Carbon nanotube supported spherical NiFe-spinel heterostructure for sensitive electrochemical detection of aristolochic acid

Abstract

Aristolochic acid (AA) has strong carcinogenicity, and it has been reported that the medicinal and edible plant Houttuynia cordata may contain AA. Among transition metals, nickel and iron have outstanding catalytic ability for nitro reduction. The multivalent NiFe₂O₄ (NFO), which effectively promotes the redox reaction, has become a promising electrochemical material. In this work, we innovatively used a one-pot hydrothermal method to prepare NFO in situ on the surface of carbon nanotubes. For the first time, the composite NiFe₂O₄@MWCNTs (NFO@CNTs) was utilized to build a sensitive electrochemical sensor for detecting AA. The NFO@CNTs/GCE exhibited strong electrochemical performance due to the synergistic effect of high catalytic activity of NFO and good conductivity of carbon nanotubes. Furthermore, in order to provide a basis for the safe use of Houttuynia cordata, the electrochemical senor was successfully applied to detect AA in Chinese herbal medicines, confirming its practicability in real samples. This work broadens the application of nickel ferrite, which is expected to be a new candidate material for sensors.

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1. Introduction

Aristolochic acid (AA) is a group of carcinogenic and nephrotoxic nitro-phenanthrene carboxylic acid compounds found in Aristolochia species and Asarum species.^1–4 In recent years, studies have shown that AA can cause genetic mutations in kidney cells. However, the abuse of AA remains high; for example, dietary supplements and drugs containing AA are still circulating in the consumer market.⁵Houttuynia cordata is a medicinal and edible plant with wide usages. Its main components have analgesic, anti-inflammatory and anti-cancer effects.^6–8 Recently, it has been reported that its edible parts contain toxic AA, causing social concern. Therefore, developing a simple and effective method to determine AA is crucial for ensuring the safety of food and the quality of drugs.

So far, many methods have been applied to detect AA, such as capillary electrophoresis (CE),⁹ high-performance liquid chromatography (HPLC),¹⁰ and liquid chromatography-tandem mass spectrometry (LC-MS/MS).¹¹ However, these methods have the disadvantages of long detection time, complex preprocessing and expensive equipment.^4,12,13 In comparison, the electrochemical technique has attracted attention due to its ease of operation, low cost and short time consumption.^14,15 Unfortunately, the non-modified electrode has some drawbacks, for example, small current response in detecting AA. In general, choosing a modifier to improve the electrode performance for effective detection of AA remains important in sensor fabrication.^13,16

Recently, spinels (AB₂O₄) have attracted much research attention in energy materials due to their high catalytic activity and chemical stability. They are expected to be used as substitutes for noble metals in applications, such as electrocatalysis and electrochemical sensors.^17–19 In addition, they exhibit higher electrochemical activity than single-component nanomaterials because of their multivalent cationic properties, unique crystal structure characteristics and great catalytic activity.^20–23 Among them, NiFe₂O₄ (NFO) exhibits excellent catalytic performance and good electrochemical activity.¹⁹ However, Ni–Fe-based oxides suffer from poor electroconductivity, which greatly limits the electron transfer process.²⁴ In order to improve the electrochemical properties of Ni–Fe-based oxides, researchers have made great efforts in this field. Extensive works have shown that low-cost carbon-based metal-free catalyst materials can be used for modifying the electrode and enhancing the catalytic performance.^25–27 Carbon nanotubes (CNTs) are widely used in energy storage and detection sensors because of their excellent specific surface area, high conductivity and good stability.^26–29 In this study, we used hydrothermal method to prepare NiFe₂O₄@MWCNTs/GCE composite material (NFO@CNTs/GCE) by combining a nickel–iron-based spinel and multi-walled carbon nanotubes; it exhibited excellent electrochemical properties and analytical performance.

2. Experimental section

2.1. Materials and reagents

Ni(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O and sodium hydroxide (NaOH) were bought from Shanghai Titan Technology Co., Ltd. MWCNTs were bought from Nanjing Xianfeng Nanomaterials Technology Co., Ltd. 0.1 M phosphate buffer solution was prepared using a suitable mixture of Na₂HPO₄ and NaH₂PO₄ solutions. Fresh Houttuynia cordata plants were obtained from Hubei Asarum medicinal materials were purchased online. All chemicals were of analytical grade.

2.2. Synthesis of NFO@CNTs

The NFO@CNTs composite with different CNTs contents was synthesized as follows (Scheme 1). In brief, NFO@CNTs containing 50 wt% CNTs was synthesized by the following procedure: 0.291 g of Ni(NO₃)₂·6H₂O, 0.808 g of Fe(NO₃)₃·9H₂O and 0.1 g of CNTs were dispersed in 20 mL DI water under stirring, named A. Then, 0.32 g of NaOH was dissolved in 10 mL DI water, named B. Next, solution B was added dropwise to solution A, and the mixture was stirred for 3 h at 25 °C, and transferred to an autoclave and warmed at 180 °C for 24 h. After the autoclave was cooled, the precipitate was washed with distilled water and dried in an oven at 80 °C. As control experiments, NFO nanoparticles were prepared using the same procedure without adding CNTs. Similarly, CNTs were subjected to the same procedure of hydrothermal treatment.

Scheme 1 Preparation of NFO@CNTs and its application in AA detection.

2.3. Modification of the working electrode

Before modifying the electrode, a bare GCE was polished with 0.05 μm alumina powder and then ultrasonically cleaned with water and ethanol for 3 min, respectively. Subsequently, 1.0 mg of NFO@CNTs electrocatalyst was dissolved in 1 mL water and then subjected to ultrasonic treatment for 10 min to get a uniform mixture. Then, a certain amount of composite suspension droplets was dropped on the cleaned GCE surface and dried for about 3 min to obtain the NFO@CNTs/GCE working electrode. For comparison, we fabricated NFO/GCE and CNTs/GCE modified electrodes using similar procedures.

2.4. Sample preparation

The washed fresh Houttuynia cordata and Asarum herbs were placed in an oven, dried at 80 °C, crushed into powder and sieved. Houttuynia cordata powder (10 g) and Asarum powder (10 g) were dissolved in methanol and extracted 3 times, 1 h each time, and the filtrate was combined. The filtrate was dried using a rotary evaporator at low pressure, fixed volume with ethanol to 100 mL, filtered with 0.22 μm organic membrane and standby.

3. Results and discussion

3.1. Morphology and structure characterization

The morphology of the prepared materials was analyzed by SEM, and a clear tubular structure can be observed in Fig. 1a. Meanwhile, NFO nanoparticles can be observed in Fig. 1b. As shown in Fig. 1c, it is evident that all the NFO nanoparticles are securely attached to the CNTs, retaining their original structure and completely covering the carbon nanotubes. As shown in the TEM images (Fig. 1d and e), many NFO nanoparticles are grown on the surface of CNTs. The hydrothermally treated CNTs in the composite material do not change their morphology and still present a clear tubular structure. Furthermore, the EDS results of NFO@CNTs are shown in Fig. 1f, which obviously suggest the uniform distribution of C, O, Ni, and Fe elements with the corresponding atomic ratio of 79.7 : 10.1 : 7.0 : 3.3 (Fig. S1, ESI†). These results are coincident with the experimental data.

Fig. 1 SEM images of (a) CNTs, (b) NFO, and (c) NFO@CNTs. (d)–(f) TEM and EDS images of the NFO@CNTs.

XRD was used to further analyze the prepared materials (Fig. 2a). The XRD spectra of NFO displayed distinct diffraction peaks located at 30.28° (220), 35.6° (311), 43.32° (400), 57.28° (511), and 62.9° (440), which matched those of NiFe₂O₄ (JCPDS 10-0325).³⁰ Moreover, a broad peak at 21.1° corresponds to CNTs. It is noteworthy that the positions of diffraction peaks in the XRD spectra of NFO@CNTs composites are similar to those of NFO, indicating that the introduction of carbon nanotubes does not disrupt the crystal structure of NFO. This indicates the successful synthesis of NFO@CNTs.

Fig. 2 (a) XRD of NFO and NFO@CNTs samples. (b) Overall spectrum for NFO@CNTs. (c)–(f) XPS spectra of C 1s, O 1s, Fe 2p, and Ni 2p of the NFO@CNTs.

To further study the materials’ chemical composition and elemental valence states, X-ray photoelectron spectroscopy (XPS) was carried out. A full scan XPS spectrum of the NFO@CNTs composite (Fig. 2b) shows the presence of C 1s, O 1s, Fe 2p, and Ni 2p peaks. The C 1s spectrum could be divided into three peaks; the peak of the C–C bond is clearly visible (Fig. 2c). As shown in Fig. 2d, the O 1s spectrum of the NFO@CNTs can be deconvoluted into three peaks. The O_L (530.65 eV) component is attributed to the typical metal-oxygen bond, and the O_V (532.88 eV) component is associated with defects/vacancy sites originating from low oxygen coordination. The O_C peak (534.38 eV) represents the –OH and the adsorbed water of NiFe₂O₄.³¹ As for NFO@CNTs, the resolution peaks at binding energies of 711.48 eV and 724.48 eV correspond to Fe 2p_3/2 and Fe 2p_1/2, respectively. In addition, the fitting peaks at 711.48 eV and 724.48 eV correspond to Fe²⁺, and those at 714.48 eV and 730.48 eV correspond to Fe³⁺. At the same time, two oscillation satellite peaks are located at 717.9 eV and 731.4 eV (Fig. 2e). As shown in Fig. 2f, there are two satellite peaks and Ni 2p_1/2 peaks and Ni 2p_3/2 peaks in the Ni 2p XPS spectrum. The peaks at 874.72 and 857.48 eV are related to the oxidation state of Ni³⁺, and the peaks with binding energies of 873.32 and 855.61 eV are related to the oxidation state of Ni²⁺. The above results demonstrate the successful preparation of NFO@CNTs and the multivalent formation of Ni and Fe.^32,33 Notably, multivalent metal ions effectively promote electron transfer and redox reactions.

3.2. Electrochemical behavior

Next, we used EIS to analyze the electron transfer of different modified electrodes. In Fig. S2a (ESI†), the Nyquist plots of bare GCE, CNTs/GCE, NFO/GCE and NFO@CNTs/GCE were analyzed with 5 mM [Fe(CN)₆]^3−/4− probe in 0.1 M KCl. And then the electrochemical impedance of different electrodes was studied using the Randles equivalent circuit. The surface modification allows the electrodes to exhibit enhanced electron transfer capability. The NFO@CNTs/GCE showed the lowest R_ct (101.6 Ω) compared to bare GCE (131 Ω), CNTs/GCE (127.7 Ω) and NFO/GCE (1907 Ω), indicating good catalytic properties and rapid charge transfer ability of NFO@CNTs/GCE.

Meanwhile, the electrochemical behavior of different modified electrodes was studied by using cyclic voltammetry (Fig. S2b†). NFO@CNTs/GCE showed the highest peak current and good reversibility compared to the other modified electrodes, and the results of CV were consistent with EIS. The electrochemical behavior of the hydrothermally treated CNTs modified GCE was compared with that of the untreated CNTs modified GCE, and the deviations of CV and EIS between hydrothermally treated and untreated samples are both very small (Fig. S6a and b, ESI†).

As shown in Fig. S3 (ESI†), the peak current of bare GCE, NFO/GCE, CNTs/GCE and NFO@CNTs/GCE in 5 mM [Fe(CN)₆]^3−/4− solution containing 0.1 M KCl increased with the scan rate increasing, and it is linearly correlated with the square root of the scan rate. To further study the electrocatalytic performance of bare GCE, CNTs/GCE, NFO/GCE and NFO@CNTs/GCE their electrochemically active surface area (ECSA) was calculated by the Randles-Sevcik eqn (1) as follows.³⁴

I_p = (2.69 × 10⁵) n^3/2ν^1/2D^1/2AC (1)

where I_p is the anodic peak current, n is the electron transport number, v is the scan rate, D is the diffusion coefficient, A is the electrochemical active surface area, and C is the concentration. The ECSA for bare GCE, CNTs/GCE, NFO/GCE and NFO@CNTs/GCE were calculated from the above equation to be 0.0706, 0.087, 0.1078 and 0.1161 cm², respectively. The results showed that the NFO@CNTs/GCE has a larger ECSA than bare GCE, CNTs/GCE and NFO/GCE. The above experiment results suggest that a synergistic interaction between NFO and CNTs is generated to maximize the electrochemically active area and effectively improve the electrocatalytic performance, leading to the electron transfer process improvement.

The chronocoulometry experiment was carried out by adding 50 μM AA into PBS. In Fig. S4a and b (ESI†), according to the Anson eqn (2),³⁵ the A in the detection process was calculated to be 1.31, 1.68, 13.87 and 19.56 cm², respectively. The results effectively prove that the current response is enhanced for AA at NFO@CNTs.

The current of different material modified electrodes to AA in a 0.1 M PBS buffer system was analyzed by using the CV method. And the results are shown in Fig. S5a (ESI†); it can be observed that the bare electrode and the electrode modified with the NiFe₂O₄ material did not show an obvious reduction peak, presenting weak electrochemical response to AA. However, CNTs/GCE had a specific electrochemical signal response, featuring a reduction peak and a reduction current of 29.42 μA, which could be attributed to the good conductivity of carbon nanotubes. The treated CNTs could still produce 29.32 μA reduction current (Fig. S6c and d, ESI†), indicating that the hydrothermal treatment didn’t affect the detection performance of CNTs. It was observed from Fig. S5b (ESI†) that the NFO@CNTs/GCE composite had an obvious reduction peak with a reduction current of 44.57 μA. The electrochemical response current was higher than that of the single material modified electrode and the bare electrode. In summary, NFO@CNTs/GCE has the smallest reduction potential and the largest reduction current, which is attributed to the synergistic effect of NFO's catalytic ability and CNTs’ conductivity. All of the results illustrate that it is feasible to achieve high sensitivity detection of AA on NFO@CNTs/GCE.

3.3. Optimization of conditions

Based on the good experimental results of basic electrochemical characterization of NFO@CNTs, we next researched the influence of various mass ratio (1 : 2, 1 : 1, 2 : 1) on the detection performance of the AA sensor by means of CV. As exhibited in Fig. S7 (ESI†), when the mass ratio of NFO to CNTs is 1 : 1, the peak current response is more obvious. The mass ratio of 1 : 1 is the best choice for subsequent NFO@CNTs preparation.

Meanwhile, the loading concentration of NFO@CNTs on the GCE is a very significant factor for AA detection. In Fig. S8 (ESI†), the peak current response was enhanced sharply as the concentration of NFO@CNTs increased from 2 to 6 μL. After the concentration exceeded 6 μL, it showed a decreasing trend as the loading concentration increased. Therefore, we can conclude that 6 μL is the optimal loading concentration for NFO@CNTs/GCE preparation.

The pH of the buffer has a significant effect on the electrochemical sensing of AA. As shown in Fig. S9a and b (ESI†), the I_p of AA raised gradually when the pH rose from 3 to 6 and when the pH exceeded 6 it decreased obviously, reaching a maximum at pH = 6. In summary, we selected 0.1 M PBS (pH = 6) for the following electrochemical experiments. Meanwhile, as the pH value increased, E_p continuously shifted to a more negative potential, indicating that the proton is involved in the reduction reaction of AA. The linear relationship was fitted as Fig. S9c (ESI†). The absolute value of the slope is 57.7 mV pH⁻¹, which is close to the theoretical value of 59 mV pH⁻¹ from the Nernst equation: d_Ep/d_pH = −2.303 (mRT/nF).³⁶ This suggests that the reduction of AA involves a reaction pathway with one electron and one proton.

3.4. Electrochemical behavior

We investigated the electrocatalytic reduction effect of NFO@CNTs/GCE on AA by changing the scan rate on I_p. As exhibited in Fig. S9d and e (ESI†), it is observed that as the scan rate varies between 0.025 and 0.25 V s⁻¹, the I_p gradually increases, while the E_p gradually shifts to the negative direction. And the current is proportional to the scan rate. The linear relationship indicated that the irreversible reduction of AA on the NFO@CNTs/GCE surface is an adsorption-controlled process.³⁷

The response mechanism of NFO@CNTs/GCE to AA was further researched. In irreversible reactions, the number of transferred electrons can be calculated by eqn (3).

where E_p is the cathode peak potential, E⁰ is the formal potential, R is the ideal gas constant, T is the temperature, α is the charge transfer coefficient, n is the number of electrons transferred in the reaction, F is Faraday's constant, K_s is the standard rate constant, and v is the scan rate.

Then, the reduction peak potential (E_p) shifts negatively as the logarithm of the scan rate (ln v) increases, indicating that E_p and lnv are proportional. And the linear regression equation was fitted as Fig. S9f (ESI†). Based on the calculation results of the above equation, the number of electrons transferred in the rate determination step is 1, which is consistent with the reported literature. Because the number of protons and electrons involved in this reaction is equal, the nitroaromatic compounds of AA can be reduced to hydroxylamine on NFO@CNTs/GCE. Therefore, we proposed a possible reaction mechanism depicted by the following two equations.³⁸ Apparently, four electrons are transferred throughout the reaction, and the slow reaction is the rate-determining step, which is consistent with previous reports.³⁹

R–NO₂ + e⁻ → R–NO₂⁻ (slow) (4)

R–NO₂⁻ + 3e⁻ + 4H⁺ → R–NHOH + H₂O (fast) (5)

The electrocatalytic mechanism of AA reduction on NFO@CNTs/GCE was studied using CV (Fig. 3). The reduction peak (R₁) has a close relationship with the change of nitro (–NO₂) to the hydroxylamine derivative groups (–NHOH). The oxidation peak (O₁) represents the oxidation of hydroxylamine to nitroso. It is worth noting that the intensity of the depletion R₁ of AA on the surface of NFO@CNTs/GCE decreased sharply in the second cycle. Therefore, R₁ was used to study the electrochemical reduction process of AA in subsequent experiments.

Fig. 3 (a)–(c) AA detection property of the NFO@CNTs/GCE sensor in 50 μM AA. (d) Reaction mechanism of electrochemical reduction of AA.

3.5. Electrochemical determination of AA using NFO@CNTs/GCE by the DPV technique

The linear range and detection limit of AA are important indicators for evaluating the sensor. The DPV technique is a more precise measurement than CV. In later experiments, we attempt to further validate the sensitivity of NFO@CNTs/GCE via DPV. As displayed in Fig. 4a, with elevating the AA concentration from 0.1 μM to 90 μM, the corresponding peak current also increases. Moreover, the I_p value increase linearly with the increase of AA concentration in two linear ranges. And the linear regression equation is given as follows (Fig. 4b):

I_p (μA) = −1.0534c (μM) −0.4874 (R² = 0.9965, 0.1–10 μM);

I_p (μA) = −0.1443c (μM) −10.441 (R² = 0.9908, 10–90 μM).

Fig. 4 (a) DPV of AA: the quantitative analysis on NFO@CNTs/GCE at different AA concentrations. (b) Peak current vs. AA concentration (μM). (c) Molecular adsorption diagram. (d) Current values of the NFO@CNTs/GCE toward different interfering substances (cation/anions/molecules). (e) Current values of the reproducibility tests. (f) Storage stability analysis results with 12 days on NFO@CNTs/GCE for AA detection.

The two linear relationships can be explained as single-layer adsorption followed by multi-layer adsorption, resulting in a gradual decrease in the current increase during the AA detection (Fig. 4c). According to the fitted equation, the limit of detection (LOD) was calculated to be 0.03 μM viaeqn (6).

LOD = 3σ/S (6)

where σ represents the standard deviation of blank solution.

In order to highlight the performance of the NFO@CNTs/GCE sensor, the detection performance of NFO/GCE for AA was also studied. In the range of 1–50 μM, the reduction peak potential moves slightly to a more negative potential (Fig. S10a, ESI†), and the current increases linearly (Fig. S10b, ESI†). The sensitivity of AA detection was evaluated by the linear equation between current density and concentration. The slope showed that the sensitivity of NFO@CNTs/GCE was 9.07 μA μM⁻¹ cm⁻², which was almost 8 times that of NFO/GCE (Fig. S10c and d, ESI†). Based on the above results, the NFO@CNTs/GCE sensor exhibited higher sensitivity. Notably, the NFO@CNTs/GCE sensor prepared in this work was compared with the previously reported sensor for the AA determination property (Table S1, ESI†), and the proposed sensor showed a wide linear range and a low detection limit.

3.6. Research on anti-interference, reproducibility, and stability

Anti-interference, repeatability, and stability are three important parameters for evaluating the performance of electrochemical sensors. To verify that interfering substances present in plant samples do not affect the AA assay (Fig. 4d and Fig. S11a, ESI†), we added 100-fold excess concentrations of inorganic ions (Na⁺, K⁺, Mg²⁺, HCO₃⁻, SO₄²⁻, and Cl⁻), and 50-fold quercetin (QR), glucose (Glu) and citric Acid (CA). Meanwhile, the relative standard deviation was calculated to be less than 7%, indicating that this sensor exhibits good anti-interference.

In addition, the reduction current response of NFO@CNTs/GCE to 50 μM AA was recorded under optimal conditions to test the reproducibility and stability. Fig. 4e and Fig. S11b (ESI†) display the reproducibility results of the prepared sensor by recording the current response of 6 electrodes. The relative standard deviation (RSD) was 2.39%, indicating that the modified electrode has high reproducibility.

Additionally, the NFO@CNTs electrode was measured for two consecutive weeks to evaluate its stability. As displayed in Fig. 4f and Fig. S11c (ESI†), the reduction current of the NFO@CNTs electrode was tested every 2 days. Surprisingly, the reduction current did not weaken obviously. It was still 92.15% of the initial value even after 12 days. Besides, the RSD value within two weeks was 2.41%, which indicates that the electrode has excellent stability.

3.7. Real samples

We used the recovery rate to study the Houttuynia cordata samples. At the same time, in order to verify the detection ability of the proposed sensor, the AA content in the Asarum plant of Aristolochiaceae was detected as a comparison. To confirm the reliability of the proposed NFO@CNTs/GCE in practical application, the results were compared with those of HPLC. Table shows the electrochemical detection results of Asarum and Houttuynia cordata samples with 0, 1, and 2 μM AA added, respectively. The recoveries for different AA concentrations were found to be 92.3–103.1% with an RSD of less than 5% (Table 1). Fig. S12 (ESI†) shows the DPV curves of NFO@CNTs/GCE for the detection of AA in real samples. And the results demonstrate that the NFO@CNTs/GCE could be successfully employed to detect AA in traditional Chinese medicines with high accuracy, which is consistent with the value measured from the HPLC method.

Table 1 Results of AA analysis in real samples

Samples	Added (μM)	Found (μM)	Recovery (%) ± RSD (%)	HPLC (μM)
Asarum	0	1.65		1.62
	1	2.73	103.1% ± 3.0	—
	2	3.37	92.3% ± 2.6	—
Houttuynia cordata	0	0	—	0
	1	0.94	94.3% ± 2.1	—
	2	2.01	100.5% ± 1.7	—

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4. Conclusion

In this work, we synthesized a novel multivalent transition metal material heterostructure NiFe₂O₄@MWCNTs in situ and successfully designed it as a modified sensor material for the determination of AA in Asarum and Houttuynia cordata. Moreover, the high catalytic ability of multivalent bimetallics and the excellent electrical conductivity of carbon nanotubes synergize with each other, resulting in NFO@CNTs/GCE with lower R_ct, larger effective active area and higher electrical conductivity. Under optimal experimental conditions, the NFO@CNTs modified electrochemical sensor had a wide detection range with good linearity and a lower detection limit. We successfully applied the sensor to the detection of AA with good recoveries, and the excellent experiment results indicated its high application value. In brief, NFO has excellent catalytic properties and is a promising material for modified electrodes. We believe that the reliable and rapid detection method of AA described here can greatly promote the detection of AA content in edible plants and medicines.

Author contributions

Hongmei Jiang: conceptualization, writing – review & editing, Supervision, project administration. Mei Zhang: conceptualization, methodology, writing – original draft. Wenjun Liu: conceptualization, data curation. Jinyu Guan: conceptualization, data curation. Qinyi Cao: visualization. Jun Fang: project administration. Hao Yao: conceptualization. Xia Wang: writing – review & editing. Jun Zhong: conceptualization. Xiaoying Liu: funding acquisition, writing – review & editing.

Data availability

Data will be made available on request.

Conflicts of 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.

Acknowledgements

The authors gratefully acknowledge the funding support provided in part by the Natural Science Foundation of Hunan Province, China [2020JJ4035], [2022JJ50043] and [2022JJ50045].

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj03817g

‡ These authors contributed equally to this work.

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