METHODS FOR SYNTHESIZING FLUORESCENT CQDs AND NITROGEN-PHOSPHORUS CO-DOPED FLUORESCENT CQDs AND APPLICATION THEREOF

Abstract

A method for synthesizing fluorescent carbon quantum dots (CQDs) and nitrogen-phosphorus co-doped fluorescent CQDs and applications are provided. Firstly, a mixture of leaf powder and deionized water is subjected to hydrothermal reaction at 200-240° C. to obtain a product A, followed by removing by-products in it and drying to obtain fluorescent CQDs; nitrogen-phosphorus co-doped fluorescent CQDs are obtained by replacing the product A with a product B and treating the product B in a same way as the product A, where product B is obtained as follows: a mixed system of leaf powder, urea phosphate and deionized water is subjected to hydrothermal reaction at 200-240° C. with a mass ratio of urea phosphate to leaf powder as less than or equal to 0.2 to obtain the product B.

Claims

1. A method for synthesizing fluorescent carbon quantum dots (CQDs), comprising: S1, performing hydrothermal reaction on a mixture containing leaf powder and deionized water at a temperature in a range of 200-240 degrees Celsius (° C.) to obtain a product; and S2, removing by-products in the product and drying the product after the removing, to obtain the fluorescent CQDs.

2. The method for synthesizing fluorescent CQDs according to claim 1, wherein in S1, the mixture is heated from room temperature to the temperature in the range of 200-240° C. at a heating rate in a range of 5-10° C. per minute (° C./min).

3. The method for synthesizing fluorescent CQDs according to claim 1, wherein in S1, the mixture is subjected to the hydrothermal reaction at the temperature in the range of 200-240° C. for a duration in a range of 5-6 hours (h) to obtain the product.

4. The method for synthesizing fluorescent CQDs according to claim 1, wherein in S2, the product is filtered by an organic filter membrane and an aqueous filter membrane for several times, followed by collecting filtered filtrate, and freeze-drying the filtrate to obtain the fluorescent CQDs.

5. The method for synthesizing fluorescent CQDs according to claim 1, wherein the mixture further contains urea phosphate, and a mass ratio of the urea phosphate to the leaf powder of less than or equal to 0.2; and the fluorescent CQDs are nitrogen-phosphorus co-doped fluorescent CQDs.

6. Fluorescent CQDs obtained by the method according to claim 1.

7. The fluorescent CQDs according to claim 6, wherein the fluorescent CQDs are nitrogen-phosphorus co-doped fluorescent CQDs.

8. A use of the nitrogen-phosphorus co-doped fluorescent CQDs according to claim 7 as a fluorescent probe in at least one of detecting a trace amount of Fe.sup.3+ in water environment and cellular imaging.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a photo of discarded apple leaves according to the present application.

[0027] FIG. 2 shows a photo of fluorescent carbon quantum dots (CQDs) aqueous solution and nitrogen-phosphorus co-doped fluorescent CQDs aqueous solution synthesized respectively in Embodiment 1 and Embodiment 3 of the present application.

[0028] FIG. 3 shows a fluorescence photo of fluorescent CQDs aqueous solution and nitrogen-phosphorus co-doped fluorescent CQDs aqueous solution synthesized respectively in Embodiment 1 and Embodiment 3 of the present application under 365 nm ultraviolet lamp.

[0029] FIG. 4 shows a TEM picture of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0030] FIG. 5 shows a high-resolution TEM image of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0031] FIG. 6 is a histogram of particle size distribution of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0032] FIG. 7 illustrates X-ray photoelectron spectroscopy (XPS) spectra of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0033] FIG. 8 shows infrared spectra of nitrogen-phosphorus co-doped fluorescent CQDs and fluorescent CQDs synthesized in Embodiment 1 and Embodiment 3 of the present application, respectively.

[0034] FIG. 9 shows excitation spectra and emission spectra of nitrogen-phosphorus co-doped fluorescent CQDs and fluorescent CQDs synthesized in Embodiment 1 and Embodiment 3, respectively.

[0035] FIG. 10 is a histogram of selective identification of Fe.sup.3+ in water by nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0036] FIG. 11 illustrates a standard curve of Fe.sup.3+ detection in water by nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0037] FIG. 12a shows variation in cytotoxicity of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0038] FIG. 12b shows diagrams of cell labeling imaging of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in Embodiment 1 of the present application.

[0039] FIG. 13 shows a TEM image of the fluorescent CQDs synthesized in Embodiment 3 of the present application (with high-resolution TEM image illustrated in the upper right corner).

[0040] FIG. 14 is a histogram showing particle size distribution of fluorescent CQDs synthesized in Embodiment 3 of the present application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] Further details of the application are described below in conjunction with specific embodiments, and the descriptions are explanatory and not limiting.

[0042] Apple leaves (including leaves from fallen leaves and dead branches, etc.) are a common agricultural and forestry waste left in the fields after the harvesting of apples. Each year, about 9 million tons of apple tree leaves are discarded all over China, which provides a good economic basis for the low-cost and large-scale production of carbon quantum dots (CQDs). Apple leaves provide a green and natural carbon source for synthesizing CQDs, as they are rich in crude fibers, vitamins, lignin, cellulose and hemicellulose. There are several ways of treating agricultural and forestry waste: firstly, by shredding and returning it to the fields, which, however, may cause pests and diseases, leading to soil pollution and affecting the growth of crops; secondly, by burning it on the spot or using it as firewood, which can lead to air pollution; thirdly, by foddering the dead leaves and branches, which imposes high requirements on raw materials and is only suitable for areas with a developed farming industry. Therefore, it is a promising strategy to efficiently utilize apple leaf waste into valuable functional carbon materials, which can reduce the releasing of carbon dioxide and play a very positive role in solving energy and ecological environment problems.

[0043] Therefore, the present application discloses an environmental-friendly and cost-effective method for synthesizing fluorescent CQDs and nitrogen-phosphorus co-doped fluorescent CQDs, including: firstly, cleaning discarded apple leaves, performing drying and grinding, then placing 1.0 gram (g) of the ground leaves in a hydrothermal reaction kettle, then adding deionized water and no more than 0.2 g of urea phosphate (used for nitrogen-phosphorus co-doped fluorescent CQDs), mixing evenly; then, carrying out hydrothermal reaction at 200-240° C. for 5-6 hours (h) at a heating rate of 5-10° C./minute (min) to ensure synthesis of high quality fluorescent CQDs and avoid excessive energy consumption; then performing filtration and freeze-drying, obtaining corresponding fluorescent CQDs.

Embodiment 1

[0044] A method for synthesizing nitrogen-phosphorus co-doped fluorescent CQDs in a green and cost-effective way and application, including the following steps:

[0045] 1) collecting discarded apple leaves as shown in FIG. 1, cleaning them with deionized water, drying them in an oven at 80° C. for 24 h, and then grinding them into powder with a mortar;

[0046] 2) dissolving 0.2 g urea phosphate in 60 milliliters (mL) of deionized water, then adding 1.0 g leaf powder and mixing well into a mixture; transferring the mixture to a 100 mL reaction kettle, then putting the reaction kettle in an oven, heating it to 240° C. at a heating rate of 5° C./min, and keeping the temperature for 6 h;

[0047] 3) taking out the reaction kettle when the oven temperature drops to room temperature, taking out lining, pouring reactants into a 100 mL beaker, and then filtering the reactants with 0.22 (filter pore size) micrometer (μm) of organic filter membrane and 0.22 μm ((filter pore size)) aqueous filter membrane in sequence for five times, or filtering them with 0.22 μm aqueous filter membrane and 0.22 μm organic filter membrane in sequence for five times; finally obtaining a dark brown liquid in the filter bottle below the filter membrane, i.e., aqueous solution of nitrogen-phosphorus co-doped fluorescent CQDs, and a physical photograph of the aqueous solution is shown in the filter bottle on the right of FIG. 2 after adjustment to black and white; this aqueous solution of nitrogen-phosphorus co-doped fluorescent CQDs fluoresces bright blue under a 365 nm UV lamp, and the aqueous solution of fluorescent CQDs synthesized in Embodiment 3 then fluoresces light blue under the 365 nm UV lamp, the aqueous solution of nitrogen-phosphorus co-doped fluorescent CQDs obtained in this embodiment fluoresces brighter blue under a 365 nm UV lamp compared to fluorescent CQDs synthesized in Embodiment 3; on the left and right of FIG. 3 are the fluorescent CQDs aqueous solution synthesized in Embodiment 3 and the nitrogen-phosphorus co-doped fluorescent CQDs aqueous solution synthesized in this embodiment with the fluorescence under a 365 nm UV lamp adjusted to black and white, respectively; the fluorescence photos of the two solutions are actually light blue and bright blue, respectively; after the graph is adjusted to black and white, it cannot reflect their actual colors, but the comparison of the color shades and transparency of the two in FIG. 3 shows that the fluorescence effect of the aqueous solution of nitrogen-phosphorus co-doped fluorescent CQDs synthesized in this embodiment is better, indicating that the co-doping of nitrogen-phosphorus improves the luminous intensity of fluorescent CQDs with high fluorescence quantum yields;

[0048] 4) freeze-drying the obtained fluorescent CQDs aqueous solution to obtain nitrogen-phosphorus co-doped fluorescent CQDs powder derived from apple tree leaves; as shown in FIGS. 4 and 5, the synthesized CQDs have high crystallinity and a quasi-spherical shape without any agglomeration, and the particle sizes as shown in histogram of FIG. 6 are fine with an average particle size of 2.0 nanometers (nm) and a narrow particle size distribution range (1.1-3.0 nm); FIG. 7 is an X-ray photoelectron spectroscopy (XPS) spectra of the obtained CQDs, showing that the CQDs are composed of four elements, carbon (C), oxygen (O), nitrogen (N) and phosphorus (P), with the content ratios of 67.97%, 23.64%, 7.64% and 0.75% respectively; and it can be seen from the infrared spectra in FIG. 8 that the nitrogen-phosphorus co-doped fluorescent CQDs synthesized by adding urea phosphate have rather rich organic functional groups such as nitrogen and phosphorus on their surface, which endow the CQDs with wonderful water solubility and excellent fluorescence performance;

[0049] 5) it can be seen from FIG. 9 that the synthesized nitrogen-phosphorus co-doped fluorescent CQDs have an optimal excitation wavelength of 330 nm, and a corresponding optimal emission wavelength of 400 nm; using quinine sulfate as fluorescence standard, obtaining fluorescence quantum yield of the nitrogen-phosphorus co-doped fluorescent CQDs aqueous solution obtained in step 3) by a reference method at the excitation wavelength of 330 nm to be about 18.1%;

[0050] 6) respectively adding various metal ions (Zn.sup.2+, Sr.sup.3+, Cr.sup.3+, Ag.sup.+, Mn.sup.2+, Fe.sup.3+, CO.sup.2+, Cd.sup.2+, Cu.sup.2+, Zr.sup.4+, Na.sup.+, Pb.sup.2+, Ba.sup.2+, Al.sup.3+, K.sup.+, Mg.sup.2+, La.sup.3+, Ca.sup.2+) at a concentration of 250 micromole (μM) to 1 mL of a solution of nitrogen-phosphorus co-doped CQDs at a concentration of 50 μg/mL, standing for 5 min at room temperature and measuring PL spectra of each mixture under light excitation at 330 nm and recording their emission intensity at 400 nm; it can be seen from FIG. 10 that only Fe.sup.3+ among various metal ions significantly quenched the fluorescence of nitrogen-phosphorus co-doped fluorescent CQDs, indicating that the synthesized nitrogen-phosphorus co-doped fluorescent CQDs can specifically identify Fe.sup.3+ in water;

[0051] 7) adding Fe.sup.3+ with a concentration ranging from 0 to 300 μM into the nitrogen-phosphorus co-doped fluorescent CQDs with the concentration of 50 μg/mL, and drawing standard curve as shown in FIG. 11 according to fluorescence intensity change of nitrogen-phosphorus co-doped fluorescent CQDs at 400 nm, where a good linear relationship between the concentration of Fe.sup.3+ ions and fluorescence intensity change can be seen in the range of 0-160 μm; as a fluorescent probe for detecting Fe.sup.3+, nitrogen-phosphorus co-doped fluorescent CQDs can quickly and sensitively detect trace amounts of Fe.sup.3+ in tap water with a detection limit of 4.0 μM, which is less than that of Fe in drinking water of 5.4 μM; and

[0052] 8) using MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) method to evaluate toxicity and imaging ability of nitrogen-phosphorus co-doped fluorescent CQDs on A549 cells with results shown in FIG. 12a and FIG. 12b, where FIG. 12a shows that when the concentration of CQDs is as high as 500 μg/mL, the cells can still keep more than 90% activity; the cells cultured with nitrogen-phosphorus co-doped fluorescent CQDs emit a bright blue fluorescence around the cells under 405 nm light excitation, but there is no obvious change in the morphology of the cells; FIG. 12b shows the effect of adjusting the diagram of cell labeling imaging of cells cultured with nitrogen-phosphorus co-doped fluorescent CQDs to black and white, as the diagram is black and white, it does not reflect the bright blue fluorescence around the cells under 405 nm light excitation, but it can be seen that the morphology of the cells has not changed significantly, indicating that the synthesized nitrogen-phosphorus co-doped fluorescent CQDs has low toxicity and good biocompatibility in addition to a great application prospect in biological imaging.

Embodiment 2

[0053] A method for synthesizing nitrogen-phosphorus co-doped fluorescent CQDs in a green land cost-effective way includes the following steps:

[0054] 1) collecting discarded apple leaves as shown in FIG. 1, cleaning them with deionized water, drying them in an oven at 80° C. for 24 h, and then grinding them into powder with a mortar;

[0055] 2) dissolving 0.1 g urea phosphate in 60 mL deionized water, adding 1.0 g leaf powder and mixing well into a mixture; transferring the mixture to a 100 mL reaction kettle, then putting the reaction kettle in an oven, heating it to 240° C. at a heating rate of 5° C./min, and keeping the temperature for 6 h;

[0056] 3) taking out the reaction kettle when the oven temperature drops to room temperature, taking out the lining, pouring reactants into a 100 mL beaker, and then filtering the reactants with 0.22 μm organic filter membrane and 0.22 μm aqueous filter membrane in sequence for five times, or filtering them with 0.22 μm aqueous filter membrane and 0.22 μm organic filter membrane in sequence for five times; finally, obtaining a brown liquid, i.e., an aqueous solution of nitrogen-phosphorus co-doped fluorescent CQDs; and

[0057] 4) freeze-drying the obtained fluorescent CQDs aqueous solution to obtain nitrogen-phosphorus co-doped fluorescent CQDs powder derived from apple tree leaves.

Embodiment 3

[0058] A method for synthesizing fluorescence CQDs in a green and cost-effective way includes the following steps:

[0059] 1) collecting discarded apple leaves as shown in FIG. 1, cleaning them with deionized water, drying them in an oven at 80° C. for 24 h, and then grinding them into powder with a mortar;

[0060] 2) adding 1.0 g of leaf powder into 60 mL of deionized water, mixing well into a mixture, then transferring the mixture to a 100 mL reaction kettle, putting the reaction kettle in an oven, and heating it at a heating rate of 5° C./min for 240° C. for 6 h;

[0061] 3) taking out the reaction kettle, taking out the lining, reactants into a 100 mL beaker, and then filtering the reactants with 0.22 μm organic filter membrane and 0.22 μm aqueous filter membrane in sequence for five times, or filtering them with 0.22 μm aqueous filter membrane and 0.22 μm organic filter membrane in sequence for five times; finally, obtaining a light brown liquid, i.e., aqueous solution of CQDs in the filter bottle below the filter membrane as shown on the left side of FIG. 2, and it emits blue fluorescence under 365 nm ultraviolet lamp as shown on the left side of FIG. 3;

[0062] 4) freeze-drying the obtained CQDs aqueous solution to obtain apple tree leaf-derived fluorescent CQDs powder; FIG. 13 shows TEM image of the synthesized fluorescent CQDs, which have high crystallinity and a quasi-spherical shape without any agglomeration, a fine particle size (an average particle size of 1.9 nm) and narrow particle size distribution (1.1-3.0 nm) as shown in FIG. 14; it can also be seen that the obtained fluorescent CQDs have rich organic functional groups on their surface, which endows them with good water solubility; and

[0063] 5) as can be seen from FIG. 9, the synthesized fluorescent CQDs have an optimal excitation wavelength of 330 nm, and a corresponding optimal emission wavelength of about 400 nm; with quinine sulfate as the fluorescence standard, the aqueous solution of fluorescent CQDs obtained in step 3) have a fluorescence quantum yield of about 8.8% by the reference method at the excitation wavelength of 330 nm.