Magnetic graphene oxide-ultrathin nickel–organic framework composite for the extraction and determination of epoxiconazole in food samples

In this work, a magnetic graphene oxide-ultrathin metal–organic framework composite (Fe3O4@SiO2-GO-Ni-MOF) was synthesized for the first time. Employing Fe3O4@SiO2-GO-Ni-MOF composite as extractant, a novel method for the separation and analysis of the pesticide epoxiconazole was established with the assistance of high performance liquid chromatography (HPLC). The adsorption mechanisms were studied including by adsorption kinetics, thermodynamic parameters and adsorption isotherms. The experimental results showed that this method was convenient, operable, effective and practical for the extraction and determination of epoxiconazole in real samples.

S3 residual permanganate and manganese dioxide. The mixture was filtered when the color changed to brilliant yellow and washed with 80 mL 5% HCl aqueous solution and deionized water. After several centrifugations the final dry GO products were obtained in vacuum at 50 o C.
Preparation of Fe 3 O 4 @SiO 2 -GO was according to the literatures with some modifications. S3,4 First, 0.01 mol Fe 3 O 4 was added in 250 mL three-port flasks, then 340.0 mL ethanol was also added twice, followed with 60.0 mL distilled water and 15.0 mL ammonia water, then ultrasound for 20 minutes. At the end of ultrasound, 1.7 mL tetraethyl orthosilicate was added and stirred for 12 hours at 60 o C in water bath. After separating the magnet, the product was washed with water and ethanol, the obtained Fe 3 O 4 @SiO 2 was dispersed with 140.0 mL isopropanol, which was added twice, followed by 0.20 mL (3aminopropyl) triethoxysilane (APTES). After ultrasonicing for 30 minutes under the protection of nitrogen, the materiaks were stirred for 6 hours at 70 o C in water bath. Fe 3 O 4 @SiO 2 -NH 2 was obtained by vacuum drying after three times of distilled water washing followed by ethanol washing. To acquire final product, 0.20 g GO was added in 100 mL three-port flask, then added 50.0 mL N, N-dimethylformamide (DMF), 0.10 g N-hydroxysuccinimide (NHS) and 0.20 g 1-ethyl-(3-dimethylaminopropyl) carbonyl diamide acetate (EDC). After ultrasonicing 20 minutes to dissolve adequately, the solution pH should be adjusted to 4.0-6.0. After the intense stirring at room temperature for 2 hours, then add Fe 3 O 4 @SiO 2 -NH 2 , continue to 12 hours. After the reaction, magnet was used to separate, distilled water was washed for 3 times, and finally air dried to get Fe 3 O 4 @SiO 2 -GO.
Prepare of Fe 3 O 4 @SiO 2 -GO-Ni-MOF. Firstly, 0.10 g Fe 3 O 4 @SiO 2 -GO was added in 100 mL threeport flask, followed by adding 0.166 g terephthalic acid (PTA) and 10.0 mL N, N-dimethylformamide (DMF), stirring 15 minutes. S5 Then, the solution was added by 2.0 mL 0.40 mol L -1 sodium hydroxide and stirred for 10 minutes. In another small beaker, 0.096 g Ni(NO 3 ) 2 ·6H 2 O and 10.0 mL N, Ndimethylformamide (DMF) were added, stirring to dissolve Ni(NO 3 ) 2 ·6H 2 O completely, then they was transferred to the above solution, stirred for 30 minutes. After the reaction, the solution was poured into the reactor and reacted for 8 hours at 100 o C. Fe 3 O 4 @SiO 2 -GO-Ni-MOF composites were prepared by air-drying after separation with magnet, washed with N, N-dimethylformamide (DMF) and absolute S4 ethanol three times respectively. Sample preparation. Fruits and Vegetables: Put the cut fruits and vegetables samples into the juicer to crush. After crushing, add 10.0 g sample, 20.0 mL of acetonitrile and 2.0 g of sodium chloride into the centrifugal tube, and shake for 15 minutes to dissolve fully. After the oscillation, the centrifugal tube was put into the centrifuge for 10 minutes. The rotational speed of the centrifuge was 8000 r min -1 . After the separation of acetonitrile and water, the upper acetonitrile was removed, and then 20.0 mL acetonitrile was added to the centrifugal tube for secondary extraction. The centrifugal operation was repeated and the acetonitrile extract obtained by two centrifugations was rotated at 100 mL. In the rotary evaporation bottle, the extract was concentrated to dry at 50 o C by rotary evaporator. 2.0 mL n-hexane was added to dissolve the steam-dried extract, which was added into Florisil after its activation. Then, 5.0ml of acetone and n-hexane were mixed respectively, eluent was collected by elution column, and the eluent was

S6
The type of adsorbent is critical to the extraction process，in order to select the best material for the extraction of epoxiconazole, the effects of Fe 3 O 4 , Fe 3 O 4 @SiO 2 , Fe 3 O 4 @SiO 2 -GO, Fe 3 O 4 @SiO 2 -GO and Fe 3 O 4 @SiO 2 -GO-Ni-MOF on the Extraction efficiencies of epoxiconazole were studied in Fig. S4 (A).
As can be seen, the extraction efficiency of Fe 3 O 4 @SiO 2 -GO-Ni-MOF is higher than the others, which may be due to the large specific surface area of GO and the good solubility of Ni-MOF to epoxiconazole. Therefore, Fe 3 O 4 @SiO 2 -GO-Ni-MOF was chosen as the adsorbent.
In order to investigate the influence of the amount on the extraction efficiency，the dosages of the adsorbent in the range of 2.0 to 30.0 mg were studied in Fig. S4 (B). When the dosage of adsorbent Extraction time is an important factor affecting the extraction efficiency. Therefore, the effect of extraction time of 2min -18 min was investigated with an interval of 2 minutes. According to the results in Fig. S4 (D), when the extraction time is less than 14 minutes, the extraction efficiency increases with the time while when the shaking time is more than 14 minutes, the extraction efficiency remains basically unchanged, so the extraction time was choosen as 14 minutes.
In the extraction process, temperature also has a certain effect on the extraction efficiency. The extraction at different temperatures (0-40 o C)and the results can be seen in Fig. S4 (E). The extraction efficiency showed an upward trend below 35 o C. When the temperature was 35 o C, the extraction S7 efficiency remained reached the maximum. The adsorption of epoxiconazole to Fe 3 O 4 @SiO 2 -GO -Ni-MOF is an endothermic process, and the increase of temperature is beneficial to the extraction (adsorption), so the extraction efficiency increases with the increase of temperature. But, when the temperature rises again, the adsorption decreases because of the increase in the velocity of the target analyte molecules. Therefore, the optimum extraction temperature was 35 o C.
The volume of the sample was optimized from 10ml to 50ml and the results were shown in the Fig.   S4 (F), which illustrated that when the sample volume was less than 30 mL, the extraction efficiency is higher than 85%, which can be used for quantitative extraction. The volume of the sample was 30mL. Optimization of elution conditions. In the process of elution of epoxiconazole, many factors will affect the elution effect. Therefore, the effects of eluent types, eluent volume, elution time and elution temperature on the elution rate of epoxiconazole extracted from composite materials were studied.
In this experiment, hydrochloric acid, methanol, ethanol, acetonitrile and acetone were used to elute epoxiconazol. As shown in Fig. S5 (A), acetone was the best of all and acetone was selected as eluent. S9 Meanwhile, the volum of eluent (Fig. S5 (B)) was investigated in the range from 1.0 mL to 6.0 mL and it was found that when the volum was 2.0 mL, the elution effect was best and and remained unchanged with the with the increase of eluent volume. Thus, the volume of eluent was 2.0 mL.
The elution time has a certain influence on the elution effect of epoxiconazole and it was studied in the range of 5-25min. The The results were presented in Fig. S5 (C), which shown that when the elution time is less than 10.0 minutes, the elution effect increases with time. When the elution time is more than 10.0 minutes, the elution rate hardly changes, so the elution time was 10.0 min.
Moreover, the elution temperature from 0-40 ℃. were optimized. The results (Fig. S5 (D)) showed that the elution effect increased with temperature when it was less than 30 o C, and remained unchanged when the elution temperature was higher than 30 o C, so the selected elution temperature was 30 o C.

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Reusability of the adsorbent. Considering the maximization of material utilization, the reusability of the adsorbent was explored in this experiment. As shown in Fig. S5, the nanocomposites can be reused for six times with the extraction efficiency more than 85%.  Influence of interfering substances. The effect of some common metal ions and six kinds of pesticides (validamycin A, prothioconazole, tebuconazole, azoxystrobin, kasugamycin and fipronil) on the determination of epoxiconazole by the proposed method was investigated. A substance was considered as interfering when it caused a variation in the determination of the sample greater than ±5%.
The results are shown in Table S2. S11