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graphene oxide nanocomposite in water and food samples
Microchemical Journal 146 (2019) 630–639
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Simultaneous magnetic solid phase extraction of acidic and basic pesticides using triazine-based polymeric network modified magnetic nanoparticles/ graphene oxide nanocomposite in water and food samples
T
Sepideh Moradi Shahrebabaka, Mohammad Saber-Tehrania, , Mohammad Farajib, , Meisam Shabanianc, Parviz Aberoomand-Azara ⁎
a b c
⁎
Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran Faculty of Food Industry and Agriculture, Department of Food Science & Technology, Standard Research Institute (SRI), Karaj, P.O. Box 31745-139, Iran Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, P.O. Box 31745-139, Iran
ARTICLE INFO
ABSTRACT
Keywords: 2,4-Dichlorophenoxyacetic acid Graphene oxide Imidacloprid Magnetic solid phase extraction Triazine-based polymeric network
In this work, a triazine-based polymeric network modified magnetic nanoparticles/graphene oxide (TPN/Fe3O4 NPs/GO) nanocomposite was successfully synthesized and used as an effective nanosorbent in simultaneous magnetic solid phase extraction of basic and acidic pesticides from food and water samples by HPLC-UV. Imidacloprid (IMI) and 2,4-dichlorophenoxyacetic acid (2,4-D) were selected as model basic and acidic pesticides, respectively. The synthesized TPN/Fe3O4/GO was characterized by field emission-scanning electron microscopy (FE-SEM), Fourier transform-infrared spectroscopy (FT-IR), vibrating sample magnetometry (VSM) and thermogravimetric analysis (TGA). Some parameters that affect the extraction efficiency such as pH of solution, sorbent amount, salt amount, extraction time, desorption solvent volume and desorption time were optimized by central composite design. However, type of desorption solvent and sample volume were optimized by one variable at a time method. Under optimum conditions, the calibration curve was linear in the range of 0.5–500 μg L−1 and 5.0–500 μg L−1 for IMI and 2,4-D, respectively. The limits of detection (LODs) for IMI and 2,4-D were 0.17 and 1.7 μg L−1, respectively. Also, the limits of quantification (LOQs) for IMI and 2,4-D were 0.5 and 5.0 μg L−1, respectively. The relative standard deviations (RSDs) were < 8.6% (n = 6). The proposed method was successfully applied for extraction and determination of IMI and 2,4-D in cucumber, tomato and water samples and satisfactory results (recoveries ≥90%) were obtained.
1. Introduction The use of pesticides in agriculture has increased dramatically and undeniably during the last few decades. Discharge of pesticides into the environment can lead to their incorporation into different kinds of matrices such as soil, crops, water and this may be a serious threat to human health [1]. Imidacloprid (IMI) and 2,4-dichlorophenoxyacetic acid (2,4-D) are important and highly used pesticides in the world. IMI is a neonicotinoid pesticide and widely used in agriculture because of its high insecticidal activity and low toxicity [2]. It acts as an agonist of acetylcholine by binding to postsynaptic nicotinic receptor in the insect central nervous system and causes the paralysis and death of insects [3]. On the other hand, 2,4-D is also widely used phenoxyacetic acid herbicide to control broad-leaf weeds due to its high efficiency [4]. Recently, 2,4-D has been considered as potential carcinogen, mutagen
⁎
and endocrine disruptor [5]. Since, maximum residue limit (MRL) value of 0.5–1.0 mg kg−1 for IMI in fruits and vegetables has been setup by Codex Alimentarius [6] but there is no MRL value for 2,4-D in fruits and vegetables in Codex Alimentarius. However, a MRL of 0.05 mg kg−1 has been set up by Plant Protection Organization of Iran for 2,4-D in fruits and vegetables [7]. Up to date, various analytical techniques such as high-performance liquid chromatography coupled with UV detection [8,9], liquid chromatography–tandem mass spectrometry [10,11] gas chromatography–mass spectrometry [12,13] and capillary electrophoresis [14,15] have been applied for determination of pesticides and herbicides. However, because of the complexity of matrix and low concentrations of pesticides in fruits and vegetables, usually a sample preparation method is required before instrumental analysis. In this regard, different sample preparation techniques including liquid-liquid
Corresponding authors. E-mail addresses: [email protected] (M. Saber-Tehrani), [email protected] (M. Faraji).
https://doi.org/10.1016/j.microc.2019.01.047 Received 17 August 2018; Received in revised form 18 January 2019; Accepted 21 January 2019 Available online 25 January 2019 0026-265X/ © 2019 Published by Elsevier B.V.
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extraction (LLE) [13,16], solid phase extraction (SPE) [17,18], dispersive liquid-liquid microextraction (DLLME) [19,20] and magnetic solid-phase extraction (MSPE) [21–35] have been developed for the extraction of pesticides. Nowadays, nanomaterials have gained much interest due to their unique mechanical, electrical, electronic, optical, magnetic, and surface properties [36]. In recent years, a new mode of SPE, magnetic solidphase extraction (MSPE), has been developed based on using magnetic sorbents. In MSPE the magnetic sorbent is added to a solution containing analytes. After extraction of analytes, the sorbent is separated from the solution by using an external magnet. Then, analytes are eluted from the sorbent and subsequently analyzed [25]. Magnetic nanoparticles (MNPs) have large surface area and short diffusion route. Therefore, high extraction capacity and high extraction efficiency can be achieved. Other property of MNPs is their easy separation by an external magnetic field. So need of centrifugation or filtration is avoided. Also, the MNPs can be easily functionalized, reused, or recycled. However, pure magnetic particles can form aggregation and their magnetic properties can be changed in complicated environmental and biological systems. Therefore, to solve the mentioned problems, a suitable coating is often necessary [36,37]. Many coatings have been used for MSPE such as C18 [29], carbon nanotube [30], graphene oxide [31,32] and polymers [33–35]. Among mentioned functional groups, polymers and graphenes present tunable surface chemistry and they introduce different extraction mechanism such as hydrophobic, hydrogen bonding and/or electrostatic attractions [31–35]. Among polymeric coatings, microporous organic polymers (MOPs) have gained much research interest due to the combined superiority of porous materials and functional polymers. MOPs possess well-defined porosity, high surface area, and tunable surface chemistry. Moreover, they can be easily functionalized [38]. Recently, MOPs have been applied for extraction of hydroxylated polycyclic aromatic hydrocarbons [39], 5-nitroimidazoles [40] and for removal of methyl orange [41]. Graphene oxide (GO) is the oxidized derivative of graphene, which contains many oxygen-containing functional groups on its surface which leading to more water dispersity and easier chemical modification in comparison with other carbon based materials [42,43]. Graphene has a two-dimensional structure and it possess large surface area, good chemical stability and other properties [42]. Therefore, preparation of magnetic nanocomposite based on MOPs and GO together could be introduced a novel magnetic nanocomposite which containing unique properties of both MOPs and GO. The aim of this study, was to design and synthesis new magnetic nanocomposite based on combination of microporous triazine-based organic polymer and graphene oxide, two advance materials, with unique properties such as having both amine and carboxylic functional groups at same time for efficient and simultaneous extraction of basic and acidic pesticides from food and water samples by HPLC-UV. Imidacloprid (IMI) and 2,4-dichlorophenoxyacetic acid (2,4-D) were selected as model basic and acidic pesticides, respectively. To the best of our knowledge, this magnetic nanocomposite has not been previously synthesized and applied for simultaneous extraction and determination of IMI. Since, first the nanocomposite were prepared and well characterized by conventional characterization techniques. Then, affecting parameters on extraction of IMI and 2,4-D were evaluated and optimized by central composite design (CCD). Distinct features of the proposed method were compared with recently published researches. Finally, the proposed method was applied for determination of the IMI and 2,4-D water and vegetable samples.
powder, ferric chloride (FeCl3·6H2O), ferrous chloride (FeCl2·4H2O), ammonia solution (25%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), melamine, terephthaldehyde, sulfuric acid, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), dichloromethane, HPLC-grade acetonitrile, acetone, acetic acid, potassium hydroxide, hydrochloric acid and sodium chloride were purchased from Merck (Darmstadt, Germany). Stock standard solution of each analyte at 1000 mg L−1 were prepared in acetonitrile and stored at 4 °C in refrigerator. Mixed standard working solutions were prepared by appropriate dilution of the stock solution in distilled water. 2.2. Apparatus Chromatographic analysis was performed on a Knauer HPLC system (Berlin, Germany) containing degasser, a K-1100 HPLC quaternary pump, a manual injector with 20 μL sample loop and a K2600 UV detector. The system was controlled by EZChrom software. The separation was performed on a Supelcosil LC-18 column (25 cm × 4.6 mm i.d, 5 μm) (Supelco. Bellefonte. PA, USA). The mobile phase consisted of (A) water containing 0.5% acetic acid and (B) acetonitrile. The gradient elution was performed as follows: from 0 to 5 min, 70% A; from 5 to 6 min, a linear gradient from 30 to 50% B; from 6 to 13 min, 50% B; from 13 to 14 min, a linear gradient from 50 to 70 A; from 15 to 20 min, 70% A. The flow rate was 1.0 mL min−1 and the detection wavelength was set at 280 nm. 2.3. Synthesize of the adsorbent Graphene oxide (GO) was synthesized according to literature [44]. 3.0 g graphite powder and 1.5 g NaNO3 were poured into a 1000 mL round bottom flask. Then 150 mL H2SO4 was added to the flask. The mixture was stirred in an ice bath for 4 h at 5 °C. 9.0 g KMnO4 was added slowly to control the temperature. Then the ice bath was removed and temperature of the mixture was increased to 45 °C and stirred for 3 h until became tasty brownish. Then, 150 mL deionized water was added to the mixture and stirred at 98 °C for about 1 h until the color changed from brown to yellow. The mixture was stirred for 15 min. Then 700 mL deionized water was added. 30 mL H2O2 (30%) was added to the mixture after 1 h. Finally, the solution was filtered and washed several times with deionized water and dried at 60 °C for 12 h to obtain GO powder. Iron oxide NPs were prepared by the reported method [45]. Briefly, 2.4 g FeCl2·4H2O and 5.4 g FeCl3·6H2O were dissolved in 20 mL deionized water under nitrogen atmosphere. Then 25 mL NH3·H2O (25 wt%) was added dropwise to the solution and the solution was heated at 70 °C for 5 h. Then, the solution temperature was increased to 85 °C to vaporize residual ammonia. The resulting black dispersion was separated with magnet and washed with deionized water. The magnetic NPs were modified by triazine-based polymeric network (TPN) via in situ method according to our previously reported method [41]. 0.939 g melamine, 1.5 g terephthaldehyde and 45 mL DMSO were stirred and degassed by argon bubbling. The mixture temperature was increased to 180 °C under an inert atmosphere. After 48 h, 1.5 g Fe3O4 NPs were added and the reaction was continued for another 24 h. After it cooled to room temperature, the resulting precipitation was collected with the magnet, washed with excess DMSO, acetone, THF and dichloromethane. The obtained TPN/MNPs were dried under vacuum at 60 °C. The GO powders and TPN/Fe3O4 were dispersed in THF separately, and then mixed together (weight ratio of 1:1) at room temperature by an ultrasonic bath. The product was separated by the magnetic field and dried at 50 °C. Field emission-scanning electron microscopy (FESEM) images were obtained using MIRA3FESEM (Tescan, Czech Republic). The FT-IR spectra were obtained by Thermo Nicolet Nexus 870 FT-IR
2. Experimental 2.1. Chemicals and reagents 2,4-Dichlorophenoxyacetic acid(2,4-D) and imidacloprid (IMI) were purchased from Sigma-Aldrich (Steinheim, Germany). Graphite 631
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spectrometer (Madison, USA). Thermogravimetric analysis (TGA) was performed on a NETZSCH TG 209 F1 (Selb, Germany) from room temperature to 800 °C at a heating rate of 10 °C min−1 in N2 atmosphere. The magnetic properties were measured by vibrating sample magnetometer (VSM) (Meghnatis Daghigh Kavir, Kashan, Iran).
3. Results and discussion 3.1. Characterization The FTIR spectra of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 1. In FTIR spectrum of Fe3O4, the band at 3390 cm−1 is attributed to OeH stretching vibration and the band at 572 cm−1 is attributed to FeeO bond. The FTIR spectrum of TPN/Fe3O4 showed a broad coupling band around 3400 cm−1 corresponding to NeH groups of TPN and OeH groups in surface of the Fe3O4 NPs. The bands at 1554 cm−1 and 1485 cm−1 are attributed to C]N group of TPN and the band at 575 cm−1 corresponds to FeeO [41]. In the FTIR spectrum of TPN/Fe3O4/GO, the characteristic absorption bands of TPN/Fe3O4 and the absorption bands of GO are observed. The broad band in the range of 2500–3600 cm−1 may attributed to OeH of carboxylic acid of GO, NeH and OeH groups in TPN/Fe3O4/GO [44]. FE-SEM images of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 2. Fig. 2a shows that the NPs have uniform size distribution while some particles are agglomerated. The size of Fe3O4 NPs is 22–50 nm. Fig. 2b shows that the size of TPN/Fe3O4 NPs is 26–70 nm. The size of TPN/Fe3O4 NPs is increased compared to Fe3O4 NPs, that may attribute to modification with TPN. SEM image of TPN/Fe3O4/GO confirmed the presence of GO (Fig. 2c). TGA curves of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 3. The TGA data including temperatures at which 5% (T5), 10% (T10) degradation occur and the residue at 800 °C are summarized in Table 1. In the TGA curve of Fe3O4 NPs, the total weight loss over the full temperature range is 1.6% that is attributed to the loss of adsorbed water and dehydration of the surface eOH groups [46]. As shown in Table 1, T5 of TPN/Fe3O4 and TPN/Fe3O4/GO is 172 °C, which is due to the loss of adsorbed water and dehydration of the surface eOH groups and some monomers. T10 of TPN/Fe3O4/GO is 192 °C which is lower than that of TPN/Fe3O4 (T10 = 242 °C), but the char residue of TPN/ Fe3O4/GO is 51% at 800 °C that is higher than that of TPN/Fe3O4 that is 46%. Magnetization curves of Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 4.The saturation magnetization of Fe3O4 and TPN/Fe3O4/GO are 70 and 19 emu/g, respectively. The saturation magnetization of TPN/ Fe3O4/GO decreased due to the present of TPN and GO. However, it is enough for magnetic separation with a magnet.
2.4. The procedure 50 mL sample solution containing the analytes (100 μg L−1) was transferred to a beaker, and the pH (7.4) and ionic strength (NaCl, 7% w/v) were adjusted. Then, 11.0 mg of TPN/Fe3O4/GO sorbent was added to the solution. The mixture was shaken for 8 min to reach the adsorption equilibrium. Afterward, the sorbent was isolated from the solution with the magnet. The supernatant was decanted and the residual was transferred to a 10 mL conical tube to elute the sorbent with smaller volumes of the desorption solvent. The adsorbed analytes were desorbed with 220 μL acetone by shaking for 6 min. Finally, by placing a magnet to the outside of the conical tube the desorption solvent containing the analytes was collected and 20 μL of it was injected into HPLC for analysis. 2.5. Sample preparation The samples including cucumber and tomato were purchased from local market (Tehran, Iran). Tap water sample was obtained from Standard Research Institute (Karaj, Iran). The water sample was filtered with filter paper. Cucumbers and tomatoes were ground and homogenized. 1.0 g of the homogenized samples was accurately weighed. Then, 2.5 mL acetonitrile and 2.5 mL distilled water were added to the samples and homogenized for 1 min. The mixtures were sonicated for 10 min and filtered. Then the filtered samples were diluted to the volume of 50 mL with distilled water. For the spiked samples, the mentioned procedure was performed and followed by adding proper amount of mixed standard solution of IMI and 2.4-D (25 mg L−1) to the weighed homogenized samples to obtain 100 μg Kg−1 spike level in the final solution.
Fig. 1. FTIR spectra of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO. 632
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Fig. 2. FE-SEM images of (a) Fe3O4 (b) TPN/Fe3O4 and (c) TPN/Fe3O4/GO.
Fig. 3. TGA curves of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO.
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larger volume of sample, due to less amount of sorbent in volume unit of the sample solution [47]. Therefore, sample volume of 50 mL was selected for subsequent experiments.
Table 1 Thermal properties data of TPN/Fe3O4 and TPN/Fe3O4/GO. Samples
T5 (° C)a
T10 (°C)b
CR (%)c
TPN/Fe3O4 TPN/Fe3O4/GO
172 172
242 192
46 51
3.2.3. Central composite design In the next step, to determine the effects of the six remaining factors at five levels, a small central composite design (SCCD) was used to reduce total number of experiments. For six variable, by using central composite design (CCD) high number of experiments should be done according to the equation N = 2k + 2k + m (where k is the number of design variables, and m is the number of center point replications (in this study, with six factor and ten center points, CCD involves 86 experiments)) [48]. SCCD under response surface methodology (RSM) as powerful mode can cost effectively decrease the number of experimental runs. It also makes it possible to optimize the effects of variables and their possible interactions on the response [49]. The factors and their ranges are as follows: pH (A; 3–11), sorbent amount (B; 5–15 mg), salt content (C; 0–10%w/v), extraction time (D; 2–14 min), desorption solvent volume (E; 200–1000 μL) and desorption time (F; 2–14 min). SCCD consisted of 34 experiments, including six variable and six central points. Since the required time to perform 34 experiments during one working day was not sufficiently short, so they were carried out in two blocks. The experiments were performed randomly in order to minimize the effect of uncontrolled variables. The design matrix and response are shown in Table 2 and Table S1, respectively. Experimental design, data analysis and response surfaces were performed by Design-Expert 7.1.3. The equation below illustrates the relationship between the six variables and the peak area:
a
Temperature at 5% weight loss. Temperature at 10% weight loss. c CR: Char residue, Weight percentage of material left after TGA analysis at a maximum temperature of 800 °C. b
Fig. 4. VSM magnetization curves of (a) Fe3O4 and (b) TPN/Fe3O4/GO.
3.2. Optimization of MSPE parameters
Peak area = + 6.3 E + 005 + 54159.4 A + 11406.6 B + 15225. 6C
The effect of several factors affecting the extraction efficiency including pH, amount of sorbent, salt content, extraction time, volume of desorption solvent, desorption time and sample volume were investigated and optimized by experimental design. However, effect of desorption solvent type and sample volume were optimized using one variable at a time method because of reducing total number of variables and also remove categoric variable in experimental design. Moreover, preliminary experiments showed that handling of large sample volumes is different from low sample volumes which it could be inserted some systematic errors in experimental design. Since, effect of desorption solvent type and sample volume was investigated separately.
+ 42150.9 D
1857.8 E
1291.3 AC
+ 2641.3 CD + 29.8 EF 588.8
D2
14543.3 F + 4564.8 AB
5483.6 AD
+ 0.8
E2
1469.0 BC 4638.8 A2
240.8
419.3 BD
1395.4 B2
734.8 C2 (3)
F2
The analysis of variance (ANOVA) was performed for modified quadratic model and the results are summarized in Table 3. p value < 0.05 indicates model terms are significant. In this case A, E and E2 are significant model terms. The model F-value of 6.13 with a pvalue of 0.0009 implies that the model is significant and there is only a 0.09% chance that a large model F-value could occur due to noise. The Lack of Fit (LOF) p-value of 0.9566 implies the LOF is not significant relative to the pure error. The quality of the model was expressed by the coefficient of determination, R2. The response equation fitted with the experimental data considering a R2-value of 0.8995, which indicates adequate correlation between responses and experimental factors according to Joglekar and May advice [50]. Fig. 5 illustrates the relationship between the explanatory and response variables in a three-dimensional representation of the response surface. As can be seen in Fig. 5a, the extraction efficiency increased with increase in pH up to 7.4. The pKa value of 2,4-D is 2.6 [51]. The molecules exist in the anionic forms when the pH is higher than pKa values. At optimum pH (pH = 7.4) amine groups of TPN/Fe3O4/GO are
3.2.1. Effect of desorption solvent type Different organic solvents including acetone, acetonitrile, mixture of acetone and 1% acetic acid solution (50:50, v/v), mixture of acetonitrile and 0.2 M Na2EDTA (50:50, v/v) and mixture of acetonitrile and 10 mM KOH (80:20, v/v) were used for desorption of analytes from the sorbent. The results showed that acetone was the superior solvent in comparison with other solvents for desorption of analytes, that is similar to many studies that have applied G-based magnetic nanocomposite as adsorbent [21]. It is noteworthy that, IMI is unstable in high acidic and alkaline pHs [3], therefore, getting low desorption efficiency for desorption solvents containing acid and base is expectable. Thus, acetone was selected as the desorption solvent for subsequent experiments.
Table 2 Experimental factors, levels and matrix of the central composite design.
3.2.2. Effect of sample volume Large volume of sample solution is required to achieve higher enrichment factor [33]. The effect of sample volume on the extraction of the analytes was studied by different sample volumes (25–200 mL) containing fixed amount of analytes (5.0 μg). The results showed that the highest extraction efficiency was obtained when sample volume was equal or lower than 50 mL. However, the extraction efficiency decreased when the sample volumes were > 50 mL. It can be attributed to the more difficult transference of analytes to the sorbent surface in
Factors
pH Sorbent amount (mg) Salt (% w/v) Extraction time (min) Desorption solvent volume (μL) Desorption time (min)
634
Symbol
A B C D E F
Level −α
−1
0
+1
+α
3 5 0 2 200 2
5 7.5 2.5 5 400 5
7 10 5 8 600 8
9 12.5 7.5 11 800 11
11 15 10 14 1000 14
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Table 3 Analysis of variance (ANOVA) for modified response surface quadratic model. Source
Sum of squares
dfa
Mean square
Block Model A-pH B-sorbent amount C-salt D-extraction time E-desorption solvent volume F-desorption time AB AC AD BC BD CD EF A2 B2 C2 D2 E2 F2 Residual Lack of fit Pure error Cor total
2.436E + 009 3.438E + 011 2.295E + 010 3.389E + 009 4.180E + 009 2.435E + 009 1.155E + 011 1.730E + 007 8.335E + 009 6.670E + 008 5.773E + 009 4.496E + 008 1.582E + 008 6.279E + 009 5.130E + 009 1.042E + 010 2.301E + 009 6.380E + 008 8.497E + 008 3.486E + 010 1.421E + 008 3.839E + 010 1.418E + 010 2.422E + 010 3.846E + 011
1 19 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 13 9 4 33
2.436E + 009 1.809E + 010 2.295E + 010 3.389E + 009 4.180E + 009 2.435E + 009 1.155E + 011 1.730E + 007 8.335E + 009 6.670E + 008 5.773E + 009 4.496E + 008 1.582E + 008 6.279E + 009 5.130E + 009 1.042E + 010 2.301E + 009 6.380E + 008 8.497E + 008 3.486E + 010 1.421E + 008 2.953E + 009 1.575E + 009 6.054E + 009
a
F value
p-Value
6.13 7.77 1.15 1.42 0.82 39.10 5.859E-003 2.82 0.23 1.95 0.15 0.054 2.13 1.74 3.53 0.78 0.22 0.29 11.80 0.048
0.0009 0.0154 0.3036 0.2554 0.3804 < 0.0001 0.9402 0.1168 0.6425 0.1855 0.7027 0.8206 0.1686 0.2103 0.0830 0.3934 0.6498 0.6008 0.0044 0.8298
Significant
0.26
0.9566
Not significant
Degree of freedom.
protonated that causes electrostatic attraction between 2,4-D and TPN/ Fe3O4/GO surface. The extraction efficiency decreased at higher pH due to the repulsion between negatively charged functional groups on the absorbent's surface and the negative charge on 2,4-D molecule. On the other hand, IMI can hydrolyze in acidic and alkaline media but it is stable in pH ranges from 6 to 7 [3]. At pH = 7.4, carboxyl groups of TPN/Fe3O4/GO are deprotonated that causes electrostatic attraction between IMI and TPN/Fe3O4/GO surface. Moreover, due to presence a lot of hydrogen bonding acceptor on the surface of the nanocomposite, extraction could be proceed via this mechanism [41]. Therefore, pH of solution was fixed at 7.4. Fig. 5a also shows that the extraction efficiency increased by increasing the sorbent amounts up to 11 mg that is because of increasing of accessible sites to the adsorption of the analytes [23]. The extraction efficiency increases by increasing salt concentration due to salting out effect [32]. However, as shown in Fig. 5b, the extraction efficiency decreases with further increase of salt concentration (at low extraction times) due to increase of solution viscosity that causes decrease of the analytes transference to the adsorbent surface [52]. However, at longer times, the analytes have enough time to transfer to the adsorbent. Therefore, the extraction efficiency increases. As can be seen in Fig. 5c, the response decreased by increasing desorption solvent volume as result of the effect of dilution. Desorption solvent volumes lower than 200 μL were not studied because of incomplete wetting of the sorbent with the desorption solvent which resulting in incomplete desorption of the analytes [47]. Also, Fig. 5c shows that desorption time had no significant effect on the desorption efficiency. Finally, according to the overall results of the optimization study, the following optimum conditions for the extraction of IMI and 2,4-D from water and food samples were selected: pH, 7.4; sorbent amount, 11 mg; salt amount, 7% w/v; extraction time; 8 min, desorption solvent volume; 220 μL and desorption time, 6 min; desorption solvent type, acetone and sample volume, 50 mL.
magnetic nanocomposite which containing MOP and GO together at the same time [35–43]. At pH = 7.4, optimum suggested pH by software, at the same time IMI is protonated and carboxylic groups of GO are deprotonated, therefore effective electrostatic interaction could be occurred between IMI and GO. Similarly, at pH = 7.4, 2,4-D is deprotonated and amine groups of MOP are protonated, therefore effective electrostatic interaction could also be occurred between 2,4-D and MOP. Moreover, according to getting good extraction efficiency in presence of relatively high salt amount (7% w/v), hydrogen bonding and π-π interactions between the analytes and nanocomposite materials (both MOP and GO) are the other probable extraction driven forces. Why so, electrostatic attraction mechanism could be remarkably inhibited by high salt amounts due to competition of salt ions with the pesticides [41]. So, it could be concluded that electrostatic attraction, π-π and hydrogen bonding interactions are the main driven forces for extraction of the analytes. 3.3. Method validation Quantitative parameters of the proposed method including linearity, limit of detection (LOD), limit of quantification (LOQ) and enrichment factor (EF) were investigated under optimized conditions. The results are summarized in Table 4. Good linearity was obtained in the range of 0.5–500 μg L−1 and in the range of 5.0–500 μg L−1 for IMI and 2,4-D, respectively (R2 ≥ 0.99). LOD based on signal-to-noise ratio of 3 for IMI was 0.17 μg L−1 and for 2,4-D was 1.7 μg L−1. LOQ based on signal-to-noise ratio of 10 for IMI was 0.5 μg L−1 and for 2,4-D was 5.0 μg L−1. The enrichment factor, defined as the ratio of the concentration of the analytes between the extracted organic phase and the initial concentration of the analytes in the aqueous, was obtained 95 for IMI and 90 for 2,4-D. The repeatability, expressed as relative standard deviations (RSDs) for six replicate measurements at 10 and 100 μg L−1 of IMI and 2,4-D were < 9.5%. A comparison of the proposed method features with other methods reported for extraction and determination of IMI and 2,4-D was made and summarized in Table 5. As can be seen from Table 5, the proposed method has wide dynamic linear range. Also, it has lower LOD than solid phase extraction methods which uses detection method such as
3.2.4. Extraction mechanism Different interactions such as electrostatic, hydrophobic, dipole-dipole, Van der Waals (dispersion), hydrogen bonding, π-π interactions could be occurred between the target analytes and the prepared 635
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Fig. 5. Three-dimensional response surface plots for interactions (a) pH and sorbent amount, (b) salt and extraction time, (c) desorption solvent volume and desorption time.
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Table 4 Analytical performance of the proposed method. Analyte
Linear range (μg L−1)
IMI 2,4-D a b c d
0.5–500 5.0–500
R2
0.9954 0.9967
LODa (μg L−1)
LOQb (μg L−1)
EFc
0.17 1.7
0.5 5.0
95 90
RSD%d (n = 6) 100 (μg L−1)
10 (μg L−1)
4.7 8.6
5.9 9.3
Limit of detection. Limit of quantification. Enrichment factor. Relative standard deviation.
HPLC-DAD [53] or cLC [55]. In contrast, LOD of method for IMI extraction is higher than some MSPE which are the basis of graphene sorbents [21,54]. However, the proposed method provides simultaneous extraction of acidic and basic pesticides at the same time. Compared to traditional SPE method, the proposed method has advantages such as shorter extraction time and less consumption of organic solvents. The sorbent can be separated from the aqueous solution by applying an external magnetic field. So, no centrifugation or filtration is necessary. Moreover, LOD of the proposed method for IMI (0.17 μg L−1) and 2,4-D (1.7 μg L−1) provide enough sensitivity determination of IMI and 2,4-D according to their MRLs (0.5–1.0 mg kg−1 for IMI and 0.05 mg kg−1 for 2,4-D in fruits and vegetables). The results indicate that the developed method is simple, rapid and efficient and can be used for trace analysis of the analytes in real samples.
Table 6 Determination of IMI and 2,4-D in cucumber, tomato and tap water samples. Sample Cucumber
Analyte
Cadded (μg Kg−1)
Cfound (μg Kg−1)
Recovery (%)
IMI
– 100 – 100 – 100 – 100 – 100 – 100
81.3 172.5 – 94.3 – 102.4 – 93.2 – 92.7 – 95.3
– 91.2 – 94.3 – 102.4 – 93.2 – 92.7 – 95.3
2,4-D Tomato
IMI 2,4-D
Tap water
IMI 2,4-D
3.4. Sample analysis
and characterized by FESEM, FTIR, TGA and VSM. Then, TPN/Fe3O4/ GO was used as efficient sorbent for simultaneous extraction of IMI and 2,4-D in cucumber, tomato and water samples. Introducing TPN and GO in magnetic nanocomposite, provide different functional (amine and carboxylic) groups at the same time which leads to simultaneous extraction of basic and acidic pesticides via hydrophobic, hydrogen bonding and or electrostatic attraction with satisfactory recoveries. Moreover, well-defined porosity, high surface area, good chemical stability and tunable surface chemistry are the other distinct features of such composites. Compared to traditional SPE methods, extraction time of this method is shorter due to rapid extraction dynamics and magnetic separation of the sorbent that no centrifugation or filtration is needed. The other advantage of this method with the proposed sorbent is high extraction capacity due to high surface area-to-volume ratio of the sorbent. The results showed that TPN/Fe3O4/GO composite can be a promising sorbent for the simultaneous extraction of basic and acidic compounds in different samples.
The proposed method was applied for the determination of IMI and 2,4-D in cucumber, tomato and tap water samples. The results showed that tap water and tomato samples were free of IMI and 2,4-D. However, 81.3 μg kg−1 IMI was detected in cucumber sample. To estimate the effect of the matrices and determine the recovery of the analytes, the samples were spiked at concentration level of 100 μg kg−1. The results are summarized in Table 6. The recoveries of IMI and 2,4-D from cucumber, tomato and water samples were in the range of 92–104%. Fig. 6 shows the chromatograms of IMI and 2,4-D in cucumber sample before and after spiking with100 μg Kg−1 of the analytes. 4. Conclusion In the present study, triazine-based polymeric network modified Fe3O4 NPs/graphene oxide nanocomposite was successfully synthesized
Table 5 Comparison of the proposed method with the other methods for extraction and determination of IMI and 2,4-D. Matrix Ground water and surface water Reservoir water, sea water and river water Tap water and riverine water Lake water, river water and reservoir water Water samples Agricultural products Pear and tomato Apple juice Cucumber, tomato and tap water a b c d e
Analyte
Extraction method
IMI IMI 2,4-D 2,4-D 2,4-D IMI IMI 2,4-D IMI 2,4-D
SPEa MSPEb SPE DLLMEc MSPE SPE MSPE SPE MSPE
Detection LC–ESI-MS–MS HPLC-UV HPLC-UV MEKCd LC–MS/MS HPLC-DAD HPLC–DAD cLCe HPLC-UV
Solid phase extraction. Magnetic solid phase extraction. Dispersive liquid–liquid microextraction. Micellar electrokinetic chromatography. Capillary liquid chromatography. 637
Linear range (μg L−1)
LOD (μg L−1)
Ref.
– 0.05–50 10–10,000 10–1000 0.1–20 20–200 0.5–100 30–170 0.5–500 5.0–500
0.009 0.006 0.15 1.56 0.02 10 0.08–0.1 7 0.17 1.7
[17] [21] [18] [20] [22] [53] [54] [55] Present work
Microchemical Journal 146 (2019) 630–639
S. Moradi Shahrebabak et al.
Fig. 6. The chromatograms of (a) non spiked cucumber sample and (b) cucumber sample spiked with 100 μg Kg−1 of IMI and 2,4-D.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.01.047.
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Simultaneous magnetic solid phase extraction of acidic and basic pesticides using triazine-based polymeric network modified magnetic nanoparticles/ graphene oxide nanocomposite in water and food samples
T
Sepideh Moradi Shahrebabaka, Mohammad Saber-Tehrania, , Mohammad Farajib, , Meisam Shabanianc, Parviz Aberoomand-Azara ⁎
a b c
⁎
Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran Faculty of Food Industry and Agriculture, Department of Food Science & Technology, Standard Research Institute (SRI), Karaj, P.O. Box 31745-139, Iran Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, P.O. Box 31745-139, Iran
ARTICLE INFO
ABSTRACT
Keywords: 2,4-Dichlorophenoxyacetic acid Graphene oxide Imidacloprid Magnetic solid phase extraction Triazine-based polymeric network
In this work, a triazine-based polymeric network modified magnetic nanoparticles/graphene oxide (TPN/Fe3O4 NPs/GO) nanocomposite was successfully synthesized and used as an effective nanosorbent in simultaneous magnetic solid phase extraction of basic and acidic pesticides from food and water samples by HPLC-UV. Imidacloprid (IMI) and 2,4-dichlorophenoxyacetic acid (2,4-D) were selected as model basic and acidic pesticides, respectively. The synthesized TPN/Fe3O4/GO was characterized by field emission-scanning electron microscopy (FE-SEM), Fourier transform-infrared spectroscopy (FT-IR), vibrating sample magnetometry (VSM) and thermogravimetric analysis (TGA). Some parameters that affect the extraction efficiency such as pH of solution, sorbent amount, salt amount, extraction time, desorption solvent volume and desorption time were optimized by central composite design. However, type of desorption solvent and sample volume were optimized by one variable at a time method. Under optimum conditions, the calibration curve was linear in the range of 0.5–500 μg L−1 and 5.0–500 μg L−1 for IMI and 2,4-D, respectively. The limits of detection (LODs) for IMI and 2,4-D were 0.17 and 1.7 μg L−1, respectively. Also, the limits of quantification (LOQs) for IMI and 2,4-D were 0.5 and 5.0 μg L−1, respectively. The relative standard deviations (RSDs) were < 8.6% (n = 6). The proposed method was successfully applied for extraction and determination of IMI and 2,4-D in cucumber, tomato and water samples and satisfactory results (recoveries ≥90%) were obtained.
1. Introduction The use of pesticides in agriculture has increased dramatically and undeniably during the last few decades. Discharge of pesticides into the environment can lead to their incorporation into different kinds of matrices such as soil, crops, water and this may be a serious threat to human health [1]. Imidacloprid (IMI) and 2,4-dichlorophenoxyacetic acid (2,4-D) are important and highly used pesticides in the world. IMI is a neonicotinoid pesticide and widely used in agriculture because of its high insecticidal activity and low toxicity [2]. It acts as an agonist of acetylcholine by binding to postsynaptic nicotinic receptor in the insect central nervous system and causes the paralysis and death of insects [3]. On the other hand, 2,4-D is also widely used phenoxyacetic acid herbicide to control broad-leaf weeds due to its high efficiency [4]. Recently, 2,4-D has been considered as potential carcinogen, mutagen
⁎
and endocrine disruptor [5]. Since, maximum residue limit (MRL) value of 0.5–1.0 mg kg−1 for IMI in fruits and vegetables has been setup by Codex Alimentarius [6] but there is no MRL value for 2,4-D in fruits and vegetables in Codex Alimentarius. However, a MRL of 0.05 mg kg−1 has been set up by Plant Protection Organization of Iran for 2,4-D in fruits and vegetables [7]. Up to date, various analytical techniques such as high-performance liquid chromatography coupled with UV detection [8,9], liquid chromatography–tandem mass spectrometry [10,11] gas chromatography–mass spectrometry [12,13] and capillary electrophoresis [14,15] have been applied for determination of pesticides and herbicides. However, because of the complexity of matrix and low concentrations of pesticides in fruits and vegetables, usually a sample preparation method is required before instrumental analysis. In this regard, different sample preparation techniques including liquid-liquid
Corresponding authors. E-mail addresses: [email protected] (M. Saber-Tehrani), [email protected] (M. Faraji).
https://doi.org/10.1016/j.microc.2019.01.047 Received 17 August 2018; Received in revised form 18 January 2019; Accepted 21 January 2019 Available online 25 January 2019 0026-265X/ © 2019 Published by Elsevier B.V.
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S. Moradi Shahrebabak et al.
extraction (LLE) [13,16], solid phase extraction (SPE) [17,18], dispersive liquid-liquid microextraction (DLLME) [19,20] and magnetic solid-phase extraction (MSPE) [21–35] have been developed for the extraction of pesticides. Nowadays, nanomaterials have gained much interest due to their unique mechanical, electrical, electronic, optical, magnetic, and surface properties [36]. In recent years, a new mode of SPE, magnetic solidphase extraction (MSPE), has been developed based on using magnetic sorbents. In MSPE the magnetic sorbent is added to a solution containing analytes. After extraction of analytes, the sorbent is separated from the solution by using an external magnet. Then, analytes are eluted from the sorbent and subsequently analyzed [25]. Magnetic nanoparticles (MNPs) have large surface area and short diffusion route. Therefore, high extraction capacity and high extraction efficiency can be achieved. Other property of MNPs is their easy separation by an external magnetic field. So need of centrifugation or filtration is avoided. Also, the MNPs can be easily functionalized, reused, or recycled. However, pure magnetic particles can form aggregation and their magnetic properties can be changed in complicated environmental and biological systems. Therefore, to solve the mentioned problems, a suitable coating is often necessary [36,37]. Many coatings have been used for MSPE such as C18 [29], carbon nanotube [30], graphene oxide [31,32] and polymers [33–35]. Among mentioned functional groups, polymers and graphenes present tunable surface chemistry and they introduce different extraction mechanism such as hydrophobic, hydrogen bonding and/or electrostatic attractions [31–35]. Among polymeric coatings, microporous organic polymers (MOPs) have gained much research interest due to the combined superiority of porous materials and functional polymers. MOPs possess well-defined porosity, high surface area, and tunable surface chemistry. Moreover, they can be easily functionalized [38]. Recently, MOPs have been applied for extraction of hydroxylated polycyclic aromatic hydrocarbons [39], 5-nitroimidazoles [40] and for removal of methyl orange [41]. Graphene oxide (GO) is the oxidized derivative of graphene, which contains many oxygen-containing functional groups on its surface which leading to more water dispersity and easier chemical modification in comparison with other carbon based materials [42,43]. Graphene has a two-dimensional structure and it possess large surface area, good chemical stability and other properties [42]. Therefore, preparation of magnetic nanocomposite based on MOPs and GO together could be introduced a novel magnetic nanocomposite which containing unique properties of both MOPs and GO. The aim of this study, was to design and synthesis new magnetic nanocomposite based on combination of microporous triazine-based organic polymer and graphene oxide, two advance materials, with unique properties such as having both amine and carboxylic functional groups at same time for efficient and simultaneous extraction of basic and acidic pesticides from food and water samples by HPLC-UV. Imidacloprid (IMI) and 2,4-dichlorophenoxyacetic acid (2,4-D) were selected as model basic and acidic pesticides, respectively. To the best of our knowledge, this magnetic nanocomposite has not been previously synthesized and applied for simultaneous extraction and determination of IMI. Since, first the nanocomposite were prepared and well characterized by conventional characterization techniques. Then, affecting parameters on extraction of IMI and 2,4-D were evaluated and optimized by central composite design (CCD). Distinct features of the proposed method were compared with recently published researches. Finally, the proposed method was applied for determination of the IMI and 2,4-D water and vegetable samples.
powder, ferric chloride (FeCl3·6H2O), ferrous chloride (FeCl2·4H2O), ammonia solution (25%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), melamine, terephthaldehyde, sulfuric acid, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), dichloromethane, HPLC-grade acetonitrile, acetone, acetic acid, potassium hydroxide, hydrochloric acid and sodium chloride were purchased from Merck (Darmstadt, Germany). Stock standard solution of each analyte at 1000 mg L−1 were prepared in acetonitrile and stored at 4 °C in refrigerator. Mixed standard working solutions were prepared by appropriate dilution of the stock solution in distilled water. 2.2. Apparatus Chromatographic analysis was performed on a Knauer HPLC system (Berlin, Germany) containing degasser, a K-1100 HPLC quaternary pump, a manual injector with 20 μL sample loop and a K2600 UV detector. The system was controlled by EZChrom software. The separation was performed on a Supelcosil LC-18 column (25 cm × 4.6 mm i.d, 5 μm) (Supelco. Bellefonte. PA, USA). The mobile phase consisted of (A) water containing 0.5% acetic acid and (B) acetonitrile. The gradient elution was performed as follows: from 0 to 5 min, 70% A; from 5 to 6 min, a linear gradient from 30 to 50% B; from 6 to 13 min, 50% B; from 13 to 14 min, a linear gradient from 50 to 70 A; from 15 to 20 min, 70% A. The flow rate was 1.0 mL min−1 and the detection wavelength was set at 280 nm. 2.3. Synthesize of the adsorbent Graphene oxide (GO) was synthesized according to literature [44]. 3.0 g graphite powder and 1.5 g NaNO3 were poured into a 1000 mL round bottom flask. Then 150 mL H2SO4 was added to the flask. The mixture was stirred in an ice bath for 4 h at 5 °C. 9.0 g KMnO4 was added slowly to control the temperature. Then the ice bath was removed and temperature of the mixture was increased to 45 °C and stirred for 3 h until became tasty brownish. Then, 150 mL deionized water was added to the mixture and stirred at 98 °C for about 1 h until the color changed from brown to yellow. The mixture was stirred for 15 min. Then 700 mL deionized water was added. 30 mL H2O2 (30%) was added to the mixture after 1 h. Finally, the solution was filtered and washed several times with deionized water and dried at 60 °C for 12 h to obtain GO powder. Iron oxide NPs were prepared by the reported method [45]. Briefly, 2.4 g FeCl2·4H2O and 5.4 g FeCl3·6H2O were dissolved in 20 mL deionized water under nitrogen atmosphere. Then 25 mL NH3·H2O (25 wt%) was added dropwise to the solution and the solution was heated at 70 °C for 5 h. Then, the solution temperature was increased to 85 °C to vaporize residual ammonia. The resulting black dispersion was separated with magnet and washed with deionized water. The magnetic NPs were modified by triazine-based polymeric network (TPN) via in situ method according to our previously reported method [41]. 0.939 g melamine, 1.5 g terephthaldehyde and 45 mL DMSO were stirred and degassed by argon bubbling. The mixture temperature was increased to 180 °C under an inert atmosphere. After 48 h, 1.5 g Fe3O4 NPs were added and the reaction was continued for another 24 h. After it cooled to room temperature, the resulting precipitation was collected with the magnet, washed with excess DMSO, acetone, THF and dichloromethane. The obtained TPN/MNPs were dried under vacuum at 60 °C. The GO powders and TPN/Fe3O4 were dispersed in THF separately, and then mixed together (weight ratio of 1:1) at room temperature by an ultrasonic bath. The product was separated by the magnetic field and dried at 50 °C. Field emission-scanning electron microscopy (FESEM) images were obtained using MIRA3FESEM (Tescan, Czech Republic). The FT-IR spectra were obtained by Thermo Nicolet Nexus 870 FT-IR
2. Experimental 2.1. Chemicals and reagents 2,4-Dichlorophenoxyacetic acid(2,4-D) and imidacloprid (IMI) were purchased from Sigma-Aldrich (Steinheim, Germany). Graphite 631
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spectrometer (Madison, USA). Thermogravimetric analysis (TGA) was performed on a NETZSCH TG 209 F1 (Selb, Germany) from room temperature to 800 °C at a heating rate of 10 °C min−1 in N2 atmosphere. The magnetic properties were measured by vibrating sample magnetometer (VSM) (Meghnatis Daghigh Kavir, Kashan, Iran).
3. Results and discussion 3.1. Characterization The FTIR spectra of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 1. In FTIR spectrum of Fe3O4, the band at 3390 cm−1 is attributed to OeH stretching vibration and the band at 572 cm−1 is attributed to FeeO bond. The FTIR spectrum of TPN/Fe3O4 showed a broad coupling band around 3400 cm−1 corresponding to NeH groups of TPN and OeH groups in surface of the Fe3O4 NPs. The bands at 1554 cm−1 and 1485 cm−1 are attributed to C]N group of TPN and the band at 575 cm−1 corresponds to FeeO [41]. In the FTIR spectrum of TPN/Fe3O4/GO, the characteristic absorption bands of TPN/Fe3O4 and the absorption bands of GO are observed. The broad band in the range of 2500–3600 cm−1 may attributed to OeH of carboxylic acid of GO, NeH and OeH groups in TPN/Fe3O4/GO [44]. FE-SEM images of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 2. Fig. 2a shows that the NPs have uniform size distribution while some particles are agglomerated. The size of Fe3O4 NPs is 22–50 nm. Fig. 2b shows that the size of TPN/Fe3O4 NPs is 26–70 nm. The size of TPN/Fe3O4 NPs is increased compared to Fe3O4 NPs, that may attribute to modification with TPN. SEM image of TPN/Fe3O4/GO confirmed the presence of GO (Fig. 2c). TGA curves of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 3. The TGA data including temperatures at which 5% (T5), 10% (T10) degradation occur and the residue at 800 °C are summarized in Table 1. In the TGA curve of Fe3O4 NPs, the total weight loss over the full temperature range is 1.6% that is attributed to the loss of adsorbed water and dehydration of the surface eOH groups [46]. As shown in Table 1, T5 of TPN/Fe3O4 and TPN/Fe3O4/GO is 172 °C, which is due to the loss of adsorbed water and dehydration of the surface eOH groups and some monomers. T10 of TPN/Fe3O4/GO is 192 °C which is lower than that of TPN/Fe3O4 (T10 = 242 °C), but the char residue of TPN/ Fe3O4/GO is 51% at 800 °C that is higher than that of TPN/Fe3O4 that is 46%. Magnetization curves of Fe3O4 and TPN/Fe3O4/GO are shown in Fig. 4.The saturation magnetization of Fe3O4 and TPN/Fe3O4/GO are 70 and 19 emu/g, respectively. The saturation magnetization of TPN/ Fe3O4/GO decreased due to the present of TPN and GO. However, it is enough for magnetic separation with a magnet.
2.4. The procedure 50 mL sample solution containing the analytes (100 μg L−1) was transferred to a beaker, and the pH (7.4) and ionic strength (NaCl, 7% w/v) were adjusted. Then, 11.0 mg of TPN/Fe3O4/GO sorbent was added to the solution. The mixture was shaken for 8 min to reach the adsorption equilibrium. Afterward, the sorbent was isolated from the solution with the magnet. The supernatant was decanted and the residual was transferred to a 10 mL conical tube to elute the sorbent with smaller volumes of the desorption solvent. The adsorbed analytes were desorbed with 220 μL acetone by shaking for 6 min. Finally, by placing a magnet to the outside of the conical tube the desorption solvent containing the analytes was collected and 20 μL of it was injected into HPLC for analysis. 2.5. Sample preparation The samples including cucumber and tomato were purchased from local market (Tehran, Iran). Tap water sample was obtained from Standard Research Institute (Karaj, Iran). The water sample was filtered with filter paper. Cucumbers and tomatoes were ground and homogenized. 1.0 g of the homogenized samples was accurately weighed. Then, 2.5 mL acetonitrile and 2.5 mL distilled water were added to the samples and homogenized for 1 min. The mixtures were sonicated for 10 min and filtered. Then the filtered samples were diluted to the volume of 50 mL with distilled water. For the spiked samples, the mentioned procedure was performed and followed by adding proper amount of mixed standard solution of IMI and 2.4-D (25 mg L−1) to the weighed homogenized samples to obtain 100 μg Kg−1 spike level in the final solution.
Fig. 1. FTIR spectra of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO. 632
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Fig. 2. FE-SEM images of (a) Fe3O4 (b) TPN/Fe3O4 and (c) TPN/Fe3O4/GO.
Fig. 3. TGA curves of Fe3O4, TPN/Fe3O4 and TPN/Fe3O4/GO.
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larger volume of sample, due to less amount of sorbent in volume unit of the sample solution [47]. Therefore, sample volume of 50 mL was selected for subsequent experiments.
Table 1 Thermal properties data of TPN/Fe3O4 and TPN/Fe3O4/GO. Samples
T5 (° C)a
T10 (°C)b
CR (%)c
TPN/Fe3O4 TPN/Fe3O4/GO
172 172
242 192
46 51
3.2.3. Central composite design In the next step, to determine the effects of the six remaining factors at five levels, a small central composite design (SCCD) was used to reduce total number of experiments. For six variable, by using central composite design (CCD) high number of experiments should be done according to the equation N = 2k + 2k + m (where k is the number of design variables, and m is the number of center point replications (in this study, with six factor and ten center points, CCD involves 86 experiments)) [48]. SCCD under response surface methodology (RSM) as powerful mode can cost effectively decrease the number of experimental runs. It also makes it possible to optimize the effects of variables and their possible interactions on the response [49]. The factors and their ranges are as follows: pH (A; 3–11), sorbent amount (B; 5–15 mg), salt content (C; 0–10%w/v), extraction time (D; 2–14 min), desorption solvent volume (E; 200–1000 μL) and desorption time (F; 2–14 min). SCCD consisted of 34 experiments, including six variable and six central points. Since the required time to perform 34 experiments during one working day was not sufficiently short, so they were carried out in two blocks. The experiments were performed randomly in order to minimize the effect of uncontrolled variables. The design matrix and response are shown in Table 2 and Table S1, respectively. Experimental design, data analysis and response surfaces were performed by Design-Expert 7.1.3. The equation below illustrates the relationship between the six variables and the peak area:
a
Temperature at 5% weight loss. Temperature at 10% weight loss. c CR: Char residue, Weight percentage of material left after TGA analysis at a maximum temperature of 800 °C. b
Fig. 4. VSM magnetization curves of (a) Fe3O4 and (b) TPN/Fe3O4/GO.
3.2. Optimization of MSPE parameters
Peak area = + 6.3 E + 005 + 54159.4 A + 11406.6 B + 15225. 6C
The effect of several factors affecting the extraction efficiency including pH, amount of sorbent, salt content, extraction time, volume of desorption solvent, desorption time and sample volume were investigated and optimized by experimental design. However, effect of desorption solvent type and sample volume were optimized using one variable at a time method because of reducing total number of variables and also remove categoric variable in experimental design. Moreover, preliminary experiments showed that handling of large sample volumes is different from low sample volumes which it could be inserted some systematic errors in experimental design. Since, effect of desorption solvent type and sample volume was investigated separately.
+ 42150.9 D
1857.8 E
1291.3 AC
+ 2641.3 CD + 29.8 EF 588.8
D2
14543.3 F + 4564.8 AB
5483.6 AD
+ 0.8
E2
1469.0 BC 4638.8 A2
240.8
419.3 BD
1395.4 B2
734.8 C2 (3)
F2
The analysis of variance (ANOVA) was performed for modified quadratic model and the results are summarized in Table 3. p value < 0.05 indicates model terms are significant. In this case A, E and E2 are significant model terms. The model F-value of 6.13 with a pvalue of 0.0009 implies that the model is significant and there is only a 0.09% chance that a large model F-value could occur due to noise. The Lack of Fit (LOF) p-value of 0.9566 implies the LOF is not significant relative to the pure error. The quality of the model was expressed by the coefficient of determination, R2. The response equation fitted with the experimental data considering a R2-value of 0.8995, which indicates adequate correlation between responses and experimental factors according to Joglekar and May advice [50]. Fig. 5 illustrates the relationship between the explanatory and response variables in a three-dimensional representation of the response surface. As can be seen in Fig. 5a, the extraction efficiency increased with increase in pH up to 7.4. The pKa value of 2,4-D is 2.6 [51]. The molecules exist in the anionic forms when the pH is higher than pKa values. At optimum pH (pH = 7.4) amine groups of TPN/Fe3O4/GO are
3.2.1. Effect of desorption solvent type Different organic solvents including acetone, acetonitrile, mixture of acetone and 1% acetic acid solution (50:50, v/v), mixture of acetonitrile and 0.2 M Na2EDTA (50:50, v/v) and mixture of acetonitrile and 10 mM KOH (80:20, v/v) were used for desorption of analytes from the sorbent. The results showed that acetone was the superior solvent in comparison with other solvents for desorption of analytes, that is similar to many studies that have applied G-based magnetic nanocomposite as adsorbent [21]. It is noteworthy that, IMI is unstable in high acidic and alkaline pHs [3], therefore, getting low desorption efficiency for desorption solvents containing acid and base is expectable. Thus, acetone was selected as the desorption solvent for subsequent experiments.
Table 2 Experimental factors, levels and matrix of the central composite design.
3.2.2. Effect of sample volume Large volume of sample solution is required to achieve higher enrichment factor [33]. The effect of sample volume on the extraction of the analytes was studied by different sample volumes (25–200 mL) containing fixed amount of analytes (5.0 μg). The results showed that the highest extraction efficiency was obtained when sample volume was equal or lower than 50 mL. However, the extraction efficiency decreased when the sample volumes were > 50 mL. It can be attributed to the more difficult transference of analytes to the sorbent surface in
Factors
pH Sorbent amount (mg) Salt (% w/v) Extraction time (min) Desorption solvent volume (μL) Desorption time (min)
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Symbol
A B C D E F
Level −α
−1
0
+1
+α
3 5 0 2 200 2
5 7.5 2.5 5 400 5
7 10 5 8 600 8
9 12.5 7.5 11 800 11
11 15 10 14 1000 14
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Table 3 Analysis of variance (ANOVA) for modified response surface quadratic model. Source
Sum of squares
dfa
Mean square
Block Model A-pH B-sorbent amount C-salt D-extraction time E-desorption solvent volume F-desorption time AB AC AD BC BD CD EF A2 B2 C2 D2 E2 F2 Residual Lack of fit Pure error Cor total
2.436E + 009 3.438E + 011 2.295E + 010 3.389E + 009 4.180E + 009 2.435E + 009 1.155E + 011 1.730E + 007 8.335E + 009 6.670E + 008 5.773E + 009 4.496E + 008 1.582E + 008 6.279E + 009 5.130E + 009 1.042E + 010 2.301E + 009 6.380E + 008 8.497E + 008 3.486E + 010 1.421E + 008 3.839E + 010 1.418E + 010 2.422E + 010 3.846E + 011
1 19 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 13 9 4 33
2.436E + 009 1.809E + 010 2.295E + 010 3.389E + 009 4.180E + 009 2.435E + 009 1.155E + 011 1.730E + 007 8.335E + 009 6.670E + 008 5.773E + 009 4.496E + 008 1.582E + 008 6.279E + 009 5.130E + 009 1.042E + 010 2.301E + 009 6.380E + 008 8.497E + 008 3.486E + 010 1.421E + 008 2.953E + 009 1.575E + 009 6.054E + 009
a
F value
p-Value
6.13 7.77 1.15 1.42 0.82 39.10 5.859E-003 2.82 0.23 1.95 0.15 0.054 2.13 1.74 3.53 0.78 0.22 0.29 11.80 0.048
0.0009 0.0154 0.3036 0.2554 0.3804 < 0.0001 0.9402 0.1168 0.6425 0.1855 0.7027 0.8206 0.1686 0.2103 0.0830 0.3934 0.6498 0.6008 0.0044 0.8298
Significant
0.26
0.9566
Not significant
Degree of freedom.
protonated that causes electrostatic attraction between 2,4-D and TPN/ Fe3O4/GO surface. The extraction efficiency decreased at higher pH due to the repulsion between negatively charged functional groups on the absorbent's surface and the negative charge on 2,4-D molecule. On the other hand, IMI can hydrolyze in acidic and alkaline media but it is stable in pH ranges from 6 to 7 [3]. At pH = 7.4, carboxyl groups of TPN/Fe3O4/GO are deprotonated that causes electrostatic attraction between IMI and TPN/Fe3O4/GO surface. Moreover, due to presence a lot of hydrogen bonding acceptor on the surface of the nanocomposite, extraction could be proceed via this mechanism [41]. Therefore, pH of solution was fixed at 7.4. Fig. 5a also shows that the extraction efficiency increased by increasing the sorbent amounts up to 11 mg that is because of increasing of accessible sites to the adsorption of the analytes [23]. The extraction efficiency increases by increasing salt concentration due to salting out effect [32]. However, as shown in Fig. 5b, the extraction efficiency decreases with further increase of salt concentration (at low extraction times) due to increase of solution viscosity that causes decrease of the analytes transference to the adsorbent surface [52]. However, at longer times, the analytes have enough time to transfer to the adsorbent. Therefore, the extraction efficiency increases. As can be seen in Fig. 5c, the response decreased by increasing desorption solvent volume as result of the effect of dilution. Desorption solvent volumes lower than 200 μL were not studied because of incomplete wetting of the sorbent with the desorption solvent which resulting in incomplete desorption of the analytes [47]. Also, Fig. 5c shows that desorption time had no significant effect on the desorption efficiency. Finally, according to the overall results of the optimization study, the following optimum conditions for the extraction of IMI and 2,4-D from water and food samples were selected: pH, 7.4; sorbent amount, 11 mg; salt amount, 7% w/v; extraction time; 8 min, desorption solvent volume; 220 μL and desorption time, 6 min; desorption solvent type, acetone and sample volume, 50 mL.
magnetic nanocomposite which containing MOP and GO together at the same time [35–43]. At pH = 7.4, optimum suggested pH by software, at the same time IMI is protonated and carboxylic groups of GO are deprotonated, therefore effective electrostatic interaction could be occurred between IMI and GO. Similarly, at pH = 7.4, 2,4-D is deprotonated and amine groups of MOP are protonated, therefore effective electrostatic interaction could also be occurred between 2,4-D and MOP. Moreover, according to getting good extraction efficiency in presence of relatively high salt amount (7% w/v), hydrogen bonding and π-π interactions between the analytes and nanocomposite materials (both MOP and GO) are the other probable extraction driven forces. Why so, electrostatic attraction mechanism could be remarkably inhibited by high salt amounts due to competition of salt ions with the pesticides [41]. So, it could be concluded that electrostatic attraction, π-π and hydrogen bonding interactions are the main driven forces for extraction of the analytes. 3.3. Method validation Quantitative parameters of the proposed method including linearity, limit of detection (LOD), limit of quantification (LOQ) and enrichment factor (EF) were investigated under optimized conditions. The results are summarized in Table 4. Good linearity was obtained in the range of 0.5–500 μg L−1 and in the range of 5.0–500 μg L−1 for IMI and 2,4-D, respectively (R2 ≥ 0.99). LOD based on signal-to-noise ratio of 3 for IMI was 0.17 μg L−1 and for 2,4-D was 1.7 μg L−1. LOQ based on signal-to-noise ratio of 10 for IMI was 0.5 μg L−1 and for 2,4-D was 5.0 μg L−1. The enrichment factor, defined as the ratio of the concentration of the analytes between the extracted organic phase and the initial concentration of the analytes in the aqueous, was obtained 95 for IMI and 90 for 2,4-D. The repeatability, expressed as relative standard deviations (RSDs) for six replicate measurements at 10 and 100 μg L−1 of IMI and 2,4-D were < 9.5%. A comparison of the proposed method features with other methods reported for extraction and determination of IMI and 2,4-D was made and summarized in Table 5. As can be seen from Table 5, the proposed method has wide dynamic linear range. Also, it has lower LOD than solid phase extraction methods which uses detection method such as
3.2.4. Extraction mechanism Different interactions such as electrostatic, hydrophobic, dipole-dipole, Van der Waals (dispersion), hydrogen bonding, π-π interactions could be occurred between the target analytes and the prepared 635
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Fig. 5. Three-dimensional response surface plots for interactions (a) pH and sorbent amount, (b) salt and extraction time, (c) desorption solvent volume and desorption time.
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Table 4 Analytical performance of the proposed method. Analyte
Linear range (μg L−1)
IMI 2,4-D a b c d
0.5–500 5.0–500
R2
0.9954 0.9967
LODa (μg L−1)
LOQb (μg L−1)
EFc
0.17 1.7
0.5 5.0
95 90
RSD%d (n = 6) 100 (μg L−1)
10 (μg L−1)
4.7 8.6
5.9 9.3
Limit of detection. Limit of quantification. Enrichment factor. Relative standard deviation.
HPLC-DAD [53] or cLC [55]. In contrast, LOD of method for IMI extraction is higher than some MSPE which are the basis of graphene sorbents [21,54]. However, the proposed method provides simultaneous extraction of acidic and basic pesticides at the same time. Compared to traditional SPE method, the proposed method has advantages such as shorter extraction time and less consumption of organic solvents. The sorbent can be separated from the aqueous solution by applying an external magnetic field. So, no centrifugation or filtration is necessary. Moreover, LOD of the proposed method for IMI (0.17 μg L−1) and 2,4-D (1.7 μg L−1) provide enough sensitivity determination of IMI and 2,4-D according to their MRLs (0.5–1.0 mg kg−1 for IMI and 0.05 mg kg−1 for 2,4-D in fruits and vegetables). The results indicate that the developed method is simple, rapid and efficient and can be used for trace analysis of the analytes in real samples.
Table 6 Determination of IMI and 2,4-D in cucumber, tomato and tap water samples. Sample Cucumber
Analyte
Cadded (μg Kg−1)
Cfound (μg Kg−1)
Recovery (%)
IMI
– 100 – 100 – 100 – 100 – 100 – 100
81.3 172.5 – 94.3 – 102.4 – 93.2 – 92.7 – 95.3
– 91.2 – 94.3 – 102.4 – 93.2 – 92.7 – 95.3
2,4-D Tomato
IMI 2,4-D
Tap water
IMI 2,4-D
3.4. Sample analysis
and characterized by FESEM, FTIR, TGA and VSM. Then, TPN/Fe3O4/ GO was used as efficient sorbent for simultaneous extraction of IMI and 2,4-D in cucumber, tomato and water samples. Introducing TPN and GO in magnetic nanocomposite, provide different functional (amine and carboxylic) groups at the same time which leads to simultaneous extraction of basic and acidic pesticides via hydrophobic, hydrogen bonding and or electrostatic attraction with satisfactory recoveries. Moreover, well-defined porosity, high surface area, good chemical stability and tunable surface chemistry are the other distinct features of such composites. Compared to traditional SPE methods, extraction time of this method is shorter due to rapid extraction dynamics and magnetic separation of the sorbent that no centrifugation or filtration is needed. The other advantage of this method with the proposed sorbent is high extraction capacity due to high surface area-to-volume ratio of the sorbent. The results showed that TPN/Fe3O4/GO composite can be a promising sorbent for the simultaneous extraction of basic and acidic compounds in different samples.
The proposed method was applied for the determination of IMI and 2,4-D in cucumber, tomato and tap water samples. The results showed that tap water and tomato samples were free of IMI and 2,4-D. However, 81.3 μg kg−1 IMI was detected in cucumber sample. To estimate the effect of the matrices and determine the recovery of the analytes, the samples were spiked at concentration level of 100 μg kg−1. The results are summarized in Table 6. The recoveries of IMI and 2,4-D from cucumber, tomato and water samples were in the range of 92–104%. Fig. 6 shows the chromatograms of IMI and 2,4-D in cucumber sample before and after spiking with100 μg Kg−1 of the analytes. 4. Conclusion In the present study, triazine-based polymeric network modified Fe3O4 NPs/graphene oxide nanocomposite was successfully synthesized
Table 5 Comparison of the proposed method with the other methods for extraction and determination of IMI and 2,4-D. Matrix Ground water and surface water Reservoir water, sea water and river water Tap water and riverine water Lake water, river water and reservoir water Water samples Agricultural products Pear and tomato Apple juice Cucumber, tomato and tap water a b c d e
Analyte
Extraction method
IMI IMI 2,4-D 2,4-D 2,4-D IMI IMI 2,4-D IMI 2,4-D
SPEa MSPEb SPE DLLMEc MSPE SPE MSPE SPE MSPE
Detection LC–ESI-MS–MS HPLC-UV HPLC-UV MEKCd LC–MS/MS HPLC-DAD HPLC–DAD cLCe HPLC-UV
Solid phase extraction. Magnetic solid phase extraction. Dispersive liquid–liquid microextraction. Micellar electrokinetic chromatography. Capillary liquid chromatography. 637
Linear range (μg L−1)
LOD (μg L−1)
Ref.
– 0.05–50 10–10,000 10–1000 0.1–20 20–200 0.5–100 30–170 0.5–500 5.0–500
0.009 0.006 0.15 1.56 0.02 10 0.08–0.1 7 0.17 1.7
[17] [21] [18] [20] [22] [53] [54] [55] Present work
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Fig. 6. The chromatograms of (a) non spiked cucumber sample and (b) cucumber sample spiked with 100 μg Kg−1 of IMI and 2,4-D.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.01.047.
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