Magnetic relaxation switching assay based on three-dimensional assembly of Fe₃O₄@ZIF-8 for detection of cadmium ions

Abstract

The design and construction of a novel magnetic resonance switch (MRS) sensor for cadmium ion (Cd²⁺) detection is described. Fe₃O₄@ZIF-8 was synthesized through seed-mediated growth of dimercaptosuccinic acid-coated Fe₃O₄. Fe₃O₄@ZIF-8 with high relaxation value (163.086 mM⁻¹ s⁻¹) and large negative zeta potential (−20.69 mV) exhibited good magnetic relaxation performance and water solubility. The successfully synthesized Fe₃O₄@ZIF-8 was used to develop an immune recognition-based MOFs-MRS sensor for highly sensitive detection of Cd²⁺. The proposed MRS detected a wide linear range of Cd²⁺ concentration from 2 to 200 ng mL⁻¹ with a low limit of detection of 0.65 ng mL⁻¹ (S/N = 3), and displayed high selectivity towards matrix interference. The robust sensing system was effective even in a complex sample matrix, enabling the quantitative analysis of Cd²⁺ content in rice samples and drinking water samples with good reliability. Recoveries of Cd²⁺ ranged from 91.50 to 112.05% for spiked drinking water and from 95.86 to 110.45% for spiked rice samples. The versatility of Fe₃O₄@ZIF-8 with customized relaxation responses could allow the adaptation of magnetic resonance platforms for food safety purposes.

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Introduction

Heavy metal pollutants have attracted increasing public attention in recent years due to their serious threat to human health and the environment.^1–5 Highly sensitive and accurate detection of heavy metal contamination in food and environmental samples is important for public health. Cadmium ions (Cd²⁺) are a typical heavy metal contaminant. Excess Cd²⁺ seriously threatens human health and can lead to serious diseases, including diarrhea, stomach pains, severe vomiting, bone fracture, reproductive failure and possibly even infertility, damage to the central nervous system and immune system, psychological disorders, and possibly DNA damage.^6–8 The World Health Organization has mandated that the concentration of Cd²⁺ in drinking water and air should be <0.005 mg L⁻¹ and <5 ng m⁻³, respectively.⁹ Traditional Cd²⁺ detection methods include inductively coupled plasma-mass spectrometry (ICP-MS)¹⁰ and atomic absorption/fluorescence spectrometry.^11,12 These methods have high detection sensitivity and reliable detection. However, requirements for complex sample pretreatment, skilled technicians, and expensive instruments have limited their widespread application in routine testing. Therefore, it to design fast, low-cost, and reliable detection methods for Cd²⁺ analysis in real samples.

The boom in nanotechnology is driving the development of traditional analytical methods. A variety of sensors have been developed to sensing metal pollutants. These include optical sensors and electrochemical sensors.^13,14 However, considering their susceptibility to interferences from the impurities, optical and electrochemical detections usually demand complicated sample pretreatments. In contrast, the magnetic relaxation switching (MRS) biosensor can be utilized in turbid and light-impermeable media without separation and purification steps.^15–17 It has been widely used to analyze different targets, including heavy metals,¹⁸ DNA,¹⁹ viruses,²⁰ bacteria,²¹ pesticide residues,^22,23 hormones,²⁴ and antibiotics.^25,26 The principles of conventional MRS biosensors generally rely on dispersion/aggregation or altered concentration of magnetic nanoparticles (MNPs).^27,28 The unstable dispersion/aggregation and broad-size distribution of traditional MNPs compromises the sensitivity of MRS sensors.²⁹ To address these issues, the synthesis of highly disperse magnetic materials with high relaxation rates magnetic materials could more effectively modulate the relaxation behavior of water molecules, thus increasing the sensitivity of MRS biosensors.

Metal–organic frameworks (MOFs), an important kind of porous inorganic–organic hybrid materials with inherent outstanding physicochemistry characteristics, can be widely applied for the facile preparation of MOFs magnetic nanoporous materials.^30–32 The porous structure and high specific surface area of MOFs magnetic nanoporous materials can effectively host significant amounts of MNPs, which prevents their aggregation.³³ In addition, the porous structures and high specific surface areas of MOFs magnetic nanoporous materials impeded the access of water molecules into the MNP particle core. It reduced the self-diffusion coefficient of the vicinity water molecules, which increased the r₂ value (r₂, the indicator of magnetic relaxation performances). The high r₂ value and exceptional chemical stability will help improve the performance of detecting MRS sensors.^34,35 However, reports of MRS biosensors based on MOFs materials have been very limited in the literature. Based on our previous findings, MOFs magnetic nanoporous materials (Fe₃O₄@NPC) are excellent signal labels for enhanced magnetic relaxation switch signals that can be applied to MRS sensors.³⁶ But, the poorly dispersed and low r₂ value of these magnetic nanoporous materials problems for the MRS sensor detection performance. Therefore, synthesizing novel MOFs magnetic nanoporous materials with higher magnetic relaxation performances is crucial for improving the performance of MRS sensors.

In this work, we reported the Fe₃O₄@zeolite imidazolate framework-8 (ZIF-8) composites obtained through seed-mediated growth of dimercaptosuccinic acid (DMSA)-coated Fe₃O₄. It was exciting to find that the Fe₃O₄@ZIF-8 with higher relaxation values and large negative zeta potentials exhibited excellent magnetic relaxation performances and good water solubility. Based on the Fe₃O₄@ZIF-8, we developed an immune recognition-based MOFs-MRS sensor for highly sensitive detection of Cd²⁺ (Scheme 1). The detection mechanism is based on the change in T₂ relaxation time caused by the assembly and disassembly of Fe₃O₄@ZIF-8 through immune recognition. In the absence of Cd²⁺, antibody (Ab)-Fe₃O₄@ZIF-8 and antigen (Ag)-Fe₃O₄@ZIF-8 assembled by immunological recognition, which makes a low change in the T₂ value. However, in the presence of Cd²⁺ strongly recognize with Ab-Fe₃O₄@ZIF-8 prevented the assembly of Ab-Fe₃O₄@ZIF-8 with Ag-Fe₃O₄@ZIF-8, which decreased the T₂ value and increased ΔT₂ (denoted as the change of the T₂ value). Experimental results validated that the MOFs-MRS exhibited good sensitivity and a low detection limit (LOD) to determine Cd²⁺ in practical applications.

Scheme 1 Principle of immune recognition-based MOFs-MRS sensor.

Results and discussion

Preparation and characterization of Fe₃O₄@ZIF-8

We prepared and characterized Fe₃O₄@ZIF-8 for the MRS sensor. The Fe₃O₄ was modified by DMSA (Fe₃O₄-DMSA), a small molecule with multiple carboxyl and mercapto functional groups. Fe₃O₄@ZIF-8 was obtained through seed-mediated growth of Fe₃O₄-DMSA. Transmission electron microscopy (TEM) was used to characterize the morphology of Fe₃O₄-DMSA and Fe₃O₄@ZIF-8. As shown in Fig. S1,† ZIF-8 exhibit the rhombic shape and have an average diameter of 179 nm. The morphology of Fe₃O₄@ZIF-8 composite is not markedly altered but the particle size is smaller (174 nm, Fig. 1C), because of the presence of heterogeneous nucleation process that typically leads to smaller crystals.^37,38

Fig. 1 (A) Synthesis process of Fe₃O₄@ZIF-8. (B and C) TEM images of the Fe₃O₄-DMSA (B) and Fe₃O₄@ZIF-8 (C).

To further confirm the synthesis of Fe₃O₄@ZIF-8, we performed Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray Photoelectron spectroscopy (XPS) for Fe₃O₄-DMSA, ZIF-8, and Fe₃O₄@ZIF-8. The Fe₃O₄-DMSA has the characteristic peak of Fe–O vibration band in 615 cm⁻¹, and the ZIF-8 has characteristic peaks in 759, 1150, and 1583 cm⁻¹. Three peaks at 759 cm⁻¹, 1150 cm⁻¹ and 1583 cm⁻¹ were observed in the spectrum of Fe₃O₄@ZIF-8, which reflected the existence of Fe₃O₄ nanoparticles.³⁹ The XRD peaks for Fe₃O₄@ZIF-8 are in line with the peaks of ZIF-8 at low angles of 5–20° and that of Fe₃O₄-DMSA at angles of 30°, 35°, 57°, and 63°, indicating the formation of Fe₃O₄@ZIF-8 composites. As shown in Fig. S3(A),† the XPS spectra of Fe₃O₄@ZIF-8 indicated the presence of carbon, nitrogen, oxygen, iron, and zinc species.⁴⁰ The Zn 2p spectra of Fe₃O₄@ZIF-8 showed that the spin orbitals of Zn 2p_1/2 and Zn 2p_3/2 have strong signals at 1045.00 eV and 1022.00 eV, respectively, demonstrating the existence of divalent Zn in the as-synthesized corresponding to characteristic peaks of ZIF-8. One peak at 615 cm⁻¹ Fe₃O₄@ZIF-8. The C 1s spectrum showed that C-sp³, C–O and C=O groups are corresponding at 284.82  eV, 285.83  eV and 288.62  eV, respectively.⁴¹ Displayed that the O 1s spectra consisting of three components with binding energies at 532.1, 331.57 eV and 531.28 eV, which were associated with C–OH, C–O, and Fe–O.⁴² We also measured the concentration of Fe by Inductive Coupled Plasma Emission Spectrometer (ICP), the Fe concentration of ZIF-8 and Fe₃O₄@ZIF-8 were 0.01% and 4.29%, which significant indicated Fe₃O₄-DMSA exists in the Fe₃O₄@ZIF-8. These characterized and confirmed the preparation of the nanocomposite we designed.

Additionally, the N₂ sorption isotherms were also examined to analyze the porous structure of the Fe₃O₄@ZIF-8.⁴³ The nitrogen adsorption/desorption curves of the Fe₃O₄@ZIF-8 were type III curves with a relative pressure in the range of 0.8 to 1.0 with a significant hysteresis loop (Fig. 2C), suggesting a micro/mesoporous structure characteristic of Fe₃O₄@ZIF-8. The average Barrett–Joyner–Halenda (BJH) adsorption pore sizes of the Fe₃O₄@ZIF-8 was 1.76 nm (Fig. 2C). These results indicated that micro-mesopores contributed significantly to the pore volumes of the Fe₃O₄@ZIF-8 nanostructures. The proton transverse relaxation performance of materials is essential for the successful construction of an MRS sensor.¹⁷ Fig. 2D and E show that the transverse relaxation rate (r₂, the indicators of magnetic relaxation performances) of Fe₃O₄@ZIF-8 (163.086 mM⁻¹ s⁻¹) was higher than Fe₃O₄-DMSA (34.164 mM⁻¹ s⁻¹). In addition, the Fe₃O₄@ZIF-8 displayed a higher r₂ value than Fe₃O₄@NPC (163.086 vs. 112.676 mM⁻¹ s⁻¹). The apparent increase in the r₂ value will help improve the performance of Fe₃O₄@ZIF-8 in MRS sensor.

Fig. 2 (A) FTIR spectroscopy of Fe₃O₄-DMSA, ZIF-8, and Fe₃O₄@ZIF-8. (B) XRD patterns of Fe₃O₄-DMSA, ZIF-8, and Fe₃O₄@ZIF-8. (C) N₂ absorption–desorption isotherms and pore distributions of Fe₃O₄@ZIF-8. (D–F) Transverse relaxation rate of Fe₃O₄-DMSA (D) Fe₃O₄@ZIF-8 (E), and Fe₃O₄@NPC.

Dynamic light scattering (DLS), transmission electron microscopy (TEM), and electrophoretic light scattering (ELS) were used to characterize the dispersity of Fe₃O₄@ZIF-8. As shown in Fig. 3A the hydrated particle diameter of Fe₃O₄@ZIF-8 was 337 nm. The average hydro-diameter of Fe₃O₄@ZIF-8 were larger than the actual diameters of that, which were similar to previous reports.^44,45

Fig. 3 (A and B) Size distribution of Fe₃O₄@ZIF-8 (A) and Fe₃O₄@NPC (B). (C and D) Zeta potential of Fe₃O₄@ZIF-8 (C) and Fe₃O₄@NPC (D). (E and F) TEM image of Fe₃O₄@ZIF-8 (E) and Fe₃O₄@NPC (F).

The particle polymer dispersity index (PDI) of Fe₃O₄@ZIF-8 (0.208) was lower than that of Fe₃O₄@NPC (0.324) (Fig. 3B). The zeta potential of Fe₃O₄@ZIF-8 of −20.69 mV (Fig. 3C) was lower than that of Fe₃O₄@NPC (−15.8 mV) (Fig. 3D). Negative zeta potentials can effectively inhibit aggregation of nanoparticles due to electric repulsion.^46,47 TEM revealed a greater dispersity of Fe₃O₄@ZIF-8 compared to Fe₃O₄@NPC (Fig. 3E and F). The collective results demonstrated the good dispersion of Fe₃O₄@ZIF-8, which is advantageous in constructing a sensitive MRS sensor. These results indicated that the Fe₃O₄@ZIF-8 exhibited good magnetic relaxation and water solubility, which will help improve the performance of the MOFs-MRS sensor.

Construction of Fe₃O₄@ZIF-8-based MRS sensor

To illustrate the assembly of Ag-Fe₃O₄@ZIF-8 and Ab-Fe₃O₄@ZIF-8, we evaluated the hydration particle size and zeta potentials using DLS and ELS. Fe₃O₄@ZIF-8 were coated with Cd²⁺-specific antibodies and Cd²⁺-specific antigens to form Ab-Fe₃O₄@ZIF-8 and Ag-Fe₃O₄@ZIF-8, respectively. The hydration particle size of Ab-Fe₃O₄@ZIF-8 increased from 337 nm to 398 nm and that of Ag-Fe₃O₄@ZIF-8 from 337 nm to 390 nm (Fig. 4B). Their zeta potentials decreased from −20.69 mV to −28.69 mV and −28.72 mV, respectively (Fig. 4A). After the biological immune recognition, the hydration particle size of assembly objects changed obviously (910.27 nm) (Fig. 4B). These results indicated that the antibody and antigen successfully modify the surface of Fe₃O₄@ZIF-8. To evaluate the feasibility of the Ag-Fe₃O₄@ZIF-8 and Ab-Fe₃O₄@ZIF-8 assembly in the construction of the MRS sensor, we measured the T₂ relaxation time of Ag-Fe₃O₄@ZIF-8, Ab-Fe₃O₄@ZIF-8, and assembly. As shown in Fig. 4C, the T₂ relaxation time of the assembly (881.864 ms) was higher than that of Ab-Fe₃O₄@ZIF-8 (689.104 ms) and Ag-Fe₃O₄@ZIF-8 (731.368 ms). The findings indicated that assembly increased the transverse relaxation time and confirmed the feasibility of Fe₃O₄@ZIF-8 to construct the MRS immunosensor.

Fig. 4 (A) Zeta potential of Fe₃O₄@ZIF-8, Ag-Fe₃O₄@ZIF-8, and Ab-Fe₃O₄@ZIF-8. (B) Hydrated particle size of Fe₃O₄@ZIF-8, Ag-Fe₃O₄@ZIF-8, Ab-Fe₃O₄@ZIF-8, and Assembly. (C) T₂ relaxation time of Ag-Fe₃O₄@ZIF-8, Ab-Fe₃O₄@ZIF-8, and Assembly.

Optimization of detection conditions

To improve the analytical performance of the MRS, we optimized the concentration of Fe₃O₄@ZIF-8, antibodies/antigens, incubation time, and TE values. The ΔT₂ value increased with increasing Fe₃O₄@ZIF-8 concentration, and then maintained a decreasing amplitude after reaching 0.15 mg mL⁻¹ (Fig. 5A). In this case, 0.15 mg mL⁻¹ was selected for further investigation. The influence of the concentration of antibodies/antigens on changes in T₂ are shown in Fig. 5B. An antibody/antigen concentration of 4 μg mL⁻¹ was the optimum concentration. Fig. 5C shows the optimized reaction times; ΔT₂ increased gradually with increasing reaction time from 20 min to 100 min. Subsequently, ΔT₂ reached a plateau after 60 min, indicating that the Cd²⁺-induced assembly of NPs was fully achieved. Therefore, 60 min was used as the optimal reaction time for the following BPA detection. As shown in Fig. 5D, the maximum value of ΔT₂ was observed at a TE of 2 ms, which was selected for the subsequent experiments. To ensure accurate results, the optimal conditions were used in subsequent detections.

Fig. 5 Condition optimization of the MRS biosensor. (A–D) Effect of the Fe₃O₄@ZIF-8 concentration (A), antigen/antibody concentration (B), incubation time (C), and echo time (D).

Detection of Cd²⁺ by MRS sensor

We employed the Ag-Fe₃O₄@ZIF-8 and Ab-Fe₃O₄@ZIF-8 to construct a MOFs-MRS sensor for the detection of Cd²⁺. The detection mechanism is based on the change in T₂ relaxation time caused by the assembly and disassembly of Fe₃O₄@ZIF-8 through immune recognition. In the absence of Cd²⁺, Ab-Fe₃O₄@ZIF-8 with Ag-Fe₃O₄@ZIF-8 leads to an immunological recognition-based assembly, which makes a low change in the T₂ value. In contrast, exposure of Cd²⁺ to the system will break the assembly process (disassembly) based on the competition effect of Cd²⁺ with Cd²⁺-Ag to strongly recognize Ab-Fe₃O₄@ZIF-8. This decreases the T₂ value and increases ΔT₂ (denoted as the change of the T₂ value) (Fig. 6A). The assembly degree of Fe₃O₄@ZIF-8 is related to the concentration of Cd²⁺. Under optimized conditions, a linear relationship was obtained between ΔT₂ and the concentration of Cd²⁺ from 2 to 200 ng mL⁻¹. The linear equation was y = 141.10lg(x) − 37.75 (R² = 0.99). The limit of detection (LOD) was 0.65 ng mL⁻¹, calculated by 3σb/slope (σb, standard deviation of the blank samples). The LOD was lower than that of the other MRS and other conventional sensitive detection methods (Table S1†).

Fig. 6 (A) Scheme of immune recognition-based MRS sensor for highly sensitive detection of the Cd²⁺. (B) Standard curve for detecting Cd²⁺. The concentrations of Cd²⁺ ranged from 2 to 200 ng mL⁻¹. Error bars represent the standard deviation of three replicates (n = 3).

Specificity analysis

To assess the selectivity of the MRS platform for Cd²⁺ detection, several metal ions, including K⁺, Na⁺, Fe²⁺, Mg²⁺, Ca²⁺, Pd²⁺, Ba²⁺, Zn²⁺, and Ag⁺, were tested in the same way. As shown in Fig. 7, Cd²⁺ resulted in a remarkable value of ΔT₂. Other metal ions failed to generate obvious ΔT₂ value in the sensing system. The data demonstrated that this sensing platform possessed excellent specificity for the Cd²⁺ assay. The control heavy metal ions did not disturb the biosensor response. This high specificity can be ascribed to the high binding capability between Cd²⁺ and antibody.

Fig. 7 Selectivity testing of the sensing system for Cd²⁺ and some control metal ions. Error bars represent the standard deviation of three replicates (n = 3).

Real sample analysis

To demonstrate the practical applications of the sensing platform, this biosensor was applied to the detection of Cd²⁺ in drinking water and rice samples. The results as depicted in Table 1. The measured Cd²⁺ in rice samples agree well with the added concentrations with the recoveries of 91.50% to 112.05% and a relative standard deviation (RSD) of 1.05% to 11.74%. Also, the measured Cd²⁺ in drinking water agreed well with the added concentrations with the recoveries of 95.85% to 110.44% as well as relative RSD of 2.14% to 9.69%.⁴⁸ The findings revealed that the magnetic sensor can be applied in the practical sample detection, with good accuracy and sensitivity for Cd²⁺ detection in food samples.

Table 1 Analytical parameters of the proposed method for determining Cd²⁺ drinking water and rice samples

Sample	Added concentration (ng mL^−1)	Detected concentration (ng mL^−1)	RSD (n = 3, %)	Recovery (n = 3, %)
Rice	50	45.75 ± 0.48	1.05	91.50 ± 0.96
	20	22.41 ± 2.63	11.74	112.05 ± 13.15
	2	1.99 ± 0.03	1.50	99.50 ± 1.50
Drinking water	50	55.22 ± 3.97	7.19	110.44 ± 7.94
	20	19.17 ± 1.86	9.70	95.85 ± 9.30
	2	1.96 ± 0.04	2.04	98.00 ± 2.00

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Conclusions

In conclusion, we generated Fe₃O₄@ZIF-8 with high relaxation values and excellent dispersion. Based on the Fe₃O₄@ZIF-8, we establish a versatile immune recognition-based MOFs-MRS sensing system for sensitive detection of the Cd²⁺ in the real sample. The optimized MOFs-MRS displayed a broad quantitative range from 2 to 200 ng mL⁻¹ and a limit of detection 0.65 ng mL⁻¹. When tested using samples of rice and drinking water, the MOFs-MRS was conferred with a satisfactory recovery (91.50% to 112.05%). The findings also detail the improved dispersion and relaxation values of MOFs magnetic nanoporous materials, and are a novel approach to improve the performance of MOFs-based MRS sensor. Moreover, the MOFs-MRS sensing system could be expected to detect other food hazard factors by simply replacing antibody/antigen.

Experimental

Preparation of Fe₃O₄@ZIF-8

Fe₃O₄-DMSA was synthesized following as previously described with slight modifications.³⁵ Then, 0.003 mol of zinc nitrate hexahydrate was dissolved in 100 mL of N, N-Dimethylformamide (DMF, liquid A), and 0.006 mol of 2-methylimidazole was dissolved in 100 mL of DMF (liquid B). 1 mL of zinc nitrate hexahydrate and 9 mL of Fe₃O₄-DMSA (1.5 mg mL⁻¹) solution were mixed in a 50 mL conical flask as the starting solution and heated in a water bath at 40 °C. 10 mL of both liquid A and liquid B were added to 10 mL of the initial solution using a syringe pump at a flow rate of 20 mL h⁻¹ at a constant speed. After 30 min of reaction, 20 mL of the solution was collected from the reaction system. This represented the first growth product. 10 mL of the first growth product were used as the starting solution. 10 mL of both liquid A and liquid B were injected using a syringe pump at a flow rate of 20 mL h⁻¹ at a constant speed. The second growth product was obtained by reaction for 1 h. For pure ZIF-8, the Fe₃O₄-DMSA is omitted and substituted with an equivalent amount of DI water.

Preparation of functional Fe₃O₄@ZIF-8

We used the covalent coupling method to immobilize the antibodies/antigens on the Fe₃O₄@ZIF-8 surfaces. Antigen-Fe₃O₄@ZIF-8 (Ag-Fe₃O₄@ZIF-8) was prepared by adding 1 mL Fe₃O₄@ZIF-8, 2 mL of 4 μg mL⁻¹ antigen solution, 0.5 mL of 10 mg mL⁻¹ EDC solution, 0.2 mL of 10 mg mL⁻¹ sulfo-NHS solution, and 2 mL of 0.5 mg mL⁻¹ Fe₃O₄@ZIF-8 solution to a 10 mL centrifuge tube. The mixture was reacted at room temperature for 12 h to allow the coupling. Excess antibody was removed by centrifugation. Preparation of Ab-Fe₃O₄@ZIF-8 was similar to that of Ag-Fe₃O₄@ZIF-8 by simply replacing antigen with antibody.

Optimization of relaxation sensing system conditions

Concentrations of Fe₃O₄@ZIF-8 were optimized at 0.05, 0.15, 0.25, 0.35 and 0.45 mg mL⁻¹ according to the ΔT₂ value. At the same concentration of Fe₃O₄@ZIF-8 (0.15 mg mL⁻¹), antibody/antigen concentrations were optimized as 2, 4, 6, 8, and 10 μg mL⁻¹. The incubation time was optimized as 20, 40, 60, 80, 100, and 120 min (0.15 mg mL⁻¹ Fe₃O₄@ZIF-8, 4 μg mL⁻¹ antibody/antigen). Finally, we selected the echo time (TE) of test parameters as 0.5, 1, 1.5, 2, and 2.5 ms.

Detection of Cd²⁺

Establishment of Cd²⁺ standard curve. 150 μL of Ab-Fe₃O₄@ZIF-8 (0.15 mg mL⁻¹) and Ag-Fe₃O₄@ZIF-8 (0.15 mg mL⁻¹) were mixed with different concentrations of Cd²⁺ (2, 5, 10, 20, 50, 100, and 200 ng mL⁻¹). The mixture was incubated at room temperature for 60 min. After 2 min of equilibrium, T₂ relaxation time was measured using NMI 20-CA at room temperature. The ΔT₂ was calculated as ΔT₂ = T₂(Blank) − T₂(Sample), where the T₂(Blank) is the relaxation time of Fe₃O₄@ZIF-8 solution without Cd²⁺and T₂(Sample) is the relaxation time of test solution with different concentrations of Cd²⁺.

Detection of Cd²⁺ in real samples. In order to test the Cd²⁺ level in drinking water and rice samples, pretreatment of the samples was carried out: the rice was ground into rice flour, 0.4 g of rice flour was put into a tetrafluoroethylene tank and soaked overnight with 5 mL nitric acid. 2 mL hydrogen peroxide solution (30%) of zinc nitrate hexahydrate was added in it, and keeping in the drying oven at 120 °C for 5 h. Then, the sample was heated to evaporate acid and make it close to dry. Pour the digestive liquid into a 25 mL volumetric flask, and use ultra-pure water for constant volume. The pre-treatment of water samples is to obtain Ipoh water from the laboratory using a 10 mL centrifuge tube, centrifuge and remove the supernatant. We take 300 μL drinking water and rice digestion solution, and T₂ measurement was performed with 0.5 T nuclear magnetic resonance (NMR) instrument. At least three tests were performed. Next, food samples were adulterated with known levels of Cd²⁺ and tested for recovery. Three concentrations of Cd²⁺ were added to mineral water and rice digestion solution, and the final concentrations of Cd²⁺ were 2 ng mL⁻¹, 20 ng mL⁻¹, and 50 ng mL⁻¹. The measured mean values were then obtained using the following dose response equation of the sensor.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the Nature Science Foundation of Hunan (2022JJ10046, 2021JJ30701), Shandong Taishan Industry Leading Talent Project High-efficiency Ecological Agriculture Innovation Project (no. 457 LJNY202004), the Research Foundation of Education Bureau of Hunan Province (21B0337), Research Project of the General Administration of Customs of the People's Republic of China (2020HK183), Foundation of State Key Laboratory of Utilization of Woody Oil Resource (GZKF202111).

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Footnote

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra03926e

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