English

• Drug-resistant tuberculosis is a global leading cause of death by an infectious disease and poses a serious therapeutic challenge. Isoniazid (INZ) is a widely used antibiotic for tuberculosis treatment [1], while hydrazine (HYD), a carcinogenic, hepatotoxic, neurotoxic substance, is a byproduct of INZ metabolism that can also severely impact the respiratory system [2,3]. Thus, rapid and straightforward monitoring of INZ and HYD metabolic changes is critical in ensuring the efficacy of this medication in combating tuberculosis, by providing timely data on drug concentrations in body fluids (e.g., blood and urine) to better guide therapy and minimize side effects [4]. Therefore, the development of an easy-to-use and efficient analytical tool for the precise and simultaneous detection of INZ and HYD is crucial for clinical diagnosis and nursing.

  Electrochemical sensing with unique features in respect of low cost, fast response, easy operation, and feasible miniaturization, is an ideal platform for the detection of INZ and HYD [5,6]. However, it requires a functionalized electrode to capture the analytes, catalyze the redox reaction of analytes, and thus realize sensitive detection [7]. The main challenge in improving the detection sensitivity is to facilitate the mass transport on the surface of the electrode, as mass transport is usually the rate-controlling step of inhomogeneous reaction [8]. C–CeO₂ nanofiber is a prominent electrocatalyst for its excellent adsorption properties and tunable physical/chemical properties of the surface [9,10]. The coexistence of Ce^Ⅲ and Ce^Ⅳ on its surface endows it with abundant active sites for the adsorbed species which would facilitate the mass transport on the surface of the electrode. In addition, according to the space charge theory, constructing a metal coordination interface could largely improve the mass transport of reactants [11]. Metal-phenolic networks (MPNs) is a versatile class of self-assembled supramolecular materials constructed from phenolic building blocks and metal ions through metal coordination [12,13]. Our group and others have recently been exploring these promising materials for energy and environmental applications [14,15]. Particularly, the native properties of polyphenols enable MPNs deposition onto various substrate materials, allowing for the rational design of functional films [16,17]. Additionally, the functionality of MPNs can be tuned through careful choice of the incorporated metal ions [18–20]. Caruso et al. demonstrated that MPNs constructed from Ni^Ⅱ are able to distribute in a way to specifically form strong interactions with the histidine and/or methionine residues of the Fc region of antibodies for enhanced targeting [21]. Therefore, we anticipate that constructing Ni^Ⅱ-incorporated MPN on C–CeO₂ nanofiber could generate an ideal interface to interact with amino-abundant INZ and HYD molecules, thus enabling highly sensitive electrochemical detection of INZ and HYD.

  In this study, we have developed a high-performance nanostructured electrode material composed of electrospun-derived C–CeO₂ nanofiber and Ni–tannic acid-based surface, also referred to as Ni–TA/C–CeO₂ (Schemes 1a and b). Mechanistic investigations demonstrate that electronegative and hydrophilic Ni–TA facilitates the mass and electron transfer in the electrooxidation reactions of INZ and HYD. Accordingly, Ni–TA/C–CeO₂ achieves low detection limits and wide linearity ranges for INZ (0.012 µmol/L and 0.1–400 µmol/L, respectively) and HYD (0.008 µmol/L and 0.015–1420 µmol/L, respectively) in the physiological condition, outperforming native C–CeO₂ nanofiber and previously reported materials [22–24]. The performance of our materials for the simultaneous detection of INZ and HYD in human plasma and urine samples suggests great promise in real-world clinical applications (Scheme 1c).

  Scheme 1

  

  Scheme 1.  Schematic representations of (a) the preparation of C–CeO₂ nanofiber, (b) the preparation of Ni–TA network on C–CeO₂ nanofiber, and (c) the possible mechanism of Ni–TA/C–CeO₂ for INZ and HYD electrochemical detection.

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  The scanning electron microscopy (SEM) image of Fig. 1a reveals that C–CeO₂ is composed of nanofibers with a mean diameter of 65 nm. The transmission electron microscopy (TEM) image of Fig. 1b further confirms that C–CeO₂ has a relatively smooth surface. As shown in Schemes 1a and b, the construction of the Ni–TA layer on C–CeO₂ is a two-step process, including the TA adsorption on the C–CeO₂ surface and the coordination reaction between the adsorbed TA and the newly added Ni^Ⅱ (see the structure of TA in Fig. S1 in Supporting information) [13,25]. Because of the strong chelation ability, TA acts as a polydentate ligand and forms the four-coordinate Ni^Ⅱ complex under alkaline conditions [13,26]. Ni^Ⅱ acts as the “joint” and links different TA molecules, thus forming a stable Ni–TA interface on C–CeO₂. As a result, the obtained Ni–TA/C–CeO₂ has a rougher surface than C–CeO₂ and a mean diameter of 70 nm (Fig. 1c). As shown in Fig. 1d, its rough surface can be seen in the TEM image. Furthermore, the thickness of Ni–TA is calculated by the nanofiber diameters before and after surface modification with the result of 5 nm. Fig. S2 (Supporting information) shows the scanning TEM (STEM) and energy-dispersive X-ray (EDX) elemental mapping images, suggesting that the C, O, Ce, and Ni elements are uniformly distributed on Ni–TA/C–CeO₂. The EDX spectrum of Ni–TA/C–CeO₂ reveals the surface exists of C, O, Ni, and Ce with the atomic ratio of 81.15:18.69:0.10:0.07 (Fig. S3 in Supporting information).

  Figure 1

  

  Figure 1.  Physicochemical Characterization. (a) SEM, and (b) TEM images of C–CeO₂. (c) SEM and (d) TEM images of Ni–TA/C–CeO₂. (e) FTIR spectra of C–CeO₂, Ni–TA/C–CeO₂, and TA. XPS spectra in the (f) C 1s and (g) O 1s regions for C–CeO₂ and Ni–TA/C–CeO₂. (h) Images displaying the dispersion stability of C–CeO₂ and Ni–TA/C–CeO₂. (i) zeta potential of C–CeO₂ and Ni–TA/C–CeO₂. (j) Contact angles of water on the surfaces of C–CeO₂ and Ni–TA/C–CeO₂.

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  The Fourier transform infrared (FTIR) spectra of C–CeO₂, Ni–TA/C–CeO₂, and TA are shown in Fig. 1e, confirming the successful formation of Ni–TA on C–CeO₂. Compared with C–CeO₂, Ni–TA/C–CeO₂ has a new peak at 1716 cm^–1, which is assigned to the C=O vibration of TA. Meanwhile, the C‒O vibrational shift from 1206 cm^–1 to 1193 cm^–1 arises via the electron back-donation from Ni d-orbitals to C‒O [27]. The bands in the 3402–3416 cm^–1 region are ascribed to ‒OH stretching of H₂O and phenol groups. The X-ray diffraction (XRD) spectra of C–CeO₂ and Ni–TA/C–CeO₂ are shown in Fig. S4 (Supporting information), with the diffraction peaks at 26.2°, 33.0°, 47.9°, 56.1°, 58.6°, 69.7°, and 76.2°. These diffraction peaks are indexed to the (111), (200), (220), (311), (222), (400), and (331) planes of CeO₂, respectively [28]. Fig. 1f displays the C 1s X-ray photoelectron spectroscopy (XPS) spectra of C–CeO₂ and Ni–TA/C–CeO₂. In contrast to C–CeO₂, Ni–TA/C–CeO₂ has more surface C‒O and C=O, and a new peak O–C=O appears, further demonstrating the successful formation of Ni–TA [29]. Meanwhile, the O 1s XPS spectra (Fig. 1g) also demonstrate that Ni–TA/C–CeO₂ has more oxygen-containing functional groups, especially the hydroxyl group. The Ce 3d XPS spectrum of Ni–TA/C–CeO₂ (Fig. S5 in Supporting information) shows the 3d_5/2 and 3d_3/2 spin-orbit splitting, with the components (885.9 and 908.1 eV) corresponding to Ce^Ⅲ species and the components (883.7, 888.8, 900.6, 904.8, and 912.1 eV) corresponding to Ce^Ⅳ species [30]. Moreover, the Ni 2p XPS spectrum of Ni–TA/C–CeO₂ (Fig. S6 in Supporting information) shows the 2p_3/2 and 2p_1/2 splitting, with the components (856.7 and 875.1 eV) corresponding to Ni^Ⅱ species [31].

  The surface properties of Ni–TA/C–CeO₂ were studied by time-settlement behavior, zeta potential analysis and water contact angle measurement. As shown in Fig. 1h, the time-settlement experiment demonstrates that the dispersion stability of Ni–TA/C–CeO₂ is much higher than C–CeO₂. In water, most of C–CeO₂ is precipitated in 10–30 min. However, the Ni–TA/C–CeO₂ suspension shows a good dispersion even after 180 min, which can be ascribed to the decoration of electronegative Ni–TA [32]. Therefore, its surface charge property was further investigated by using the zeta potential analysis. As shown in Fig. 1i, the zeta potential value of C–CeO₂ is −5.07±0.90 mV in water. The Ni–TA functional layer decreases the zeta potential of Ni–TA/C–CeO₂ to −31.13±1.03 mV. Meanwhile, the zeta potential of Ni–TA/C–CeO₂ decreases with the increment of the Ni^Ⅱ/TA ratio (Fig. S7 in Supporting information), suggesting the quantitative introduction of Ni^Ⅱ sites. These results suggest that the Ni–TA layer enhances the electronegativity of the C–CeO₂ surface, which significantly improves the dispersion stability of the Ni–TA/C–CeO₂ suspension. The wettability of electrode material significantly influences its mass transport and thus affects the detection performance [33]. Therefore, water contact angle measurement was then conducted to analyze the surface wetting properties of C–CeO₂ and Ni–TA/C–CeO₂. As shown in Fig. 1j, the water contact angle of C–CeO₂ is as high as 131.7°. However, the water droplet can completely spread over the surface of Ni–TA/C–CeO₂. It confirms that Ni–TA/C–CeO₂ is highly hydrophilic, allowing efficient transport of analytes from the aqueous electrolyte to the electrode surface.

  The adsorption properties of Ni–TA/C–CeO₂ towards INZ and HYD were investigated by batch experiments and kinetics studies. Time-dependent adsorption capacities (Q_t) are shown in Figs. 2a and b, demonstrating that Ni–TA could accelerate mass transport in the heterogeneous reaction. Figs. 2c and d show that the adsorption capacities at equilibrium (Q_e) increase with the initial concentrations (C₀), where Q_e is obtained by Eq. S1 (Supporting information) [34]. It can be attributed to the collision between adsorption sites and solutes in the high-concentration solution. The maximum Q_e of C–CeO₂ towards INZ and HYD are 5.06 mg/g and 7.03 mg/g, respectively. The maximum Q_e of Ni–TA/C–CeO₂ towards INZ and HYD are 16.93 mg/g and 18.22 mg/g, respectively. Figs. S8 and S9 (Supporting information) show the fitted adsorption isotherms for Langmuir and Freundlich models, respectively. The related values are obtained by Eqs. S2 and S3 (Supporting information) [34]. From the model constants in Table S1 (Supporting information), both INZ and HYD adsorption processes follow the Langmuir model which is usually applied to describe monolayer adsorption. The calculated maximum adsorption capacity (Q_m) of C–CeO₂ towards INZ and HYD are 5.32 mg/g and 6.94 mg/g, respectively. The calculated Q_m of Ni–TA/C–CeO₂ towards INZ and HYD are 16.67 mg/g and 20.16 mg/g, respectively. Figs. S10 and S11 (Supporting information) display the fitted kinetic models for pseudo-first-order and pseudo-second-order kinetics, respectively. From the correlation coefficients (R²), both INZ and HYD adsorption obey pseudo-second-order kinetics. The related values of Table S2 (Supporting information) are obtained by Eqs. S4 and S5 (Supporting information), where the calculated Q_e and rate constant (k) are obtained from the intercept and slope by plotting t against ln(Q_e−Q_t) [35]. The k of C–CeO₂ towards INZ and HYD are 0.075 g mg^–1 h^–1 and 0.047 g mg^–1 h^–1, respectively. The k of Ni–TA/C–CeO₂ towards INZ and HYD are 0.091 g mg^–1 h^–1 and 0.062 g mg^–1 h^–1, respectively. The enhanced k demonstrates Ni–TA is beneficial for mass transport in INZ and HYD detection.

  Figure 2

  

  Figure 2.  Adsorption property and electrochemical performance. Relationship between Q_t and time: (a) INZ, (b) HYD. Relationship between Q_e and C₀: (c) INZ, (d) HYD. (e) CV curves, and (f) Nyquist plots of CP, C–CeO₂ and Ni–TA/C–CeO₂ in the potassium ferricyanide solution. (g) DPV curves of CP, C–CeO₂ and Ni–TA/C–CeO₂ in the 0.1 mol/L PBS solution (pH 7.0) containing 90 µmol/L INZ and 60 µmol/L HYD.

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  Cyclic voltammetry (CV) and alternating-current (AC) impedance measurements were carried out to evaluate the electrochemical performance of Ni–TA/C–CeO₂. As shown in Fig. 2e, the anodic peak current (I_pa) from Ni–TA/C–CeO₂ is much higher than that of C–CeO₂ and bare carbon paper (CP), suggesting faster charge transfer efficiency of the Ni–TA/C–CeO₂. As shown in the Nyquist plots (Fig. 2f), the charge transfer resistances of bare CP, C–CeO₂, and Ni–TA/C–CeO₂ are 705, 417, and 56 Ω, respectively. The impedance of Ni–TA/C–CeO₂ in the high-frequency range is much lower than that of CP and C–CeO₂, revealing that the charge transfer of Ni–TA/C–CeO₂ is easier than that of CP and C–CeO₂. To evaluate the electrochemical active surface area (EASA), CV measurements of C–CeO₂ and Ni–TA/C–CeO₂ were performed using various scan rates (Figs. S12a and b in Supporting information). As shown in Figs. S12c and d (Supporting information), the relationship between I_pa and the square root of scan rate (v^1/2) is linear. According to the Randles-Sevcik Eq. S6 (Supporting information) [36], the EASA of Ni–TA/C–CeO₂ is calculated to be 0.29 cm², which is larger than that of C–CeO₂ (0.12 cm²). According to the Laviron Eq. S7 (Supporting information) [37], the charge-transfer rate (k_s) of Ni–TA/C–CeO₂ is calculated to be 0.22 s^–1, which is higher than that of C–CeO₂ (0.14 s^–1). The calculated results demonstrate that Ni–TA functional layer could accelerate the mass and electron transfer between the solution and C–CeO₂ nanofiber. As shown in Fig. 2g, the current responses of analytes obtained on Ni–TA/C–CeO₂ are much higher than that of CP and C–CeO₂. These results reveal that Ni–TA functional layer could accelerate the mass and electron transfer of C–CeO₂ nanofiber, and thus enhance the current responses of INZ and HYD. To optimize the detection performance, pH conditions and Ni/TA ratio were investigated by comparative experiments. As shown in Fig. S13 (Supporting information), Ni–TA/C–CeO₂ exhibits the maximum response for the analytes in pH 7.0 electrolyte. Moreover, the peak potentials (E_pa) for both analytes have negative linear relationships with the pH of the electrolyte, satisfying the equation E_pa = −0.056 pH + 0.595 (R² = 0.980) for INZ and E_pa = −0.054 pH + 0.728 (R² = 0.998) for HYD. The slope values are 0.056 V/pH and 0.054 V/pH, respectively. They both are close to the Nernst theoretical value (0.059 V/pH), indicating equal numbers of proton and electron transfer are involved in the electrooxidation reactions [38]. The possible electrooxidation mechanisms of INZ and HYD can be expressed as Eqs. S8 and S9 (Supporting information), respectively [2,39]. As shown in Fig. S14 (Supporting information), Ni–TA/C–CeO₂ exhibits the maximum response when the ratio of Ni to TA is 0.35.

  Differential pulse voltammetry (DPV) which is more sensitive to the trace detection of organic compounds was carried out to evaluate the detection performance of Ni–TA/C–CeO₂. As shown in Fig. 3a, the peak current gradually increases with INZ concentration increasing from 0 to 400 µmol/L (Fig. S15 in Supporting information). As shown in Fig. 3b, the corresponding calibration curve of INZ satisfies the equation of I_INZ = 1.17C_INZ + 13.00 (R² = 0.991). As shown in Fig. 3c, the peak current gradually increases with HYD concentration increasing from 0 µmol/L up to 1420 µmol/L (Fig. S16 in Supporting information). As shown in Fig. 3d, the corresponding calibration curves of HYD satisfy the equation of I_HYD = 1.18C_HYD + 10.68 (R² = 0.995) in the concentration range from 0.015 µmol/L to 70 µmol/L and the equation of I_HYD = 0.38C_HYD + 69.71 (R² = 0.994) in the concentration range from 150 µmol/L to 1420 µmol/L. The slope in the first concentration section is higher than that in the second concentration section, demonstrating a higher sensitivity in the first concentration section. This is because the electrooxidation reaction is a diffusion-controlled process, which can be demonstrated by the following kinetics study. The kinetics of INZ and HYD electrooxidation on the surface of Ni–TA/C–CeO₂ was investigated by using CV measurements with various scan rates. As shown in Fig. S17 (Supporting information), the oxidation peak current of INZ and HYD is linearly increased with the square root of the scan rate, satisfying the regression equation of I_INZ = 16.57v^1/2 – 21.16 (R² = 0.995) and I_HYD = 32.67v^1/2 – 36.45 (R² = 0.997), respectively. Moreover, the increase in the scan rate causes a positive shift in the oxidation peak potential. These demonstrate that the electrooxidation reaction of INZ and HYD is a diffusion-controlled process.

  Figure 3

  

  Figure 3.  Detection performance of Ni–TA/C–CeO₂. (a) DPV curves to different concentrations of INZ, and (b) corresponding calibration curve. (c) DPV curves to different concentrations of HYD, and (d) corresponding calibration curves. (e) Simultaneous detection of INZ and HYD, and (f) corresponding calibration curves.

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  Furthermore, the detection limits of INZ and HYD are calculated to be 0.012 µmol/L and 0.008 µmol/L (S/N = 3), respectively. The comparison of the detection performance with other electrodes is summarized in Tables S3 and S4 (Supporting information). It should be stressed that Ni–TA/C–CeO₂ compares favorably with previously reported electrodes. Simultaneous detection of INZ and HYD was also carried out by DPV. Fig. 3e shows two well-separated peaks, which are ascribed to INZ and HYD respectively. The peak currents gradually increase with the INZ and HYD concentrations increasing. Fig. 3f shows the regression equation of INZ, determined to be I_INZ = 1.15C_INZ – 4.57 (R² = 0.998) in the concentration range from 0 to 265 µmol/L. Besides, the regression equations of HYD are determined to be I_HYD = 0.96C_HYD – 5.14 (R² = 0.992) in the concentration range from 0 to 90 µmol/L and to be I_HYD = 0.36C_HYD + 58.16 (R² = 0.999) in the concentration range from 140 µmol/L to 450 µmol/L. These results verify the feasibility of simultaneous detection of INZ and HYD using Ni–TA/C–CeO₂.

  With high response and low background, 0.20 V and 0.40 V are demonstrated as the optimal working potentials for the amperometric tests of INZ and HYD, respectively (Fig. S18 in Supporting information). Accordingly, the optimal working potentials are conducted to evaluate the detection performance of Ni–TA/C–CeO₂ if not specified. As shown in Fig. 4a, the response current gradually increases during the successive addition of INZ. The response current of INZ exhibits a wide linearity range from 0.1 µmol/L to 460 µmol/L (Fig. S19 in Supporting information), satisfying the equation of I_INZ = 0.20C_INZ + 0.86 (R² = 0.990). Meanwhile, the response current of HYD also gradually increases during its successive addition (Fig. 4b). The response current of HYD has two linearity ranges from 0.02 µmol/L to 42.93 µmol/L and from 135.50 µmol/L to 1490 µmol/L (Fig. S20 in Supporting information), satisfying the equations of I_HYD = 0.40C_HYD + 1.73 (R² = 0.994) and I_HYD = 0.08C_HYD + 13.02 (R² = 0.998), respectively. The excellent performance of Ni–TA/C–CeO₂ could be ascribed to the accelerated mass and electron transfer.

  Figure 4

  

  Figure 4.  Practicability of Ni–TA/C–CeO₂. Amperometric responses to the successive addition of (a) INZ and (b) HYD. (c) Amperometric responses towards INZ and HYD in the presence of interfering substances. (d) Stability tests for the simultaneous detection of INZ and HYD. DPV curves of the analyzed samples containing (e) human plasma and (f) human urine.

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  Selectivity, stability, and repeatability are essential factors influencing practical application. The interfering substances (Na⁺, Cl⁻, K⁺, Fe³⁺, DA, Asc, UA, Lac, Glu, Suc, D-Met, D-Lys) with a concentration of 500 µmol/L were adopted to discuss the influence of coexisting substances on the response current of Ni–TA/C–CeO₂. As shown in Fig. 4c, the response currents from 90 µmol/L INZ and 60 µmol/L HYD are insignificantly affected by these interfering substances. In addition, the response currents are also not affected by these interfering substances in the absence of INZ and HYD (Fig. S21 in Supporting information). It demonstrates that Ni–TA/C–CeO₂ has excellent selectivity for INZ and HYD detection. The stability was then assessed by testing the concentration of INZ and HYD for 50 days on the same working electrode (Fig. 4d). During the whole process, the relative standard deviations (RSDs) are calculated to be only 1.3% and 1.9% for INZ and HYD, respectively. The repeatability was investigated by measuring the response currents of the same electrolyte on eight as-prepared electrodes (Fig. S22 in Supporting information). RSDs are only 0.73% and 0.67% for INZ and HYD, respectively. The XRD and XPS spectra before and after use also indicate that Ni–TA/C–CeO₂ has excellent stability and reproducibility (Fig. S23 in Supporting information). These results suggest that Ni–TA/C–CeO₂ is a promising electrode material for analyzing biological samples. Therefore, the feasibility of the established electrochemical sensor in the clinical application was verified by detecting INZ and HYD in human plasma and urine samples using the standard addition method. The samples were collected from patients in the clinical laboratory with the approval of the Ethics Committee of the hospital. After adding three groups of INZ and HYD with different concentrations to the human plasma samples, the recoveries for INZ and HYD are in the ranges of 97.76%−99.20% and 98.33%−100.83%, respectively (Fig. 4e and Table S6 in Supporting information). After adding three groups of INZ and HYD with different concentrations to the human urine samples, the recoveries for INZ and HYD are in the range of 97.97%−98.23% and 97.62%−101.77%, respectively (Fig. 4f and Table S7 in Supporting information). For comparison, high-performance liquid chromatography (HPLC) methodology was also used to detect INZ and HYD in human plasma and urine samples, which agrees with the result measured by the DPV technique (Tables S6 and S7). These results suggest that Ni–TA/C–CeO₂ has high accuracy and precision in the analysis of complex plasma and urine samples in human-based samples.

  Our study utilizes the polyphenol-based surface functionalization strategy to prepare Ni–TA/C–CeO₂ for the sensitive and simultaneous detection of INZ and HYD. The SEM, TEM, FTIR, XPS, EDX mapping, zeta potential analysis, contact angle measurement and time-settlement experiment are conducted to demonstrate the formation of the Ni–TA layer and analysis the surface properties. Adsorption kinetics and electrochemical characterizations reveal that Ni–TA/C–CeO₂ has strong adsorption and efficient mass transport of analyte molecules and electrons. In a physiological condition, Ni–TA/C–CeO₂ exhibited a wide linearity range for INZ (0.1–400 µmol/L) and for HYD (0.015–1420 µmol/L). Ni–TA/C–CeO₂ also exhibits low detection limits for INZ (0.012 µmol/L) and HYD (0.008 µmol/L), respectively. In the simultaneous detection of INZ and HYD, Ni–TA/C–CeO₂ has two well-separated peaks with the peak currents increasing linearly with the concentrations of INZ and HYD, verifying the feasibility of simultaneous detection. Furthermore, Ni–TA/C–CeO₂ exhibits high sensitivity, selectivity, stability, repeatability, and excellent practicability for analyzing biological samples. These results endow Ni–TA/C–CeO₂ with great potential applications in clinical diagnosis and nursing.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by Cooperative Education Program of the Ministry of Education, China (Nos. 202101256027 and 202102070134), National Excellent Young Scientists Found (No. 00308054A1045), National Key R & D Program of China (No. 2022YFA0912800), National Natural Science Foundation of China (No. 22178233), Talents Program of Sichuan Province, Double First Class University Plan of Sichuan University, State Key Laboratory of Polymer Materials Engineering (No. sklpme 2020-03-01), and Sichuan Tianfu Emei Project (No. 2022-EC02-00073-CG).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109113.

  
  
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  Scheme 1  Schematic representations of (a) the preparation of C–CeO₂ nanofiber, (b) the preparation of Ni–TA network on C–CeO₂ nanofiber, and (c) the possible mechanism of Ni–TA/C–CeO₂ for INZ and HYD electrochemical detection.

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  Figure 1  Physicochemical Characterization. (a) SEM, and (b) TEM images of C–CeO₂. (c) SEM and (d) TEM images of Ni–TA/C–CeO₂. (e) FTIR spectra of C–CeO₂, Ni–TA/C–CeO₂, and TA. XPS spectra in the (f) C 1s and (g) O 1s regions for C–CeO₂ and Ni–TA/C–CeO₂. (h) Images displaying the dispersion stability of C–CeO₂ and Ni–TA/C–CeO₂. (i) zeta potential of C–CeO₂ and Ni–TA/C–CeO₂. (j) Contact angles of water on the surfaces of C–CeO₂ and Ni–TA/C–CeO₂.

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  Figure 2  Adsorption property and electrochemical performance. Relationship between Q_t and time: (a) INZ, (b) HYD. Relationship between Q_e and C₀: (c) INZ, (d) HYD. (e) CV curves, and (f) Nyquist plots of CP, C–CeO₂ and Ni–TA/C–CeO₂ in the potassium ferricyanide solution. (g) DPV curves of CP, C–CeO₂ and Ni–TA/C–CeO₂ in the 0.1 mol/L PBS solution (pH 7.0) containing 90 µmol/L INZ and 60 µmol/L HYD.

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  Figure 3  Detection performance of Ni–TA/C–CeO₂. (a) DPV curves to different concentrations of INZ, and (b) corresponding calibration curve. (c) DPV curves to different concentrations of HYD, and (d) corresponding calibration curves. (e) Simultaneous detection of INZ and HYD, and (f) corresponding calibration curves.

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  Figure 4  Practicability of Ni–TA/C–CeO₂. Amperometric responses to the successive addition of (a) INZ and (b) HYD. (c) Amperometric responses towards INZ and HYD in the presence of interfering substances. (d) Stability tests for the simultaneous detection of INZ and HYD. DPV curves of the analyzed samples containing (e) human plasma and (f) human urine.

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