CN111874890B

CN111874890B - Red light carbon dot, red light carbon dot-cellulose composite film and preparation method thereof

Info

Publication number: CN111874890B
Application number: CN202010691107.0A
Authority: CN
Inventors: 陈叶青; 朱丽娜
Original assignee: Wuyi University
Current assignee: Wuyi University
Priority date: 2020-07-17
Filing date: 2020-07-17
Publication date: 2022-04-08
Anticipated expiration: 2040-07-17
Also published as: CN111874890A

Abstract

A red light carbon dot-cellulose composite film has absorption peaks of 300nm, 450nm and 600nm, and emission peaks of 750nm at 550 nm. A preparation method of a red light carbon dot-cellulose composite film comprises the following steps: placing the cellulose membrane in boiling ethanol for pretreatment; and placing the red carbon dots into the pretreated cellulose membrane, and soaking the cellulose membrane in water for reaction to obtain the red carbon dot-cellulose composite membrane. According to the invention, the red-light carbon dot cellulose composite membrane is prepared by compounding the red-light carbon dot and the cellulose membrane in a chemical combination mode. The adhesion stability of the carbon dots on the cellulose membrane is researched, and the result shows that the structure of the membrane matrix is not damaged, the cellulose membrane is endowed with unique luminescent characteristics, the quantum efficiency of the composite membrane is improved by 5 times, and the luminescent intensity is stronger than that of pure carbon dots. The invention provides a new method and thought for realizing solid luminescence of the red carbon dots.

Description

Red light carbon dot, red light carbon dot-cellulose composite film and preparation method thereof

Technical Field

The invention belongs to the technical field of novel nanometer functional materials, and relates to a red light carbon dot and a preparation method thereof, and a preparation method for preparing a red light carbon dot-cellulose composite membrane by compounding the red light carbon dot with a cellulose membrane.

Background

Photoluminescence (PL) nanomaterials have received extensive scientific attention for their wide application. Since Carbon Dots (CDs) are first reported and found in 2004, many studies have focused on the nanomaterial, and CDs have the advantages of ultra-small volume, adjustable fluorescence emission, high thermal stability, low cost, easy preparation, environmental friendliness, good biocompatibility, easy surface functionalization and the like. More importantly, the nano-silver nanoparticle has no inherent toxicity or contains toxic elements, so that the nano-silver nanoparticle has wide application in the fields of biological imaging, photocatalysis, sensing and the like. It is currently common for CDs to emit predominantly blue or green, and polychromatic emission of CDs cannot be achieved by merely changing the particle size. In fact, in most cases, PL color is related to surface state, but not to size. Long wavelength emission has shown significant importance in applications such as bio-imaging, multi-color patterning and White Light Emitting Diodes (WLEDs), as well as sensor arrays and full color displays, and there is therefore a great need to produce efficient red emitting carbon dots. Most preferablyNear, Ge, etc^[1]Red-emitting CDs were prepared using polythiophene derivatives as a carbon source, however, a plurality of reaction steps were required to obtain a starting material, and PL fluorescence quantum efficiency of the prepared CDs was very low. Ding et al^[2]The red light emitting CDs prepared from p-phenylenediamine and urea can be separated by silica gel column chromatography to obtain correct separation fraction, but the operation of silica gel column chromatography equipment requires great patience and attention. At present, the red carbon dot has the problem of unclear light-emitting mechanism, and more importantly, the preparation difficulty, low efficiency and poor stability become main factors for restricting the application of the red carbon dot.

Because many defect sites exist on the surface of the CDs, such as dangling bonds, non-radiative states, free radicals and the like, the photoluminescence efficiency is low, and the photoelectron performance of the CDs is reduced, so that the photoelectron performance of the CDs is in a weaker state. Surface passivation is the most effective way to increase the photoluminescence intensity. Sun et al^[3]Silica is reported to form composites with carbon dots, providing multiple enhancements and confinement of embedded CDs through Si-O networks, resulting in ultra-long RTP emission and unique afterglow characteristics. In addition, non-linear polyethylene glycol (PEG), chitosan, Polyethyleneimine (PEI), 4,7, 10-trioxydiamine-1, 13-tridecyldiamine (TTDDA) and the like have also been used as polymer carriers for preparing and compounding carbon dots, and the action mechanism of the polymer carriers on the surface of the carbon dots is to amplify the emission intensity caused by irradiation by cutting off other non-emission paths. The quantum efficiency of CDs is significantly increased after passivation by polymers or other organic molecules. However, whether silica, hydrogel or a composite material formed by compounding a polymer material such as PEI, PEG, etc. with carbon dots, the method for improving the quantum efficiency and the luminescence stability of the carbon dots belongs to a physical mixture, and the method mostly provides a grid matrix structure, reduces non-radiative transitions by hindering the polymerization of the carbon dots, and has a limited improvement on the quantum efficiency and the luminescence stability.

Reference documents:

[1]Ge J,Jia Q,Liu W,et al.Red-Emissive Carbon Dots for Fluorescent,Photoacoustic,and Thermal Theranostics in Living Mice[J].Advanced Materials,2015,27(28):4169-4177.

[2]Ding H,Yu S B,Wei J S,et al.Full-Color Light-Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism[J].Acs Nano,2016,10(1):484-491.

[3]Sun Y P,Zhou B,Lin Y,et al.Quantum-sized carbon dots for bright and colorful photoluminescence[J].Journal of the American Chemical Society,2006,128(24):7756-7757.

disclosure of Invention

The applicant finds through research that the physical combination of the carbon dot and the cellulose membrane depends on the intercalation or electrostatic attraction of the nano-particles, while the chemical bonding depends on the coupling of the water phase carbodiimide, and finally confirms the superiority of the covalent connection in the application of the carbon dot cellulose composite membrane relative to the physical intercalation of the polymer and the electrostatic attraction. The applicant realizes the compounding of cellulose and carbon dots by a chemical method, and researches and prepares a light-emitting stable cellulose film with red light emission characteristics, namely a red light carbon dot-cellulose composite film aiming at the purposes of improving the stability of red light carbon dots and improving the quantum efficiency.

In order to overcome the defects and shortcomings in the prior art, the invention aims to prepare the composite functional nanomaterial by taking the cellulose membrane as a structural support and using a simple direct soaking method for the prepared red-light carbon dots, realize immobilization of the red-light carbon dots through chemical bond combination, and successfully improve the quantum efficiency and photoluminescence stability of the carbon dots on the basis of not damaging the cellulose membrane.

Based on the above problems, one technical solution adopted by the present invention to solve the technical problems is:

a red carbon dot, the absorption spectrum of which comprises absorption peaks at 270-340nm, 340-450nm and 450-700 nm; the emission spectrum comprises an emission peak at 550-750 nm.

Preferably, the absorption spectrum of the red carbon dot comprises absorption peaks at 300nm, 300nm and 600nm of 250-; the emission spectrum comprises emission peaks at 600-700 nm.

In some embodiments of the invention, the red carbon dots have a particle size of about 1-5 nm; preferably, the red carbon dots have an average particle size of about 2 nm. In some embodiments of the invention, the red carbon dot has a fluorescence lifetime of about 4.6-5.2 ns.

According to some embodiments of the invention, the raw materials for preparing the red carbon dots comprise, by weight: 1-5 parts of citric acid, 10-20 parts of ethanol and 10-20 parts of formamide.

According to the invention, citric acid and formamide are used as precursors, and a hydrothermal method is adopted to synthesize carbon dots. The invention also provides a preparation method of the red light carbon dots, which comprises the following steps:

adding citric acid into ethanol, and stirring to obtain a citric acid solution;

and adding formamide into the obtained citric acid solution, stirring, heating to obtain a red light carbon dot solution, and filtering to obtain a red light carbon dot.

According to some embodiments of the present invention, citric acid is added to ethanol and stirred at 300-1000rpm to obtain a citric acid solution. Preferably, the ethanol is anhydrous ethanol.

According to some embodiments of the invention, formamide is added dropwise to the resulting citric acid solution; stirring at 300-; heating at 140 ℃ and 180 ℃ for 6-10h to obtain a red light carbon dot solution, and filtering to obtain a red light carbon dot. Preferably, the formamide is 99% pure.

The invention also provides a red light carbon dot-cellulose composite membrane which comprises a cellulose membrane and a red light carbon dot combined with the cellulose membrane. Specifically, the red carbon dot is bound to cellulose of the cellulose film.

According to some embodiments of the invention, the cellulose membrane has a molecular weight cut-off of 500-; the cellulose film is preferably in the form of a pouch.

According to some embodiments of the present invention, the absorption spectrum of the red carbon dot-cellulose composite film includes absorption peaks at 270-340nm, 340-450nm and 450-700 nm; the emission spectrum comprises an emission peak at 550-750 nm.

Preferably, the absorption spectrum of the red carbon dot-cellulose composite film comprises absorption peaks at 275-325nm, 350-425nm and 480-650 nm; the emission spectrum comprises an emission peak at 600-725 nm.

According to a preferred embodiment of the present invention, in the red light carbon dot-cellulose composite film, the red light carbon dot and the cellulose film are chemically bonded. The quantum efficiency of the red light carbon dot-cellulose composite film is improved by 5 times compared with that of the red light carbon dot. The luminous intensity of the red-light carbon dot-cellulose composite film after ultrasonic treatment is kept unchanged. The red light carbon dot-cellulose composite film has unchanged luminous intensity within one month to three months after preparation. In some embodiments of the invention, the red light carbon dot-cellulose composite film has a fluorescence lifetime of about 5.2-5.6 ns.

According to some embodiments of the invention, the cellulose of the cellulose film is selected from at least one of: cellulose nitrate, cellulose acetate, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose and regenerated cellulose. Cellulose is one of the most abundant biopolymers, and is inexpensive and readily available. On the molecular level, it consists of d-glucose polymeric units linked in linear polymer molecules, these macromolecules being organized in a fibrous structure with alternating amorphous and crystalline segments.

Accordingly, the cellulose membrane is selected from at least one of: nitrocellulose membranes, cellulose acetate membranes, methylcellulose membranes, hydroxypropylmethylcellulose membranes, hydroxyethylcellulose membranes, carboxymethylcellulose membranes, and regenerated cellulose membranes.

The invention takes a cellulose membrane as a substrate, directly puts red carbon dots into the cellulose membrane after the cellulose membrane is pretreated, reacts in water by a direct soaking method, and obtains a composite film of the red carbon dots and cellulose after cleaning. The invention also provides a preparation method of the red light carbon dot-cellulose composite film, which comprises the following steps:

placing the cellulose membrane in boiling ethanol for pretreatment;

and placing the red carbon dots into the pretreated cellulose membrane, and soaking the cellulose membrane in water for reaction to obtain the red carbon dot-cellulose composite membrane.

According to some embodiments of the invention, the cellulose membrane is pre-treated in boiling ethanol for a pre-treatment time of 10-30 min.

According to some embodiments of the invention, the red carbon dots are placed in a pre-treated cellulose membrane and soaked in water for reaction for 12-48 h. Preferably, the water change is performed at intervals. More preferably, the water is changed every 2-6 hours.

Specifically, the preparation method for preparing the red light carbon dot-cellulose composite membrane by compounding the red light carbon dot and the cellulose membrane comprises the following steps:

(1) adding citric acid into ethanol, and stirring to obtain a citric acid solution;

(2) adding formamide into the obtained citric acid solution, stirring, heating to obtain a red light carbon dot solution, and filtering to obtain a red light carbon dot;

(3) placing the cellulose membrane in boiling ethanol for pretreatment;

(4) and placing the red carbon dots into the pretreated cellulose membrane, and soaking the cellulose membrane in water for reaction to obtain the red carbon dot-cellulose composite membrane.

More specifically, the preparation method for preparing the red light carbon dot-cellulose composite membrane by compounding the red light carbon dot and the cellulose membrane comprises the following steps:

(1) adding 1-5 parts of citric acid into 10-20 parts of ethanol, and stirring at 300-1000rpm until the citric acid is completely dissolved to obtain a citric acid solution;

(2) adding 10-20 parts of formamide into the obtained citric acid solution, stirring at 1000rpm under 300-;

(3) placing the cellulose membrane in boiling ethanol for pretreatment, wherein the pretreatment time is 10-30 min;

(4) and placing the red carbon dots into the pretreated cellulose membrane, soaking the cellulose membrane in water, and reacting for 12-48h to obtain the red carbon dot-cellulose composite membrane.

Drawings

The invention is further described below with reference to the accompanying drawings.

FIG. 1 is a transmission electron micrograph of a red carbon dot obtained in example 3.

FIG. 2 is a graph showing a distribution of the particle sizes of red carbon dots obtained in example 3.

Fig. 3 is an atomic force microscope image of the red light carbon dot-cellulose composite film of example 3.

FIG. 4 is a FT-IR spectrum of a red carbon dot and red carbon dot-cellulose composite film of example 3.

FIGS. 5a and 5b are XPS spectra of the red carbon dot and red carbon dot-cellulose composite film of example 3

FIG. 6 shows absorption spectra of red-light carbon dots, red-light carbon dot-cellulose composite films, and cellulose films.

FIG. 7 shows the ultraviolet absorption spectrum and the red light emission spectrum of the red carbon dot of example 3.

FIG. 8 is an ultraviolet absorption spectrum and a red light emission spectrum of the red light carbon dot-cellulose composite film according to example 3.

FIG. 9 shows the UV absorption spectra of red carbon dot-cellulose composite films prepared with different carbon dot mass numbers.

FIG. 10 is the emission spectra of the red carbon dots of example 3 at different excitation wavelengths.

FIG. 11 is an emission spectrum of the red light carbon dot-cellulose composite film of example 3 at different excitation wavelengths.

Fig. 12 shows the emission peak position and the emission intensity of the red carbon dot and the red carbon dot-cellulose composite film at different times of the ultrasonic treatment.

FIG. 13 shows the emission peak positions of red carbon dot-cellulose composite films treated under different pH conditions for different periods of time.

Fig. 14 is an emission spectrum measured after the red carbon dot-cellulose composite film was left in the air for one month and three months.

FIG. 15 shows the change in fluorescence of red carbon dots over eleven days.

FIG. 16 is a fluorescence attenuation curve of a red carbon dot-cellulose composite film.

Detailed Description

The invention will now be described in detail with reference to specific examples, which are intended to illustrate the invention but not to limit it further.

Preparation of red carbon dots

According to the invention, citric acid and formamide are used as precursors, and a hydrothermal method is adopted to synthesize the red light carbon dots.

In a preferred embodiment, 10-20 parts by weight formamide is added to 10-20 parts ethanol containing 1-5 parts citric acid and stirred at 300-1000rpm until the citric acid is completely dissolved. Then, heating can be carried out by any heating method known in the art, heating is carried out for 6-10h at 140-180 ℃, and red carbon points are obtained by filtering. Optionally, the resulting red carbon dots can be collected after drying and stored at low temperature.

In some embodiments of the invention, for the purpose of avoiding the effects of impurities, the citric acid is citric acid monohydrate (A.R.); the ethanol is anhydrous ethanol (A.R.); the formamide is formamide (A.R.) with the purity of 99%; citric acid, ethanol, formamide were purchased from Beijing chemical plant.

Certain embodiments of the present invention employ, for illustrative purposes herein, a specific compound of higher purity as a starting material for preparation, such as, for example, analytical grade (A.R.), but this is not intended to limit or represent the invention to only one specific embodiment. It is to be understood that the invention is not limited to any single specific embodiment or to the variations listed. Many modifications, variations, and other embodiments of the invention will occur to those skilled in the art to which the invention pertains, and the invention is intended to cover such modifications, variations, and embodiments.

Preparation of red light carbon dot-cellulose composite film

The invention takes a cellulose membrane as a matrix, the cellulose membrane is pretreated by using boiling ethanol, red carbon dots are directly filled into a cellulose membrane bag, the cellulose membrane bag reacts in water for 12-48h by using a direct soaking method, then redundant liquid in the cellulose membrane bag is removed, and the cellulose membrane is washed by water for several times to obtain the composite membrane of the red carbon dots and cellulose.

In the present application, the cellulose of the cellulose film is selected from at least one of the following: cellulose nitrate, cellulose acetate, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose and regenerated cellulose. Accordingly, the cellulose membrane is selected from at least one of: nitrocellulose membranes, cellulose acetate membranes, methylcellulose membranes, hydroxypropylmethylcellulose membranes, hydroxyethylcellulose membranes, carboxymethylcellulose membranes, and regenerated cellulose membranes.

In some embodiments of the present invention, a Regenerated Cellulose (RC) membrane is used, the RC membrane is a hydrophobic filter membrane, which has the characteristics of low non-specific adsorption, strong chemical compatibility, and being able to tolerate most organic solvents, and the like, and the RC membrane as a biopolymer does not contain heavy metals and harmful substances, has the advantages of good biocompatibility, relatively low cost, sustainability, and easy surface functionalization, and is beneficial to realizing the compounding of compounds as a matrix structure, so that the RC membrane becomes an ideal material for a new generation of polymer matrix.

In some embodiments of the present invention, the water used may be deionized water for the purpose of avoiding the effects of impurities, and in fact, the selection of the types of water and cellulose films is intended to be illustrative only and should not be construed as limiting the particular embodiments of the present invention.

Example 1

A preparation method for preparing a red light carbon dot-cellulose composite membrane by compounding a red light carbon dot and a cellulose membrane comprises the following steps:

(1) adding 2.5g of citric acid monohydrate into 15mL of absolute ethyl alcohol, and stirring at 600rpm until a clear solution is obtained, so as to obtain a citric acid solution;

(2) dropwise adding 15mL of formamide with the purity of 99% into the obtained citric acid solution, stirring at 600rpm, transferring the stirred solution to a 50mL polytetrafluoroethylene reaction kettle, and heating in an oven at 160 ℃ for 8 hours to obtain a red light carbon dot solution;

(3) and (3) carrying out suction filtration on the obtained red light carbon dot solution, removing unreacted precursors and large-particle byproducts to obtain a brown solution, adding sodium hydroxide and ethanol to carry out centrifugal rotation speed adjustment to 8000r/min, extracting lower-layer red CDs (cadmium sulfide) which are red light carbon dots, collecting and storing at low temperature.

(4) Placing about 5-6cm of MD44 (molecular weight cut-off is 1000) regenerated cellulose membrane in boiling ethanol solution for 15min for pretreatment to remove unnecessary impurities;

(5) putting 1.5g of the obtained red carbon dots into a regenerated cellulose membrane, putting the regenerated cellulose membrane into 1000mL of deionized water for reaction for 24 hours, and changing water every three hours on average to obtain a red carbon dot-cellulose composite membrane;

(6) and pouring out the residual solution in the red light carbon dot-cellulose composite membrane, cleaning in deionized water, and airing at normal temperature for later use.

Example 2

(1) adding 4.0g of citric acid monohydrate into 15mL of absolute ethyl alcohol, and stirring at 600rpm until a clear solution is obtained, so as to obtain a citric acid solution;

Example 3

(1) adding 3g of citric acid monohydrate into 15mL of absolute ethyl alcohol, and stirring at 600rpm until a clear solution is obtained, so as to obtain a citric acid solution;

(2) dropwise adding 15mL of formamide with the purity of 99% into the obtained citric acid solution, stirring at 600rpm, transferring to a 50mL polytetrafluoroethylene reaction kettle after stirring, heating in an oven at 160 ℃ for 8h to obtain a red light carbon dot solution, and reserving part of the red light carbon dot solution for later use;

The red Carbon Dots (CDs) obtained in example 3, and the red carbon dot-cellulose composite films (RC-CDs) were subjected to optical property and characteristic structure tests.

Examples 4 to 7

Examples 4-7 the preparation of red carbon dot and red carbon dot-cellulose composite films was carried out in a similar manner to example 3, except that in step (5) of example 4, no red carbon dot (i.e., 0g of red carbon dot) was placed in the regenerated cellulose film, and in steps (5) of examples 5-7, 0.5g, 1g, and 2g of red carbon dot were taken, respectively.

Morphology and Structure characterization of CDs and RC-CDs films

The CDs prepared in example 3 were characterized by morphology and structure using a Transmission Electron Microscope (TEM), which uses a FEI Tecnai G2S-Twin transmission electron microscope, and the field emission gun was operated at 200kv, and the results are shown in FIG. 1. The particle size distribution of CDs was measured using a particle size analyzer (Nano ZS, Malvern Instruments Ltd, England) and the CDs were shown to be quasi-spherical between 1 and 5nm (histogram of particle size distribution as shown in FIG. 2) with an average diameter of 2nm, indicating that the samples separated well without aggregation.

FIG. 3 is an Atomic Force Microscope (AFM) image of RC-CDs of example 3 using a size-icon scanning probe microscope (Bruker AXS, Marne la Valle, France) equipped with a nanoscopy control station.

Chemical characterization of CDs and RC-CDs films

The chemical composition, structure and surface state of the CDs of example 3 were further analyzed by fourier transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS) for comparison. Fourier transform Infrared Spectroscopy (FT-IR) measurements were carried out using the KBr pellet technique using a Vertex Perkin Elmer 580BIR spectrophotometer (Bruker). x-ray photoelectron spectroscopy (XPS) a VG ESCALAB MK II electron spectrometer Mg K α (1200eV) was used as the excitation source.

From the infrared spectrum of CDs in the FT-IR spectrum shown in FIG. 4, it was revealed that the surface of CDs contained-OH/-NH (3100 3500cm-1) and the carbon point at this peak point was 3452cm^-1Peak position 2835cm^-1An O-H bond vibrational peak associated with a hydrogen bond; located at 1612cm^-1The strong absorption peak is the absorption vibration peak of double bonds such as C ═ N/C ═ C/C ═ O and the like, and is 1500cm due to 1600-^-1The carbon point is a C ═ C bond on a benzene ring skeleton, and more than 1 strong peak is formed, which indicates that a benzene ring structure exists in the carbon point; at 1458cm^-1Is a C-H bond in the plane of the aromatic ring structure, and 1300-1475cm^-1Is methyl, absorption vibration peak of methylene, 1300-1050cm^-1Is the stretching vibration peak of the C-O/C-C/C-N bond and is at 1070-^-1The flat substituted vibration peak of the cyclic compound is shown, so that the simultaneous existence of the bonds and the benzene ring structure in the carbon points is proved, and the polyaromatic hydrocarbon structure is formed in CDs. The surface of CDs presents-OH, -NH2, and shows good hydrophilicity.

From the infrared spectrum of RC-CDs in the FT-IR spectrum shown in fig. 4, it is shown that the composite film after carbon recombination has C ═ O, C ═ C/C ═ N, and the same carbon as the original carbonThe C-O-C/C-N bond is coincident with that of the other bond, and is obviously positioned at 1020cm^-1The left side and the right side are attributed to C-O bonds, which are caused by the fact that the cellulose membrane contains a large amount of C-O, and after the carbon points are compounded, the bonding of the carbon points can be found in the composite membrane, namely the composite membrane has all infrared vibration absorption peaks of the carbon points and simultaneously has various types of bonding in the carbon points.

XPS spectra of CDs and RC-CDs as shown in fig. 5a and 5b, C1s and O1s were similarly clearly observed in both samples, indicating that the composition of CDs and RC-CDs films remained unchanged before and after compounding. Fig. 5a shows two peaks at 530.8, 535.5eV, belonging to C-O, C-O-C, respectively, where the ratio of C ═ O is highest, and CDs surfaces show a large amount of C ═ O and more from formamide. And only C-O-C bonds at 531eV in the RC-CDs film disappear, which indicates that carbon-oxygen single bonds are consumed in the internal reaction of the composite material to generate C-O, and the C-O-C bonds on the surfaces of the carbon points are decomposed in the reaction process to form a new structure with the molecular chains on the surfaces of the cellulose.

A typical C1s high-resolution wavelength band (fig. 5b) in CDs is divided into two peaks at 284 and 288eV, which are respectively assigned to C/C-C, C ═ O/C ═ N, that is, CDs are composed of C, N, O and elements. In contrast, the high resolution display band of C1s (fig. 5b) typical of RC-CDs films is divided into three peaks at 284, 286, 288eV, which are respectively assigned to C/C-C, C-N/C-O, C ═ O/C ═ N. Compared with carbon points, the appearance of the peak position of C-N/C-O shows that after the composite cellulose is compounded, the carbon points react with the cellulose to generate new bond charges, namely, amino groups on the surfaces of the carbon points and carboxyl groups on the surfaces of the cellulose undergo dehydration condensation reaction to form C-N/C-O single bonds.

Optical Properties of CDs and RC-CDs films

Through the above studies of characterization data, CDs consist of a large number of amorphous carbon cores and surface structures, and the surface has abundant functional groups. In the solid existing form of the RC-CDs film, the carbon points are fixed and react with surface molecules of the RC-CDs film to generate new chemical bonds, so that the light-emitting property of the carbon points is ensured, and the stability of the carbon points is enhanced. The optical properties of the RC-CDs films, CDs and RC films were measured by UV-visible spectrophotometry and photoluminescence spectrophotometry. Tests were carried out with 2mL of the carbon dot solution or with 1cm by 1cm of the RC-CDs film or RC film, unless otherwise stated. Wherein the UV-visible absorption spectrum value is measured by a U-3310 spectrophotometer (Hitachi); performing two-dimensional fluorescence matrix scanning on an Edinburgh 980 spectrophotometer, wherein the excitation wavelength is 200-600nm, and the emission wavelength is 300-850 nm; photoluminescence (PL) measurements were performed using a Hitachi F-7000 spectrophotometer, with a 150w xenon lamp as the excitation source. It is worth noting that in the PL performance comparison of the composite film and the carbon dots, the fluorescence quantum efficiency of the red carbon dots is about 1%, the fluorescence quantum efficiency of the red carbon dot-cellulose composite film is about 5%, and the fluorescence quantum efficiency of the RC-CDs film is improved by 5 times compared with the carbon dot luminescence.

As can be seen from the absorption spectra of CDs, the RC-CDs film, and the RC film (fig. 6), the absorption spectra of the RC film are shown as a straight line, which indicates that the RC film itself does not absorb ultraviolet light. The absorption and emission spectra (FIG. 7) of the red carbon dot show that the three absorption bands are respectively located at 250-300nm, 300-450nm and 450-600nm, and the three absorption peaks are respectively located at 270nm, 366nm and 520-556 nm; the emission band is 550-750nm, the maximum emission peak is 638nm, and the energy released by the surface state absorption in the corresponding ultraviolet absorption spectrum emits light. The absorption band at 250-300nm is carbon nuclear absorption, and the highest energy level is related to the transition of sp2 carbon domain. The second absorption band is located at 300-450nm, and belongs to the absorption of the CDs edge state. A third broad absorption band is observed at 450-600nm and extends to longer wavelengths consisting of a set of low energy absorption tails, which are considered surface state absorptions of CDs.

The absorption and emission spectra (figure 8) of the RC-CDs composite film are in red shift relative to the carbon points, the red shift phenomenon of the composite film luminescence is derived from the surface state change after the carbon points are combined with cellulose, three absorption wave bands are respectively positioned at 340nm, 450nm and 450nm, and three absorption peaks are respectively positioned at 285nm, 366nm and 535 nm; the emission band is at 550-750nm, and the maximum emission peak is at 638 nm. However, the nuclear absorption and edge band absorption peak intensities of the RC-CDs film are very small, and are obviously lower than the intensity of the surface band absorption peak. The reason is that the surface functional group of the carbon point after the cellulose is compounded reacts with the surface molecule of the cellulose, so that the carbon point is grafted on the cellulose, the surface state of the carbon point and the cellulose is a cellulose long chain, the rest functional groups on the surface of the carbon point still exist, carboxyl in the cellulose long chain also exists as the surface state of the carbon point and participates in surface state luminescence, and the absorption of the surface state at 553nm shows a graphical result. Carbon dot absorption enhances stable luminescence from complexing with cellulose fixing the carbon dots, and the appearance of new energy levels is believed to be single bonds and aromatic-like structures generated by the reaction of red carbon dots with cellulose, consistent with the analysis of infrared spectroscopy and XPS energy spectra. The transition of the carbon dot nucleus state and the edge state is negligible compared with the surface state in the cellulose-carbon dot composite, and the absorption peak intensity is greatly reduced.

Optical characteristics of compounding carbon points with different masses with cellulose

To further explain the manner in which carbon dots bind to cellulose, the optical properties of the RC-CDs obtained from examples 3-7 in combination at different concentrations of carbon dots were determined by UV-visible spectrophotometry and photoluminescence spectrophotometry. The ultraviolet absorption spectrum of FIG. 9 shows that the absorption spectrum forms of the synthesized RC-CDs films with different mass numbers of carbon dots are the same, which indicates that the combination mode of the carbon dots and the RC film is stable, the surface state absorption intensity at 553nm of the RC-CDs film is gradually increased along with the increase of the mass number of the carbon dots due to the combination of the positions of the absorption spectrum, and the absorption intensity is the highest when the mass number of the carbon dots is 1.5g, and then the reduction begins to appear. Because the surface state bindable sites of a certain mass of carbon dots are limited, when the carbon dots are supplied and requested, physical accumulation occurs, and the concentration quenching phenomenon occurs to influence the absorption and emission of the carbon dots. This also indicates that the bonding of the carbon spot to the RC film is bound by the intrinsic site bonding so that the increase in the intensity of the carbon spot light can be maintained. The carbon points and the RC film are compositely connected in a chemical mode, chemical bonding is formed between the carbon points and the RC film, and the influence of the concentration of the carbon points on an absorption spectrum is not obvious.

Since the luminescent color of the carbon dots and the composite film is red, the luminescent source can be determined by analyzing the luminescent center of the carbon dots and the composite film through PL spectrum. Fig. 10 is an emission spectrum of a red light portion of a red carbon dot solution at different excitation wavelengths, and it can be seen that as the excitation wavelength increases, the emission center remains almost unchanged, showing a characteristic unrelated to excitation except that the emission intensity increases as the excitation wavelength increases, but when the excitation wavelength reaches 560nm, the fluorescence intensity increases and decreases as the excitation wavelength continues to increase. In addition, the surface state luminescence of the carbon dots is independent of the excitation wavelength. Fig. 11 is an emission spectrum of the RC-CDs film at different excitation wavelengths, and as in the solution state, the red light emission is still independent of the excitation and only the emission intensity increases with the increase of the excitation wavelength, which indicates that the luminescence of the composite film comes from the surface state, because the bonding of the carbon dots to the RC film is essentially the reaction between the carbon dot surface state molecules or functional groups and cellulose, the carbon dots are fixed on the film, but the cellulose does not participate in the luminescence, the luminescence property of the carbon dots before and after the recombination is not changed, and only the carbon dots are changed from the liquid state to the film state, and the good stability is maintained.

Stability testing of CDs and RC-CDs films

The complex pattern of carbon sites is generally considered to include physical intercalation as well as chemical bonding. The invention adopts a direct soaking method to combine the carbon dots with the RC film to prepare the composite film, and after the cleaning steps of the embodiment, the carbon dots have well-known good water solubility, and the physically embedded carbon dots are nearly not existed in the RC-CDs film.

To more specifically exclude physical mixing and explain the superiority of chemical bond recombination, the present example contemplates treating the sample with ultrasound at an excitation wavelength of 540nm with carbon dots and an RC-CDs film, and recording and comparing the emission peak positions and emission intensities of the two at different sonication times. As shown in fig. 12, W represents the wavelength of the emission peak position, and I represents the emission intensity. It can be seen from the figure that the emission peak positions of the carbon dot solution and the composite film are not changed within 24h, the emission peak position of the carbon dot solution is kept at about 612nm, and the emission peak position of the composite film is kept at 655nm, which indicates that the light-emitting sources of the two samples are not changed by ultrasonic treatment, and the corresponding light-emitting intensity of the carbon dot solution is rapidly reduced after ultrasonic treatment, because the carbon dots are poor in stability and are easily oxidized and inactivated. The light intensity of the RC-CDs film after ultrasonic treatment is kept unchanged and even tends to rise, because the carbon dots are stably combined with the RC film, and the advantage of solid-state light emission is shown again, so that a new idea is provided for the combination mode of the composite film, namely the combination mode of the composite film is not physical embedding (if the physical embedding exists, the fluorescence center of the composite film is reduced after the ultrasonic treatment for a long time, and the intensity is reduced to some extent).

In order to further eliminate the influence of electrostatic attraction, the RC-CDs film was soaked in solvents with different pH, and the emission peak position of the RC-CDs film at 540nm excitation wavelength was recorded as the soaking time increased, so that as shown in fig. 13, the emission peak of the RC-CDs film did not change much all the time in 0 to 36 hours when the pH was 5, 7, and 10, respectively, i.e., the carbon spot bound to the cellulose film did not change itself. Therefore, the recombination process of the carbon dots and the RC film is not electrostatic attraction or physical embedding, but rather is more favorable for chemical bonding.

Aiming at the time stability of the carbon dot solution and the RC-CDs film, the emission spectra of the carbon dot solution and the RC-CDs film with the change of the luminous intensity along with the time are respectively tested, and the excitation wavelength is 540 nm. As shown in fig. 14, the composite membrane exhibited a constant luminous intensity for a period of one month to three months after preparation. Whereas the strength of the carbon dot solution had almost disappeared after 11 days (fig. 15). This is because the red carbon dots are denatured by oxidation due to exposure of their surface functional groups to air, while the carbon dots in the RC-CDs film are unchanged before and after the recombination, indicating that the carbon dots are more stably bonded to the cellulose film by chemical bonds, and thus it is very advantageous to bond the carbon dots to the RC film.

Fluorescence lifetime of CDs and RC-CDs films

According to the invention, the red carbon dot and the cellulose membrane are connected through a chemical bond, so that a functional group on the surface of the carbon dot is in a stable state, and the energy level structure of the surface state of the carbon dot is changed to a certain extent, and further the fluorescence life of the carbon dot is influenced, so that the two lives are different. To verify this hypothesis, emission lifetimes were also measured, as shown in fig. 16, the lifetime of the RC-CDs films was significantly greater than the carbon dot solution, and the fitted decay curves for the CDs and RC-CDs films were both exponential, with specific lifetimes of 4.88ns and 5.42ns, respectively. In the RC-CDs film, the carbon dots are fixed due to the combination of the cellulose structure and the amino groups on the surfaces of the carbon dots, so that the surface state related to nitrogen is richer, the red light emission is stable, and the situation that the luminous intensity is enhanced and the luminous stability of the carbon dots is greatly improved through chemical bond connection is shown.

In conclusion, the invention provides a novel red light carbon dot and cellulose membrane composite material. The red light carbon dot-cellulose composite membrane is prepared by a direct soaking method at room temperature, the quantum efficiency is improved while the carbon dots are kept to emit red light, the stability of the carbon dots is improved, the structure of the composite membrane is characterized by infrared spectrum and XPS spectrum, and functional groups and bonding change prove that the RC-CDs membrane is combined chemically, namely amino on the surface of the carbon dots reacts with carboxyl on the surface of cellulose molecules, so that the carbon dots are combined with a regenerated cellulose membrane instead of simple physical embedding or electrostatic attraction. The advantages of the composite membrane are proved when pH change, emission spectrum of ultrasonic treatment determination and stability comparative analysis in air are carried out on the carbon dots and the composite membrane, the carbon dot aggregation quenching effect is overcome, a stable solid red light emission carbon dot composite is obtained, the fluorescence life is proved by the fluorescence life to enable the fluorescence life to be enhanced by fixing the carbon dots on the composite membrane of the carbon dots and the regenerated cellulose membrane, and the superiority of chemical bond combination and new discovery of solid red light carbon dot light emission are proved again. The fluorescence of the composite material of the red carbon dot and the cellulose membrane is a unique chemical phenomenon and has great technical potential. The red light carbon dot-cellulose composite film has the advantages of convenient preparation, cheap reagent, simple synthesis method, convenient output light recording and the like. The idea proposed by the invention is universal and can be implemented in the sensing and optical fields and applied to the combination of various carbon dots and polymers.

The invention has been disclosed in its preferred embodiments, the invention is not limited to the examples given, and any equivalent alterations to the technical solution of the invention which are obvious to those skilled in the art from reading the description of the invention are intended to be covered by the claims of the invention.

Claims

1. The red-light carbon dot-cellulose composite membrane is characterized by comprising a cellulose membrane and a red-light carbon dot bonded to the cellulose membrane; wherein the red carbon dot is chemically bound to the cellulose membrane;

the preparation method of the red light carbon dot-cellulose composite film comprises the following steps:

placing the cellulose membrane in boiling ethanol for pretreatment;

2. The red-light carbon dot-cellulose composite film according to claim 1, wherein the absorption spectrum comprises absorption peaks at 270-340nm, 340-450nm and 450-700 nm; the emission spectrum comprises an emission peak at 550-750 nm.

3. The red-light carbon dot-cellulose composite film according to claim 2, wherein the absorption spectrum comprises absorption peaks at 275-325nm, 350-425nm and 480-650 nm; the emission spectrum comprises an emission peak at 600-725 nm.

4. The red carbon dot-cellulose composite film according to claim 1, wherein the absorption spectrum of the red carbon dot comprises absorption peaks at 300nm, 300-450nm and 450-600 nm; the emission spectrum comprises an emission peak at 550-750 nm.

5. The red carbon dot-cellulose composite film according to claim 4, wherein the absorption spectrum of the red carbon dot comprises absorption peaks at 300nm, 400nm and 600nm, respectively, of 250-300nm, 300-400nm and 500-600 nm; the emission spectrum comprises emission peaks at 600-700 nm.

6. The red carbon dot-cellulose composite film according to claim 1, wherein the raw materials for preparing the red carbon dots comprise, in parts by weight: 1-5 parts of citric acid, 10-20 parts of ethanol and 10-20 parts of formamide.

7. The red carbon dot-cellulose composite film according to claim 1, wherein the method for preparing the red carbon dot comprises the steps of:

adding citric acid into ethanol, and stirring to obtain a citric acid solution;

8. The red carbon dot-cellulose composite film according to claim 1, wherein the cellulose of the cellulose film is selected from at least one of: cellulose nitrate, cellulose acetate, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose and regenerated cellulose.

9. The method for preparing a red light carbon dot-cellulose composite film according to any one of claims 1 to 8, comprising the steps of:

placing the cellulose membrane in boiling ethanol for pretreatment;