shown in Figure 4 and 8 , under ultraviolet light excitation, N-CDs@PVA showed a significant absorption peak centered at 423 nm, accompanied by a wide absorption range of 230 to 700 nm, which allowed effective excitation of the triplet state in the visible light range. However, in the undoped CDs, the absorption band was mainly in the wavelength range of 230 to 600 nm, and the absorption peak showed a clear blue shift trend compared with N-CDs. As shown in Figure 5 and 6 , N-CDs@PVA showed a PL emission peak at 496 nm (with a lifetime of 4.59 ns) when excited at 399 nm, which was similar to N-CDs. The RTP spectral distribution of N-CDs@PVA was dependent on the excitation wavelength, and the emission maximum was located at 558 nm under the optimal excitation of 417 nm. As shown in Figure 7 , the dynamic phosphorescence lifetime of the N-CDs@PVA film could be adjusted from 0.12 s to 2.1 s by changing the excitation wavelength. As shown in Figure 9 and 10As shown, the RTP luminescence intensity increases when the visible light (420nm) excitation aperture is increased from 20% to 100%. Furthermore, for N-CDs@PVA films, an ultralong RTP can be achieved with a rapid excitation time of only 2 seconds. Figure 11 As shown, the nearly identical phosphorescence intensities under nitrogen and air conditions indicate that the PVA matrix can effectively isolate and protect the RTP of N-CDs from oxygen quenching.

[0058] like Figure 12 As shown, N-CDs have a lattice band gap of 0.21 nm. Figure 13 As shown, the average particle size of the N-CDs is 2.57 nm. Figure 14 As shown, the X-ray diffraction pattern of N-CDs exhibits a broad peak at 26.5°, indicating that it is a carbon material, which corresponds to the lattice band gap observed in the TEM. Figures 15-16 As shown, infrared spectroscopy and X-ray photoelectron spectroscopy results indicate that N-CDs@PVA is mainly composed of four elements: C, H, O, and N, and contains C-C bonds, C=C bonds, CO bonds, CN bonds, CH bonds, OH bonds, and NH bonds. Figure 17 As shown, the glass transition temperature (T) of N-CDs@PVA thin films g The temperature (112.8℃) is significantly higher than that of the original N-CDs (77.5℃), indicating that the N-CDs@PVA film can create a rigid environment to reduce thermal motion and triplet deactivation caused by hydrogen bonding between the N-CDs and the PVA matrix. Figure 18 As shown, compared to the N-CDs@PVA film, the original N-CDs exhibit a higher electron paramagnetic resonance (EPR) peak at a lower magnetic field, suggesting that intermolecular hydrogen bonding interactions may hinder electron transfer between triplet excitons and triplet oxygen in the N-CDs@PVA film. Figure 19 As shown, the simplified N-CDs structure and the N-CDs@PVA system's natural transition orbitals (NTOs) topology and energy level diagrams of related photophysical processes are presented. DFT calculations show that the N-CDs and N-CDs@PVA systems possess two effective intersystem crossover (ISC) channels (S1→T2, T3), and the band gap (Δ) between the singlet and triplet states is significant. EST The value is very small. After embedding PVA, the spin-orbit coupling (SOC) constants ξ(S1→T2, T3) of N-CDs decrease from 0.430 and 0.119 cm⁻¹, respectively. -1 Significantly increased to 1.244 and 1.460 cm. -1 .

[0059] like Figure 20As shown in Fig. 4a, a "butterfly" pattern was fabricated on N-CDs@PVA film using Chinese paper-cutting technology. When exposed to 365 nm UV light, the pattern showed strong blue fluorescence, followed by green afterglow after the light excitation was stopped. Notably, when the WLED light irradiation was turned off, the "butterfly" pattern emitted bright yellow afterglow, which could be easily recognized by the naked eye under ambient conditions. As shown in Fig. 4b, the N-CDs@PVA solution was applied as anti-counterfeiting ink on the five-pointed star component of a paper folding artwork. When exposed to UV light, this complex artwork, featuring a blue five-pointed star and a red cork base, was revealed. After the UV light source was removed, the blue five-pointed star turned green, while the cork base became invisible. However, once the WLED light was turned off, the green five-pointed star turned yellow. As shown in Fig. 4c, the cotton yarn passed through a coating tank containing N-CDs@PVA solution, resulting in functional cotton yarn of RTP color, which could be used to produce anti-counterfeiting knitted products in cross-stitch technology. As shown in Fig. 4d, the yarns were used to embroider the logo "QILU INDUSTRIAL UNIVERSITY" on an experimental garment. Under UV irradiation, the logo appeared blue, and after the UV light or WLED excitation light was turned off, the green or yellow afterglow of the logo could be seen by the naked eye. As shown in Fig. 4e, the yarns were used to make an embroidery security pattern that showed green and yellow afterglow under UV and WLED lamps. Figure 20 Figure 20 Figure 20 Figure 20

[0060] As shown in Fig. 5a, information storage based on Morse code was achieved by using N-CDs@PVA to represent "dots" or "lines". Figure 21

[0061] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Those skilled in the art can make various modifications and changes to the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.​​​​​

Claims

1. A method for preparing a long-lifetime phosphorescent carbon dot thin film excited by visible light, characterized in that, Includes the following steps: S1. Polyvinyl alcohol and anhydrous ethylenediamine are dissolved in deionized water and subjected to a hydrothermal reaction to obtain a brown suspension. Acetone is added to the brown suspension and the precipitate is removed by centrifugation. The transparent solution obtained by centrifugation is further purified in deionized water using a dialysis membrane to obtain a nitrogen-doped carbon dot solution. S2. Add the nitrogen-doped carbon dot solution to the polyvinyl alcohol solution, and heat the ultrasonically mixed solution to obtain the visible light-excited long-life phosphorescent carbon dot film. In step S1, the volume ratio of anhydrous ethylenediamine to water is 1:20, and the ratio of anhydrous ethylenediamine to polyvinyl alcohol is 1 mL:1 g; in step S1, the temperature of the hydrothermal reaction is 219-221 ℃, and the time of the hydrothermal reaction is 5.9-6.1 h.

2. The preparation method according to claim 1, characterized in that, In step S1, the ratio of ethylenediamine to polyvinyl alcohol in the polyvinyl alcohol solution in step S2 is 1 mL: 8.9-9.1 g.

3. The preparation method according to claim 1, characterized in that, In step S2, the ultrasound duration is 0.9-1.1 hours.

4. The preparation method according to claim 1, characterized in that, In step S2, the heating temperature is 59-61 ℃, and the heating time is 1.9-2.1 h.

5. The preparation method according to claim 1, characterized in that, The degree of alcoholysis of the polyvinyl alcohol is 78.5-81.5 mol%, and the viscosity is 45.0-51.0 mPa·s.

6. A long-lifetime phosphorescent carbon dot thin film excited by visible light, characterized in that, It is obtained by the visible light-excited long-life phosphorescent carbon dot film preparation method as described in any one of claims 1-5.

7. The visible light-excited long-lifetime phosphorescent carbon dot thin film as described in claim 6, characterized in that, The visible light-excited long-life phosphorescent carbon dot film contains nitrogen-doped carbon dots with a particle size of 2.28-3.16 nm.

8. The application of the visible light-excited long-life phosphorescent carbon dot film as described in claim 6 or 7 in the fields of anti-counterfeiting security and / or optical information storage.

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