Electrochemical study of the Cu²⁺ sensor based on ZIF-67/MWCNTs/Nafion

1. Introduction

Heavy metals are highly toxic and difficult to be degrade, posing a serious threat to the environment and human health [1]. Although copper ions are an essential trace element of the human body, excessive participation will cause damage to human function. Therefore, it is particularly critical to find an efficient, fast and sensitive method to detect the content of copper ions in a precise way. To date, various analytical and detection technologies have been used for the detection of heavy metals, such as high-performance liquid chromatography (HPLC) [2], inductively coupled plasma‒mass spectrometry (ICP‒MS) [2] and UV‒visible spectroscopy (UV‒Vis) [3], while the disadvantages of large equipment, high price, and complex operation limit their practical application. Compared with traditional detection methods, electrochemical sensors have the advantages of high sensitivity, good selectivity, rapid detection, low price and easy operation [4,5]. However, bare electrode detection has problems such as low selectivity and repeatability. How to further select the electrode modification materials and construct an electrochemical sensing platform to improve the detection performance of heavy metal ions is still a challenge in electrochemical sensor research. Therefore, exploring a high-performance electrode modification material plays an important role in the preparation of electrochemical sensors.

Metal organic framework (MOF) materials are coordination polymers composed of metal units and organic linkers [6]. It is considered as an ideal material for electrode modification due to its large surface area, uniform distribution of pores and good adsorption properties. The cobalt-based imidazolate zeolite skeleton (ZIF-67) with rhombic dodecahedral structure is widely used in the field of sensors for food analysis [7,8], biomedicine[9,10], and environmental monitoring [11,12,13] due to its good adsorption capacity. Therefore, ZIF-67 was selected as an electrode modification material to construct an electrochemical sensing platform for detecting heavy metal ions. Unfortunately, the low conductivity and poor stability of the material in aqueous solution seriously hinder its application in electrochemical sensors. To solve the abovementioned problem of the ZIF-67 electrode, postsynthetic modification methods [14,15,16] were used to enhance the electrochemical activity and conductivity of pure ZIF-67 [17].

Multiwalled carbon nanotubes (MWCNTs) have a more efficient detection area and faster electron transfer rate than ordinary electrode-modified materials due to their unique tubular structure [18,19] and have been deemed potential materials to modify ZIF-67. At the same time, Nafion is the most commonly used ion exchange polymer in the determination of heavy metals. It has important properties such as chemical inertia, nonelectroactivity, hydrophilicity, and insolubility in water [20] and has been widely used for electrode surface modification [21,22]. In addition, Nafion can not only effectively adsorb heavy metal ions but also be used as a modifier to firmly attach the modified material to the electrode surface, thus improving its stability. Thus, Nafion was used to improve the stability of the ZIF-67/MWCNTs-modified layer to obtain the final composite material (ZIF-67/MWCNTs/Nafion) to detect Cu²⁺ ions in an electrochemical sensing system. Cu²⁺ detection by ZIF-67/MWCNTs/Nafion/GCE as a sensing platform has not been reported.

Based on the above analysis, in this work we introduced MWCNTs into ZIF-67 to synthesize ZIF-67/MWCNTs/Nafion composites, dedicated to the efficient detection of Cu²⁺ in domestic water (Scheme 1). The effects of pH, deposition time, deposition potential, interfering ions, and Cu²⁺ concentration on with the detection of Cu²⁺ were investigated. The kinetic and isotherm characteristics of Cu²⁺ adsorption by ZIF-67/MWCNTs/Nafion and the adsorption mechanism were investigated. This work is expected to provide an effective strategy for the rational modification of MOF-based adsorbents for effective application in the detection of Cu²⁺ in domestic water.

2. Experiment

2.1. Chemical reagents

2-Methylimidazole (2-MI, 98%) and Cobalt nitrate hexahydrate (Co(NO₃)₂∙6H₂O, 99%) were obtained from Shanghai Macklin Technology Co., Ltd. Sodium hydroxide (NaOH, AR), Methanol (CH₃OH, AR) and Potassium ferrocyanide (K₃[Fe(CN)₆], AR) were obtained from Tianjin Tianli Chemical Reagent Co., Ltd. Multiwalled carbon nanotube (MWCNTs, 97%) was obtained from Kaisa New Materials Co., Ltd. Nafion (5%, AR) was purchased from Shanghai Hesen Electric Co., Ltd. Ammonia (NH_{3 •} H₂O, AR) and Acetic acid (HAc, AR) were supplied by Xilong Chemical Co., Ltd. Sodium acetate (NaAc, AR) was purchased from Tianjin Beichen Fangzheng Reagent Factory. Ammonium chloride (NH₄Cl, AR) was obtained from Tianjin Shentai Chemical Reagent Co., Ltd. Potassium chloride (KCl, AR) was supplied by Tianjin Guangfu Technology Co., Ltd. The solutions in the experiments were all obtained from deionized water (18.2 MΩ∙cm) as the water source, which was purified by the UPC system. Real water samples were selected for testing from the tap water of Taiyuan Institute of Technology, Jiancaoping District, Taiyuan City, and mineral water from Cestbon. All materials and reagents are non-toxic and harmless. Additionally, all the materials and chemicals were used as received directly from the suppliers without any further treatment.

2.2. Preparation of ZIF-67

According to previous reports, the synthesis steps of ZIF-67 are as follows [23] : First, 1.17 g Co(NO₃)₂·6H₂O and 1.4 g 2-MI were weighed with an electronic balance and added into 50 mL of CH₃OH, respectively. After stirring with a glass rod to clarify, the two were mixed immediately and stirred in a magnetic stirrer for 30 min. Second, the mixed solution was centrifuged at a rotational speed of 8000 r/min. Finally, the centrifuged product was dried at 60℃ for 12 h and ground to obtain ZIF-67.

2.3. Purification Treatment of MWCNTs

Purification of MWCNTs can improve the agglomeration of MWCNTs by increasing the content of oxygen-containing functional groups such as hydroxyl and carboxyl groups [24]. The steps are as follows: MWCNTs were put into 20 mL V(H₂SO₄):V(HNO₃)= 1:3 mixed acid at 40℃ continuous reflux for 6 h, and then washed with water to neutral. Finally, the product was dried in an oven.

2.4. Fabrication of ZIF-67/GCE, ZIF-67/MWCNTs/GCE, ZIF-67/MWCNTs/Nafion/GCE

ZIF-67 (1 mg/mL) and MWCNTs (0.5 mg/mL) were ultrasonicated for 5 min to obtain a uniform dispersion solution. Nafion (5%) was diluted with methanol. Before modifying the electrode, the working electrode was polished by Al₂O₃ and cleaned by ultrasonication. After drying, ZIF-67, ZIF-67/MWCNTs, and ZIF-67/MWCNTs/Nafion modification solutions were applied in drops and dried under a red lamp to obtain the corresponding electrode.

2.5. Instruments and measurements

The morphology and microstructure of the modified materials ZIF-67 and MWCNTs were characterized by scanning electron microscopy (SEM, HITACHI SU8010) and transmission electron microscopy (TEM, FEI Tecnai G2F30). The crystal structure of the modified materials ZIF-67 and MWCNTs were examined by X-ray diffraction (XRD, Bruker D8 Advance). The functional groups contained in MWCNTs before and after the modification treatment were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet lS10); The N₂ adsorption-desorption (BET, Micromeritics ASAP 3020) technique was used to determine the surface area and pore structure of the modified materials ZIF-67, ZIF-67/MWCNTs. X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250XI) was used to examine the elemental composition and valence of ZIF-67.

All electrochemical experiments in this work were carried out at Princeton ParSTAT MC multichannel electrochemical workstation. A glassy carbon electrode (L-type, 3 mm) was used as the working electrode, a saturated Ag/AgCl electrode as the reference electrode, and a platinum sheet electrode as the counter electrode. The electrolyte was 0.1 M HAc-NaAc buffer solution (pH=5.0). The pH of the HAc-NaAc buffer was adjusted with 0.1 M HCl and 0.1 M NaOH. Square wave dissolution voltammetric curves were measured at a deposition potential of -0.4 V, a deposition time of 150 s, and a measurement range of -0.4~0.6 V. Cyclic voltammetry (CV) and electrochemical impedance (EIS) measurements were carried out in K₃[Fe(CN)₆] solution in 1 M KCl. Cyclic voltammetric curves were measured in the potential range of -0.2 V ~ 0.8 V with a scan rate of 20 mV/s. EIS was performed in the frequency range of 100,000 ~ 0.1 Hz.

3. Results and discussion

3.1. Characterization of electrode modification materials

The structures of ZIF-67 and MWCNTs were analyzed by XRD, and the physical and chemical properties of the materials were determined. The XRD pattern of ZIF-67 (Figure 1a) shows that the diffraction peak is basically consistent with that of simulated ZIF-67, which proves that ZIF-67 has been successfully synthesized [25]. The diffraction peaks of the acidified MWCNTs (Figure 1b) at 26.35^◦ and 44.12^◦ corresponded to the characteristic peaks of graphic-carbon standard card PDF#75-1612, indicating that the skeleton was intact and intact after acid treatment [26].

FTIR plays an important role in identifying functional groups. Figure 1c is the FTIR spectrum of MWCNTs before and after acidification treatment. The absorption peak caused by the C=O stretching vibration in -COOH appeared at 1624 cm^-1 [27], and the stretching vibration peak of -OH caused by intramolecular association appeared at 3435 cm^-1 [28,29]. This result shows that there are a certain amount of -OH and -COOH functional groups in the MWCNTs. The absorption peak of acid-treated MWCNTs was sharper at 1624 cm^-1, indicating that the mixed acid introduced more C=O to the MWCNTs. At the same time, C=O was further oxidized into -COOH, which increased the carboxyl functional groups. The absorption peak at 3435 cm^-1 widened, indicating that acidification introduced more -OH into the surface. In conclusion, acid treatment increases the amount of carboxyl and hydroxyl groups on the surface of MWCNTs, which greatly improves the dispersity of MWCNTs.

Specific surface area and pore size distribution are important parameters affecting the performance of electrode materials. Thus, the N₂ adsorption-desorption isotherms and pore size distributions of ZIF-67 and ZIF-67/MWCNTs were investigated. As shown in Figure 1d, both ZIF-67 and ZIF- 67/MWCNTs exhibited type I isotherms. The average pore size was 4.8478 nm, which was in the range of 2-50 nm, indicating that the pores were mainly composed of mesopores.The BET specific surface area (SBET) of the ZIF-67 and ZIF-67/MWCNTs nanocomposites were 1324.1918 m²/g and 1237.6814 m²/g, respectively.The prepared ZIF-67/MWCNTs had relatively low specific surface area, which may be due to the fact that MWCNTs occupy a part of the pore volume of ZIF-67 in their attachment to ZIF-67. Overall, the ZIF-67/MWCNTs exhibited a large specific surface area, which is expected to play a positive role for the adsorption and conductivity properties of Cu²⁺.

The element types and chemical states in the sample ZIF-67 were analyzed by XPS. Figure 1e is the full XPS spectrum of ZIF-67, which confirms that C, O, N and Co coexist in ZIF-67. Peaks of C1s, O1s, N1s and Co 2p are clear without obvious impurity peaks. The results are in good agreement with the XRD and SEM test results, which proves that the material has been successfully synthesized. The C1s spectrum of ZIF-67 (Figure 1f) show that there are mainly three different types of C in C1s, which are C-C (284.7 eV), C-O (286.4 eV) and C=O (288.8 eV). Meanwhile, the O1s spectrum (Figure 1g) corresponds to the Co-O bond and C-O bond at 532.1 eV and 533.7 eV, respectively. The N1s XPS spectrum shows that the peaks at 398.6 eV and 400.2 eV, respectively, correspond to two states of nitrogen existence in the material: pyridine-N and pyrrorole-N [30,31,32] (Figure 1h). According to the XPS spectrum of Co 2p, the peaks at 796.7 eV and 781.2 eV are characteristic peaks of Co 2p_1/2 and Co 2p_3/2, respectively [33,34] (Figure 1i). In addition, there are two satellite peaks, 802.1 eV and 786.2 eV, which confirm the presence of Co²⁺ in ZIF-67 [35,36]. This indicates that cobalt in ZIF-67 exists in the form of divalent ions.

The morphology of the electrode modification solution after sonication was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2a,b, the skeleton structure of ZIF-67 is intact, uniform in shape and clearly visible in outline. It has a typical and regular rhomboidal dodecahedron structure, part of which is broken into fine particles under the action of ultrasound. As shown in Figure 2c,d, the acidified MWCNTs showed a tubular structure, uniform diameter, smooth surface, and well-ordered dispersion. From the TEM results (Figure 2g,h), it was further demonstrated that the MWCNTs after acidification were in a hollow tubular structure, which provided conditions for Cu²⁺ transport. The modification liquid after ultrasonic mixing of ZIF-67 and MWCNTs in a certain proportion is shown in Figure 2e,f. ZIF-67 was completely broken into nanoparticles and wrapped together with tubular MWCNTs.

3.2. Electrochemical characterization of electrodes

The electrochemical properties of the GCE, ZIF-67/GCE, MWCNTs/GCE and ZIF-67/MWCNTs/GCE were tested by CV and EIS in a 5 mM K₃[Fe(CN)₆] solution containing 1 M KCl. Figure 3a shows the CV curves of different modified materials with a scanning range of -0.2V to 0.8 V and a scanning rate of 20 mV/s. After comparison of the four electrodes, it was found that MWCNTs/GCE showed the highest peak current and the lowest peak potential difference (the difference between the oxidation peak and the reduction peak, 57 mV), which can be attributed to the peak potential difference being inversely proportional to the electron transfer rate constant [37]. MWCNTs have the highest electron transfer rate constant among the four, indicating the good electrical conductivity of MWCNTs. Compared with ZIF-67/GCE, the oxidation peak current of ZIF-67/MWCNTs/GCE increased by approximately 37%, while the peak potential difference decreased from 110 mV to 90 mV, indicating that the addition of MWCNTs improved the conductivity of ZIF-67. In order to investigate the charge transfer resistance between the electrode modification material and the electrolyte interface, electrochemical impedance spectroscopy (EIS) test method was used. The solution resistance (Rs) denotes the intersection of the semicircle in the high frequency region with the x-axis. The charge transfer resistance (Rct) indicates the charge transfer rate, and the size is the diameter of the semicircle [38]. The smaller the diameter of the semicircular arc, the smaller the resistance to electron transfer, the faster the rate of electron movement, and the better the conductivity of the material modified on the electrode surface. From the Nyquist plot in Figure 3b, it can be seen that the diameter of the semicircular arc of MWCNTs/GCE is the smallest, indicating that MWCNTs have the best electrical conductivity. Compared with ZIF-67/GCE, ZIF-67/MWCNTs/GCE showed a smaller semicircle diameter and a correspondingly smaller electron transport resistance, indicating that MWCNTs improved the conductivity of ZIF-67, which was consistent with the CV results.

3.3. Optimization of test conditions

Through the above studies, ZIF-67/MWCNTs/Nafion/GCE was used as the electrochemical sensor for the detection of Cu²⁺. To obtain the best sensing performance, the key experimental parameters were optimized. The Optimization parameters are as follows: type of electrolyte solution, pH of electrolyte solution, deposition time and deposition potential (Figure 4). In short, the following experimental conditions were found to have the best results: (a) 0.1 M HAc-NaAc electrolyte solution with pH 5; (b) The deposition potential was -0.4 V, and the deposition time was 150 s, taking into account factors such as modification material, electrode loss and test efficiency. The following experiments were carried out under the above conditions.

3.4. Detection of Cu²⁺ by SWSV

The electrodes of different modified materials were analyzed by SWSV in HAc-NaAc solution containing 0.2 μM Cu²⁺. As shown in the SWSV curve shown in Figure 5a, the stripping peak appearing at approximately -0.1 V is the stripping peak potential of Cu²⁺. The peak current response of ZIF-67/GCE was three times higher than that of the bare GCE, which was attributed to the good adsorption performance and catalytic activity of ZIF-67. The MWCNTs/GCE showed a higher electrochemical signal but a weaker dissolution peak for Cu²⁺, reflecting the indispensability of ZIF-67. When the electrode was modified by ZIF-67 and MWCNTs, the response of Cu²⁺ significantly increased, which was attributed to the increase in the number of active site of MWCNTs and the superior conducting ability. When Nafion is added, the sensor performance of the electrode is further improved, which is due to the ability of Nafion to enhance the adsorption capacity of heavy metal ions through electrostatic attraction. Meanwhile, the modification of Nafion can improve the stability of the electrode and effectively prevent the accessories on the electrode surface from falling off. Therefore, ZIF-67/MWCNTs/Nafion was selected as the electrode modification material.

The sensitivity of the sensor was tested by using ZIF-67/MWCNTs/Nafion/GCE as the working electrode under optimized experimental conditions. In the illustration of Figure 5b, it can be found that the peak current increases with increasing Cu²⁺ concentration in the range of 0.2~0.8 μM. Correspondingly, in the calibration curve in Figure 4, the Cu²⁺ concentration is positively correlated with the stripping peak of Cu²⁺_. The linear equation is y=57.5x+2.3152 (R²=0.9904). The sensitivity of the sensor constructed by ZIF-67/MWCNTs/Nafion to detect Cu²⁺ is 57.5 μA/μM. According to the 3σ/S criterion (where σ is the standard deviation and S is the slope of the calibration graph) [39], the limit of detection (LOD) was calculated to be 15.0 nM. This shows that the electrochemical sensor constructed by ZIF-67, MWCNTs and Nafion has a synergistic effect on the detection of Cu²⁺, and Cu²⁺ can be effectively reduced and stripped to realize the trace detection of Cu²⁺.

3.5. Effect of scan rate

To determine the mechanism of electrochemical oxidation of Cu²⁺ by ZIF-67/MWCNTs/Nafion/GCE. The effect of scan rate was investigated using CV. Figure 5c shows the cyclic voltammograms obtained for 0.2 μM Cu²⁺ at different scan rates (0.04-0.2 V/s). From Figure 5d, it can be seen that the oxidation peak current (Ipa) of Cu²⁺ gradually increases with the increase of the scan rate. the I_pa of Cu²⁺ is linearly correlated with the scan rate, and the linear equation is: I_pa(μA) =582.54 ν+33.44(V/s) (R²=0.969). From the equation, it can be seen that the current is linearly related to the scan rate. Therefore, the redox behavior of Cu²⁺ on ZIF-67/MWCNTs/Nafion/GCE is a typical adsorption-controlled process [40]. In addition, it can be seen from Figure 5e that the oxidation peak potential (E_pa) tends to be positively shifted with the increase of scanning rate, and its linear equation is E_Pa = 0.0244 lnν + 0.4103 (R²=0.977). These results indicate that the electrochemical oxidation process of Cu²⁺ on ZIF-67/MWCNTs/Nafion/GCE is irreversible. The electron transfer number (n) can be obtained according to the Laviron equation [41].

$$E_{pa}=E^{\theta }+\frac{RT}{\alpha nF}\ln \frac{RT{K}^{\theta }}{\alpha nF}+\frac{RT}{\alpha nF}\ln v$$

where α, n, k^θ, and T are the electron transfer coefficient, electron transfer number, electron transfer rate constant, and absolute temperature, respectively. Based on this equation, the value of αn was calculated to be 1.053. Using the recognized value of α of 0.5 for irreversible reactions, the number of electrons involved in the electrochemical oxidation of Cu²⁺ was calculated to be 2.12. This indicates that the electrochemical oxidation of Cu²⁺ on the surface of the sensor based on ZIF-67/MWCNTs/Nafion/GCE is the behavior of the two protons and two electrons involved.

3.6. Effect of Co²⁺ on the detection of Cu²⁺

A constant deposition potential of -0.4 V was applied to the working electrode. Electrons are transferred from the working electrode to the ZIF-67/MWCNTs /Nafion/GCE interface, where Cu²⁺ obtains electrons and is reduced to Cu. When the forward scanning voltage is applied and the Cu²⁺ stripping peak potential (-0.1V) is reached, the Cu deposited on the electrode is oxidized into Cu²⁺ and returned to the solution. At the same time, electrons return to the surface of the electrode, and the stripping current signal of Cu²⁺ is detected. In this process, ZIF-67 has a good adsorption and catalytic capacity for Cu²⁺. The addition of MWCNTs and Nafion significantly improves the enrichment degree of Cu²⁺ and the stability of the electrode. For the catalytic ability of ZIF-67, different amounts of Co²⁺ were added to the HAc-NaAc solution to verify the effect on the adsorption of heavy metals. As shown in Figure 5f, the stripping peak of Cu²⁺ is significantly higher than that without the addition of Co²⁺ after the addition of excess Co²⁺ in the HAc-NaAc solution. It can be concluded that Co²⁺ in ZIF-67 plays a catalytic role in the detection of heavy metals [42].

3.7. Research on the repeatability, reproducibility, stability and anti-interference ability of the sensor

In addition to the sensitivity and LOD of the sensor, the repeatability, reproducibility, stability and anti-interference ability of the sensor are also important indicators to detect the performance of the sensor. SWSV tests were performed on ZIF-67/MWCNTs/Nafion/GCE seven consecutive times to verify the repeatability of the electrodes. As shown in Figure 6a, the peak stripping potential of Cu²⁺ was almost consistent in seven tests, and the relative standard deviation (RSD) was 0.49%, which further demonstrated that the electrode had good repeatability. Seven electrodes were used under the same conditions to test the reproducibility of the electrodes. As shown in Figure 6b, the peak stripping potential of the seven electrodes did not deviate significantly, and the RSD was 2.21%, less than 5%, which proved that the sensor had good reproducibility. To test the anti-interference ability of the electrode, 4 μM interfering metal ions such as Zn²⁺, Co²⁺, Ba²⁺, Mn²⁺, K⁺, Fe²⁺, Na⁺ , Mg²⁺, Cr⁶⁺, Fe²⁺, Hg²⁺, Cd²⁺ and Pb²⁺ were added to the solution containing 0.2 μM Cu²⁺. It can be clearly observed from Figure 6c that after the addition of a high concentration of interfering ions, the detection of Cu²⁺ (within 8%) was not affected. This indicates that the sensor has high selectivity. Finally, stability tests were conducted on the original electrode, and the electrode was placed seven days later. As shown in Figure 6d, the peak current seven days later was 95% of the original, indicating that the sensor had good stability.

3.8. Testing of real water samples

The feasibility of ZIF-67/MWCNTs/Nafion/GCE in practical applications was further studied, and SWSV was determined for Cu²⁺ in mineral water and tap water. The results obtained at a ratio of 2:8 between the water sample and HAc-NaAc solution are shown in Table 1. The recovery rate of tap water and mineral water was 98.6%-103%, and the RSD was less than 5%. This indicates that the electrochemical sensor based on ZIF-67/MWCNTs/Nafion can be used to detect Cu²⁺ in actual water samples.

4. Conclusion

In conclusion, ZIF-67/MWCNTs/Nafion composites were prepared in a simple and environmentally friendly way. ZIF-67/MWCNTs/Nafion/GCE detected the SWSV of Cu²⁺ in the linear range (0.2~0.8 μM), and the LOD was 15.0 nM, which realized the trace detection of Cu²⁺. Moreover, the sensor showed high sensitivity, selectivity and stability for Cu²⁺. The applicability of this method is verified by the detection of Cu²⁺ in real water samples, and satisfactory results are obtained. These results indicate that the Cu²⁺ sensor platform based on ZIF-67/MWCNTs/Nafion has great application potential.