Patent Description:
Luminescent nanocarbon (carbon dots) is a novel carbon nanomaterial that has been recently discovered in soot. Unlike graphene and other nanocarbon materials, the luminescent nanocarbon has a feature of exhibiting strong luminescence. Moreover, materials used as the carbon source compounds are organic molecules rather than materials of highly toxic cadmium compounds such as cadmium sulfide (CdS) and cadmium selenide (CdSe) and rare metals such as europium which are used in semiconductor quantum dots. For these reasons, the luminescent nanocarbon attracts attention as a novel luminescent material capable of replacing semiconductor quantum dots which may have toxicity.

In recent years, various methods of synthesizing the luminescent nanocarbon have been reported. Examples of such methods include a method of chemically processing soot synthesized in a gas phase and a batch method of synthesizing luminescent nanocarbon using a reaction in a liquid phase. The inventors of the present invention have proposed a method of synthesizing luminescent nanocarbon using a micro reaction field forming apparatus (Patent Literature <NUM>) that utilizes electrostatic interaction between micro-droplets synthesized by electrospray (electrostatic spraying) (Non-Patent Literature <NUM>). Another method has also been proposed for manufacturing an inorganic oxide fine particle powder using an apparatus similar to the above micro reaction field forming apparatus described in Patent Literature (Patent Literature <NUM>, Non-Patent Literature <NUM>). <CIT> describes methods for preparing carbogenic nanoparticles and photoluminescent carbogenic nanoparticles. <CIT> describes a carbon nano point with high fluorescence quantum efficiency and a preparation method thereof.

However, the conventional manufacturing method using a micro reaction field forming apparatus has a problem in that the luminescent nanocarbon cannot be efficiently manufactured in large amounts.

An object of the present invention is therefore to provide a method of manufacturing luminescent nanocarbon with which the luminescent nanocarbon can be efficiently manufactured in large amounts.

According to the present invention, the carbon source compound and nitrogen source compound in the raw material aqueous solution may be reacted in water at high temperature and high pressure and the luminescent nanocarbon can thereby be efficiently manufactured in large amounts. Moreover, when the reaction step and the cooling step are configured to progress continuously and concurrently, the luminescent nanocarbon can be more efficiently manufactured. Luminescence characteristics may be adjusted by changing the composition of the raw material aqueous solution and the reaction condition in the reaction step, and luminescent nanocarbons having various luminescence characteristics can thereby be manufactured.

One or more embodiments of the method of manufacturing luminescent nanocarbon according to the present invention will be described hereinafter.

The method of the present invention is a method of manufacturing luminescent nanocarbon from a raw material aqueous solution that contains organic acid or sugar as a carbon source compound and aliphatic amine as a nitrogen source compound.

Examples of organic acids are hydroxy acid and saccharic acid. An example of sugars is glucose. Examples of the hydroxy acid include citric acid, malic acid, tartaric acid, galactaric acid (<NUM>,<NUM>,<NUM>,<NUM>-tetrahydroxyadipic acid, mucic acid), quinic acid, glyceric acid, gluconic acid, glucuronic acid, ascorbic acid, and gallic acid.

Examples of the aliphatic amine include monoamines such as hexylamine and N,N-dimethylethylenediamine and diamines such as ethylenediamine. Among these, the ethylenediamine which is an aliphatic amine is preferred from the viewpoint that the luminescent nanocarbon with high emission intensity can be synthesized. In an embodiment, the luminescence characteristics of the synthesized luminescent nanocarbon can be readily controlled by changing the concentration of ethylenediamine to adjust the ratio to the carbon source compound.

The method of manufacturing luminescent nanocarbon according to the present embodiment includes a reaction step of heating the raw material aqueous solution in a reaction container to react the raw material aqueous solution at a temperature of <NUM> to <NUM> and a cooling step of cooling a reaction solution that contains a reaction product generated in the reaction step. In the present invention, the raw material aqueous solution refers to an aqueous solution that contains the carbon source compound and the nitrogen source compound, and the reaction solution refers to a solution that contains the reaction product generated from the raw material aqueous solution. A solution in a state in which a part of the carbon source compound and a part of the nitrogen source compound in the raw material aqueous solution react to generate the reaction product, therefore, corresponds to both the raw material aqueous solution and the reaction solution.

The raw material aqueous solution contains water as a solvent for dissolving the carbon source compound and the nitrogen compound. Examples of solvents other than water include alcohols. Since the raw material aqueous solution contains water as a solvent, the reaction step of reacting the carbon source compound and the nitrogen compound is hydrothermal synthesis in which the compound is synthesized under the presence of high-temperature and high-pressure water.

The reaction step is a step of heating the raw material aqueous solution in a state of being closed in the reaction container to synthesize the luminescent nanocarbon as the reaction product which results from the reaction under a reaction temperature of <NUM> or higher and <NUM> or lower. The carbon source compound and the nitrogen source compound are reacted under a condition in which the raw material aqueous solution uniformly exists in the reaction container, that is, in the reaction solution (raw material aqueous solution) of a uniform state with a higher pressure than that in the vapor-liquid equilibrium. The uniform state refers to a state in which no steady-state interface exists between the vapor phase and the liquid phase, that is, a state in which no interface exists because the vapor phase and the liquid phase are mixed together or a state in which an interface exists but its position is not fixed and fluctuates.

For example, an example of the state in which no interface exists, among the uniform states, is a state in which the raw material aqueous solution in the reaction container forms a supercritical phase (supercritical fluid) that has both the diffusivity of a vapor and the solubility of a liquid. The supercritical phase of the raw material aqueous solution can be formed by setting the temperature and pressure in the reaction container at a critical temperature or higher and a critical pressure or higher, respectively.

Examples of the state in which the position of an interface is not fixed and fluctuates include a case in which a small amount of bubbles exists in the reaction solution (raw material aqueous solution) in the reaction container. Such a state in which a small amount of bubbles exists in the reaction solution (raw material aqueous solution) may be obtained in some cases, such as when a gas is generated due to a side reaction of the synthesis reaction of the luminescent nanocarbon, and the like.

The temperature (reaction temperature) in the reaction container in the reaction step is set at <NUM> or higher and <NUM> or lower. Setting the reaction temperature at <NUM> or higher can accelerate the hydrothermal reaction. According to the present invention, the reaction temperature is <NUM> or higher and preferably <NUM> or higher from the viewpoints of improving the conversion rate of the raw material aqueous solution to the luminescent nanocarbon in the reaction step and suppressing the generation of insoluble components of which the luminescence has been lost.

To suppress the generation of insoluble components due to further progression of the conversion reaction in the reaction step, the reaction temperature is <NUM> or lower and most preferably <NUM> or lower.

When the reaction step is carried out through a batch-type reaction, the raw material aqueous solution is charged in such an amount that the raw material aqueous solution in the reaction container has a higher density in the reaction step than that in the vapor-liquid equilibrium state. When the luminescent nanocarbon is manufactured through a flow-through-type (continuous-type) reaction, the reaction container is adjusted to such a temperature and pressure that the raw material aqueous solution in the reaction container in the reaction step has a higher density than that in the vapor-liquid equilibrium state. This allows the raw material aqueous solution to be in a uniform state in the reaction step, that is, a state in which the vapor and the liquid are mixed together in the reaction container. The carbon source compound and nitrogen source compound contained in the raw material aqueous solution, therefore, do not have to be in a combination that forms a salt. Thus, the materials are not limited to those, such as citric acid and amine, which form a salt, and any combination of raw materials, such as grape sugar (glucose) and amine, for example, which do not form a salt can be used. Moreover, the reaction can efficiently progress.

The reaction container used in the reaction step has pressure resistance against high-temperature and high-pressure conditions.

When the reaction step is carried out through a batch-type reaction, the entire reaction container charged with the raw material aqueous solution is heated to make the inside of the reaction container as a whole to predetermined temperature and pressure so that the reaction step progresses in the entire reaction container. For example, a tube-type high-pressure container is used as the reaction container, which may be put into an electric furnace set at a predetermined temperature to progress the reaction step.

After completion of the reaction step, the tube-type high-pressure container is taken out to room-temperature air and air-cooled to cool the reaction solution which contains the reaction product (cooling step).

When the reaction step is carried out through a batch-type reaction, the raw material aqueous solution is charged in such an amount that the raw material aqueous solution in the reaction container has a higher density than that in the vapor-liquid equilibrium state. For example, the saturation density of water (liquid phase) is <NUM>/cm<NUM>, <NUM>/cm<NUM>, <NUM>/cm<NUM>, and <NUM>/cm<NUM> at reaction temperatures of <NUM>, <NUM>, <NUM>, and <NUM>, respectively. According to the present invention, the reaction temperature in the batch-type reaction is <NUM> to <NUM>, and the raw material aqueous solution is charged in an amount that occupies about <NUM>% or more of the volume of the reaction container. The raw material aqueous solution is charged in such an amount thereby to allow the raw material aqueous solution to exist in a uniform state in the reaction step, and the reaction efficiency improves.

When the reaction step is carried out through a continuous reaction, a pressure-resistant reaction container having a long continuous internal space, such as an elongated tube, is used. A part of the reaction container having a sufficient length is heated while being supplied with the raw material aqueous solution from one end of the elongated tube, and the reaction can thereby continuously progress in the partial region to carry out the reaction step. In the reaction step, the pressure in the reaction container is regulated so that the pressure of the raw material aqueous solution becomes higher than the saturated vapor pressure at which the vapor-liquid equilibrium is established. The saturated vapor pressure of water is <NUM> MPa at <NUM>, <NUM> MPa at <NUM>, <NUM> MPa at <NUM>, <NUM> MPa at <NUM>, and <NUM> MPa at <NUM>. The pressure in the reaction container is therefore regulated to be not lower than the saturated vapor pressure at the reaction temperature. This allows the raw material aqueous solution to exist in a uniform state in the reaction step, and the reaction efficiency thus improves.

Another portion connected to the portion in which the reaction step is carried out is cooled (cooling step) thereby to obtain the luminescent nanocarbon from the reaction solution. The cooling step is carried out, for example, by rapidly cooling a part of the elongated tube, which is the reaction container, using an ice bath or a water bath.

In a configuration in which the reaction step and the cooling step are progressed in different portions of the reaction container and the portion in which the reaction step progresses and the portion in which the cooling step progresses are connected to each other, the raw material aqueous solution and the reaction solution move in the reaction container thereby to allow the reaction step and the cooling step to progress continuously and concurrently.

When the reaction step is carried out in a partial region using the above-described reaction container and the cooling step is carried out in another region, the reaction step and the cooling step progress continuously and concurrently, and a large amount of the luminescent nanocarbon can therefore be efficiently manufactured in a short time.

The adjustment step is a step of adjusting luminescence characteristics of the luminescent nanocarbon through manufacturing a plurality of luminescent nanocarbons that are obtained by changing at least one of the raw material aqueous solution and the manufacturing condition, and evaluating the luminescence characteristics of the plurality of luminescent nanocarbons. In order to manufacture the luminescent nanocarbon having desired luminescence characteristics, a plurality of luminescent nanocarbons is manufactured and evaluated using different raw materials and/or different manufacturing conditions.

Different raw materials refer to those in which the ratio and/or type (combination) of the carbon source compound and nitrogen source compound contained in the raw material aqueous solution are different. By using different raw materials, luminescent nanocarbons having various luminescence characteristics can be manufactured. For example, when a raw material aqueous solution that contains quinic acid as the carbon source compound and ethylenediamine as the nitrogen source compound is used, luminescent nanocarbon that emits light having substantially the same wavelength can be manufactured regardless of the wavelength of excitation light. Specifically, when excitation light having a wavelength of <NUM> to <NUM> is used, luminescent nanocarbon having an emission wavelength within a range of <NUM> to <NUM> can be manufactured. The emission wavelength is a wavelength at which the emission intensity of the emission spectrum of the luminescent nanocarbon is maximum.

The manufacturing conditions refer, for example, to conditions of the reaction step, such as the shape of the reaction container, the flow rate (supply amount) of the reaction solution, heating temperature, heating time, and pressure and conditions of the cooling step, such as cooling temperature and cooling speed, etc. These conditions can also be adjusted thereby to manufacture luminescent nanocarbons having various different luminescence characteristics.

Hereinafter, examples of the present invention will be described in which luminescent nanocarbons were manufactured using a batch-type reaction apparatus and a flow-through-type (continuous-type) reaction apparatus.

Citric acid (abbreviated as "CA," hereinafter), which is hydroxy acid, was used as the carbon source compound, and ethylenediamine (abbreviated as "EDA," hereinafter) was used as the nitrogen source compound. A raw material aqueous solution containing CA with a concentration of <NUM> and EDA with a concentration of <NUM> was used. <CHM>
<CHM>
<CHM>.

A tube-type high-pressure container (reaction container) of SUS316 having an inner volume of about <NUM> was used as the batch-type reactor. The reaction container closed therein with about <NUM> of the raw material aqueous solution was put into an electric furnace and the raw material aqueous solution was reacted at <NUM> for a predetermined time (<NUM> hours, <NUM> hours, <NUM> hours) (reaction step). Since the saturation density of water at <NUM> is <NUM>/cm<NUM>, it can be said that, in the reaction step, the raw material solution existed in the reaction container in a substantially uniform state with a higher density than that in the vapor-liquid equilibrium. Thereafter, the reaction container was taken out and then air-cooled (cooling step) to produce luminescent nanocarbon. The heating rate in the heating step was <NUM>/min and the cooling rate in the cooling step was -<NUM>/min.

The following method was employed to purify the reaction solution after cooling, containing the luminescent nanocarbon as the reaction product. Dialysis was performed in water for about <NUM> hours using a dialyzing membrane (MWCO100-<NUM>) to remove low-molecular-weight components containing the unreacted raw material aqueous solution, and insoluble components were removed using a syringe filter (pore diameter: <NUM>). To observe the TEM image of each example, the luminescent nanocarbon was purified. The emission spectra were measured using unpurified luminescent nanocarbon.

The synthesized luminescent nanocarbon was dispersed in water and the photoluminescence (PL) was measured using a spectrophotofluorometer F-<NUM> available from Hitachi High-Technologies Corporation. For transmission electron microscope (TEM) measurement, H-<NUM> available from Hitachi High-Technologies Corporation was used.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing CA with a concentration of <NUM> and EDA with a concentration of <NUM> was used.

<FIG> are graphs of emission spectra when irradiating the luminescent nanocarbons of Example <NUM> (<NUM>), Example <NUM> (<NUM>), and Example <NUM> (<NUM>) with excitation light of a wavelength of <NUM>. In each graph, the vertical axis represents emission intensity and the horizontal axis represents emission wavelength. <FIG> are graphs illustrating the results of a reaction time of two hours, a reaction time of four hours, and a reaction time of eight hours, respectively. As illustrated in <FIG>, the luminescent nanocarbons were obtained with different luminescence characteristics by changing the concentration of EDA with respect to CA and changing the heating time in the reaction step.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of CA as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used and the reaction step was carried out at <NUM> for two hours.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of CA as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used.

<FIG> are graphs illustrating dependency of the photoluminescence intensity (PL intensity) of the luminescent nanocarbons of Examples <NUM> and <NUM> on the emission wavelength and the excitation wavelength. In <FIG>, the vertical axis represents the wavelength of excitation light (excitation wavelength) while the horizontal axis represents the emission wavelength, and the shading in the graphs represents the PL intensity.

As illustrated in <FIG>, the luminescent nanocarbon of Example <NUM> synthesized using a raw material aqueous solution having a higher EDA concentration than the CA concentration was excited with ultraviolet light around <NUM> to emit strong blue light around <NUM>.

As illustrated in <FIG>, the luminescent nanocarbon of Example <NUM> synthesized using a raw material aqueous solution having the same CA concentration and EDA concentration was excited with ultraviolet light around <NUM> to emit weakened blue light and also excited with visual light around <NUM> to emit stronger yellow light around <NUM>.

From <FIG>, it has been found that the emission characteristics of nanocarbon can be controlled by the composition of the raw material aqueous solution.

<FIG> is a substitute photograph for drawing that presents a transmission electron microscopic image (TEM image) of the luminescent nanocarbon of Example <NUM>. <FIG> is a graph illustrating the particle diameter distribution of the luminescent nanocarbon of Example <NUM>. In <FIG>, the horizontal axis represents the particle diameter and the vertical axis represents the frequency of occurrence. From the results of <FIG> and <FIG>, it has been found that the luminescent nanocarbon is obtained with an average particle diameter of <NUM>.

The TEM image and the particle diameter distribution of the luminescent nanocarbon of Example <NUM> are almost the same as those of Example <NUM>, and the particle diameter distribution scarcely depends on the EDA concentration.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of malic acid (abbreviated as "MA") as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used. The luminescent nanocarbon was purified in the same manner as in Example <NUM>.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of tartaric acid (abbreviated as "TA") as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used. The luminescent nanocarbon was purified in the same manner as in Example <NUM>.

<FIG> is a graph illustrating dependency of the PL intensity of the luminescent nanocarbon of Example <NUM> on the excitation wavelength and the emission wavelength. Items represented by the vertical axis, the horizontal axis, and the shading are the same as those in <FIG>. The luminescent nanocarbon of Example <NUM> obtained using malic acid as the carbon source compound exhibits emission having a maximum around <NUM> when excided with an excitation wavelength around <NUM> and also exhibits weaker PL than that of the luminescent nanocarbon of Example <NUM>, which is obtained using CA as the raw material, in a shorter wavelength region than that of the luminescent nanocarbon of Example <NUM>.

<FIG> is a graph illustrating dependency of the PL intensity of the luminescent nanocarbon of Example <NUM> on the excitation wavelength and the emission wavelength. It has been found that the luminescent nanocarbon obtained using tartaric acid as the carbon source compound has an emission wavelength similar to that of the luminescent nanocarbon of Example <NUM> obtained using malic acid as the carbon source compound but the dark shading, which represents strong emission intensity, appears around <NUM> and the emission intensity is thus stronger than that of Example <NUM>.

In both of them, the average particle size estimated from the TEM image was <NUM>. It has thus been found that, even though the particle diameter is the same, the luminescence characteristics of the obtained luminescent nanocarbon vary depending on the molecular structure of the carbon source compound contained in the raw material aqueous solution.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of quinic acid (abbreviated as "QA") as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used. The luminescent nanocarbon for observing the TEM image was purified in the same manner as in Example <NUM>.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of glucose as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used. The luminescent nanocarbon for observing the TEM image was purified in the same manner as in Example <NUM>.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of gluconic acid (abbreviated as "GcoA") as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used.

Luminescent nanocarbon was produced in the same manner as in Example <NUM> except that a raw material aqueous solution containing <NUM> of gluconic acid as the carbon source compound and <NUM> of EDA as the nitrogen source compound was used.

<FIG> is a graph of emission spectra exhibiting dependency of the PL intensity of the luminescent nanocarbon of Example <NUM> on the excitation wavelength and the emission wavelength. Items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. Each emission spectrum is illustrated with the wavelength of the excitation light. The luminescent nanocarbon manufactured using the raw material aqueous solution containing quinic acid as the carbon source compound and EDA as the nitrogen source compound exhibited emission of only yellow.

As can be seen from the evaluation results of the luminescent nanocarbons obtained using other raw material aqueous solutions (<FIG>, <FIG>, and <FIG>), the emission wavelength of the luminescent nanocarbon at which the emission intensity is maximum varies in general depending on the wavelength of the excitation light. The use of quinic acid as the carbon source compound allows the luminescent nanocarbon to be obtained in which, when the excitation light having a wavelength of <NUM> to <NUM> is used, the emission wavelength at which the emission intensity of the emission spectrum is maximum is within a range of <NUM> to <NUM> irrespective of the wavelength of the excitation light and the emission wavelength is substantially the same, also irrespective of the wavelength of the excitation light. The graph of <FIG> illustrates the results when measuring the reaction product without diluting (no dilution).

<FIG> is a substitute photograph for drawing that presents a TEM image of the luminescent nanocarbon of Example <NUM>. <FIG> is a graph illustrating the particle diameter distribution of the luminescent nanocarbon of Example <NUM>, and items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. From the results of <FIG> and <FIG>, it has been found that the luminescent nanocarbon is generated with an average diameter of <NUM>.

<FIG> is a graph of emission spectra exhibiting dependency of the PL intensity of the luminescent nanocarbon of Example <NUM> on the excitation wavelength and the emission wavelength. Items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. Each emission spectrum is illustrated with the wavelength of the excitation light. The raw material aqueous solution containing glucose as the carbon source compound and EDA as the nitrogen source compound does not contain a salt of the carbon source compound and the nitrogen source compound, but the luminescent nanocarbon was obtained as in the other examples. The graph of <FIG> illustrates the results when measuring the reaction product diluted <NUM>,<NUM> times (×<NUM> dilution).

<FIG> is a substitute photograph for drawing that presents a TEM image of the luminescent nanocarbon of Example <NUM>. <FIG> is a graph illustrating the particle diameter distribution of the luminescent nanocarbon of Example <NUM>, and items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. From the results of <FIG> and <FIG>, it has been found that the luminescent nanocarbon is generated with an average particle diameter of <NUM>.

<FIG> are graphs of emission spectra exhibiting dependency of the PL intensity of luminescent nanocarbons on the excitation wavelength and the emission wavelength. <FIG> illustrates the result of Example <NUM>, <FIG> illustrates the result of Example <NUM>, and <FIG> illustrates the result of Example <NUM>. Items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. Each emission spectrum is illustrated with the wavelength of the excitation light. The graphs of <FIG> illustrate the results when measuring the reaction products diluted <NUM> times (×<NUM> dilution).

From <FIG>, it has been found that the luminescence characteristics of the luminescent nanocarbon obtained using gluconic acid as the carbon source compound depend on the excitation wavelength and vary in accordance with the concentration of ethylenediamine contained in the raw material aqueous solution.

«Flow-through-type (Continuous-type) Reaction Apparatus».

<FIG> is a schematic view illustrating the outline of a flow-through-type reaction apparatus used in Examples <NUM> to <NUM>. As illustrated in the figure, a SUS316 tube having an outer diameter of <NUM>/<NUM> inch and an inner diameter of <NUM> was used as the reaction container, and the raw material aqueous solution was delivered from a reservoir so as to be of a predetermined amount per one minute using a pump. The reactor tube length of a furnace (electric furnace) in which the reaction step in the reaction container would progress was <NUM>, and the sample temperature (reaction solution temperature) in the vicinity of the exit of the electric furnace was determined as the reaction temperature. A portion of the reaction container through which the reaction solution after the reaction would flow was rapidly cooled in an ice bath (cooling step), and the pressure was reduced with a back pressure regulator to recover the luminescent nanocarbon solution. When the raw material aqueous solution is supplied in an amount of <NUM> per one minute, the residence time in the furnace is about <NUM> minutes.

In the flow-through-type reaction apparatus, the pressure in the portion (reaction part) in which the reaction step is carried out is monitored with a pressure gauge and regulated with the back pressure valve so as to be a pressure (about <NUM> MPa) higher than the saturated vapor pressure.

Continuous synthesis of luminescent nanocarbon was carried out at a flow rate of <NUM>/min and a reaction temperature of <NUM> using the flow-through-type reaction apparatus of <FIG> and using a raw material aqueous solution containing CA with a concentration of <NUM> and EDA with a concentration of <NUM>.

The reaction solution after the cooling step was developed in ethanol, the supernatant liquid was recovered by filtration, and the solvent was then removed to obtain luminescent nanocarbon for TEM measurement. When continuous synthesis is carried out using a raw material aqueous solution of CA:EDA=<NUM>:<NUM> as in this example, the reaction product can be purified by the above simple method because unreacted raw materials are not mixed in the reaction product.

Continuous synthesis of luminescent nanocarbon was carried out under the same condition as in Example <NUM> except that the flow rate was <NUM>/min.

Continuous synthesis of luminescent nanocarbon was carried out under the same condition as in Example <NUM> except that the reaction temperature was <NUM>.

<FIG> is a graph of emission spectra exhibiting dependency of the PL intensity of the luminescent nanocarbon of Example <NUM> on the excitation wavelength and the emission wavelength. Items represented by the vertical axis, the horizontal axis, and the shading are the same as those in <FIG>. As illustrated in the figure, the luminescent nanocarbon of Example <NUM> exhibits strong blue light emission around <NUM> when irradiated with ultraviolet light around <NUM>.

<FIG> is a substitute photograph for drawing that presents a TEM image of the luminescent nanocarbon of Example <NUM>. <FIG> is a graph illustrating the particle diameter distribution of the luminescent nanocarbon of Example <NUM>, and items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. From <FIG> and <FIG>, it has been found that the nanocarbon is generated with an average particle diameter of <NUM>.

The yield rate of nanocarbon with respect to the input raw material is about <NUM>% at <NUM> because the reaction does not sufficiently progress, but the yield rate is improved to about <NUM>% at <NUM>. Thus, continuous synthesis of about <NUM> of the luminescent nanocarbon per one hour has been successfully carried out.

The yield rates of the luminescent nanocarbons obtained in Examples <NUM> to <NUM> are listed in the following table. As listed in the table, it has been found that, when the feed rate of the raw material aqueous solution is set at <NUM> per one minute, the yield rate sharply increases as the reaction temperature increases from <NUM> to <NUM> while the yield rate moderately decreases as the reaction temperature further increases from <NUM> to <NUM>. From this result, it can be said that the reaction temperature is preferably around <NUM> (about <NUM> to <NUM>) from the viewpoint of yield rate.

<FIG> are graphs of emission spectra exhibiting dependency of the PL intensity of the luminescent nanocarbons of Examples <NUM> to <NUM> on the excitation wavelength and the emission wavelength. Items represented by the vertical axis and the horizontal axis are the same as those in <FIG>. Each emission spectrum is illustrated with the wavelength of the excitation light. As illustrated in these figures, the obtained luminescent nanocarbons have different luminescence characteristics depending on the wavelength of the excitation light.

From the comparison between <FIG>, it has been found that the feed rate of the raw material aqueous solution affects the luminescence characteristics of the luminescent nanocarbon. From the comparison between <FIG> and <FIG>, it has been found that the reaction temperature affects the luminescence characteristics of the luminescent nanocarbon. It has thus been found that luminescent nanocarbons having different luminescence characteristics can be obtained from the same raw material aqueous solution by adjusting the reaction condition such as a flow rate and reaction temperature.

Luminescent nanocarbons were produced using raw material aqueous solutions in which the molar ratio of CA and EDA was <NUM>: <NUM> but the concentration was changed from <NUM> to <NUM>. When the obtained luminescent nanocarbons were diluted so as to have the same raw material concentration and the PL measurement was performed, the difference due to the raw material concentration was only reflected on a small difference in the emission intensity. In other words, the luminescence characteristics of the luminescent nanocarbons scarcely changed even through the raw material concentration of the raw material aqueous solution was significantly changed. From this result, it has been found that, when using raw material aqueous solutions containing CA and EDA at the same molar ratio, the reaction characteristics (luminescence characteristics) of the luminescent nanocarbons are substantially not affected by the concentrations of the carbon source compound and nitrogen source compound contained in the raw material aqueous solutions, provided that the heating condition at the time of the reaction is the same.

As described above, from the comparison between <FIG>, it can be said that the feed rate of the raw material aqueous solution affects the emission characteristics of the light emitting nanocarbon. It can be estimated that this is caused by the change in the heating condition of the reaction step because the residence time (reaction time) in the furnace varies due to the change of flow rate in the flow-through-type reactor. For example, when the flow rate is halved, the residence time is about doubled, so the reaction is carried out under the same heating condition for a long time.

In contrast, even when the concentrations of the carbon source compound and nitrogen source compound in the raw material aqueous solution are changed while the flow rate remains the same, the heating condition (temperature, time) in the reaction step does not change. It can thus be said that the emission characteristics of the obtained luminescent nanocarbons substantially did not change. However, the effect of the raw material concentration in the reaction step can vary depending on the reaction mechanism. When another raw material aqueous solution is used, therefore, nanocarbon having different emission characteristics may be obtained due to the concentration effect in the reaction step.

When the flow-through-type reaction apparatus illustrated in <FIG> was used, about <NUM> of the luminescent nanocarbon was able to be manufactured per one hour by supplying <NUM> of <NUM> of the raw material aqueous solution per one minute and reacting it.

<FIG>) is a schematic view illustrating the outline of a flow-through-type reaction apparatus used in Examples <NUM> to <NUM>. In the flow-through-type reaction apparatus illustrated in the figure, high-temperature and high-pressure water is supplied by a preheater to reduce the time required for temperature rise in the reaction step. The use of a SUS316 tube having an outer diameter of <NUM>/<NUM> inch and an inner diameter of <NUM> as the reaction container was the same as in the apparatus of <FIG>, but the length of a heater was <NUM>. The flow rate of water supplied to the preheater was <NUM> per one minute (<NUM>/min).

Continuous synthesis of luminescent nanocarbons was carried out at a flow rate of <NUM>/min and predetermined reaction temperatures (<NUM>, <NUM>, <NUM>, <NUM>, and <NUM>) using the flow-through-type reaction apparatus of <FIG> and using an aqueous solution containing malic acid with a concentration of <NUM> and EDA with a concentration of <NUM> as the raw material aqueous solution.

Continuous synthesis of luminescent nanocarbons was carried out at a flow rate of <NUM>/min and predetermined reaction temperatures (<NUM>, <NUM>, <NUM>, <NUM>, and <NUM>) using the flow-through-type reaction apparatus of <FIG> and using an aqueous solution containing galactaric acid with a concentration of <NUM> and EDA with a concentration of <NUM> as the raw material aqueous solution.

Continuous synthesis of luminescent nanocarbons was carried out at a flow rate of <NUM>/min and predetermined reaction temperatures (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>) using the flow-through-type reaction apparatus of <FIG> and using an aqueous solution containing tartaric acid with a concentration of <NUM> and EDA with a concentration of <NUM> as the raw material aqueous solution.

Continuous synthesis was carried out at a flow rate of <NUM>/min and a reaction temperature of <NUM> using the flow-through-type reaction apparatus of <FIG> and using an aqueous solution containing glucose with a concentration of <NUM> and EDA with a concentration of <NUM> as the raw material aqueous solution, and it was confirmed that luminescent nanocarbon was synthesized.

<NUM>(a) to <NUM>(c) are graphs illustrating dependency of the PL intensity of luminescent nanocarbons on the emission wavelength and the reaction temperature. <NUM>(a) (<FIG>) illustrates the measurement result of the luminescent nanocarbon of Example <NUM>, FIG. <NUM>(b) (<FIG>) illustrates the measurement result of the luminescent nanocarbon of Example <NUM>, and FIG. <NUM>(c) illustrates the measurement result of the luminescent nanocarbon of Example <NUM>. From these figures, it can be found that luminescent nanocarbons having different emission characteristics are obtained depending on the reaction temperature. From the comparison of FIGS. <NUM>(a) to <NUM>(c), it has been found that the effect of the reaction temperature on the emission characteristics of the luminescent nanocarbons is not uniform and varies depending on the type of the raw material used as the carbon source compound.

Continuous synthesis of luminescent nanocarbons was carried out at predetermined flow rates (<NUM>/min and <NUM>/min) and a reaction temperature of <NUM> using the flow-through-type reaction apparatus of <FIG> and using aqueous solutions containing citric acid (CA) with a concentration of <NUM> and urea with predetermined concentrations (<NUM> and <NUM>) as the raw material aqueous solutions.

<FIG> illustrates emission spectra measured when the reaction products of Example <NUM> are diluted <NUM> times and the excitation wavelength is <NUM>. As illustrated in the figure, regardless of the content of urea and the feed rate of the raw material aqueous solution, emission spectra having the maximum emission intensity around <NUM> were obtained. With regard to the feed rate of the raw material aqueous solution, luminescent nanocarbons having higher emission intensity were obtained at a feed rate of <NUM>/min than <NUM>/min regardless of the concentration of urea in the raw material aqueous solution. On the other hand, with regard to the concentration of urea in the raw material aqueous solution, when the feed rate of the raw material aqueous solution was <NUM>/min, luminescent nanocarbon having higher emission intensity was obtained at a concentration of urea of <NUM> than <NUM>, while when the feed rate was <NUM>/min, luminescent nanocarbon having higher emission intensity was obtained at a concentration of urea of <NUM> than <NUM>.

Claim 1:
A method of manufacturing luminescent nanocarbon from a raw material aqueous solution containing organic acid or sugar as a carbon source compound and aliphatic amine as a nitrogen source compound, the method comprising:
a reaction step of heating the raw material aqueous solution in a reaction container to react the raw material aqueous solution at a reaction temperature of <NUM> or higher and <NUM> or lower; and
a cooling step of cooling a reaction solution containing a reaction product generated from the raw material aqueous solution in the reaction step,
wherein the reaction step is carried out through a batch-type reaction or a continuous-type reaction;
when the reaction step is carried out through the batch-type reaction, the raw material aqueous solution is charged in such an amount that the raw material aqueous solution in the reaction container has a higher density than that in the vapor-liquid equilibrium state by carrying out the reaction step under a condition in which the reaction temperature is <NUM> or higher and <NUM> or lower, and the raw material aqueous solution is charged in an amount that occupies about <NUM>% or more of the volume of the reaction container;
when the reaction step is carried out through the continuous-type reaction, the pressure in the reaction container is regulated so that the pressure of the raw material aqueous solution becomes higher than the saturated vapor pressure at which the vapor-liquid equilibrium is established.