Process for the electrochemical synthesis of non-conjugated fluorescent polymer dots

Abstract

The application relates to a preparation method of electrochemical synthesis of non-conjugated fluorescent polymer dots, which comprises the following steps: fluorescent polymer dot preparation: citric acid and ethylenediamine are dissolved in deionized water, stirred uniformly to obtain an electrolyte, an electrode is inserted into the electrolyte, direct current is introduced into the electrolyte to perform electrochemical reaction, and fluorescent polymer dots are obtained after the electrochemical reaction is completed. The electrolyte is filtered, then separated and purified, and then the purified aqueous solution is freeze-dried to remove the solvent, so that fluorescent polymer dot powder is obtained. The prepared non-conjugated fluorescent polymer dot powder is simple in operation, low in price, high in stability, friendly to the environment, and has excellent fluorescent properties. The electrochemical synthesis method has the advantages of low synthesis temperature, simple process, safety and environmental protection. The nitrogen-containing and oxygen-containing groups on the surface of the non-conjugated fluorescent polymer dots can react with heavy metal ions, so that efficient and sensitive detection of the heavy metal ions is realized.

Description

Technical Field

[0001] This invention relates to a method for preparing non-conjugated fluorescent polymer dots by electrochemical synthesis, belonging to the field of fluorescent nanomaterial preparation technology. Background Technology

[0002] Polymer dots are a class of organic fluorescent materials with typical spherical nanoparticle sizes, mainly including conjugated polymer dots and non-conjugated polymer dots (NCPDs). Conjugated polymer dots typically have a π-conjugated structure, while NCPDs do not contain aromatic groups and are usually compounds containing electron-rich atoms (N, O, etc.) or electron-rich groups (-C=O, -COOH, -NH2, etc.). These electron-rich groups can absorb ultraviolet or visible light and are chemical groups with weak fluorescence, called chromophores. Many conjugated polymer dots have low water solubility and their fluorescence gradually quenches with increasing concentration. Compared with conjugated polymer dots, NCPDs have the advantages of good water solubility, low biotoxicity, and simple synthesis. Because non-conjugated NCPDs are rich in highly electronegative atoms, they can often coordinate with metal ions or other small molecules, changing their aggregation state and thus their emission state, exhibiting a "turn-on" or "turn-off" fluorescence phenomenon. Therefore, they can be used for the specific detection of metal ions or small molecules. Therefore, the unique polymer structure and photoluminescence properties of NCPDs make them promising emerging fluorescent nanomaterials with broad application prospects in fields such as chemical sensing and bioimaging.

[0003] Currently, the preparation methods for NCPDs are mainly hydrothermal, covalent cross-linking, and self-assembly. Electrochemical synthesis of NCPDs has never been reported. Compared with other methods, electrochemical synthesis avoids high-temperature and high-pressure synthesis, and has the advantages of low synthesis temperature, simple process, and safety and environmental friendliness. Therefore, this invention proposes a method for preparing non-conjugated fluorescent polymer dots by electrochemical synthesis. The fluorescence of the non-conjugated polymer dots prepared by this method originates from electron-rich groups (-C=O, -COOH, -NH2, etc.). Due to the cross-linking of the polymer formed by the electrochemical reaction, strong intramolecular / intermolecular interactions are formed between molecules, and the molecular conformation is rigidified. This suppresses the vibration and rotation of NCPDs in solid powder compared to liquid, reducing the nonradiative rate, thus enhancing the fluorescence in the solid state compared to the liquid state. Simultaneously, the nitrogen- and oxygen-containing groups on the surface of NCPDs can interact with Fe... 3+ Ionic reactions to achieve Fe 3+ Highly efficient and sensitive detection of ions. Summary of the Invention

[0004] Objective of the Invention: Aiming at the above-mentioned existing problems and deficiencies, the objective of the present invention is to provide a preparation method for electrochemically synthesizing non-conjugated fluorescent polymer dots. The prepared non-conjugated fluorescent polymer dot (NCPD) powder is simple to operate, low in price, high in stability, environmentally friendly, and has excellent fluorescence properties.

[0005] Technical Solution: To achieve the above-mentioned objective of the invention, the present invention adopts the following technical solutions: A preparation method for electrochemically synthesizing non-conjugated fluorescent polymer dots, comprising the following steps: Step 1: Preparation of Fluorescent Polymer Dots: Dissolve citric acid and ethylenediamine in deionized water, stir evenly to obtain an electrolyte solution, insert electrodes into the electrolyte solution, and pass direct current into the electrolyte solution for an electrochemical reaction. After completion, fluorescent polymer dots are obtained. Step 2: Separation and Purification of Polymer Dots: Filter the electrolyte solution in Step 1, then separate and purify it, and then freeze-dry the purified aqueous solution to remove the solvent to obtain fluorescent polymer dot powder.

[0006] Further, the specific steps of Step 1 are as follows: Step 1.1: Weigh citric acid and ethylenediamine in a molar ratio of 1:X (0 < X ≤ 1), and dissolve them in deionized water according to a mass ratio of 1:(40 - 70) based on the sum of the two solutes of citric acid and ethylenediamine, and mix and stir evenly to obtain a clear and transparent electrolyte solution. Step 1.2: Insert two platinum electrode plates as working electrodes into the electrolyte solution obtained in Step 1.1, pass 10 - 25V direct current into the electrolyte solution, and carry out an electrochemical reaction for 0.5 - 2h. After the reaction is completed, fluorescent polymer dots are obtained.

[0007] Further, in Step 2, a filter membrane with a pore size of 0.20 - 0.22 microns is used to filter the electrolyte solution obtained in Step 1.2, and a dialysis bag with a molecular weight of 500 - 1000 is used for separation and purification.

[0008] Further, the freeze-drying temperature in Step 2 is -80 to -10°C, and the pressure is below 10 Pa.

[0009] Advantageous Effects: Compared with the prior art, the present invention has the following advantages: The prepared non-conjugated fluorescent polymer dot (NCPD) powder is simple to operate, low in price, high in stability, environmentally friendly, and has excellent fluorescence properties.

[0010] Compared with current NCPD preparation technologies, electrochemical synthesis offers advantages such as lower synthesis temperature, simpler process, and greater safety and environmental friendliness. Furthermore, non-conjugated fluorescent polymer dots (NCPDs) powder is readily soluble in water and can be formulated into solutions, exhibiting blue fluorescence in both aqueous and solid states. The nitrogen- and oxygen-containing groups on the surface of NCPDs can react with heavy metal ions, enabling efficient and sensitive detection of these ions. Attached Figure Description

[0011] Figure 1 These are transmission electron microscope images of the fluorescent polymer dots NCPDs1 prepared in Example 1 of this invention. Figure 2 This is the infrared spectrum of the fluorescent polymer dot NCPDs1 prepared in Example 1 of the present invention. In the image: 3462 cm -1 The vibration corresponding to the OH bond at 3000 cm -1 The stretching vibration corresponding to the CH bond at 1713 cm⁻¹ -1 and 1640 cm -1 The vibration corresponding to the C=O bond is 1570 cm. -1 The corresponding bending vibration of NH is 1386 cm. -1 The vibration corresponds to the CN bond at that location; Figure 3 This is the 13C nuclear magnetic resonance spectrum of the fluorescent polymer dot NCPDs1 prepared in Example 1 of the present invention; Figure 4 This is the spectrum of the aqueous solution of the fluorescent polymer dot NCPDs1 prepared in Example 1 of the present invention. In the picture: Figure 4 (a) — Ultraviolet absorption spectrum, Figure 4 (b) — Photoluminescence spectra under excitation light of different wavelengths; Figure 5 These are photoluminescence spectra of the solid powder of fluorescent polymer dot NCPDs1 prepared in Example 1 of the present invention under excitation light of different wavelengths; Figure 6 This is a photograph of the fluorescent polymer dots NCPDs1 prepared in Example 1 of this invention under a UV lamp. In the picture: Figure 6 (c) — Solid powder, Figure 6 (d) — Aqueous solution; Figure 7 This is the spectrum of the fluorescent polymer dot NCPDs3 prepared in Example 1 of the present invention. In the picture: Figure 7 (e)—at 1 mM Ni 2+ Cu 2+ Fe3+ Photoluminescence spectrum in solution, Figure 7 (f) — The relative fluorescence intensity ratio in aqueous solution of heavy metal ions, where I represents the fluorescence intensity in the presence of heavy metal ions and I0 represents the fluorescence intensity in the absence of heavy metal ions. Implementation

[0012] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims. Example

[0013] 192 mg of citric acid and 60 μL of ethylenediamine (molar ratio 1:1) were weighed and dissolved in 10 mL of deionized water. The resulting mixture was stirred thoroughly to form the electrolyte. Two equal-sized platinum plates (2 cm × 2 cm) were used as the working electrode and the counter electrode, respectively, and immersed in the electrolyte, with a distance of 3 cm between the two platinum plates. A Keithley 2400 Sourcemeter instrument provided the voltage for the electrochemical reaction, which was 10 V, and the reaction time was 1 hour. The electrolyte after the reaction was filtered through a 0.22 μm aqueous filter to remove insoluble impurities. Then, the polymer dots were separated and purified using a dialysis bag with a molecular weight of 500-1000 for 48 hours. Finally, the solution in the dialysis bag was freeze-dried (at -80 °C) to completely remove the aqueous solution, yielding a dry polymer dot powder, named NCPDs1.

[0014] Figure 1 The morphology of the polymer dots is shown, and they are quasi-spherical. The particle size statistics show that the average size is 4.5 nm, and the size distribution is 2–7 nm. Figure 2 Infrared spectroscopy confirmed the presence of abundant functional groups at the polymer dots, including OH, CH, C=O, NH, and CN. Figure 3 As shown, nuclear magnetic resonance spectroscopy verified the presence of the aforementioned functional groups and revealed the molecular structure of the polymer dots. N-(C)3 corresponds to the tertiary amine structure in the branched polyethyleneimine formed by the polymerization of ethylenediamine. The R-COO-R ester group and O-CH2 ether group demonstrate that citric acid undergoes a decarboxylation copolymerization reaction via a free radical mechanism under electrolytic conditions, and is grafted onto the polymerization product of the aforementioned ethylenediamine. Figure 4 (a) is the UV absorption spectrum of the polymer spot aqueous solution, with the strongest absorption peak at 330 nm; Figure 4 (b) The emission spectrum shows obvious excitation-dependent fluorescence characteristics, with the strongest emission peak at 425 nm, which is excited by a wavelength of 330 nm; under this excitation light, its absolute quantum yield is 0.86%. Figure 5The emission spectrum of the polymer dot solid powder is shown. The strongest fluorescence is excited at a wavelength of 390 nm, with a blue fluorescence peak at 485 nm and an absolute quantum yield of 2.8%. The solid-state fluorescence quantum yield is higher than that of the liquid-state fluorescence quantum yield because the non-conjugated polymer dots lack π-conjugated structures, restricting molecular rotation in the solid state, resulting in enhanced fluorescence compared to liquid fluorescence. Therefore, both the aqueous solution and solid powder of the polymer dots prepared in this invention emit blue fluorescence under ultraviolet light excitation, as shown in the figure. Figure 6 (c)~ Figure 6 As shown in (d). Example

[0015] Based on the polymer dots NCPDs obtained in Example 1, and keeping other conditions unchanged, the voltage of the electrochemical reaction was varied to 15V, 20V, and 25V, resulting in NCPDs with different degrees of electrochemical polymerization and crosslinking, named NCPDs2, NCPDs3, and NCPDs4, respectively. The absolute quantum yields of these polymer dots in low-concentration aqueous solutions (3 mg / ml) were 1.83%, 1.46%, and 0.62%, respectively, while their absolute quantum yields in solid powder were 3.3%, 1.5%, and 0.62%, respectively. Therefore, the performance at 10V was optimal in Example 1, and the performance at 15V was optimal in Example 2.

[0016] Comparative Example 1: 192 mg of triglyceride (instead of citric acid) was weighed and dissolved in 10 mL of deionized water along with 60 μL of ethylenediamine (molar ratio 1:1), keeping other conditions unchanged. The resulting mixture was stirred thoroughly and used as the electrolyte. Two equal-sized platinum plates (2 cm × 2 cm) were used as the working and counter electrodes, respectively, and immersed in the electrolyte, with a distance of 3 cm between them. A Keithley 2400 Sourcemeter instrument provided a voltage of 10 V for the electrochemical reaction. The electrochemical reaction was not initiated because the final product could not be synthesized. The two chemical reaction equations are as follows:

[0017] The electrochemical polymerization experiment can only be initiated by the presence of hydroxyl groups in citric acid; without hydroxyl groups, electrochemical polymerization cannot be initiated, and therefore the target substance cannot be obtained.

[0018] Comparative Example 2: Weigh 192 mg of citric acid and 240 μL of ethylenediamine (molar ratio 1:4), dissolve in 10 mL of deionized water, and stir the resulting mixture thoroughly to use as the electrolyte. React at 15 V for 1 hour. Other procedures are the same as in Example 1. Two equal-sized platinum plates (2 cm × 2 cm) are used as the working electrode and counter electrode, respectively, immersed in the electrolyte, with a distance of 3 cm between the two platinum plates. A Keithley 2400 Sourcemeter instrument provides the voltage for the electrochemical reaction.

[0019] However, due to the excess of ethylenediamine, the product formed is mainly polyethyleneimine (PEI) rather than polymer nanodots. The excess ethylenediamine forms polymer macromolecules, which lose their fluorescent properties. Therefore, the target product cannot be obtained.

[0020] Comparative Example 3: The molar ratio of citric acid to ethylenediamine was less than 1:1, and other conditions were the same as in Example 1. Due to the higher content of citric acid and lower content of ethylenediamine, fewer polymer dots were formed, resulting in a lower yield that did not meet production requirements.

[0021] Therefore, the ratio of citric acid to ethylenediamine in this invention can yield the most non-conjugated fluorescent polymers.

[0022] Fluorescent probe detection: The polymer dots NCPDs3 obtained in Example 2 were prepared into a 1 mg / mL aqueous solution and used as a fluorescent probe to detect heavy metal ions Fe. 3+ The existence of. Figure 7 (e)~ Figure 7 (f) The results showed that the surface of NCPDs3 was rich in amino groups, Fe 3+ It can react with these groups, causing static quenching of the blue fluorescence emission peak at 425 nm. Under the same conditions, Ni... 2+ Cu 2+ Fluorescence detection was performed, and no fluorescence quenching was observed. Therefore, the polymeric dots NCPDs3 are effective for Fe... 3+ The detection has high selectivity, a detection limit of 1 mM, and is characterized by high sensitivity and speed.

[0023] Therefore, the non-conjugated fluorescent polymer dots (NCPDs) powder prepared by this invention is simple to operate, inexpensive, highly stable, environmentally friendly, and has excellent fluorescent properties.

[0024] Compared with current NCPD preparation technologies, electrochemical synthesis offers advantages such as lower synthesis temperature, simpler process, and greater safety and environmental friendliness. Furthermore, non-conjugated fluorescent polymer dots (NCPDs) powder is readily soluble in water and can be formulated into solutions, exhibiting blue fluorescence in both aqueous and solid states. The nitrogen- and oxygen-containing groups on the surface of NCPDs can react with heavy metal ions, enabling efficient and sensitive detection of these ions.

Claims

1. A method for preparing non-conjugated fluorescent polymer dots by electrochemical synthesis, characterized in that: Includes the following steps: Step 1: Preparation of fluorescent polymer dots: Weigh citric acid and ethylenediamine in a molar ratio of 1:1, dissolve citric acid and ethylenediamine in deionized water, stir evenly to obtain an electrolyte, insert the electrode into the electrolyte, and pass a 10~25V DC current into the electrolyte to carry out an electrochemical reaction. After the reaction is completed, fluorescent polymer dots are obtained. Step 2: Polymer dot separation and purification: Filter the electrolyte from step 1, then separate and purify it, and then freeze-dry the purified aqueous solution to remove the solvent, thereby obtaining fluorescent polymer dot powder.

2. The method for preparing non-conjugated fluorescent polymer dots by electrochemical synthesis according to claim 1, characterized in that: The specific steps of step 1 are as follows: Step 1.1: Dissolve the sum of citric acid and ethylenediamine as two solutes in deionized water at a mass ratio of 1:(40~70), mix and stir evenly to obtain a clear and transparent electrolyte; Step 1.2: Insert two platinum electrode sheets as working electrodes into the electrolyte obtained in step 1.1 and carry out an electrochemical reaction for 0.5~2 hours. After the reaction is completed, fluorescent polymer dots are obtained.

3. The method for preparing non-conjugated fluorescent polymer dots by electrochemical synthesis according to claim 2, characterized in that: In step 2, the electrolyte obtained in step 1.2 is filtered using a 0.20-0.22 micrometer filter membrane, and separated and purified using a dialysis bag with a molecular weight of 500-1000.

4. The method for preparing non-conjugated fluorescent polymer dots by electrochemical synthesis according to claim 3, characterized in that: The freeze-drying temperature in step 2 is -80 to -10°C, and the pressure is below 10 Pa.

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