A new magnetic tailor made polymer for separation and trace determination of cadmium ions by flame atomic absorption spectrophotometry

Abstract

Magnetic ion imprinted polymers have been prepared and applied for the selective extraction and trace monitoring of cadmium ions in food samples. The characterization of synthesized sorbent was carried out by Fourier transform infrared spectroscopy, elemental analysis, scanning electron microscopy, X-ray diffraction, and thermal analysis. The separation of magnetic nanoparticles from the extraction solution could be easily carried out by applying an external magnetic field. Flame atomic absorption spectrophotometry was used for determination of the extracted target ion. The detection limit and linear dynamic range for the coupling of the proposed sample preparation method under the optimized conditions with FAAS were 0.02 μg L⁻¹ (based on 3S_b/m) and 0.1–500 μg L⁻¹, respectively. Finally, the applicability of the coupling of the solid phase extraction method with flame atomic absorption spectrophotometry was investigated by extraction and monitoring of cadmium ions in food samples. Therefore, the introduced technique can be applied by food scientists as a reliable pre-concentration stage before determination of cadmium by flame atomic absorption spectrophotometry.

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1. Introduction

Heavy metal ions are increasingly being released into the environment, leading to serious pollution, particularly as a result of industrialization. Cadmium is a very toxic element for animals and humans, even at low concentrations.^1–3 The International Agency for Research on Cancer classified cadmium as a human carcinogen.⁴ Due to the toxicity of cadmium for humans and animals, its concentration should be monitored in the environment. The appropriate guideline values for cadmium content for drinking water by the WHO⁵ and the USEPA⁶ are 3.0 μg L⁻¹ and 5.0 μg L⁻¹, respectively.

Cadmium enters the organism primarily via the alimentary and respiratory tract. The sources of this metal are food, drinking water and air. Roughly 15 000 t of cadmium is produced worldwide each year for nickel–cadmium batteries, pigments, chemical stabilizers, metal coatings and alloys. So its usage is becoming wider and wider. However, as the levels of cadmium in geological and environmental samples are low, a preconcentrative separation and determination of trace cadmium from the natural water is essential and needs much more attention.^7–10 Molecularly imprinted polymers (MIPs) and ion imprinted polymers (IIPs) are artificial polymers, which are formed in the presence of a target molecule or a target ion that finally is removed by the proper solvents; therefore, the obtained specific cavities are complementary to the target molecule or target ion. The use of MIPs and IIPs has been developed in various fields such as extraction,^11–21 sensors,^22,23 catalysis^24,25 and drug delivery.²⁶ In recent years, the combination of MIP and IIPs and other sample preparation techniques like solid phase extraction, solid-phase micro extraction and matrix solid-phase dispersion has opened a new window for selective extraction and recognition of target molecules or ions from the complex matrices.^27–29 The conventional method for the preparation of MIPs and IIPs is bulk polymerization,^30,31 which is traditional and exhibit high selectivity. There are some defects for this polymerization method such as imperfect removal of template molecules or ions, slow mass transfer of template molecules or ions from the polymer backbone, heterogeneous distribution of the binding sites, poor site accessibility and small binding capacity in some cases. Therefore, surface polymerization has been suggested for the improving of accessibility to the target molecules and ions, more suitable mass transfer and complete removal of templates.³² In this regard, the surface grafting MIPs and IIPs have been developed on the different surfaces such as silica particles,^33,34 titanium dioxide particles³⁵ and polymeric supports.³⁶ Nowadays, magnetic nanoparticles (MNPs) because of their attractive properties are used in the separation methods,^37–42 bioscience⁴³ and environmental remediation.⁴⁴ The MNPs possess significant advantages, including small size, high surface-to-volume ratio, high magnetic susceptibility and effective ability for binding. On the other hand, easy and efficient separation can be performed with an external magnetic field without any additional centrifugation or filtration procedures.^45,46

In this work, a surface imprinting technique was used for modification of magnetic nanoparticles with ion imprinted polymer and it was applied as an efficient solid phase extraction technique for rapid and selective pre-concentration of cadmium ions in the presence of interfering compounds in complex matrices. Magnetic ion imprinted polymer used all advantages of Fe₃O₄ nanoparticles (easy separation from extraction medium) and IIP (with selective recognition site) for pre-concentration of cadmium ions. Characterization of this polymer was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), elemental analysis (CHN) and thermogravimetric analysis (TGA). The influencing parameters on the extraction of cadmium ions by the tailor made sorbent were examined and optimized. Finally, the prepared sorbent with highly selective extraction sites was used for extraction of cadmium ions before its trace monitoring by FAAS.

2. Experimental

2.1. Apparatus

The information about apparatus was presented in the ESI.†

2.2. Material and reagents

Applied standard solutions of ions (Titrisol solutions), methacrylic acid (MAA) with high purity, ethylene glycol dimethacrylate (EGDMA), 4-(2-pyridylazo) resorcinol, 2,2′-azobisisobutyronitrile (AIBN) and acidic and basic solutions was prepared from Merck (Darmstadt, Germany, http://www.merck.de), Fluka (Buchs SG, Switzerland, http://www.sigmaaldrich.com), and Acros Organics (New Jersey, USA). A Milli-Q system from Millipore from Bedford, MA, USA was used to prepare double distilled water. The applied certified reference material to validate the proposed sample preparation method was white cabbage [IRRM (BCR-679)].

2.3. Synthesis of modified magnetic nanoparticles with silica

For preparation of Fe₃O₄ nanoparticles, 100 mL of degassed deionized water with nitrogen gas was used for dissolving of 10.4 g of FeCl₃·6H₂O and 4.0 g of FeCl₂·4H₂O and then the resulted solution was heated to 80 °C. After this step, 15 mL of ammonium hydroxide (NH₄OH) (32% solution) was added dropwise to the mentioned solution and after 15 min the obtained black solid sorbent was separated by a magnet. To remove the impurities from the sorbent, the separated magnetic nanoparticles washed three times with 0.1 mol L⁻¹ sodium chloride (NaCl). Ultrasound wave was used for dispersion of Fe₃O₄ nanoparticles (1.5 g) in 300 mL mixture of ethanol and double distilled water (4 : 1, v/v) for 15 min. In the next step, 15 mL of NH₄OH and 3.1 mL of tetraorthosilicate were simultaneously added into the reaction container. The reaction was kept for 12 h at 40 °C and in the final step the obtained modified magnetic nanosorbent was collected by a magnet field.

2.4. Surface modification of Fe₃O₄@SiO₂ with 3-methacryloxypropyl trimethoxysilane

Fe₃O₄@SiO₂ nanoparticles (1.5 g) were added to 5.0 mL of 3-methacryloxypropyl trimethoxysilane (MPTS) in 30 mL toluene solution for chemically modification and the mixture were purged under nitrogen atmosphere for 24 h at 120 °C. Finally the resulted mixture of Fe₃O₄@SiO₂@MPTS was collected using magnet and washed with toluene and distilled water in order to remove impurity.

2.5. Preparation of the MIIPs

The magnetic ion imprinted polymers (MIIPs) were prepared by surface molecular imprinting technique. Briefly, cadmium (0.5 mmol) as the template ion and 4-(2-pyridylazo) resorcinol (1 mmol) as the cadmium-binding ligand were dissolved in 50 mL methanol : acetonitrile (50 : 50 v/v) and then methacrylic acid (4 mmol) as the functional monomer was added to this solution. After that, 1.5 g of Fe₃O₄@SiO₂@MPTS nanoparticles was added to the mixture, and stirred for 2 h. Subsequently, ethylene glycol dimethacrylate (24.0 mmol) and 2,2′-azobisisobutyronitrile (100 mg) were added into the system and the mixture was degassed in an ultrasonic bath for 15 min. Then, reaction vassal was flashed with nitrogen gas for 10 min to remove oxygen. The polymerization was performed at 60 °C with nitrogen protection for 24 h. The MIIPs were collected by a magnet, and washed by nitric acid (HNO₃) (1 mol L⁻¹) to remove the cadmium ions until no cadmium ions was detected by FAAS. Finally, the modified magnetic nanoparticles were dried in the vacuum. The magnetic non-imprinted polymers (MNIPs) were prepared by the same method as MIIPs without the addition of template. Fig. 1 illustrates a scheme for synthesis of MIIPs.

Fig. 1 Schematic illustration of imprinting process for the preparation of magnetic ion imprinted polymer by surface imprinting technique.

2.6. Sample pretreatment for metal analysis

2.6.1. Shrimp, crab and fish samples. The information about the section was presented in the ESI.†

2.6.2. Agricultural products. The information about the section was presented in the ESI.†

2.7. Extraction procedure

10 mg of dried MIIPs was added to 25 mL of cadmium sample solution at optimized pH (pH = 8) in a 50 mL sample tube. To facilitate sorption of cadmium ions onto the recognition sites of MIIPs, the solution was shaken for 5 min. The applied sorbent with magnetic characteristic was easily separated from extraction solution using a strong flat permanent magnet and then supernatants were decanted directly. To remove the interferences, the modified magnetic nanosorbents washed with 5 mL deionized water. The sorbent after the mentioned steps was transferred to a 2 mL sample vial and subsequently to leach the extracted cadmium ions by MIIPs, 1 mL of HNO₃ (1 mol L⁻¹) was added to the sorbent and shaken for 5 min to complete elution of target ions and after this step the phase separation was done using a strong flat permanent magnet. Finally, the separated phase containing pre-concentrated cadmium ions was measured by FAAS. The mentioned easy and efficient sample preparation method was successfully used for monitoring of cadmium ions in different food samples.

Eqn (1) was used to calculate the MIIP adsorption efficiency.

In this equation C₀ is initial and C_r is the final concentrations of cadmium in the extraction solution.

3. Results and discussion

3.1. Characterization of MIIPs

SEM was used to evaluate the surface morphology of the tailor made polymer and the obtained micrographs are presented in Fig. 2. The resulted particle size from SEM evaluation was approximately 25, 35 and 65 nm for Fe₃O₄, Fe₃O₄@SiO₂@MPTS and Fe₃O₄@SiO₂@MPTS@IIP, respectively. As can be obvious from this data, the imprinted shell thickness on the Fe₃O₄@SiO₂@MPTS is about 15 nm.

Fig. 2 The scanning electron microscopy images of (A) Fe₃O₄ (B) Fe₃O₄@SiO₂@MPTS and (C) Fe₃O₄@SiO₂@MPTS@MIIP.

Fig. 1S (ESI†) shows the XRD patterns of the Fe₃O₄ and MIIPs. In the diffraction peaks of two mentioned materials several relatively strong diffractions can be obvious in the region of 20–70° that related to the identical peaks of magnetic nanoparticles.¹⁷ The obtained data can prove that Fe₃O₄ and MIIPs nanoparticles were composed of Fe₃O₄.

FT-IR spectra were used for further evaluation of successful synthesis of the prepared materials. According to Fig. 2S (ESI†), the bands at 590 cm⁻¹ is related to the Fe–O–Fe bond. The presence of Fe–O–Fe bond in the FT-IR spectra of Fe₃O₄ and MIIPs can prove successful embedded of Fe₃O₄ in the synthesized sorbent. Si–O–Si stretching vibration can be seen around 1150 cm⁻¹. The presence of C=O peak at 1730 cm⁻¹ on the spectra of leached MIIPs can show the synthesized of the hybrid tailor made polymer on magnetic nanoparticles via the polymerization of EGDMA and MAA. The presence of 4-(2-pyridylazo) resorcinol on the network of the synthesized tailor made material can prove with stretching vibration bands of OH (3298–3730 cm⁻¹) and –N=N– group (1447 cm⁻¹).

Thermal analysis graphs of the synthesized Fe₃O₄@SiO₂, Fe₃O₄@SiO₂@MPTS and MIIPs are shown in Fig. 3S (ESI†). 11.5% weight loss in the temperature range of 200 to 400 °C was obvious in the thermogram of Fe₃O₄@SiO₂@MPTS which can be related to the loss of MPTS layer from the mentioned material. 78.5% weight loss was recognized in the thermogram of Fe₃O₄@SiO₂@MPTS@IIP which probably related to the loss of MPTS and imprinted polymer layers. The weigh residue in the thermogram of the synthesized sorbent can be attributed to Fe₃O₄ magnetite particles with high thermal stability. The magnetic encapsulation efficiency for the synthesized material was 21.5% according to the TGA analysis data and it is considerably high and appropriate. The obtained data from these analyses prove successfully synthesis of MIIPs.

Elemental analysis (EA) was used for further characterization of leached MIIPs and the percentage of elements by EA was found to be: carbon (55.87%); hydrogen (8.42%) and nitrogen (1.542%). All observations from characterization of the synthesis sorbent confirm the successful preparation of magnetic tailor made material.

3.2. The optimization of experimental parameters

Several parameters can effect on the extraction and pre-concentration of cadmium on MIIPs. The main influencing parameters are solution pH, sorption time, type, volume and concentration of eluent, elution time and volume of sample. To efficient extract and pre-concentrate of cadmium ions by the synthesized sorbent, the mentioned parameters should be optimized.

3.2.1. Evaluation of pH. To examine the effect of pH on the retention of cadmium by the synthesized sorbent, pH was changed between 2.0 and 10.0 for 10 mL of different sample solutions containing 1 mg L⁻¹ of target ion. It should be noted that other parameters for retention of target ions was kept constant at optimum conditions as follow: the amount of sorbet: 10 mg and sorption time of 5 min. The obtained data for evaluation of pH effect on the retention of cadmium ions with magnetic-imprinted polymer (MIIP) and magnetic-non imprinted polymer (MNIP) was shown in Fig. 3. The quantitatively retention of cadmium ions with MIIPs can be achieved in the pH of 8.0. Therefore for quantitatively retention of cadmium ions by the sorbent pH 8.0 was chosen for further experiments. By decreasing the pH value of the solution, the quantitative recovery of the sorbent was also decreased due to the electrostatic repulsion of the protonated active sites on the sorbent with the positively charged cadmium ions. The mild reduce in adsorption efficiency of cadmium ions by the sorbent in pH higher than 8.0 is due to the hydrolysis of target ions.

Fig. 3 The effect of sample pH on the MIIP and MNIP adsorption efficiency (%) of cadmium ions (the obtained results are the mean of three measurements).

3.2.2. Evaluation of the sorbent amounts. The information about the section was presented in the ESI.†

3.2.3. Sorption time evaluation. Several shaking time was used to examine the retention of analyte by the recognition sites of magnetic tailor made polymer in the range of 2–15 min. It should be noted that other parameters for retention of target ions was kept constant at optimum conditions as follow: the ph of solution: 8 and the amount of sorbet: 10 mg. As obtained results in Fig. 4S (ESI†), 5 min was enough for quantitative retention of the target ions by active sites of the sorbent. The small required time for retention of cadmium by the sorbent is attributed to the high surface area of the nanoparticles. Therefore, 5 min was chosen as a shaking time for quantitative extraction of cadmium ions by the sorbent in the subsequent experiments.

3.2.4. Evaluation of desorption conditions. To elute of cadmium ions from magnetic imprinted and non-imprinted polymer, three selected eluents, comprising hydrogen chloride (HCl), HNO₃ and acetic acid (HOAC) were applied for leaching of cadmium ions. It should be noted that retention parameters for sorption of target ions was kept constant at optimum conditions as follow: the pH of solution: 8, the amount of sorbet: 10 mg and sorption time of 5 min. The applied acids for leaching of Cd²⁺ ions from the magnetic-IIP and magnetic-NIP nanoparticles was tested by using 5 mL portions of 2 mol L⁻¹ HNO₃, 2 mol L⁻¹ HCl and 2 mol L⁻¹ HOAC, which obtained data for leaching efficiency was >99%, 93% and 45% for magnetic-IIP and 44%, 39% and 13% for magnetic-NIP. Thus, 2 mol L⁻¹ HNO₃ was chosen as elution solvent for its better desorption characteristics more than two other acids. Three 5 mL of nitric acid solutions as the elution solvent with various concentrations (i.e., 0.5, 1 and 2 mol L⁻¹) were applied for optimization of desorption cadmium ions from the recognition holes in the polymer network. It was observed that quantitative leaching of cadmium ions was achieved by 1 mol L⁻¹ of HNO₃ (i.e., with leaching efficiency of 90, >99%, and >99% (for 0.5, 1 and 2 mol L⁻¹)).

The effect of desorption solvent volume was tested for quantitative desorption of cadmium ions from the imprinted polymer sites. As a result 1.0 mL of HNO₃ (1 mol L⁻¹) (recovery > 99%) is adequate for quantitative elution of cadmium ions. Therefore, 1.0 mL of eluent was used for leaching of cadmium ions in the further steps.

Several times in the range of 2–15 min were applied to elute of cadmium ions from magnetic tailor made polymer while other parameters were maintained in optimized conditions. The obtained data show that 5 min is necessary for leaching of cadmium ions and further increase in the elution time do not have significant effect on desorption of target ions. Hence, 5 min is selected as an optimum condition for elution of cadmium ions in subsequent steps.

3.3. Selectivity study

The following formula was used to evaluate the distribution ratio (mL g⁻¹) of cadmium ions between the magnetic tailor made polymer and aqueous solution:

In this equation, C_i (mg L⁻¹) is defined as concentrations before retention of cadmium ions and C_f (mg L⁻¹) is its concentration after retention by the prepared sorbent, V is the applied volume for extraction and m is defined as the sorbent mass. The selectivity of the sorbent for cadmium ions toward potentially interfering ions was examined by two factor including selectivity coefficients (K) and relative selectivity coefficients (K′) and presented as follow:

K_{Cd²⁺/Mⁿ⁺} = K_d(Cd²⁺)/K_d(Mⁿ⁺) (3)

K′ = (K_{Cd²⁺/Mⁿ⁺})_MIIP/(K_{Cd²⁺/Mⁿ⁺})_MNIP (4)

In this formula K_d(Cd²⁺) and K_d(Mⁿ⁺) are defined as distribution ratios of cadmium and tested interfering ions, respectively.

Various batch extractions containing pairs of target ions and tested interfering ions were carried out using 10.0 mg of magnetic tailor made polymer at optimized pH (pH of 8.0) for examination of the selectivity of synthesized sorbent. The selective retention of target ion over the studied competing ions such as Co²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ for MIIP and MNIP nanoparticles was evaluated under optimized experimental conditions (the concentration of target ion and competing ions for the experiment was 2 mg L⁻¹) and subsequently the distribution ratios (K_d), selectivity coefficients (K) and relative selectivity coefficients (K′) for target ions toward interfering ions were computed using eqn (2)–(4), respectively. The obtained data for the mentioned experiments are shown in Table 1. From the data in Table 1, the calculated sorption capacity of magnetic ion imprinted polymer nanoparticles is higher than magnetic non-imprinted polymer for retention of cadmium ions. Relative selectivity coefficients (K′) in Table 1 can obviously illustrate the higher selectivity of tailor made imprinted polymer related to non-imprinted polymer and the observation can prove the effect of imprinting process on the selectivity extraction of target ions. To synthesis of magnetic tailor made imprinted polymer, the complex between target ions and ligands act as template and create a selective recognition cavity in the network of polymer after removing of target ions from its structure, however, the control non-imprinted polymer is synthesized in the absence of target ions, therefore in this synthesis condition the creation of selective cavities in the polymer structure are impossible.

Table 1 Distribution ratio (K_d), selectivity coefficient (K) and relative selectively coefficient (K′) values of MIIP and MNIP for different cations

Interfering ion	Kd(MIIP) (mL g^−1)	Kd(MNIP) (mL g^−1)	KCd^2+/M^n+(MIIP)	KCd^2+/M^n+(MNIP)	K′
Cd^2+	38 000.0	2444.2	—	—	—
Cu^2+	353.0	3000.0	107.6	0.8	134.5
Pb^2+	500.0	2444.2	76.0	1.0	76.0
Zn^2+	222.2	1636.4	171.0	1.5	114.0
Ni^2+	222.2	2000.0	171.0	1.2	142.5
Co^2+	353.0	2444.2	107.6	1.0	107.6

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The created specific three dimensional structures and size for cadmium ions in the magnetic tailor made imprinted polymer is more suitable for target ions in comparison of other ions. According to this description, the cadmium ions can selectively be extracted from complex medium by using the synthesized tailor made material. The lower selectivity of non-imprinted polymer is due to the absence of the selective recognition sites in its structure. From the obtained data, a significant difference was obvious for binding ability of cadmium ions by magnetic ion imprinted polymer in comparison to magnetic-non imprinted polymer, and this finding can prove the high capability of synthesized tailor made material for selective extraction and pre-concentration of cadmium ions from complex matrices.

3.4. Evaluation of sample volume effect

The information about the section was presented in the ESI.†

3.5. Evaluation of sorption capacity for magnetic imprinted polymer and magnetic non-imprinted polymer

An important factor to evaluate the successful synthesis of imprinted materials is sorption capacity and defined as the maximum amount of target ions can be sorbed by 1.0 gram of the sorbent. The calculation of sorption capacity for the synthesized materials was carried out with 10 mL of a solution containing 0.1 mg of target ions at the optimum experimental conditions. Sorption capacity was calculated as the difference between concentration of the solution before and after extraction of cadmium ions. The sorption capacities for magnetic tailor made imprinted polymer and magnetic non-imprinted polymer were 9.9 mg g⁻¹ and 2.1 mg g⁻¹, respectively. According to the presence of selective and suitable cavity in the structure of imprinted polymer, the sorption capacity for this material is significantly higher than non-imprinted polymer.

3.6. Evaluation of repeated usage of the sorbent

The same sorbent was repeatedly used to investigate the reusability of the magnetic tailor made polymer (sorption–desorption cycle) for extraction and removal of target ions. The regeneration of the sorbent for subsequent usage was carried out by 1 mL of 1 mol L⁻¹ HNO₃. The retention–elution cycles were applied for ten times. The following optimized extraction experiment was used for each step: initial cadmium concentration, 100 μg L⁻¹; sorbent amount, 20 mg; sample pH, 8.0. According to the obtained data no significant decrease was observed in the initial binding tendency of the recognition holes on magnetic-IIP (recovery was >95% after ten retention–elution cycles).

3.7. The analytical figure of merits

The analytical performance of the coupling of MIIPs with FAAS was examined at the optimized experimental parameters. Figure of merits for the proposed method such as detection limit, coefficient of determination (r²), regression equations, linear dynamic ranges (LDRs), extraction recoveries (ER%) and enhancement factors (EFs) were evaluated. The sketched calibration curve for spiked cadmium ions was linear in the range of 0.1–500 μg L⁻¹. The precision of method was evaluated using eight separate experiments for determination of 50.0 μg L⁻¹ of target ions in 100 mL of water sample. Relative standard deviation for the experiments was 2.8%. The limit of detection (LOD) and limit of quantification (LOQ) for the applied coupling solid phase extraction method with FAAS was 0.02 μg L⁻¹ and 0.1 μg L⁻¹, respectively. The extraction recovery was tested using 100 μg L⁻¹ of cadmium ions in water sample and the obtained extraction recovery for the experiment was 98.2%. The enhancement factor for the proposed sample preparation method was calculated as the ratio between the slopes of calibration curves obtained by the method (Y = 21.85X + 0.0008, r²; 0.997) and direct monitoring of cadmium standards by FAAS (Y = 0.23X + 0.0744, r²; 0.998). The calculated EF for analysis of cadmium ions was 95 (21.85/0.23 = 95).

Certified reference materials (white cabbage [IRRM (BCR-679)]) were used for validation of the obtained data by the introduced method for trace monitoring of cadmium ions. As Table 1S (ESI†) shows, a good agreement is obvious for the measured value by the proposed method and the certified value in standard reference material. For further validation of the proposed SPE method, the obtained data for analysis of selected samples by the mentioned method was compared with the obtained data by higher sensitive instrument (graphite furnace atomic absorption spectrophotometry). As can be seen in the Table 2S,† there is a satisfactory agreement between the results obtained by the proposed method and by GFAAS. Therefore, the proposed SPE method can be reliably used for trace monitoring of cadmium ions in real samples with complex matrices (the optimum operation parameters for determination of cadmium by GFAAS were as Table 2S†).

3.8. Food analysis by the introduced method

To illustrate the power of introduced sample preparation method for selective pre-concentration of cadmium ions in complex matrices, the coupling of MIIPs with FAAS was used for trace detection of targets in several food samples. The precision of the method for trace detection of cadmium ions was evaluated by the mean relative standard deviation (RSD). Relative recovery (RR) was used to prove the accuracy of the obtained data by proposed sample preparation method. Relative recovery percentage is calculated according to the following formula:

RR (%) = (C_found − C_real/C_added) × 100 (5)

In this equation, C_found, C_real, and C_added was defined as the concentrations of analyte after addition of a specific amount of cadmium, the concentration of cadmium ions in real sample and the concentration of a known concentration of cadmium, which was added to the real sample, respectively.

The obtained data for trace detection of cadmium ions in real samples can prove our claim about reliable application of the proposed sample preparation method for trace monitoring of target ions with high selectivity in food samples. Table 2 provide the data for analysis of food samples by the proposed sample preparation method.

Table 2 Determination of cadmium ions in different food samples (N = 3)

Sample	Element	Real sample (μg kg^−1)	Added (μg kg^−1)	Found (μg kg^−1)	RR^a [%]	RSD [%]
a Relative recovery.
Shrimp	Cadmium	7.40	1.00	8.35	95.0	2.9
		7.40	10.00	17.20	98.0	2.1
		7.40	100.00	106.50	99.1	3.4
Fish	Cadmium	8.80	1.00	9.75	95.0	3.1
		8.80	10.00	18.90	101.0	2.8
		8.80	100.00	106.80	98.0	3.9
Crab	Cadmium	7.10	1.00	8.15	105.0	3.2
		7.10	10.00	16.90	98.0	2.9
		7.10	100.00	105.20	98.1	2.9
Persimmon	Cadmium	—	1.00	1.00	100.0	2.8
		—	10.00	9.90	99.0	2.5
		—	100.00	97.90	97.9	3.5
Apple	Cadmium	—	1.00	0.96	96.0	3.0
		—	10.00	9.85	98.5	2.1
		—	100.00	98.20	98.2	3.8
Tomato	Cadmium	—	1.00	0.97	97.0	3.5
		—	10.00	9.84	98.4	2.8
		—	100.00	97.90	97.9	3.1
Mushroom	Cadmium	0.50	1.00	1.46	96.0	2.9
		0.50	10.00	10.54	100.4	3.1
		0.50	100.00	98.50	98.0	3.5
Potato	Cadmium	—	1.00	0.98	98.0	3.1
		—	10.00	10.14	101.4	3.4
		—	100.00	96.9	96.9	3.8

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3.9. Comparison of the sample preparation method with previous works

In this section, the presented study was compared with other sample preparation methods that published recently. The obtained LOQ by coupling of magnetic ion imprinted polymers with flame atomic absorption spectrophotometry for trace detection of cadmium ions was satisfactory. Table 3 provide a comparison between the analytical performances of the applied sample preparation method in this article with recently published articles for trace monitoring of cadmium ions.^14,47–55 The obtained figure of merits for the proposed method can proof the robustness of the introduced sample preparation method for analysis of cadmium ions in the different real samples with complex matrices. Also, the prepared magnetic nanoparticles modified with imprinted polymer have several advantages in comparison of the conventional imprinted polymers.

Table 3 Comparison of the synthesized MIIP with literatures

Method	LOQ (μg L^−1)	RSD (%)	Extraction time (min)	EF^a; SV^b (mL)	Ref.
a Enhancement factor.b Sample volume.c Phenol-formaldehyde–Cd(II)–2-(p-sulphophenylazo)-1,8-dihydroxynaphthalene-3,6-disulphonate.d Poly-Cd(II)-diazoaminobenzene-vinyl pyridine.e Imprinted polymer.f Ion imprinted polymer.g Multiwalled carbon nanotubes.h Diphenylcarbazide.
PF–Cd(II)–SPANDS^c	—	—	50	—	47
Poly-Cd(II)–DAAB–VP^d	0.21	3.7	60	200; 2000	48
Cd(II)-IP^e	0.47	2.4	20	200; 1000	49
Cd(II)-IIP^f	0.5	4.2	7	60; 150	14
Modified-MWCNTs^g	0.1	2.4	95	150; 750	50
Modified Fe3O4@MCM-41	0.1	2.9	5	—, 100	51
DPC^h–SBA-15	1	3.4	50	100, 500	52
NNGT–MSADLLME	10	1.89	10	280, 50	53
Functionalized silica gel	10	—	—	12.3, —	54
L-Cystine modified zeolite	0.1	1.4	50	400, 100	55
Magnetic-IIP	0.1	3.4	5	95; 100	This work

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(1) Could easily collected from the studied samples.

(2) The method is faster than conventional imprinted polymer as the sample preparation method.

(3) The access to recognition sites in the structure of the imprinted polymer for target ions is significantly higher than conventional imprinted polymers.

(4) The analytical figure of merits by the applied sample preparation method was improved.

4. Conclusion

The prepared tailor made material with the advantages of magnetic nanoparticles and ion imprinted polymers was used for highly selective and rapid extraction of cadmium ions from food samples. It is should be noted that, the sample clean up with the mentioned solid phase extraction method is significantly higher than traditional sorbents. Therefore, the potentially interfering compounds do not have any significant effect on the extraction and pre-concentration of target ions. Also, the preconcentration of cadmium ions with the introduced sample preparation method is very fast and easy, because the separation of the sorbent can be achieved with the aid of an external magnetic field. The figure of merits for analysis of cadmium ions by the coupling procedure (MIIPs-FAAS) at trace levels is satisfactory. Finally, we can claim that, the applied sample preparation method can be reliably used in several fields such as environmental, water and food analysis. In this work to prove the applicability of the introduced sample preparation method, the coupling of magnetic ion imprinted polymer nanoparticles with flame atomic absorption spectrophotometry was performed for trace monitoring of cadmium ions in several food and agricultural products.

Acknowledgements

The authors express their appreciation to the Behbahan Faculty of Medical Sciences for financial support of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23688j

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