A method for preparing coffee grounds-based carbon dots

Abstract

The present invention belongs to the field of nanomaterial technology, specifically to a method for preparing coffee grounds-based blue fluorescent carbon dots. The method provides a method for synthesizing stable fluorescent carbon dots using waste coffee grounds as a precursor. The method comprises acid-hydrolyzing the waste coffee grounds, dispersing them in an ethanol solution to prepare a homogenate, and then performing a one-step hydrothermal reaction to prepare biomass carbon dots with excellent size distribution, fluorescence properties, biocompatibility, and water solubility. The method has the potential for widespread application in multiple 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 (Cdots) are carbon-based fluorescent nanomaterials that have attracted widespread attention due to their excellent biocompatibility, photoluminescence properties, bioactivity, and ease of synthesis. Currently, most Cdot precursors are simple compounds, such as ascorbic acid, citric acid, and ethylenediamine. However, with increasing awareness of environmental protection, the precursors for Cdot preparation have gradually shifted to biomass waste, such as food waste and agricultural waste straw, because these are rich in carbon and can serve as a good carbon source. Furthermore, they are rich in nitrogen, sulfur, and other elements, facilitating the self-doping of heteroelements during the synthesis of Cdots.

[0003] Coffee is the world's second most valuable commodity after petroleum and its derivatives. A large amount of biowaste, coffee grounds, is generated annually. These waste coffee grounds are rich in polysaccharides, proteins, and lipids, making them excellent precursors for the preparation of carbon dots. However, the primary treatment methods for waste coffee grounds are currently incineration and landfill, which not only pollutes the environment but also hinders their resource utilization. Exploring the preparation of fluorescent carbon dots using coffee grounds as precursors offers another avenue for reusing waste coffee grounds.

[0004] Although methods for preparing carbon dots using coffee grounds as precursors have been reported, the preparation process requires high-temperature carbonization and a strict temperature gradient. This preparation method is subject to problems such as expensive equipment, strict experimental conditions, and difficult operation, and is difficult to achieve large-scale production. Furthermore, most of the coffee ground-based carbon dots reported so far emit green fluorescence, which is limited by the monochromatic nature of their fluorescence emission. Therefore, it is of great significance to explore the preparation of blue-emitting carbon dots using waste coffee grounds as precursors through a one-step hydrothermal method. Summary of the Invention

[0005] The present invention aims to solve the technical problem of preparing carbon dots using waste coffee grounds as precursors and provides a method for synthesizing carbon dots via a one-step hydrothermal method. The steps are as follows:

[0006] A method for preparing coffee grounds-based carbon dots uses waste coffee grounds as a carbon source. It first pre-treats them under acidic conditions, then disperses the treated waste coffee grounds in a 50% ethanol solution, and produces 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, and then filtering them;

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

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

[0012] (5) adding 50% ethanol solution to the above precipitate, stirring and ultrasonicating it to fully disperse it to prepare 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 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 in step (3) are 70% concentrated sulfuric acid, a hot water bath temperature of 70° C., and an acid hydrolysis time of 45 minutes.

[0016] In 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 200° C. for 12 h.

[0018] The separation conditions in step (7) are a rotation speed of 10,000 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 with a dialysis bag (<1,000 Da) for 48 hours, changing the water every 8 hours.

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

[0020] Using the above preparation method, a carbon dot nanomaterial with blue fluorescence emission was synthesized. Its average particle size was 11.37±2.17 nm, and it exhibited excellent fluorescence properties and water solubility. In Example 2, the coffee grounds-based carbon dots exhibited a maximum fluorescence emission wavelength of 430 nm at a maximum excitation wavelength of 360 nm. In Example 3, the coffee grounds-based carbon dots exhibited a rich surface area with functional groups, which suggests a broad range of applications.

[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 possess more excellent fluorescence properties.

[0024] (3) In the present invention, waste coffee grounds were treated under acidic conditions and then fully dispersed in a 50% ethanol solution. Coffee ground-based carbon dots were then synthesized via a one-step hydrothermal method. This method eliminates the need for high-temperature carbonization pretreatment of the waste coffee grounds, effectively shortening the synthesis process. The method is easy to operate and time-efficient, while also enabling 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 path 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 TEM images of carbon dots prepared from coffee ground precursors of different masses in Example 1;

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

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

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

[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 solutions 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 intended to limit the present invention.

[0034] Example 1

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

[0036] (2) Reagents: All reagents are 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 them. The residues are then oven-dried at 60°C for 24 hours. Next, pass the dried coffee grounds through a 60-mesh sieve. Accurately weigh 0.5g, 1.0g, and 1.5g of the sieved grounds, wash them three times with 50mL of anhydrous ethanol, and filter. The residues are then washed three times with 50mL of deionized water and filtered. The residues are placed in a beaker and acid-hydrolyzed in 10mL of 70% concentrated sulfuric acid in a 70°C waterbath for 45 minutes. After cooling, dilute with deionized water, centrifuge, discard the supernatant, and wash the precipitate three times with deionized water before filtering. Add 25mL of 50% ethanol solution to the precipitate, stir, and sonicate for 30 minutes to fully disperse it in the 50% ethanol solution to create a homogenate. The homogenate is then transferred to a 50mL hydrothermal reactor, which is then heated in a dry oven at 200°C for 12 hours. After the reaction was complete, the hydrothermal reactor was cooled to room temperature and the reaction mixture was transferred to a centrifuge tube and centrifuged (10,000 rpm for 10 minutes). The supernatant was filtered through a 0.22 μm filter membrane and dialyzed using a dialysis bag (<1000 Da) for 48 hours, with the water changed every 8 hours. The dialyzed solution was freeze-dried to obtain coffee ground carbon dot powder, and a portion was dissolved to prepare a coffee ground carbon dot aqueous solution for particle size analysis.

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

[0042] Example 2

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

[0044] (2) Reagents: All reagents are 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 times were 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 at different hydrothermal reaction times show that a 12-hour reaction time yields a higher yield of waste coffee grounds and a more uniform particle size distribution. Shorter hydrothermal reaction times result in incomplete carbonization of the coffee grounds, while longer reaction times lead to aggregation of the dispersed carbon dots.

[0050] Example 3

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

[0052] (2) Reagents: All reagents are 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 TEM images of carbon dots prepared from coffee grounds at different hydrothermal reaction temperatures show that the particle size distribution of waste coffee grounds is optimal at 200°C. Lower hydrothermal temperatures result in incomplete carbonization of the coffee grounds, resulting in larger particles. However, higher temperatures lead to excessive carbonization of the coffee grounds, resulting in direct loss of the precursor as a gas.

[0057] Example 4

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

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

[0060] (3) Instruments and equipment

[0061]

[0062] Preparation of waste coffee grounds-based carbon dots. Based on Example 1, the mass of waste coffee grounds was 1 g, the hydrothermal reaction time was 12 h, and the hydrothermal reaction temperature was 200°C. All 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 observed the ultraviolet-visible absorption spectrum of coffee grounds-based carbon dots and found that Figure 5 As shown, the coffee grounds-based carbon dots exhibit a broad absorption peak around 270 nm, attributed to the π-π* transition of C=C. When excited at a wavelength of 360 nm, the coffee grounds-based carbon dots exhibit strong blue fluorescence emission, with a maximum emission wavelength at 430 nm, demonstrating their photoluminescence properties. Normalized photoluminescence spectra reveal an excitation-dependent fluorescence emission. Specifically, as the excitation wavelength increases from 300 to 400 nm, the fluorescence emission peak of the coffee grounds-based carbon dots gradually redshifts, moving toward 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 of the 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, XPS analysis of coffee grounds-based carbon dots reveals that the full XPS spectrum confirms the presence of abundant C, N, and O elements on the surface of coffee grounds-based carbon dots. Na and S are also present. Na originates from the raw material, while S is due to the pretreatment of SCG with sulfuric acid. The component detected at 284.8 eV in the C1s spectrum of coffee grounds-based carbon dots is likely CC or OC=O. In the O 1s spectrum, O=CO is detected at 531.4 eV. The peak at 401.5 eV in the N1s spectrum is attributed to -NH.

[0066] Through the above characterization analysis, it was found that the coffee grounds-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 embodiment of the present invention, but is not limited to the above embodiments. For technicians in the technical field to which the present invention belongs, they can make some improvements and optimizations without departing from the content of the present invention, which should be regarded as belonging to the scope of protection of the present invention.

Claims

1. A method for preparing coffee grounds-based carbon dots, characterized in that: The specific plan is as follows: (1) Wash the waste coffee grounds with deionized water, dry them in an oven, and sieve them; (2) Weigh and sieve the waste coffee grounds, wash them with anhydrous ethanol and deionized water, and then filter them; (3) Place the filter residue in a beaker, add concentrated sulfuric acid and acid hydrolyze in a hot water bath; (4) Cool the acid hydrolysis solution, add deionized water to dilute and centrifuge, discard the supernatant, then wash the precipitate with deionized water and filter; (5) Add 50% ethanol solution to the above precipitate, stir and ultrasonicate to fully disperse it to prepare a homogenate; (6) Transfer the homogenate to a hydrothermal reactor and place it in a drying oven for hydrothermal reaction; (7) Separating and purifying the hydrothermal reaction solution and then freeze-drying it to obtain carbon dot powder; The acid hydrolysis conditions in step (3) are 70% concentrated sulfuric acid, a hot water bath temperature of 70°C, and an acid hydrolysis time of 45 min; In step (5), the mass volume ratio of waste coffee grounds to anhydrous ethanol is 1 g: (25-30) mL; The hydrothermal reaction conditions of step (6) are 200° C. for 12 h.

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

3. The method for preparing coffee grounds-based carbon dots according to claim 1, wherein: The stirring and ultrasonication time in step (5) is 30 min.

4. The method for preparing coffee grounds-based carbon dots according to claim 1, wherein: The separation conditions in step (7) are a rotation speed of 10,000 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 with a dialysis bag for 48 h, changing the water every 8 h.

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