Preparation method and application of multicolor room-temperature phosphorescent carbon dots in water environment

Abstract

The invention relates to the technical field of nano materials, in particular to a preparation method and application of multicolor room-temperature phosphorescent carbon dots in a water environment, and provides a universal and simple method for synthesizing multicolor RTP CDs with ultra-long service life in the water environment by adjusting the conjugation degree and reabsorption radiation energy transfer. Blue RTP CDs (B-CDs (at) g-C3N4) with an ultra-long service life of 1.37 seconds in a water environment and green RTP CDs (G-CDs (at) CA) with a service life of up to 1.70 seconds are prepared by one-step pyrolysis of a mixture of arylboronic acid and urea / biuret with different conjugation degrees. A reabsorption radiation energy transfer system between G-CDs (at) CA and RhB is constructed, and the system shows red afterglow as long as 1.45 seconds in a water environment. The B-CDs (at) g-C3N4, the G-CDs (at) CA and the G-CDs (at) CA (at) RhB have ultra-long phosphorescence lifetime, good stability and multicolor RTP characteristics, and show huge potential in the aspects of information encryption and delayed illumination application. The strategy provides precious reference for synthesizing the color-adjustable super-long-life RTP material in the water environment, and the application range of the RTP material is expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and in particular to a method for preparing multicolor room temperature phosphorescent carbon dots in an aqueous environment and applications thereof. Background Art

[0002] Due to their unique optical properties, including long lifetime, large Stokes shift, and lack of background fluorescence interference, RTP materials have shown broad application prospects in delayed illumination, bioimaging, sensors, information anti-counterfeiting and other fields. Although traditional RTP materials are mainly divided into organometallic complexes and metal-free organic compounds, they generally have some problems, such as the need to introduce precious metals, high toxicity, high cost, and complex synthesis and purification processes. In addition, these materials are easily quenched by dissolved oxygen and solvent relaxation in aqueous environments, which seriously limits their application in aqueous phases. Therefore, there is an urgent need to develop a new RTP material that is not only easy to prepare, low in cost, and low in toxicity, but also can maintain excellent optical properties in aqueous environments.

[0003] Carbon dots (CDs), as an emerging carbon nanomaterial, offer numerous advantages, including simple synthesis, tunable optical properties, high quantum yield, excellent optical stability, good biocompatibility, environmental friendliness, and low cost. However, CDs typically exhibit weak phosphorescence and short lifetimes due to weak spin-orbit coupling and low intersystem crossing (ISC) efficiency. Currently, two main strategies are used to enhance the RTP performance of CDs: one is to embed CDs into rigid matrices (such as melamine, PVA, PMMA, SiO2, MOFs, polyurethane, cyanuric acid (CA), urea, LDHs, B2O3, HOFs, and inorganic metal salts) to create a rigid microenvironment that isolates oxygen and suppresses molecular vibrations and rotations, thereby inhibiting nonradiative transitions. The other approach is to enhance spin-orbit coupling by doping with heteroatoms (such as P, N, O, Cl, S, F, B, Na, etc.) and aromatic carbonyl groups, thereby improving the intersystem crossing (ISC) efficiency from the singlet to triplet state. As an emerging material with unique luminescence properties, RTP carbon dots (RTP) have found widespread application in fields such as information security and encryption, sensors, bioimaging, fingerprint detection, and light-emitting diodes. Despite recent advances in room-temperature phosphorescent carbon dots (RTP-CDs), most RTP-CDs are limited to blue or green emission, and reports on RTP-CDs with excellent optical properties in aqueous environments are limited. Therefore, the preparation of multicolor RTP-CDs with excellent optical properties in aqueous environments remains challenging. Summary of the Invention

[0004] The present invention aims to solve the above technical problems and proposes a method for preparing multicolor room temperature phosphorescent carbon dots in an aqueous environment and its application.

[0005] The technical solution of the present invention is: In the first aspect, a method for preparing multicolor room-temperature phosphorescent carbon dots in an aqueous environment is characterized in that multicolor room-temperature phosphorescent carbon dots with ultra-long lifespan are synthesized by pyrolyzing a mixture of aryl boronic acids with different degrees of conjugation and urea or biuret; the aryl boronic acids include 4-carbamoylphenylboronic acid and 9-phenanthreneboronic acid; the pyrolysis process includes heating the mixture at a high temperature to embed the carbon dots into a rigid matrix generated by urea or biuret, forming a rigid microenvironment that isolates dissolved oxygen, thereby enhancing the room-temperature phosphorescence performance.

[0006] Preferably, the specific steps of the pyrolysis process are: mixing arylboronic acid and urea or biuret with water, ultrasonically treating, heating in an oven at 200° C. for 3 hours, and cooling to obtain the room temperature phosphorescent carbon dots.

[0007] Preferably, the rigid matrix is g-C3N4 or cyanuric acid.

[0008] Preferably, the color of the room temperature phosphorescent carbon dots can be tuned by adjusting the conjugation degree of the arylboronic acid.

[0009] Preferably, the room temperature phosphorescent carbon dots include blue room temperature phosphorescent carbon dots and green room temperature phosphorescent carbon dots, and the phosphorescence lifetimes thereof in an aqueous environment can reach 1.37 seconds and 1.70 seconds, respectively.

[0010] Furthermore, through reabsorption of radiation energy transfer, green room-temperature phosphorescent carbon dots were used as energy donors and rhodamine B as energy acceptors to prepare red room-temperature phosphorescent materials, whose phosphorescence lifetime in an aqueous environment can reach 1.45 seconds.

[0011] Preferably, the room temperature phosphorescent carbon dots B-CDs@g-C3N4, G-CDs@CA and G-CDs@CA@RhB are used in information encryption, anti-counterfeiting and delayed lighting.

[0012] In the second aspect, an application of the room temperature phosphorescent carbon dots prepared in the first aspect in an aqueous environment.

[0013] Compared with the prior art, the present invention has the following beneficial effects: Here, we synthesize multicolor RTP carbon dots (Cds) with exceptionally long lifetimes in aqueous environments via the pyrolysis of arylboronic acid and urea / biuret mixtures. The color of the RTP CDs is tunable by adjusting the degree of conjugation of the precursor arylboronic acid and the energy transfer of reabsorbed radiation. B-CDs@g-C3N4 were synthesized via the high-temperature pyrolysis of 4-carbamoylphenylboronic acid (4-CPBA) and urea, while G-CDs@CA were produced via the pyrolysis of 9-phenanthreneboronic acid (Phe9-B) and biuret.

[0014] This excellent RTP performance is attributed to multiple effects, including a rigid environment and hydrogen bonding interactions, which effectively suppress molecular vibrations and isolate dissolved oxygen. Furthermore, the doping of N, B, and O promotes intersystem crossing (ISC). The phosphorescence emission wavelength can be further tuned by adjusting the degree of conjugation of the precursor.

[0015] As the number of benzene rings in the arylboronic acid increases, the optical band gap of the prepared carbon dots gradually narrows, resulting in a red shift in the RTP emission, thereby exhibiting excellent RTP performance in the blue to green range. Further research found that urea generates g-C3N4 at high temperature, while biuret generates cyanuric acid (CA) through thermal condensation.

[0016] During the pyrolysis process, carbon dots are in situ embedded in the g-C3N4 and cyanuric acid (CA) matrix, creating a rigid microenvironment that isolates dissolved oxygen and suppresses nonradiative transitions, resulting in ultralong phosphorescence lifetimes for B-CDs@g-C3N4 and G-CDs@CA in aqueous environments. Using G-CDs@CA as an energy donor and rhodamine B (RhB) as an energy acceptor, red-afterglow G-CDs@CA@RhB was prepared via reabsorption radiative energy transfer between G-CDs@CA and RhB. The resulting red afterglow lasted up to 1.45 seconds in aqueous environments. Based on these excellent properties, B-CDs@g-C3N4, G-CDs@CA, and G-CDs@CA@RhB have been successfully applied in information encryption and multicolor light-emitting diodes, showing broad application prospects in information security and delayed lighting. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0018] Figure 1 Schematic diagram of the synthesis process of B-CDs@g-C3N4 and G-CDs@CA in this application, as well as a schematic diagram of regulating the afterglow performance through reabsorption radiation energy transfer between G-CDs@CA and rhodamine B (RhB); Figure 2(a) Transmission electron microscopy image of B-CDs@g-C3N4 (inset: particle size distribution histogram of B-CDs@g-C3N4); (b) high-resolution transmission electron microscopy image of B-CDs@g-C3N4; (c) X-ray diffraction pattern of B-CDs@g-C3N4; (d) transmission electron microscopy image of G-CDs@CA (inset: particle size distribution histogram of G-CDs@CA); (e) high-resolution transmission electron microscopy image of G-CDs@CA; (f) X-ray diffraction patterns of G-CDs@CA and CA; Figure 3 (a) Enlarged XRD patterns of G-CDs@CA and CA; (b) Schematic diagram of the hydrogen bond framework formed between G-CDs and CA; Figure 4 (a) Fourier transform infrared spectra (FT-IR) of B-CDs@g-C3N4 and G-CDs@CA of this application; (bc) full X-ray photoelectron spectroscopy (XPS) spectra of B-CDs@g-C3N4 and G-CDs@CA; (df) high-resolution XPS spectra of C 1s, N 1s, and O 1s of B-CDs@g-C3N4; (gi) high-resolution XPS spectra of C 1s, N 1s, and O 1s of G-CDs@CA; Figure 5 High-resolution B 1s XPS spectra of (ab) B-CDs@g-C3N4 and G-CDs@CA of this application; Figure 6 UV-visible absorption spectra, optimal fluorescence and phosphorescence excitation spectra of the (ab) B-CDs@g-C3N4 and G-CDs@CA solid powders of this application; Figure 7 Optimal fluorescence and phosphorescence emission spectra of the solid-state B-CDs@g-C3N4 (a) and G-CDs@CA (b) of this application; (c) time-resolved phosphorescence spectra of solid-state B-CDs@g-C3N4 and G-CDs@CA; (d) fluorescence and phosphorescence photos of solid-state B-CDs@g-C3N4 and G-CDs@CA when the UV light is turned on and off; Figure 8 The absolute quantum yields of (ab) B-CDs@g-C3N4 and G-CDs@CA solid powders of this application; Figure 9 The CIE color coordinates corresponding to the phosphorescence emission spectra of B-CDs@g-C3N4 and G-CDs@CA of this application; Figure 10 Density functional theory calculation results of the precursors 4-CPBA and Phe9-B of this application; Figure 11 Comparison of the optical band gaps corresponding to the phosphorescence emission spectra of B-CDs@g-C3N4 and G-CDs@CA; Figure 12 Optimal fluorescence and phosphorescence emission spectra of aqueous solutions of B-CDs@g-C3N4 (a) and G-CDs@CA (b) in this application; (c) time-resolved phosphorescence spectra of B-CDs@g-C3N4 and G-CDs@CA in aqueous environment; (d) fluorescence and phosphorescence photos of B-CDs@g-C3N4 and G-CDs@CA in aqueous environment with UV light turned on and off.

[0019] Figure 13 (ab) Phosphorescence emission spectra of B-CDs@g-C3N4 and G-CDs@CA at different temperatures; (cd) Effect of temperature on the phosphorescence intensity of B-CDs@g-C3N4 and G-CDs@CA; Figure 14 The changes in fluorescence intensity of (ab) B-CDs@g-C3N4 and G-CDs@CA of this application under continuous irradiation with UV light for one hour; Figure 15 The changes in phosphorescence intensity of (ab) B-CDs@g-C3N4 and G-CDs@CA of this application under continuous irradiation with UV light for one hour; Figure 16 (ab) Comparison of the phosphorescence emission spectra of B-CDs@g-C3N4 and G-CDs@CA before and after storage in air for one month for this application; Figure 17 (a) UV-visible absorption spectrum of RhB and phosphorescence emission spectrum of G-CDs@CA; (b) phosphorescence emission spectra of G-CDs@CA and G-CDs@CA@RhB, and fluorescence emission spectrum of RhB; (c) normalized phosphorescence emission spectra of G-CDs@CA with different volumes of 5 mM RhB added; (d) fluorescence and phosphorescence photographs of G-CDs@CA with different volumes of 5 mM RhB added when 310 nm UV light is turned on and off; (e) schematic diagram of the reabsorption radiation energy transfer mechanism between G-CDs@CA and RhB in an aqueous environment; Figure 18 (a) Morse code encryption and decryption process; (b) anti-counterfeiting pattern photo under 254 nm excitation; (c) digital anti-counterfeiting; (d) delayed illumination photos of B-LED, G-LED, and R-LED for this application. DETAILED DESCRIPTION

[0020] In the following description, specific details such as specific system structures and techniques are provided for purposes of illustration rather than limitation to facilitate a thorough understanding of the embodiments of the present application. However, it will be apparent to those skilled in the art that the present application may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to avoid obscuring the description of the present application with unnecessary detail.

[0021] 1. Experimental 1.1 Experimental Reagents Bicarbamide, 4-carbamoylphenylboronic acid (4-CPBA), 9-phenanthreneboronic acid (Phe9-B), and rhodamine B were purchased from Adamas, while urea and ammonia (NH3·H2O) were purchased from Greagent. All reagents were of analytical grade and used directly without further purification. Distilled water was used in this experiment.

[0022] 1.2 Characterization The morphologies of B-CDs@g-C3N4 and G-CDs@CA were investigated using a high-resolution transmission electron microscope (HR-TEM) on a Tecnai G2 F30 model from FEI (USA). X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance system (Bruker, Germany). X-ray photoelectron spectroscopy (XPS) data were collected using a K-Alpha instrument from Thermo Fisher Scientific (USA). Fourier transform infrared (FT-IR) spectroscopy was performed using a PerkinElmer (USA) infrared spectrometer. Ultraviolet-visible (UV-vis) and fluorescence spectra were recorded using a Shimadzu UV-3600 (Japan) UV-visible spectrophotometer and a Hitachi F-7000 (Japan) fluorescence spectrophotometer, respectively. The photoluminescence quantum yield (PLQY) was measured using an FLS1000 fluorescence spectrometer from Edinburgh (UK). Density functional theory (DFT) calculations were performed using Materials Studio 7.0, using the Compass force field and water as the implicit solvent. The molecular structure was optimized at the GGA / BLYP / DND level of theory. The optical band gap energy was calculated according to formula (1): E = hc / λ (1) Where E is the energy of the photon (J), h is Planck's constant, which is approximately 6.626×10 -34 (J·s), c is the speed of light, approximately 3×10 8 (m / s), λ is the wavelength of the photon (m).

[0023] 1eV = 1.602×10 -19 J 1.3 Synthesis of B-CDs@g-C3N4 First, 0.05 g of 4-carbamoylphenylboronic acid and 3 g of urea were accurately weighed and poured into a 25 mL beaker. 10 mL of distilled water was then added. The mixture was then ultrasonically treated for 10 minutes to mix thoroughly. The beaker was sealed with tin foil and heated in an oven at 200°C for 3 hours. After cooling to room temperature, the resulting product was ground to obtain B-CDs@g-C3N4.

[0024] 1.4 Synthesis of G-CDs@CA First, accurately weigh 0.05 g of 9-phenanthreneboronic acid and 3 g of biuret into a 25 mL beaker. Add 10 mL of distilled water and 0.4 mL of aqueous ammonia. Then, sonicate the mixture for 10 minutes to mix thoroughly, and seal the beaker with tin foil. The subsequent synthesis process is identical to that for B-CDs@g-C3N4.

[0025] 1.5 Synthesis of G-CDs@CA@RhB First, accurately weigh 0.35 g of G-CDs@CA into a 25 mL beaker. Then, add 10 mL of distilled water and various volumes of 5 mmol / L RhB solutions to the beaker and mix thoroughly. This yields G-CDs@CA@RhB.

[0026] Results and Discussion 2.1 Morphology and structure characterization of B-CDs@g-C3N4 and G-CDs@CA By adjusting the degree of conjugation of arylboronic acid groups, multicolor RTP materials with ultralong lifetimes in aqueous environments were prepared. An ultralong-lived blue RTP material (B-CDs@g-C3N4) was synthesized at high temperature using urea and 4-carbamoylphenylboronic acid, while an ultralong-lived green RTP material (G-CDs@CA) was prepared using biuret and 9-phenanthreneboronic acid. The morphologies of B-CDs@g-C3N4 and G-CDs@CA were characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM).

[0027] like Figure 2 a and Figure 2 As shown in d, B-CDs@g-C3N4 and G-CDs@CA particles exhibited good dispersion and spherical morphology, with average sizes of 2.00 nm and 1.12 nm, respectively. HR-TEM images ( Figure 2 b and Figure 2e) shows that both B-CDs@g-C3N4 and G-CDs@CA have obvious lattice fringes with a lattice spacing of 0.21 nm, which corresponds to the (100) crystal plane of graphite.

[0028] like Figure 2 As shown in Figure c, the diffraction peak at 27.8° in the XRD pattern of B-CDs@g-C3N4 corresponds to the (002) diffraction peak of g-C3N4, which indicates that g-C3N4 is generated by urea at high temperature.

[0029] like Figure 2 As shown in Figure d, the XRD patterns of G-CDs@CA and CA are very similar. In the XRD pattern of G-CDs@CA, the diffraction peak at 26.5° is attributed to the (002) crystal plane of the graphite structure, while the diffraction peaks observed at 19.8°, 22.7°, 26.5°, and 29.7° are consistent with the four characteristic diffraction peaks of CA. The above results indicate that biuret is thermally condensed to form cyanuric acid (CA). As shown in the enlarged XRD patterns of G-CDs@CA and CA ( Figure 3 As shown in a), after the formation of G-CDs@CA, the peak position of CA at 29.94° shifted. In the magnified X-ray diffraction (XRD) pattern, the peak position shifted from 29.94° to 29.72°, indicating that a hydrogen bond framework may have formed between the functional groups (such as -OH and -NH2) on the surface of carbon dots (CDs) and cyanuric acid (CA). The schematic diagram of the hydrogen bond framework between CDs and CA is shown in Figure 3 b. XRD results show that urea and biuret form different rigid matrices (g-C3N4 and CA), respectively, at high temperatures. During the heating process, CDs are in situ embedded in the rigid matrix (g-C3N4 / CA), creating a rigid environment that isolates dissolved oxygen, restricts molecular rotation and vibration, and suppresses non-radiative transitions, which contributes to the ultralong RTP emission of B-CDs@g-C3N4 and G-CDs@CA in aqueous solution.

[0030] The chemical composition and surface functional groups of B-CDs@g-C3N4 and G-CDs@CA were analyzed by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Figure 4 a) at 3213-3461 cm -1 The absorption peak is at 1692 cm -1 、1604cm -1 、1340 cm -1 and 1050 cm -1The absorption peaks correspond to the stretching vibrations of C=O, C=N, CN and C-OH. In addition, the peak at 1458 cm -1 / 1412 cm -1 、1130 cm -1 , 957 cm -1 and 762 cm -1 The peaks correspond to the stretching vibrations of BO, B-OH, BC, and BNB, respectively. FT-IR results indicate that B-CDs@g-C3N4 and G-CDs@CA possess abundant hydrophilic groups, including -OH, -COOH, and -NH2. Hydrogen bonding between the hydrophilic groups on the CDs surface and the g-C3N4 / CA matrix creates a rigid environment, which plays a crucial role in the ultralong RTP emission of B-CDs@g-C3N4 and G-CDs@CA in aqueous solution.

[0031] like Figure 4 b and Figure 4 As shown in c, B-CDs@g-C3N4 and G-CDs@CA are mainly composed of elements B, C, N and O, which are 1.19%, 45.05%, 31.25% and 22.51%, and 1.69%, 41.52%, 30.43% and 26.36%, respectively. The above results show that compared with G-CDs@CA, B-CDs@g-C3N4 has a higher degree of carbonization and a lower degree of oxidation. The high-resolution (HR) C 1s spectra of B-CDs@g-C3N4 and G-CDs@CA are decomposed into four peaks, corresponding to BC, CC / C=C, C=O and C=N, respectively, as shown in Figure 3. Figure 4 d and Figure 4 g. N 1s spectrum of B-CDs@g-C3N4 ( Figure 4 e) can be fitted into three peaks at 398.42 eV (NB), 399.23 eV (NC) and 400.04 eV (NH). The N 1s spectrum of G-CDs@CA ( Figure 4 h) shows three peaks at 398.70 eV (NB), 399.46 eV (NC) and 400.13 eV (NH). High-resolution O 1s spectrum ( Figure 4 f and 4i) confirmed the presence of BO, CO and C=O in B-CDs@g-C3N4 and G-CDs@CA. The high-resolution B 1s spectrum of B-CDs@g-C3N4 showed two characteristic peaks at 189.82 eV and 190.60 eV, corresponding to BC and BN bonds (Figure 5a). In contrast, the high-resolution B 1s spectrum of G-CDs@CA ( Figure 5b) shows three different peaks at 189.96 eV, 190.79 eV and 191.95 eV, which are attributed to BC, BN and BO, respectively. The results of X-ray photoelectron spectroscopy (XPS) are consistent with those of Fourier transform infrared spectroscopy (FT-IR). B-CDs@g-C3N4 and G-CDs@CA contain a large number of nitrogen-containing functional groups and carbonyl groups, which are conducive to the transition of electrons from the lowest singlet state (S1) to the lowest triplet state (T1), thereby promoting the efficiency of intersystem crossing (ISC). The empty p orbital of the boron atom can interact with the lone electron of the electron-rich atoms (N, O), thereby reducing the energy gap (Δ EST ), protecting the excited triplet excitons from quenching and enhancing the RTP emission. The doping of nitrogen, oxygen, and boron elements is very important for improving the RTP performance of B-CDs@g-C3N4 and G-CDs@CA in aqueous solution.

[0032] 2.2 Study on the optical properties of B-CDs@g-C3N4 and G-CDs@CA

[0033] The optical properties of B-CDs@g-C3N4 and G-CDs@CA in solid and aqueous solutions were investigated by UV-visible and fluorescence spectroscopy. Figure 6 b) shows that the broad absorption band in the range of 200–280 nm can be attributed to aromatic sp 2 The absorption band at 280-400 nm is attributed to the π-π* transition of C=C in the domain, while the absorption band at 280-400 nm is attributed to the n-π* transition of C=N / C=O bonds. Figure 6 The fluorescence excitation spectrum of B-CDs@g-C3N4 shown in a has two peaks at 271 nm and 340 nm. In contrast, the phosphorescence excitation spectrum of B-CDs@g-C3N4 exhibits a single peak at 269 nm, indicating that the afterglow of B-CDs@g-C3N4 originates from the π-π* transition of C=C. As shown in Figure 6b, both the fluorescence and phosphorescence excitation spectra of G-CDs@CA show two peaks, with the fluorescence excitation maxima appearing at 270 nm and 312 nm, respectively, while the phosphorescence excitation maxima appear at 275 nm and 316 nm, indicating that the phosphorescence emission of G-CDs@CA originates from the π-π* transition of the C=C bond and the n-π* transition of the C=N / C=O bond. When n-π* and π-π* transitions exist simultaneously in a molecule, the two states will be strongly mixed through perpendicular vibrations between planes, thereby increasing the ISC rate and achieving enhanced phosphorescence emission of G-CDs@CA. As shown in Figure 6b, the fluorescence excitation spectrum of G-CDs@CA exhibits two peaks, with the fluorescence excitation maxima appearing at 270 nm and 312 nm, respectively, and the phosphorescence excitation maxima appearing at 275 nm and 316 nm, respectively. This indicates that the phosphorescence emission of G-CDs@CA originates from the π-π* transition of the C=C bond and the n-π* transition of the C=N / C=O bond. When n-π* and π-π* transitions exist simultaneously in a molecule, the two states will be strongly mixed through perpendicular vibrations between planes, thereby increasing the ISC rate and achieving enhanced phosphorescence emission of G-CDs@CA. Figure 7As shown in a, the fluorescence emission spectrum of B-CDs@g-C3N4 solid powder shows two peaks at 345 nm and 426 nm, and its maximum phosphorescence emission peak is located at 425 nm. For G-CDs@CA solid powder, the maximum fluorescence emission peak and phosphorescence emission peak are located at 381 nm and 510 nm, respectively ( Figure 7 b) Average phosphorescence lifetime of solid powders of B-CDs@g-C3N4 and G-CDs@CA ( Figure 7 c) are 1.57 and 1.96 seconds long, respectively, which are calculated according to equation (2).

[0034] (2) The above results are consistent with the experimental phenomena. B-CDs@g-C3N4 emits blue fluorescence when irradiated by 254 nm or 365 nm UV light, but the blue afterglow only appears after the 254 nm UV light is turned off, and it lasts for 24 seconds visible to the naked eye. G-CDs@CA, on the other hand, exhibits blue fluorescence under 254 nm or 365 nm UV light, and shows a green afterglow after the 254 nm or 365 nm UV light is turned off. Under the excitation of 310 nm UV light, the green afterglow of G-CDs@CA solid powder is visible to the naked eye for 29 seconds, as shown in Figure 2. Figure 7 d. The absolute PLQY of B-CDs@g-C3N4 and G-CDs@CA are 4.12% and 10.73%, respectively ( Figure 8 a and Figure 8 b) The phosphorescence CIE coordinates of B-CDs@g-C3N4 and G-CDs@CA are (0.148, 0.0562) and (0.183, 0.473), respectively. Figure 9 As shown. This indicates that G-CDs@CA has a significant red shift in the phosphorescence emission wavelength compared to B-CDs@g-C3N4. The arylboronic acids used to synthesize B-CDs@g-C3N4 and G-CDs@CA differ in the number of benzene rings and the type of functional groups. In order to explore the relationship between the precursor structure and the RTP performance, the structures of 4-CPBA and Phe9-B were optimized. Further calculations showed that the energy level differences between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of 4-CPBA and Phe9-B are 3.696 eV and 3.238 eV, respectively ( Figure 10 ). Generally, the increase in the degree of molecular conjugation leads to an increase in the degree of electron delocalization in the conjugated system, thereby reducing the energy gap between HOMO and LUMO. The optical band gaps of the optimal phosphorescent emission of B-CDs@g-C3N4 and G-CDs@CA are 2.918 eV and 2.431 eV, respectively ( Figure 11), which is consistent with the trend of energy gap changes in the precursor. As the degree of conjugation of the arylboronic acid increases, the RTP emission peak undergoes a red shift. These results indicate that the RTP emission wavelength can be tuned by adjusting the degree of conjugation of the precursor compound.

[0035] Room temperature phosphorescent (RTP) materials are easily quenched by dissolved oxygen and solvent relaxation in aqueous environments, which makes it extremely challenging to prepare multicolor RTP materials with ultra-long phosphorescence lifetimes in aqueous environments. In this study, urea generates g-C3N4 at high temperature, while biuret generates CA through thermal condensation reaction. Carbon dots (CDs) are in situ embedded in the matrix of g-C3N4 and CA to form a rigid microenvironment, effectively isolating dissolved oxygen and suppressing non-radiative transitions. Surprisingly, B-CDs@g-C3N4 and G-CDs@CA exhibit ultra-long-lifetime multicolor phosphorescence emission in aqueous environments. The fluorescence spectrum of B-CDs@g-C3N4 aqueous solution shows three peaks at 360 nm, 429 nm and 448 nm, and its maximum phosphorescence emission peak is located at 451 nm ( Figure 12 a). Meanwhile, the maximum fluorescence and phosphorescence emission peaks of G-CDs@CA aqueous solution are located at 388 nm and 496 nm, respectively ( Figure 12 b). The phosphorescence lifetimes of B-CDs@g-C3N4 and G-CDs@CA in water environment are as long as 1.37 seconds and 1.70 seconds respectively ( Figure 12 c). After turning off the 254 nm UV lamp, the blue afterglow of the B-CDs@g-C3N4 aqueous solution was visible for 14 seconds. After turning off the 310 nm UV lamp, the green afterglow of the G-CDs@CA aqueous solution was visible for 16 seconds ( Figure 12 d). Phosphorescence emission spectra were recorded at different temperatures to study the mechanism of afterglow phenomenon in B-CDs@g-C3N4 and G-CDs@CA. As the temperature increases, the phosphorescence intensity of B-CDs@g-C3N4 and G-CDs@CA shows a gradual decrease, as shown in Figure 5. Figure 13 As shown. This phenomenon can be attributed to the fact that as the temperature increases, the rate of non-radiative transitions accelerates, resulting in a decrease in the phosphorescence intensity. On the contrary, when the temperature decreases, the molecular vibration is suppressed, thereby enhancing the phosphorescence intensity. The above phenomena indicate that the afterglow emission observed in B-CDs@g-C3N4 and G-CDs@CA originates from room temperature phosphorescence (RTP) rather than thermally activated delayed fluorescence (TADF). B-CDs@g-C3N4 and G-CDs@CA exhibit good photostability. Even after one hour of irradiation under a 254 nm UV lamp, their fluorescence and phosphorescence intensities remain essentially unchanged, as shown in Figure 2. Figure 14 and 15Comparison of the phosphorescence emission spectra of B-CDs@g-C3N4 and G-CDs@CA before and after storage at room temperature for one month showed that their phosphorescence intensities remained at 90.6% and 91.3% of their original intensities, respectively. Figure 16 a and Figure 16 b). These results indicate that the synthesized B-CDs@g-C3N4 and G-CDs@CA possess good resistance to photobleaching and stability.

[0036] 2.3 Tunable afterglow color based on reabsorption radiation energy transfer between G-CDs@CA and RhB There is an overlap between the phosphorescence emission spectrum of G-CDs@CA and the absorption spectrum of RhB (e.g. Figure 17 As shown in a), the premise for reabsorption radiation energy transfer to occur is that the emission spectrum of the energy donor overlaps with the absorption spectrum of the energy acceptor. Therefore, it is feasible to adjust the afterglow color by reabsorption radiation energy transfer, using G-CDs@CA as the energy donor and RhB as the energy acceptor. In this work, different volumes of 5 mM RhB were added to the aqueous solution of G-CDs@CA to prepare G-CDs@CA@RhB composite materials. Figure 17 As shown in (b), the phosphorescence spectrum of G-CDs@CA@RhB exhibits two distinct emission peaks, located near 496 nm and 600 nm, respectively. The phosphorescence emission peak near 496 nm is attributed to the energy donor (G-CDs@CA), which is similar to the emission peak of G-CDs@CA, while the emission peak near 600 nm is associated with RhB, similar to the fluorescence emission of RhB.

[0037] like Figure 17 As shown in Figure c, with the increase of the volume of 5 Mm RhB added, the emission peak near 600 nm gradually red-shifted, which may be due to the interaction between RhB molecules at high concentrations, while the emission peak at about 496 nm gradually blue-shifted. Figure 17 As shown in (d), the afterglow color of G-CDs@CA with varying volumes of RhB also gradually changes from green to red. τ1 is the average TP lifetime of the phosphorescence emission peak at 496 nm, and τ2 is the average RTP lifetime of the phosphorescence emission peak at 600 nm. The gradual blue shift of the short-wavelength emission peak (approximately 496 nm) and the consistent donor and acceptor lifetimes are consistent with a mechanism of energy transfer via reabsorption of radiation. Figure 17 e shows the possible mechanism of reabsorption radiation energy transfer between G-CDs@CA and RhB.

[0038] 2.4 Applications of B-CDs@g-C3N4, G-CDs@CA, and G-CDs@CA@RhB in Stimulus-Response Information Encryption and Delayed Lighting

[0039] The rampant spread of counterfeit and substandard goods and false information has caused enormous economic losses and even seriously threatened personal safety. Against this backdrop, the secure storage and transmission of information is of paramount importance. Optical storage technology, with its unique stimuli-responsiveness, massive storage capacity, and cost-effectiveness, has emerged as a promising solution for information storage. B-CDs@g-C3N4, G-CDs@CA, and G-CDs@CA@RhB, due to their extremely long phosphorescence lifetimes, excellent stability, and multicolor room-temperature phosphorescence (RTP) properties, show great potential for applications in information encryption and delayed illumination. Leveraging these materials' unique RTP properties, a multidimensional dynamic information storage technology has been developed that enables information storage based on both color and time scales. G-CDs@CA and G-CDs@CA@RhB have been used to develop a Morse code system that exhibits both time- and color-dependent properties. In Morse code, G-CDs@CA represents the dot (.), while G-CDs@CA@RhB represents the slash (-). These materials were made into information-encrypted dot patterns, where G-CDs@CA is represented by a green circle and G-CDs@CA@RhB is represented by a red circle, as shown in Figure 18 As shown in a. When placed under 310 nm ultraviolet light, G-CDs@CA emits bright blue fluorescence, while G-CDs@CA@RhB emits rose-red fluorescence; when the ultraviolet light is turned off, G-CDs@CA transforms into bright green RTP, while G-CDs@CA@RhB emits a red afterglow, and the decoded Morse code message is "FLY", which is actually a wrong, deceptive signal. Because the red afterglow emitted by G-CDs@CA@RhB lasts for a short time, only 8 seconds, the green RTP of G-CDs@CA becomes the only visible emission light, which is decoded into the Morse code message "SSE". At the same time, G-CDs@CA and G-CDs@CA@RhB powders are loaded into a tulip-shaped mold to create a complex multi-dimensional dynamic anti-counterfeiting pattern, as shown in the figure. Figure 18(b) Under 254 nm UV light, the tulip flowers emit a bright rose-red fluorescence, while the tulip leaves emit a blue fluorescence. When the UV light is turned off, the tulip flowers transform into a red afterglow, while the tulip leaves transform into a green RTP. After 8 seconds, only the green RTP of the tulip leaves remains visible. Furthermore, because B-CDs@g-C3N4 exhibits no phosphorescence under 365 nm UV light, an excitation wavelength-responsive information encryption technology was developed using B-CDs@g-C3N4 and G-CDs@CA. These two materials were incorporated into an "8888" pattern, with B-CDs@g-C3N4 representing the blue portion of the digital design and G-CDs@CA representing the green portion. When the 254 nm UV light is turned off, the pattern displays a blue-green dual-color "8888"; conversely, when the 365 nm UV light is turned off, the pattern transforms into the word "HOPE." Due to the excellent RTP performance of B-CDs@g-C3N4 and G-CDs@CA in water environment, the aqueous solution was dropped on the “8888” pattern. The phenomenon of the wet “8888” pattern after extinguishing the 254 / 365 nm UV light was basically the same as that of the dry “8888” pattern, as shown in Figure 2. Figure 18 As shown in c. Since B-CDs@g-C3N4, G-CDs@CA and G-CDs@CA@RhB have good photostability and resistance to photobleaching, they are used as light conversion layers and combined with 280 nm UV chips to manufacture blue (B-LED), green (G-LED) and red (R-LED) light-emitting diodes (LEDs). These LEDs are turned off after the power is turned off ( Figure 18 d), emitting blue, green, and red afterglow, respectively, which lasted for approximately 12, 14, and 8 seconds, respectively. These experimental results indicate that the prepared B-CDs@g-C3N4, G-CDs@CA, and G-CDs@CA@RhB materials have broad application prospects in information encryption and delayed lighting.

[0040] This application proposes a general and simple method for preparing multi-color RTP CDs with ultra-long lifetimes in aqueous environments. The method is achieved by adjusting the conjugation degree of the precursor and the reabsorption radiation energy transfer. Specifically, by one-step pyrolysis of a mixture of arylboronic acid and urea / biuret with different degrees of conjugation, blue RTP CDs (B-CDs@g-C3N4) with an ultra-long lifetime of 1.37 seconds in aqueous environments and green RTP CDs (G-CDs@CA) with a lifetime of up to 1.70 seconds were prepared. This excellent RTP performance is attributed to multiple effects, including a rigid environment and hydrogen bonding interactions, which effectively suppress molecular vibrations and isolate dissolved oxygen. In addition, the doping of N, B and O promotes intersystem crossing (ISC). By adjusting the conjugation degree of the precursor, the phosphorescence emission wavelength can be adjusted. A reabsorption radiation energy transfer system (G-CDs@CA@RhB) was constructed using G-CDs@CA as an energy donor and RhB as an energy acceptor. This system exhibited a red afterglow lasting up to 1.45 seconds in aqueous solution. B-CDs@g-C3N4, G-CDs@CA, and G-CDs@CA@RhB exhibited extremely long phosphorescence lifetimes, excellent stability, and multicolor RTP properties, demonstrating great potential for applications in information encryption and delayed lighting. This strategy provides a valuable reference for the synthesis of color-tunable, ultra-long-lifetime RTP materials in aqueous solutions, expanding the application range of RTP materials.

[0041] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A method for preparing multicolor room temperature phosphorescent carbon dots in an aqueous environment, characterized in that: Multicolor room-temperature phosphorescent carbon dots with ultra-long lifetimes are synthesized by pyrolyzing a mixture of arylboronic acids with different degrees of conjugation and urea or biuret; the arylboronic acids include 4-carbamoylphenylboronic acid and 9-phenanthreneboronic acid; the pyrolysis process includes heating the mixture at high temperature to embed the carbon dots into a rigid matrix generated by the urea or biuret reaction, forming a rigid microenvironment that isolates dissolved oxygen, thereby enhancing the room-temperature phosphorescent performance.

2. The preparation method according to claim 1, characterized in that The specific steps of the pyrolysis process are: mixing arylboronic acid and urea or biuret with water, ultrasonically treating, heating in an oven at 200° C. for 3 hours, and cooling to obtain the room temperature phosphorescent carbon dots.

3. The preparation method according to claim 1, characterized in that The rigid matrix is g-C3N4 or cyanuric acid.

4. The preparation method according to claim 1, characterized in that The color of the room temperature phosphorescent carbon dots can be tuned by adjusting the conjugation degree of the arylboronic acid.

5. The preparation method according to claim 1, characterized in that The room temperature phosphorescent carbon dots include blue room temperature phosphorescent carbon dots and green room temperature phosphorescent carbon dots, and the phosphorescence lifetimes of the blue room temperature phosphorescent carbon dots and green room temperature phosphorescent carbon dots in an aqueous environment can reach 1.37 seconds and 1.70 seconds, respectively.

6. The preparation method according to claim 1, characterized in that Furthermore, through reabsorption of radiation energy transfer, green room-temperature phosphorescent carbon dots were used as energy donors and rhodamine B as energy acceptors to prepare red room-temperature phosphorescent materials, whose phosphorescence lifetime in an aqueous environment can reach 1.45 seconds.

7. The preparation method according to claim 1, characterized in that The room temperature phosphorescent carbon dots B-CDs@g-C3N4, G-CDs@CA and G-CDs@CA@RhB are used in information encryption, anti-counterfeiting and delayed lighting.

8. Use of the room temperature phosphorescent carbon dots prepared according to the method of claim 1 in an aqueous environment.

Images