A fluorescent paper-based polarity sensor, a preparation method and application thereof

Abstract

This invention belongs to the field of fluorescent materials technology and discloses a fluorescent paper-based polar sensor, its preparation method, and its application. Using p-phenylenediamine and o-phenanthroline as precursors, nitrogen-doped red carbon dots sensitive to solvent polarity are prepared via a one-step solvothermal method. N-RCDs are fixed onto general qualitative filter paper using an immersion method to prepare polar test papers. The colors of the test papers with different polarities are extracted under 365nm flashlight illumination using a mobile app, and a colorimetric card is created. The colors of the test paper photos are converted into RGB data, and a linear relationship curve between the R / G value and the water content in the organic solvent is generated. The paper-based polar sensor is simple to process, low in cost, easy to operate, and portable. It exhibits high selectivity for solvent polarity response, a wide dynamic range, good stability, and strong environmental adaptability. It can be used for visual semi-quantitative analysis of solvent polarity and accurate quantification of water content in organic solvents.

Description

Technical Field

[0001] This invention belongs to the field of fluorescent materials technology, and relates to a fluorescent paper-based polar sensor, its preparation method, and its application. In particular, it relates to the preparation and application of a paper-based polar sensor based on red carbon dots. Background Technology

[0002] Polarity is a fundamental characteristic of liquids, reflecting the non-uniformity of charge distribution within molecules and the degree of polarity of intermolecular chemical bonds. It is commonly described and compared quantitatively using dipole moment and polarizability. Detecting the polarity of solutions helps in understanding and predicting the properties and behavior of substances in various chemical and biological environments, and is of great significance for scientific research, industrial production, and experimental design. Commonly used methods for polarity determination include Karl Fischer titration, gas chromatography, infrared spectroscopy, nuclear magnetic resonance, and polarity sensors based on different principles such as electrochemistry and photochemistry. While these methods provide accurate analytical results, they still have limitations, such as complex testing procedures, high costs, and the need for specialized experimental personnel and equipment.

[0003] The luminescence behavior of fluorescent substances exhibits a solvent effect. The emission spectral position and intensity of fluorescent substances change significantly in solutions of different polarities, thus they can be used as fluorescent probes to construct fluorescent polarity sensors. While commonly used polarity sensors based on fluorescent dye molecules offer high sensitivity, they also have some drawbacks: the dye molecules themselves possess broad hydrophilicity, leading to limited selectivity; fluorescence intensity is significantly affected by environmental factors and illumination, impacting the sensor's accuracy and stability; dye fluorescence signals typically vary within a limited solvent polarity range, failing to cover a wide polarity range or detect low concentrations of polar molecules; and some dye molecules possess potential toxicity, limiting their applications.

[0004] Carbon dots (CDs) are a novel type of carbon nanomaterial with excellent luminescent properties, low toxicity, chemical stability, and resistance to photobleaching. By controlling parameters such as the surface functional groups, size, and structure of carbon dots, fluorescence responses to solvents of different polarities can be achieved. Currently, most reported CDs used for polar sensing are blue-green fluorescent, with relatively short emission wavelengths. In practical applications, they are susceptible to interference from the blue background fluorescence of complex samples or the sensor carrier itself, as well as changes in sample performance caused by high-energy excitation light.

[0005] In addition, most existing fluorescent polar sensors are large in size, expensive to manufacture, and cumbersome to operate. They require a fluorescence spectrometer to work and cannot achieve rapid on-site detection, which limits the widespread application and commercialization of the sensors. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention specifically provides a paper-based polar sensor based on red-light carbon dots, its preparation method, and its applications. Using p-phenylenediamine and o-phenanthroline as precursors, nitrogen-doped red-light carbon dots (N-RCDs) sensitive to solvent polarity are prepared via a one-step solvothermal method. The N-RCDs are fixed onto general qualitative filter paper using an immersion method to prepare polar-sensitive test paper. The colors of the test paper under 365nm flashlight illumination with different polar solvents are extracted using a mobile app, and a colorimetric chart is created corresponding to the solvent polarity parameter Δf. The test paper images are converted into RGB data, and a linear relationship curve between the R / G value and the water content in the organic solvent is generated. The paper-based polar sensor is simple to process, low in cost, easy to operate, and portable. It exhibits high selectivity for solvent polarity response, a wide dynamic range, good stability, and strong environmental adaptability. It can be used for visual semi-quantitative analysis of solvent polarity and accurate quantification of water content in organic solvents.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution:

[0008] The first objective of this invention is to claim protection for a nitrogen-doped red carbon dot (N-RCDs) prepared by a one-step solvothermal method using p-phenylenediamine and o-phenanthroline as precursors, which is sensitive to solvent polarity.

[0009] The second objective of this invention is to claim protection for the preparation method of the aforementioned N-RCDs, specifically: 0.0437 g of p-phenylenediamine is weighed into 10 mL of DMF, and o-phenanthroline is added in multiples of 0.33 to 3 moles. After ultrasonic stirring to dissolve, the solution is transferred to a 25 mL polytetrafluoroethylene-lined reactor, placed in a forced-air drying oven, and reacted at 200°C for 4 to 24 hours. After cooling to room temperature, a dark red liquid is obtained. The dark red liquid is passed through an aqueous filter membrane, the filtrate is collected, and dialyzed using a dialysis bag with a molecular weight cutoff of 1000 Da to remove unreacted precursor molecules. The dialysate is freeze-dried to obtain black powder N-RCDs, which are sealed and stored at 4°C for later use. Before use, the powder is ultrasonically dispersed in a suitable solvent.

[0010] The third objective of this invention is to request protection for the application of nitrogen-doped red carbon dots (N-RCDs) in the fabrication of fluorescent paper-based sensors.

[0011] The fourth objective of this invention is to protect a method for preparing a fluorescent paper-based sensor, specifically: selecting general qualitative filter paper as a substrate, cutting it into circular pieces with a diameter of 6-10 mm using a punch, immersing it in an N-RCDs aqueous dispersion of 0.02-0.5 mg / mL for 20 seconds, and drying it in an oven at 60°C to obtain a polar test paper-type fluorescent paper-based sensor, which is then stored in a dark and dry place. The fifth objective of this invention is to protect a method for preparing a fluorescent polar colorimetric card using the above-mentioned polar test paper-type fluorescent paper-based sensor, specifically: adding a solvent with a known polarity parameter Δf to the test paper, irradiating it at 365 nm in a dark room, extracting the color of each test paper after 30 seconds using a mobile phone "Color Picker" app, and listing the polarity parameter Δf of the solvent used below the corresponding extracted color to create a fluorescent polar colorimetric card with Δf in the range of 0.0133-0.3211.

[0012] The sixth objective of this invention is to seek protection for the application of the above-mentioned fluorescent paper-based sensor and fluorescent polar colorimetric card in solution polarity measurement and low-concentration water detection in organic solvents.

[0013] The specific applications are as follows: the luminescence intensity of the N-RCDs decreases with increasing solution polarity. Based on the linear correlation between fluorescence intensity and Δf, as well as the water content in the organic solvent, a fluorescence spectrometer can be used to quantitatively detect solvent polarity and low concentrations of water in the organic solvent. The emission wavelength of the N-RCDs redshifts with increasing solution polarity. Paper-based sensors and polarity color charts based on N-RCDs can be used for semi-quantitative detection of solvent polarity by visual visualization. By combining the R / B value of the test strip color extracted by the "ColorCare" app on a smartphone with the linear relationship between the water content in the organic solvent, the water content in the organic solvent can be quantitatively detected.

[0014] A fluorescent paper-based polarity sensor; this fluorescent paper-based polarity sensor fixes nitrogen-doped red carbon dots (N-RCDs) onto ordinary qualitative filter paper using an immersion method. Under 365nm flashlight illumination in a dark room, it can visually distinguish test solutions of different polarities. By using the "ColorCare" app on a mobile phone to extract the color of the test paper and the corresponding solvent polarity parameter Δf to create a colorimetric card, polarity can be visualized and semi-quantitatively determined. By converting the color of the polarity test paper photo into RGB data, the water content in the solvent can be accurately quantified.

[0015] The advantages of this invention compared to the prior art are:

[0016] This invention utilizes a simple one-step solvothermal method, using p-phenylenediamine and o-phenanthroline (molar ratio 1:2) as precursors, reacting at 200℃ for 12 h, to prepare nitrogen-doped red carbon dots (N-RCDs) with an average particle size of 4.38 nm, an emission wavelength of 600 nm, and a relative quantum yield of 24.1%. These CDs exhibit excitation wavelength independence, resistance to photobleaching, salt tolerance, and low cytotoxicity. Certain concentrations of most metal ions, anions, and amino acids do not significantly affect their fluorescence. The fluorescence properties of the N-RCDs prepared by the solvothermal method are sensitive to changes in solution polarity.

[0017] This invention tested the fluorescence properties of N-RCDs in eight single solvents: 1,4-dioxane, toluene, chloroform, ethyl acetate, acetone, ethanol (EtOH), dimethyl sulfoxide (DMSO), and water (H2O). It was found that as the polarity parameter Δf increases in the range of 0.0133–0.3211, the excitation and emission wavelengths redshift, and the relative quantum yield decreases. The fluorescence spectra of N-RCDs in binary solvents containing EtOH and DMSO mixed with H2O in different proportions were also tested. A good linear relationship was found between fluorescence intensity and Δf in the range of 0.269–0.319, establishing a method for detecting the polarity of N-RCDs in solutions. Utilizing the polarity effect, a method for detecting low concentrations of water in MeOH, EtOH, DMF, and DMSO was established. The linear ranges were 0.100v%–10.0v%, 0.100v%–6.00v%, 0.100v%–10.0v%, and 0.100v%–10.0v%, respectively, with detection limits of 0.032v%, 0.088v%, and 0.056v%, respectively. This method is simple to operate, has a wide dynamic range of polarity response, and exhibits high sensitivity and low detection limits for detecting water content in organic solvents. This demonstrates the polarity sensing capability of N-RCDs in liquid systems and shows potential for further device-based applications in field detection.

[0018] This invention uses general qualitative filter paper as a carrier and employs an immersion method to prepare a paper-based sensor based on N-RCDs. Under 365nm flashlight illumination in a dark room, the color of the test paper can be visually distinguished by solvents of different polarities. A polarity colorimetric card is created by extracting the color of the test paper and the corresponding solvent Δf using a mobile app. This allows for semi-quantitative polarity sensing of solvents within the Δf range of 0.0133 to 0.3211. Photos of EtOH and DMSO solvents with different water contents are taken with a mobile phone, and the colors are converted into detailed RGB data. Utilizing the linear correlation between the R / G value and the water content in the solvent, rapid and accurate readings of water content within the range of 0% to 100% are achieved. This paper-based polarity sensor is simple to prepare, low in cost, easy to operate, and portable. It possesses excellent performance characteristics such as a wide dynamic response range, high sensitivity, and strong stability, offering practical advantages for rapid on-site analysis and detection. Attached Figure Description

[0019] Figure 1 This is a quantum yield diagram of N-RCDs prepared with different precursor molar ratios.

[0020] Figure 2 This is a quantum yield diagram of N-RCDs prepared at different reaction times.

[0021] Figure 3 This is a scanning electron microscope image of N-RCDs.

[0022] Figure 4 These are transmission electron microscopy (TEM) images and particle size distribution diagrams of N-RCDs; image A is a TEM image of N-RCDs. Image B is a particle size distribution diagram of N-RCDs.

[0023] Figure 5 These are atomic force microscopy and height distribution maps of N-RCDs; where map A is the atomic force microscopy image of N-RCDs; and map B is the height distribution map of N-RCDs.

[0024] Figure 6 This is the XRD pattern of N-RCDs.

[0025] Figure 7 These are XPS plots of N-RCDs. Figure A is the full spectrum. Figure B is the C1s spectrum; Figure C is the N1s spectrum; and Figure D is the O1s spectrum.

[0026] Figure 8 This is the infrared spectrum of N-RCDs.

[0027] Figure 9 This is the UV-vis absorption spectrum of N-RCDs; (c CDs =0.1 mg / mL, solvent: DMSO).

[0028] Figure 10 This is a 3D fluorescence spectrum of N-RCDs; (c N-RCDs =0.1 mg / mL, solvent: DMSO).

[0029] Figure 11 These are fluorescence spectra of N-RCDs under different excitation wavelengths; the inset shows photographs of N-RCDs solutions under natural light (left) and 365nm flashlight illumination (right); (c N-RCDs =0.1 mg / mL, solvent: DMSO).

[0030] Figure 12 Figure A shows the concentration effect of N-RCDs; Figure B shows the fluorescence spectra of N-RCDs at different concentrations; Figure B shows the fluorescence intensity of N-RCDs as a function of concentration; Solvent: DMSO.

[0031] Figure 13 This is a graph showing the effect of continuous irradiation with 560nm excitation light for 60 min on the fluorescence intensity of N-RCDs; (c N-RCDs =0.1 mg / mL).

[0032] Figure 14 This is a graph showing the effect of pH on the fluorescence properties of N-RCDs; (c N-RCDs =0.1 mg / mL).

[0033] Figure 15 This is a graph showing the effect of NaCl concentration on the fluorescence intensity of N-RCDs; (c N-RCDs =0.1 mg / mL).

[0034] Figure 16 This is a graph showing the effect of different metal ions on the fluorescence intensity of N-RCDs; (c N-RCDs =0.1 mg / mL, c anions =5×10 -4 M).

[0035] Figure 17 This is a graph showing the effect of different anions on the fluorescence intensity of N-RCDs; (c N-RCDs =0.1 mg / mL, c anions =5×10 -4 M).

[0036] Figure 18 This is a graph showing the effect of different amino acids on the fluorescence intensity of N-RCDs; (c N-RCDs =0.1 mg / mL, c aminoacids =1×10 -3 M).

[0037] Figure 19These are fluorescence performance graphs of N-RCDs in a single solvent; Figure A shows the normalized fluorescence spectrum; Figure B shows the effect of absorption spectra; Figure C is a visualization of the N-RCDs in the liquid phase; (c N-RCDs =0.1 mg / mL).

[0038] Figure 20 This is the UV-Vis absorption spectrum of N-RCDs in DMSO and EtOH; (c N-RCDs =0.15mg / mL).

[0039] Figure 21 These are fluorescence spectra of N-RCDs in two binary solvents at excitation wavelengths of 360 nm (A, B), 420 nm (C, D), and 540 nm (E, F); (c N-RCDs =0.15mg / mL).

[0040] Figure 22 This is the fluorescence spectrum of N-RCDs in a 1,4-Dioxane / H2O mixture (inset: a graph showing the relationship between fluorescence intensity and water content).

[0041] Figure 23 This is a graph showing the relationship between fluorescence intensity and polarity of N-RCDs in a 1,4-Dioxane / H2O mixed system.

[0042] Figure 24 Figure A shows the fluorescence performance of N-RCDs in the EtOH / H2O mixed system; Figure B shows the fluorescence spectrum of N-RCDs in the EtOH / H2O mixed system (inset: fluorescence intensity versus water content curve); Figure B shows the fluorescence intensity versus polarity curve.

[0043] Figure 25 Figure A shows the fluorescence performance of N-RCDs in the DMSO / H2O mixed system; Figure B shows the fluorescence spectrum of N-RCDs in the DMSO / H2O mixed system (inset: fluorescence intensity versus water content curve); Figure B shows the fluorescence intensity versus polarity curve.

[0044] Figure 26 The fluorescence spectra of N-RCDs in MeOH with different water contents are shown in the inset: (F0-F) / F0 versus water content curve.

[0045] Figure 27 This is a linear relationship curve between the fluorescence intensity of N-RCDs and the water content in MeOH.

[0046] Figure 28Figure A shows the fluorescence performance of N-RCDs in EtOH with different water contents; Figure B shows the fluorescence spectra of N-RCDs in EtOH with different water contents (inset: the relationship curve between (F0-F) / F0 and water content); Figure B shows the linear relationship curve between the fluorescence intensity of N-RCDs and water content.

[0047] Figure 29 Figure A shows the fluorescence performance of N-RCDs in DMF with different water contents; Figure B shows the fluorescence spectra of N-RCDs in DMF with different water contents (inset: the relationship curve between (F0-F) / F0 and water content); Figure B shows the linear relationship curve between the fluorescence intensity of N-RCDs and water content.

[0048] Figure 30 Figure A shows the fluorescence performance of N-RCDs in DMSO with different water contents; Figure B shows the fluorescence spectrum of N-RCDs in DMSO with different water contents (inset: the relationship curve between (F0-F) / F0 and water content); Figure B shows the linear relationship curve between the fluorescence intensity of N-RCDs and water content.

[0049] Figure 31 These are photographs showing the uniformity of N-RCDs test strips under natural light, under natural light with a 365nm flashlight, and under dark room with a 365nm flashlight.

[0050] Figure 32 The graph shows the response of N-RCDs test strips of different concentrations to solvents under natural light, natural light and 365nm flashlight illumination, and dark room and 365nm flashlight illumination.

[0051] Figure 33 This is a visual response diagram of N-RCDs test paper to solvents of different polarities.

[0052] Figure 34 It is an N-RCDs test paper polarity colorimetric card.

[0053] Figure 35 It is the polar response of N-RCDs test paper to EtOH / H2O mixtures of different proportions and the corresponding precision colorimetric card.

[0054] Figure 36 This is a linear relationship curve between the R / G ratio of the color extracted by a smartphone from N-RCDs test strips and the water content in EtOH.

[0055] Figure 37 It shows the polar response of N-RCDs test paper to DMSO / H2O mixtures of different proportions and the corresponding precision colorimetric cards.

[0056] Figure 38This is a linear relationship curve between the R / G ratio of the color extracted by a smartphone from N-RCDs test strips and the water content in DMSO.

[0057] Figure 39 The graphs show the daytime reuse of N-RCDs test strips; Figure A shows the EtOH / H2O system; and Figure B shows the DMSO / H2O system. Detailed Implementation

[0058] The present invention is described in detail below through specific embodiments, but this does not limit the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can all be obtained commercially.

[0059] p-Phenylenediamine (C6H8N2) was purchased from Beijing Bailingwei Chemical Reagent Co., Ltd., and o-phenanthroline (C6H8N2) was also purchased from the same company. 12 H8N2 was purchased from Tianjin Kemio Chemical Reagent Co., Ltd.; ultra-dry N,N-dimethylformamide (C3H7NO), citric acid (C6H8O7), terephthalic acid (C8H6O4), and p-hydroxybenzoic acid (C7H6O3) were purchased from Tianjin Kemio Chemical Reagent Co., Ltd. Dialysis bags (MW: 1000Da) were purchased from Beijing Solarbio Biotechnology Co., Ltd. Ultra-dry methanol (CH3OH) was purchased from Tianjin Kemio Chemical Reagent Co., Ltd.

[0060] Histidine (His), phenylalanine (Phe), aspartic acid (Asp), threonine (Thr), citrulline (Cit), arginine (Arg), proline (Pro), leucine (Leu), glutamic acid (Glu), serine (Ser), and tyrosine (Tyr) were purchased from Dalian Meilun Biotechnology Co., Ltd.; cysteine ​​(Cys), sodium chloride, calcium chloride, magnesium chloride, mercuric chloride, zinc chloride, barium chloride, ammonium acetate, sodium carbonate, sodium bicarbonate, potassium dihydrogen phosphate, and disodium hydrogen phosphate were purchased from Tianjin Kemei Chemical Reagent Co., Ltd.; alanine (Ala) was purchased from Sinopharm Chemical Reagent Co., Ltd.; ferric chloride, manganese chloride, and copper sulfate were purchased from Tianjin Damao Chemical Reagent Factory; nickel nitrate and lead chloride were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; potassium chloride was purchased from Tianjin Tianhe Chemical Reagent Factory; silver nitrate, sodium hypochlorite, sodium thiosulfate, and sodium nitrite were purchased from Shanghai Maclean Biochemical Technology Co., Ltd.; and sodium sulfide was purchased from Shanghai Tongya Huagong Technology Development Co., Ltd.

[0061] Toluene (C7H8), chloroform (CHCl3), ethyl acetate (C4H8O2), anhydrous ethanol (C2H6O), acetone (C3H6O), ultra-dry dimethyl sulfoxide (C2H6OS), and ultra-dry 1,4-dioxane (C4H8O2) were all purchased from Tianjin Kemio Chemical Reagent Co., Ltd.

[0062] Rhodamine B(C)28 H 31 ClN2O3) stock solution (1.0×10 -3 M): Weigh 1.1975 g of Rhodamine B into a 50 mL beaker, dissolve in a small amount of water, and transfer to a 25 mL volumetric flask and dilute to the required concentration before use.

[0063] Phosphate buffer solution (PBS, 0.1M): Weigh 3.4g of potassium dihydrogen phosphate (KH₂PO₄) and 8.96g of disodium hydrogen phosphate (Na₂HPO₄·12H₂O) into 50mL beakers, dissolve them in a small amount of water, and transfer them to a 250mL volumetric flask and dilute to volume. Prepare the required pH buffer solution according to the volume ratio in the experiment.

[0064] N-RCDs stock solution (1.5 mg / mL): Accurately weigh 30 mg of freeze-dried N-RCDs solid and dissolve it in 20 mL of 1,4-dioxane, EtOH and DMSO. Prepare fresh solution before each use and disperse by sonication.

[0065] All reagents used in the experiment were of analytical grade, and the water used in the experiment was double-distilled water.

[0066] Example 1

[0067] 0.0437 g of p-phenylenediamine was weighed into 10 mL of DMF, and the precursor o-phenanthroline was added. The molar ratio of p-phenylenediamine to precursor was 1:2. After ultrasonic stirring to dissolve, the solution was transferred to a 25 mL polytetrafluoroethylene-lined reactor and placed in a drying oven at 200 °C for 12 h. After cooling to room temperature, a dark red liquid was obtained. The solution was dialyzed using a dialysis bag with a molecular weight cutoff of 1000 Da to remove unreacted precursor molecules. The dialysate was freeze-dried to obtain a black powder N-RCDs, which was sealed and stored at 4 °C for later use. Before use, the powder was ultrasonically dispersed in a suitable solvent.

[0068] Example 2

[0069] The molar ratio of p-phenylenediamine to the precursor was 3:1; other aspects were the same as in Example 1.

[0070] Example 3

[0071] The molar ratio of p-phenylenediamine to the precursor was 2:1; other aspects were the same as in Example 1.

[0072] Example 4

[0073] The molar ratio of p-phenylenediamine to the precursor was 1:1; other aspects were the same as in Example 1.

[0074] Example 5

[0075] The molar ratio of p-phenylenediamine to the precursor was 1:3; other aspects were the same as in Example 1.

[0076] Example 6

[0077] The reaction was carried out at 200°C for 4 hours, and the rest was the same as in Example 1.

[0078] Determination of the relative fluorescence quantum yield (PLQY) of N-RCDs prepared in Examples 1-5

[0079] Using Rhodamine B as a reference, the fluorescence emission integral intensity and optical density of N-RCDs and Rhodamine B were measured respectively, and the relative fluorescence quantum yield Φ was calculated according to Equation 1-1:

[0080]

[0081] In the formula, Φ represents the fluorescence quantum yield of the analyte N-RCDs; Φ′ represents the fluorescence quantum yield of the reference substance Rhodamine B in ethanol (Φ′=0.95); I is the integrated intensity of the emission spectrum of N-RCDs; I′ is the integrated intensity of the emission spectrum of Rhodamine B; A represents the optical density of N-RCDs; A′ represents the optical density of Rhodamine B; n is the refractive index of the solvent of N-RCDs (dimethyl sulfoxide), with a value of 1.48; n′ is the refractive index of the solvent of Rhodamine B (ethanol), with a value of 1.36.

[0082] Different precursor molar ratios lead to different structures in the prepared N-RCDs, thus affecting their luminescent properties. In Examples 1-5, the quantum yields of the products obtained when the molar ratios of p-phenylenediamine to o-phenanthroline were 1:2, 3:1, 2:1, 1:1, and 1:3, respectively, were as follows: Figure 1 As shown, the quantum yield first increases and then decreases with increasing proportion of o-phenanthroline. The quantum yield of N-RCDs prepared at a molar ratio of 1:2 is the highest, which may be because N-RCDs prepared from this precursor ratio have the best degree of carbonization and the highest nitrogen content.

[0083] The reaction time determines the degree of carbonization of N-RCDs, which in turn affects their size and quantum yield. Figure 2 As can be seen, when the reaction time is 4h (Example 6), the quantum yield of the prepared N-RCDs is only 1.1%, while the quantum yield increases to 24.1% after 12h of reaction. If the reaction time is extended further, the quantum yield shows a decreasing trend. Therefore, 12h is the optimal preparation time for N-RCDs.

[0084] Using p-phenylenediamine and o-phenanthroline as precursors, when their molar ratio is 1:2 and the reaction is carried out at 200°C for 12 hours, N-RCDs with high quantum yield and long emission wavelength can be obtained. This is the preparation method described in Example 1.

[0085] Comparative Example 1

[0086] The precursor is p-phenylenediamine; other aspects are the same as in Example 1.

[0087] Comparative Example 2.

[0088] The precursors are p-phenylenediamine and citric acid; other aspects are the same as in Example 1.

[0089] Comparative Example 3

[0090] The precursors are p-phenylenediamine and terephthalic acid; other aspects are the same as in Example 1.

[0091] Comparative Example 4

[0092] The precursors are p-phenylenediamine and p-hydroxybenzoic acid; other aspects are the same as in Example 1.

[0093] The results of the products prepared in Comparative Examples 1-4 and Example 1 are listed in Table 1.

[0094] Table 1. Luminescent properties of CDs / N-RCDs prepared from different precursors.

[0095]

[0096] The results showed that CDs prepared using citric acid and p-phenylenediamine as precursors had a slightly higher quantum yield than CDs prepared using p-phenylenediamine alone, but shorter excitation and emission wavelengths. CDs prepared using p-phenylenediamine and terephthalic acid had the highest quantum yield, but slightly shorter excitation and emission wavelengths than CDs prepared using p-phenylenediamine alone. Although the emission wavelength of CDs prepared using p-phenylenediamine and p-hydroxybenzoic acid was longer than that of CDs prepared using p-phenylenediamine alone, the quantum yield was lower. However, N-RCDs prepared using p-phenylenediamine and o-phenanthroline as precursors had not only longer excitation and emission wavelengths but also higher quantum yields compared to CDs prepared using p-phenylenediamine alone, and were red-light N-RCDs.

[0097] Characterization of N-RCDs prepared in Example 1

[0098] ① Scanning electron microscope

[0099] N-RCDs were ultrasonically dispersed and diluted, then evenly dropped onto a clean glass slide and dried at 60°C in an electric thermostatic drying oven. The dried N-RCDs sample was repeatedly picked up from one side of the conductive adhesive and attached to the other side of the sample stage using a scanning electron microscope.

[0100] ② Transmission electron microscopy

[0101] The N-RCDs were dissolved and diluted with ethanol, then dropped onto a copper mesh and tested using a transmission electron microscope.

[0102] ③ Atomic force microscope

[0103] After ultrasonic dispersion of N-RCDs, a few drops of dilute solution were dropped onto a clean silicon wafer surface using a capillary tube and dried in an electrically heated constant temperature drying oven. The wafer was then tested using an atomic force microscope.

[0104] ④X-ray diffraction

[0105] After N-RCDs were freeze-dried, the powder was tested using an X-ray powder diffractometer.

[0106] ⑤ X-ray photoelectron spectroscopy

[0107] After N-RCDs were freeze-dried, the powder was tested using an X-ray photoelectron spectrometer.

[0108] ⑥ Fourier transform infrared spectroscopy

[0109] After freeze-drying, the N-RCDs powder was mixed with spectrally pure KBr at a ratio of 1:100 and compressed into tablets. Fourier transform infrared spectroscopy was used to obtain images at 400-4000 cm⁻¹. -1 The infrared spectrum.

[0110] ⑦ Ultraviolet-Visible Spectrum

[0111] N-RCDs were ultrasonically dispersed in the appropriate solvent, and ultraviolet spectrophotometers were used to measure their ultraviolet spectra.

[0112] ⑧Fluorescence spectrum

[0113] N-RCDs were ultrasonically dispersed in the appropriate solvent, and fluorescence spectrometry was performed using a fluorescence spectrometer.

[0114] Figure 3 The image shows the SEM image of the N-RCDs. As can be seen from the image, the prepared N-RCDs have good dispersion and exhibit a small and uniform spherical structure. Figure 4 Image A in the middle is a TEM image of N-RCDs. It can be seen that N-RCDs are well dispersed and have a relatively uniform size distribution. Figure 4 Figure B shows the particle size distribution of N-RCDs, which are mainly distributed in the range of 1.5–7.5 nm, with an average particle size of 4.38 nm. AFM testing was performed to obtain the thickness of the N-RCDs. The results are as follows... Figure 5 As can be seen, its thickness is between 1 and 4 nm. The thickness of a typical single atomic layer is around 1 nm, indicating that the N-RCDs synthesized in this invention likely consist of approximately 1 to 4 atomic layers with good dispersion. The XRD measurement results of the N-RCDs are as follows: Figure 6 As shown in the figure, a broad peak appears centered at 24°, which is the (002) crystal plane of the graphite structure. The low intensity diffraction peak indicates that the carbon crystallization degree is low, so the prepared N-RCDs are amorphous carbon.

[0115] The surface composition and elemental chemical state of N-RCDs were characterized by XPS, and the results are as follows: Figure 7 As shown. Raw XPS data showed that the prepared N-RCDs contained three elements: C (63.89%), N (23.41%), and O (12.70%). In the XPS total spectrum ( Figure 7 (Figure A in the diagram) The signal peaks at 285.3 eV, 400.1 eV, and 531.7 eV correspond to C1s, N1s, and O1s, respectively. The high-resolution spectrum of C1s is shown in Figure A. Figure 7 As shown in Figure B, three distinct bonding peaks are observed at 284.7, 285.6, and 287.5 eV, attributed to the CC / C=C, CN / CO, and C=O components, respectively. The high-resolution N 1s spectrum is shown below. Figure 7 As shown in Figure C, the bonding energy peaks at 399.5, 400.3, and 401.8 eV correspond to the pyridine-type N, amino-type N, and pyrrole-type N components, respectively. The high-resolution spectrum of O 1s is shown below. Figure 7 As shown in Figure D, the bonding energy peaks at 531, 531.8, and 532.7 eV correspond to CO / OH, C=O, and O=CO groups, respectively.

[0116] The functional groups that may be present on the surface of N-RCDs were characterized by FT-IR, and the results are as follows: Figure 8 As shown. At 3411cm -1 The broad absorption peaks at 2925 and 1618 cm⁻¹ are due to the stretching vibrations of OH and NH. -1 The peak at 1506 cm⁻¹ originates from the stretching vibration of CH and C=O. -1 The peaks at 1416 and 1133 cm⁻¹ are attributed to the bending vibrations of NH₂. -1 The peaks correspond to the stretching vibration peaks of C=C and CN. The results indicate that the surface functional groups of N-RCDs mainly include amino and amide groups.

[0117] Figure 9 This is the UV-Vis absorption spectrum of N-RCDs in the wavelength range of 300-800 nm. It can be seen that three relatively obvious absorption peaks appear at 360 nm, 420 nm and 560 nm, which correspond to the np* transitions of C=O and C=N in the N-RCDs structure and the p-system transitions of aromatic compounds, respectively.

[0118] Study on the luminescence properties of N-RCDs prepared in Example 1

[0119] Figure 10The 3D fluorescence spectrum of N-RCDs shows that the obtained N-RCDs exhibit wavelength-independent photoluminescence behavior in the red light emission range (550–650 nm). As the excitation wavelength gradually increases, the maximum emission wavelength remains constant at 600 nm. The two-dimensional fluorescence spectrum of N-RCDs is also shown. Figure 11 Further analysis showed that within the excitation wavelength range of 320–580 nm, a maximum emission peak was observed at 600 nm; under 365 nm flashlight illumination, the N-RCDs solution exhibited bright red fluorescence. Figure 11 (Inner illustration). This wavelength-independent photoluminescence phenomenon may originate from the molecular state effect of surface groups on N-RCDs, that is, the precursor residues or the overall molecule on the surface of N-RCDs determine the luminescence properties of N-RCDs.

[0120] Study on the effect of mass concentration on luminescence properties of N-RCDs prepared in Example 1

[0121] The study found that the fluorescence spectra of N-RCDs are concentration-dependent. Figure 12 Figure A in the table shows the fluorescence spectra of N-RCDs at different concentrations with an excitation wavelength of 520 nm. It can be seen that as the concentration of N-RCDs gradually increases, the emission wavelength exhibits a certain red shift (see Table 2 for specific data). Figure 12 As shown in Figure B, the fluorescence intensity gradually increases as the concentration of N-RCDs increases from 0.018 mg / mL to 0.150 mg / mL; however, the fluorescence intensity decreases as the concentration increases from 0.150 mg / mL to 0.500 mg / mL. This phenomenon may be because the N-RCDs, doped with the more electronegative nitrogen, form electron-rich CDs. At lower concentrations, the fewer luminescent particles result in lower fluorescence intensity, which increases with increasing concentration. At higher concentrations, self-absorption reduces fluorescence intensity, and nanoparticle aggregation leads to electron system expansion and a narrowing of the band gap, causing a redshift in emission wavelength and a decrease in fluorescence intensity. Therefore, in subsequent quantitative application experiments, the concentration of N-RCDs should be controlled at 0.15 mg / mL or below.

[0122] Table 2. Emission intensity and wavelength of N-RCDs with different concentrations at an excitation wavelength of 520 nm

[0123]

[0124] Study on the photobleaching resistance of N-RCDs prepared in Example 1

[0125] Fluorescence sensing often requires prolonged sample irradiation, and resistance to photobleaching is a crucial indicator for the practical application of fluorescent probes. Therefore, the effect of irradiation time on the fluorescence properties of N-RCDs was investigated. A time scan was performed on the N-RCDs at their maximum excitation wavelength of 560 nm for a duration of 60 min. Figure 13 It can be seen that the fluorescence intensity of N-RCDs decreased by only 1.64%, which is not significant, indicating that the prepared N-RCDs are not easily bleached under long-term light exposure and have excellent photostability.

[0126] Acidity stability study of N-RCDs prepared in Example 1

[0127] Depend on Figure 14 It can be seen that when the pH is in the range of 3 to 6, the fluorescence stability of N-RCDs is good, with intensity changes within the range of -4.5%; while when the pH is in the range of 6 to 10, the fluorescence intensity decreases and shows a linear relationship with pH (F = 3564.8 - 182.9 [pH], R 2 =0.9785). This may be because the amino, pyrrole nitrogen, or pyridine nitrogen on the surface of N-RCDs undergoes deprotonation in an alkaline environment, resulting in a narrowing of the band gap between HOMO and LUMO, thus reducing the fluorescence intensity.

[0128] Salt tolerance study of N-RCDs prepared in Example 1

[0129] The effect of NaCl concentration on the fluorescence intensity of N-RCDs was studied, and the results are as follows: Figure 15 As shown, N-RCDs exhibit strong salt tolerance; when the NaCl concentration increases to 200 mM, their fluorescence intensity remains within the allowable range of ±10% error, showing no significant change.

[0130] Study on the effects of metal ions, anions, and amino acids on N-RCDs prepared in Example 1

[0131] Study 5×10 -4 M's Al 3+ Ca 2+ Co 2+ Cu 2+ Fe 3+ K + Mg 2+ Mn 2+ Na + Zn 2+ Pb 2+ Ba 2+ The effects of 12 metal ions on the fluorescence intensity of N-RCDs. Figure 16 The display shows that Cu 2+ and Fe 3+Both metal ions significantly quenched the fluorescence of N-RCDs. Cu 2+ The maximum quenching degree of N-RCD fluorescence was 86%, Fe 3+ The quenching degree was 53%, while other metal ions were not significantly affected. This may be due to the presence of precursor p-phenylenediamine residues on the surface of N-RCDs, which interact with Cu. 2+ and Fe 3+ Energy transfer occurs after coordination, leading to a decrease in fluorescence intensity.

[0132] CH3COO was investigated - PO4 3- S 2- SO4 2- S2O3 2- I - ,ClO - NO2 - HPO4 2- H2PO4 - CO3 2- HCO3 - The effects of 12 anions on the fluorescence intensity of N-RCDs were investigated. The results are as follows: Figure 17 As shown, within a ±10% error tolerance range, none of the anions had a significant effect on the fluorescence of N-RCDs. The fluorescence of N-RCDs at a concentration of 1×10⁻⁶ was investigated. -3 The effects of 12 amino acids at mol / L on the fluorescence intensity of N-RCDs are shown in the following results. Figure 18 As shown, within the allowable error range of ±10%, all the amino acids studied had no significant effect on the fluorescence intensity of N-RCDs.

[0133] Study on solvent polarity effect and detection of low concentration water in organic solvents of N-RCDs prepared in Example 1

[0134] Experimental methods

[0135] (1) Fluorescence properties of N-RCDs in a single solvent system

[0136] N-RCDs were dispersed in eight single solvents—1,4-dioxane, toluene, chloroform, ethyl acetate, acetone, ethanol, dimethyl sulfoxide (DMSO), and water (H2O)—at a concentration of 0.1 mg / mL. The fluorescence spectrum, absorption spectrum, and relative quantum yield of each solution were measured, and the polarity parameter—directional polarizability Δf—of each solvent was calculated according to Equation 1-2.

[0137]

[0138] Where ε is the dielectric constant of the solvent, and n is the refractive index of the solvent. Δf is the result of the combined effect of the electron mobility in the solvent and the dipole moment of the solvent molecules, explaining the influence of the new directional rearrangement of solvent molecules on the spectral properties of fluorescent substances.

[0139] (2) Determination of fluorescence properties and polarity of N-RCDs in different aqueous binary solvent systems

[0140] N-RCDs were dispersed in binary solvents containing water, 1,4-Dioxane, EtOH, and DMSO, respectively. The solvent-to-water ratios in these binary mixtures were 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, and 10:0, respectively, with an N-RCD concentration of 0.15 mg / mL. The fluorescence spectra of each solution were measured using appropriate excitation wavelengths, and the maximum fluorescence intensity value F was recorded. The polarity parameter Δf of each solution was calculated using Equation 1-2. The polarity of the test solution was determined based on the relationship curve between F and Δf.

[0141] (3) Method for determining low concentrations of water in organic solvents

[0142] A series of N-RCDs (final concentration 0.15 mg / mL) solutions in ultra-dry methanol (MeOH), anhydrous EtOH, ultra-dry DMF, and ultra-dry DMSO with different water contents were prepared. Fluorescence spectra were scanned at appropriate excitation wavelengths, and the maximum fluorescence value was recorded. The fluorescence intensity of N-RCDs in pure solvents was F0, and the fluorescence intensity after adding water was F. The water content of a given solvent was determined based on the relationship curve between (F0-F) / F0 and water content.

[0143] Experimental results

[0144] Polarity effect of N-RCD luminescence in a single solvent

[0145] Figure 19 Table 3 shows the fluorescence performance test results of N-RCDs in eight single solvents. Figure A shows the normalized fluorescence spectra in different solvents, Figure B shows the absorption spectra, and Figure C shows the visualization in the liquid phase. It can be seen that the luminescence performance of the prepared N-RCDs is significantly correlated with solvent polarity. As Δf increases from 0.0133 to 0.3211, there is an overall trend of redshift in absorption and emission wavelengths and a decrease in relative quantum yield. This phenomenon provides the possibility for using N-RCDs to determine the polarity of solvent systems with a wide polarity range.

[0146] Table 3 Absorption wavelength, emission wavelength, and quantum yield of N-RCDs in different solvents

[0147]

[0148] Study on the polar effect of N-RCD luminescence in aqueous binary solvent systems

[0149] Figure 20 The UV-Vis absorption spectra of N-RCDs in single solvents DMSO and EtOH show three absorption peaks around 350–360 nm, 400–420 nm, and 500–560 nm in both solvents. The fluorescence spectra of N-RCDs in binary mixed solvents DMSO / H₂O and EtOH / H₂O were also measured at excitation wavelengths of 360 nm, 420 nm, and 540 nm. Figure 21 It can be seen that when N-RCDs are dispersed in two binary mixed solvents, DMSO / H2O and EtOH / H2O, different fluorescence spectra will appear under different excitation wavelengths. Figure 21 Figure A and Figure 21 Figure B shows that when the excitation wavelength is 360 nm, N-RCDs have two strong emission peaks at 470 nm and 600 nm in the DMSO / H2O system, while they have two weaker emission peaks at 405 nm and 600 nm in the EtOH / H2O system. Figure 21 Figure C and Figure 21 Figure D shows that when the excitation wavelength is 420 nm, N-RCDs exhibit a strong emission peak at 500 nm and a weaker emission peak at 600 nm in the DMSO / H2O system, and a broad emission peak in the 500–600 nm range in the EtOH / H2O system. Figure 21 Figure E and Figure 21 Figure F shows that when the excitation wavelength is 540 nm, N-RCDs exhibit strong emission peaks with similar wavelengths in both DMSO / H2O and EtOH / H2O, at 600 nm and 590 nm, respectively. Furthermore, in binary solvents composed of DMSO and EtOH / H2O, with increasing water content (i.e., increased polarity), N-RCDs show a red shift in emission peak positions and a decrease in fluorescence intensity at all three excitation wavelengths.

[0150] Relationship between luminescence intensity of N-RCDs and solvent polarity and water content

[0151] (1) 1,4-Dioxane / H2O mixed system

[0152] The Δf of 1,4-Dioxane is 0.0205, with the largest Δf range (0.228–0.316) obtained when mixed with water. The polarity sensing performance of N-RCDs was tested under optimal experimental conditions. Figure 22As shown, the fluorescence intensity of N-RCDs gradually decreases with increasing water content in the system. The inset shows the linear relationship between fluorescence intensity and water content in the system within the range of 10% to 90%, with the regression equation being F = -17.45[H2O] + 1714(R). 2 =0.9861). The relationship between fluorescence intensity and Δf in the range of 0.228 to 0.316 is shown in the figure. Figure 23 The fitted equation is F = -7.949exp([Δf] / 0.0568) + 2157(R). 2 =0.9956).

[0153] (2) EtOH / H2O mixed system

[0154] EtOH is a commonly used protic solvent in laboratories and is miscible with water in any proportion. Figure 24 Figure A shows the fluorescence spectrum of N-RCDs in the EtOH / H2O binary mixture system. The inset shows the linear relationship between fluorescence intensity and water content in the range of 10% to 90%, with the regression equation being F = -44.89[H2O] + 3825(R). 2 =0.9805). Fluorescence intensity and Δf showed a good linear correlation in the range of 0.295–0.319, as shown in the linear relationship curve. Figure 24 In Figure B, the regression equation is F = -1.568E5Δf + 5.016E4(R). 2 =0.9907).

[0155] (3) DMSO / H2O mixed system

[0156] Figure 25 Figure A shows the fluorescence spectrum of N-RCDs in the DMSO / H2O binary mixture. The inset shows the linear relationship between fluorescence intensity and water content in the range of 10%–90%, with the regression equation being F = -61.65[H2O] + 5695(R). 2 =0.9966). Fluorescence intensity and Δf showed a good linear correlation in the range of 0.269–0.315, as shown in the linear relationship curve. Figure 25 Figure B shows the regression equation as F = -1.072E5Δf + 3.392E4(R). 2 =0.9964).

[0157] Detection of low concentrations of water in organic solvents

[0158] Water is considered an impurity in organic solvents in some cases. In most organic reactions, even low concentrations of water can cause catastrophic reactions, or even explosions and fires. Therefore, detecting low concentrations of water in organic solvents is essential. N-RCDs were used to detect low concentrations of water in four commonly used organic solvents: MeOH, EtOH, DMF, and DMSO.

[0159] (1) Detection of low concentration water in MeOH

[0160] like Figure 27 As shown, (F0–F) / F0 exhibits a linear relationship with water content in the range of 0.100 v% to 10.0 v% and the regression equation is (F0-F) / F0 = 0.0136 + 0.0173 [H2O](R 2 =0.9789). Eleven parallel experiments were performed on MeOH with a water content of 0.100 v%, and the relative standard deviation (RSD) was 1.1%. The limit of detection for water in MeOH was calculated to be 0.032 v% according to the method recommended by IUPAC.

[0161] (2) Detection of low concentration water in EtOH

[0162] like Figure 28 As shown in Figure B, (F0–F) / F0 exhibits a linear relationship with the water content in EtOH within the range of 0.100 v% to 6.00 v% , with the regression equation being (F0–F) / F0 = 0.0883 + 0.0453 [H2O](R 2 =0.9795). Eleven parallel experiments were performed on EtOH with a water content of 0.100 v%, and the relative standard deviation (RSD) was 2.6%. The limit of detection for water in EtOH was calculated to be 0.088 v% according to the method recommended by IUPAC.

[0163] (3) Detection of low concentration water in DMF

[0164] like Figure 29 As shown in B, (F0-F) / F0 exhibits a linear relationship with the water content in DMF within the range of 0.100 v% to 10.0 v% , with the regression equation being (F0-F) / F0 = 0.0846 + 0.0351 [H2O](R 2 =0.9957). Eleven parallel experiments were conducted on DMF with a water content of 0.100 v%, and the relative standard deviation (RSD) was 4.1%. The limit of detection for water in DMF was calculated to be 0.082 v% according to the method recommended by IUPAC.

[0165] (4) Detection of low concentration water in DMSO

[0166] like Figure 30As shown in Figure B, (F0-F) / F0 shows a linear relationship with the water content in DMF within the range of 0.100 v% to 10.0 v% and the regression equation is (F0-F) / F0 = 0.0137 + 0.0402[H2O](R 2 =0.9956). Eleven parallel experiments were conducted on DMSO with a water content of 0.100 v%, and the relative standard deviation (RSD) was 3.7%. The limit of detection for water in DMSO was calculated to be 0.056 v% according to the method recommended by IUPAC.

[0167] The specific analytical performance of N-RCDs in determining low concentrations of MeOH, EtOH, DMF, and DMSO in water is summarized in Table 4.

[0168] Table 4 Analytical performance of N-RCDs in determining low concentrations of MeOH, EtOH, DMF, and DMSO in water.

[0169]

[0170] The N-RCDs prepared in Example 1 were applied to the fabrication of paper-based polarity sensors;

[0171] N-RCDs stock solution was prepared using the N-RCDs prepared in Example 1; N-RCDs stock solution (1.5 mg / mL): 15 mg of N-RCDs solid was dispersed in 10 mL of the corresponding organic solvent, stored in a sealed container at 4 °C, and diluted to the required concentration before use.

[0172] Experimental instruments

[0173] The 365nm flashlight was purchased from the Qianxi brand's self-operated store on Taobao.

[0174] The smartphone (iPhone Xs Max) was purchased from Apple Electronics Trading (Beijing) Co., Ltd. The "Color" app for the phone was supplied by WizEyes Tech Inc., version number 2.6.8.

[0175] The qualitative filter paper was purchased from Dalian Kaimi Chemical Engineering Supporting Co., Ltd.

[0176] All solvents used in the experiment were ultra-dry solvents; the water used in the experiment was double-distilled water.

[0177] Application examples

[0178] Fabrication of paper-based sensors

[0179] Generally, qualitative filter paper is cut into circular filter paper pieces with a diameter of 6 mm using a punch. After soaking in an aqueous solution of N-RCDs prepared in Example 1 at a concentration of 0.02–0.5 mg / mL for 20 seconds, it is dried in an oven at 60°C to obtain light red test paper. This paper should be stored in a dry, dark place and thoroughly dried before use.

[0180] Semi-quantitative and quantitative studies of organic solvent polarity and water content using paper-based sensors

[0181] Figure 31 The images show photographs of dry test strips prepared before and after soaking with different concentrations of N-RCDs under three testing environments. It can be seen that test strips of the same concentration exhibit uniform color under different testing environments; the color intensity of test strips with different concentrations under the same testing conditions is concentration-dependent, and concentrations above 0.1 mg / mL completely cover the original white color (blue fluorescence) of the test strip; the test strip exhibits the most obvious fluorescence color when illuminated with a 365 nm flashlight in a dark room. Therefore, it can be concluded that soaking with N-RCDs at concentrations above 0.1 mg / mL is a simple and effective method for preparing test strips, and the colors are clearer and brighter when photographed under a 365 nm flashlight in a dark room.

[0182] To further determine the effectiveness of the test strip soaking concentration and the distinguishing effect of the shooting environment on the solvent, a certain proportion of mixed EtOH / H2O solution was dropped onto each of the above groups of test strips, and photographs were taken under three different lighting conditions, such as... Figure 32 It can be seen that the digital photos obtained under 365nm flashlight illumination in a dark room have the brightest and most vivid colors, and the test strips prepared with 0.1mg / mL N-RCDs show the most obvious color differentiation between different test solutions.

[0183] In summary, subsequent experiments used N-RCDs aqueous solution with a concentration of 0.1 mg / mL (optimal) to prepare paper-based sensors, and photographs were taken in a dark room using a 365 nm flashlight to visualize and distinguish the polarity and type of the test solution.

[0184] The prepared N-RCDs test paper was used to visually distinguish solvents of different polarities and to perform semi-quantitative analysis of polarity.

[0185] Figure 33Digital photos were taken with a mobile phone after adding one drop (0.04 mL) of 1,4-dioxane, toluene, ethyl acetate, chloroform, acetone, ethanol, dimethyl sulfoxide, and water sequentially to N-RCDs test paper and then illuminating the paper with a 365 nm flashlight at different times. It can be seen that as the solvent polarity increases, the color of the N-RCDs test paper gradually turns redder, and the brightness decreases. This phenomenon is basically consistent with the fluorescence spectra of N-RCDs in the above solvents under liquid phase conditions. Furthermore, the color of the test paper changes to varying degrees over time after adding the reagents. At 2 minutes, the color of the test paper is similar to that at 30 seconds, and the differentiation between the added solvents is still relatively high. After 5 minutes, the more volatile organic solvents such as chloroform, ethyl acetate, and acetone have almost completely evaporated, and the test paper returns to its dry color. After 10 minutes, almost all solvents except DMSO and H2O have evaporated. After 20 minutes, only the hygroscopic DMSO remains. The above phenomena suggest that N-RCDs test strips can be used to visually distinguish solvents of different polarities. When using them, the appropriate color observation time should be selected according to the volatility of the test solution.

[0186] To achieve semi-quantitative analysis of polarity using N-RCDs test strips, Figure 33 The colors of each test strip were extracted using the "Color Picker" app on a mobile phone after the solvent was added dropwise for 30 seconds. The polarity parameter Δf of the solvent used was listed below the corresponding extracted color, and a sample was prepared as follows: Figure 34 The solvent polarity and N-RCDs test paper colorimetric card are shown. N-RCDs test paper can provide semi-quantitative analysis of solvent polarity within a relatively large range of Δf = 0.0133 to 0.3211. It should be noted that, generally, DMSO has a higher polarity than EtOH and acetone, but a lower polarity than water. However, the Δf value calculated using Equation 1-3 is lower than that of acetone. This is because the factors determining solvent polarity are generally complex, and the calculation of DMSO's Δf only considers the interaction between the solute and solvent, ignoring other interactions, but this does not affect the semi-quantitative determination of polarity. Furthermore, although the Δf of 1,4-dioxane is greater than that of toluene, N-RCDs emit a deep green fluorescence at a wavelength of 549 nm in 1,4-dioxane, which is an anomaly.

[0187] To evaluate the accuracy of the semi-quantitative colorimetric card, a solution of DMSO and water in an 8:2 ratio (Δf = 0.2747) was dropped onto the test paper and compared with the color of the colorimetric card. It can be seen that the color of the test paper is within the corresponding Δf range (0.2636 to 0.3211) of the colorimetric card, proving that the colorimetric card and test paper can visualize the polarity of the semi-quantitative sample.

[0188] The application of N-RCDs polar test paper prepared using the application example in the detection of water content in organic solvents.

[0189] A series of binary organic solvents, DMSO and EtOH, with different water contents were tested using N-RCDs test strips. One drop (0.04 mL) of solvent / water mixtures with ratios of pure solvent, 9:1, 8:2, 7:3…1:9 were sequentially added to two rows of uniformly arranged N-RCDs test strips, and digital photos were taken under different conditions. The RGB data of the test strip colors after solvent addition were obtained using the "ColorCare" app on a mobile phone, and the water content of the target solvent was quantitatively analyzed using the R / G value.

[0190] (1) Response of N-RCDs test paper to EtOH / H2O mixed system

[0191] Figure 35 Under dark conditions and illumination with a 365nm flashlight, N-RCDs polar test papers with appropriate EtOH / H2O mixed solvents were added. It can be observed that as the water content in EtOH increases, the color of the N-RCDs test paper gradually changes from orange-red to brown, and the color brightness decreases. This is consistent with the fluorescence spectra of N-RCDs tested in EtOH / H2O systems with different mixing ratios: as the water ratio in the system increases, the polarity of the solvent increases, the fluorescence intensity of the N-RCDs decreases, and the emission wavelength redshifts.

[0192] The colors of the corresponding N-RCDs test strips were extracted using the "Color" app on a mobile phone, and a precise colorimetric card with a Δf range of 0.2889 to 0.3211 was created. The colors of each test strip were converted into detailed RGB color data, which are listed in Table 5. The R / G value shows a linear relationship with the water content in the EtOH / H2O mixture from 0% to 100%, with the regression equation being [R / G] = 1.781 - 4.758E⁻³[H₂O](R 2 =0.9739), such as Figure 36 As shown.

[0193] Table 5 shows the RGB data of the response of N-RCDs test paper to the EtOH / H2O mixed system.

[0194]

[0195] (2) Response of N-RCDs test paper to DMSO / H2O mixture

[0196] Figure 37 N-RCDs test papers were prepared under dark conditions and 365nm flashlight illumination, with appropriate DMSO / H2O mixed solvents added. It can be seen that as the water content in DMSO increases, the color of the N-RCDs test paper gradually changes from dark red to light brown, and the color brightness decreases. This is consistent with the fluorescence spectra of N-RCDs tested in DMSO / H2O systems with different mixing ratios.

[0197] The colors of the corresponding N-RCDs test strips were extracted using the "ColorCare" app on a smartphone, and a precise colorimetric card with a Δf range of 0.2646 to 0.3211 was created. The colors of each test strip were converted into detailed RGB color data, which are listed in Table 6. It was found that the R / G value has a linear relationship with the water content in the DMSO / H2O mixture from 0% to 100%, with the regression equation being [R / G] = 1.839 - 5.186E-3[H2O](R 2 =0.9935), such as Figure 38 As shown.

[0198] Table 6 shows the RGB data of the response of N-RCDs test paper to the DMSO / H2O mixture.

[0199]

[0200] In addition, the reusability of used N-RCDs test strips was tested the day after the test solution had completely evaporated. Figure 39 The test results were from the following day. Figure 39 Figure A shows the polarity sensing of the EtOH / H2O system by N-RCDs test paper. Figure 39 Figure B shows the polarity sensing of the DMSO / H2O system by N-RCDs test paper, and... Figure 35 and Figure 37 The results are largely consistent with those of the present invention. It can be seen that the N-RCDs test strip of this invention exhibits good repeatability every other day and excellent experimental reproducibility, indicating its potential for practical application.

[0201] The embodiments described above are merely preferred embodiments of the present invention, and not all feasible embodiments of the present invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A nitrogen-doped red carbon dot N-RCD, characterized in that, Nitrogen-doped red carbon dots (N-RCDs) sensitive to solvent polarity were prepared by a one-step solvothermal method using p-phenylenediamine and o-phenanthroline as precursors. The one-step solvothermal method used N,N-dimethylformamide as the reaction solvent. The N-RCDs had an average particle size of 4.38 nm, a nitrogen content of 23.41%, and an emission wavelength of 600 nm. The emission wavelength red-shifted with increasing solvent polarity parameter Δf, and the fluorescence intensity decreased with increasing solvent polarity parameter Δf. The changes in emission wavelength and fluorescence intensity were attributed to the solvation stabilization effect of the intramolecular charge-transfer luminescent centers. The preparation method of N-RCDs is as follows: 0.0437 g of p-phenylenediamine is weighed into 10 mL of DMF, and the precursor o-phenanthroline is added. The molar ratio of p-phenylenediamine to the precursor is 1:

2. After ultrasonic stirring and dissolution, the mixture is transferred to a 25 mL polytetrafluoroethylene-lined reactor, placed in a forced-air drying oven, and reacted at 200 °C for 12 h. After cooling to room temperature, a dark red liquid is obtained. The dark red liquid is passed through an aqueous filter membrane, the filtrate is collected, and dialyzed with a dialysis bag with a molecular weight cutoff of 1000 Da to remove unreacted precursor molecules. The dialysate is freeze-dried to obtain black powder N-RCDs, which are then sealed and stored at 4 °C for later use.

2. The application of nitrogen-doped red carbon dots (N-RCDs) as described in claim 1 in the fabrication of fluorescent paper-based sensors.

3. A fluorescent paper-based sensor prepared using nitrogen-doped red carbon dots (N-RCDs) as described in claim 1, characterized in that the fluorescence... The preparation method of the paper-based sensor is as follows: select general qualitative filter paper as the substrate, cut it into circular paper pieces with a diameter of 6~10 mm using a punch, soak it in 0.02~0.5 mg / mL N-RCDs aqueous dispersion for 20 s, and dry it in an oven at 60 ℃ to obtain a polar test paper type fluorescent paper-based sensor, which is stored in a dark and dry place.

4. The fluorescent polar colorimetric card prepared from the fluorescent paper-based sensor as described in claim 3, characterized in that, A solvent with a known polarity parameter Δƒ was dropped onto the test paper and irradiated at 365 nm in a dark room. The color of each test paper was extracted after 30 s using the "Color Pick" app on a mobile phone. The polarity parameter Δƒ of the solvent used was listed below the corresponding extracted color to create a fluorescent polar colorimetric card with Δƒ in the range of 0.0133 to 0.3211.

5. The application of the fluorescent paper-based sensor as described in claim 3 in solution polarity measurement and low-concentration water detection in organic solvents.

6. The application of the fluorescent polar colorimetric card as described in claim 4 in the measurement of solution polarity and the detection of low concentrations of water in organic solvents.

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