A pHEMA / ionic liquid / mxene composite gel electrode, a preparation method thereof and application thereof in electrochemical detection of p-nitrophenol

Abstract

This invention proposes a PHEMA / ionic liquid / MXene / Ti composite hydrogel electrode and develops its application method in the electrochemical detection of phenolic pollutants. Using few-layer MXene as a conductive substrate, an ionic liquid and hydroxyethyl methacrylate are introduced. A homogeneous precursor system is constructed through vortex mixing and ultrasonic dispersion. A titanium mesh is used as the electrode substrate, and UV-initiated polymerization is employed to prepare the composite hydrogel electrode in one step. The entire preparation process is simple, efficient, and low-cost. This electrode utilizes the structural support of the three-dimensional PHEMA network, the high ionic conductivity and intercalation effect of EMIMBF4, and the high conductivity and abundant surface functional groups of MXene to form a synergistic effect. This not only effectively solves the technical problems of insufficient conductivity of PHEMA alone and the easy stacking of MXene sheets, but also significantly improves the electrocatalytic activity and structural stability of the electrode. When applied to the electrochemical detection of the phenolic pollutant p-nitrophenol, this electrode exhibits excellent detection performance, with a convenient detection process, high sensitivity, and accurate and reliable results.

Description

Technical Field

[0001] This invention relates to the field of electrochemical sensor technology, specifically to the preparation of a PHEMA / ionic liquid / MXene composite gel electrode and its application in the electrochemical detection of phenolic pollutants. Background Technology

[0002] p-Nitrophenol (p-NP) is an important chemical intermediate widely used in dyes, textiles, fertilizers, explosives, and pharmaceuticals. Due to its high resistance to biodegradation and strong toxicity, p-NP can cause severe skin irritation, irreversible damage to the central nervous system and other organs in humans and animals, and even death. Therefore, establishing a simple and accurate method for detecting p-nitrophenol is of great significance for environmental monitoring. Phenolic compounds are detected using various techniques, such as high-performance liquid chromatography (HPLC) and fluorescent probes; however, these techniques have drawbacks, such as long sample preparation times and relatively high costs. Electrochemical detection of p-nitrophenol offers advantages such as portability, low power consumption, and high sensitivity. Poly(hydroxyethyl methacrylate) (PHEMA) is a biocompatible polymer material whose three-dimensional network structure provides abundant active sites and is often used for electrode modification to enhance electrochemical performance. Ionic liquids (such as EMIMBF4) are widely used to improve the catalytic activity and stability of electrodes due to their high ionic conductivity, enrichment properties, wide electrochemical window, and good thermal stability. Hydrophilic MXenes possess high conductivity, abundant surface functional groups (-OH, -O, and -F), large interlayer spacing, and high specific surface area. MXene nanosheets can not only be uniformly distributed within hydrogels but also enhance the mechanical properties and conductivity of hydrogels by forming hydrogen bonds with polymers. Therefore, constructing MXene-based hydrogel sensors is a promising approach to broaden their applications in aquatic environments. Summary of the Invention

[0003] The purpose of this invention is to provide a PHEMA / ionic liquid / MXene composite hydrogel electrode for detecting phenolic contaminants. Therefore, this invention employs a PHEMA / ionic liquid / MXene composite hydrogel to modify the electrode, constructing a sensitive composite hydrogel electrode for the detection of p-nitrophenol. This composite hydrogel electrode exhibits excellent electrocatalytic activity, a wide linear range, and a low detection limit for p-nitrophenol.

[0004] To achieve the above objectives, the technical solution adopted in this invention is: a PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode, the preparation method of which includes the following steps:

[0005] A few-layer MXene was added to deionized water. Then, ionic liquid, hydroxyethyl methacrylate (HEMA), and 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) were slowly added dropwise to the aqueous solution of the few-layer MXene. The mixture was vortexed for 10 min and ultrasonically vibrated for 30 min. The cut titanium mesh was folded at 90° and placed at the bottom of a square polytetrafluoroethylene (PTFE) mold. The mixture was poured into the square PTFE mold to uniformly cover the titanium mesh. Polymerization was initiated by irradiation with a UV lamp for 30 min to obtain the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode.

[0006] The above-mentioned PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode is characterized in that the preparation method of the few-layer MXene includes the following steps:

[0007] NaF was added to hydrochloric acid and stirred to obtain an etchant. Ti3AlC2 powder was gradually added to the etchant over 30 min and stirred to obtain a mixture. The mixture was washed several times by centrifugation with deionized water. When the pH of the supernatant was ≥5, the precipitate was collected and dried under vacuum. The product was dissolved in 25% tetrapropylammonium hydroxide (TPAOH) aqueous solution and sonicated for 0.5 h under a nitrogen atmosphere. The mixture was filtered, washed until the pH of the supernatant was 7, sonicated to disperse, centrifuged, and the supernatant was collected and freeze-dried to obtain a few-layer MXene.

[0008] The aforementioned PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode is characterized in that the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate.

[0009] The aforementioned PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode is characterized in that the concentration of the hydrochloric acid is 6 M.

[0010] The aforementioned PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode is characterized in that the stirring is carried out at 160°C for 9 hours.

[0011] The above-mentioned PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode is used in the electrochemical detection of phenolic pollutants.

[0012] In the above applications, the phenolic contaminant is p-nitrophenol.

[0013] The above application is performed as follows: using the above-mentioned PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as the working electrode, the Ag / AgCl electrode as the reference electrode, and the platinum wire as the counter electrode, the three-electrode system is placed in a phosphate buffer solution containing p-nitrophenol for electrochemical detection.

[0014] In the above application, the concentration of p-nitrophenol in the phosphate buffer solution is 5 × 10⁻⁶. -7 M-2.05×10 -4 M, phosphate buffer solution, pH = 7.

[0015] The beneficial effects of this invention are:

[0016] 1. This invention combines an ionic liquid (EMIMBF4) with few-layer MXene, utilizing the intercalation effect of imidazole cations in the ionic liquid to effectively expand the interlayer spacing of MXene. This structural design not only significantly enhances the stability of MXene nanosheets in hydrogel systems, preventing stacking collapse during preparation and application, but also fully exposes their high specific surface area and abundant surface functional groups. Ultimately, this composite strategy synergistically improves the overall conductivity of the material, laying a solid material foundation for constructing high-performance electrodes.

[0017] 2. This invention improves the problem of insufficient conductivity of PHEMA when used alone by introducing ionic liquid and MXene into the three-dimensional network of PHEMA hydrogel. A composite system is formed by photo-initiated polymerization, and the synergistic effect of the three is used to improve the sensitivity and detection limit of electrochemical detection of p-nitrophenol.

[0018] 3. This invention successfully fabricates a high-performance composite hydrogel electrode through the ingenious design and optimized ratio of electrode material composition (PHEMA, ionic liquid, MXene), combined with the control of photopolymerization process parameters. This electrode exhibits excellent comprehensive performance in the detection of p-nitrophenol: a wide linear detection range (0.5-205 μM), an extremely low detection limit (28.2 nM), high sensitivity, and good stability. This not only significantly improves the reliability and accuracy of the detection results, but also provides a highly promising solution for the rapid, economical, and sensitive detection of phenolic pollutants in the environment due to its simple preparation process and low cost. Attached Figure Description

[0019] Figure 1 These are SEM images of PHEMA / EMIMBF4 / MXene composite hydrogels with different mass fractions of EMIMBF4.

[0020] Among them, A and B: 10 wt%; C and D: 20 wt%; E and F: 30 wt%.

[0021] Figure 2 These are infrared spectra of PHEMA, EMIMBF4, PHEMA / EMIMBF4, and PHEMA / EMIMBF4 / MXene.

[0022] Figure 3 This is a TEM image of MXene.

[0023] Figure 4 Different modified electrodes in [Fe(CN)6] 3- / 4- Electrochemical impedance spectroscopy in solution.

[0024] These are Ti, PHEMA / EMIMBF4 / Ti, PHEMAEMIMBF4 / Ti, and PHEMA / EMIMBF4 / MXene / Ti, respectively.

[0025] Figure 5 This is a CV graph showing the detection of p-nitrophenol using different modified electrodes in PBS at pH 7.

[0026] Figure 6 The image shows the CV curves of a PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode measuring a 1 mM p-nitrophenol concentration in PBS solution at pH=7 at different scan rates.

[0027] Figure 7 It is a linear graph of the scan rate versus the peak current of the p-nitrophenol reduction.

[0028] Figure 8 It is a differential pulse voltammogram at different concentrations of p-nitrophenol.

[0029] Figure 9 This is a calibration graph showing the peak current as a function of p-nitrophenol concentration. Detailed Implementation

[0030] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] Example 1: Fabrication of a PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode

[0032] (a) The preparation method is as follows

[0033] 1. Preparation of few-layer MXene

[0034] 1) Add 0.5 g NaF to 40 mL of 6 M hydrochloric acid to obtain an etchant, and stir for 10 min to ensure complete dissolution. Gradually add 0.5 g Ti3AlC2 powder to the above etchant over 30 min, and react at 160 °C for 9 h to obtain a mixture.

[0035] 2) Washing: The product obtained in step 1) is washed several times by centrifugation with deionized water. Each wash is centrifuged at 3500 rpm for 5 min. After each wash, the supernatant is removed as waste. When the pH of the supernatant is ≥5, the precipitate is collected and vacuum dried at 60°C for 12 h.

[0036] 3) Dissolve the product obtained in step 2) in 10.0 mL of 25% tetrapropylammonium hydroxide (TPAOH) aqueous solution and sonicate for 0.5 h under a nitrogen atmosphere.

[0037] 4) Filter the product obtained in step 3), wash the solid with deionized water until the pH of the supernatant is 7, sonicate for 0.5 h, centrifuge at 5000 r / min for 1 h, collect the supernatant and freeze dry.

[0038] 2. Preparation of PHEMA / EMIMBF4 / MXene composite hydrogel precursor solution

[0039] 20 mg of few-layer MXene was dissolved in 0.5 g of deionized water. Then, 0.18 g, 0.40 g, 0.69 g of EMIMBF4, 1.0 g of HEMA, and 0.1 g of HMPP were slowly added dropwise to the MXene aqueous solution. The mixture was vortexed for 10 min and ultrasonically vibrated for 30 min to obtain PHEMA / EMIMBF4 / MXene composite hydrogel precursor solutions with EMIMBF4 mass fractions of 10%, 20%, and 30%, respectively. (Both the PHEMA hydrogel precursor solution and the PHEMA / EMIMBF4 composite hydrogel precursor solution were prepared according to the above monomer mass ratios and the same preparation steps. Neither was loaded with titanium mesh; photoinitiated polymerization was performed directly in a mold, followed by infrared spectroscopy.)

[0040] 3. Electrode pretreatment

[0041] The cut 1.0 cm × 1.5 cm titanium mesh was ultrasonically washed with ethanol and ultrapure water for 3 min each. It was then dried with high-purity nitrogen gas for later use.

[0042] 4. Preparation of PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode

[0043] The pretreated 1.0 cm × 1.5 cm titanium mesh was folded at 90 degrees and placed at the bottom of a square polytetrafluoroethylene mold. 0.2 ml of the PHEMA / EMIMBF4 / MXene composite hydrogel precursor solution obtained in step 2 was poured into the square polytetrafluoroethylene mold (1.0 cm × 1.0 cm). The polymerization was initiated by irradiation with a UV lamp for 30 min to obtain the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode.

[0044] The PHEMA / Ti composite hydrogel electrode and the PHEMA / EMIMBF4 composite hydrogel electrode were prepared by replacing the PHEMA / EMIMBF4 / MXene composite hydrogel precursor with PHEMA / Ti composite hydrogel precursor and PHEMA / EMIMBF4 composite hydrogel precursor, respectively.

[0045] (II) Testing

[0046] Figure 1 These are SEM images of PHEMA / EMIMBF4 / MXene composite hydrogels with different mass fractions of EMIMBF4. A strong interaction exists between EMIMBF4 and the hydrogel, therefore, variations in its content affect various properties of the composite hydrogel. As shown in the figure, the composite hydrogel network maintains a porous structure regardless of the EMIMBF4 content. With increasing EMIMBF4 content, the pore walls gradually thicken, reaching 5-20 μm in some areas when the EMIMBF4 content reaches 30 wt%. This is because the hydrogel samples undergo lyophilization during the SEM pretreatment stage, and due to hydrogen bonding and electrostatic interactions with the composite hydrogel, EMIMBF4 becomes entangled with the polymer network and fills into the gel network.

[0047] Figure 2 These are the infrared spectra of PHEMA, EMIMBF4, PHEMA / EMIMBF4, and PHEMA / EMIMBF4 / MXene composite hydrogels. Figure 2 It can be seen that at 3445 cm -1 The absorption band observed nearby belongs to the -OH absorption peak, reflecting the hydrophilicity of MXene. At 620 cm⁻¹ -1 The characteristic peak at 1700-1500 cm⁻¹ indicates Ti-O. -1 The absorption bands within the range correspond to the vibrations of the C=N bonds in the EMIM⁺ cation, and the 1063 cm⁻¹ band. -1 The characteristic BF peak at 2850-2950 cm⁻¹ confirms the presence of EMIMBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate). -1 The CH stretching vibration in the region is the methyl / methylene of PHEMA, and the spectrum of the PHEMA / EMIMBF4 / MXene composite material shows the characteristic absorption peaks of MXene and EMIMBF4.

[0048] Figure 3 This is a TEM image of MXene. (By...) Figure 3As can be seen, the lattice fringe spacing obtained by etching the few-layer MXene and taking the average value is d=2.328 / 9=0.25 nm. According to the comparison with (JCPDS no.52-0875), it can be determined to be the MXene (110) crystal plane, indicating that the MXene preparation was successful.

[0049] Example 2: Application of PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode in the detection of p-nitrophenol

[0050] The method is as follows: Using the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode prepared in Example 1 as the working electrode, the Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode, the three-electrode system was placed in a phosphate buffer solution containing p-nitrophenol. Electrochemical measurements were performed using a CHI660e electrochemical workstation, employing cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. A standard electrochemical curve was plotted based on the electrochemical signal data. The concentration range of p-nitrophenol in the phosphate buffer solution was determined to be 5 × 10⁻⁶. -7 M-2.05×10 -4 M, phosphate buffer solution, pH=7.

[0051] The working electrodes were selected as follows: Ti electrode, PHEMA / Ti composite hydrogel electrode, and PHEMA / EMIMBF4 composite hydrogel electrode, respectively, replacing the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode prepared in Example 1.

[0052] (I) PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode, Ti electrode, PHEMA / Ti composite hydrogel electrode, PHEMA / EMIMBF4 composite hydrogel electrode in [Fe(CN)6] 3- / 4- Electrochemical response in KCl solution

[0053] Take 10 mL containing 5 mM [Fe(CN)6] 3- / 4- The 0.1 M KCl solution was placed in a beaker. A three-electrode system was used, with the Ag / AgCl electrode as the reference electrode, the platinum wire as the counter electrode, and the working electrodes being the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode, the Ti electrode, the PHEMA / Ti composite hydrogel electrode, and the PHEMA / EMIMBF4 composite hydrogel electrode, respectively. Electrochemical impedance spectroscopy was performed using a CHI660e electrochemical workstation. The test results are as follows: Figure 4 As shown.

[0054] Figure 4These are PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrodes, Ti electrodes, PHEMA / Ti composite hydrogel electrodes, and PHEMA / EMIMBF4 composite hydrogel electrodes in [Fe(CN)6]. 3- / 4- Electrochemical impedance spectroscopy in solution. Figure 4 As can be seen, the charge transfer resistance (Rct) of the PHEMA / EMIMBF4 / Ti and PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrodes are 432.7 Ω and 176.3 Ω, respectively, which are much lower than the Rct of Ti and the Rct of PHEMA / Ti. This is because the high conductivity of MXene and EMIMBF4 promotes electron transfer and reduces the Rct value.

[0055] (II) Detection performance of PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode, Ti electrode, PHEMA / Ti composite hydrogel electrode, and PHEMA / EMIMBF4 composite hydrogel electrode

[0056] Take 10 mL of 0.2 mol / L phosphate buffer solution with pH 7 and place it in a beaker. Then add p-nitrophenol to the beaker to make its concentration 1 mM and mix well.

[0057] A three-electrode system was used, with the Ag / AgCl electrode as the reference electrode and the platinum wire as the counter electrode. The working electrodes were a PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode, a Ti electrode, a PHEMA / Ti composite hydrogel electrode, and a PHEMA / EMIMBF4 composite hydrogel electrode, respectively. Electrochemical measurements were performed using a CHI660e electrochemical workstation with a potential scan range of -0.4–0.8 V. p-Nitrophenol was detected, and the results are as follows: Figure 5 As shown.

[0058] Figure 5 This is a CV graph showing the detection of p-nitrophenol using different electrodes in PBS at pH 7. (From...) Figure 5 It is evident that the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode exhibits the strongest current response, demonstrating that the addition of EMIMBF4 and MXene can effectively improve the conductivity of the hydrogel and its electrocatalytic performance for p-nitrophenol, further enhancing the detection signal of the electrode.

[0059] (III) Catalytic detection performance of PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode for different concentrations of p-nitrophenol

[0060] The effect of scan rate on peak current was investigated using cyclic voltammetry in the range of 25–400 mV / s. Figure 6As shown, the reduction peak current of p-nitrophenol increases with increasing scan rate from 25 mV / s to 400 mV / s. The results obtained after linear fitting are as follows: Figure 7 As shown, the reduction peak current of p-nitrophenol is linearly related to the scan rate. The equation shows that the reduction peak current of p-nitrophenol is proportional to the first power of the scan rate. The results indicate that the typical mechanism controlling the oxidation kinetics of p-nitrophenol on the PHEMA / EMIMBF4 / MXene / Ti hydrogel electrode surface is diffusion-controlled electrochemical process.

[0061] p-Nitrophenol was added to a phosphate buffer solution at pH 7 to achieve concentrations of 0.5, 8, 15, 20, 25, 28, 31, 35, 75, 125, 165, and 205 μM. The PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode prepared in Example 1 was used as the working electrode, the Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. Electrochemical measurements were performed using a CHI660e electrochemical workstation with a potential scan range of -0.8–0.8 V. p-Nitrophenol was detected. The test results are as follows: Figure 8 As shown

[0062] Figure 8 It is a differential pulse voltammogram at different concentrations of p-nitrophenol. Figure 9 This is a calibration graph showing the peak current as a function of p-nitrophenol concentration. (From...) Figure 8 As shown, the reduction peak current of the composite hydrogel electrode increases with increasing concentration of p-nitrophenol in the phosphate buffer solution, demonstrating that the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode prepared in this invention exhibits good catalytic performance for p-nitrophenol. Figure 9 As shown, the DPV reduction current gradually increases with increasing p-nitrophenol concentration. A good linear relationship exists between the reduction peak current and the p-nitrophenol concentration in the range of 0.5 μM–205 μM, and the linear equation in the range of 0.5 μM–35 μM is -I(A) = 8.35720 × 10⁻⁶. -6 C(μM) -8.79107×10 -4 (R 2 =0.996), and the linear equation in the range of 35 μM-205 μM is -I(A)=3.03456×10 -7 C(μM) -1.16×10 -3 (R 2 =0.991), and the detection limit is 28.2 nM.

Claims

1. A PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode, characterized in that, The preparation method includes the following steps: A few-layer MXene was added to deionized water. Then, ionic liquid, hydroxyethyl methacrylate (HEMA), and 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) were slowly added dropwise to the aqueous solution of the few-layer MXene. The mixture was vortexed for 10 min and ultrasonically vibrated for 30 min. The cut titanium mesh was folded at 90° and placed at the bottom of a square polytetrafluoroethylene (PTFE) mold. The mixture was poured into the square PTFE mold to uniformly cover the titanium mesh. Polymerization was initiated by irradiation with a UV lamp for 30 min to obtain the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode.

2. The PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as described in claim 1, characterized in that, The preparation method of the few-layer MXene includes the following steps: NaF was added to hydrochloric acid and stirred to obtain an etchant. Ti3AlC2 powder was gradually added to the etchant over 30 min and stirred to obtain a mixture. The mixture was washed several times with deionized water by centrifugation. When the pH of the supernatant was ≥5, the precipitate was collected and dried under vacuum. The product was dissolved in 25% tetrapropylammonium hydroxide (TPAOH) aqueous solution and sonicated for 0.5 h under a nitrogen atmosphere. The mixture was filtered, washed until the pH of the supernatant was 7, sonicated to disperse, centrifuged, and the supernatant was collected and freeze-dried to obtain a few-layer MXene.

3. The PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as described in claim 2, characterized in that, The ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate.

4. The PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as described in claim 2, characterized in that, The concentration of the hydrochloric acid is 6 M.

5. The PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as described in claim 2, characterized in that, The stirring was carried out at 160°C for 9 hours.

6. The application of the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as described in claim 1 in the electrochemical detection of phenolic pollutants.

7. The application as described in claim 6, characterized in that, The phenolic pollutant is p-nitrophenol.

8. The application as described in claim 7, characterized in that, The method is as follows: using the PHEMA / EMIMBF4 / MXene / Ti composite hydrogel electrode as described in claim 1 as the working electrode, the Ag / AgCl electrode as the reference electrode, and the platinum wire as the counter electrode, the three-electrode system is placed in a phosphate buffer solution containing p-nitrophenol for electrochemical detection.

9. The application as described in claim 8, characterized in that, In a phosphate buffer solution containing p-nitrophenol, the concentration of p-nitrophenol is 5 × 10⁻⁶. -7 M-2.05×10 -4 M, phosphate buffer solution, pH = 7.

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