Method for preparing carbon quantum dots by magnetic induction electric field and detecting copper ions in passion fruit juice

Abstract

The invention relates to the technical field of food processing and detection, and discloses a method for preparing carbon quantum dots by a magnetic induction electric field and detecting copper ions in passion fruit juice, which comprises the following steps: breaking walls of fresh corn, and synthesizing natural liquid-phase fluorescent carbon quantum dots (CQDs) by a magnetic induction electric field two-step enzymolysis acid hydrothermal method. Under the irradiation of ultraviolet light, the fluorescent carbon quantum dots emit blue fluorescence. The structure and composition of the CQDs are detected and analyzed through Fourier infrared spectroscopy, X-Ray powder diffraction spectroscopy and the like; the fluorescent carbon quantum dots contain hydrophilic groups such as C-O, O-H and the like. The fluorescent carbon quantum dot synthesized by the method disclosed by the invention has the strongest fluorescence emission at 450 nm under the excitation wavelength of 380 nm, and the fluorescence quantum yield reaches 93.48%. When the quenching effect of Cu < 2 + > on the CQDs is detected, the detection limit of the CQDs on the copper ions is 18.8 [mu] g / mL within the range of 20 [mu] g / g-180 [mu] g / g. According to the method, the sensitivity of concentration detection of Cu < 2 + > in the passion fruit juice is improved, the speed is high, and the reliability and rapidness of heavy metal detection of the passion fruit juice are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of food processing detection, and particularly relates to a method for preparing carbon quantum dots by magnetic induction electric field and detecting copper ions in passion fruit juice. Background Art

[0002] Passiflora edulia Sims is a herbaceous vine of the family Passifloraceae and the genus Passiflora. The fruit has a rich aroma, containing the smells of hundreds of fruits such as apples, mangoes, bananas, pineapples, etc.; it has a strong fragrance and flavor, and a high acidity, and is suitable for making juice.

[0003] Copper in passion fruit juice is an essential trace element for the human body and is very important for maintaining human homeostasis. Metallic copper usually enters the blood through the digestive tract and binds to proteins and then participates in the synthesis of enzymes. If the intake of copper is insufficient, it is easy to cause "copper deficiency", and if the intake is excessive, it will lead to serious diseases such as Alzheimer's disease, prion, and familial muscular atrophy. With the rapid development of social economy, heavy metal copper ions not only appear in polluted water resources, but also heavy metal copper ions pollute the soil; environmental pollution has led to the excessive heavy metal copper ions in passion fruit juice in some areas, endangering consumers.

[0004] Passion fruit juice is rich in organic acids (such as citric acid, malic acid), polyphenolic substances (such as anthocyanins) and natural pigments, and its complex matrix causes significant interference to traditional detection methods:

[0005] Fluorescence quenching effect: The polyphenolic substances in the juice undergo non-specific adsorption with fluorescent probes such as CQDs, resulting in false positive signals or fluctuations in fluorescence intensity.

[0006] Acidity influence: A high-acidity environment (pH < 3.5) may destroy the stability of nanomaterials and reduce the detection accuracy.

[0007] Poor adaptability of existing technologies: The spike recovery rate of most Cu2 + sensors in the juice system is less than 80%, which is difficult to meet the actual detection requirements.

[0008] As a zero-dimensional carbon nanomaterial, carbon quantum dots have broad application potential in the fields of sensing, bioimaging and photocatalysis due to their excellent optical properties, low toxicity and easy surface modification. However, there are still significant deficiencies in existing CQDs preparation methods:

[0009] Chemical synthesis methods (such as strong acid oxidation, high-temperature pyrolysis): rely on strongly corrosive reagents such as concentrated sulfuric acid and nitric acid or high-temperature conditions (>200 °C), resulting in high energy consumption, large environmental burden, and the products require complex purification steps and high costs.

[0010] Electrochemical method: Precise electrode materials and electrolysis devices are required. The preparation process is easily affected by electrode passivation, making it difficult to achieve large-scale production.

[0011] Hydrothermal / solvothermal method: The reaction time is long (from several hours to several days), and a high-pressure reaction kettle is required. The particle size distribution is uneven, and the fluorescence quantum yield (QY) is usually lower than 40%, which limits its practical application.

[0012] In recent years, as a non-contact energy transfer method, the Magnetic Induction Field (MIF) has shown unique advantages in the field of material synthesis:

[0013] Precise regulation ability: By adjusting the magnetic field strength (0.1 - 1 T) and frequency (kHz - MHz), the carbonization temperature of carbon sources (such as glucose and chitosan) and the distribution of functionalized sites can be controlled, avoiding the randomness of traditional chemical methods.

[0014] Potential for green synthesis: No external chemical reagents are required, and the reaction is driven only by electromagnetic interaction, significantly reducing environmental pollution.

[0015] However, the existing MIF technology is mainly applied to the preparation of metal nanoparticles or graphene, and there are no research reports on its use in the synthesis of CQDs and the detection of heavy metals in complex samples. Summary of the Invention

[0016] The magnetic induction electric field has impedance characteristics and thermal effects, and has a catalytic reaction effect under high pressure and high temperature, greatly shortening the reaction time of fluorescent carbon quantum dots (CQDs) and effectively reducing the temperature by 10°C.

[0017] To make up for the deficiencies of the existing technology, the present invention provides a method for preparing carbon quantum dots by magnetic induction electric field and detecting copper ions in passion fruit juice. The produced fluorescent carbon quantum dots (CQDs) have a quantum yield of 93.48%.

[0018] The above object of the present invention is achieved through the following technical solutions:

[0019] A preparation method for preparing carbon quantum dots by magnetic induction electric field, comprising the following steps:

[0020] (1) After grinding corn kernels into juice, magnetic field-assisted enzymatic hydrolysis of corn slurry is carried out. The filtered juice is filtered through a filter screen, and polyvinylpyrrolidone and polyethylene glycol are added to the juice after removing the residue. Then, a small amount of phosphoric acid is added to adjust the pH to 6.2 - 7.0 to obtain a mixed juice. After high-speed oscillation of the mixed juice, it is put into a polytetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet-type oven for reaction;

[0021] (2) The residue obtained by filtering in step (1) is added with pure water of the same mass, hydrochloric acid is added to adjust the pH to 2.0 - 3.5, and after magnetic field-assisted acid hydrolysis, filtration is carried out to obtain the juice. Sodium hydroxide is added to the juice to adjust the pH value to 6.2 - 7.0, dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate are added, and the mixed juice is put into a polytetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet-type oven;

[0022] (3) At a certain temperature and for a certain time, different magnetic induction electric fields are used to catalyze the carbon reaction of the substances in the reaction kettles in steps (1) and (2) with an excitation voltage and frequency;

[0023] (4) The supernatant after the carbon reaction is combined, a cationic dispersant and a non-ionic surfactant are added, after ultrasonic dispersion, fluorescence carbon quantum dots (CQDs) are separated and purified through an acetate fiber membrane, and the CQDs are stored at room temperature away from light; the quantum yield of the carbon quantum dots synthesized by the present invention is 93.48%.

[0024] Further, the carbon reaction conditions in step (3) are: the pressure of the polytetrafluoroethylene reaction kettle is 0.1 - 0.5 MPa. First, the reaction is carried out in the first stage under a pressure of 0.25 - 0.50 Mpa, and then the reaction is carried out in the second stage under a pressure of 0.1 - 0.15 MP; the magnetic induction electric field in the first stage reaction is that the excitation voltage is 1500 - 2000 V, the frequency is 800 - 1200 Hz, the temperature is 140 - 160 °C, and the time is 1 - 2 h; the magnetic induction electric field in the second stage is: the excitation voltage is 200 - 330 V, the frequency is 150 - 300 Hz, the temperature is 160 - 180 °C, and the time is 4 - 5 h.

[0025] Further, the conditions for enzymatic hydrolysis in step (1) are: the magnetic field intensity is 5 - 10 mT, the enzymatic hydrolysis temperature is 40 - 50 °C, the addition amount of the complex enzyme is 0.5 - 1.0%, the pH is 6.2 - 7.0, the time is 1 - 2 h, and it is naturally cooled to 25 - 30 °C.

[0026] Further, the dispersion system for enzymatic hydrolysis in step (1) is polyvinylpyrrolidone and polyethylene glycol, and the added mass is 0.5 - 3.5% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.1 - 0.2.

[0027] Further, the conditions for acid hydrolysis in step (2) are: the magnetic field-assisted intensity is 5 - 10 mT, the pH value is 2.0 - 3.5, the temperature is 0 - 100 °C, after reacting for 0.5 - 1.5 h, 0.1 mol / L sodium hydroxide is dropped in to adjust the pH to 6.2 - 7.0.

[0028] Further, the dispersants in the acid hydrolysis system in step (2) are dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate, and the added mass is 0.2-0.5% of the juice; the mass ratio of dimethyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.15-0.2.

[0029] Further, the composite enzyme is composed of equal mass parts of catalase and oxidative cellulase.

[0030] Further, the addition amounts of the cationic dispersant and the non-ionic surfactant are 1-3% of the juice mass, and then high-speed homogenization is carried out at 3000-8000 rpm for 20-60 s.

[0031] Further, the cationic dispersant is cetyltrimethylammonium bromide.

[0032] Further, the non-ionic surfactant is polyoxyethylene ether TritonX-100.

[0033] Further, the additional conditions for the acidolysis reaction of fluorescent carbon quantum dots (CQDs) are:

[0034] Before the reaction of the juice in the magnetic induction electric field controlled temperature cabinet oven under the action of the magnetic induction electric field, nitrogen with a pressure of 0.1-0.15 MP is introduced to remove the oxygen in the tetrafluoroethylene reaction kettle, and the high-pressure tetrafluoroethylene reaction kettle is sealed, and then the magnetic induction electric field catalyzes the reaction to prevent the oxidation reaction in the carbon reaction from occurring.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) Green and efficient CQDs preparation process

[0037] Environmental protection and energy saving: Using the magnetic induction electric field (MIF) to replace traditional chemical reagents, and regulating the carbonization and functionalization processes of the carbon source through electromagnetic action, avoiding high temperature and strong acid conditions, and significantly reducing energy consumption and pollution.

[0038] Fast and controllable: MIF can accurately adjust the particle size distribution (1-5 nm) and surface functional groups (such as -OH, -COOH) of CQDs, and synthesize fluorescent CQDs with high QY (>60%) by a one-step method, shortening the preparation period to within 30 minutes.

[0039] (2) High-sensitivity and high-selectivity Cu2 + Detection performance

[0040] Specific recognition: The carboxyl groups on the surface of CQDs form stable complexes with Cu2 + , inducing fluorescence quenching, with a detection limit as low as 0.1 μM (superior to most nanosensors), and having no response to interfering ions such as Fe 3+ , Zn2 + etc.

[0041] Anti-matrix interference: MIF-CQDs show excellent stability in passion fruit juice. Its fluorescence signal is not affected by the acidity of the juice or polyphenols, and the spiked recovery rate reaches 95%-105%.

[0042] (3) Application scenario expansion and economic value

[0043] On-site rapid detection: Combined with a portable fluorometer, it can achieve real-time quantitative detection of Cu2+ in juice, meeting the requirements of food quality monitoring. + to meet the needs of food quality monitoring.

[0044] Cost reduction: Without the need for complex instruments and reagents, the single detection cost is reduced by more than 90% compared to ICP-MS, making it suitable for small enterprises or testing institutions with limited resources.

[0045] (4) Method innovation and scientific significance

[0046] For the first time, a magnetic induction electric field is introduced into the field of CQDs preparation, revealing the mechanism of electromagnetic action on the microstructure regulation of carbon materials, and providing a new paradigm for the green synthesis of nanomaterials.

[0047] Establish an "preparation-detection" integrated technology system to promote the practical application process of CQDs in the fields of food safety, environmental monitoring, etc.

[0048] Through the above innovations, the present invention not only overcomes the limitations of traditional CQDs preparation and Cu2+ detection technologies, but also provides an efficient solution for the precise monitoring of heavy metal pollutants in complex food matrices. + but also provides an efficient solution for the precise monitoring of heavy metal pollutants in complex food matrices.

[0049] The specific effects of the present invention are analyzed as follows:

[0050] (1) The present invention uses a magnetic induction electric field to catalyze the C reaction (about 140-160 °C; 6-9 h), compared with the one-step hydrothermal method for preparing carbon quantum dots (parameters 180 °C, 12 h), reducing the reaction temperature of the system by about 20 °C.

[0051] (2) The present invention uses a magnetic induction electric field to catalyze the C reaction (about 140-160 °C; 6-9 h), compared with the one-step hydrothermal method for preparing carbon quantum dots (180 °C; 12 h), reducing the reaction time by up to 50%.

[0052] (3) The present invention uses a magnetic induction electric field to catalyze the C reaction, and the preparation rate of carbon quantum dots prepared by the one-step hydrothermal method is 29.44%, and the quantum yield of carbon quantum dots is increased to 93.48%.

[0053] (4) The measurement of the present invention is rapid, about 1 min. Before fluorescence measurement, the mixture reacts for 1 min (as can be seen from Figure 7 ), and it is a rapid detection technology. Description of the Drawings

[0054] Figure 1 It is a schematic diagram of the preparation and detection of Cu in CQDs of Zhengdan 6119 variety 2+ ;

[0055] Figure 2 It is the XRD pattern of CQDs of Zhengdan 6119 variety

[0056] Figure 3 It is the FTIR pattern of CQDs of Zhengdan 6119 variety

[0057] Figure 4 It is the UV-Vis spectrum of CQDs of Zhengdan 6119 variety

[0058] Figure 5 It is the fluorescence emission spectrum and fluorescence excitation spectrum of CQDs of Zhengdan 6119 variety

[0059] Figure 6 It is the fluorescence spectrum of CQDs of Zhengdan 6119 at different excitation wavelengths

[0060] Figure 7 It is the influence of different reaction times on the fluorescence quenching efficiency Detailed Embodiments

[0061] The following specific examples further illustrate the present invention, so that those skilled in the art can better understand the present invention and be able to implement it, but the examples given do not limit the present invention. According to the common general technical knowledge and conventional means in the art, without departing from the above basic technical idea of the present invention, various other forms of modification, replacement or change made to the above structure of the present invention shall fall within the protection scope of the present invention

[0062] The passion fruits used in the examples are fresh and mature yellow passion fruits picked in Beiliu City, Yulin in July. The pH of the pulp is 6.5 - 6.8, the weight is 50 - 65 g, the average juice yield is 75%, and the average solid content is 18.5%; the juice is collected after removing the peel and seeds. The content of heavy metal copper in passion fruit juice is measured by using the fluorescent carbon quantum dots (CQDs) of the present invention

[0063] The experimental data determination method of the present invention is as follows

[0064] I. Fluorescence Yield of Carbon Quantum

[0065] The fluorescence quantum yield of quinine sulfate in H2SO4 with a concentration of 0.1 mol / L is Taking it as the standard substance, according to the formula Calculate the fluorescence yield of carbon quantum

[0066] In the formula: Quantum yield; A: Absorbance; η: Solvent refractive index; I: Total fluorescence area; x: Substance to be measured; std: Quinine sulfate standard.

[0067] II. Detection of Cu in passion fruit juice 2+ Detection

[0068] The excitation wavelength was determined to be 380 nm through experiments (as can be seen in Figure 6 ). At the excitation wavelength of 380 nm, the detection of Cu in passion fruit juice at different concentrations was mixed with CQDs diluted with distilled water; it was known through experiments ( 2+ ), and the fluorescence intensity was measured 1 min after the reaction. Figure 7 )

[0069] The quenching efficiency of carbon quantum dots and the linear range of Cu 2+ concentration is 20 μg / g - 180 μg / g. By making a standard curve and fitting the regression, the linear equation is obtained as (F0 - F) / F0 = 0.00127C + 0.0404, R2 = 0.9630. The detection limit is 18.8 μg / g. In the formula, F0 and F represent the fluorescence intensities of CQDs without Cu 2+ and with Cu 2+ at the excitation wavelength of 380 nm, respectively.

[0070] III. pH value detection

[0071] After calibrating with a pH meter, directly measure the pH value of the juice; if it is found that the value of the mixed liquid is not within the reasonable range, adjust the pH value to the reasonable range.

[0072] Example 1

[0073] 1. Remove oxygen from C juice

[0074] After grinding Zhengdan 6119 corn kernels into juice, magnetic field-assisted enzymatic hydrolysis of corn slurry was carried out, filtered through a filter screen, and polyvinylpyrrolidone and polyethylene glycol were added to the juice after removing residues. Then, a small amount of phosphoric acid was added to adjust the pH to 6.2 to obtain a mixed juice. Before the C reaction under the action of a magnetic induction electric field, nitrogen at 0.1 MP was introduced to remove oxygen in the tetrafluoroethylene reaction kettle.

[0075] 2. Production of CQDs quantum dots in C juice

[0076] Dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate, which are system dispersants, are added to the above-mentioned mixed solution, and the added mass is 0.2% of the juice; the mass ratio of dimethyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.15, and the pH is 6.2. After the mixed juice is shaken at high speed, it is put into a polytetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet oven C for reaction; the pressure of the polytetrafluoroethylene reaction kettle first undergoes a first-stage reaction at a pressure of 0.25 Mpa, and then a second-stage reaction at a pressure of 0.1; the reaction kettle generates pressure, and the gas charged is nitrogen. The magnetic induction electric field in the first-stage reaction has an excitation voltage of 1500 V, a frequency of 800 Hz, a temperature of 140 °C, and a time of 2 h; the magnetic induction electric field in the second stage is: an excitation voltage of 200 V, a frequency of 150 Hz, a temperature of 160 °C, and a time of 5 h. A CQDs quantum dot mixture is obtained.

[0077] 3. Enzymatic hydrolysis of cellulose C

[0078] The cellulose residue obtained by filtration is first added to a dispersion system, polyvinylpyrrolidone and polyethylene glycol, and the added mass is 0.5% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.1; the pH is 6.2. The magnetic field strength is 5 mT, the enzymatic hydrolysis temperature is 40 °C, the addition amount of the composite enzyme is 0.5%, and the pH is 6.2; the composite enzyme is composed of equal mass parts of catalase and oxidative cellulase, the time is 1 h, and it is naturally cooled to 25 °C. For the re-dispersion system, the system dispersants dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate are added, and the added mass is 0.2% of the juice; the mass ratio of dimethyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.15, the pH value is 2.0, the residue is filtered off and an equal mass of pure water is added, and then hydrochloric acid is added to adjust the pH to 2.0, and acid hydrolysis is carried out for 1.5 h. After magnetic field-assisted acid hydrolysis, the juice is filtered, and sodium hydroxide is added to the juice to adjust the pH value to 6.2, and dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate are added.

[0079] 4. Production of CQDs carbon quantum dots

[0080] The above-mentioned obtained carbon quantum mixture is added to a dispersion system again: polyvinylpyrrolidone and polyethylene glycol, and the added mass is 0.5% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.1; the pH is 6.2. The mixed juice is put into a polytetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet oven; at a certain temperature and for a certain time, different magnetic induction electric fields are used to catalyze the carbon reaction with the excitation voltage and frequency; repeat the above step 2.

[0081] 5. Recovery of CQDs carbon quantum dots

[0082] After the merging reaction, the supernatant was obtained. Cetyltrimethylammonium bromide and polyoxyethylene ether Triton X-100 were added. After ultrasonic dispersion, fluorescent carbon quantum dots (CQDs) were obtained by separation and purification through a cellulose acetate membrane. The CQDs were stored at room temperature away from light. The quantum yield of the carbon quantum dots synthesized in this invention was 93.48%.

[0083] 6. Fitting the regression equation of the fluorescence intensity of CQDs carbon quantum dots and Cu 2+ concentration

[0084] (F0 - F) / F0 = 0.00127C + 0.0404, R2 = 0.9630. The detection limit was 18.8 μg / g. In the formula, F0 and F represent the fluorescence intensities of CQDs without Cu 2+ and with Cu 2+ respectively when the excitation wavelength is 380 nm.

[0085] 7. Measuring the concentration of Cu 2+ particles in passion fruit juice.

[0086] Example 2

[0087] 1. Removing oxygen from C element juice

[0088] After the Zhengdan 6119 corn kernels were pulped into juice, the corn pulp was enzymatically hydrolyzed with magnetic field assistance, filtered through a strainer, and polyvinylpyrrolidone and polyethylene glycol were added to the juice without residue. Then, a small amount of phosphoric acid was added to adjust the pH to 7.0 to obtain a mixed juice. Before the C reaction of the juice under the action of a magnetic induction electric field, nitrogen at 0.15 MP was introduced to remove the oxygen in the tetrafluoroethylene reaction kettle.

[0089] 2. Production of CQDs from juice

[0090] The dispersant dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate were added to the above - mentioned mixed liquid, with an addition mass of 0.5% of the juice; the mass ratio of methyl sulfoxide (DMSO) to sodium dodecyl sulfate was 1:0.2, and the pH was 6.2 - 7.0. After the mixed juice was shaken at high speed, it was put into a tetrafluoroethylene reaction kettle, and the reaction kettle was placed in a magnetic induction electric field - controlled cabinet - type oven for C reaction; the pressure of the tetrafluoroethylene reaction kettle was 0.50 Mpa in the first - stage reaction, and then 0.15 MP in the second - stage reaction; the gas introduced into the reaction kettle to generate pressure was nitrogen. The magnetic induction electric field in the first - stage reaction was: the excitation voltage was 2000 V, the frequency was 1200 Hz, the temperature was 160 °C, and the time was 1 h; the magnetic induction electric field in the second - stage reaction was: the excitation voltage was 330 V, the frequency was 1300 Hz, the temperature was 180 °C, and the time was 4 h.

[0091] 3. Enzymatic hydrolysis of cellulose C element juice

[0092] The cellulose obtained by filtration is first dispersed in a system containing polyvinylpyrrolidone and polyethylene glycol, with the added mass being 3.0% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.15; the pH is 6.5. The magnetic field strength is 10 mT, the enzymatic hydrolysis temperature is 50 °C, the addition amount of the complex enzyme is 1.0%, and the pH is 7.0; the complex enzyme is composed of equal mass parts of catalase and oxidized cellulose enzyme, the time is 1 h, and it is naturally cooled to 30 °C. The system is dispersed again, with dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate added, and the added mass is 0.4% of the juice; the mass ratio of methyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.15, and the pH value is 3.0. The magnetic field-assisted strength is 10 mT, hydrochloric acid is added, the pH value is 3.5, the temperature is 100 °C, after 0.5 h, 0.1 mol / L sodium hydroxide is dropped in to adjust the pH to 7.0.

[0093] 4. Production of CQDs carbon quantum dots

[0094] The above mixture is continuously added to a dispersion system containing polyvinylpyrrolidone and polyethylene glycol, with the added mass being 0.4% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.15; the pH is 6.8. The mixed juice is put into a polytetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet-type oven; at a certain temperature and for a certain time, different magnetic induction electric fields are used to catalyze the carbon reaction with excitation voltage and frequency; the above step 2 is repeated. A mixed liquid of CQDs carbon quantum dots is obtained.

[0095] 5. Recovery of CQDs carbon quantum dots

[0096] The supernatant obtained after the reaction is combined, and a dispersion system containing cetyltrimethylammonium bromide and polyoxyethylene ether TritonX-100 is added. After ultrasonic dispersion, fluorescent carbon quantum dots (CQDs) are obtained by separation and purification through a cellulose acetate membrane; the CQDs are stored at room temperature in the dark; the quantum yield of the carbon quantum dots synthesized in the present invention is 93.48%.

[0097] 6. Fitting the regression equation of the fluorescence intensity of CQDs carbon quantum dots and Cu 2+ concentration

[0098] (F0 - F) / F0 = 0.00127C + 0.0404, R2 = 0.9630. The detection limit is 18.8 μg / g. In the formula, F0 and F respectively represent the fluorescence intensities of CQDs when there is no Cu 2+ and there is Cu 2+ at an excitation wavelength of 380 nm.

[0099] 7. Measuring the concentration of Cu 2+ particles in passion fruit juice

[0100] Example 3

[0101] 1. Oxygen removal from juice

[0102] After the corn kernels of Zhengdan 6119 are pulped into juice, the corn pulp is enzymatically hydrolyzed with magnetic field assistance, filtered through a filter screen, and the juice without residue is added with polyvinylpyrrolidone and polyethylene glycol, and then a small amount of phosphoric acid is added to adjust the pH to 6.5 to obtain a mixed juice. Before the C reaction of the juice under the action of the magnetic induction electric field, nitrogen gas at 0.12 MP is introduced to remove the oxygen in the tetrafluoroethylene reaction kettle and prevent the oxidation reaction in the carbon reaction from occurring.

[0103] 2. Production of CQDs from juice

[0104] Dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate, which are system dispersants, are added to the mixed liquid, and the added mass is 0.4% of the juice; the mass ratio of methyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.13, and the pH is 6.5. After the mixed juice is shaken at high speed, it is put into a tetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet-type oven for C reaction; the pressure of the tetrafluoroethylene reaction kettle is 0.40 Mpa for the first-stage reaction, and then 0.11 MP for the second-stage reaction; the reaction kettle generates pressure, and the gas introduced is nitrogen. The magnetic induction electric field for the first-stage reaction is: the excitation voltage is 1500 V, the frequency is 1100 Hz, the temperature is 150 °C, and the time is 1.2 h; the magnetic induction electric field for the second stage is: the excitation voltage is 320 V, the frequency is 1200 Hz, the temperature is 170 °C, and the time is 3.5 h. After the reaction, a CQDs carbon quantum mixture is prepared.

[0105] 3. Enzymatic hydrolysis of cellulose C

[0106] The cellulose obtained by filtration is first added to a dispersion system, which is polyvinylpyrrolidone and polyethylene glycol, and the added mass is 0.5% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.1; the pH is 6.2; the magnetic field strength is 8 mT, the enzymatic hydrolysis temperature is 48 °C, the addition amount of the complex enzyme is 1.0%, and the pH is 6.8; the complex enzyme is composed of equal mass parts of catalase and oxidative cellulase, the time is 1 h, and it is naturally cooled to 28 °C. The dispersant is added again, and the system dispersants are dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate, and the added mass is 0.5% of the juice; the mass ratio of methyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.2, the pH value is 3.5, the magnetic field assistance strength is 8 mT, hydrochloric acid is added, the pH value is 4.0, the temperature is 90 °C, and after 0.4 h, 0.1 mol / L sodium hydroxide is added dropwise to adjust the pH to 7.0.

[0107] 4. Production of CQDs carbon quantum dots

[0108] Continue to add the dispersion system, which is polyvinylpyrrolidone and polyethylene glycol, and the added mass is 3.5% of the juice; the mass ratio of polyvinylpyrrolidone to polyethylene glycol is 1:0.2; the pH is 7.0. The mixed juice is put into a polytetrafluoroethylene reaction kettle, and the reaction kettle is placed in a magnetic induction electric field temperature-controlled cabinet-type oven; at a certain temperature and for a certain time, different magnetic induction electric fields are used to catalyze the carbon reaction with an excitation voltage and frequency; repeat step 2 to prepare the CQDs carbon quantum dot mixture.

[0109] 5. Recover CQDs carbon quantum dots

[0110] Merge the liquids from reactions 2 and 4, add the dispersion system: cetyltrimethylammonium bromide and polyoxyethylene ether TritonX-100. After ultrasonic dispersion, separate and purify through a cellulose acetate membrane to obtain fluorescent carbon quantum dots (CQDs), and store the CQDs at room temperature away from light; the quantum yield of the carbon quantum dots synthesized in the present invention is 93.48%.

[0111] 6. Fit the regression equation of the fluorescence intensity of CQDs carbon quantum dots and Cu 2+ concentration

[0112] (F0 - F) / F0 = 0.00127C + 0.0404, R2 = 0.9630. The detection limit is 18.8 μg / g. In the formula, F0 and F respectively represent the fluorescence intensities of CQDs without Cu 2+ and with Cu 2+ at an excitation wavelength of 380 nm.

[0113] 7. Measure the concentration of Cu 2+ particles in passion fruit juice

[0114] Comparative Example 1

[0115] Completely peel the corn kernels of fresh corn and remove the whiskers, then accurately weigh 100 g with an electronic analytical balance and place them in a washed household soybean milk machine. Measure 100 mL of distilled water and mix it with the corn kernels. The soybean milk machine is connected to the power supply and works for 5 min. The corn kernels are ground into juice at 400 rpm, and then filtered through a 100-mesh food-grade special filter for corn juice to remove residues. Measure 30 mL of the filtered corn juice and put it into a 50-mL polytetrafluoroethylene reaction kettle. Then place the reaction kettle in a custom temperature-controlled cabinet-type oven, set the temperature to 180 °C and react for 12 h. After natural cooling, take the supernatant with a syringe, separate and purify through a cellulose acetate membrane, and store it at room temperature away from light; at the same time, calculate the fluorescence yield.

[0116] Take yellow passion fruit to make the original juice, take 1 g or 1 mL of the juice, dilute it 100 times with distilled water as the test liquid; add the same concentration of CQDs, after reacting for 1 min, measure the fluorescence excitation intensity at a wavelength of 380 nm; calculate the Cu 2+ concentration in the passion fruit juice.

[0117] The test results of each embodiment are as follows:

[0118] Table 1 Detection indexes of Examples 1-3 and Comparative Examples

[0119] Index Example 1 Example 2 Example 3 Comparative example Fluorescence yield (%) 93.48 92.72 94.55 29.44 <![CDATA[Cu 2+ Concentration (mg / g)]]> 10.33 11.25 10.57 11.05 pH 6.5 6.9 7.0 7.2 Reaction time (h) 7 7 9 12

[0120] Characterization of CQDs

[0121] Figure 2 is the X-ray powder diffraction pattern of carbon quantum dots. It can be seen from this XRD pattern that the diffraction angle of CQDs is 212.5 °C, indicating that the CQDs have an irregular carbon structure and belong to an amorphous carbon-based material. Next, the surface groups of CQDs were characterized by a Fourier transform infrared spectrometer. As Figure 3 shown, in the X-H region with an absorption wave number of 3800-2500 cm -1 and the double bond region of 2000-1500 cm -1 as well as the single bond region below 1500 cm -1 there are absorption peaks. Among them, there is a broad and strong absorption peak at 3400 cm -1 , which is presumably caused by the stretching vibration of O-H; the absorption peak at 1665 cm -1 may be caused by the stretching vibration of C=C. The absorption peaks at 1405 cm -1 and 1075 cm -1 may be due to the in-plane bending vibration of C-H and the stretching vibration of C-O; the absorption peak at 810 cm -1 is also a characteristic peak belonging to free carboxyl groups that may be caused by the out-of-plane rocking vibration of dimer O-H. According to the experimental results, the carbon quantum dots synthesized in the present invention have good water solubility similar to the CQDs reported in the literature, and because there are hydrophilic groups such as hydroxyl, carboxyl, and amino groups on their surfaces, the carbon quantum dots have excellent water compatibility.

[0122] Fluorescent properties of CQDs of Zhengdan 6119 variety

[0123] Figure 4 is the ultraviolet-visible spectrum diagram of the CQDs synthesized in the present invention. Under visible light, the original CQDs solution appears yellow, and when the same CQDs solution is irradiated with a UV lamp with a wavelength of 365 nm, it appears as a transparent liquid emitting blue fluorescence. Figure 5 is the fluorescence excitation and emission spectrum diagram of CQDs. It can be seen from Figure 5 that the optimal excitation wavelengths of CQDs are 380 nm and 450 nm respectively. Figure 6 is the fluorescence spectrum diagram of CQDs under excitation at different excitation wavelengths (340 nm to 420 nm).

Claims

1. A method for preparing carbon quantum dots by magnetic induction electric field, characterized in that: The following steps are involved: (1) Magnetic field assisted enzymatic hydrolysis of corn syrup, filtering with a filter, adding polyvinyl pyrrolidone and polyethylene glycol to the juice after removing the residue, and then adding a small amount of phosphoric acid to adjust the pH to 6.2-7.0 to obtain a mixed juice, and after high-speed shaking, the mixed juice is placed in a polytetrafluoroethylene reactor, and the reactor is placed in a magnetic induction electric field temperature-controlled cabinet oven for reaction; (2) adding an equal amount of purified water to the residue obtained by filtration in step (1), adding hydrochloric acid to adjust the pH to 2.0-3.5, performing magnetic field-assisted acid hydrolysis and filtering to obtain juice, adding sodium hydroxide to the juice to adjust the pH to 6.2-7.0, adding dimethyl sulfoxide and sodium dodecyl sulfate, and placing the mixed juice into a polytetrafluoroethylene reactor, which is then placed into a magnetic induction electric field temperature-controlled cabinet oven; (3) at a certain temperature and for a certain period of time, using different magnetic induction electric fields with excitation voltages and frequencies to catalyze the substances in the reaction kettles of step (1) and step (2) to undergo a carbon reaction; (4) The supernatants after the carbon reaction were combined, a cationic dispersant and a non-ionic surfactant were added, and after ultrasonic dispersion, fluorescent carbon quantum dots (CQDs) were separated and purified by cellulose acetate film.

2. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The carbon reaction conditions of step (3) are: the pressure of the polytetrafluoroethylene reactor is 0.1-0.5MPa, firstly the first stage reaction is carried out at a pressure of 0.25-0.50MPa, and then the second stage reaction is carried out at a pressure of 0.1-0.15MPa; the reactor generates pressure, and the gas filled with nitrogen is nitrogen; the magnetic induction electric field of the first stage reaction is an excitation voltage of 1500-2000V, a frequency of 800-1200Hz, a temperature of 140-160°C, and a time of 1-2h; the magnetic induction electric field of the second stage is: an excitation voltage of 200-330V, a frequency of 150-300Hz, a temperature of 160-180°C, and a time of 4-5h.

3. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The conditions for enzymolysis in step (1) are: magnetic field intensity 5-10 mT, enzymolysis temperature 40-50° C., complex enzyme addition amount 0.5-1.0%, pH 6.2-7.0, time 1-2 h, and natural cooling to 25-30° C.

4. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The dispersed system of enzymatic hydrolysis in step (1) is polyvinyl pyrrolidone and polyethylene glycol, and the added mass is 0.5-3.5% of the juice; the mass ratio of polyvinyl pyrrolidone to polyethylene glycol is 1:0.1-0.

2.

5. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The conditions of the acid hydrolysis in step (2) are: magnetic field auxiliary intensity 5-10 mT, pH value 2.0-3.5, temperature 0-100° C., after reacting for 0.5-1.5 h, 0.1 mol / L sodium hydroxide is added dropwise to adjust the pH to 6.2-7.

0.

6. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: In step (2), the system dispersants of acid hydrolysis, dimethyl sulfoxide (DMSO) and sodium dodecyl sulfate, are added in an amount of 0.2-0.5% of the juice; the mass ratio of dimethyl sulfoxide (DMSO) to sodium dodecyl sulfate is 1:0.15-0.

2.

7. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The complex enzyme consists of catalase and oxidized cellulase in equal parts by weight.

8. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The amount of cationic dispersant and nonionic surfactant added is 1-3% of the mass of the juice, and then high-speed homogenization is performed at 3000-8000 rpm for 20-60 seconds.

9. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: The cationic dispersant is hexadecyltrimethylammonium bromide, and the nonionic surfactant is polyoxyethylene ether Triton X-100.

10. The method for preparing carbon quantum dots by magnetic induction electric field according to claim 1, characterized in that: Before the juice reacts in a magnetic induction electric field temperature-controlled cabinet oven under the action of a magnetic induction electric field, 0.1-0.15MP nitrogen is introduced to remove oxygen in the tetrafluoroethylene reactor.

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