Carbon quantum dots and method for producing carbon quantum dots

Abstract

Carbon quantum dots that are characterized by having a zeta potential of -44–-1.1 mV when dispersed in water and that are also characterized in that (1) the lattice spacing of the lattice fringe from the (100) plane of the graphite in a transmission electron microscope image thereof is 0.200–0.234 nm or in that (2) the average particle size (D50) thereof as obtained by dynamic light scattering is 3.1–8.7 nm. These carbon quantum dots can exhibit favorable and stable fluorescence characteristics with favorable reproducibility.

Description

[Technical field]

[0001] The present invention relates to carbon quantum dots (hereinafter also referred to as C-QDs) and a method for producing carbon quantum dots. [Background technology]

[0002] In recent years, carbon quantum dots have been attracting attention due to their chemical stability, excellent solubility in water, inexpensive raw materials, low manufacturing costs, and good and stable fluorescent properties. In particular, carbon quantum dots, unlike conventional metal-based (e.g., Cd, Se, etc.) quantum dots, have excellent properties such as low toxicity and biocompatibility. In addition, because they are chemically stable and have good water compatibility, they are considered to be particularly promising for use and applications in the bio field.

[0003] Under these circumstances, various methods for producing carbon quantum dots using so-called "green" (environmentally friendly) substances as carbon sources, that is, various natural materials that are relatively inexpensive, have been tried. For example, there are methods for producing carbon quantum dots using raw materials such as chitosan, orange juice, lemon peel (Non-Patent Document 1), egg yolk oil (Non-Patent Document 2), bee pollen (Non-Patent Document 3), collagen (Non-Patent Document 4), humic substances (Non-Patent Document 5), hair (Non-Patent Document 6), peanut shells (Non-Patent Document 7), soy milk (Non-Patent Document 8), cashew resin (Non-Patent Document 9), and garlic (Non-Patent Document 10).

[0004] However, the above-mentioned natural materials, especially those derived from plants, are subject to seasonal variations in quality, shape, size, etc. In addition, the quality and shape often vary depending on the geographical conditions and weather of the production area. When using raw materials that are subject to such seasonal and geographical differences, the carbon quantum dots obtained are prone to various variations. For example, variations in particle size, particle size distribution, types of impurities, and their amount of inclusion. It is known that there is a correlation between the particle size of quantum dots and the emission wavelength obtained from them, and variations in the particle size of quantum dots for each production lot cause inconvenience. In addition, if the type, size, and amount of impurities differ for each raw material lot, the fluorescent properties of the resulting carbon quantum dots will naturally vary, resulting in problems with the emission intensity and lifespan of C-QDs.

[0005] By the way, there are two main methods for producing carbon quantum dots. One is the "top-down" method, in which carbon-containing materials (such as graphite, carbon fiber, candle soot, etc.) are broken down in some way to produce carbon quantum dots. The other is the "bottom-up" method, in which carbon-containing precursor molecules are polymerized to produce carbon quantum dots. In the former method, carbon quantum dots are produced by breaking down a carbon-containing substance using techniques such as electrical discharge, laser ablation, and oxidation reactions, but it is extremely difficult to control the particle size distribution of the resulting carbon quantum dots. In the latter method, precursor molecules are polymerized using a hydrothermal synthesis method, a solvothermal method, or the like, but the manufacturing process is complicated and requires troublesome manufacturing process control. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Xin Liu, et al., Sci Rep. 6, 31100 (2016) [Non-Patent Document 2] Yan Zhao, et al., Sci Rep. 7, 4452 (2017) [Non-Patent Document 3] Xiaochen Guo, et al., ChemPhotoChem, 1, 116-119 (2017) [Non-Patent Document 4] Qiao-Ling Chen, et al., Sci Rep., 6, 19382 (2016) [Non-Patent Document 5] Yongming Dong, et al., Sci Rep. 5, 10037 (2015) [Non-Patent Document 6] Yongming Dong, et al., Sci Rep., 6 35795 (2016) [Non-Patent Document 7] Mingyue Xue, et al., New J. Chem. 40, 1698-1703 (2016) [Non-Patent Document 8] Chengzhou Zhu, et al., Chem. Commun., 48, 9367-9369 (2012) [Non-Patent Document 9] Natalia R. Pires, et al., J. Braz. Chem. Soc., 26 (6),1274-1282, (2015) [Non-Patent Document 10] Shaojing Zhao, et al., ACS Appl. Mater. Interfaces 7, 17054-17060 (2015) Summary of the Invention [Problem to be solved by the invention]

[0007] In the production of carbon quantum dots, the carbon source material should be environmentally friendly, economical, and less susceptible to seasonal variations and differences in production area. In addition, the obtained carbon quantum dots should have good fluorescent properties.

[0008] However, the carbon quantum dots and the manufacturing methods thereof that have been proposed thus far are often insufficient in terms of the cost, the difficulty of manufacturing, and the fluorescent emission performance or characteristics of the obtained carbon quantum dots, and satisfactory carbon quantum dots and methods for manufacturing such carbon quantum dots at low cost and easily have not yet been obtained.

[0009] The present invention has been made in view of the above circumstances, and aims to provide carbon quantum dots having good fluorescent properties that are environmentally friendly, are made from economical raw materials, and are produced by a relatively easy method, and a method for producing such quantum dots. [Means for solving the problem]

[0010] The present inventors provide the following invention. [1] The zeta potential when dispersed in water is -44 to -1.1 mV. The lattice spacing of the lattice fringes due to the (100) plane of graphite, as seen in transmission electron microscope images, is in the range of 0.200 to 0.234 nm. A carbon quantum dot characterized by: [2] The zeta potential when dispersed in water is -44 to -1.1 mV. The average particle size (D50) obtained by dynamic light scattering is 3.1 to 8.7 nm. A carbon quantum dot characterized by: [3] The carbon quantum dots according to [2], characterized in that the full width at half maximum of the particle size distribution obtained by dynamic light scattering is 0.6 to 11.7 nm. [4] The carbon quantum dot according to [2] or [3], characterized in that the lattice spacing of lattice fringes resulting from the (100) plane of graphite, as seen in an image taken by a transmission electron microscope, is in the range of 0.200 to 0.234 nm. [5] The carbon quantum dot according to any one of [1] to [4], characterized in that it is crystallized in a hexagonal system. [6] The ratio of carbon and nitrogen elements by X-ray photoelectron spectroscopy C: 55~80% N: Less than 0.5% The carbon quantum dot according to any one of [1] to [5], [7] The carbon quantum dot according to [6], characterized in that the elemental ratio of oxygen measured by X-ray photoelectron spectroscopy is 20 to 45%. [8] The ratio of carbon and nitrogen elements by X-ray photoelectron spectroscopy C: 55~75% N: 2-10% The carbon quantum dot according to any one of [1] to [5], [9] The carbon quantum dot according to [8], characterized in that the elemental ratio of oxygen measured by X-ray photoelectron spectroscopy is 20 to 40%.

[10] A method for producing carbon quantum dots, comprising a step of pyrolyzing plant seeds to obtain a carbonized product.

[11] The method for producing carbon quantum dots according to

[10] , wherein the plant seeds are fennel seeds.

[12] The method for producing carbon quantum dots according to

[10] or

[11] , wherein the thermal decomposition is performed by electromagnetic induction.

[13] The method for producing carbon quantum dots according to any one of

[10] to

[12] , wherein the pyrolysis is carried out at 330 to 600°C.

[14] The method for producing carbon quantum dots according to

[13] , wherein the heating time in the pyrolysis is 2 to 4 hours.

[15] The method for producing carbon quantum dots according to any one of

[10] to

[14] , wherein the seeds of the plant are crushed before the pyrolysis is carried out.

[16] A method for producing carbon quantum dots, comprising the step of carbonizing a carbon-containing substance by a plasma CVD method.

[17] The method for producing carbon quantum dots according to

[16] , wherein the carbon-containing substance has an elemental ratio of carbon of 40 to 95%.

[18] The method for producing carbon quantum dots according to

[16] or

[17] , wherein the carbon-containing substance contains nitrogen.

[19] The method for producing carbon quantum dots according to

[18] , wherein the carbon-containing material is a plant seed.

[20] The method for producing carbon quantum dots according to

[19] , wherein the plant seeds are fenugreek seeds.

[21] The method for producing carbon quantum dots according to

[18] , wherein the carbon-containing substance is a protein.

[22] The method for producing carbon quantum dots according to

[21] , wherein the protein is glucose isomerase or papain.

[23] The method for producing carbon quantum dots according to any one of

[16] to

[22] , wherein the plasma CVD method is a plasma CVD method using hydrogen gas. Effect of the Invention

[0011] The carbon quantum dots according to the present invention can exhibit excellent and stable fluorescent properties with good reproducibility. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram showing a flow of a method for producing carbon quantum dots (thermal decomposition method) according to one embodiment of the present invention. [Diagram 2] FIG. 4 is a diagram showing a flow of a method (plasma method) for producing carbon quantum dots according to another embodiment of the present invention. [Diagram 3] FIG. 2 is a diagram showing an optical emission spectroscopy spectrum of the hydrogen plasma used in Examples 2 and 3. [Figure 4] 1 shows photographs of the C-QDs dispersion obtained in Example 1 taken under normal light and under black light irradiation. [Diagram 5] FIG. 1 shows the ultraviolet-visible light absorption spectrum of the C-QDs obtained in Example 1. [Figure 6]TEM images of C-QDs obtained in Example 1, (A) is a low-resolution image, and (B) is a high-resolution image. [Figure 7] 1 is a graph showing the particle size distribution of C-QDs obtained in Example 1. [Figure 8] 1 is a high-resolution TEM image of the C-QDs obtained in Example 1. [Figure 9] FIG. 2 is a diagram of an output spectrum showing the results of elemental analysis using an energy dispersive X-ray analyzer. [Figure 10] 1 is a graph showing the measurement results of the particle size distribution of C-QDs using dynamic light scattering. [Figure 11] 1 is a graph showing the results of measuring the zeta potential of the C-QDs obtained in Example 1. [Figure 12] Photographs of C-QDs-containing water immediately after production ((A) and (C)) and C-QDs-containing water 6 months after production ((B) and (D)). (A) and (B) are photographs taken under normal light, while (C) and (D) are photographs taken under black light. [Figure 13] 1 shows FT-IR spectra, where (a) is the FT-IR spectrum of the fennel seeds used in Example 1, and (b) is the FT-IR spectrum of the C-QDs obtained in Example 1. [Figure 14] 1 shows X-ray photoelectron spectroscopy (XPS) spectra, where (a) is the XPS spectrum of the C-QDs obtained in Example 1, and (b) is the XPS spectrum of the crushed fennel seeds used as the raw material. [Figure 15] Figures showing X-ray photoelectron spectroscopy (XPS) spectra. (a) shows the result of deconvoluting the peak corresponding to the carbon 1s orbital obtained by XPS into three major peaks, and (b) shows the result of deconvoluting the peak corresponding to the oxygen 2p orbital obtained by XPS into two major peaks. [Figure 16] FIG. 1 shows fluorescence emission spectra for excitation light of different wavelengths, where (a) shows the observed spectrum and (b) shows the normalized spectrum. [Figure 17] 1 is a graph showing the change over time in the fluorescence emission intensity of C-QDs obtained in Example 1 when the C-QDs were continuously irradiated with light from a xenon lamp. [Figure 18] 1 is a graph showing the fluorescence emission spectra of the C-QDs obtained in Example 1 under various pH conditions. [Figure 19] This is a graph showing the fluorescence emission obtained when 800 nm excitation light was irradiated onto the C-QDs obtained in Example 1, and shows that upconversion is occurring. [Figure 20] FIG. 1 is a schematic diagram showing a plasma CVD apparatus used in Examples 2 and 3. [Figure 21] 1A and 1B are photographs of the C-QDs dispersion obtained in Example 2 taken under normal light and under black light irradiation. [Figure 22] FIG. 1 shows the ultraviolet-visible light absorption spectrum of the C-QDs obtained in Example 2. [Diagram 23] TEM images of C-QDs obtained in Example 2, where (A) is a low-resolution image and (B) is a high-resolution image. [Figure 24] 1 is a graph showing the particle size distribution of C-QDs obtained in Example 2. [Diagram 25] 1 is a high-resolution TEM image of the C-QDs obtained in Example 2. [Figure 26] FIG. 2 is a diagram of an output spectrum showing the results of elemental analysis using an energy dispersive X-ray analyzer. [Figure 27] 1 is a graph showing the measurement results of the particle size distribution of C-QDs using dynamic light scattering. [Figure 28] 1 is a graph showing the results of measuring the zeta potential of the C-QDs obtained in Example 2. [Figure 29] Photographs of C-QDs-containing water immediately after production ((A) and (C)) and C-QDs-containing water 6 months after production ((B) and (D)). (A) and (B) are photographs taken under normal light, while (C) and (D) are photographs taken under black light. [Diagram 30] 1 shows FT-IR spectra, where (A) is the FT-IR spectrum of the fenugreek seeds used in Example 2, and (B) is the FT-IR spectrum of the C-QDs obtained in Example 2. [Diagram 31] 1 shows X-ray photoelectron spectroscopy (XPS) spectra, where (A) is the XPS spectrum of the C-QDs obtained in Example 2, and (B) is the XPS spectrum of the crushed fenugreek seeds used as the raw material. [Diagram 32] These are figures showing X-ray photoelectron spectroscopy (XPS) spectra. (A) shows the results of deconvoluting the peak corresponding to the carbon 1s orbital obtained by XPS into three major peaks, and (B) shows the results of deconvoluting the peak corresponding to the oxygen 1s orbital obtained by XPS into two major peaks. [Diagram 33] FIG. 1 shows Raman spectrum analysis results. [Diagram 34] 1 is a graph showing the change over time in the fluorescence emission intensity of C-QDs obtained in Example 2 when the C-QDs were continuously irradiated with light from a xenon lamp. [Diagram 35] FIG. 1 shows fluorescence emission spectra for excitation light of different wavelengths, where (A) shows the observed spectrum and (B) is a graph of the normalized spectrum. [Diagram 36] 1 is a graph showing that the C-QDs obtained in Example 2 exhibit two shifts (dual mode), a blue shift and a red shift, in fluorescence emission. [Figure 37] 1 is a graph showing the fluorescence emission spectra of the C-QDs obtained in Example 2 under various pH conditions, where (A) shows the observed spectrum and (B) shows the normalized spectrum. [Figure 38] Photograph (A) of water containing C-QDs produced from papain in Example 3 under normal light and (B) under black light irradiation. [Figure 39]Photograph (A) of water containing C-QDs produced from glucose isomerase in Example 3 under normal light and (B) under black light irradiation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described.

[0014] [Carbon quantum dots] The carbon quantum dots (C-QDs) according to one embodiment of the present invention have a zeta potential of -44 to -1.1 mV when dispersed in water. (1) The lattice spacing of the lattice fringes resulting from the (100) plane of graphite as seen in a transmission electron microscope image is in the range of 0.200 to 0.234 nm; or (2) The average particle size (D50) obtained by dynamic light scattering is 3.1 to 8.7 nm. Carbon quantum dots (C-QDs). The C-QDs according to this embodiment may satisfy either one of the above (1) and (2), or may satisfy both.

[0015] The zeta potential of the C-QDs according to this embodiment when dispersed in water is preferably -44 to -1.1 mV. A more preferable range of the zeta potential is -30 to -1.8 mV. C-QDs having a zeta potential in this range are well dispersed in an aqueous medium without aggregation even when stored for a long period of time, and as a result, the fluorescent properties of the C-QDs are stable for a long period of time.

[0016] Among the C-QDs according to this embodiment, the C-QDs produced by the thermal decomposition method described below often have a zeta potential of −30 to −1.1 mV when dispersed in water, and more often −26 to −11 mV. On the other hand, among the C-QDs according to this embodiment, the plasma law The C-QDs produced by this method often have a zeta potential of −44 to −1.8 mV when dispersed in water, and more often have a zeta potential of −37 to −2.5 mV.

[0017] From various studies so far, it has been found that C-QDs produced from natural materials and C-QDs synthesized from carbon-containing precursors generally have an amorphous form. To be more specific, the C-QDs produced by the methods proposed so far have either an amorphous structure as a whole or a structure in which an amorphous shell is formed around a core having a crystalline structure. However, the carbon quantum dots according to the present invention have a substantially crystallized structure. In addition, the crystal structure is preferably a hexagonal system.

[0018] In the C-QDs according to this embodiment, the lattice spacing of the lattice fringes due to the (100) plane of graphite is preferably in the range of 0.200 to 0.234 nm as measured by transmission electron microscope (TEM). The more preferable lattice spacing is 0.205 to 0.225 nm. C-QDs with a lattice spacing in the above range can be said to have a high degree of crystallinity. Incidentally, the spacing of the lattice fringes due to the (100) plane in completely crystallized graphite is 0.213 nm, but it is known that the crystallinity decreases as the measured lattice spacing becomes larger than 0.213 nm. The C-QDs of this embodiment, which have the lattice spacing described above, i.e., have a high degree of crystallinity, have emission wavelengths that are less dependent on the pH of the medium and are less susceptible to fading.

[0019] In this embodiment, the average particle size (D50) of the C-QDs obtained by dynamic light scattering is preferably in the range of 3.1 to 8.7 nm. The particularly preferred range of the average particle size (D50) obtained by dynamic light scattering is 3.1 to 7.5 nm. In addition, the full width at half maximum (FWHM) of the particle size distribution of the C-QDs obtained by dynamic light scattering is preferably 0.6 to 11.7 nm. More preferably, the full width at half maximum (FWHM) of the particle size distribution of the C-QDs obtained by dynamic light scattering is 0.9 to 5.1 nm. The C-QDs having the average particle size as described above can emit good blue fluorescence.

[0020] Among the C-QDs according to this embodiment, the C-QDs produced by the pyrolysis method described below often have an average particle size (D50) of 3.6 to 8.7 nm, or more often 4.2 to 7.5 nm, obtained by dynamic light scattering. The C-QDs produced by the pyrolysis method often have a full width at half maximum (FWHM) of the particle size distribution of the C-QDs obtained by dynamic light scattering, or more often 2.4 to 11.7 nm, or more often 3.3 to 5.1 nm.

[0021] On the other hand, among the C-QDs according to this embodiment, the plasma law The average particle size (D50) of the C-QDs produced by the plasma method is often in the range of 3.1 to 6.0 nm, and more often 3.1 to 4.8 nm, as determined by dynamic light scattering. law The C-QDs produced by the above method often have a full width at half maximum (FWHM) of the particle size distribution of the C-QDs obtained by dynamic light scattering method of 0.6 to 2.1 nm, and more often 0.9 to 1.8 nm.

[0022] In a preferred embodiment of the C-QDs according to this embodiment, the ratio of the constituent elements obtained by X-ray photoelectron spectroscopy (XPS) is within the following range: C: 55~80% N: Less than 0.5% A more preferable ratio of the constituent elements is as follows: C: 60~75% N: Less than 0.3% It is.

[0023] Furthermore, the elemental ratio of oxygen as determined by X-ray photoelectron spectroscopy is preferably 20 to 45%, and more preferably 25 to 40%.

[0024] C-QDs having an elemental ratio in the above range, particularly C-QDs having an elemental ratio of nitrogen in the above range, can emit fluorescence of a constant wavelength, regardless of the wavelength of the excitation light. Such C-QDs can be produced, for example, by employing a pyrolysis method in the production method of C-QDs described later.

[0025] C-QDs having the ratio of the constituent elements obtained by XPS analysis in the above range can emit fluorescence having a wavelength of 300 to 630 nm when exposed to various excitation light having a wavelength in the range of 200 nm to 360 nm. In particular, when exposed to excitation light having a wavelength of 240 to 350 nm, it can exhibit an emission spectrum in the range from 350 nm to 630 nm. The peak of the emission spectrum observed when receiving excitation light having a wavelength of 240 to 350 nm is preferably located in the range of 300 to 630 nm, and the full width at half maximum of the emission spectrum is preferably 50 to 90 nm.

[0026] In another preferred embodiment of the C-QDs according to this embodiment, the ratio of the constituent elements obtained by X-ray photoelectron spectroscopy (XPS analysis) is within the following range: C: 55~75% N: 2-10% A more preferable ratio of the constituent elements is as follows: C: 61.0-65.8% N: 2.8-7.5% It is.

[0027] Furthermore, the elemental ratio of oxygen as determined by X-ray photoelectron spectroscopy is preferably 20 to 40%, and more preferably 29.2 to 34.3%.

[0028] C-QDs having elemental ratios in the above ranges, particularly C-QDs having an elemental ratio of nitrogen in the above range (2 to 10%), tend to have excitation light-dependent fluorescence emission as described below. lawThe carbon source may be a carbon-containing substance containing a nitrogen element, or a nitrogen-containing gas may be turned into plasma.

[0029] C-QDs having the above-mentioned ratio of the constituent elements obtained by XPS analysis, particularly C-QDs having the above-mentioned ratio of the nitrogen element (2-10%), can emit fluorescence having a wavelength of 340-680 nm when exposed to various excitation light having a wavelength in the range of 220 nm to 400 nm. In particular, when irradiated with excitation light having a wavelength of 260-360 nm, an emission spectrum can be expressed in the range from 359 nm to 612 nm. The center of the emission spectrum is preferably in the range of 334-490 nm. In addition, the half-width at that time is preferably 30-56 nm. In addition, the C-QDs according to this embodiment may become carbon quantum dots that have the ability to emit fluorescence depending on the excitation light, that is, the peak value of the fluorescence emission changes depending on the wavelength of the excitation light. Furthermore, the C-QDs according to this embodiment preferably exhibit both a blue shift and a red shift in fluorescence emission (dual mode) due to differences in the wavelengths of excitation light. The dependency of excitation light and the dual mode exhibiting both blue and red shifts will be described in detail later.

[0030] Among the C-QDs according to this embodiment, the C-QDs produced by the pyrolysis method described later also have the property of emitting fluorescence with higher energy than the excitation light when irradiated with low-energy excitation light. That is, they have the properties of an upconversion phosphor. The reason for this is not entirely clear, but it is believed that the characteristics of the excited state of the obtained C-QDs give them this property.

[0031] In terms of upconversion performance, C-QDs produced by the method of a preferred embodiment of this embodiment can emit fluorescence having a wavelength of 400 to 630 nm when exposed to excitation light having a wavelength in the range of 750 nm to 800 nm.

[0032] [Method for producing carbon quantum dots] The carbon quantum dots (C-QDs) according to the above-described embodiment can be produced, for example, by the following two methods: [A] A manufacturing method comprising a step of pyrolyzing plant seeds to obtain a carbonized product; or [B] Carbon-containing material is subjected to plasma law The method for producing the carbonized material includes the step of obtaining the carbonized material by the steps of:

[0033] [A] Production of C-QDs by pyrolysis A method for producing carbon quantum dots according to one embodiment of the present invention includes a step of pyrolyzing plant seeds to obtain a charcoal.

[0034] 1 is a diagram showing a flow of a particularly preferred embodiment of the method for producing carbon quantum dots according to the present embodiment. Hereinafter, the present embodiment will be described in detail with reference to FIG. In this embodiment, fennel seeds, a type of herb, are used as a preferred material (carbon source). In a preferred embodiment of the present invention, fennel seeds are first crushed and then pyrolyzed. The pyrolyzed carbonized material is then placed in water and ultrasonically dispersed. The resulting suspension is centrifuged, and water-insoluble materials are removed by dialysis. Through this series of steps, water containing C-QDs that emits ultraviolet wavelength fluorescence is obtained.

[0035] In the method for producing carbon quantum dots according to the present embodiment, plant seeds are used as the carbon source, not grown plants. This is because seeds are generally stable in quality. Also, they are easy to store for a long time under certain conditions. Furthermore, it is possible to eliminate the influence of impurities that the plant may take in during its growth process as much as possible, and furthermore, it is possible to eliminate the variation in quality of grown plants caused by weather, geographical conditions, etc. Incidentally, the structure of a seed is composed of a seed coat, an endosperm, a cotyledon, and an embryo axis, and the components thereof are carbohydrates (carbohydrates), lipids, proteins, water, and nutritional components such as trace minerals and vitamins. Although fennel seeds are used as an example in the preferred embodiment and examples below, the carbon source in this embodiment should not be limited to fennel seeds.

[0036] In a preferred aspect of this embodiment, the seeds are pulverized into fine granules because the subsequent pyrolysis step allows uniform heat treatment (i.e., uniform carbonization) to be performed even inside the granules. There are no particular restrictions on the method for pulverizing the seeds, and the method can be performed using a commonly used method, such as a pulverizing means such as a mixer or a mill. There are also no particular restrictions on the size (particle size) of the pulverized product obtained, but it is preferable to pulverize the seeds to about 70 to 90 μm.

[0037] Next, the crushed seeds are heated to perform pyrolysis and carbonization. As a heating method for pyrolysis, for example, heating by electromagnetic induction can be suitably used. In addition, an electric hot plate that can relatively easily control the heating temperature can also be used. Heating can be performed in the air, but it is preferable to perform it in an inert gas atmosphere and under reduced pressure so that the seed powder, which is the raw material, is not burned.

[0038] The heating temperature in pyrolysis is preferably 330 to 600°C. The heating time in the pyrolysis step is preferably 2 to 4 hours, depending on the amount of sample to be carbonized. If the heating temperature in pyrolysis is lower than 330°C, the carbonization of the sample may not proceed sufficiently. If the temperature exceeds 600°C, problems such as combustion and vaporization may occur. A more preferable heating temperature is 350 to 500°C. On the other hand, if the heating time is less than 2 hours, the sample may not be sufficiently carbonized. After the heating is completed, the sample may be left to stand and slowly cooled to room temperature.

[0039] Incidentally, whether or not a sample (plant seed) has been sufficiently carbonized can be determined by setting the heating temperature and heating time to the desired values ​​and conducting a preliminary pyrolysis experiment, measuring the ultraviolet-visible light absorption spectrum of the obtained substance (substance thought to have been carbonized) using, for example, an FT-IR spectrometer, and checking whether or not there are peaks seen in hydrocarbons (for example, a peak due to CH2) in the obtained spectral chart.

[0040] Next, the material (carbonized material) obtained by pyrolysis is put into water, and an aqueous dispersion of the carbonized material is obtained using, for example, an ultrasonic disperser. There are no particular constraints on the ultrasonic dispersion at this time, but it is preferable to set the frequency of the ultrasonic disperser to 30 to 50 kHz. In addition, it is preferable to set the ultrasonic dispersion time to about 5 to 10 minutes. This ultrasonic dispersion process allows the carbonized material aggregated in the pyrolysis process to be uniformly and finely dispersed in water.

[0041] The water in which the pyrolysate is dispersed is not particularly limited, but deionized water (ion-exchanged water), distilled water, and pure water can be suitably used.

[0042] Next, water-insoluble matters are removed from the ultrasonically dispersed liquid by centrifugation. There are no particular limitations to this centrifugation step, but it is preferable to carry out the centrifugation step at 13,000 to 15,000 rpm for 8 to 10 minutes.

[0043] The supernatant obtained in the above centrifugation step becomes C-QDs-containing water, but if the product (C-QDs) is to be further purified, further steps such as centrifugal sedimentation and filtration may be added. For example, as in the present embodiment, it is preferable to use a dialysis method after the above centrifugation step to further remove insoluble matter remaining in the C-QDs-containing water. This dialysis method can be performed using a commercially available dialysis kit, for example, using a filter with a pore size of 0.1 μm.

[0044] Through the above steps, water containing C-QDs can be obtained by the manufacturing method of this embodiment. The obtained solution can be subjected to appropriate pressure reduction or the like to obtain carbon quantum dots.

[0045] [B] Production of C-QDs by plasma method In another embodiment of the present invention, a method for producing carbon quantum dots is to use a carbon-containing substance as a carbon source, and to irradiate the carbon source with plasma. law to obtain a carbide.

[0046] Carbon source In the method for producing carbon quantum dots according to the present embodiment, the carbon element ratio of the carbon-containing material used as the carbon source is preferably 40 to 95%. More preferably, the carbon element ratio is 50 to 90%, and even more preferably, 55 to 85%. Here, the carbon content ratio (percentage) is obtained by X-ray photoelectron spectroscopy (XPS analysis) and refers to the abundance ratio of carbon atoms in all elements (excluding hydrogen and helium) in the carbon-containing material. The same applies to other elements described below.

[0047] In a preferred aspect of this embodiment, a carbon-containing substance containing a nitrogen element can be used as a carbon source for producing carbon quantum dots. The nitrogen element ratio in the carbon-containing substance serving as the carbon source is preferably 0.5 to 20%, more preferably 1 to 10%.

[0048] A specific example of a carbon-containing material that can be suitably used is a plant body. The plant body is easily available and is environmentally friendly. In particular, it is preferable to use plant seeds as a carbon source. The reason why plant seeds are preferable as a carbon source is the same as the reason for the pyrolysis method described above. Furthermore, plants other than plant seeds can also be used, for example, seaweed such as Ulva.

[0049] In addition to the above-mentioned plants, examples of the substances include proteins, carbohydrates, lipids, etc. In particular, since proteins contain nitrogen, plasma According to this method, the carbon quantum dots obtained tend to contain nitrogen, i.e., the carbon quantum dots having the characteristics (excitation light-dependent fluorescence) described below are easily obtained. As the protein, industrially widely used and easily available enzymes, collagen, etc. can be used, and as the enzyme, plant-derived papain, bromelain, etc. can be used. In addition, enzymes such as glucose isomerase can also be used.

[0050] Using a specific protein means that a homogeneous substance with a certain chemical composition (certain element ratio) is used as the carbon source substance. Therefore, by setting appropriate manufacturing conditions in the carbon quantum dot manufacturing process, it becomes easy to obtain homogeneous and good carbon quantum dots, and as a result, it becomes possible to obtain carbon quantum dots with good and stable fluorescent properties.

[0051] The present invention will be described in detail below using an example in which the seeds of fenugreek, a type of herb, are used as a carbon source as a preferred embodiment.

[0052] Fenugreek is a commonly cultivated plant worldwide, which means that its seeds are a sustainable and easily available carbon source. Fenugreek seeds are eco-friendly (it is a natural material that contains substances with antibacterial and antioxidant properties), are produced in large quantities around the world, and are inexpensive. More importantly, fenugreek seeds have little seasonal variation in quality.

[0053] plasma Fabrication of C-QDs by the method 2 is a diagram showing a flow of a particularly preferred embodiment of the method for producing carbon quantum dots according to the present embodiment. First, the flow of the production method of the present embodiment will be outlined with reference to FIG. 2, and then each step will be described in detail.

[0054] In a preferred embodiment of this embodiment, fenugreek seeds are first crushed, and the crushed powder is subjected to a one-step process using hydrogen gas. plasma The crushed fenugreek powder is carbonized using a carbonization process (the crushing process is not shown in Figure 2). 2A shows a schematic diagram of a piece of powder of crushed fenugreek seeds, which is the carbon precursor of the present embodiment, and also shows that the precursor naturally contains an organic compound. (B) to (D) in Fig. 2 show the results using hydrogen gas. plasma This is the stage corresponding to the method, but for the sample (fenugreek seed powder) plasma The changes in the sample during the process are shown in (B) through (D) in the order shown. The details of (B) through (D) will be explained later. Next, the obtained carbonized material is added to water and dispersed ultrasonically (Figure 2(E)). Furthermore, the suspension obtained in (E) above is centrifuged (FIG. 2(F)). Thereafter, if desired, water-insoluble substances are removed by dialysis (the step of removing water-insoluble substances by dialysis is not shown in FIG. 2). Through this series of steps, water containing C-QDs (Figure 2(G)) is obtained, which emits fluorescence in the ultraviolet wavelength range. Incidentally, hydrogen gas plasma According to the law, plasma Although it depends on the discharge frequency, power, and amount of raw material in the method, charcoal can be obtained in about 5 minutes.

[0055] Next, each step in the manufacturing method will be described. Seed grinding In a preferred embodiment of the present invention, the seeds are milled into fine granules (or powder) using a subsequent plasma Carbonization This is because the inside of the granular material (powder) can be carbonized easily and uniformly in the process. There is no particular restriction on the method for crushing the seeds, and it can be carried out using a commonly used method, for example, a crushing means such as a mixer or a mill. There is also no particular restriction on the size (particle size) of the resulting crushed material, but it is preferable to crush it to about 50 to 200 μm, and more preferably 70 to 90 μm.

[0056] Plasma Carbonization Next, the crushed seeds are placed in a plasma CVD apparatus to carbonize them. law In this experiment, hydrogen gas is introduced into the device and fenugreek seeds are carbonized using hydrogen plasma. law is also possible.

[0057] plasma law The discharge frequency in the plasma is preferably in the range of 2.10 to 3.01 GHz. The high-frequency power in the plasma is preferably in the range of 300 to 1000 W. law In a preferred embodiment, when hydrogen gas is used, the hydrogen gas pressure in the sample chamber within the hydrogen gas plasma CVD apparatus is preferably in the range of 0.020 to 0.133 Pa. More preferably, the hydrogen gas pressure is 0.020 to 0.05 Pa. In addition, hydrogen plasma lawWhen the above is applied to crushed seeds, no special heating of the sample (crushed material) is required.

[0058] Hydrogen Plasma law Analysis of plasma discharge in the plasma is important. The optical emission spectroscopy (OES) spectrum of the excited hydrogen plasma used in this embodiment is shown in FIG. 3. In the spectrum shown in FIG. 3, the large peaks seen at wavelengths of 658.2, 486.9, 434.7, and 463.8 nm are considered to correspond to Hα, Hβ, Hγ, and secondary hydrogen, respectively. Also, as can be seen from FIG. 3, the Fulcher Band (a collection of many small peaks to the left of the Hα peak) has a smaller intensity than the above-mentioned peaks, which indicates that ionization of hydrogen molecules has progressed considerably in the plasma. That is, FIG. 3 shows that various ions derived from hydrogen molecules are present in large quantities in the hydrogen plasma. Therefore, hydrogen plasma law By using this method, natural carbon sources such as plant seeds can be efficiently converted into C-QDs.

[0059] The generation of C-QDs is believed to consist of the following steps: (a) Destruction of carbon precursors (destruction of molecular species containing carbon atoms in the carbon source material (ground fenugreek seed powder)): (corresponding to the schematic diagram shown in Figure 2(B)), (b) The step of nucleation of C-QDs after the destruction of the carbon precursor: (corresponding to the schematic diagram shown in FIG. 2(C)), and (c) The step in which the nuclei of the formed C-QDs grow (corresponding to the schematic diagram shown in Figure 2(D)). It is.

[0060] First, in step (a) above, hydrogen gas is introduced into the PECVD chamber, and hydrogen molecules are ionized by plasma irradiation. As mentioned above, there is no need to heat the sample holder in the PECVD chamber. The high-density plasma causes hydrogen ions with large kinetic energy and a large number of electrons with high energy to collide with the carbon source (crushed plant seed powder). As a result, a sufficient amount of energy is transferred (given) to the crushed plant seed powder, destroying (fragmenting) the carbon source (molecules containing carbon) and changing it into atomic species (carbon, hydrogen, and oxygen atoms). That is, in this step, the organic compounds (precursors) present in the sample are fragmented, as shown diagrammatically in FIG. 2B. Next, in step (b), the carbon atoms produced in the above steps gather together and unite to form small nuclei, probably by diffusion, as shown diagrammatically in FIG. 2(C). Furthermore, in step (c), these small nuclei gradually grow larger by diffusion as long as there is still carbon source around them. The growth of these nuclei continues until there is no carbon source around them, and the growth of C-QDs finally stops (see Fig. 2(D)).

[0061] Incidentally, whether the sample (plant seed) was sufficiently carbonized or not was determined by the above-mentioned plasma law The discharge frequency, power, etc. were set to desired values, and a plasma was Carbonization This can be determined by measuring the ultraviolet-visible light absorption spectrum of the obtained material (material that is thought to have been carbonized) using, for example, an FT-IR spectrometer, and checking whether or not peaks seen in hydrocarbons (for example, peaks due to CH2) are present in the obtained spectral chart. C-QDs can be obtained as described above, but further steps such as ultrasonic dispersion, centrifugation, and removal of impurities by dialysis may be carried out as described below.

[0062] Ultrasonic dispersion process Next, the plasma lawThe material (carbonized material) obtained in step 2 is added to water, and then, for example, an ultrasonic disperser is used to obtain an aqueous dispersion of the carbonized material. There are no particular constraints on the ultrasonic dispersion, but it is preferable to set the frequency of the ultrasonic disperser to about 30 to 50 kHz. The ultrasonic dispersion time is preferably about 5 to 15 minutes. This ultrasonic dispersion process produces a plasma law The carbonized material that is aggregated in the carbonization process can be uniformly and finely dispersed in water.

[0063] plasma law There is no particular limitation on the water in which the carbonized material is dispersed later, but deionized water (ion-exchanged water), distilled water, and pure water can be suitably used.

[0064] Centrifugation process Next, water-insoluble matters are removed from the ultrasonically dispersed liquid by centrifugation. There are no particular limitations to this centrifugation step, but it is preferable to carry out the centrifugation step at 3,000 to 20,000 rpm for 8 to 15 minutes.

[0065] The supernatant obtained in the above centrifugation step becomes C-QDs-containing water, but if the product (C-QDs) is to be further purified, further steps such as centrifugation, filtration, etc. may be added. In addition, after the above centrifugation step, a dialysis method can be used to further remove insoluble matter remaining in the C-QDs-containing water. This dialysis method can be performed using a commercially available dialysis kit, for example, using a filter with a pore size of 0.1 μm.

[0066] Through the above steps, water containing C-QDs can be obtained by the manufacturing method of this embodiment. The obtained solution can be subjected to appropriate pressure reduction or the like to obtain carbon quantum dots.

[0067] The above-described embodiments are described for the purpose of facilitating understanding of the present invention, and are not described for the purpose of limiting the present invention. Therefore, each element disclosed in the above embodiment is intended to include all design modifications and equivalents that fall within the technical scope of the present invention. EXAMPLES

[0068] The present invention will be described in more detail below by showing examples, but the present invention is not limited to the following examples in any way.

[0069] Example 1: Production of C-QDs from fennel seeds by pyrolysis Fennel seeds (Swati seeds) from India were crushed to a particle size of about 80 μm using a mixer (SKS-G700(V)) manufactured by Tiger Corporation. 0.2 g of the resulting crushed fennel seeds was placed in a crucible (2-300-12 manufactured by AS ONE Corporation) and placed on a hot plate (CHP-170AF manufactured by AS ONE Corporation), and pyrolysis of the fennel seeds was carried out under the conditions of 500 °C for 3 hours in an air atmosphere. After heating at 500 °C for 3 hours, the hot plate was turned off and allowed to cool to room temperature. The crushed fennel seeds before the pyrolysis process were green, but the sample in the crucible after the pyrolysis process was dark gray and had become aggregates.

[0070] The dark gray sample in the crucible after cooling was placed in a bottle containing 2 ml of deionized water and ultrasonically dispersed for 10 minutes at 38 kHz using an ultrasonic disperser. The black suspension obtained by ultrasonic dispersion was then centrifuged at 15,000 rpm for 10 minutes to obtain a supernatant.

[0071] The obtained supernatant was filtered by dialysis using a filter with a pore size of 100 nm manufactured by Pall Acrodisc Japan and a Float-A-Lyzer G2 Dialysis kit to separate and remove insoluble matter, and water containing C-QDs was obtained. Using this C-QDs-containing water, various tests described below were performed to investigate the properties, physical properties, chemical properties, etc. of the C-QDs obtained in this example.

[0072] The water containing C-QDs obtained in Example 1 above is a colorless and transparent liquid in sunlight (normal ambient light) as shown in Figure 4(A), and when irradiated with ultraviolet light, it turns blue as shown in Figure 4(B). Incidentally, the wavelength of the irradiated ultraviolet light (excitation light) in the sample in Figure 4(B) was 365 nm.

[0073] Next, the absorption spectrum in the ultraviolet to visible light region was measured using a JASCO UV-Vis-IR spectrophotometer (V-530). The spectrum obtained is shown in Figure 5. From this spectrum, it can be seen that there is a very strong peak near the wavelength of 220 nm, and a small shoulder peak near the wavelength of 338 nm. The strong peak near the wavelength of 220 nm is thought to correspond to the π-π* transition of the C=C bond (carbon-carbon double bond). In addition, the small peak near 338 nm is thought to correspond to the n-π* transition of the C=O bond (carbonyl group).

[0074] Furthermore, a TEM image was obtained using a transmission electron microscope (TEM: model number JEM2002-FS) manufactured by JEOL Ltd. The obtained TEM image is shown in Figure 6. Here, Figure 6(A) is a relatively low-resolution image, and (B) is a high-resolution image. As can be seen from Figure 6(B), the particles are arranged in a very neat hexagonal pattern. This can be seen as evidence that the obtained C-QDs particles are monodispersed.

[0075] Using the obtained TEM image, the particle size distribution of the C-QDs nanoparticles was obtained by the following method. First, 148 nanoparticles in the TEM image were focused on. Next, 42 high-resolution images were taken at random positions within the area where the 148 nanoparticles were present, and the particle diameters were measured from the obtained images to obtain the particle size distribution. A graph of the particle size distribution of the obtained C-QDs is shown in FIG. 7. According to the analysis of this TEM image, the C-QDs obtained in this example have an average diameter of 3.90±0.91 nm. It can also be seen that the particles are highly monodispersed.

[0076] The D50 calculated from the particle size distribution of C-QDs shown in Figure 7 was 3.9 nm. The full width at half maximum (FWHM) in the graph of Figure 7 was 1.1 nm.

[0077] FIG. 8 shows a high-resolution TEM image of the C-QDs obtained in this example. As can be seen from this image, a neatly arranged lattice fringe was confirmed inside the circular area corresponding to one C-QDs particle. The line spacing in this lattice fringe was 0.21 nm. Lattice fringes are fringes formed by the interference of light transmitted through a crystal structure and light reflected from a certain crystal plane in the crystal structure, and are a phenomenon in which a light and dark stripe pattern corresponding to the interplanar spacing of the crystal planes is obtained, but clear lattice fringes were observed in this TEM image. This indicates that the C-QDs obtained in this example have a high degree of crystallinity. The line spacing of the lattice fringes was calculated from the image shown in FIG. 8, and was 0.21 nm. This suggests that the reflected light from the (100) plane in the carbon crystal is involved in the formation of the lattice fringes.

[0078] Next, an elemental analysis of the obtained C-QDs was performed using an energy dispersive X-ray analyzer (JEM-2300F manufactured by JEOL Ltd.). The results are shown in FIG. 9. As can be seen from this figure, only peaks corresponding to carbon and oxygen are present in the C-QDs sample obtained in this example. Incidentally, the two peaks seen on the right side of the spectrum in FIG. 9 are due to copper, but these are thought to be peaks caused by the grid of the device.

[0079] Next, using a Malvern Zeta-sizer (Nano-ZS 90 Zeta sizer), the particle size of the C-QDs obtained in this example was measured by dynamic light scattering, and the zeta potential of the C-QDs particles in the aqueous dispersion was measured. The results of the diameter measurement of the C-QDs particles by this dynamic light scattering method are shown in Figure 10. According to this, the particle size of the C-QDs particles has a distribution centered on 6.1 nm. In addition, the full width at half maximum of the peak in the particle size distribution shown in Figure 10 was 3.3 nm.

[0080] The diameter of C-QDs obtained by dynamic light scattering is larger than the diameter of 3.90 nm obtained from the analysis of TEM images described above, but this is not unnatural. In other words, the so-called "hydrodynamic diameter" of a particle dispersed in a solvent such as water is generally larger than the diameter of the same particle in a vacuum state. This is because molecules or functional groups present in the solvent that have an opposite charge to the charge on the surface of the particle are attached to the surface of the particle, resulting in the formation of an electric double layer with a certain thickness that surrounds the particle.

[0081] Next, the zeta potential of the C-QDs-containing water obtained in this example was measured using the above-mentioned Zeta-sizer. The results are shown in FIG. 11. As can be seen from the graph shown in FIG. 11, the zeta potential of the C-QDs in this example was -23 mV, and the full width at half maximum of this peak was 10.01 mV. The absolute value of the zeta potential being as large as 23 means that the C-QDs continue to be dispersed stably in the aqueous solvent. That is, one C-QDs particle and other C-QDs particles present in the vicinity are firmly charged to the same potential, so they repel each other, making it difficult for the C-QDs particles to aggregate with each other. Incidentally, the zeta potential is the potential at the "slip surface" where liquid flow begins to occur in the electric double layer formed around the fine particles in the solution. This "slip surface" is the boundary surface conveniently defined as existing between the "ion fixing layer" formed around the fine particles and the "ion diffusion layer" formed outside it.

[0082] The negative zeta potential in this example means that the surface of the C-QDs obtained in this example is also negatively charged. Since C-QDs have such properties, the dispersion of C-QDs in water is maintained well for a long period of time. (A) and (C) in FIG. 12 are photographs showing the state of the C-QDs-containing solution under normal light and ultraviolet light immediately after production by the method of the present invention, respectively, and (B) and (D) are photographs showing the state of the C-QDs-containing solution under normal light and ultraviolet light six months after production, respectively. When comparing the degree of fluorescence emission of the solution six months after production with that of the solution immediately after production, there is no inferiority in the fluorescence emission properties. It can be seen that the C-QDs obtained in this example maintain good fluorescence emission properties even after long-term storage.

[0083] Furthermore, the chemical state of the surface of the C-QDs particles was analyzed using FT-IR analysis. The instrument used was a PerkinElmer Spectrum 100. For comparison, the FT-IR spectrum was also measured for the raw material (crushed fennel seeds) used in the production of C-QDs. The results are shown in Figure 13. Figure 13(a) shows the spectrum of fennel seeds, and Figure 13(b) shows the spectrum of the C-QDs produced in this example.

[0084] In the spectrum shown in FIG. 13(a), the wave numbers are 3296, 2921, 2851, 1742, 1597 and 1027 cm -1 These peaks correspond to the OH stretching motion, symmetric CH2 stretching, asymmetric CH2 stretching, C=O of carboxyl / carbonyl groups, C=C bonds, and CH bending, respectively. The presence of peaks at 2921 and 2851 in the FTIR spectrum of fennel seeds, which correspond to the symmetric CH2 stretching and asymmetric CH2 stretching, indicates that fennel seeds contain hydrocarbons.

[0085] On the other hand, in the graph of FIG. 13(b) showing the FTIR spectrum of the C-QDs obtained in the examples, the wave numbers are 3350, 1640, 1405, 1023, and 859 cm -1 These peaks correspond to OH, C=O, C=C, (CH or CO), and CH bends, respectively. The presence of a peak corresponding to C=C (carbon-carbon double bond) indicates that C-QDs have a graphite structure. Incidentally, the presence of peaks corresponding to OH, C=O, and OH bends is evidence that groups with such bond structures exist on the surface of C-QDs.

[0086] Zeta potential measurement and FTIR measurement are complementary to each other, and it is preferable to comprehensively analyze the results of both. The analysis in this example revealed that the surface of C-QDs particles is negatively charged, and the zeta potential in water is also large at -23 mV. Therefore, C-QDs particles can exist stably in water for a long period of time without agglomeration. In addition, the FTIR spectrum shows that the wave numbers 2921 and 2851 cm, which correspond to the symmetry and asymmetry of CH2, are -1 It can be seen that the two peaks in the graph have disappeared, and in this example, the hydrocarbons in the fennel seeds have been substantially completely carbonized.

[0087] In addition, when carbonizing the hydrocarbons that serve as the carbon source, depending on the conditions of the carbonization process, sufficient carbonization may not be achieved, and the resulting C-QDs may not exhibit good fluorescent properties. To prevent this, one effective way to determine whether carbonization has progressed sufficiently is to use the FTIR method to check for the presence or absence of peaks due to CH2.

[0088] Elemental analysis was performed on both the crushed fennel seeds and the C-QDs obtained in the examples using an X-ray photoelectron spectroscopy (XPS analysis). The results are shown in Figure 14. Figure 14(a) shows the spectrum of the C-QDs obtained in the examples, and Figure 14(b) shows the spectrum of the crushed fennel seeds.

[0089] The measurement conditions for the XPS analysis are as follows: X-ray source: Al Kα1 (1.4866keV) (analysis diameter: 100μm, output: 28.8W 15kV) Photoelectron extraction angle: 45° Raster area: 10μm×10μm Pass Energy:69eV,Step Size:0.13eV Neutralization gun: 20μA 1.0V

[0090] In FIG. 14(a), a peak corresponding to the carbon 1s orbital and a peak corresponding to the oxygen 1s orbital are seen. That is, it can be seen that the C-QDs obtained in this embodiment have carbon and oxygen. Here, the ratio of the intensity of the peak corresponding to the carbon 1s orbital to the intensity of the peak corresponding to the oxygen 1s orbital was 71.2:28.8. Incidentally, this spectrum also contains peaks corresponding to the silicon 2s orbital and 2p orbital, but this is because the sample was placed on a silicon plate and the measurement was performed. No peaks due to other elements were seen. This indicates that the C-QDs obtained in this embodiment are of high purity.

[0091] On the other hand, in Figure 14(b), peaks corresponding to the carbon 1s orbital, oxygen 1s orbital, and nitrogen 1s orbital are observed. No peaks due to other elements were observed. This result was obtained because fennel seeds also contain nitrogen element.

[0092] Figure 15(a) is a graph showing the results of deconvoluting the peak corresponding to the carbon 1s orbital obtained by the above-mentioned XPS analysis into three main peaks. Here, the first peak at 286.16 eV corresponds to the sp2 orbital of the carbon atom in the graphite structure. The second peak at 286.16 eV and the third peak at 287.2 eV are peaks attributable to the C-O bond (carbon-oxygen single bond) and the C=O bond (carbon-oxygen double bond), respectively. The intensity ratio of the first, second, and third peaks was 70.2:20.6:9.2. This indicates that about one-quarter of the total carbon atoms are bonded to oxygen to form the functional groups described above.

[0093] Figure 15(b) is a graph showing the results of deconvoluting the peak corresponding to the oxygen 2p orbital obtained by the above-mentioned XPS analysis into two main peaks. Here, the peak at 531.1 eV, which corresponds to about 88.5% of the total peak, is due to C=O functional groups (free -OH, C=O, or OC=O). The presence of such functional groups on the surface of C-QDs is also thought to be one of the factors that allows C-QDs to maintain colloidal stability for a long period of time.

[0094] 16(a) and (b) are graphs showing the intensity of the fluorescence emission spectrum of C-QDS for various excitation lights having different wavelengths, where (a) shows the observed spectrum and (b) shows the normalized representation of each spectrum. As can be seen from this, the C-QDs according to the present invention show a broad peak that is asymmetrical in the wavelength range of 355 to 627 nm even if the wavelength of the excitation light is different (independent of the wavelength of the excitation light). The fact that the wavelength of the fluorescence emission does not depend on the wavelength of the excitation light is believed to indicate that the particle diameter of the C-QDs obtained in this example is extremely uniform.

[0095] The fluorescence emission spectrum shown in Fig. 16 generally has the following two peaks: a main peak (full width at half maximum is 50 nm) with a maximum emission intensity at about 417 nm, and a shoulder peak (full width at half maximum is 90 nm) that appears at about 470 nm to the right of the main peak.

[0096] Figure 17 is a graph showing the results of investigating the change over time in the intensity of fluorescence emission of C-QDs obtained in the example when light was continuously irradiated from the outside. In this experiment, a 150W xenon lamp was used to continuously irradiate light for 4 hours, while the intensity of fluorescence emitted by the C-QDs was continuously measured. As shown in Figure 17, the C-QDs obtained in this example did not show a significant change in the intensity of fluorescence emission during 4 hours of continuous light irradiation from a 150W xenon lamp.

[0097] FIG. 18 is a graph showing the fluorescence emission spectrum of C-QDs obtained in the example under various pH conditions. First, hydrochloric acid was added to the water containing C-QDs to make the pH of the C-QDs-containing solution acidic at 3 or 5, and excitation light of 260 nm was applied to measure the fluorescence emission intensity of the C-QDs. In addition, sodium hydroxide was added to the C-QDs-containing solution to make the pH of the C-QDs-containing solution 9, 11, or 13, and excitation light of 260 nm was applied to measure the fluorescence emission intensity of the C-QDs. Under each pH condition, a spectrum with a large peak near 419 nm was obtained. As can be seen from FIG. 18, it can be seen that the wavelength of the fluorescence emitted by the C-QDs obtained in this example does not depend on the pH of the medium. This is thought to indicate that functional groups affected by pH are not present in large amounts on the surface of the C-QDs, and that the C-QDs themselves have a high degree of crystallinity.

[0098] The C-QDs obtained in this example were irradiated with 800 nm excitation light. Then, fluorescence emission with a shorter wavelength (higher energy) than the excitation light was observed. The spectrum of the emission intensity measured at this time is shown in FIG. 19. As can be seen from FIG. 19, the C-QDs obtained in this example emits fluorescence in the visible region of 450 to 470 nm wavelength when exposed to 800 nm excitation light. This shows that the C-QDs obtained in this example have upconversion emission ability. The mechanism why the C-QDs according to this example have upconversion emission ability is not clear at this time, but it is thought to be related to the peculiarities of the energy level in the excited state of C-QDs.

[0099] Example 2 plasma Preparation of C-QDs from fenugreek seeds by a novel method Indian fenugreek seeds (Swati seeds) were crushed to a particle size of about 80 μm using a mixer (SKS-G700(V)) manufactured by Tiger Corporation. 0.2 g of the resulting crushed fenugreek seeds was placed in a crucible (2-300-12 manufactured by AS ONE Corporation) and placed on the sample holder of a plasma CVD apparatus (a device in which the plasma source of ULVAC's CN-CVD-200 was modified to a coaxial type dielectric hollow tube) as shown diagrammatically in Figure 20. Here, the plasma CVD apparatus 1 has a microwave inlet 2 (microwave generator not shown), a coaxial waveguide 4, a quartz disk 6 which serves as a hydrogen radical generation source, a hydrogen inlet 8 which introduces hydrogen gas from a hydrogen supply device (not shown), a gas outlet 10 which is connected to a vacuum pump (not shown), a sample holder 12 on which a sample is placed, a heater 14 placed on the sample holder, a power supply device 16 which supplies electricity to the heater 14, and a vacuum gauge 18. A crucible 20 containing a sample is placed on the sample holder 12.

[0100] The crucible 20 containing the sample (pulverized fenugreek seeds) was placed in the apparatus 1 as shown in FIG. 20, and the apparatus was first degassed. Then, the microwave generator was set to an output of 500 W. Next, 30 cm of hydrogen gas was introduced. 3The crucible was placed in the apparatus for 5 minutes at a flow rate of 1 / min. The pressure inside the apparatus 1 was maintained at 30 Pa (0.23 Torr) throughout the experiment. The distance between the tip of the plasma 30 generated by the microwaves (the lower end of the plasma 30 in FIG. 20) and the upper end of the crucible was 40 mm. Note that in this experiment, no heating operation was actually performed using a heater, but the temperature of the sample holder on which the crucible 20 was placed was measured using a thermocouple (not shown in FIG. 20) attached to the sample holder, and was found to be below 70°C. Plasma Carbonization Before the operation, the sample in the crucible (ground fenugreek seeds) was greenish, Plasma Carbonization Afterwards the sample turned a dark grey colour. plasma The dark gray sample in the crucible that had been carbonized by the above-mentioned method was placed in a bottle containing 2 ml of deionized water, and ultrasonic dispersion was performed for 5 minutes at 38 kHz using an ultrasonic disperser.

[0101] Next, the black suspension obtained by ultrasonic dispersion was centrifuged at 15,000 rpm for 10 minutes to obtain a supernatant. The obtained supernatant was filtered through a filter with a pore size of 100 nm manufactured by Nippon Pall Acrodisc Co., Ltd. to separate and remove insoluble matter, and water containing C-QDs was obtained. The obtained water containing C-QDs was stored in a small glass bottle.

[0102] Using the C-QDs-containing water obtained by the above experiment, various tests described below were carried out to investigate the properties, physical properties, chemical properties, etc. of the C-QDs obtained in this example. The details are described below.

[0103] The water containing C-QDs obtained above is a colorless and transparent liquid in sunlight (normal ambient light) as shown in Figure 21(A), but when irradiated with ultraviolet light, it turns blue as shown in Figure 21(B). Incidentally, the wavelength of the irradiated ultraviolet light (excitation light) in the sample in Figure 21(B) was 365 nm.

[0104] Next, an absorption spectrum in the ultraviolet to visible light region was measured using a UV-Vis-IR spectrophotometer (V-530) manufactured by JASCO Corporation. The spectrum obtained is shown in Figure 22. In the graph in Figure 22, it can be seen that there are peaks at wavelengths of 270 nm and 338 nm (as shoulders of a large peak). These two peaks are thought to correspond to the π-π* transition of C=C (carbon double bond) and the n-π* transition of C=O (carbonyl group), respectively.

[0105] Furthermore, a TEM image was obtained using a transmission electron microscope (TEM: model number JEM2002-FS) manufactured by JEOL Ltd. The obtained TEM image is shown in Figure 23. Here, Figure 23(A) is a relatively low-resolution image (accelerating voltage: 200 kV, magnification: 200,000 times), and Figure 23(B) is a high-resolution image (accelerating voltage: 200 kV, magnification: 500,000 times). As can be seen from Figure 23(B), the particles are arranged in a very neat hexagonal pattern. This can be seen as evidence that the obtained C-QDs particles are monodispersed.

[0106] Furthermore, the particle size distribution of the C-QDs nanoparticles was determined by the following method using the obtained high-resolution TEM image. First, attention was paid to 134 nanoparticles in the TEM image. Next, 65 high-resolution images were taken at random positions within the area where the 134 nanoparticles of interest exist, and the particle diameters were measured from the obtained images to determine the particle size distribution. A graph of the particle size distribution of the obtained C-QDs is shown in FIG. 24. According to the analysis of this high-resolution TEM image, the C-QDs obtained in this example had an average diameter of 4.25 nm, with a standard deviation of 0.56 nm. It was found that the diameters of more than 90% of the C-QDs particles were in the range of 3.1 to 4.8 nm. It can be seen from the high-resolution TEM image that the particles are highly monodispersed.

[0107] Incidentally, the D50 calculated from the graph showing the particle size distribution of C-QDs shown in Figure 24 was 4.2 nm. Also, the full width at half maximum (FWHM) in the graph of Figure 24 was 1.2 nm.

[0108] FIG. 25 shows a high-resolution TEM image of the C-QDs obtained in this example. As can be seen from this image, a neatly arranged lattice fringe was confirmed inside the circular area corresponding to one C-QDs particle. The line spacing of this lattice fringe was 0.21 nm. Lattice fringes are fringes formed by the interference of light transmitted through a crystal structure and light reflected from a certain crystal plane in the crystal structure, and are a phenomenon in which a light and dark stripe pattern corresponding to the interplanar spacing of the crystal planes is obtained, but clear lattice fringes were observed in this TEM image. This indicates that the C-QDs obtained in this example have a high degree of crystallinity. Incidentally, the theoretical value of the lattice fringe spacing due to the (100) plane in completely crystallized graphite is 0.213 nm, but as mentioned above, the line spacing of the lattice fringes obtained from the TEM image was 0.21 nm, so it is thought that the reflected light of light on the (100) plane of the C-QDs obtained in this example is involved in the formation of the lattice fringes.

[0109] Next, an elemental analysis of the obtained C-QDs was performed using an energy dispersive X-ray analyzer (JEM-2300F manufactured by JEOL Ltd.). The results are shown in FIG. 26. As can be seen from this figure, according to the energy dispersive X-ray analysis, only peaks corresponding to carbon and oxygen are present in the C-QDs sample obtained in this example. Incidentally, the two peaks seen on the right end of the spectrum in FIG. 26 are due to copper, but these are thought to be peaks caused by the grid of the device.

[0110] Furthermore, using a Malvern Zeta-sizer (Nano-ZS 90 Zeta sizer), the particle size of the C-QDs obtained in this example was measured by dynamic light scattering, and the zeta potential of the C-QDs particles was also measured. These measurements were performed at 23°C using water as a dispersion medium. The results of the diameter measurement of the C-QDs particles by this dynamic light scattering method are shown in Figure 27. According to this, the particle size of the C-QDs particles was 4.37±0.41 nm, and the half-value width was 1.2 nm.

[0111] The diameter of C-QDs obtained by dynamic light scattering is slightly larger than the average diameter of 4.25 nm obtained from the analysis of TEM images described above, but this is not unnatural. In other words, the so-called "hydrodynamic diameter" of a particle dispersed in a solvent such as water is generally larger than the diameter of the same particle in a vacuum. This is because molecules or functional groups present in the solvent that have an opposite charge to that of the particle's surface are attached to the particle's surface, resulting in the formation of an electric double layer with a certain thickness that surrounds the particle.

[0112] Next, the zeta potential of the C-QDs-containing water obtained in this example was measured using the Zeta-sizer meter described above. The results are shown in FIG. 28. As can be seen from the graph shown in FIG. 28, the zeta potential of the C-QDs in this example was -32 mV, and the full width of this peak was 14.8 mV. The absolute value of the zeta potential being as large as 32 means that the C-QDs continue to be dispersed stably in the aqueous solvent. That is, one C-QDs particle and other C-QDs particles present in the vicinity are firmly charged to the same potential, so they repel each other, making it difficult for the C-QDs particles to aggregate with each other. Incidentally, the zeta potential is the potential at the "slip surface" where liquid flow begins to occur in the electric double layer formed around the fine particles in the solution. This "slip surface" is the boundary surface conveniently defined as existing between the "ion fixing layer" formed around the fine particles and the "ion diffusion layer" formed outside it.

[0113] The negative zeta potential of the C-QDs produced in this example means that the surface of the C-QDs is also negatively charged. Because the C-QDs have such properties, the dispersion of the C-QDs in water is maintained well for a long period of time. (A) and (C) in FIG. 29 are photographs showing the state of the C-QDs-containing solution under normal light and ultraviolet light immediately after production by the method of the present invention, respectively, and (B) and (D) are photographs showing the state of the C-QDs-containing solution under normal light and ultraviolet light, respectively, six months after production. When the fluorescence emission level of the solution six months after production is compared with that immediately after production, there is no inferiority in the fluorescence emission characteristics. It can be seen that the C-QDs obtained in this example maintain good fluorescence emission characteristics even after long-term storage.

[0114] Furthermore, the chemical state at the surface of the C-QDs particles was analyzed using FT-IR analysis. The FTIR-4100 manufactured by JASCO Corporation was used as the instrument. For comparison, the FT-IR spectrum was also measured for the raw material (crushed fenugreek seeds) used in the production of C-QDs. The results are shown in FIG. 30. Here, FIG. 30(A) is the spectrum of crushed fenugreek seeds, and FIG. 30(B) is the spectrum of the C-QDs produced in this example.

[0115] In the spectrum shown in FIG. 30(A), the wave numbers are 3283, 2925, 2855, 1740, 1638 and 1032 cm -1 These peaks correspond to OH stretching motion, symmetric CH2 stretching, asymmetric CH2 stretching, C=O of carboxyl / carbonyl groups, C=C bond, and CH bending, respectively. The presence of peaks at 2925 and 2855 in the FTIR spectrum of fenugreek seeds, which correspond to symmetric CH2 stretching and asymmetric CH2 stretching, indicates that fenugreek seeds contain hydrocarbons.

[0116] On the other hand, in the graph of FIG. 30(B) showing the FTIR spectrum of the C-QDs obtained in the examples, the wave numbers 3323 and 1630 cm -1 These peaks correspond to OH and C=O, respectively. In addition, in the graph (spectrum) of Figure 30(B), the wave numbers 2925 and 2855 cm, which correspond to the symmetry and asymmetry of CH2, -1 It can be seen that the two peaks of OH, C=O, and OH have disappeared. This indicates that the hydrocarbons in the fenugreek seeds obtained in this example have been substantially completely carbonized. Incidentally, the presence of peaks corresponding to the bending of OH, C=O, and OH is evidence that groups having such a bond structure are present on the surface of C-QDs.

[0117] Zeta potential measurement and FTIR measurement are complementary to each other, and it is preferable to comprehensively analyze the results of both. The analysis in this example revealed that the surface of C-QDs particles is negatively charged, and their zeta potential in water is also large at -32 mV. Therefore, C-QDs particles can exist stably in water for a long period of time without agglomeration.

[0118] In addition, when carbonizing the hydrocarbons that serve as the carbon source, depending on the conditions of the carbonization process, sufficient carbonization may not be achieved, and the resulting C-QDs may not exhibit good fluorescent properties. To prevent this, one effective way to determine whether carbonization has progressed sufficiently is to use the FTIR method to check for the presence or absence of peaks due to CH2.

[0119] Elemental analysis was performed on both the crushed fenugreek seeds and the C-QDs obtained in the examples using an X-ray photoelectron spectrometer (PHI Quantes, manufactured by ULVAC-PHI, Inc.) (XPS analysis). The results are shown in Figure 31. Figure 31(A) shows the spectrum of the C-QDs obtained in the examples, and Figure 31(B) shows the spectrum of the crushed fenugreek seeds.

[0120] The measurement conditions for the XPS analysis are as follows: X-ray source: Al Kα1 (1.4866keV) (analysis diameter: 100μm, output: 28.8W 15kV) Photoelectron extraction angle: 45° Raster area: 10μm×10μm Pass Energy:69eV,Step Size:0.13eV Neutralization gun: 20μA 1.0V

[0121] In Figure 31(A), a peak corresponding to the carbon 1s orbital, a peak corresponding to the nitrogen 1s orbital, and a peak corresponding to the oxygen 1s orbital are observed. In other words, it can be seen that the C-QDs obtained in this example contain carbon, oxygen, and nitrogen. In addition, from the results of the XPS analysis (Figure 31(A)), no peaks due to other elements were observed. This indicates that the C-QDs obtained in this example are of high purity. Next, the peak intensity corresponding to the carbon 1s orbital, the peak intensity corresponding to the oxygen 1s orbital, and the peak intensity corresponding to the nitrogen 1s orbital were compared in the graph of Figure 31(A) to determine the abundance ratio of each element in the obtained C-QDs. The results are shown in Table 1.

[0122] [Table 1] From the above XPS analysis, it is found that approximately one-third of all carbon atoms in the C-QDs are bonded to oxygen atoms to form functional groups, which means that a considerable number of functional groups are formed in the obtained C-QDs.

[0123] FIG. 32(A) shows a graph of the deconvolution of the carbon 1s peak observed above. As shown in this graph, the carbon 1s peak is deconvoluted into the following three peaks: (1) A main peak (77.5 at.%) centered at 283.6 eV corresponds to the sp2 hybrid orbital in the graphite structure, and (2) A peak (13.5 at.%) due to CO (carbon-oxygen single bond) centered at 285.0 eV and (3) A peak (8.9 at.%) due to C=O (carbon-oxygen double bond) centered at 286.8 eV It is. Next, a graph of the deconvolution of the peaks corresponding to the oxygen 1s orbital is shown in Figure 32(B). In this graph, the peak centered at 529.58 eV (65.9 at.%) is due to -OH groups / C=O groups present on the surface of C-QDs, while the peak centered at 531.01 eV (34.1 at.%) is due to C-O bonds.

[0124] The results of the XPS analysis complement those of the FT-IR analysis. In the FT-IR analysis, the presence of mainly C-O and C=O bonds was detected. On the other hand, the amount ratio (atomic percentage: at.%) of elements other than hydrogen and helium can be obtained in the XPS analysis. Therefore, the results of the XPS analysis and the FT-IR analysis combined provide information about the functional groups present in the obtained C-QDs.

[0125] As with the C-QDs described above, the proportions of each element in the crushed fenugreek seeds used as the raw material were determined from the results of XPS analysis, and were C: 57.6%, O: 40.4%, and N: 2.0%.

[0126] The measurement results of the Raman spectrum of the C-QDs produced in the example (excitation light wavelength: 785 nm) are shown in Figure 33. The laser Raman spectrometer used was Lab RAM HR-800 manufactured by Horiba Jobin Yvon. As can be seen from Figure 33, the Raman spectrum has a wave number of 1362 cm -1 The D band has a peak at 1590 cm -1The G band has a peak at 100 nm, and the D band has a peak at 150 nm. Here, the G band corresponds to the sp2 hybrid orbital of graphite. Therefore, the G band is an indicator of the degree of graphitization of the C-QDs. On the other hand, the D band corresponds to the sp3 hybrid orbital, which is an indicator of the amount of defects or functional groups. The intensity ratio of the G band to the D band (G / D) in the C-QDs obtained in this example was 1.68, which was higher than any of the G / D ratios of C-QDs produced using other natural products as carbon sources reported so far. A large G / D ratio means that the obtained C-QDs have a high degree of crystallinity (i.e., many graphite crystal structures). The high-resolution TEM image shown above shows that the C-QDs obtained in this example do not have an amorphous structure, and the Raman spectrum measurement shows that the D band exists as described above. Taken together, it can be seen that the C-QDs obtained in this example are moderately functionalized (have functional groups).

[0127] FIG. 34 is a graph showing the results of investigating the change over time in the intensity of the fluorescence of C-QDs obtained in the example when light was continuously irradiated from the outside. In this experiment, a 150W xenon lamp was used to continuously irradiate light for 4 hours, while the intensity of the fluorescence emitted by the C-QDs was continuously measured. As shown in FIG. 34, the C-QDs obtained in this example did not show a significant change in the intensity of the fluorescence even when they were continuously irradiated with light from a 150W xenon lamp for 4 hours. This means that the obtained C-QDs have excellent photostability. Unlike amorphous C-QDs, they are completely carbonized and have good crystallinity, which is thought to be why they have such good photostability.

[0128] The mechanism of fluorescence emission of C-QDs is extremely complex and has not yet been fully elucidated. Various discussions and proposals have been made on this topic. For example, in recent years, various proposals have been made to explain the mechanism using particle size distribution, surface traps, quantum effects, charge transfer, intramolecular hydrogen bonds, and the generation of aromatic molecules. Among these, the most promising theories are band gap transition due to quantum incorporation and transitions from various surface states.

[0129] Figure 35(A) is a graph showing the intensity of fluorescence measured when the C-QDs obtained in this example were dispersed in water and made to emit fluorescence using various excitation lights, with the excitation lights being 260, 280, 300, 320, 340, 360, or 380 nm. Figure 35(B) shows the normalized fluorescence graphs corresponding to each excitation light. The observed fluorescence emission shows excitation light dependence. It also has a broad and asymmetric peak ranging from 357 to 665 nm. Among the various excitation lights mentioned above, the maximum emission intensity was observed with an excitation light of 340 nm. Regarding the shape of the fluorescence emission peak, a typical fluorescence emission spectrum has a shape that seems to be a combination of two peaks. That is, there is a main peak centered at 414 nm with a full width at half maximum (FWHM) of 46 nm, and a peak centered at 468 nm with a FWHM of 88 nm.

[0130] It is noteworthy that excitation light-dependent C-QDs can be easily produced from precursors consisting of synthetic or natural carbon-containing substances, and the obtained C-QDs always have a red shift. However, the C-QDs obtained in this example (C-QDs obtained from natural carbon sources) are also excitation light-dependent carbon dots. However, surprisingly, the C-QDs obtained in this example exhibit both a blue shift and a red shift (have dual mode). FIG. 36 is a graph showing the intensity of fluorescence emission (normalized) obtained when seven different excitation lights having wavelengths of 260 to 380 nm are irradiated to the C-QDs obtained in this example. From this figure, it can be seen that by changing the wavelength of the excitation light, the observed fluorescence emission spectrum may shift to the long wavelength side or to the short wavelength side. Specifically, in the case of excitation light having a wavelength of 260 to 320 nm, a blue shift (shift to the short wavelength side) of about 22 nm is shown, while in the case of excitation light in the wavelength range of 340 to 380 nm, a red shift (shift to the long wavelength side) of about 40 nm is shown. The sum of both shifts (sum of blue shift and red shift) shows a change of 62 nm. Here, although it is possible to explain the occurrence of a red shift as the wavelength of the excitation light increases using the quantum capture theory, the existence of a blue shift cannot be explained (with respect to the phenomenon of fluorescence emission) using the quantum capture theory alone. Considering that the C-QDs obtained in this example have CO and C=O functional groups and a certain degree of functional group content (<25%), it is natural to assume that the various surface states make some contribution to the fluorescence emission.

[0131] Essentially, two or more fluorescence mechanisms are believed to contribute to the fluorescence emission in the C-QDs obtained in this example. Specifically, the presence of activated functional groups on the carbon dot surface and the doping of C-QDs with heteroatoms (atoms other than carbon) are believed to have an important effect on the fluorescence emission. In particular, the electronegativity of heteroatoms (nitrogen atoms and sulfur atoms) is believed to play a central role in the various fluorescences. The fluorescence emission peak shows a red shift or blue shift when C-QDs are doped with sulfur or nitrogen, respectively. Since the C-QDs obtained in this example have nitrogen atoms, it is believed that they mainly exhibit a blue shift. That is, the intensity of the fluorescence emission (photoluminescence) of the C-QDs obtained in this example depends on the excitation light, but this fluorescence emission is believed to be due to the synergistic effect of quantum incorporation, functional groups, and the presence of heteroatoms (nitrogen atoms).

[0132] Incidentally, the C-QDs obtained by this embodiment show a small red shift (40 nm) as mentioned above. This is smaller than the red shift width (100 nm) of carbon quantum dots produced by hydrothermal method and the red shift width (150 nm) of carbon quantum dots produced by microwave pyrolysis reported so far. It is considered that the reason why the C-QDs produced by this embodiment have such a narrow red shift is because the particle size distribution of the C-QDs is narrow. In particular, doping with nitrogen atoms causes the surface state of the carbon quantum dots to minimize the O state (a new energy state due to C-O bonds formed by the introduction of oxygen in the carbon quantum dots) and promote the N state (a new energy state due to C-N bonds formed by the introduction of nitrogen). This causes fluorescence emission in a narrow region.

[0133] It is known that the fluorescence emission (photoluminescence) of C-QDs is often sensitive to external factors such as pH and the type of solvent the C-QDs are dispersed in. Therefore, we investigated the relationship between the pH of the C-QDs dispersion water and the fluorescence emission spectrum. FIG. 37 is a graph showing the fluorescence emission spectrum of C-QDs obtained in the examples under various pH conditions. First, hydrochloric acid was added to the water containing C-QDs to make the C-QDs-containing solution acidic at pH 5, and 320 nm excitation light was applied to measure the fluorescence emission intensity of C-QDs. In addition, nothing was added to the water containing C-QDs, and the fluorescence emission intensity of C-QDs was measured at pH 7 by applying 320 nm excitation light. Furthermore, sodium hydroxide was added to the C-QDs-containing solution to make the pH of the C-QDs-containing solution 9, 11, or 13, and the fluorescence emission intensity of C-QDs was measured by applying 320 nm excitation light. As can be seen from FIG. 37, the emission spectrum is broad with a peak at 412 nm at any pH. As the pH of the water containing dispersed C-QDs changes from 5 to 7, the fluorescence intensity increases, and then as the pH changes to 9 and then to 13, the fluorescence intensity decreases. That is, the fluorescence intensity is maximum at pH 7. In other words, as the pH of the water in which C-QDs are dispersed moves away from the neutral state, the emission intensity decreases. This phenomenon is considered to occur because the protonation-deprotonation mechanism and surface trap states are involved. The surface trap states referred to here include functional groups formed on the surface of C-QDs, dangling bonds, and carbon atoms in C-QDs with sp2 or sp3 hybrid orbitals.

[0134] In addition, when the peak position of the emission spectrum shown in FIG. 37 is examined in detail, the peak position shifts (changes) to the longer wavelength side as the pH increases, but the change is only 2.4 nm. In other words, when the same excitation light is used, the peak position of the fluorescence emission is almost independent of the pH. Until now, it has been believed that various surface states control the fluorescence emission phenomenon. For example, it was understood that when a certain combination of surface states is activated, the fluorescence emission shifts to the longer wavelength side (red shift). However, in the C-QDs obtained in this example, the wavelength range of the fluorescence emission does not basically change over the range of pH 5 to 13, despite the fact that the C-QDs have a number of functional groups on the surface. Considering the above comprehensively, it is considered that the functionalization of carbon quantum dots may help stabilize the colloid (carbon quantum dots exist stably in water), but it is not basically involved in the characteristics of the fluorescence emission of carbon quantum dots.

[0135] Example 3 C-QDs were obtained (dispersed in water) in the same manner as in Example 2, except that papain (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., derived from Carica Papaya) and glucose isomerase (manufactured by Sigma-Aldrich, derived from Streptomyces murinus) were used instead of ground fenugreek seeds.

[0136] Photographs of the obtained C-QDs-containing water under normal light and under black light irradiation are shown in Figure 38 (C-QDs from papain) and Figure 39 (C-QDs from glucose isomerase). Both types of C-QDs were colorless and transparent liquids under normal light (Figures 38(A) and 39(A)), and showed a blue color when irradiated with ultraviolet light (365 nm) (Figures 38(B) and 39(B)). [Industrial Applicability]

[0137] The carbon quantum dots according to the present invention have a narrow particle size distribution, are crystallized, and exhibit good and stable fluorescent properties with good reproducibility. In addition, since the fluorescent properties last for a long period of time, they are expected to be used and applied in various fields. In particular, they can be used as quantum dots that are non-toxic to living organisms, and are expected to be widely used in the field of bioresearch, including medicine and physiology. Furthermore, since they exhibit upconversion luminescence, they are expected to be widely used in various LEDs, high efficiency solar cells, and renewable energy technology. In addition, in the method for producing carbon quantum dots by pyrolysis, it is possible to produce carbon quantum dots by a relatively easy method, namely pyrolysis, using plant seeds, which are inexpensive and readily available materials, as a carbon source. on the other hand, plasma In the method for producing carbon quantum dots by this method, various carbon-containing substances such as plant seeds and enzymes can be used as carbon sources, and the plant seeds serving as the carbon source can be carbonized in a single step in a relatively short time without unnecessary heating.

Claims

1. A method for producing carbon quantum dots, comprising the step of pyrolyzing fennel seeds to obtain a carbide.

2. 2. The method for producing carbon quantum dots according to claim 1, wherein the pyrolysis is performed by electromagnetic induction.

3. 3. The method for producing carbon quantum dots according to claim 1, wherein the pyrolysis is carried out at a temperature of 330 to 600°C.

4. The method for producing carbon quantum dots according to claim 3, wherein the heating time in the pyrolysis is 2 to 4 hours.

5. The method for producing carbon quantum dots according to any one of claims 1 to 4, characterized in that the fennel seeds are crushed before the pyrolysis is carried out.

6. A method for producing carbon quantum dots, comprising the steps of: The method includes a step of placing a powder of a carbon-containing material in a plasma CVD apparatus and carbonizing the carbon-containing material by colliding a plasma gas with the carbon-containing material, The carbon-containing material further contains nitrogen and oxygen, the carbon element ratio being 40 to 95% and the nitrogen element ratio being 0.5 to 20%. A method for producing carbon quantum dots, comprising:

7. A method for producing carbon quantum dots, comprising the steps of: The method includes a step of carbonizing a powder of a carbon-containing material by colliding the powder with a plasma gas, The carbon-containing material further contains nitrogen and oxygen, the carbon element ratio being 40 to 95% and the nitrogen element ratio being 0.5 to 20%. A method for producing carbon quantum dots, comprising:

8. The method for producing carbon quantum dots according to claim 6 or 7, wherein the carbon-containing material is a plant seed.

9. The method for producing carbon quantum dots according to claim 8, wherein the plant seeds are fenugreek seeds.

10. The method for producing carbon quantum dots according to claim 6 or 7, wherein the carbon-containing substance is a protein.

11. The method for producing carbon quantum dots according to claim 10, wherein the protein is glucose isomerase or papain.

12. The method for producing carbon quantum dots according to any one of claims 6 to 11, wherein the gas plasma is hydrogen gas plasma.

13. The resulting quantum dots are The zeta potential when dispersed in water is −44 to −1.1 mV, The lattice spacing of the lattice fringes due to the (100) plane of graphite, as seen in a transmission electron microscope image, is in the range of 0.200 to 0.234 nm. The method for producing carbon quantum dots according to any one of claims 1 to 12.

14. The resulting quantum dots are The zeta potential when dispersed in water is −44 to −1.1 mV, The average particle size (D50) obtained by dynamic light scattering is 3.1 to 8.7 nm. The method for producing carbon quantum dots according to any one of claims 1 to 12.

15. The method for producing carbon quantum dots according to claim 14, characterized in that the resulting quantum dots have a full width at half maximum of 0.6 to 11.7 nm in particle size distribution measured by dynamic light scattering.

16. The method for producing carbon quantum dots according to claim 14 or 15, characterized in that the lattice spacing of the lattice fringes due to the (100) plane of graphite, as seen in an image taken by a transmission electron microscope, is in the range of 0.200 to 0.234 nm.

17. The method for producing carbon quantum dots according to any one of claims 13 to 16, characterized in that the obtained quantum dots are crystallized in a hexagonal system.

18. The resulting quantum dots are The ratio of carbon and nitrogen elements by X-ray photoelectron spectroscopy was C: 55-80% N: less than 0.5% The method for producing carbon quantum dots according to any one of claims 1 to 5,

19. The method for producing carbon quantum dots according to claim 18, characterized in that the obtained quantum dots have an elemental ratio of oxygen of 20 to 45% by X-ray photoelectron spectroscopy.

20. The resulting quantum dots are The ratio of carbon and nitrogen elements by X-ray photoelectron spectroscopy was C: 55-75% N: 2-10% The method for producing carbon quantum dots according to any one of claims 6 to 12, characterized in that

21. The method for producing carbon quantum dots according to claim 20, characterized in that the obtained quantum dots have an elemental ratio of oxygen of 20 to 40% by X-ray photoelectron spectroscopy.

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