阅读说明：本技术 一种快速荧光检测润滑油老化程度的方法 (Method for rapidly detecting aging degree of lubricating oil by fluorescence ) 是由 胡恩柱 陈妍洁 苏恩豪 刘书生 王剑平 胡坤宏 刘俊生 唐丽霞 于 2021-07-14 设计创作，主要内容包括：一种快速荧光检测润滑油老化程度的方法,涉及荧光性纳米材料的应用技术领域。首先向模拟润滑油中加入稀释的碳量子点溶液、金属离子溶液和/或者模拟酸,在荧光光谱仪中测定荧光强度变化,从而确定金属离子的浓度、模拟酸与碳量子点的荧光强度之间存在的线性关系。然后向待测润滑油中添加碳量子点溶液,在荧光光谱仪中测定荧光强度,通过对比前述线性关系,计算出待测润滑油中金属离子浓度以及酸值,从而综合判断润滑油的老化程度。本发明可以快速诊断润滑油的老化程度,技术操作简单、可靠且稳定,为润滑油快速监测提供了理论依据和实际意义。(A method for rapidly detecting the aging degree of lubricating oil by fluorescence relates to the technical field of application of fluorescent nano materials. Firstly, diluted carbon quantum dot solution, metal ion solution and/or simulated acid are added into simulated lubricating oil, and fluorescence intensity change is measured in a fluorescence spectrometer, so that the linear relation existing between the concentration of metal ions, the simulated acid and the fluorescence intensity of the carbon quantum dots is determined. And then adding a carbon quantum dot solution into the lubricating oil to be detected, measuring the fluorescence intensity in a fluorescence spectrometer, and calculating the concentration and acid value of metal ions in the lubricating oil to be detected by comparing the linear relation so as to comprehensively judge the aging degree of the lubricating oil. The invention can quickly diagnose the aging degree of the lubricating oil, has simple, reliable and stable technical operation, and provides theoretical basis and practical significance for quick monitoring of the lubricating oil.)

1. A method for rapidly detecting the aging degree of lubricating oil by fluorescence is characterized by mainly comprising the following steps:

firstly, adding diluted carbon quantum dot solution and metal ion solution with a certain gradient into simulated lubricating oil, and measuring fluorescence intensity change in a fluorescence spectrometer, thereby determining the linear relation between the concentration of metal ions and the fluorescence intensity of carbon quantum dots; adding diluted carbon quantum dot solution and simulated acid with a certain gradient amount into simulated lubricating oil, and measuring the change of fluorescence intensity in a fluorescence spectrometer, thereby determining the linear relation between the concentration of the simulated acid and the fluorescence intensity of the carbon quantum dot;

secondly, adding a carbon quantum dot solution into the lubricating oil to be detected, measuring the fluorescence intensity in a fluorescence spectrometer, and calculating the concentration and acid value of metal ions in the lubricating oil to be detected by comparing the linear relationship so as to comprehensively judge the aging degree of the lubricating oil;

and finally, setting a lubricating oil aging replacement threshold, and prompting to replace the lubricating oil when the metal ion concentration and the acid value of the lubricating oil to be detected reach the threshold.

2. The method for rapid fluorescence detection of the aging degree of lubricating oil according to claim 1, wherein the metal ion is selected from Fe³⁺、Ni²⁺。

3. The method for rapid fluorescence detection of the aging degree of lubricating oil according to claim 1, wherein polyethylene glycol 400(PEG400) is used as the simulated lubricating oil.

4. The method for rapid fluorescence detection of the aging degree of lubricating oil according to claim 1, wherein acetic acid is used as the mimetic acid.

5. The method for rapid fluorescence detection of the aging degree of lubricating oil according to claim 1, characterized by the steps of:

1) preparation of carbon quantum dot solution

Weighing 1g of citric acid monohydrate, dissolving the citric acid monohydrate into 30mL of absolute ethanol, weighing 30mL of N-aminoethyl-3-aminopropyltrimethoxysilane KH792, uniformly mixing the citric acid monohydrate and the absolute ethanol, transferring the mixture into a polytetrafluoroethylene inner container of a 100mL hydrothermal reaction kettle, putting the inner container into a constant-temperature air-blowing drying oven, reacting at 140 ℃ for 4 hours, taking out the mixture after the reaction is finished and cooled, diluting the mixture by 10000 times of absolute ethanol, and sealing and storing the mixture to obtain a blue-fluorescence ethanol solution of nitrogen-silicon co-doped carbon quantum dots;

2) determination of the Linear relationship between the concentration of Metal ions and the fluorescence intensity of the carbon Quantum dots

Firstly, 1mmol/L Fe is prepared³⁺、Ni²⁺The solution of (4), further diluting the solution to a concentration of 1. mu.M, 5. mu.M, 10. mu.M, 20. mu.M, 50. mu.M, 60. mu.M, 70. mu.M, 80. mu.M, 90. mu.M, 100. mu.M, 200. mu.M, 400. mu.M, 600. mu.M, 800. mu.M, or 1000. mu.M, respectively, and preserving the same;

weighing 10mL of polyethylene glycol 400, putting the polyethylene glycol 400 into a 10mL transparent screw glass bottle, adding 200 mu L of the carbon quantum dot solution diluted by ethanol in the step 1), uniformly shaking, then adding 200 mu L of each metal ion solution with various concentrations, uniformly shaking, waiting for 2h, and then carrying out fluorescence test;

adding 3mL of the mixed solution into a four-way quartz cuvette with an optical path of 10mm, setting the excitation wavelength to be 370nm, measuring the peak intensity of a fluorescence emission spectrum in a sample, and making a relational graph according to the relationship between metal ion solutions with different concentrations and the fluorescence intensity;

3) determination of the Linear relationship between the concentration of the mock acid and the fluorescence intensity of the carbon Quantum dots

Weighing 10mL of polyethylene glycol 400, putting the polyethylene glycol 400 into a 10mL transparent screw glass bottle, adding 200 mu L of the carbon quantum dot solution diluted by the ethanol in the step 1), uniformly shaking, then adding 200 mu L, 400 mu L, 600 mu L, 800 mu L and 1mL of acetic acid, uniformly shaking, waiting for 2 hours, and then performing fluorescence test;

adding 3mL of the mixed solution into a four-way quartz cuvette with an optical path of 10mm, setting the excitation wavelength to be 370nm, measuring the peak intensity of a fluorescence emission spectrum in a sample, and making a relational graph according to the relationship between acetic acid with different addition amounts and the fluorescence intensity;

4) detection of used lubricating oil

Adding 200 mu L of carbon quantum dot solution diluted by ethanol in the step 1) into 10mL of used lubricating oil to be detected, measuring fluorescence intensity in a fluorescence spectrometer, and calculating the concentration and acid value of metal ions in the lubricating oil to be detected by comparing the linear relationship obtained in the step 2) and the step 3), thereby comprehensively judging the aging degree of the lubricating oil;

5) prompt for replacement

And setting a lubricating oil aging replacement threshold, and prompting to replace the lubricating oil when the metal ion concentration and the acid value of the lubricating oil to be detected reach the threshold.

Technical Field

The invention relates to the technical field of application of fluorescent nano materials, in particular to a method for rapidly detecting the aging degree of lubricating oil by fluorescence.

Background

In recent years, the automobile holding capacity has rapidly increased, and the demand for lubricating oil for automobiles has rapidly increased. During the operation of the internal combustion engine of the automobile, part of the lubricating oil enters the combustion chamber of the internal combustion engine to form harmful emissions such as gaseous pollutants or particles. The consumption of lubricating oil varies with the technical level of the vehicle and the degree of wear, and generally, the consumption of lubricating oil should be less than 1% (volume ratio) of the consumption of fuel. But the lubricating oil consumption of a severely worn engine will be higher. In addition, a large amount of waste lubricating oil is generated after the vehicle is used or scrapped, and although most of the waste lubricating oil is recycled, a considerable part of the waste lubricating oil enters a water system or soil to cause environmental pollution.

The lubricating oil is gradually aged and deteriorated due to high temperature and oxidation of air during use, and metal powder abraded off the friction member, moisture introduced into the oil due to respiration and other causes, and impurities introduced from the environment contaminate the lubricating oil and promote oxidation of the lubricating oil, thereby possibly causing various troubles of the machine. Therefore, the lubricating oil must be replaced after the lubricating oil is used for a certain period of time and the deterioration reaches a certain degree.

The carbon quantum dots have the advantages of incomparable optical performance, small size, low toxicity, good biocompatibility, functional modification, low preparation cost, mild reaction conditions and the like, so that the carbon quantum dots become the first choice of the fluorescent probe, and the fluorescence quenching phenomenon of the carbon quantum dots in the lubricating oil can well detect the oxidation degree of the lubricating oil and the content of heavy metal ions (mainly iron and chromium ions) in the lubricating oil, thereby playing a good guiding role in reasonable discharge and treatment of waste oil.

Disclosure of Invention

The invention aims to provide a method for rapidly detecting the aging degree of lubricating oil by fluorescence, which can rapidly diagnose the aging degree of the lubricating oil, has simple, reliable and stable technical operation and provides theoretical basis and practical significance for rapidly monitoring the lubricating oil.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows: a method for rapidly detecting the aging degree of lubricating oil by fluorescence mainly comprises the following steps:

firstly, adding diluted carbon quantum dot solution and metal ion solution with a certain gradient into simulated lubricating oil, and measuring fluorescence intensity change in a fluorescence spectrometer, thereby determining the linear relation between the concentration of metal ions and the fluorescence intensity of carbon quantum dots; adding diluted carbon quantum dot solution and simulated acid with a certain gradient amount into simulated lubricating oil, and measuring the change of fluorescence intensity in a fluorescence spectrometer, thereby determining the linear relation between the concentration of the simulated acid and the fluorescence intensity of the carbon quantum dot;

secondly, adding a carbon quantum dot solution into the lubricating oil to be detected, measuring the fluorescence intensity in a fluorescence spectrometer, and calculating the concentration and acid value of metal ions in the lubricating oil to be detected by comparing the linear relationship so as to comprehensively judge the aging degree of the lubricating oil;

and finally, setting a lubricating oil aging replacement threshold, and prompting to replace the lubricating oil when the metal ion concentration and the acid value of the lubricating oil to be detected reach the threshold.

As the preferred technical scheme of the method for rapidly detecting the aging degree of the lubricating oil by fluorescence, the metal ions in the detection method are selected from Fe³⁺、Ni²⁺Polyethylene glycol 400(PEG400) was used as the simulated lubricating oil, and acetic acid was used as the simulated acid.

Compared with the prior art, the invention has the following advantages:

(1) particle size analysis and HRTEM analysis show that the prepared carbon quantum dots have the size distribution of 1-6 nm and the average particle size of 2.9nm, and have good dispersibility, small size and stable structure. XRD (X-ray diffraction) junctionAs a result, the CQDs produced showed only one peak at around 22 ℃ indicating that it was amorphous. Fourier infrared transform spectrum results show that the prepared CQDs have the silane group and the amino group of KH792 on the surface, and the modifier is used for successfully modifying and modifying the surface of the carbon quantum dot. XPS results showed that CQDs contains 46.16% C, 23.83% N, 17.37% O and 12.64% Si, and the chemical bonds have C-C and-NH₂、N-C＝O、Si-C。

(2) The excitation wavelength of the prepared carbon quantum dot is 370nm, when the excitation wavelength is 300-400 nm, the carbon quantum dot emits bright blue light, the emission peak is located at 450nm, when the excitation wavelength is increased from 300nm, the intensity of an emission spectrum is increased firstly and then reduced, the position of the emission peak is obviously blue-shifted firstly, and when the position of the emission peak is 370nm, the emission peak is red-shifted.

(3) The fluorescence quantum yield of the prepared carbon dots is 46% by calculation, the fluorescence yield can also change along with the amount of reactants, the reaction time and the reaction temperature, and after single-factor experiments and optimization, the reaction conditions for preparing the carbon dots are determined to be that the addition amount of KH792 is 30mL, the addition amount of absolute ethyl alcohol is 30mL, the addition amount of citric acid is 1g, the reaction temperature is 140 ℃ and the reaction time is 4 h.

(4) Detection of heavy metal ions (Fe) in polyethylene glycol by using biomass carbon quantum dots as fluorescent probes³⁺、Mn²⁺、Cu^{2 +}、Cr³⁺、Ni²⁺) And an acidic substance, preferably, optimal detection conditions are determined. The research is carried out on the regression equation of the fluorescence intensity and the concentration, the correlation coefficient and the quenching constant, and the result shows that:

CQDs detecting Fe³⁺The results are accurate, the quenching constant is 22687 at 1-100 mu M (low concentration), and the method can be used for detecting low concentration Fe³⁺The relation curve equation of the fluorescent probe at low concentration is-0.22687 x +119.52458, and the linear correlation coefficient is R²0.96667, the linear curve equation is-0.05499 x +105.90959 at 200-1000 μ M (high concentration), and the linear correlation coefficient is R²0.9173, the quenching constant is 5499.

② CQDs is detectedMn²⁺On the other hand, the detection result is inaccurate at low concentration, the correlation coefficient is only 0.48064, and the detection result is accurate at high concentration, the quenching constant is 2129, and the method can be used for detecting high-concentration Mn²⁺The relation curve equation of the fluorescent probe at high concentration is-0.02129 x +118.49863, and the linear correlation coefficient is R²＝0.87797。

Detecting Cu with CQDs²⁺On the other hand, the overall detection result is inaccurate between 1 mu M and 1mM, the correlation coefficient is only 0.58639, the concentration is more accurate when the concentration is divided into 1 mu M to 100 mu M (low concentration) and 200 mu M to 1000 mu M (high concentration) for single detection, the curve equation is-0.3382 x +121.28889 at low concentration, and the linear correlation coefficient is R²0.89111, quenching constant 33820, at high concentration y-0.01805 x +89.16164, linear correlation coefficient R²Since the quenching constant is 1805 at 0.81461, the method can be used for detecting low-concentration Cu²⁺The fluorescent probe of (2) is slightly inaccurate in detection result at a high concentration, has a small quenching constant, and cannot be specifically identified.

CQDs detecting Cr³⁺The detection result is inaccurate, the correlation coefficient is only 0.62136, after the data is subjected to piecewise fitting, the linear correlation coefficient at low concentration is 0.74583, the linear correlation coefficient at high concentration is 0.73391, the correlation is not good, and the method cannot be used for detecting Cr³⁺However, the quenching constant at low concentration is 22223, and can be used for specific detection of Cr³⁺Means of (4).

CQDs detecting Ni²⁺On the other hand, the overall detection result is more accurate between 1 mu M and 1mM, the correlation coefficient is 0.81438, the concentration is further divided into 1 mu M to 100 mu M (low concentration) and 200 mu M to 1000 mu M (high concentration) for individual detection, the curve equation is-0.21864 x +126.24406, the linear correlation coefficient is 0.84939 and the quenching constant is 21864 at low concentration, and the curve equation is-0.03808 +106.34247, the linear correlation coefficient is 0.93665 and the quenching constant is 3808 at high concentration, so that the method can be used for detecting high-concentration Ni²⁺The fluorescent probe has a common effect in low-concentration detection.

The CQDs has accurate result in the aspect of detecting acetic acid in polyethylene glycol, the linear relation curve equation is that y is-0.04914 x +121.5713, the linear correlation coefficient is 0.97857, the linear relation is good, the quenching constant is 4914, the CQDs can be used as a fluorescent probe for detecting acidic substances in polyethylene glycol, and the detection result is accurate.

(5) Based on the detection of the carbon quantum dots on metal ions and acetic acid, the invention mainly provides a method for rapidly detecting the aging degree of the lubricating oil by fluorescence, which can rapidly diagnose the aging degree of the lubricating oil, has simple, reliable and stable technical operation, and provides theoretical basis and practical significance for rapid monitoring of the lubricating oil.

Drawings

Fig. 1 is a particle size distribution diagram of carbon quantum dots.

FIG. 2 is an X-ray diffraction pattern of carbon quantum dots.

FIG. 3 is an infrared spectrum of citric acid, silane coupling agent KH792, and carbon quantum dots.

Fig. 4 shows XPS full spectrum (a) of carbon quantum dots and high resolution spectrum (b) of contained elements.

Fig. 5 shows the dispersion of the carbon quantum dot solution in various solvents (PEG400, ethanol, isopropanol, water, oleylamine, and n-hexane, respectively, from left to right) (a) and the emission thereof under irradiation with an excitation wavelength of 370nm (b).

FIG. 6 is a fluorescence emission spectrum of a carbon quantum dot solution in different solvents.

Fig. 7 shows an excitation spectrum and an emission spectrum of the carbon quantum dot solution.

FIG. 8 shows fluorescence emission spectra of carbon quantum dots at different excitation wavelengths.

FIG. 9 is a fitting equation (a) of fluorescence intensity of carbon quantum dots under different pH influences and a fitting equation (b) of fluorescence intensity of a carbon quantum dot solution with pH changes.

FIG. 10 shows fluorescence intensities of carbon quantum dot solutions at different temperatures.

FIG. 11 is a graph of fluorescence intensity of a carbon quantum dot solution versus time.

FIG. 12 is a graph showing the relationship between the reaction time and the fluorescence yield.

FIG. 13 is a graph showing the effect of reaction temperature on fluorescence yield.

FIG. 14 is a graph (a) showing the relationship between the amount of carbon quantum dots added to polyethylene glycol 400 and the fluorescence intensity and a fitted equation (b).

Fig. 15 is an infrared spectrum of polyethylene glycol 400 and polyethylene glycol 400 to which carbon quantum dots are added.

FIG. 16 shows different concentrations of Fe³⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M.

FIG. 17 shows Mn concentrations²⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M.

FIG. 18 shows Cu concentrations²⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M.

FIG. 19 shows Cr concentrations³⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M.

FIG. 20 shows different concentrations of Ni²⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M.

FIG. 21 is a graph (a) showing the fitting equation of the fluorescence intensity of carbon quantum dots with different amounts of acetic acid added and a graph (b) showing the influence relationship.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments.

Example 1

The preparation method of the carbon quantum dot solution comprises the following steps:

weighing 1g of citric acid monohydrate, dissolving the citric acid monohydrate into 30mL of absolute ethanol, weighing 30mL of N-aminoethyl-3-aminopropyltrimethoxysilane KH792, uniformly mixing the citric acid monohydrate and the absolute ethanol, transferring the mixture into a polytetrafluoroethylene inner container of a 100mL hydrothermal reaction kettle, putting the inner container into a constant-temperature air-blowing drying oven, reacting at 140 ℃ for 4 hours, taking out the mixture after the reaction is finished and cooled, diluting the mixture by 10000 times of absolute ethanol, and sealing and storing the mixture to obtain the blue-fluorescence nitrogen-silicon co-doped carbon quantum dot ethanol solution.

In order to facilitate detection of the carbon quantum dots, the carbon quantum dots are prepared into powder by the following method:

and (3) dialyzing the diluted carbon quantum dot solution for 48h by using a 1000D dialysis bag (the dialysis external liquid is absolute ethyl alcohol), evaporating the dialyzed solution to about 10mL by using a rotary evaporator, and drying the solution into powder by using a freeze dryer at the temperature of minus 30 ℃ to obtain carbon quantum dot powder (CQDs).

Example 2

Characterization and fluorescence properties of prepared carbon quantum dots

1. Particle size distribution

The particle size distribution of the carbon quantum dots was analyzed by a particle size analyzer, and as can be seen from the particle size distribution diagram shown in fig. 1, the particle size distribution of the carbon quantum dots was between 1 and 6nm, and the average particle size was 2.9 nm.

2. Structural analysis

Phase analysis was performed on the carbon quantum dots using XRD, as shown in fig. 2. Only one broad peak centered around 22 ° is shown in the figure, indicating that the prepared carbon quantum dots are amorphous.

The infrared spectrum of the substance is the reflection of the molecular structure, the absorption peak in the infrared spectrum corresponds to the vibration form of each group in the molecule, and the molecular structure can be estimated through the absorption peak of the functional group. For further analysis, the surface groups of the synthesized carbon quantum dots were characterized by fourier infrared spectroscopy as shown in fig. 3.

Compared with citric acid and a silane coupling agent KH792, the prepared nitrogen-silicon co-doped carbon dots show different absorption peaks. The carbon quantum point is 1250cm by analyzing the map^-1The peak of stretching vibration of-C ═ O at the amide group appears, which indicates that the carboxyl group in citric acid and the amino group in the silane coupling agent KH792 are subjected to acylation reaction during the reaction; at 1005cm^-1The peak of (a) belongs to the stretching vibration peak of Si-O-Si; at 3325cm^-1And 735cm^-1The peaks appearing at are respectively CH of the carbon chain skeleton₂The stretching vibration peak and the antisymmetric deformation peak of (1); 2970cm^-1Peak of (a) is bound-NH₂Of (2)And (4) contracting the vibration peak.

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopy technique for measuring the elemental composition of materials, and the chemical and electronic states of the elements contained therein, and is commonly used for the analysis of the material surface and the auxiliary research of the reaction mechanism in the development and preparation processes of novel materials. By XPS analysis of the material, not only can the chemical composition of the material surface be detected (except H, He), but the chemical valence state of the element can be determined. The prepared carbon quantum dot powder was subjected to XPS characterization, and the results were further processed with XPS software as shown in fig. 4.

Fig. 4a is an XPS full spectrum of a carbon quantum dot, and full spectrum analysis shows that the surface of the carbon dot contains C, N, O, Si four elements, the percentage contents of the elements are: 46.16% C, 23.83% N, 17.37% O, and 12.64% Si, indicating that the sample is carbon as the major constituent element. And performing high-resolution scanning on each element on the basis of full-spectrum scanning, and performing peak separation processing on spectrograms of the four elements respectively to obtain different combination forms of each element (shown in fig. 4 b). The narrow spectrum of C1s, fitted to give 1 fitted curve, indicates the presence of one form of C. The fitted peak at 284.83eV corresponds to a C-C single bond of sp2 form, constituting the backbone of the carbon quantum dot. The fitting peak of 398.53eV in the spectrum of N1s belongs to-NH₂A key. The peak 531.68eV in the spectrum of O1s belongs to N-C ═ O, indicating the formation of an amide group. Two fitting peaks are obtained in the spectrum of Si2p, and the peak of 101.93eV belongs to Si-C bonds on a carbon chain.

3. Fluorescence mechanism of carbon quantum dots

The luminescence of the carbon quantum dots originates from the Cabbeen structure with saw-tooth edges. It is well known that the cabbene structure is sensitive to the environment and that if the cabbene structure is protonated, the fluorescence is quenched. In contrast, a defect point is formed after amino modification, and an active point which is easily attacked by the original Cabbeen structure is protected, so that the quenching of fluorescence is inhibited. After excited pi electrons in the quantum dots are transited, different excited states pass through in the process of transition to the ground state, and only when the first excited state, namely the electron of the lowest excited state, transits to the ground state, fluorescence is emitted, otherwise, heat is emitted in the form of phosphorescence or heat radiation.

Radovic and Bockrath have built structural models aimed at demonstrating that the edges of graphene sheets are caboben structures in the triplet ground state without hydrogen atoms. The maximum excitation wavelength of CQDs of 370nm corresponds to the transition of electrons from the homoorbital to the Cabbeen's LUMO orbital, while the emitted fluorescence at 450nm is due to electron-hole recombination. On the other hand, amino radicals generated from amino groups are readily coupled to single electrons on the Cabbeen-structured herz orbital by the Gomberg-Bachmann reaction. While a single electron on another orbit can have three types of homing: accept an electron to form a negative charge, lose an electron to form a positive charge, or retain the original free radical state. No matter how the single electron on the orbit finally belongs, the defect with energy bandwidth is generated at the active point, and the size of the bandwidth depends on the influence of the grafted amino group on the conjugated system of the carbon quantum dot.

It is the generation of such defect states that different amino-modified carbon quantum dots have different fluorescence excitation peaks and fluorescence emission peaks. The principle is mainly that the electron-hole recombination generating radiation is increased at the defect. The fluorescence luminescence mechanism of the quantum dots in the application is similar to the structure model established by Radovic, and different fluorescence excitation peaks and fluorescence emission peaks exist because different modifiers enable the surfaces of the quantum dots to have different positions or numbers of active sites in the modification process.

4. Fluorescent properties of carbon quantum dots

4.1 fluorescence representation of carbon quantum dots in liquid phase

The carbon quantum dot solution prepared by the hydrothermal synthesis method has good dispersibility, good water solubility, excellent fluorescence performance and capability of being dispersed in organic solvents such as n-hexane, methanol, acetone and ethanol. Fig. 5a shows the effect of dispersing in various solvents (PEG400, ethanol, isopropanol, water, oleylamine, n-hexane), and it is understood that the dispersion is excellent in various solvents. FIG. 5b is a photograph under UV light, which shows colorless in daylight and emits bright blue fluorescence under 370nm fluorescent light.

Respectively adding 200 mu L of 10000-fold diluted carbon quantum dot solution into 10mL of solvent (normal hexane, isopropanol, ethanol, water and PEG400), uniformly mixing to obtain carbon quantum dot solutions with different solvents and the same concentration, and testing the fluorescence intensity of the carbon quantum dots in the different solvents. Fig. 6 is a fluorescence emission spectrum of a carbon quantum dot solution in various solvents, and it can be seen from fig. 6 that the fluorescence intensity of the carbon quantum dots in an organic solvent is greater than that in ultrapure water.

4.2 carbon quantum dot emission and excitation spectra

200 mu L of the carbon quantum dot solution diluted by 10000 times is added into 10mL of ethanol, and then the carbon quantum dot solution is subjected to fluorescence measurement by using a fluorescence spectrometer to obtain an excitation spectrum and an emission spectrum (figure 7) of the carbon quantum dot solution, and as can be seen from figure 7, the maximum excitation wavelength of the carbon quantum dot solution is 370nm, and the maximum emission wavelength is 450 nm.

FIG. 8 is a graph of the emission spectra of the carbon dot solution at different excitation wavelengths, and it can be seen from FIG. 8 that the emission peak is at 450nm and the half-width is 62.5nm, and the relationship between the fluorescence intensity and the excitation wavelength is: when the excitation wavelength is increased from 300nm, the intensity of the emission spectrum is increased and then reduced, the position of an emission peak is obviously red-shifted firstly, and then blue-shifted when the wavelength reaches 370 nm.

4.3 Effect of pH Change on fluorescence Properties of carbon Quantum dot solutions

Since the pH of the solution has a very significant influence on the fluorescence performance of the carbon quantum dot, the influence tests of different pH values are carried out on the prepared silane modified carbon dot solution, and the results are analyzed. As a result, as shown in fig. 9a, the fluorescence intensity of the carbon dot solution sharply decreases under a strong acidic condition (pH 1), which indicates that the strong acidic condition has a quenching effect on the fluorescence of the carbon dot. As the pH is increased from 3 to 11, the fluorescence intensity of the carbon dot solution is gradually increased, and the relationship between the pH and the fluorescence intensity of the carbon dot solution can be more intuitively shown in FIG. 9b, the relationship is basically linear, the data is linearly fitted, and the correlation coefficient R of the fitting equation is²0.95031, the relation between pH and the fluorescence intensity of the carbon dot solution can be well expressed by the equation, and the line between the pH and the fluorescence intensity can be illustratedThe sexual relationship is relatively close.

4.4 Effect of temperature on the fluorescence Properties of carbon Quantum dot solutions

In order to examine the change of the fluorescence intensity at the solution temperature of 10 ℃, 20 ℃, 30 ℃ and 40 ℃, the research shows that the fluorescence intensity of the carbon quantum dot solution is also influenced by the temperature, and the figure 10 shows. The prepared carbon dot solution is sensitive to temperature, and the fluorescence intensity of the carbon quantum dots is reduced along with the increase of the temperature, so that the influence of the temperature is considered in a fluorescence experiment, otherwise, the result measured by the experiment is inaccurate. It is worth mentioning that the fluorescence intensity almost returns to the original intensity level when the solution is cooled from a higher temperature to a lower temperature.

Meanwhile, a cycle test is carried out aiming at the influence of temperature on the fluorescence property of the carbon quantum dots, the obtained fluorescence intensity of the carbon dots still has recoverability after multiple temperature cycles, and the result shows that the structure of the carbon quantum dots does not change after the temperature changes, and the fluorescence intensity is reversible, so that the prepared carbon quantum dot solution has good thermal stability.

Fluorescence stability of 4.5 carbon quantum dot solution

The fluorescence intensity of the carbon quantum dots changes with time. In order to investigate the change of the fluorescence property of the carbon quantum dots prepared in the present application with time, experiments were performed on the relationship between the fluorescence intensity of the carbon quantum dot solution and time, and the fluorescence intensities of the carbon quantum dots after 0d, 1d, 2d, 3d, 4d, 5d, 6d, and 7d were respectively tested, fig. 11 is a graph showing the relationship between the fluorescence intensity of the carbon quantum dot solution and time, as can be seen from fig. 11, the fluorescence intensity of the carbon dot solution gradually decreased when the time was 0-5 d, and after 5d, the fluorescence intensity of the carbon dot solution remained almost unchanged, and the fluorescence intensity was about 95% of that of the original 0 d. It can be seen that the fluorescence intensity of the carbon dot solution remains substantially constant over time and is substantially stable.

4.6 fluorescence Quantum yield of silane-modified carbon Quantum dots

4.6.1 measurement of fluorescence yield of carbon Quantum dots

The fluorescence quantum yield of the carbon quantum dot solution is measured by adopting a reference methodFirstly preparing 1mol/L sulfuric acid, then weighing a small amount of quinine sulfate to dissolve in the sulfuric acid, measuring the absorbance of the quinine sulfate to be 0.14, and stabilizing the absorbance of the solution to be below 0.05 by continuously adjusting the dosage of the quinine sulfate, wherein the accurate measurement value of the experiment is 0.04. The concentration of the carbon quantum dot solution was adjusted so that the absorbance was stabilized to 0.05 or less, and the measured value was 0.04. Sequentially measuring the fluorescence intensity of the quinine hemisulfate solution and the carbon dot solution under the condition of excitation wavelength of 350nm, wherein the obtained results are that the fluorescence intensity of the quinine hemisulfate solution is 72, the fluorescence intensity of the carbon dot solution is 91, the fluorescence quantum yield of the quinine hemisulfate solution under the excitation wavelength of 350nm is 0.54, and the data are substituted into the formulaThe fluorescence quantum yield of the silane modified carbon quantum dot solution was calculated to be 46%.

4.6.2 Effect of reaction conditions on fluorescence yield of carbon Quantum dot solution

1) Influence of reactant ratio on fluorescence yield of carbon dot solution

First, in order to examine the influence of the mixture ratio of reactants on the experiment, the reaction temperature of the experiment was designed to be 140 ℃, the reaction time was designed to be 4h, the amount of 30mL of KH792 was kept unchanged, and the experiment results are shown in table 1 by comparing the fluorescence quantum yields of different mixture ratios, wherein the fluorescence quantum yield was the highest at 37% when the amount of citric acid was 1g and ethanol was added. In this experiment, when ethanol was added, the fluorescence yield was increased because citric acid was not well soluble in the silane coupling agent and the reactants were not sufficiently contacted and reacted completely, and thus, ethanol served as a solvent for citric acid to be mixed with the silane coupling agent and the reactants were completely contacted.

TABLE 1 Effect of different reactant ratios on fluorescence yield

2) Effect of reaction time on fluorescence yield of carbon dot solution

The reaction time affects the extent of the reaction, thereby causing a change in the fluorescence efficiency of the product. 1g of citric acid was dissolved in 30mL of ethanol, mixed with 30mL of KH792, and added into 100mL of a polytetrafluoroethylene liner, the reaction temperature was 140 ℃, and the reaction time was measured at 0.5h, 1h, 2h, 3h, 4h, 6h, and 12h, to investigate the effect of the reaction time on the fluorescence yield, and the results are shown in FIG. 12. It is known that the carbon dot solution produced in 0.5h reaction time at 140 ℃ has 15% fluorescence yield, which indicates that the time has less influence on the fluorescence efficiency of the product. After the reaction time is prolonged, the fluorescence efficiency of the product shows a trend of increasing and then decreasing, the peak is 4h, and the fluorescence efficiency is 46%, which shows that the reaction degree can also increase along with the extension of the time, after the reaction degree reaches 4h, the reaction degree is complete, and then the reaction time is prolonged, but the carbon quantum dots have an agglomeration phenomenon, so that the size of the carbon quantum dots is increased, and the fluorescence efficiency becomes low.

3) Influence of reaction temperature on fluorescence yield of carbon dot solution

The temperature is the most main factor influencing the fluorescence yield of the synthesized carbon quantum dots, and the carbonization degree of reactants is controlled by controlling the temperature of the reaction, so that the fluorescence yield is influenced. The same amount of reactants as above were used, the reaction time was 4 hours, and the reaction temperatures were set to 110 deg.C, 120 deg.C, 130 deg.C, 140 deg.C, and 150 deg.C, respectively, for the experiments. As shown in FIG. 13, the reaction product has a very low carbonization degree and a very low fluorescence yield at a temperature of 110 ℃, and the carbonization degree of the reaction product is higher and higher with the increase of the temperature, and reaches the maximum at 140 ℃, so that the fluorescence quantum yield is the highest. As the temperature continues to rise, the carbon quantum dots also increase in size and the fluorescence efficiency decreases.

Example 2

Relationship between fluorescence of carbon quantum dots and metal ions and acid value in simulated oil product

1. Selection of addition amount of carbon quantum dots

Because the components of the oil product in practical application are more complex, the base oil polyethylene glycol 400 is adopted to replace the base oil polyethylene glycol 400 for simulation experiments. 10mL of polyethylene glycol 400 was pipetted into a 10mL transparent screw glass bottle using a pipette, 100. mu.L, 200. mu.L, 400. mu.L, 600. mu.L, and 800. mu.L of the carbon quantum dot solution diluted in example 1 were added thereto, 3mL of the above-mentioned mixed solution was added to a four-way quartz cuvette having a 10mm light path, the excitation wavelength was set at 370nm, and the peak intensity of the fluorescence emission spectrum in the sample was measured.

2. Detection of metal ions

In order to simulate the metal ions formed after the actual oil product is used, Fe is selected according to the metal elements in the general GCr15 high-carbon chromium bearing steel³⁺、Cu²⁺、Cr³⁺、Mn²⁺、Ni²⁺A total of 5 metal ions were tested for their effect on the fluorescence of the carbon quantum dots at an excitation wavelength of 370 nm.

Firstly, 1mmol/L Fe is prepared³⁺、Cu²⁺、Cr³⁺、Mn²⁺、Ni²⁺The solution of (4) is further diluted to a concentration of 1. mu.M, 5. mu.M, 10. mu.M, 20. mu.M, 50. mu.M, 60. mu.M, 70. mu.M, 80. mu.M, 90. mu.M, 100. mu.M, 200. mu.M, 400. mu.M, 600. mu.M, 800. mu.M or 1000. mu.M, and stored. 10mL of polyethylene glycol 400 is measured and put into a 10mL transparent screw glass bottle, 200 mu L of the diluted carbon quantum dot solution in the embodiment 1 is added into the glass bottle, the glass bottle is uniformly shaken, 200 mu L of each metal ion solution with various concentrations is added into the glass bottle, the glass bottle is uniformly shaken, and after 2 hours, a fluorescence test is carried out. 3mL of the mixed solution is added into a four-way quartz cuvette with a light path of 10mm, the excitation wavelength is set to be 370nm, the peak intensity of a fluorescence emission spectrum in a sample is measured, and a relational graph is drawn according to the relationship between the solution and the concentration of the fluorescence intensity of the added metal ion solutions with different concentrations.

3. Detection of acid value of oil product

The acid value is the amount of potassium hydroxide used to neutralize the acidic substances in the oil, and is expressed in terms of mgKOH/g. Thus, its definition is expressed as the total amount of acidic species present in the lubricating oil. The acidic species generated in these oils can cause various levels of corrosion to the machinery. In addition, the acid value of the lubricating oil gradually increases due to deterioration caused by oxidation during storage and use, and the lubricating oil to be used must be replaced when the acid value reaches a specific value. However, since the acid content of the used oil is too complex, the experiment was simulated by replacing it with acetic acid.

Weighing 10mL of polyethylene glycol 400, putting the polyethylene glycol 400 into a 10mL transparent screw glass bottle, adding 200 mu L of carbon quantum dot solution, uniformly shaking, then adding 200 mu L, 400 mu L, 600 mu L, 800 mu L and 1mL of acetic acid, uniformly shaking, waiting for 2h, and then carrying out fluorescence test; 3mL of the mixed solution is added into a four-way quartz cuvette with a light path of 10mm, the excitation wavelength is set to be 370nm, the peak intensity of a fluorescence emission spectrum in a sample is measured, and a relational graph is drawn according to the relationship between acetic acid with different addition amounts and the fluorescence intensity.

4. Selection of addition amount of carbon quantum dots

In order to find out a proper addition amount of the carbon quantum dots so that the carbon quantum dots have proper fluorescence intensity on a fluorescence spectrometer and can well display the relationship between the fluorescence intensity and the ion concentration in the polyethylene glycol 400, a selection experiment of the addition amount of the carbon quantum dots is performed.

As can be seen from fig. 14, the addition amount of the carbon quantum dot and the fluorescence intensity of the carbon quantum dot in the polyethylene glycol 400 have a good linear relationship, and the correlation coefficient is 0.99253, which indicates that the carbon quantum dot emits light stably, and does not lose or decrease the fluorescence intensity at high concentration due to polymerization or other reactions caused by the addition amount, and the fluorescence intensity tends to increase steadily and linearly with the increase of the addition amount. Meanwhile, since the concentration of the metal ions to be detected is low, when the fluorescence intensity is too high, the change rate may be too small by adding the metal ions, and if the addition amount is too low, the fluorescence of the solution at a high concentration may completely disappear, and the measurement result is inaccurate, so that the addition amount of the carbon quantum dots is 200 μ L.

FIG. 15 is a Fourier transform infrared spectrum of polyethylene glycol 400 and polyethylene glycol with carbon quantum dots added, and it can be seen from FIG. 15 that the infrared spectra of the two are very similar, the only difference is that the solution with carbon quantum dots added is 2905cm^-1There is a smallPeak convex, which is-NH₂The peak is not obvious because the amount added is too small compared to the solvent. This indicates that the structure of polyethylene glycol 400 is not destroyed after adding carbon quantum dots, and the use of the polyethylene glycol is not affected.

5. Relationship between fluorescence of carbon quantum dots and various metal ions

According to the Stern-Volmer equation (I)₀/I)-1＝K_svC^[84]The resulting fluorescence quenching standard curve was fitted. I is₀: fluorescence intensity of carbon quantum dots when no heavy metal ions are added; i: respectively adding heavy metal ions with different concentrations to obtain fluorescence intensity of the carbon quantum dots; c: the concentration of the added heavy metal ions; k_sv: quenching constant (L/mol).

5.1 different concentrations of Fe³⁺Influence on fluorescence intensity of carbon quantum dots

FIG. 16 shows different concentrations of Fe³⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M. As shown in FIG. 16a, in polyethylene glycol 400 solution of CQDs, when Fe is added³⁺When the concentration is increased continuously, the fluorescence intensity of the whole solution is reduced gradually, and after linear fitting is carried out on the whole data, Fe between 1 mu M and 1mM is obtained³⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.06582 x +111.87475, and the linear correlation coefficient is R²The linear relationship is better when 0.91801 is added, but it was observed that when a lower concentration of Fe is added³⁺The CQDs have a large decrease in fluorescence intensity in solution; when higher concentration of Fe is added³⁺When the CQDs are in solution, the fluorescence intensity of the CQDs is reduced to a smaller extent, so that piecewise fitting is carried out, linear fitting is carried out once between 1 mu M and 100 mu M, when the heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced, a good linear relation is presented, and Fe between 1 mu M and 100 mu M is obtained³⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.22687 x +119.52458, and the linear correlation coefficient is R²0.96667; linear fitting is carried out again between 200 mu M and 1000 mu M when the concentration of heavy metal ions is changedWhen the carbon quantum dot solution is added from small to large, the fluorescence intensity is gradually reduced, and a good linear relation is presented, so that Fe between 200 mu M and 1000 mu M is obtained³⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.05499 x +105.90959, and the linear correlation coefficient is R²0.9173. The quenching constant between 1 and 100. mu.M is 22687 and 5499 according to the Stern-Volmer equation. This result indicates that the CQDs can be used as a detection of Fe³⁺The fluorescent probe has accurate detection and can detect heavy metal ions.

5.2 different concentrations of Mn²⁺Influence on fluorescence intensity of carbon quantum dots

FIG. 17 shows Mn concentrations²⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M. As shown in FIG. 17, in the polyethylene glycol solution of CQDs, Mn was added when Mn was added²⁺The fluorescence intensity of the whole solution is gradually reduced when the concentration is increased continuously, and after linear fitting is carried out on the whole data, Mn between 1 mu M and 1mM is obtained²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.02175 x +118.62583, and the linear correlation coefficient is R²The linear relationship is better when 0.83725 is added, but it is observed that when lower concentrations of Mn are added²⁺The CQDs have a large decrease in fluorescence intensity in solution; when Mn is added at a higher concentration²⁺When the solution is used, the fluorescence intensity of the CQDs is reduced to a smaller extent, so that the piecewise fitting is carried out, the linear fitting is carried out between 1 mu M and 100 mu M (figure 17b), when the heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced, but the linear relation is not good, and the Mn between 1 mu M and 100 mu M is obtained²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.07719 x +120.77427, and the linear correlation coefficient is R²0.48064; linear fitting is performed again between 100 mu M and 1000 mu M (figure 17c), when the heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced and presents a good linear relation, and the fluorescence intensity between 100 mu M and 1000 mu M is obtainedM Mn²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.02129 x +118.49863, and the linear correlation coefficient is R²0.87797. The calculated quenching constant of 7719 and 2129 between 1 and 100. mu.M and 200 to 1000. mu.M according to the Stern-Volmer equation. This result indicates that the CQDs cannot be used as a detection Mn at low concentrations²⁺The fluorescent probe of (1) can be used for detecting Mn at high concentration²⁺The fluorescent probe has an inaccurate detection result, and the effect of the fluorescent probe cannot be used for detecting heavy metal ions under an ideal state.

5.3 different concentrations of Cu²⁺Influence on fluorescence intensity of carbon quantum dots

FIG. 18 shows Cu concentrations²⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M. As shown in FIG. 18a, in polyethylene glycol solution of CQDs, when Cu is added²⁺When the concentration is increased continuously, the fluorescence intensity of the whole solution is reduced gradually, and after linear fitting is carried out on the whole data, Cu between 1 mu M and 1mM is obtained²⁺The equation of the relation curve between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.04416 x +106.40369, and the linear correlation coefficient is R²The linear relationship was poor at 0.58639, but it was observed that when a lower concentration of Cu was added²⁺The CQDs have a large decrease in fluorescence intensity in solution; when adding higher concentration of Cu²⁺When the solution is used, the fluorescence intensity of the CQDs is reduced in a small range, so that piecewise fitting is carried out, linear fitting is carried out once between 1 mu M and 100 mu M (figure 18b), when the heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced, a good linear relation is presented, and the Cu between 1 mu M and 100 mu M is obtained²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.3382 x +121.28889, and the linear correlation coefficient is R²0.89111; linear fitting is carried out again between 200 mu M and 1000 mu M (figure 18c), when the heavy metal ions are added into the carbon quantum dot solution from small to large, the fluorescence intensity is gradually reduced, and a good linear relation is presented, so that Cu between 200 mu M and 1000 mu M is obtained²⁺With carbon quantumThe relation curve equation between the point solutions is that y is-0.01805 x +89.16164, and the linear correlation coefficient is R²0.81461. The quenching constant between 1 and 100. mu.M is 33820 and that between 200 and 1000. mu.M is 1805, calculated according to the Stern-Volmer equation. This result indicates that the CQDs can be used as the detection of Cu²⁺The fluorescent probe has an inaccurate detection result, and the effect of the fluorescent probe cannot be used for detecting heavy metal ions under an ideal state.

5.4 Cr of different concentrations³⁺Influence on fluorescence intensity of carbon quantum dots

FIG. 19 shows Cr concentrations³⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M. As shown in FIG. 19a, in polyethylene glycol 400 solution of CQDs, when Cr is added³⁺When the concentration is increased continuously, the fluorescence intensity of the whole solution is reduced gradually, and after linear fitting is carried out on the whole data, Cr between 1 mu M and 1mM is obtained²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.03459 +116.92512, and the linear correlation coefficient is R²The linear relationship was poor at 0.62136, but it was observed that when lower concentrations of Cr were added³⁺The CQDs have a large decrease in fluorescence intensity in solution; when adding higher concentration of Cr³⁺When the CQDs is in solution, the fluorescence intensity of the CQDs is reduced to a smaller extent, so that piecewise fitting is carried out, linear fitting is carried out once between 1 mu M and 100 mu M (figure 19b), when heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced, a good linear relation is presented, and Cr between 1 mu M and 100 mu M is obtained³⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.22223 x +129.95106, and the linear correlation coefficient is R²0.74583; linear fitting is carried out again between 200 mu M and 1000 mu M (figure 19c), when the heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced, and a good linear relation is presented, thus obtaining Cr between 200 mu M and 1000 mu M³⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.01523 +103.53699, and the linear correlation coefficient is R²＝0.73391. The quenching constant between 1 and 100 mu M is 22223 and the quenching constant between 200 and 1000 mu M is 1523 according to the Stern-Volmer equation. This result indicates that the CQDs cannot be used as the detection Cr³⁺The fluorescent probe of (1).

5.5 different concentrations of Ni²⁺Influence on fluorescence intensity of carbon quantum dots

FIG. 20 shows different concentrations of Ni²⁺A curve graph is fitted to the influence of the fluorescence intensity of carbon quantum dots, wherein (a) the curve graph is 1 mu M-1 mM, (b) the curve graph is 1 mu M-100 mu M, and (c) the curve graph is 200 mu M-1000 mu M. As shown in FIG. 20a, in polyethylene glycol solution of CQDs, Ni was added when adding²⁺When the concentration is increased continuously, the fluorescence intensity of the whole solution is reduced gradually, and after linear fitting is carried out on the whole data, Ni between 1 mu M and 1mM is obtained²⁺The relation curve equation between the carbon quantum solution is that y is-0.05546 x +118.48074, and the linear correlation coefficient is R²The linear relationship is better when 0.81438 is added, but it is observed that when a lower concentration of Ni is added²⁺The CQDs have a large decrease in fluorescence intensity in solution; when adding higher concentration of Ni²⁺When the CQDs are in solution, the fluorescence intensity of the CQDs is reduced in a small range, so that the data are subjected to piecewise fitting, linear fitting is performed between 1 mu M and 100 mu M (figure 20b), when heavy metal ions are added into the carbon quantum dot solution from small to large in concentration, the fluorescence intensity of the CQDs is gradually reduced and presents a good linear relation, and Ni between 1 mu M and 100 mu M is obtained²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.21864 x +126.24406, and the linear correlation coefficient is R²0.84939; linear fitting is performed again between 200 mu M and 1000 mu M (figure 20c), when the heavy metal ions are added into the carbon quantum dot solution from small concentration to large concentration, the fluorescence intensity is gradually reduced, and a good linear relation is presented, so that Ni between 200 mu M and 1000 mu M is obtained²⁺The relation curve equation between the carbon quantum dot solution and the carbon quantum dot solution is that y is-0.03808 +106.34247, and the linear correlation coefficient is R²0.93665. The quenching constant between 1 and 100. mu.M is 2186 and 3808 between 200 and 1000. mu.M calculated according to the Stern-Volmer equation. This result indicates that the CQDs are present at low concentrationsWhile detecting Ni as a fluorescent probe²⁺The effect of (2) is not accurate enough, and the detection result is more accurate when the concentration is high.

6. Relationship between fluorescence and acid value of carbon quantum dots

The acid number indicates the degree of oxidation of the oil. The high acid value can easily corrode the machine and deteriorate the oil product, which can reduce the service life of the machine, so the detection of the acid value of the lubricating oil is very important.

FIG. 21 shows the effect of different amounts of acetic acid on the fluorescence intensity of carbon quantum dots, and it can be seen from FIG. 21a that in the polyethylene glycol solution of CQDs, the fluorescence intensity of the whole solution gradually decreases as the amount of acetic acid added increases. Also, it can be seen from FIG. 21b that the peak of fluorescence is slightly red-shifted after the addition of acetic acid, because the entire solution turned from colorless to yellow and appeared greenish under fluorescence after the addition of acetic acid. After linear fitting is carried out on the whole data, the relation curve equation between the acetic acid and the carbon quantum dot solution is obtained in the addition amount of 0-1 mL, the relation curve equation is-0.4914 x +121.5713, and the linear correlation coefficient is R²A linear relationship is better at 0.97857. The quenching constant is 4914, and the result shows that the prepared CQDs can be used as fluorescent probes for detecting acidic substances in polyethylene glycol, and the detection result is accurate.

Example 3

Based on the research result of example 2, the method for rapidly detecting the aging degree of the lubricating oil by fluorescence mainly comprises the following steps:

1) determination of the Linear relationship between the concentration of Metal ions and the fluorescence intensity of the carbon Quantum dots

Firstly, 1mmol/L Fe is prepared³⁺、Ni²⁺The solution of (4), further diluting the solution to a concentration of 1. mu.M, 5. mu.M, 10. mu.M, 20. mu.M, 50. mu.M, 60. mu.M, 70. mu.M, 80. mu.M, 90. mu.M, 100. mu.M, 200. mu.M, 400. mu.M, 600. mu.M, 800. mu.M, or 1000. mu.M, respectively, and preserving the same;

weighing 10mL of polyethylene glycol 400, putting the polyethylene glycol 400 into a 10mL transparent screw glass bottle, adding 200 mu L of carbon quantum dot solution (diluted in example 1, the same below), uniformly shaking, adding 200 mu L of metal ion solutions with various concentrations, uniformly shaking, waiting for 2 hours, and performing fluorescence test;

adding 3mL of the mixed solution into a four-way quartz cuvette with an optical path of 10mm, setting the excitation wavelength to be 370nm, measuring the peak intensity of a fluorescence emission spectrum in a sample, and making a relational graph according to the relationship between metal ion solutions with different concentrations and the fluorescence intensity;

2) determination of the Linear relationship between the concentration of the mock acid and the fluorescence intensity of the carbon Quantum dots

Weighing 10mL of polyethylene glycol 400, putting the polyethylene glycol 400 into a 10mL transparent screw glass bottle, adding 200 mu L of carbon quantum dot solution, uniformly shaking, then adding 200 mu L, 400 mu L, 600 mu L, 800 mu L and 1mL of acetic acid, uniformly shaking, waiting for 2h, and then carrying out fluorescence test;

adding 3mL of the mixed solution into a four-way quartz cuvette with an optical path of 10mm, setting the excitation wavelength to be 370nm, measuring the peak intensity of a fluorescence emission spectrum in a sample, and making a relational graph according to the relationship between acetic acid with different addition amounts and the fluorescence intensity;

3) detection of used lubricating oil

Adding 200 mu L of carbon quantum dot solution into 10mL of used lubricating oil to be detected, measuring fluorescence intensity in a fluorescence spectrometer, and calculating the concentration and acid value of metal ions in the lubricating oil to be detected by comparing the linear relationship obtained in the step 2) and the step 3), thereby comprehensively judging the aging degree of the lubricating oil;

4) prompt for replacement

And setting a lubricating oil aging replacement threshold, and prompting to replace the lubricating oil when the metal ion concentration and the acid value of the lubricating oil to be detected reach the threshold.

The foregoing is merely exemplary and illustrative of the principles of the present invention and various modifications, additions and substitutions of the specific embodiments described herein may be made by those skilled in the art without departing from the principles of the present invention or exceeding the scope of the claims set forth herein.