CN114891502A - Method for auxiliary synthesis of fluorescent probe by eutectic solvent and application - Google Patents

Method for auxiliary synthesis of fluorescent probe by eutectic solvent and application Download PDF

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CN114891502A
CN114891502A CN202210564360.9A CN202210564360A CN114891502A CN 114891502 A CN114891502 A CN 114891502A CN 202210564360 A CN202210564360 A CN 202210564360A CN 114891502 A CN114891502 A CN 114891502A
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梁潇
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Beijing Detong Xing Pharmaceutical Polytron Technologies Inc
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Abstract

The invention relates to a method for auxiliary synthesis of a fluorescent probe by a eutectic solvent and application thereof. The synthesis method comprises the following steps: (1) mixing a hydrogen bond donor and a hydrogen bond acceptor to obtain a mixed solution, heating the mixed solution until the mixed solution becomes a uniform and transparent solution to obtain a metal eutectic solvent, namely MDES; (2) preparing MDES solution from MDES; (3) heating the MDES solution for reaction, and cooling to obtain a carbon dot solution; (4) and filtering and dialyzing the carbon dot solution to obtain a solution containing the fluorescent probe. The probe synthesized by the invention can be used for detecting nitrite in food and monitoring the pH value of water environment. The invention has the advantages of environmental protection, low cost, stable fluorescence, wide sources, simple preparation, high quantum yield, good water solubility, strong selectivity, wide linear range, good detection sensitivity and the like.

Description

Method for auxiliary synthesis of fluorescent probe by eutectic solvent and application
Technical Field
The invention belongs to the technical field of fluorescent nano material preparation, food and environment science, and particularly relates to a method for auxiliary synthesis of a fluorescent probe by using a eutectic solvent and application of the fluorescent probe.
Background
Nitrites, commonly known as industrial salts, are widely used as colorants, preservatives, antibacterial agents, and as additives in the food industry. However, excessive intake of nitrite can be detrimental to human health. It can be converted into various nitrogen oxides in human body, and can react with protein metabolites such as amine in acidic medium to form nitrosamine, so that it has serious carcinogenic risk. In addition, excess nitrite in the blood converts hemoglobin to methemoglobin, which interferes with the oxygen transport system in the body, resulting in hypoxia and a decrease in blood pressure. Nitrite is not only widely used in the food industry, but also widely present in fertilizer degradation, acid rain, industrial waste, and the like. Therefore, monitoring of nitrite ion content is becoming increasingly important for human health. This has prompted the need to develop a highly sensitive, highly selective method. In addition, the addition of nitrite can greatly influence the pH value, and the detection of stability and fluorescence performance under different pH values is particularly important, so that a foundation is laid for the application of the nitrite in water environment pH monitoring in the future.
At present, nitrite is detected by a chromatographic method, a capillary electrophoresis method, an electrochemical method, a fluorescence spectrometry method and the like. Colorimetric methods are among the most common methods and have been used for decades. The method is simple, rapid, low in cost and wide in application, but has the defects of low sensitivity, high toxicity, easiness in interference of other matrixes and anions and the like. The chromatographic method and the electrochemical method have high sensitivity and good selectivity, but are long in time consumption and not suitable for trace detection.
At present, the fluorescent nano materials used as fluorescent sensors are various, such as semiconductor quantum dots, polymer dots, fluorescent nano-diamond, fluorescent nanoclusters, and the like. However, a series of fluorescent materials for synthesizing carbon dots are expensive and harmful, and most of fluorescent probes need to satisfy acidic conditions for detecting nitrite ions. Therefore, it is still necessary to develop a green alternative material.
Therefore, aiming at the food problems caused by large-scale production of nitrite, the development of the preparation method of the precursor of the green-friendly carbon quantum dots has important significance in realizing effective detection of nitrite in food.
Disclosure of Invention
In view of the problems in the prior art, the invention provides a method for synthesizing a fluorescent probe by using a eutectic solvent in an auxiliary manner and application thereof, and the method has the advantages of environmental friendliness, low cost, stable fluorescence, wide source, simplicity in preparation, high quantum yield, good water solubility, strong selectivity, wide linear range, good detection sensitivity and the like.
The technical scheme for solving the technical problems is as follows:
the invention provides a method for auxiliary synthesis of a fluorescent probe by a eutectic solvent, which comprises the following steps:
(1) mixing a hydrogen bond donor and a hydrogen bond acceptor to obtain a mixed solution, heating the mixed solution until the mixed solution becomes a uniform and transparent solution to obtain a metal eutectic solvent, namely MDES;
(2) preparing MDES solution from MDES;
(3) heating the MDES solution for reaction, and cooling to obtain a carbon dot solution;
(4) and filtering the carbon dot solution to obtain a solution containing the fluorescent probe.
The beneficial effects of adopting the above scheme include:
the fluorescent probe prepared by the invention is based on a metal type eutectic solvent as a precursor and an iron-doped carbon quantum dot fluorescent probe, and the eutectic solvent has great thermal stability, higher atom economy and biodegradability, conforms to the concept of 'green chemistry', and is a strong substitute of a toxic organic solvent.
The fluorescent probe synthesized by the method has the advantages of environmental friendliness, low cost, stable fluorescence, wide source, simplicity in preparation, high quantum yield, good water solubility, strong selectivity, wide linear range, good detection sensitivity and the like. And the nitrite detection in food and environment can be realized, the unique color change under different pH can be used as an environmental pH indicator.
Further, the hydrogen bond donor is DL-malic acid; the hydrogen bond acceptor comprises ethylene glycol and a metal; the mol ratio of DL-malic acid, glycol and metal is 2:8: (0.2-1.2).
The beneficial effects of adopting the above scheme include: the inventors have found unexpectedly that the ratio of the doped metal greatly affects the response of MDES-CDs to nitrite. The quality of the fluorescent probe is improved by adopting the proper proportion.
Further, the metal is selected from one or a combination of more of Cu, Mn, Fe, Zn and Co.
Preferably, the hydrogen bond donor is selected from DL-malic acid and the hydrogen bond acceptor comprises ethylene glycol, metallic Fe.
The beneficial effects of adopting the above scheme include: the fluorescence probe prepared from the eutectic solvent synthesized from DL-malic acid-metal Fe-glycol has high fluorescence intensity, and can realize the determination of nitrite.
Further, in the step (1), the heating temperature is 60-80 ℃; in the step (3), the heating reaction conditions include: the temperature is 120-; in the step (4), a 0.22 mu m microporous filter membrane is adopted for filtration.
The beneficial effects of adopting the above scheme include:
the heating temperature is 60-80 ℃, which is beneficial to accelerating the reaction speed and leading the MDES to be heated evenly, and the synthesis of the MDES is relatively stable in the temperature range. The temperature is too low and the heating is slow, so that the temperature requirement for forming MDES can not be met; if the temperature is too high, the bottom is easy to be burnt or cracked, and the safety problem can occur.
The temperature is 120-200 ℃, the reaction time is 2-10h, and the condition is the optimal condition obtained by researching the influence of the synthesis temperature and time variable on the fluorescence intensity, so that the MDES-CDs can obtain higher fluorescence quantum yield.
The solution is filtered by a 0.22 mu m microporous membrane to remove macromolecular impurities, so that the MDES-CDs solid has better luminescence property.
Further comprises the steps of filtering, dialyzing and freeze-drying to obtain MDES-CDs; conditions for freeze-drying include: vacuum-80 deg.C, and freeze drying for 24-48 hr.
The beneficial effects of adopting the above scheme include: the obtained MDES-CDs solid has better luminescence property.
Further, the method also comprises the step of preparing MDES-CDs solution from the MDES-CDs; the concentration of MDES-CDs in the MDES-CDs solution is 10-50 mg/mL -1 。
The beneficial effects of adopting the above scheme include: the concentration of the mixture is 10-50 mg/mL -1 The MDES-CDs solution has low concentration, low fluorescence intensity and high dosage; an excessively high concentration error becomes large and a self-quenching phenomenon may occur.
The invention provides a reagent or a kit for detecting nitrite and/or pH, which comprises a fluorescent probe synthesized by the method.
The beneficial effects of adopting the technical scheme include: the reagent or the kit provided by the invention has ultrahigh sensitivity and selectivity, excellent pH-dependent luminescence and unique color change, and has wide application prospect.
The invention provides the application of the fluorescent probe in one or more of (1), (2), (3) and (4),
(1) detecting nitrite;
(2) detecting the pH value;
(3) a reagent or kit for making or for detecting nitrite;
(4) making or using the reagent or kit for detecting pH.
The invention provides a method for detecting nitrite, which comprises the following steps: and (3) making a standard curve of the fluorescence intensity relative to the nitrite concentration, mixing and reacting the fluorescent probe with the sample to be detected, detecting the fluorescence intensity, and calculating the nitrite concentration of the sample to be detected according to the standard curve.
For example: whether to carry out the pretreatment or not can be selected according to actual needs. Then mixing the sample to be tested with MDES-CDs solution, wherein the concentration of MDES-CDs in the MDES-CDs solution can be 10-50 mg/mL -1 The volume of the sample to be tested and the MDES-CDs solution can be 1:10, the reaction is carried out for 20min at 60 ℃ under the condition that the pH value is 2, and the fluorescence intensity of the reacted solution is tested. Substituting the numerical value into a linear regression equation to calculate the concentration of the nitrite in the sample to be detected.
The beneficial effects of adopting the technical scheme include: the fluorescent probe provided by the invention is environment-friendly, has ultrahigh sensitivity and selectivity to sodium nitrite, has detection limit as low as 50nm, and can be used for large-scale production and detection of nitrite in foods and environments.
The invention provides a method for detecting pH, which comprises the following steps: and (3) making a standard curve of the fluorescence intensity relative to the pH value, mixing the fluorescence probe and the sample to be detected for reaction, detecting the fluorescence intensity, and calculating the pH value of the sample to be detected according to the standard curve.
For example: whether to carry out the pretreatment or not can be selected according to actual needs. Then mixing the sample to be tested with MDES-CDs solution, wherein the concentration of MDES-CDs in the MDES-CDs solution can be 10-50 mg/mL -1 The reaction was carried out at 60 ℃ for 20min, and the fluorescence intensity of the reacted solution was measured. And substituting the numerical value into a linear regression equation to calculate the pH value of the sample to be measured.
The beneficial effects of adopting the technical scheme include: the fluorescent probe provided by the invention has excellent pH-dependent luminescence and unique color change, and can be used as a pH indicator for detecting water environment. And has good application value and prospect in the fields of food and environmental monitoring and the like.
Drawings
FIG. 1 is a schematic diagram of the preparation principle and the detection principle of an Fe-doped carbon quantum dot fluorescent probe for detecting nitrite and pH.
FIG. 2 shows the optimization of the synthesis conditions for MDES-CDs: a metal species (A); iron doping ratio (B); synthesis temperature (C); time (D).
FIG. 3 shows the results of the study of the morphology, composition and surface structure of MDES-CDs; a is the transmission electron microscopy and lattice analysis of MDES-CDs; b is the particle size distribution diagram of MDES-CDs; c is the infrared spectrogram of MDES-CDs.
FIG. 4 is an X-ray photoelectron spectrum of MDES-CDs: a full scan spectrum (A); c1s spectrogram (B); an O1 s spectrogram (C); fe 2p spectrum (D).
FIG. 5 is a result of verifying the optical characteristics of MDES-CDs by ultraviolet-visible absorption spectroscopy (UV-Vis) and fluorescence spectroscopy; ultraviolet absorption spectra and optimal fluorescence excitation and emission spectra (A) of MDES-CDs (inset is solution color contrast plot of MDES-CDs under sunlight and ultraviolet lamp); fluorescence emission pattern (B) of MDES-CDs at different excitation wavelengths (350-520 nm).
FIG. 6 shows the effect of high ion intensity and irradiation time on the fluorescence intensity of MDES-CDs; MDES-CDs in different concentration salt solution (0.1-1 mol. L) -1 ) A spectrum (A) of the salt stability and a spectrum (B) of the photobleaching resistance under continuous excitation (0-100min) at the optimum excitation wavelength.
FIG. 7 shows the optimization of the detection conditions for nitrite by MDES-CDs: a pH value (A); temperature (B); time (C).
FIG. 8 is a graph showing the fluorescence emission spectra of MDES-CDs solutions at different nitrite concentrations.
FIG. 9 is a graph of the linear relationship between the different concentrations of nitrite and the fluorescence intensity of MDES-CDs.
FIG. 10 is a graph comparing the effect of different interfering ions on the fluorescence intensity of MDES-CDs solutions.
FIG. 11 is a graph showing fluorescence emission spectra of MDES-CDs at different pH.
FIG. 12 is a graph of the fluorescence intensity of MDES-CDs as a function of pH.
Detailed Description
The principles and features of this invention are described below in conjunction with the following drawings, which are set forth by way of illustration only and are not intended to limit the scope of the invention.
As shown in figure 1, the invention provides a method for preparing a fluorescent probe with the assistance of a metal eutectic solvent and application thereof.
The invention provides a method for preparing a fluorescent probe by using a metal eutectic solvent in an auxiliary manner, which comprises the following steps:
(1) preparing a metal type eutectic solvent: mixing the hydrogen bond donor and the hydrogen bond acceptor, and heating at 60-80 ℃ until the mixture becomes a uniform and transparent solution, namely the metal eutectic solvent; hydrogen bond donors include: DL-malic acid; the hydrogen bond acceptor comprises ethylene glycol and metal (one or more selected from Cu, Mn, Fe, Zn and Co). Preferably, DL-malic acid acts as a hydrogen bond donor and ethylene glycol acts as a hydrogen bond acceptor with metallic Fe. The fluorescence probe prepared from the eutectic solvent synthesized from DL-malic acid-metal Fe-glycol has high fluorescence intensity, and can realize the determination of nitrite. DL-malic acid: ethylene glycol: the molar ratio of the metal phase Fe is 2:8: (0.2-1.2).
(2) Accurately weighing 5mL of MDES, dissolving in 10mL of deionized water, and performing ultrasonic treatment until the system is uniformly mixed to obtain an MDES solution.
(3) And (3) placing the MDES solution mixed in the step (2) in a polytetrafluoroethylene inner container high-pressure reaction kettle, reacting for 2-10h in an oven at 120-200 ℃, and cooling the system to room temperature to obtain a carbon dot solution.
(4) The carbon dot solution cooled to room temperature in step (3) was filtered through a 0.22 μm filter and dialyzed for 24 hours using a dialysis bag (MW:500 Da).
(5) And (3) carrying out vacuum (-80 ℃) freeze drying on the solution obtained in the step (4) for 24-48h to obtain a novel fluorescent probe (MDES-CDs), and storing at a low temperature of 4 ℃ for later use.
(6) Adding the MDES-CDs solid in the step (5) into ultrapure water to prepare an MDES-CDs solution (the concentration is 10-50 mg. mL) -1 ) And storing at 4 deg.C.
The metal-doped carbon quantum dot fluorescent probe (namely, the MDES-CDs fluorescent probe) can be prepared by the preparation method.
The MDES-CDs fluorescent probe prepared by the method can be applied to nitrite detection and environmental pH detection.
The invention provides a method for detecting nitrite, which comprises the following steps: and (3) making a standard curve of the fluorescence intensity relative to the nitrite concentration, mixing and reacting the fluorescent probe with the sample to be detected, detecting the fluorescence intensity, and calculating the nitrite concentration of the sample to be detected according to the standard curve.
For example: whether to carry out the pretreatment or not can be selected according to actual needs. Then mixing the sample to be tested with MDES-CDs solution, wherein the concentration of MDES-CDs in the MDES-CDs solution can be 10-50 mg/mL -1 The sample to be tested and MDES-CDs solutionThe volume of (a) may be 1:10, and the reaction is performed at 60 ℃ for 20min under the condition of pH 2, and the fluorescence intensity of the solution after the reaction is measured. Substituting the numerical value into a linear regression equation to calculate the concentration of the nitrite in the sample to be detected.
The invention provides a method for detecting pH, which comprises the following steps: and (3) making a standard curve of the fluorescence intensity relative to the pH value, mixing the fluorescence probe and the sample to be detected for reaction, detecting the fluorescence intensity, and calculating the pH value of the sample to be detected according to the standard curve.
For example: whether to carry out the pretreatment or not can be selected according to actual needs. Then mixing the sample to be tested with MDES-CDs solution, wherein the concentration of MDES-CDs in the MDES-CDs solution can be 10-50 mg/mL -1 The reaction was carried out at 60 ℃ for 20min, and the fluorescence intensity of the reacted solution was measured. And substituting the numerical value into a linear regression equation to calculate the pH value of the sample to be measured.
The fluorescent probe prepared based on the metal type eutectic solvent is environment-friendly, low in cost, high in fluorescence intensity and good in stability. MDES-CDs have relatively uniform size and good dispersibility in aqueous solutions without significant aggregation. The surface of the probe contains rich oxygen-containing functional groups, and MDES-CDs can be used as a stable and excellent fluorescent probe for sensing measurement.
Compared with the prior art, the invention has the following characteristics:
(1) the fluorescent probe prepared by the invention has wide raw material source and is green.
(2) The fluorescent probe provided by the invention is simple to prepare, high in quantum yield and good in water solubility.
(3) The fluorescent probe provided by the invention has stronger photobleaching stability and keeps basic stability under higher salt concentration.
(4) The fluorescent probe is MDES-CDs with blue fluorescence, and the addition of the nitrite can quench the fluorescence of the MDES-CDs. And the fluorescence intensity has a good linear relation with the nitrite concentration within a certain range.
(5) The fluorescent probe shows super-strong selectivity to nitrite, has wide linear range and good detection sensitivity, and is successfully used for sample detection of food and environment.
(6) The MDES-CDs fluorescent probe also has unique pH sensitivity property, can realize excellent linear relation with the pH in the range of 2-7, has obvious quenching of MDES-CDs fluorescence intensity, and can be used as a color indicator for pH detection due to the color transition of a solution from light yellow to green in sunlight.
(7) Based on the excellent sensing ability of MDES-CDs on nitrite and pH. So that the method has good application value in the fields of food safety, environmental pollution and the like.
The following description is given by way of specific examples.
In the present invention, the parameters of the fluorescence spectrophotometer can be set as follows: scanning speed (1000nm/min), excitation bandwidth (10nm), emission bandwidth (10nm), gain (medium, 650V).
Example 1
A preparation method of an MDES-CDs fluorescent probe taking a metal eutectic solvent as a precursor comprises the following steps:
(1) preparing a metal type eutectic solvent (MDES) by using DL-malic acid as a hydrogen bond donor and using ethylene glycol and metal as hydrogen bond acceptors. The molar ratio is DL-malic acid: ethylene glycol: metal phase 2:8: 1. magnetically stirring the mixture at 80 deg.C for 30min to obtain uniform transparent liquid phase, i.e. MDES. Naturally cooling to room temperature, and placing in a closed container for later use.
(2) Accurately weighing 5mL of MDES, dissolving in 10mL of deionized water, and carrying out ultrasonic treatment until the system is uniformly mixed. The mixed solution is placed in an oven and reacted for 6h at 200 ℃, and after the system is cooled to room temperature, the product is filtered by a 0.22 mu m filter membrane.
The synthesis conditions of the MDES-CDs are optimized, the influences of doping metal species, doping metal proportion, synthesis temperature and synthesis time on the fluorescence property of the MDES-CDs are respectively considered, and the experimental result is shown in FIG. 2.
The influence of the doping metal species on the fluorescence properties of the MDES-CDs was examined. Different metals including Cu, Mn, Fe, Zn and Co are selected and respectively mixed with DL-malic acid and ethylene glycol according to the same proportion as the steps, and the mixture is placed at 80 ℃ for magnetic stirring for 30 min. After cooling to room temperature, 5mL of each was taken, 10mL of deionized water was added thereto, and the mixture was mixed by sonication. And then placing the mixture in a high-pressure reaction kettle with a polytetrafluoroethylene liner to react for 6 hours at 200 ℃ in an oven, cooling the system to room temperature, filtering the mixture by using a 0.22-micron filter membrane, and then testing the fluorescence intensity of the mixture under the excitation wavelength of 410nm to find that only MDES-CDs prepared and synthesized by Fe-DES has obvious quenching on nitrite (shown in figure 2A), wherein the inset in the upper left corner of figure 2A shows different MDES-CDs colors under sunlight (from left to right, metal type eutectic solvents prepared by Cu, Mn, Fe, Zn and Co respectively adopt a hydrothermal method to prepare a carbon dot solution under the same conditions of 180 ℃ and 6 hours), and the fluorescence intensity is the highest under the condition that metal Fe exists, which indicates that the effect is the best when the doped metal is Fe. The following experiment was performed with the doping metal selected as Fe.
The influence of the doping metal ratio on the fluorescence properties of the MDES-CDs was examined. DL-malic acid was studied: ethylene glycol: the mol ratio of the doped Fe elements is 2:8: (0.2-1.2) on the fluorescence intensity of MDES-CDs, the fluorescence intensity of the Fe element with the doping ratio of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 was tested under the same conditions (other steps are the same as the steps for synthesizing MDES-CDs), the experimental result is shown in FIG. 2B, the result shows that when the Fe element ratio reaches 1.0, the fluorescence intensity of MDES-CDs is obviously enhanced, the growth trend is reduced at 1.2 molar ratio, and finally the molar ratio of the doped Fe is set as 1.
The influence of the synthesis temperature on the fluorescence properties of the MDES-CDs was examined. We placed them in an oven at 120 deg.C, 140 deg.C, 160 deg.C, 180 deg.C, 200 deg.C, respectively, and reacted for 6h (other steps are the same as those described above for MDES-CDs synthesis), and then tested their fluorescence intensity. As can be seen from FIG. 2C, the enhancement of the fluorescence intensity of MDES-CDs is not significant at 120 ℃ to 180 ℃, and the fluorescence intensity is substantially increased when the reaction temperature reaches 200 ℃, indicating that 200 ℃ is the optimal reaction temperature.
The influence of the synthesis time on the fluorescence properties of the MDES-CDs was examined. The influence of the synthesis time on the fluorescence intensity of MDES-CDs was investigated, and the reaction was carried out at 200 ℃ for 2h, 4h, 6h, 8h, and 10h, respectively (other steps were the same as those for synthesizing MDES-CDs described above), and then the fluorescence intensity was measured, respectively. As shown in FIG. 2D, the synthesis time of 6h was chosen because the fluorescence difference was not significant after 6 h.
Example 2
A preparation method of an MDES-CDs fluorescent probe taking a metal eutectic solvent as a precursor comprises the following steps:
(1) preparing a metal type eutectic solvent (MDES) by using DL-malic acid as a hydrogen bond donor and using ethylene glycol and metal (Fe) as hydrogen bond acceptors. The molar ratio is DL-malic acid: ethylene glycol: metal phase 2:8: 1. magnetically stirring the mixture at 80 deg.C for 30min to obtain uniform transparent liquid phase, i.e. MDES. Naturally cooling to room temperature, and placing in a closed container for later use.
(2) Accurately weighing 5mL of MDES, dissolving in 10mL of deionized water, and carrying out ultrasonic treatment until the system is uniformly mixed. The mixed solution was placed in an oven and reacted at 200 ℃ for 6 hours, after the system was cooled to room temperature, the product was filtered with a 0.22 μm filter, and then dialyzed with a dialysis bag (MW:500Da) for 24 hours, followed by freeze-drying to obtain MDES-CDs. Then, the appropriate powder was dispersed in ultrapure water to prepare a dispersion having a concentration of 30. mu.g.mL -1 MDES-CDs solution, stored at 0-6 deg.C for further characterization and application.
The shape, composition and surface structure of MDES-CDs obtained by the above optimization method were studied, and the results are shown in FIG. 3. The MDES-CDs morphology was analyzed by Transmission Electron Microscopy (TEM) and, as shown in FIG. 3A, the MDES-CDs had relatively uniform size and good dispersibility in aqueous solution without significant aggregation. Furthermore, the carbon lattice distance was analyzed by HR-TEM image to be 0.21nm, corresponding to the lattice spacing of the graphite structure (inset in the upper right corner of FIG. 3A). FIG. 3B shows that the size distribution of MDES-CDs is mainly in the range of 2-5nm and the average particle size is 3.72 nm. The functional groups in MDES-CDs were then identified using Fourier transform infrared spectroscopy (FT-IR). In FIG. 3C, from bottom to top, MDES-CDs and MDES-CDs + NO, respectively 2 - Infrared spectrum of (D). Notably, 3423cm -1 The broad and strong absorption peak at (a) is the tensile vibration of O-H. In addition, MDES-CDs and MDES due to out-of-plane bending vibration of O-H-CDs+NO 2 - At 1398cm -1 And 1384cm -1 The presence of hydroxyl groups is further confirmed by the absorption peaks at (a). 1635cm -1 And 1637cm -1 Infrared absorption peak at (D) is caused by stretching vibration of carbonyl group C ═ O, and 1041cm -1 The absorption peak at (a) can be assigned to the tensile vibration of C — O, indicating the possible presence of carboxyl groups in the structure. At 626cm -1 The peak of (2) is the stretching vibration of the C-H bond. From the above characterization results, it can be deduced that the surface of MDES-CDs may contain abundant oxygen-containing groups, and these functional hydrophilic groups greatly improve the water solubility and stability of MDES-CDs and provide an action site for sensing nitrite.
X-ray photoelectron spectroscopy (XPS) also confirmed the elemental composition and chemical bonding of MDES-CDs (FIG. 4), and was essentially consistent with elemental measurements of FT-IR measurements. As shown in fig. 4A, three characteristic peaks centered approximately at 285, 583 and 710eV, attributable to the binding energies of C1s, O1 s and Fe 2p, respectively, indicate successful doping of Fe element. Fig. 4B is a high resolution XPS spectrum of C1s (spectrum from left to right, C C, C-O, C ═ O), which shows three fitted peaks at 284.0, 285.1 and 287.2eV, respectively, assigned to C ═ C (sp2 hybridized carbon), C-O and C ═ O. This indicates that the graphitic structure of the prepared MDES-CDs has sp2 hybridized carbon atoms and confirms the presence of carbonyl functional groups. As shown in fig. 4C, the three fitted peaks at 530.1, 531.1, and 532.2eV represent the three types of oxygen-containing functional groups, i.e., C-O, O-H, and C ═ O bonds, respectively. As shown in fig. 4D, in the high-resolution electron spectrum of Fe 2p, absorption peaks at 710.8 and 722.2eV indicate the presence of iron in the oxidized state. These results further confirm the doping of the Fe element and the formation of oxygen-containing functional groups on the surface of MDES-CDs.
The optical properties of the MDES-CDs were verified by ultraviolet-visible absorption spectroscopy (UV-Vis) and fluorescence spectroscopy (FIG. 5). Without any further chemical modification, MDES-CDs exhibited excellent water solubility. As shown in FIG. 5A (from left to right, results from MDES-CDs, Excitation, and Emission, respectively), the MDES-CDs solution was clear and yellowish in sunlight and emitted bright bluish fluorescence under Excitation of a 410nm UV lamp. The UV-Vis spectrum (black line in fig. 5A) shows the different characteristic absorption bands of MDES-CDs, including two UV absorption peaks at 241 and 351nm, due to the pi-pi transition of C ═ C bonds and the n-pi transition of C ═ O bonds, respectively. As shown in FIG. 5B, in order to investigate whether the fluorescence emission wavelength of MDES-CDs depends on the excitation wavelength, the fluorescence properties thereof were tested at different excitation wavelengths of 350nm to 520 nm. The results show that MDES-CDs has a maximum emission of about 473nm at 410nm excitation and shows a pronounced excitation wavelength dependence. Meanwhile, the absolute quantum yield (Abs-QY) of MDES-CDs was 15.59%.
In addition, the influence of high ion intensity and irradiation time on the fluorescence intensity of MDES-CDs was examined (FIG. 6). MDES-CDs was mixed with NaCl solutions of 0mol/L, 0.1mol/L, 0.2mol/L, 0.4mol/L, 0.6mol/L, 0.8mol/L, and 1.0mol/L, respectively, and vortexed to mix them uniformly, and then the fluorescence intensity was measured. FIG. 6A shows that the fluorescence intensity can be kept substantially constant at NaCl concentrations up to 1M. For the irradiation time, we place the UV lamp under the wavelength of 410nm to continuously irradiate for 90min, and test the fluorescence intensity every 5min, and it can be seen from FIG. 6B that the fluorescence intensity can be maintained at 75% or more within 90min of the 410nm UV irradiation. The MDES-CDs can be used as a stable and excellent fluorescent probe for sensing determination.
Example 3
A method of detecting nitrite comprising the steps of:
accurately weighing 3.0mg of MDES-CDs, placing in a 100mL volumetric flask, and fixing the volume to obtain a corresponding stock solution. mu.L of MDES-CDs solution (30. mu.g/mL in concentration) -1 ) Mixing with 200 μ L of sample to be tested in a centrifuge tube, adjusting pH value, and adding water to 1 mL. And shaking the reaction solution in a constant-temperature water bath for 20min, and then carrying out fluorescence test.
Detection conditions are as follows: 410nm as the excitation wavelength and 475nm as the fluorescence value as NO 2 - The emission slit width was set at 10nm and the test conditions were in triplicate (i.e. triplicate samples were prepared for the assay).
When a standard curve is made, the sample to be detected can be NaNO with different concentrations 2 And (3) solution. AdoptAfter the reaction of the method, the fluorescence intensity is detected, and a standard curve of the fluorescence intensity relative to the nitrite concentration is drawn. According to the fluorescence intensity of the sample to be detected with unknown concentration and the standard curve, the nitrite concentration in the sample to be detected can be calculated.
In the case of the actual sample measurement, the actual sample may be first subjected to appropriate pretreatment such as pulverization, centrifugation, filtration, dilution, and the like. Then, it was mixed with MDES-CDs, reacted at 60 ℃ for 20min at pH 2, and the fluorescence intensity was measured. Substituting the numerical value into a linear regression equation to calculate the concentration of the nitrite in the actual sample.
Example 4
(1) And preparing an MDES-CDs probe system for optimizing nitrite detection conditions. The influence of the solution acidity, reaction time, temperature and other variables on the nitrite detection performance of MDES-CDs is respectively examined, and the result is shown in FIG. 7.
In order to examine the influence of the acidity of the solution on the performance of nitrite detection, solutions with the pH values of 2.08, 2.59, 3.15, 3.64, 3.93, 4.26, 4.66, 5.40, 5.90, 6.50 and 6.98 are prepared by using an HCl solution, MDES-CDs and a nitrite solution are added into the solutions, the solutions are mixed uniformly by vortex, the reaction is carried out for 20min at the temperature of 60 ℃, and then the fluorescence intensity of the solutions is tested by using a fluorescence spectrophotometer. The influence of the acidity (pH) of the solution is shown in FIG. 7A, where the fluorescence intensity of MDES-CDs itself rapidly decreases in the pH range of 2 to 7. When pH reached 7, its autofluorescence disappeared, as pH 2 was chosen as the optimal condition for analytical detection.
For the optimization of the reaction temperature, MDES-CDs and a nitrite solution are mixed and put under the condition of pH 2, and are respectively reacted for 20min at 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃, 50 ℃, 55 ℃ and 60 ℃, and then the fluorescence intensity is tested. As shown in FIG. 7B, the nitrite quenching degree of MDES-CDs is increased with the temperature, the reaction speed is promoted at higher temperature, and the fluorescence quenching degree is maximum at 60 ℃, so that 60 ℃ is finally selected as the optimal reaction temperature.
Fig. 7C studies the trend of the change of fluorescence intensity with time after adding nitrite, we mixed MDES-CDs with nitrite solution and reacted at pH 2 and 60 ℃ for 0min, 0.66min, 3min, 5min, 10min, 15min, 19min, 22min, 25min, 27min, 29min, and 32min, respectively, and found that the fluorescence intensity at 473nm was quenched 59% in 5min and reached 96% after 20min, indicating that the reaction was completed in 20 min. Therefore, 20min was selected as the optimal reaction time.
(2) The response of MDES-CDs to nitrite was investigated under the optimal conditions for MDES-CDs synthesis (200 ℃, 6h, molar ratio DL-malic acid: ethylene glycol: Fe ═ 2:8: 1). MDES-CDs was mixed with different concentrations of nitrite at pH 2, reacted at 60 deg.C for 20min, and then measured for fluorescence intensity. FIG. 8 shows fluorescence spectra in the presence of different concentrations of nitrite, the curves from top to bottom being kb (blank), 0.2. mu.M, 1. mu.M, 5. mu.M, 10. mu.M, 20. mu.M, 30. mu.M, 40. mu.M, 50. mu.M, 60. mu.M, 80. mu.M, 100. mu.M, 150. mu.M, 300. mu.M, and it can be seen from FIG. 8 that the fluorescence intensity of MDES-CDs gradually decreases with increasing nitrite concentration (0-300. mu.M).
(3) As shown in FIG. 9, we plotted the fluorescence intensity values against nitrite concentration, and found that the fluorescence intensity followed the nitrite concentration in the range of 0.2-80. mu.M (0.2. mu.M, 3. mu.M, 7. mu.M, 15. mu.M, 20. mu.M, 30. mu.M, 40. mu.M, 50. mu.M, 60. mu.M, 70. mu.M, 80. mu.M) to give a regression equation of y-39.62 [ NO [ (-39.M) 2 - ]+4212, coefficient of linearity R 2 Is 0.9932. Limit of detection (LOD, s/n is 3).
The detection limit of nitrite concentration was calculated as 50nM by parallel measurement of 11 blank solutions and calculation of their 3-fold standard deviation versus the slope of the linear equation.
When the actual sample is detected, for example, the sausage is subjected to proper pretreatment, then the pretreated sausage is added into a mixed solution of nitrite and MDES-CDs, the reaction is carried out under the same condition as the step (2), the fluorescence value of the reaction is tested, and the fluorescence value is substituted for a linear equation, so that the concentration of the nitrite in the sausage can be obtained. This enables us to accurately quantify nitrite in real samples.
(4) A comparison of the effect of different interfering ions on the fluorescence intensity of MDES-CDs solutions is shown in FIG. 10.
Considering the selectivity of MDES-CDs sensing platform in nitrite detection, a plurality of 1mL centrifuge tubes are taken, and 20 μ L and 200 μ L of MDES-CDs solution in example 2 are added into the 1mL centrifuge tubes respectively, so as to obtain interfering substances with five times concentration, wherein the interfering substances include anions, metal ions and some additives, the anions are as follows: NO 2 - 、F - 、Cl - 、Br - 、I - 、OH - 、NO 3 - 、SO 4 2- 、SO 3 2- 、CO 3 - 、CH 3 COO - 、 HCO 3 - Etc., metal ions such as: fe 3+ 、Mg 2 、K + 、Na + 、Cu 2+ 、Zn 2+ 、Ca 2+ 、Fe 2+ And the like, additives such as: DL-malic acid, L-cysteine, lactic acid, ascorbic acid, citric acid, glucose, glutamic acid, glutathione and the like, and the fluorescence intensity was measured and recorded.
Experimental results as shown in fig. 10A, it can be observed that only nitrite can cause a sharp decrease in fluorescence intensity, while no significant change in fluorescence is observed in the presence of other anions.
In addition, under the condition of coexistence of MDES-CDs and nitrite, five times concentration of the above-mentioned interfering substances including anions, metal ions, additives, and the like were added thereto at 200. mu.L, respectively, and fluorescence intensity was measured and recorded, and it was observed that the degree of quenching was not much different from that in the presence of only nitrite, as shown in FIG. 10B, indicating that fluorescence intensity of MDES-CDs was slightly affected when the interference experiment was performed under the condition of coexistence of nitrite and interfering ions. This indicates that the MDES-CDs thus prepared have high selectivity for nitrite detection.
Example 5
A method of detecting pH comprising the steps of:
accurately weighing 3.0mg of MDES-CDs, placing in a 100mL volumetric flask, and fixing the volumeAnd obtaining the corresponding stock solution. In this experiment, 20. mu.L of MDES-CDs solution (30. mu.g. mL in concentration) was added -1 ) Placing the sample into a centrifuge tube containing a sample to be measured, and fixing the volume to 1 mL. Detection conditions are as follows: the emission slit width was set to 10nm with 410nm as the excitation wavelength and the fluorescence at 475nm as the measured value of pH, and the test conditions were run in triplicate (i.e. triplicate samples were prepared for the assay).
When a standard curve is made, the samples to be tested are solutions with different pH values. And measuring the fluorescence intensity after the reaction by adopting the method, and drawing a standard curve of the fluorescence intensity relative to the pH value. And calculating the pH value of the sample to be detected according to the fluorescence intensity of the sample to be detected with unknown pH value and the standard curve.
When the detection of the actual sample is carried out, the fluorescence intensity can be measured by first subjecting the actual sample (e.g., water) to an appropriate pretreatment (centrifugation, filtration, dilution, etc.), then mixing it with MDES-CDs, and reacting it with MDES-CDs at 60 ℃ for 20 min. And (4) substituting the value into a linear regression equation to obtain the pH value of the actual sample.
Example 6
And preparing an MDES-CDs probe system for optimizing and detecting pH.
1) Accurately weighing 3.0mg of MDES-CDs, placing in a 100mL volumetric flask, and fixing the volume to obtain a corresponding stock solution. In this experiment, 20. mu.L of MDES-CDs solution (30. mu.g. mL in concentration) was added -1 ) Placing the solution in centrifuge tubes containing solutions with different pH values, and fixing the volume to 1 mL. Detection conditions are as follows: the emission slit width was set to 10nm with 410nm as the excitation wavelength and the fluorescence at 475nm as the measured value of pH, and the test conditions were run in triplicate (i.e. triplicate samples were prepared for the assay).
2) Under the optimal synthesis conditions for MDES-CDs (200 ℃, 6h, DL-malic acid: ethylene glycol: fe ═ 2:8:1, under which conditions MDES-CDs synthesized showed the best fluorescence), the ability of MDES-CDs to respond to pH was investigated. The fluorescence spectra are shown in FIG. 11, which are curves from top to bottom with pH values of 2.08, 2.59, 3.64, 3.96, 4.26, 4.66, 5.30, 5.91, 6.50, 6.98, respectively, and it can be seen from FIG. 11 that as the pH value is increased from 2 to 7, the fluorescence intensity of MDES-CDs is quenched significantly, and the color change of the solution from pale yellow-green to the sample provides excellent characteristics as a color indicator for pH detection.
3) As shown in FIG. 12, the fluorescence intensity of MDES-CDs decreased linearly with pH between 2 and 7 (pH 2.08, 2.59, 3.15, 3.64, 3.93, 4.26, 4.66, 5.40, 5.90, 6.50, 6.98, different pH solutions could be obtained by adjusting with HCl and NaOH), and the linear equation was y-923.2 [ pH]+6660，R 2 ＝0.9931。
For the sample to be tested, the same operation as above is carried out, and the fluorescence intensity is detected, and the pH can be obtained by the linear equation. The method lays a foundation for pH detection of practical samples such as various water environments.
The above examples are only for illustrating the technical solutions of the present invention, and are only for facilitating the understanding and use of the invention by those of ordinary skill in the art, but not for limiting the same; the person skilled in the art can modify the embodiments of the invention or substitute parts of the technical features equally; and to apply the general principles described herein to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications within the scope of the present invention based on the disclosure of the present invention.

Claims (10)

1. A method for synthesizing a fluorescent probe under the assistance of a eutectic solvent is characterized by comprising the following steps:
(1) mixing a hydrogen bond donor and a hydrogen bond acceptor to obtain a mixed solution, heating the mixed solution until the mixed solution becomes a uniform and transparent solution to obtain a metal eutectic solvent, namely MDES;
(2) preparing MDES solution from MDES;
(3) heating the MDES solution for reaction, and cooling to obtain a carbon dot solution;
(4) and filtering the carbon dot solution to obtain a solution containing the fluorescent probe.
2. The method for synthesizing the fluorescent probe assisted by the eutectic solvent as claimed in claim 1, wherein the hydrogen bond donor is DL-malic acid; the hydrogen bond acceptor comprises ethylene glycol and a metal; the mol ratio of DL-malic acid, glycol and metal is 2:8: (0.2-1.2).
3. The method for synthesizing the fluorescent probe assisted by the eutectic solvent as claimed in claim 2, wherein the metal is selected from one or more of Cu, Mn, Fe, Zn and Co.
4. The method for synthesizing the fluorescent probe assisted by the eutectic solvent as claimed in claim 1, wherein in the step (1), the heating temperature is 60-80 ℃; in the step (3), the heating reaction conditions include: the temperature is 120-; in the step (4), a 0.22 mu m microporous filter membrane is adopted for filtration.
5. The method for synthesizing the fluorescence probe assisted by the eutectic solvent according to any one of claims 1 to 4, further comprising the steps of filtering, dialyzing, and freeze-drying to obtain MDES-CDs; conditions for freeze-drying included: vacuum-80 deg.C, and freeze drying for 24-48 hr.
6. The method for the eutectic solvent-assisted synthesis of fluorescent probe according to claim 5, further comprising the steps of preparing MDES-CDs into MDES-CDs solution; the concentration of MDES-CDs in the MDES-CDs solution is 10-50 mg/mL -1 。
7. A reagent or kit for detecting nitrite and/or pH, comprising a fluorescent probe synthesized by the method of any one of claims 1 to 6.
8. The use of the fluorescent probe synthesized by the method of any one of claims 1 to 6 in one or more of (1), (2), (3) and (4),
(1) detecting nitrite;
(2) detecting the pH value;
(3) a reagent or kit for making or for detecting nitrite;
(4) making or using the reagent or kit for detecting pH.
9. A method of detecting nitrite, comprising the steps of: and (3) making a standard curve of the fluorescence intensity relative to the nitrite concentration, mixing and reacting the fluorescent probe with the sample to be detected, detecting the fluorescence intensity, and calculating the nitrite concentration of the sample to be detected according to the standard curve.
10. A method of detecting pH comprising the steps of: and (3) making a standard curve of the fluorescence intensity relative to the pH value, mixing the fluorescence probe and the sample to be detected for reaction, detecting the fluorescence intensity, and calculating the pH value of the sample to be detected according to the standard curve.
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