Preparation method of high-fluorescent cellulose-based carbon quantum dots

Abstract

The application relates to a preparation method of high-fluorescent cellulose-based carbon quantum dots, which comprises cellulose enzymolysis, carbon source acquisition and CQD preparation. The application realizes the purpose of improving the fluorescent performance of the CQDs by preparing the CQDs after performing enzymolysis pretreatment on natural cellulose.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of composite materials, in particular to a preparation method of high-fluorescent cellulose-based carbon quantum dots. BACKGROUND

[0002] Cellulose, as one of the most abundant renewable organic resources on earth, widely exists in plant cell walls. However, due to the strong mechanical structure and high chemical stability of cellulose, it is difficult to be directly utilized. Through enzymatic hydrolysis, cellulose is degraded into monosaccharides or oligosaccharides such as glucose, which can realize the resource utilization of cellulose and improve its economic value. Among the cellulose enzymatic hydrolysates, glucose has become an excellent raw material for preparing CQDs due to its simple structure and easy-to-control process.

[0003] At present, the common problem of cellulose-based CQDs is that they cannot meet the expected fluorescence performance requirements. The known common solution is to incorporate heteroatoms, for example, amines and alkali metals as reaction ligands. Some researchers believe that nitrogen (N) doping will enter the carbon nucleus formation process and participate in the formation of a graphene-like structure. The superior fluorescence stability of N-doped cellulose-based CQDs and the selectivity of Fe 3+ confirm the effectiveness of the dopant. In recent years, some researchers have proposed a new method of synthesizing CQDs by double-element doping. Cellulose extracted from sawdust can be co-doped with Mg(OH)2 and ethylenediamine to produce strong green-fluorescent cellulose-based CQDs. Jiang Fan et al. prepared high-fluorescent quantum yield CQDs by hydrothermal method with a certain proportion of microcrystalline cellulose as carbon source and polyethyleneimine as nitrogen source, and the CQDs had good selectivity for Fe 3+ . Huang Hai-long et al. prepared CQDs with different particle sizes from cellulose hydrogel precursors, which can be used for detecting heavy metal ions Hg 2+ . Hu Dong-hao et al. prepared fluorescent carbon dots capable of simultaneously detecting Fe 3+ and Mn 2+ by microwave-assisted synthesis method using 2,2,6,6-tetramethylpiperidine-1-oxyl radical-mediated oxidized cellulose nanofiber and 4,7,10-trioxa-1,13-tridecanediamine as precursors.

[0004] At present, the method for regulating the fluorescence performance of C-CQDs is mainly to dope heteroatoms, and the research on improving the fluorescence performance of CQDs by pretreating natural cellulose is almost not found. SUMMARY

[0005] In view of the above, it is necessary to provide a preparation method of high-fluorescent cellulose-based carbon quantum dots, which prepares CQDs by pretreating natural cellulose through enzymatic hydrolysis, so as to realize the purpose of improving the fluorescence performance thereof.

[0006] To achieve the above object, the technical scheme adopted by the present application is:

[0007] A preparation method of high-fluorescent cellulose-based carbon quantum dots, the method comprising the following steps:

[0008] (1) Enzymatic hydrolysis of cellulose: Put cellulose and cellulase into a conical flask, use sodium citrate buffer to keep the pH value of the reaction system at 4.5-5.0, and then perform enzymatic hydrolysis;

[0009] (2) Obtain carbon source: Inactivate the cellulase in the reaction system of step (1) at high temperature, and the obtained enzymatic hydrolysate is the carbon source, which is ready for use;

[0010] (3) Preparation of CQDs: Put the carbon source obtained in step (2) into a polytetrafluoroethylene-lined reaction kettle, then place the reaction kettle in an environment of 180-220℃ for 10-15h, and naturally cool to room temperature, then perform suction filtration on the product in the reaction kettle, collect the supernatant, perform dialysis, concentration, and freeze-drying, and the solid CQDs finished product can be obtained.

[0011] Further, the cellulose concentration in step (1) is 1.5-2.5% w / v, and the enzyme activity of cellulase is 10-20 FPU / g.

[0012] Further, the temperature of enzymatic hydrolysis in step (1) is 45-55℃, and the time of enzymatic hydrolysis is 5-40min.

[0013] Further, the molecular weight cut-off of the dialysis bag in step (3) is 1000Da.

[0014] Further, the method of obtaining the carbon source can be: inactivating the cellulase in the reaction system of step (1) at high temperature, then washing and suction filtering the solid product after cellulase hydrolysis with deionized water, drying, then mixing the carbon source with deionized water according to the proportion of 1g dry matter to 25ml deionized water, and obtaining the carbon source.

[0015] The present application has at least the following beneficial effects:

[0016] The present application uses the enzymatic hydrolysate obtained after cellulase hydrolysis of cellulose as a carbon source to prepare CQDs, and the test proves that the CQDs obtained by the method of the present application have high fluorescence performance, which proves that the interaction between cellulose and its enzymatic hydrolysate during the formation of CQDs is operable to improve the fluorescence performance of CQDs. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A schematic diagram of the formation process of cellulose-based CQDs. DETAILED DESCRIPTION

[0018] In order to make the above objectives, features and advantages of the present application more apparent, the specific embodiments of the present application are described in detail below. In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the scope of the present application, and therefore the present application is not limited to the specific embodiments disclosed below.

[0019] Example 1

[0020] The present embodiment is a preparation method of high-fluorescent cellulose-based carbon quantum dots, which comprises the following steps:

[0021] (1) Enzymatic hydrolysis of cellulose: 1.5% w / v cellulose and 10 FPU / g cellulase are placed in a conical flask, and a sodium citrate buffer is used to maintain the pH value of the reaction system at 4.5, followed by enzymatic hydrolysis at 45°C for 5 min at a rotation speed of 150 rpm;

[0022] (2) Obtain carbon source: the cellulase in the reaction system of step (1) is inactivated at high temperature, and the obtained enzymatic hydrolysis product is the carbon source, which is ready for use;

[0023] (3) Preparation of CQDs: the carbon source obtained in step (2) is added to a 50 ml polytetrafluoroethylene-lined reaction kettle, and then the reaction kettle is placed in an environment at 180°C for 10 h, and then naturally cooled to room temperature. The product in the reaction kettle is suction filtered, the supernatant is collected, dialysis with a molecular weight cut-off of 1000 Da is performed, concentrated, and freeze-dried to obtain solid CQDs finished product.

[0024] Example 2

[0025] The present embodiment is a preparation method of high-fluorescent cellulose-based carbon quantum dots, which comprises the following steps:

[0026] (1) Enzymatic hydrolysis of cellulose: 2.0% w / v cellulose and 15 FPU / g cellulase are placed in a conical flask, and a sodium citrate buffer is used to maintain the pH value of the reaction system at 4.8, followed by enzymatic hydrolysis at 50°C for 40 min at a rotation speed of 150 rpm;

[0027] (2) Obtain carbon source: the cellulase in the reaction system of step (1) is inactivated at high temperature, and the obtained enzymatic hydrolysis product is the carbon source, which is ready for use;

[0028] (3) Preparation of CQDs: The carbon source obtained in step (2) was added to a 50 ml polytetrafluoroethylene-lined reactor, and the reactor was placed in an environment of 200 ° C for 12 h. The reaction was naturally cooled to room temperature. The product in the reactor was filtered, and the supernatant was collected and dialyzed to a molecular weight cutoff of 1000 Da. The product was concentrated and freeze-dried to obtain a solid CQD product.

[0029] Example 3:

[0030] This embodiment is a method for preparing highly fluorescent cellulose-based carbon quantum dots, which comprises the following steps:

[0031] (1) Enzymatic hydrolysis of cellulose: 2.5% w / v cellulose and 20 FPU / g cellulase were placed in a conical flask, and the pH value of the reaction system was maintained at 5.0 with sodium citrate buffer. The enzymatic hydrolysis was then carried out at 55°C for 40 min at a rotation speed of 150 rpm.

[0032] (2) Obtaining a carbon source: inactivating the cellulase in the reaction system of step (1) at high temperature, and the resulting enzymatic hydrolysis product is the carbon source for later use;

[0033] (3) Preparation of CQDs: The carbon source obtained in step (2) was added to a 50 ml polytetrafluoroethylene-lined reactor, and the reactor was placed in an environment of 220°C for 15 h. The reaction was naturally cooled to room temperature. The product in the reactor was filtered, and the supernatant was collected and dialyzed to a molecular weight cutoff of 1000 Da. The product was concentrated and freeze-dried to obtain a solid CQD product.

[0034] Example 4:

[0035] This embodiment is a method for preparing highly fluorescent cellulose-based carbon quantum dots, which comprises the following steps:

[0036] (1) Enzymatic hydrolysis of cellulose: 2.0% w / v cellulose and 15 FPU / g cellulase were placed in a conical flask, and the pH of the reaction system was maintained at 4.8 with sodium citrate buffer. The enzymatic hydrolysis was then carried out at 50°C for 40 min at a rotation speed of 150 rpm.

[0037] (2) Obtaining a carbon source: The cellulase in the reaction system of step (1) was inactivated at high temperature, and then the solid product after the cellulolysis was washed, filtered, and dried in deionized water. Then, 0.5 g of the obtained dry product was mixed with 25 ml of deionized water to obtain a carbon source for use;

[0038] (3) Preparation of CQDs: The carbon source obtained in step (2) was placed in a 50 ml polytetrafluoroethylene-lined reaction kettle, and then the reaction kettle was placed in an environment of 200°C for 12 h, and naturally cooled to room temperature. The product in the reaction kettle was suction filtered, the supernatant was collected, dialysis was performed with a molecular weight cutoff of 1000 Da, concentrated, and freeze-dried to obtain solid CQDs.

[0039] Test example:

[0040] In a 500 mL conical flask, a reaction system of 200 mL was set up, 2% (w / v) of cellulose and 15 FPU / g of cellulase were added, and by controlling the enzymolysis time, products with different degrees of polymerization were obtained, respectively: 0 min, 1 min, 5 min, 40 min. Except for the different enzymolysis time, other ways were respectively according to the way described in Example 2 and the way described in Example 4, and the only difference between the two was the way of obtaining carbon source;

[0041] Cellulose with the above eight degrees of polymerization was used as carbon source to prepare cellulose-based carbon quantum dots by hydrothermal method; in Example 4, the enzymolysis time of 0 min, 1 min, 5 min and 40 min correspond to C-CQDs0, C-CQDs1, C-CQDs5 and C-CQDs 40 , respectively; in Example 2, the enzymolysis time of 1 min, 5 min and 40 min correspond to E-CQDs1, E-CQDs5 and E-CQDs 40 ;

[0042] The physicochemical properties and optical properties of C-CQDs and E-CQDs were characterized, and the relationship between the physicochemical properties and optical properties of C-CQDs and E-CQDs and the degree of polymerization of cellulose was explored.

[0043] Characterization:

[0044] The degree of polymerization of the cellulose after enzymolysis was determined by copper ethylenediamine method through a Ubbelohde viscometer. The degree of hydrolysis at the corresponding stage was measured by measuring the glucose content in the system by high performance liquid chromatography (HPLC). The cellulose powder samples before and after enzymolysis and the CQDs powder samples were tested by X-ray diffractometer (Rigaku D / MAX 2500V, Japan).

[0045] The scanning range was 2θ = 5°-80°. The UV-Vis absorption spectrum of the CQDs solution was determined by ultraviolet visible spectrophotometer (UV-Vis, Hitachi U-4100 UV-Vis, Japan) in the range of 200-800 nm.

[0046] Fourier transform infrared spectrometer (FTIR, TENSOR II, Bruker, Germany) was used to determine the chemical structure of CQDs in attenuated total reflectance (ATR) mode. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-alpha+, USA) was used to determine the content of elements such as carbon and oxygen and the composition of functional groups on the surface of CQDs. A transmission electron microscope (TEM, FEI TECNAI G2 F30, USA) was used to observe the microstructure of CQDs. A micro-Raman spectrometer (Raman) was used to analyze the degree of graphitization of CQDs. The laser wavelength of the instrument was 630 nm and the power was 0.10 mW. A fluorescence spectrometer (Hitachi RF-5301PC, Japan) was used to measure the fluorescence spectrum (FLs) and quantum yield of CQDs. When measuring FLs, the width of the excitation slit and the emission slit were both 3 nm. Through the above characterization, the formation mechanism of cellulose-based CQDs can be inferred, such as Figure 1 shown.

[0047] result:

[0048] The fluorescence intensity and fluorescence quantum yield at each enzymatic hydrolysis time were tested, and the data were recorded as shown in Table 1:

[0049] Table 1

[0050] fluorescence intensity fluorescence quantum yield (%) [C-CQDs0] 56539 0.7％ [C-CQDs1] 67375 0.71％ [C-CQDs5] 93000 0.99％ C-CQDs 40 ]]> 103314 1.87％ [E-CQDs1] 117628 1.98％ [E-CQDs5] 127373 2.51％ E-CQDs 40 ]]> 143497 2.77％

[0051] The data in Table 1 demonstrates that using enzymatically hydrolyzed cellulose as a carbon precursor to prepare CQDs improves their fluorescence properties. Furthermore, the test results show that using the enzymatic hydrolyzate directly as a carbon source (E-CQDs) exhibits higher fluorescence intensity and quantum yield than using the enzymatic hydrolyzate washed with deionized water, dried, and then mixed with deionized water as a carbon source (C-CQDs).

[0052] The examples described above merely illustrate several embodiments of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that variations and modifications are possible within the scope of the present invention, and such variations and modifications are within the scope of the present invention. Therefore, the scope of the present invention shall be determined by the appended claims.

Claims

1. A method for preparing highly fluorescent cellulose-based carbon quantum dots, characterized in that: The method comprises the following steps: (1) Enzymatic hydrolysis of cellulose: Cellulose and cellulase were placed in a conical flask, and the pH value of the reaction system was maintained at 4.5-5.0 with sodium citrate buffer, followed by enzymatic hydrolysis; (2) Obtaining a carbon source: inactivating the cellulase in the reaction system of step (1) at high temperature, and the resulting enzymatic hydrolysis product is the carbon source for later use; (3) Preparation of CQDs: The carbon source obtained in step (2) was added to a polytetrafluoroethylene-lined reactor, and the reactor was placed in an environment of 180-220°C for reaction for 10-15 hours, and naturally cooled to room temperature. The product in the reactor was filtered, and the supernatant was collected, dialyzed, concentrated, and freeze-dried to obtain a solid CQDs product.

2. The method according to claim 1, characterized in that The cellulose concentration in step (1) is 1.5-2.5% w / v, and the cellulase activity is 10-20 FPU / g.

3. The method according to claim 1, characterized in that The enzymatic hydrolysis temperature in step (1) is 45-55° C., and the enzymatic hydrolysis time is 1-40 min.

4. The method according to claim 1, wherein The molecular cutoff of the dialysis bag in step (3) is 1000Da.

Images