A method for synthesizing solid carbon dots

Abstract

The present invention relates to a method for synthesizing solid carbon dots, characterized by the steps of: dissolving phenolic compound in a nitrile compound (1) to form a phenolic- nitrile solution; cooling the phenolic-nitrile solution (2); cooling a mineral acid (3); adding the mineral acid in a drop-wise manner into the phenolic-nitrile solution (4) and stirring (5) to form a mixture; heating the mixture at room temperature (6); heating the mixture to obtain carbon dots (7); cooling the carbon dots to room temperature (8); adding the carbon dots into an organic solvent (9); sonicating the carbon dots in organic solvent (10); centrifuging carbon dots in organic solvent (11); and washing (12) the carbon dots with organic solvent.

Description

[0001]A METHOD FOR SYNTHESIZING CARBON DOTS Background of the Invention Field of the Invention 5 This invention relates to a method for synthesizing solid carbon dots, particularly for synthesizing multicolour emission and high purity emission colour solid carbon dots for solid-state emitters in a simple, straightforward, sustainable and feasible manner applicable for industrial scale-up and large-scale synthesis. 10 Description of Related Arts Carbon dots (C-dots) are nanoscale carbon-based materials with sizes ranging from a few nanometers to several tens of nanometers. They have gained significant attention in recent years due to their unique optical properties, often stemming from surface functional groups such as hydroxyl, amino, or carboxyl groups. These properties make C-dots promising for 15 various applications, including white light emitting diodes (WLEDs), lasers, security printing, photodetectors, and multiplexed bioimaging optoelectronics. These achievements demonstrate that C-dots can serve as viable alternatives to inorganic QDs. However, carbon dots (nanographene) production faces critical issues such as consuming 20 immense energy for material conversion, insufficient yield, unselective products and an expensive separation process. One Chinese patent, CN 104059644A, offers a straightforward and rapid method for preparing nitrogen-doped carbon quantum dots. This patent method involves treating a 25 mixture of an alcohol amine organic substance or an alcohol amine organic substance and an oxidant at high temperatures to obtain nitrogen-doped carbon quantum dots. However, the method also consumes a high amount of energy, and it causes serious surface oxidative damage to the prepared nitrogen-doped carbon quantum dots. This damage, in turn, affects the quantum yield and catalytic activity of the quantum dots. 30 Another Chinese patent CN108525691A, provides a method for synchronous reduction to prepare carbon-doped quantum dots and prepared carbon-doped quantum dots and application. The method comprises the steps: preparing a carbon-based material dispersion; mixing a hydroxylamine with the carbon-based material dispersion so as to obtain a carbon-based hydroxylamine composite material dispersion; putting a hydrogen peroxide solution into the carbon-based hydroxylamine composite material dispersion to 5 implement reactions so as to obtain a carbon-doped quantum dot dispersion; optionally, purifying carbon-doped quantum dots so as to obtain a purified functionalized carbon-doped quantum dot dispersion; optionally, performing vacuum freeze-drying on the carbon-doped quantum dot dispersion or the purified functionalized carbon-doped quantum dot dispersion, thereby obtaining solid carbon-doped quantum dots or solid purified functionalized carbon- 10 doped quantum dots. The method provided by the invention is a metal-independent high-class oxidation method, and the prepared carbon-doped quantum dots are low in cost, gentle in reaction condition, controllable in preparation process, low in energy consumption, and free of secondary pollution and can be further applied to multiple fields. However, this patent has a drawback: graphitic structures produced using bottom-up methods tend to have issues with 15 purity and low conversion yield. Carbon dot is one of them, and the crude mixture usually requires further separation and purification to obtain the pure carbon dots. Eventually, this stage is costly, not environmentally friendly and produces a relatively small product yield. Another Chinese granted patent, CN113666359B, relates to a green chemical method for large-scale solid-phase synthesis of fluorescent carbon dots in air. The method is to first 20 prepare a carbon source into nanoparticles of uniform size, and then uniformly disperse the carbon source nanoparticles into an inorganic solid catalyst. Compact, then heat in air, cool and then undergo water washing and organic solvent extraction to obtain fluorescent carbon dots. The method in said patent does not require high-temperature and high-pressure reaction equipment, does not require a large amount of organic solvents and wastewater treatment 25 liquids, is green, environmentally friendly, low-cost, easy to operate, has high yield, and is suitable for a new route of industrialization. Such large-scale manufacturing of carbon dots is expected to be used in lighting display, fluorescence anti-counterfeiting, analysis and detection, photoelectrocatalysis and other fields. However, this patent has a drawback: the heating process takes several hours, which is time-consuming, and the resulting carbon dots 30 may not exhibit high purity. One granted US patent, US8747517B2, to a method for isolating a nanoparticle. A medium containing a nanoparticle is The medium is acidified with a weak acid. An alcoholic solvent is added to induce the nanoparticle to precipitate from the medium. The precipitated nanoparticles are separated from the medium. The methods outlined in this 5 patent are versatile and can be used to isolate various types of nanoparticles in different media. They are applicable to the isolation of quantum dots and similar nanoparticles from both core growth media used to grow core nanocrystals and shell growth media used to produce the shell portion of core-shell nanocrystals. They may also be applicable for isolation of aggregates of nanoparticles. Besides, said invention delivers high yields of clean and 10 consistent nanoparticles, free of undesired impurities or contaminants, and which would involve manipulations that are readily applied to large-scale nanoparticle preparation. However, the method in this invention uses fairly high temperature to heat complex mixture. Preparing CDs using solvothermal / hydrothermal conditions, can be time consuming and energy-intensive, as the reaction must be conducted at high temperatures, usually around 15 200 °C, yet the resulting conversion yield is still low. To address the shortcomings of the previous methods, the present invention utilizes direct heating with innovative steps to significantly enhance the quality and yield of the product at lower temperatures and ambient pressure. Cooling the reactants before the rigorous heating stage helps the size control and the emission of the products. The emission wavelengths of20 the products are also strongly related to the ratio of the concentrated acid and the phenolic- acetonitrile solution. The consistent d-spacing of the products showed the formation of graphitic structure, while the elemental analyses suggested the N and S successfully doped in the carbon dots. Therefore, the pre-treatment of the cooling stage to the reactant mixtures resulted in tuneable and high-colour-purity carbon dots. 25 Accordingly, it can be seen in the prior arts that there exists a need to eliminate the tedious, time-consuming and expensive purification processes associated with the preparation of CDs, graphene quantum dots, and other related nanoparticles and for overcoming the limitations of the prior arts. Summary of Invention It is an objective of the present invention provide a novel direct heating method that produces multi emission colour and high purity emission colour solid-state CDs with a high product yield up to 99.5% with complete reaction in under 6 minutes, without the need for 5 tedious purification steps. It is also an objective of the present invention to exhibit a highly crystalline structure, and their emission can be effectively tuned by adjusting the amount of concentrated sulfuric acid used. 10 Accordingly, these objectives may be achieved by following the teachings of the present invention. The present invention relates to a method for synthesizing solid carbon dots, characterized by the steps of: dissolving phenolic compound in a nitrile compound (1) to form a phenolic-nitrile solution; cooling the phenolic-nitrile solution (2) preferably at the 15 range of 0 °C- 5°C; cooling a mineral acid (3) preferably at the range of 0 °C- 5°C; adding the mineral acid in a drop-wise manner into the phenolic-nitrile solution (4) and stirring (5) preferably at the range of 5 – 30 minutes to form a mixture; heating the mixture at room temperature (6); heating the mixture preferably at the range of 80°C to 300°C for 1-12 minutes to obtain carbon dots (7); cooling the carbon dots to room temperature (8); adding 20 the carbon dots into an organic solvent (9); sonicating the carbon dots in organic solvent (10); centrifuging carbon dots in organic solvent (11) preferably at the range of 5000-6000 rpm for 5-10 minutes; and washing (12) the carbon dots with organic solvent, wherein sonicating, centrifuging and washing are carried out routinely for at least 3 times. 25 Brief Description of the Drawings The features of the invention will be more readily understood and appreciated from the following detailed description when read in conjunction with the accompanying drawings of the preferred embodiment of the present invention, in which: Figures 1 is a flowchart of the simplified method of the present invention. Figure 2 is the morphology carbon dots Transmission electron microscopy (TEM) images (left) and High-resolution electron microscopy HRTEM (right) of a CDs-A; b CDs-B; c CDs-1; d CDs-2; e CDs-3; f CDs-4; g CDs-5; h CDs-6; and i CDs-7. Figure 3 is the chemical structure and composition analysis of carbon dots (CDs): a FTIR 5 spectra; b Raman spectra; c1H NMR; and d13C NMR. Figure 4 is the optical analysis of a Solid-state absorption; b UV–Vis absorption spectra of 100 mg / L; c photoluminescence emission spectra and d CIE chromaticity index of all seven CDs. 10 Detailed Description of the Invention As required, a detailed embodiment of the present invention is disclosed herein; however, it is to be understood that the disclosed embodiment is merely an exemplary of the invention, which may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting but merely as a basis for claims. It should be understood that the 15 drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims. As used throughout this application, the word "may" is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning 20 must). Similarly, the words "include," "including," and "includes" mean including, but not limited to. Further, the words "a" or "an" mean "at least one” and the word "plurality" means one or more, unless otherwise mentioned. Where the abbreviations or technical terms are used, these indicate the commonly accepted meanings as known in the technical field. The present invention will now be described with reference to Figures 1. 25 The present invention relates to a method for synthesizing solid carbon dots, characterized by the steps of: dissolving phenolic compound in a nitrile compound (1) to form a phenolic-nitrile solution; 30 cooling the phenolic-nitrile solution (2) preferably at the range of 0 °C- 5°C; cooling a mineral acid (3) preferably at the range of 0 °C- 5°C; adding the mineral acid in a drop- manner into the phenolic-nitrile solution (4) and stirring (5) preferably at the range of 5 – minutes to form a mixture; heating the mixture at room temperature (6); heating the mixture preferably at the range of 80°C to 300°C for 1-12 minutes to 5 obtain carbon dots (7); cooling the carbon dots to room temperature (8); adding the carbon dots into an organic solvent (9); sonicating the carbon dots in organic solvent (10) for 5- 10 minutes; centrifuging carbon dots in organic solvent (11) preferably at the range of 5000-6000 10 rpm for 5-10 minutes; and washing (12) the carbon dots with organic solvent, wherein sonicating, centrifuging and washing are carried out routinely for at least 3 times. In a preferred embodiment of the present invention, the phenolic compound comprising of an 15 aromatic ring and one and / or more hydroxyl groups. In a preferred embodiment of the present invention, the phenolic compound may be comprising of 1,3,5-trihydroxybenzene (phloroglucinol), 1,3-dihydroxybenzene (resorcinol) and / or combination. 20 In a preferred embodiment of the present invention, the nitrile compound may be comprising of acetonitrile, propionitrile and / or combination. In a preferred embodiment of the present invention, the mineral acid may be comprising of 25 sulfuric acid, phosphoric acid, nitric acid and / or combination. In a preferred embodiment of the present invention, the organic solvent may comprise of acetone and / or other solvents and / or mixed solvents that can precipitate the carbon dots. In a preferred embodiment of the present the adding the mineral acid in a drop- wise manner into the phenolic-nitrile and stirring to form the mixture, produces conversion yield preferably at the range of 50- 99%. 5 Below is an example of a method for synthesizing solid carbon dots, the phenolic compound used in this example is 1,3-dihydroxybenzene (resorcinol), the nitrile compound used is acetonitrile and the mineral acid used is sulfuric acid from which the advantages of the present invention may be more readily understood. It is to be understood that the following 10 example is for illustrative purpose only and should not be construed to limit the present invention in any way. Example of the Method of the Preferred Embodiment 15 A 0.42 g (3.815 mmol) resorcinol was dissolved in 0.50 mL (9.57 mmol) acetonitrile (ACN) (1) in a 50 mL Erlenmeyer flask. Seven amount of concentrated sulfuric acid (such as 3.52 mL, 3.00 mL, 2.49 mL, 1.87 mL, 1.45 mL, 1.35 mL, and 1.04 mL) was then added into seven Erlenmeyer flasks. 20 Both Erlenmeyer flasks containing the resorcinol solution and concentrated sulfuric acid were cooled to 0 °C (2, 3). Subsequently, the cooled concentrated sulfuric acid (such as 3.52 mL, 3.00 mL, 2.49 mL, 1.87 mL, 1.45 mL, 1.35 mL, and 1.04 mL) was added dropwise into the resorcinol solution (4) while stirring (5) and were labelled carbon dots (CDs): CDs-1, -2, -3, - 25 4, -5, -6, and -7. This is to ensure complete protonation before initiating the heating process. Furthermore, sulfuric acid can be replaced with other mineral acids such as phosphoric acid and nitric acid, as they act as conjugation catalyst, and sources of other heteroatoms comprising of sulfur and phosphor maybe added together with mineral acids in a drop-wise manner into the phenolic-nitrile solution. 30 The reaction mixture was left to warm to room temperature (6) before heating on a hot plate at 200 °C for 6 minutes to obtain the CDs (7). Direct heating on a hot plate was chosen as it is convenient to set up, accessible and can produce CDs in bulk. After cooling to room temperature (8), the were dispersed in acetone via sonication (9, 10) followed by centrifugation (11) (5000 rpm, 5 minutes) with acetone washing (12) three times. Morphology and Analysis of Carbon Dots (CDs) 5 The morphology of theCDs(as CDs-A.)was determined under high resolution transmission electron microscopy (HRTEM). 10 Figure 2a shows that the size of CDs-A was around 5 nm and it also displayed two distinct fringes with lattice spacing of 0.21 nm, and 0.32 nm. The promising test reaction data warranted further investigation with increased amounts of concentrated sulfuric acid. Subsequently, the amount of concentrated sulfuric acid was increased to 1.25 mL (23.0 15 mmol). This was carried out to find a suitable amount of concentrated sulfuric acid to be used in this method. This led to a significant improvement in the yield of the product, which was named as CDs-B (21.3%). HRTEM analysis in Figure 2b revealed that the size of CDs-B was 31 nm, with two different 20 fringes having lattice spacings of 0.23 nm and 0.32 nm. Subsequently, the amount of acid was further increased to 3.11 mL (57.22 mmol), but the resulting CDs produced from this test reaction had poor colour purity. Therefore, the reaction was then optimized to achieve CDs with high colour purity in the solid-state. 25 It was then discovered that reacting 0.42 g resorcinol with 0.5 mL (9.58 mmol) ACN in the presence of 3.52 mL (64.77 mmol), 3.00 mL (55.20 mmol), 2.49 mL (45.82 mmol), 1.87 mL (34.41 mmol), 1.45 mL (26.68 mmol), 1.35 mL (24.82 mmol), and 1.04 mL (19.14 mmol) of concentrated sulfuric acid would yield solid-state CDs-1, -2, -3, -4, -5, -6, and -7 with better colour purity than CDs-A and CDs-B. The conversion yield for CDs-1, -2, -3, -4, -5, -6, and - 30 7 were 52.0%, 58.1%, 92.8%, 96.1%, 98.7%, 98.6%, and 99.5%, respectively. As shown in Fig. 2c–e, CDs-1 to -7 formed highly aggregated structures with gradually increasing sizes ranging from 24 to 44 nm. Specifically, CDs-1, -2, -3, -4, -5, -6, and -7 had average sizes of 24 nm, 24.5 nm, 25 nm, nm, 32 nm, 38 nm, and 44 nm, respectively. CDs-1 to CDs-7 exhibited similar lattice of 0.23 nm, 0.26 nm, 0.42 nm, and 0.51 nm. In short, the smallest carbon dots are made of 5 nm diameter with a blue emission, and the 5 largest are 44 nm carbon dots with deep red emission. All products show the graphitic lattice spacing from 0.23 nm to 0.51 nm, indicating structural similarity. The Chemical Structure and Composition Analysis of Carbon Dots (CDs) 10 Energy dispersive X-ray (EDX) analysis, Fourier-transform infrared (FTIR) and Raman spectroscopy were carried out to study the possible chemical structure and composition of the CDs. EDX was carried out to confirm the elemental composition at the surface of the CDs and 15 confirm their purity. According to Table 1, it is clear that the atomic percentage of C, N, and O for all seven CDs showed only relatively small differences, while S increased as the amount of concentrated sulfuric acid was reduced. The elemental study shows a relatively consistent ratio trend of C, 20 N and O. Percentages of sulfur, on the other hand, increase by size. Table 1: Atomic percentages of C, N, O and S. C (%) N (%) O (%) S (%) In addition, the FTIR spectra in Fig.3a shows that all seven CDs exhibited broad and strong 25 absorption bands at around 3420–3200 cm−1which corresponded to the NH / OH groups. They also displayed weak to medium, stretching vibrations of C=C and C–N at around 1602 cm−1and 1424 cm−1, and cm−1, respectively. The C=C stretching vibrations of these CDs were shifted to lower frequency compared to resorcinol (at around 1608 cm−1and 1490 cm−1) due to the increase in the conjugation. A strong and sharp absorption band was observed for these CDs at around 1056 cm−1due to the presence of C–O. CDs-1 and CDs-2 did not display any stretching vibrations of C=O (at around 1704 cm−1 ) and SO3H (at around 1362 cm−1and 1020 cm−1), while the other CDs did. This also means that CDs-1 and CDs-2 are primarily comprised of aromatic rings with less surface group. Moreover, the Raman spectra of resorcinol of Fig.3b show that resorcinol has peaks at 1532 cm−1and 1338 cm−1. CDs- 1 to -4 also exhibited peaks at 1532 cm−1and 1338 cm−1which are known as G and D bands, respectively. The former represents the E2gvibration mode of sp2hybridized carbon, while the latter corresponds to the A1g(zone-edge) breathing vibration. The graphitization degree and ratio of sp2 / sp3carbon (IG / ID) of the CDs were 1.12, 0.94, 1.23, and 1.07 for the CDs-1,-2, -3, and -4, respectively. No Raman shift was obtained for CDs-5, -6 and -7 as their emission was about the same as the laser source (785 nm). These CDs exhibited a high degree of graphitization or crystal. These findings were also in agreement with HRTEM and XRD. The NMR of the products was also investigated to observe the chemical changes, especially in the aromatic structure. In the1H nuclear magnetic resonance (NMR) spectra (DMSO-d6), the signal corresponding to aromatic protons was observed in the range of 6–8 ppm, whereas the proton signal from the hydroxyl groups was located at around 10.5 ppm for all seven CDs (Fig.3c). These CDs also displayed the signal of alkane groups at around 2 ppm. However, two singlet of alkane groups around 3–4 ppm were only observed for CDs-5, -6 and -7. Furthermore,13C NMR spectra (DMSO-d6) in Fig.3d show numerous signals of aromatic sp2 carbons in the range of 100–120 ppm. The signal for carbon bonded with hydroxyl group was observed at around 156 ppm; meanwhile, the signals of alkane groups were in the range of 30–70 ppm. 5 Our solid-state emission CDs were found to be of high purity from these NMR spectra. These pure and sharp NMR signals are first reported, suggesting that the carbon dots have consistent shapes. 10 The Optical Analysis of Carbon Dots (CDs) The emission wavelength of the products was pre-determined using a fluorescence spectrometer in solid and solution forms. This stage is to ensure the products can fluoresce and turn into carbonised form. 15 Figure 5 shows the solid-state absorption, Ultraviolet–visible spectroscopy (UV–Vis) absorption and solid-state photoluminescence emission spectra of these CDs. From the solid- state absorption spectra, the absorption peaks at 459 nm, 461 nm, 464 nm, 467 nm, 488 nm, 476 nm, and 510 nm belonged to CDs-1, -2, -3, -4, -5, -6, and -7, respectively. 20 Overall, bathochromic shifting was observed as the absorbance peaks of CDs-1 to -7 shift from 459 to 510 nm. In addition, the UV–Vis absorption spectra were obtained when these CDs were suspended in 25 ethanol with a concentration of 100 mg / L. The spectra revealed that these CDs exhibited multiple electronic transitions, and they were assigned by bands I, II, III and IV for simplicity. Bands I and II which were located at around 200 nm to 270 nm were assigned to the π→π∗transition from the aromatic sp2domains of C=C. Whereas, band III from 300 to 350 nm was 30 ascribed to n→π∗transitions of C=O. Band IV (450 nm to 550 nm) was assigned to the surface groups transition. The CDs-1 and CDs-2 did not exhibit both band III and IV like the rest of the CDs, since they lacked surface groups such as C=O and SO3H and this result was found to be consistent with FTIR. The emission peaks of CDs-1, -2, -3, -4. -5, -6, and -7 were centered at 484, 486, 529, 530, 529, 615, and 650 nm, respectively. These emission peaks were the result of the 5 excitation in the visible region (460–480 nm). The colour purity of the emissions is indicated by the full width half maximum (FWHM), where the range is in between 65 nm (CDs-2) to 90 nm (CDs-6 and CDs-7). From these emissions, the chromaticity index (CIE) coordinates were assigned to further 10 evaluate the colour purity of the CDs. Figure 5d shows that CDs-1, -2, -3, -4. -5, -6, and -7 had CIE coordinates at (0.13, 0.50), (0.15, 0.59), (0.38, 0.60), (0.28, 0.66), (0.34, 0.64), (0.65, 0.35), and (0.71, 0.29), respectively. Thus, CDs-3, -6, and -7 were found to exhibit high- colour purity as the CIE coordinates were located near or on the spectral energy locus wavelength. 15 The appearance / colour of the CDs became more reddish and less bright as the amount of concentrated sulfuric acid was reduced, and the emission wavelength shifted from 484 to 650 nm accordingly. 20 The photoluminescence spectra and CIE coordinates confirmed that each reaction would produce CDs with specific multicolour emission and high colour-purity. The direct heating method developed herein, has demonstrated high-colour purity CDs with simple a purification process. 25 The dots' optical properties were investigated from absorption, fluorescence and CIE spectrometer. Although the present invention has been described with reference to a specific embodiment, 30 also shown in the appended figures, it will be apparent for those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined in the following claims.

Claims

Claims I / We claim:

1. A method for synthesizing solid carbon dots, characterized by the steps of: 5 dissolving phenolic compound in a nitrile compound (1) to form a phenolic-nitrile solution; cooling the phenolic-nitrile solution (2) preferably at the range of 0 °C- 5°C; cooling a mineral acid (3) preferably at the range of 0 °C- 5°C; adding the mineral acid in a drop-wise manner into the phenolic-nitrile solution (4) 10 and stirring (5) preferably at the range of 5 – 30 minutes to form a mixture; heating the mixture at room temperature (6); heating the mixture preferably at the range of 80°C to 300°C for 1-12 minutes to obtain carbon dots (7); cooling the carbon dots to room temperature (8); 15 adding the carbon dots into an organic solvent (9); sonicating the carbon dots in organic solvent (10) for 5- 10 minutes; centrifuging carbon dots in organic solvent (11) preferably at the range of 5000-6000 rpm for 5-10 minutes; and washing (12) the carbon dots with organic solvent, 20 wherein sonicating, centrifuging and washing are carried out routinely for at least 3 times.

2. The method for synthesizing solid carbon dots according to claim 1, wherein the phenolic compound comprising of an aromatic ring and one and / or more hydroxyl groups.

3. The method for synthesizing solid carbon dots according to claim 1 and claim 2, wherein25 the phenolic compound comprising of 1,3,5-trihydroxybenzene (phloroglucinol), 1,3- dihydroxybenzene (resorcinol) and / or combination.

4. The method for synthesizing solid carbon dots according to claim 1, wherein the nitrilecompound comprising of acetonitrile, and / or combination.

5. The method for synthesizing solid carbon dots according to claim 1, wherein the mineral acid comprising of sulfuric acid, phosphoric acid, nitric acid and / or combination. 5 6. The method for synthesizing solid carbon dots according to claim 1, wherein sources of other heteroatoms comprising of sulfur and phosphor may be added together with the mineral acid in a drop-wise manner into the phenolic-nitrile solution. 10 7. The method for synthesizing solid carbon dots according to claim 1, wherein the adding the mineral acid in a drop-wise manner into the phenolic-nitrile solution and stirring to form the mixture, produces conversion yield preferably at the range of 50- 99%.

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