CN114410299B

CN114410299B - Carbon dot and preparation method of carbon dot-based composite material

Info

Publication number: CN114410299B
Application number: CN202210096992.7A
Authority: CN
Inventors: 卢思宇; 王柏扬; 孙震
Original assignee: Zhengzhou University
Current assignee: Zhengzhou University
Priority date: 2022-01-27
Filing date: 2022-01-27
Publication date: 2023-09-26
Anticipated expiration: 2042-01-27
Also published as: CN114410299A

Abstract

The invention belongs to the technical field of carbon dot preparation, and particularly discloses a carbon dot and a preparation method of a carbon dot-based composite material. The invention adopts green pollution-free ethylene diamine tetraacetic acid as the raw material for synthesizing carbon dots, and the carbon dots can be obtained by adding a proper amount of sodium hydroxide and heating for a certain time by a microwave oven, and the synthesis process is extremely simple and easy to operate; and then respectively adding polyvinyl alcohol or polyacrylamide into the obtained carbon dot solution for microwave heating to obtain a carbon dot matrix composite (coated carbon dots), wherein the carbon dot matrix composite has RTP characteristic and long afterglow emission characteristic, and can be applied to the fields of photoelectric equipment, biological imaging, display equipment, sensors and the like.

Description

Carbon dot and preparation method of carbon dot-based composite material

Technical Field

The invention belongs to the technical field of carbon dot preparation, and particularly relates to a carbon dot and a preparation method of a carbon dot-based composite material.

Background

The stimulus-responsive optical material is receiving more and more attention because of its wide application prospect in fields such as photoelectric equipment, biological imaging, display equipment, sensors, etc., especially in the field of information encryption, the development of novel stimulus-responsive intelligent material is very important for avoiding imitation anti-counterfeiting technology. Room Temperature Phosphorescent (RTP) materials are widely focused on due to their long-life phosphorescence, large stokes shift and small background interference, and are expected to be promising candidates in this field. To achieve efficient RTP, efficient spin-orbit coupling is typically required to achieve both singlet to triplet intersystem crossing (ISC) and stable triplet excited states. However, many RTP materials suffer from short phosphorescent lifetimes, low quantum yields, the need for stringent deoxygenation, and incompatibility with printable inks due to the spin-forbidden transition characteristics of the triplet excited states and triplet exciton deactivation resulting from non-radiative transition processes. The most reported RTP materials, such as organometallic and pure organic compounds, also suffer from the disadvantages of high cost, poor stability, complex preparation and high toxicity.

Carbon Dots (CDs), a zero-dimensional carbon material with a size within 10nm, has low toxicity, good biocompatibility and excellent optical properties, and is easy to prepare and can be used for mass production, so that the method has wide application in the fields of biosensors, solar cells, light-emitting diodes and catalysis. In recent years, many scholars have extensively explored the fluorescent properties of carbon dots, but reports on their RTP properties are very limited. Also because of the problems of unstable triplet excited species, oxygen-induced phosphorescence quenching, and ineffective intersystem crossing (ISC), development of room temperature phosphorescent CDs remains a great challenge.

The first report on afterglow of CDs can be traced back to 2012 when Lin et al used Pb ²⁺ Fluorescent CDs are caused to emit strong and stable room temperature phosphorescence, but this heavy atom-assisted approach is toxic and therefore of little interest. In 2013, deng et al reported phosphorescence of carbon dots in a polyvinyl alcohol (PVA) matrix, phosphorescence was attributed to c=o bonds at the surface of the carbon dots, and had a long lifetime (380 ms). This is the phosphorescence that CDs first achieve without heavy atomic doping, since then the afterglow of CDs formally enters the field of view of the researchers.

CDs have two key preconditions to achieve afterglow emission: (1) CDs should have a suitable energy level structure to generate triplet excitons for electron transition, and at S ₀ And T ₁ With a small deltaest in between to achieve an efficient ISC; (2) There must be a specific structure that can protect the triplet excitons generated by CD from non-radiative deactivation. The origin of afterglow emission in CDs is mainly due to c=o/c=n related functional groups, which exhibit strong spin-orbit coupling ability, and triplet excitons can be efficiently generated. Thus, the first condition can be satisfied well by doping heteroatoms such as N, P into CDs. Not only can heteroatom doping greatly promote spin-orbit coupling, but it can create rich surface states on CDs, making them an efficient ISC process. In order to meet the second requirement of protecting triplet excitons from non-radiative transitions, matrix assist systems and self-protecting structures can be designed and constructed to immobilize CDs, resulting in efficient RTP emission. Afterglow of CDs in organic matrices such as polymers and organic compounds is mainly due to hydrogen bondingIt is formed that suppresses intramolecular vibration and stabilizes the triplet state, thereby producing afterglow emission. The afterglow mechanism of CDs in different inorganic matrices is quite different and highly dependent on the various interactions (e.g., rigid structural covalent bonds, structural limitations, energy conversion, etc.) between the CDs and the different matrices. Self-protecting CDs always have a polymer-like structure and can be used as a rigid matrix to activate afterglow emission.

In recent years, there have been two main forms reported for constructing CDs-based RTP materials: one is a matrix assist system that immobilizes CDs in a rigid matrix and forms a quencher barrier through the matrix coating, resulting in efficient RTP emission; the other is the self-protective structure of CDs, i.e. the surface of the CDs carries many polymer chains, or the CDs have a polymer-like structure, which can act as a rigid matrix to protect them from the external environment, thus obtaining RTP emission.

Some of the presently reported materials relating to stimulus-responsive phosphors are practically unsuitable for use in conventional applications for information encryption and anti-counterfeiting due to their short lifetime, the need for stringent deoxidizing conditions or incompatibility with the manufacture of printable inks. Therefore, it is desirable to develop Room Temperature Phosphorescent (RTP) materials with long lifetimes, but it is very challenging to fabricate such materials due to the spin-forbidden transition characteristics of the triplet excited state.

Disclosure of Invention

The invention aims to provide a carbon dot and a preparation method of a carbon dot-based composite material.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:

a preparation method of carbon dots comprises the following preparation steps:

(a) Putting tetra sodium ethylenediamine tetraacetate and NaOH into a 1# container, adding water for dissolution, then heating the 1# container by a microwave oven until black carbonized blocks are generated, taking out the 1# container and putting the 1# container to room temperature;

(b) Removing black carbonized blocks in the No. 1 container, retaining white and brown alternate solids, adding water into the No. 1 container, centrifuging after the solids are completely dissolved, discarding the precipitate, retaining supernatant, and freeze-drying to obtain carbon dot powder;

wherein the dosage ratio of the raw materials is that the ratio of the ethylene diamine tetraacetic acid to the NaOH= (0.5-1.5) g to (0.3-1.0) g.

Preferably, the raw materials are mixed by tetra sodium ethylenediamine tetraacetate, naOH and water in the step (a) and water in the step (b) = (0.5-1.5) g and (0.3-1.0) g and (10-50) mL and (10-20) mL.

Preferably, in step (a) and step (b), dissolution is performed under ultrasonic conditions.

Preferably, in the step (a), the power of the microwave oven is 560-700W, and the heating of the microwave oven is 100-150 s.

Preferably, in the step (a), the 1# container is taken out and the solution is uniformly shaken every 15-20 s in the heating process of the microwave oven.

The preparation method of the carbon dot matrix composite material comprises the following preparation steps:

(a) Putting tetra sodium ethylenediamine tetraacetate and NaOH into a 1# container, adding water for dissolution, then heating the 1# container by a microwave oven until black carbonized blocks are generated, taking out the 1# container after heating is finished, and cooling to room temperature;

(b) Removing black carbonized blocks in the No. 1 container, retaining white and brown alternate solids, adding water into the No. 1 container, centrifuging after the solids are completely dissolved, discarding the precipitate, and retaining supernatant, namely a carbon dot solution;

(c) Placing the polymer in a No. 2 container, adding water according to the mass-volume ratio of the polymer to water= (0.2-0.5) g to (25-50) mL, dispersing, and dispersing the dispersion; the polymer is polyvinyl alcohol PVA or polyacrylamide PAM;

(d) Uniformly mixing the carbon dot solution obtained in the step (b) and the dispersion liquid obtained in the step (c) in a volume ratio of (2-8) to 5 in a 3# container; heating the mixed solution to a 3# container by a microwave oven until the solution is evaporated to dryness, cooling to room temperature, adding water (10-20 mL) for dissolution, and freeze-drying to obtain a carbon dot matrix composite;

Preferably, in step (a) and step (b), dissolution is performed under ultrasonic conditions; in step (c), dispersing under ultrasonic conditions; in the step (d), the mixture is uniformly mixed under the ultrasonic condition.

Preferably, in step (a) and step (d), the microwave oven power is 560-700W; heating 100-150 s by a microwave oven in the step (a); and (d) heating 80-120 s by a microwave oven.

Preferably, in the step (a) and the step (d), the 1# container or the 3# container is taken out and the solution is shaken evenly every 15-20 s in the heating process of the microwave oven.

The beneficial effects are that: the invention adopts green pollution-free ethylene diamine tetraacetic acid as the raw material for synthesizing carbon dots, and the carbon dots can be obtained by adding a proper amount of sodium hydroxide and heating for a certain time by a microwave oven, and the synthesis process is extremely simple and easy to operate; and then respectively adding polyvinyl alcohol or polyacrylamide into the obtained carbon dot solution for microwave heating to obtain a carbon dot matrix composite (coated carbon dots), wherein the carbon dot matrix composite has RTP (real-time transport protocol) characteristics and long afterglow emission characteristics and can be applied to the fields of photoelectric equipment, biological imaging, display equipment, sensors and the like.

Drawings

FIG. 1 is a TEM image (a) and an HR-TEM image (b) of CDs prepared in example 1.

FIG. 2 shows AFM images (a, b) of CDs prepared in example 1.

FIG. 3 is an XRD pattern for CDs@PVA prepared in example 2.

FIG. 4 shows the phosphorescence intensity spectra of CDs (a), CDs@PVA (b) and CDs@PAM (c) prepared in examples 1 to 3.

FIG. 5 is a fluorescence spectrum of CDs prepared in example 1 using different excitation wavelengths.

FIG. 6 is a graph showing the phosphorescence intensity spectra of CDs prepared in example 1 using excitation wavelengths 285 and 345 and nm, respectively.

FIG. 7 is a graph showing the phosphorescence intensity spectra of CDs prepared in example 1 using different excitation wavelengths.

FIG. 8 is a phosphorescent lifetime spectrum of CDs and 5CDs@5PVA prepared in examples 1-2 excited with excitation wavelength 345 nm.

FIG. 9 is a photograph of phosphorescence attenuation of CDs prepared in example 1: a-excitation wavelength 254nm, b-excitation wavelength 365nm.

FIG. 10 shows FT-IR of CDs, 5CDs@5PVA, 5CDs@5PAM prepared in examples 1-3.

FIG. 11 shows the UV-visible diffuse reflectance spectra (UV Vis DRS) of CDs, 5CDs@5PVA, 5CDs@5PAM prepared in examples 1-3.

FIG. 12 is an X-ray photoelectron spectroscopy (XPS) analysis of CDs prepared in example 1: a-total spectrum, b-C1s fine spectrum, C-N1s fine spectrum, d-O1s fine spectrum.

Fig. 13 is a graph of a temperature swing phosphorescence test plot of 5cds@5pva prepared in example 2 under excitation of 345 nm.

FIG. 14 is a blue-violet phosphorescent temperature-shift lifetime spectrum of 5CDs@5PVA prepared in example 2 under excitation of 285 nm.

FIG. 15 is a graph of green phosphorescent temperature-shift life of 5CDs@5PVA prepared in example 2 under 330nm excitation.

Detailed Description

The present invention will be described in further detail below for the purpose of making the present invention clearer and more specific. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

Example 1 preparation of Carbon Dots (CDs)

Weighing 0.5 g ethylene diamine tetraacetic acid (EDTA.4Na) and 0.3 g NaOH on an analytical balance, transferring to a small beaker, adding 10 mL deionized water, and performing ultrasonic treatment for 3 min until the solid is completely dissolved; heating the small beaker 120 s by a microwave oven (power 700W) until black carbonized blocks are generated (the beaker has no water before the black carbonized blocks are generated; the beaker is taken out to shake the solution every 15 s in the heating process), and taking out the beaker to be at room temperature after the heating is finished; removing black carbonized blocks with tweezers, taking out white solid and brown solid phase in beaker, adding 10 mL water, ultrasonic treating for 3 min until the solid is completely dissolved, transferring to centrifuge tube, centrifuging in centrifuge for 15 min, discarding precipitate, retaining supernatant, namely CDs solution, and lyophilizing to obtain CDs powder.

Example 2 preparation of carbon dot based composite CDs@PVA

Weighing 0.2. 0.2 g polyvinyl alcohol (PVA) solid in a small beaker, adding 25. 25 mL water to dissolve and ultrasonically treating for 3 min to obtain PVA dispersion liquid for later use; the CDs solution obtained in example 1 was divided into 2 mL, 5 mL and 8 mL in three beakers, 5 mL of PVA dispersion was added per portion, and sonicated for 3 min until mixing was uniform; and then heating 100 s parts of the solution by a microwave oven (power 700W) respectively until the solution is evaporated to dryness (the solution is uniformly shaken by taking out a beaker every 15 s in the heating process), taking out the beaker after heating, putting the beaker to room temperature, adding 10 mL water for dissolution, and freeze-drying to obtain carbon dot matrix composite CDs@PVA powder, wherein the carbon dot matrix composite CDs@PVA powder corresponding to 2 mL, 5 mL and 8 mL of CDs solution is marked as follows: 2CDs@5PVA, 5CDs@5PVA, 8CDs@5PVA.

Example 3 preparation of carbon dot based composite CDs@PAM

The difference from example 1 is that: the polyvinyl alcohol (PVA) in example 2 was replaced with polyacrylamide PAM, and the resulting product was carbon dot matrix composite cds@pam powder, labeled as carbon dot matrix composite cds@pam powder corresponding to 2 mL, 5 mL and 8 mL of CDs solution, respectively: 2cds@5pam, 5cds@5pam, 8cds@5pam.

Experimental analysis test:

FIG. 1 is a TEM image (a) and an HR-TEM image (b) of CDs prepared in example 1. As can be seen from fig. 1: the carbon dots exist, the size of the carbon dots is 1-10 nm, and the lattice stripes are not uniform in orientation and cross.

FIG. 2 shows AFM images (a, b) of CDs prepared in example 1. As can be seen from fig. 2: the thickness of the carbon dots is 0.1-6. 6 nm.

FIG. 3 is an XRD pattern for CDs@PVA prepared in example 2. From fig. 3, characteristic peaks belonging to carbon can be observed.

FIG. 4 shows the phosphorescence intensity spectra of CDs (a), CDs@PVA (b) and CDs@PAM (c) prepared in examples 1 to 3, respectively, at an excitation wavelength of 330 nm. As can be seen from fig. 4 a: the weak phosphorescence of the raw materials is greatly enhanced, and CDs powder emits green phosphorescence of 519 nm under the excitation wavelength of 330 nm; as can be seen from fig. 4 b: in the composite comparison of CDs solutions with different contents and PVA, the 5CDs@5PVA with moderate content of coated CDs shows better phosphorescence characteristic; as can be seen from fig. 4 c: in the composite comparison of CDs with different contents and PAM, the phosphorescence intensity of 5CDs@5PAM is relatively similar to that of 8CDs@5PAM.

FIG. 5 is a fluorescence spectrum of CDs prepared in example 1 using different excitation wavelengths. As can be seen from fig. 5: the emission wavelength of CDs does not significantly shift.

FIG. 6 is a graph showing the phosphorescence intensity spectra of CDs prepared in example 1 using excitation wavelengths 285 and 345 and nm, respectively. As can be seen from fig. 6: excitation wavelengths 285 and 345 nm correspond to blue-violet phosphorescent emission (Em 1) and green phosphorescent emission (Em 2), respectively.

FIG. 7 is a graph showing the phosphorescence intensity spectra of CDs prepared in example 1 using different excitation wavelengths. A clear phosphorescent excitation dependence can be observed from fig. 7.

FIG. 8 is a phosphorescent lifetime spectrum of CDs and 5CDs@5PVA prepared in examples 1-2 excited with excitation wavelength 345 nm. As can be seen from fig. 8: the phosphorescent lifetimes of CDs and 5CDs@5PVA were not significantly different, and the average phosphorescent lifetimes of the CDs and the 5CDs@5PVA were obtained by fitting using a three-exponential equation and were 0.75s and 0.70 s, and the percentage content of each part is shown in Table 1.

FIG. 9 is a photograph of phosphorescence attenuation of CDs prepared in example 1: a-excitation wavelength 254nm, b-excitation wavelength 365nm. As can be seen from fig. 9 a: 254 Under the excitation of nm, blue-violet afterglow can reach 2 s; as can be seen from fig. 9 b: 365 Under the excitation of nm, the green afterglow can reach 7s.

FIG. 10 shows FT-IR of CDs, 5CDs@5PVA, 5CDs@5PAM prepared in examples 1-3. By comparing with the raw materials, it can be seen that the three samples have similar functional group vibration, and have hydroxyl, carboxyl, C-N and the like, but two absorption peaks of carboxyl in CDs synthesized later and 5CDs@5PVA and 5CDs@5PAM coated with different polymers are also reserved, so that the fact that no decarboxylation reaction occurs to a large extent in the microwave heating process is deduced, and the carbonization degree is weaker. The change in C-N is due to the fact that a large amount of N leaves in the form of gas during the reaction.

FIG. 11 shows the UV-visible diffuse reflectance spectra (UV Vis DRS) of CDs, 5CDs@5PVA, 5CDs@5PAM prepared in examples 1-3. From the ultraviolet visible diffuse reflected light curve of CDs: no decarboxylation reaction occurs to a large extent in the microwave heating process, the carbonization degree is weak, and part of N element is NH in the heating process ₃ And the like, and loss into the air in a possible form; from the 5CDs@5PVA, 5CDs@5PAM ultraviolet visible diffuse reflection light curves, it is known that: the absorption peak of PVA or PAM coated carbon dots has a significant red shift relative to the absorption peak of CDs, which is believed to be the result of the secondary microwave heating during polymer coating, resulting in a further increase in the degree of carbonization of the carbon dots.

FIG. 12 is an X-ray photoelectron spectroscopy (XPS) analysis of CDs prepared in example 1: a-total spectrum, b-C1s fine spectrum, C-N1s fine spectrum, d-O1s fine spectrum. The molecular formula of EDTA is known: compared with the percentage of N element in EDTA, the content of N element in CDs is reduced, and the fact that the N element is NH in the microwave heating process is further deduced ₃ And the like may be lost to the atmosphere.

To eliminate the possibility that long afterglow is derived from thermally delayed fluorescence (TADF), we performed a temperature-variable phosphorescence test on 5cds@5pva using 345 nm excitation light source, and as a result, as shown in fig. 13, the non-radiative transition process such as vibration rotation of triplet excitons becomes intense due to the increase of temperature, so that the triplet excitons are deactivated, and the phosphorescence intensity becomes weak. We then also analyzed the 5cds@5pva temperature dependent transient photoluminescence decay process, the afterglow intensity of the blue-violet phosphorescence at 285nm excitation being as shown in fig. 14, the afterglow intensity of the yellow-green phosphorescence at 330nm excitation being as shown in fig. 15, decreasing with increasing temperature during the decay process, which is characteristic of room temperature phosphorescence, since the rate constant of non-radiative deactivation becomes greater with increasing temperature. Unlike the characteristics of TADF, the enhancement of triplet exciton vibration and rotation increases the possibility that triplet excitons return to singlet energy levels through the reverse intersystem crossing process as the temperature increases, and the heat activation energy activates afterglow, and thus the proportion of the retardation component increases.

With the data as a support, we can further determine the source of phosphorescence, and can see that carbon points are in an aggregation crosslinking state from the morphology under TEM, EDTA 4Na raw materials are carbonized under the strong alkaline condition in an acceleration way, a graphitized conjugated structure is formed to a certain extent, the carbon points are in a compact carbon nuclear state, alkyl chains and carboxylate groups around a conjugated framework are further crosslinked through interactions such as hydrogen bonds, electrostatic forces and the like, a small amount of CDs are aggregated and crosslinked into clusters, and after PVA/PAM is compounded, the carbon clusters are dispersed, embedded and crosslinked in a PVA/PAM network structure through interactions such as hydrogen bonds, electrostatic forces and the like. From the analysis of FT-IR, UV Vis DRS and XPS on carbon dot structure, it can be seen that the N- > pi transition of C=O/C-O/C-N is the main source of phosphorescence, the cross-linked aggregation of carbon dots and the network structure of PVA/PAM provide a rigid environment for the carbon dot, limit the non-radiative transition process, protect triplet excitons, and play a good role in blocking oxygen for carbon clusters embedded in PVA/PAM, and avoid the oxygen quenching process.

Claims

1. The preparation method of the carbon dot is characterized by comprising the following preparation steps:

(a) Putting the tetra sodium ethylenediamine tetraacetate and NaOH into a 1# container, adding water for dissolution, putting into a microwave oven, taking out the 1# container to shake the solution at intervals of 15-20 s in the heating process of the microwave oven, heating the 1# container by the microwave oven until black carbonized blocks are generated, taking out the 1# container, and cooling to room temperature;

wherein the dosage ratio of the raw materials is that the water in the step (a) is= (0.5-1.5) g to (0.3-1.0) g to (10-50) mL.

2. The method for producing a carbon dot according to claim 1, wherein: the ratio of the raw materials is tetra sodium ethylenediamine tetraacetate to NaOH to water in the step (a) to water in the step (b) = (0.5-1.5) g to (0.3-1.0) g to (10-50) mL to (10-20) mL.

3. The method for producing a carbon dot according to claim 1, wherein: in step (a) and step (b), dissolution is performed under ultrasonic conditions.

4. The method for producing a carbon dot according to claim 1, wherein: in the step (a), the power of the microwave oven is 560-700W, and the heating of the microwave oven is 100-150 s.

5. The preparation method of the carbon dot matrix composite material is characterized by comprising the following preparation steps:

(a) Putting the tetra sodium ethylenediamine tetraacetate and NaOH into a 1# container, adding water for dissolution, putting into a microwave oven, taking out the 1# container every 15-20 s in the heating process of the microwave oven, shaking the solution uniformly, heating the 1# container by the microwave oven until black carbonized blocks are generated, and taking out the 1# container to be at room temperature after heating;

(c) Placing the polymer in a No. 2 container, adding water according to the mass-volume ratio of the polymer to water= (0.2-0.5) g to (25-50) mL, dispersing, and dispersing the dispersion; the polymer is PVA or PAM;

(d) Uniformly mixing the carbon dot solution obtained in the step (b) and the dispersion liquid obtained in the step (c) in a volume ratio of (2-8) to 5 in a 3# container; heating the mixed solution to a 3# container by a microwave oven until the solution is evaporated to dryness, cooling to room temperature, adding water for dissolution, and freeze-drying to obtain a carbon dot matrix composite;

6. The method for preparing a carbon dot matrix composite according to claim 5, wherein: the ratio of the raw materials is tetra sodium ethylenediamine tetraacetate to NaOH to water in the step (a) to water in the step (b) = (0.5-1.5) g to (0.3-1.0) g to (10-50) mL to (10-20) mL.

7. The method for preparing a carbon dot matrix composite according to claim 5, wherein: in step (a) and step (b), dissolving under ultrasonic conditions; in step (c), dispersing under ultrasonic conditions; in the step (d), the mixture is uniformly mixed under the ultrasonic condition.

8. The method for preparing a carbon dot matrix composite according to claim 5, wherein: in the step (a) and the step (d), the power of the microwave oven is 560-700W; heating 100-150 s by a microwave oven in the step (a); and (d) heating 80-120 s by a microwave oven.

9. The method for preparing a carbon dot matrix composite according to claim 8, wherein: in the step (d), the 3# container is taken out and the solution is uniformly shaken every 15-20 s in the heating process of the microwave oven.