Preparation method of coffee residue-based carbon dots

Abstract

The invention belongs to the technical field of nano materials, and particularly relates to a preparation method of coffee-residue-based blue fluorescent carbon dots. The invention provides a preparation method for synthesizing stable fluorescence emission carbon dots by taking waste coffee grounds as precursors, which mainly comprises the following steps of: performing acidolysis on the waste coffee grounds, dispersing the acidolysis waste coffee grounds in an ethanol solution to prepare homogenate, and performing one-step hydrothermal reaction to prepare biomass carbon dots with good size distribution, fluorescence characteristic, biocompatibility and water solubility. The method has wide application potential in a plurality of fields.

Description

Technical Field

[0001] The invention belongs to the technical field of nanomaterials, and particularly relates to a method for preparing coffee grounds-based blue fluorescent carbon dots. Background Art

[0002] Carbon dots are a type of carbon-based fluorescent nanomaterials that have attracted widespread attention due to their good biocompatibility, photoluminescent properties, biological activity, and ease of synthesis. At present, most carbon dots are prepared from simple compounds such as ascorbic acid, citric acid, and ethylenediamine. However, as people's awareness of environmental protection continues to increase, they have gradually turned their precursors to biomass waste such as food waste and agricultural waste straw, because they are rich in carbon and can serve as a good carbon source. At the same time, they are rich in nitrogen, sulfur, and other elements, which facilitates the self-doping of heteroelements in the synthesis of carbon dots.

[0003] Coffee is the world's second most valuable commodity after petroleum and its derivatives. Every year, a large amount of biowaste coffee grounds is produced. Waste coffee grounds are rich in polysaccharides, proteins, lipids and other substances, which can be used as a good precursor for the preparation of carbon dots. However, the main treatment methods for waste coffee grounds are incineration and landfill, which not only causes environmental pollution, but also fails to realize the resource utilization of coffee grounds. Exploring the preparation of fluorescent carbon dots using coffee grounds as precursors is another way to realize the resource utilization of waste coffee grounds.

[0004] Although a method for preparing carbon dots using coffee grounds as precursors has been reported, the preparation process requires high-temperature carbonization treatment and strictly requires a carbonization temperature gradient. This preparation method has problems such as expensive equipment, strict experimental conditions, and difficult operation, and it is difficult to achieve large-scale production. At the same time, most of the coffee grounds-based carbon dots reported so far emit green fluorescence, and their fluorescence emission has a monochromatic limitation. Therefore, it is of great significance to explore the use of waste coffee grounds as precursors to prepare blue fluorescent carbon dots through a one-step hydrothermal method. Summary of the invention

[0005] The purpose of the present invention is to solve the technical problem of preparing carbon dots using waste coffee grounds as precursors, and to provide a method for synthesizing carbon dots by a one-step hydrothermal method, the steps of which are as follows:

[0006] A method for preparing coffee grounds-based carbon dots, using waste coffee grounds as a carbon source, first pre-treating the waste coffee grounds under acidic conditions, then dispersing the treated waste coffee grounds in a 50% ethanol solution, and obtaining light yellow carbon dot powder through a one-step hydrothermal reaction.

[0007] A method for preparing coffee grounds-based carbon dots, the specific scheme is as follows:

[0008] (1) washing the waste coffee grounds with deionized water, drying them in an oven, and sieving them;

[0009] (2) weighing and sieving the waste coffee grounds, washing them with anhydrous ethanol and deionized water respectively, and then filtering them;

[0010] (3) Place the filter residue in a beaker and add concentrated sulfuric acid to perform acid hydrolysis in a hot water bath;

[0011] (4) cooling the acid hydrolysis solution, adding deionized water to dilute and centrifuge, discarding the supernatant, then washing the precipitate with deionized water and filtering;

[0012] (5) adding 50% ethanol solution to the above precipitate, stirring and ultrasonicating to fully disperse it to obtain a homogenate; (6) transferring the above homogenate to a hydrothermal reactor and placing it in a drying oven for hydrothermal reaction;

[0013] (7) The hydrothermal reaction liquid is separated, purified, and then freeze-dried to obtain carbon dot powder.

[0014] The drying condition of step (1) is drying at 60° C. for 12 h.

[0015] The acid hydrolysis conditions of step (3) are 70% concentrated sulfuric acid, a hot water bath temperature of 70° C., and an acid hydrolysis time of 45 min.

[0016] In the step (5), the mass volume ratio of the waste coffee grounds to anhydrous ethanol is 1 g: (25-30) mL; and the stirring and ultrasonication time is 30 min.

[0017] The hydrothermal reaction conditions of step (6) are to react at 200° C. for 12 hours.

[0018] The separation conditions in step (7) are a rotation speed of 10000 rpm and a separation time of 10 min; the purification conditions are to pass the reaction solution through a 0.22 μm filter membrane and then dialyze it using a dialysis bag (<1000 Da) for 48 hours, with the water being changed every 8 hours.

[0019] A method for preparing coffee grounds-based carbon dots can produce carbon dots with blue fluorescence emission.

[0020] According to the above preparation method, a carbon dot nanomaterial with blue fluorescence emission was synthesized, with an average particle size of 11.37±2.17nm, good fluorescence properties and water solubility. In Example 2, when the maximum excitation wavelength is 360nm, the maximum fluorescence emission wavelength of the coffee residue-based carbon dots is 430nm. In Example 3, the surface of the coffee residue-based carbon dots contains abundant functional groups, which makes it have a wide range of application prospects.

[0021] Compared with the prior art, the technical method of the present invention has the following beneficial effects:

[0022] (1) The present invention provides a method for preparing carbon dots that emit blue fluorescence. The prepared carbon dots emit stable blue fluorescence under ultraviolet irradiation conditions, which helps to solve the problem of the single color of fluorescence emission of coffee grounds-based carbon dots.

[0023] (2) The coffee grounds-based carbon dots prepared in the present invention have a uniform particle size distribution, an average particle size of less than 20 nm, and have more excellent fluorescence properties.

[0024] (3) In the present invention, the waste coffee grounds are treated under acidic conditions and then fully dispersed in a 50% ethanol solution, and then coffee grounds-based carbon dots are synthesized by a one-step hydrothermal method. There is no need to perform high-temperature carbonization pretreatment on the waste coffee grounds, which effectively reduces the synthesis process flow, has the characteristics of easy operation and short time consumption, and can also achieve large-scale production.

[0025] (4) The present invention enriches the sources of precursors for the preparation of carbon dots. Compared with traditional compound carbon-based materials, the present invention uses waste coffee grounds as precursors, which is green and pollution-free, and at the same time broadens the resource utilization channels of waste coffee grounds. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Flow chart of the preparation process of coffee grounds-based carbon dots;

[0027] Figure 2 Example 1 TEM images of carbon dots prepared from coffee grounds precursors of different masses;

[0028] Figure 3 Example 2 TEM images of coffee grounds-based carbon dots at different hydrothermal reaction times;

[0029] Figure 4 Example 3 TEM images of coffee grounds-based carbon dots at different hydrothermal reaction temperatures;

[0030] Figure 5 Example 4: (a) shows the UV-visible absorption spectrum of coffee grounds-based carbon dots, (b) shows the photoluminescence spectrum, and (c) shows the normalized photoluminescence spectrum;

[0031] Figure 6 Example 4 Fourier transform infrared spectroscopy (FTIR) of coffee grounds based carbon dots;

[0032] Figure 7 Example 4 X-ray Photoelectron Spectroscopy (XPS) of Coffee Grounds-based Carbon Dots. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] Example 1

[0035] (1) Raw materials: Waste coffee grounds

[0036] (2) Reagents: All reagents were analytical grade

[0037] (3) Instruments and equipment

[0038]

[0039] (4) The present invention provides a method for preparing carbon dots using waste coffee grounds as a precursor, the specific steps of which are as follows:

[0040] First, wash the coffee grounds with deionized water and filter, then put the filter residue in an oven and dry it at 60°C for 24h. Secondly, pass the dried coffee grounds through a 60-mesh sieve, accurately weigh 0.5g, 1.0g, and 1.5g of the sieved coffee grounds, wash them three times with 50mL of anhydrous ethanol, and filter; wash the filter residue three times with 50mL of deionized water, and filter; place the filter residue in a beaker, add 10mL of 70% concentrated sulfuric acid, and acid hydrolyze it in a 70°C water bath for 45min; after cooling, add deionized water to dilute and centrifuge, discard the supernatant, wash the precipitate with deionized water three times, and filter. Add 25mL of 50% ethanol solution to the above precipitate, stir and ultrasonicate for 30min, so that it is fully dispersed in the 50% ethanol solution to obtain a homogenate. Transfer the above homogenate to a 50mL hydrothermal reactor, then place the hydrothermal reactor in a drying oven and heat it at 200°C for 12h. After the reaction is completed, the hydrothermal reactor is cooled to room temperature, and the reaction solution is transferred to a centrifuge tube for centrifugation (10000rpm, 10min). The supernatant after centrifugation is filtered through a 0.22μm filter membrane, and the filtered solution is dialyzed with a dialysis bag (<1000Da) for 48h, with water changed every 8h. The dialyzed solution is freeze-dried to obtain coffee grounds carbon dot powder, and a portion of it is dissolved to prepare a coffee grounds carbon dot aqueous solution for particle size testing.

[0041] Figure 2 TEM images of carbon dots prepared from different weights of coffee grounds precursors. The results show that the carbonization effect of waste coffee grounds is best when the weight is 1g. When the amount is too much, cross-linking is likely to occur, while when the amount is too little, the yield is low.

[0042] Example 2

[0043] (1) Raw materials: Waste coffee grounds

[0044] (2) Reagents: All reagents were analytical grade

[0045] (3) Instruments and equipment

[0046]

[0047]

[0048] (4) Based on Example 1, the mass of the waste coffee grounds was determined to be 1 g; the hydrothermal reaction time was set to 8 h, 12 h, and 16 h, respectively; the other steps and parameters were the same as in Example 1.

[0049] Figure 3 TEM images of carbon dots prepared from coffee grounds precursors at different hydrothermal reaction times. The results show that the yield of waste coffee grounds is better when the reaction time is 12h, and the particle size distribution is more uniform. When the hydrothermal reaction time is short, the carbonization of coffee grounds is not sufficient, while if the reaction time is too long, the carbonized and dispersed carbon dots will aggregate.

[0050] Example 3

[0051] (1) Raw materials: Waste coffee grounds

[0052] (2) Reagents: All reagents were analytical grade

[0053] (3) Instruments and equipment

[0054]

[0055] (4) Based on Example 1, the mass of the waste coffee grounds was determined to be 1 g, the hydrothermal reaction time was 12 h, and the hydrothermal reaction temperatures were set to 160° C., 200° C., and 240° C., respectively. The other steps and parameters were the same as in Example 1.

[0056] Figure 4 The TEM images of carbon dots prepared from coffee grounds precursor at different hydrothermal reaction temperatures show that the particle size distribution of waste coffee grounds is optimal when the reaction temperature is 200°C. When the hydrothermal reaction temperature is low, the coffee grounds are not completely carbonized and the particle size is large, while too high a reaction temperature leads to excessive carbonization of the coffee grounds precursor and direct loss in the form of gas.

[0057] Example 4

[0058] (1) Raw materials: Waste coffee grounds

[0059] (2) Reagents: All reagents were analytical grade

[0060] (3) Instruments and equipment

[0061]

[0062] Preparation of waste coffee grounds-based carbon dots. Based on Example 1, the mass of the waste coffee grounds was determined to be 1 g, the hydrothermal reaction time was 12 h, the hydrothermal reaction temperature was 200° C., and the other steps and parameters were the same as in Example 1. The optical properties and surface structure of the prepared coffee grounds-based carbon dots were characterized.

[0063] The present invention observes the ultraviolet-visible absorption spectrum of coffee grounds-based carbon dots and finds that Figure 5 As shown in the figure, it has a broad absorption peak at around 270nm, which is attributed to the π-π* transition of C=C. When the excitation wavelength is 360nm, the coffee grounds-based carbon dots show strong blue fluorescence emission with a maximum emission wavelength of 430nm, indicating that the prepared coffee grounds-based carbon dots have photoluminescence properties. After normalizing the photoluminescence spectrum, it was observed that the fluorescence emission of the coffee grounds-based carbon dots showed an excitation-dependent characteristic, that is, as the excitation wavelength increases from 300nm to 400nm, the fluorescence emission peak of the coffee grounds-based carbon dots gradually red-shifts and moves in the direction of increasing wavelength.

[0064] like Figure 6 As shown in the figure, the present invention performs Fourier transform infrared spectroscopy observation and analysis on coffee grounds-based carbon dots. The coffee grounds-based carbon dots have a peak at 3202.11 cm -1 The characteristic peak at 3039.32 cm is attributed to the -OH stretching vibration on the surface of coffee grounds-based carbon dots, while the peak at 3039.32 cm -1 The characteristic peak at 1409.89 cm is the -COOH or -NH vibration of the dimer. -1 and 1074.58cm -1 The former is attributed to the bending vibration of olefin CH or the stretching vibration of amide CN, while the latter is formed by the stretching vibration of C-OH.

[0065] like Figure 7 As shown, the present invention analyzes the XPS spectrum of coffee grounds-based carbon dots and finds that the full XPS spectrum confirms that the surface of coffee grounds-based carbon dots contains abundant C, N, and O elements, as well as Na, S and other elements. The Na element comes from the raw materials, and the S element appears because of the pretreatment of SCG with sulfuric acid in the early stage. The component detected at 284.8eV in the C1s spectrum of coffee grounds-based carbon dots may be CC or OC=O; in the O 1s spectrum, O=CO is detected at 531.4eV; and the peak at 401.5eV in the N1s spectrum is attributed to -NH.

[0066] Through the above characterization analysis, it is found that the coffee residue-based carbon dots prepared in the present invention have excellent particle size distribution, stable fluorescence emission, rich surface functional groups, and have good application prospects in multiple fields.

[0067] The above content describes the technical principles, beneficial effects and features of the present invention. It should be pointed out that the above is only a preferred implementation mode of the present invention, but is not limited to the above embodiments. For technicians in the technical field to which the present invention belongs, some improvements and optimizations can be made without departing from the content of the present invention, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for preparing coffee grounds-based carbon dots, characterized in that: Waste coffee grounds were used as the carbon source and were first pretreated under acidic conditions. The treated waste coffee grounds were then dispersed in an ethanol solution and a light yellow carbon dot powder was obtained through a one-step hydrothermal reaction.

2. The method for preparing coffee grounds-based carbon dots according to claim 1, wherein: The specific plan is as follows: (1) washing the waste coffee grounds with deionized water, drying them in an oven, and sieving them; (2) weighing and sieving the waste coffee grounds, washing them with anhydrous ethanol and deionized water respectively, and then filtering them; (3) Place the filter residue in a beaker and add concentrated sulfuric acid to perform acid hydrolysis in a hot water bath; (4) cooling the acid hydrolysis solution, adding deionized water to dilute and centrifuge, discarding the supernatant, then washing the precipitate with deionized water and filtering; (5) adding 50% ethanol solution to the above precipitate, stirring and ultrasonicating to fully disperse it to obtain a homogenate; (6) transferring the homogenate to a hydrothermal reaction kettle and placing it in a drying oven for hydrothermal reaction; (7) The hydrothermal reaction liquid is separated, purified, and then freeze-dried to obtain carbon dot powder.

3. The method for preparing coffee grounds-based carbon dots according to claim 2, wherein: The drying condition of step (1) is drying at 60° C. for 12 hours.

4. The method for preparing coffee grounds-based carbon dots according to claim 2, wherein: The acid hydrolysis conditions of step (3) are 70% concentrated sulfuric acid, a hot water bath temperature of 70° C., and an acid hydrolysis time of 45 min.

5. The method for preparing coffee grounds-based carbon dots according to claim 2, wherein: In the step (5), the mass volume ratio of the waste coffee grounds to anhydrous ethanol is 1 g: (25-30) mL.

6. The method for preparing coffee grounds-based carbon dots according to claim 2, wherein: The stirring and ultrasonication time in step (5) is 30 minutes.

7. The method for preparing coffee grounds-based carbon dots according to claim 2, wherein: The hydrothermal reaction conditions of step (6) are to react at 200° C. for 12 hours.

8. The method for preparing coffee grounds-based carbon dots according to claim 2, wherein: The separation conditions in step (7) are a rotation speed of 10000 rpm and a separation time of 10 min; the purification conditions are to filter the reaction solution through a 0.22 μm filter membrane and then dialyze it with a dialysis bag for 48 hours, changing the water every 8 hours.

9. The carbon dots prepared by the method for preparing coffee grounds-based carbon dots as described in claims 1-8 have blue fluorescence emission.

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