Chinese Chemical Letters  2020, Vol. 31 Issue (7): 1843-1846   PDF    
Sulfonic acid-based metal organic framework functionalized magnetic nanocomposite combined with gas chromatography-electron capture detector for extraction and determination of organochlorine
Ying Wanga, Qing Yea,*, Menghuan Yua, Xijun Zhanga, Chunhui Dengb,*     
a School of chemistry and Environmental Science, Shangrao Normal University, Shangrao 334001, China;
b Department of Chemistry, Fudan University, Shanghai 200433, China
Abstract: The metal organic framework functionalized with sulfonic acid was combined with magnetic nanoparticles to fabricate a new nanocomposite (denoted as Fe3O4@PDA@Zr-SO3H). By combining with gas chromatography-electron capture detector, the resulting Fe3O4@PDA@Zr-SO3H nanocomposite was successfully used as a high-efficiency adsorbent for pre-concentrating eight organochlorine pesticides from water sample in environment. Apart from the ability of fast separation, the as-prepared Fe3O4@PDA@Zr-SO3H nanocomposite also exhibited high adsorption capacity for organochlorine pesticides. With the use of optimal experimental conditions, the linear relationship can be obtained in the range of 0.05~300 μg/L, the correlation coefficient was over 0.9978, and the relative standard deviation was located in 2.5%-7.7%. Moreover, the limit of detection and quantification was between 0.005-0.016 μg/L and 0.017~0.050 μg/L. Finally, the nanocomposite was used for the determination of organochlorine pesticides from environmental water samples, and displayed the recovery of 82%-118%.
Keywords: Organochlorine pesticides    Metal-organic framework    Magnetic nanocomposite    Water    Gas chromatography-electron capture detector    

Organochlorine pesticides (OCPs), primarily consisting of chlorine element, carbon element and hydrogen element, which are a class of man-made chemical pesticides and been regarded as one of the most poisonous and persistent organic contaminants in our environment. It has been reported that the production of hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs), both of which are the members of organochlorinated compounds family, is respectively approximate to 400, 000 and 4, 900, 000 tons from the 1950s to 1983 in China [1]. In order to exterminate pests, OCPs were still used in china until early 1990s, resulting in serious contamination especially the water contamination [2]. As a consequence, the monitor of OCPs level in water environment is quite significant by appropriate analytical tool.

Gas chromatography-mass spectrometry (GC–MS) and gas chromatography-electron capture detector (GC-ECD) have been widely adopted for the detection of organochlorine pesticides [3]. Generally speaking, the sample preparation step, namely purification and enrichment of target analytes, is necessary for the determination of OCPs prior to GC analysis [4] because of the high matrix effects. Up to now, great efforts have been devoted to develop different pretreatment methods for OCPs including solidphase extraction (SPE) [5, 6], liquid-liquid extraction (LLE) [7], Soxhlet extraction [8], liquid-liquid microextraction (LLMP) [9], micro-solid-phase extraction coupled with dispersive liquid-liquid microextraction (μ-SPE-DLLME) [10], ultrasound-assisted liquid phase microextraction (USA-LPME) [11], solid-phase microextraction (SPME) [4, 12-14], and magnetic solid-phase extraction (MSPE) [15]. Among these advanced methods, MSPE has received considerable attention in recent years. The enrichment process of MSPE can be directly carried out in crude samples of containing suspended adsorbents without the requirement additional operations such as centrifugation or filtration, which makes the separation easier and more rapidly. Therefore, improving the performance of MSPE adsorbent is the key to develop new ones [16-19]. Current researches in novel MSPE adsorbents have been concentrated on improving their adsorption capacity and selectivity for analytes, as well as enhancing their dispersity in aqueous environment.

As one of the significant porous materials, metal-organic frameworks (MOFs) have combination of advantages, which have shown many applications with great promise in molecule adsorption. The coordination of inorganic ions or clusters with organic ligands results in a large number of unsaturated metal coordination sites which are available, and adjustable pore size, as well as large specific surface area [20]. These fascinating characteristics make them attracting much attention in chemical science and have been explored as excellent adsorbents for the removel of various environmental contaminants [21-24]. However, the mechanical strength of most MOFs is relatively low and the water sensitivity is high, moreover, most MOFs are insoluble in water, which limits the application of MOFs in separation greatly, especially water matrix. Therefore, for a better application in sample pre-treatment, and improvement in hydrophilicity of MOFs is urgently demanded [25-28]. Sulfonic acid is excellently hydrophilic and chemically stable [29], and thereby it can be used as a good candidate in advancing hydrophilicity of MOFs. In this work, we synthesized the sulfonic acid-based ultrahydrophilic MOFs-functionalized magnetic nanocomposite (denoted as Fe3O4@PDA@Zr-SO3H) successfully, and used the magnetic nanocomposite as adsorbent to separate and determine OCPs from water sample prior to GC-ECD.

The methods of synthesize Fe3O4@PDA@Zr-SO3H nanocomposite are showed in the supporting information. Fig. 1 shows the SEM and TEM image of the Fe3O4@PDA (Figs. 1a and c) and Fe3O4@PDA@Zr-SO3H (Figs. 1b and d) nanoparticles. From the SEM and TEM images of Fe3O4@PDA, there was a relatively thin layer outside of the Fe3O4. After the sulfonic acid functionalized MOFs were grafted, the surface of nanosphere became rough and thick. Fig. S1 (Supporting information) shows the FTIR spectrum of Fe3O4@PDA@Zr-SO3H. The peak at 583 cm-1 can be assigned to the stretching vibration of Fe-O-Fe in Fe3O4. The adsorption peaks at 1405 and 1585 cm-1 were attributed to the aromatic ring in both of the PDA and ligands of MOFs. The adsorption peak at 3388 cm-1 resulted from -SO3H in the MOF layer. Fig. S2 (Supporting information) shows the wide-angle XRD pattern of the Fe3O4@PDA@Zr-SO3H nanocomposite. The typical diffraction peaks around 8.0°, 13.0°, 30.8° were from the Zr-SO3H, while 36.7°, 43.3°, 54.7°, 57.4° and 63.9° were from the Fe3O4. Fig. S3 (Supporting information) shows the EDX spectrum of Fe3O4@PDA@Zr-SO3H. The mass fractions of O, C, N, Fe, Zr and S were 43.04%, 18.83%, 5.83%, 19.37%, 11.26% and 1.67%, respectively. All these characterizations suggested the fabrication of Fe3O4@PDA@Zr-SO3H nanocomposite was successful.

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Fig. 1. SEM (a, b) and TEM (c, d) images of Fe3O4@PDA and Fe3O4@PDA@Zr-SO3H microspheres. Scale bars: (a, b) 3 μm; (c, d) 100 nm.

Fe3O4@PDA@Zr-SO3H nanocomposite was used to extract OCPs in 40 mL of water sample which containing OCPs with a concentration of 20 μg/L. In order to achieve satisfactory extraction efficiency, several important experimental parameters, including the type and dosage of eluent, the dosage of Fe3O4@PDA@Zr-SO3H, the extraction time and elution time, were investigated. A mimic polluted water sample was selected for optimizing the extraction condition and GCECD was used for tandem analysis. The parameters of GC-ECD analysis are showed in Supporting information.

In order to elude the extracts off Fe3O4@PDA@Zr-SO3H effectively, ethanol, n-hexane and ethyl acetate were chosen as eluting solvents for comparison. As can be seen from Fig. S4 (Supporting information), n-hexane exhibited the best desorption ability, this probably should be attributed to the similar polarity of n-hexane to the target molecule. The n-hexane was therefore selected as the eluting solvent for the following experiments. The dosage of eluentwas also investigated by using different volumes (0.4, 0.6, 0.8 and 1.0 mL). The results showed the best extraction efficiency of Fe3O4@PDA@ZrSO3H towards target molecules can be acquired when 0.6 mL was selected as the elution dosage (Fig. S5 in Supporting information)

The selection of proper dosage of adsorbent is necessary for the MSPE method. Here, we chose different amounts of particles (5, 10, 15 and 20 mg) to explore the appropriate dosage. As shown in Fig. S6 (Supporting information), the extracted amount of analytes increased along with the increase of Fe3O4@PDA@Zr-SO3H nanocomposite. When the dosage of Fe3O4@PDA@Zr-SO3H nanocomposite reached to 10 mg, the curves flattened out and the extraction efficiency had no obvious increase. So 10 mg Fe3O4@PDA@Zr-SO3H was used for the following experiments.

The influence of extraction time and elution time on extraction efficiency of the Fe3O4@PDA@Zr-SO3H nanocomposite for OCPs was also explored. Firstly, different extraction time (3–15 min) was applied during the experimental process while keeping other parameters unchanged. As seen in Fig. S7 (Supporting information), the extraction efficiency presented rising trend in the range of 2–8 min, then maintained stable. As a result, 8 min was chosen as the following extraction time. Next, different elution time (5–20 min) were applied during the experimental process while keeping other parameters unchanged. According to Figs. S8 (Supporting information), the best elution efficiency for all analytes can be obtained when adopting 10 min. Therefore, 10 min of elution time was applied in the following experiments.

The reusability of the Fe3O4@PDA@Zr-SO3H adsorbent was investigated furtherly. Before each cycling, the Fe3O4@PDA@Zr-SO3H adsorbent was washed by 5 mL of n-hexane twice for 30 min and driedin 50℃ of vacuumfor 24 h, the aim is tomake sure that thereis no residual analyte on Fe3O4@PDA@Zr-SO3H adsorbent. And the results showed the reusability of Fe3O4@PDA@Zr-SO3H adsorbent was excellent, there was no significant decrease of the adsorption capacity when the Fe3O4@PDA@Zr-SO3H has been used for 10 times.

In order to validate method of the analysis, several quantitative parameters such as correlation coefficient (r), linear range, precision, limit of detection (LOD) and limit of quantification (LOQ) were evaluated and listed in Table 1. The linear curve can be acquired ranging from 0.05 μg/L to 300 μg/L, and the correlation coefficients are over 0.9978. The repeatability test was conducted by five parallel experiments. In each experiment, each of the OCPs adopted 1 μg/L as sample concentration. The precision of this method was around 2.5%–7.7%. The LOD was estimated based on the S/N ratio of 3 and the values of the target analytes from 0.005 μg/L to 0.016 μg/L. The LOQ was estimated based on the S/N ratio of 10 and the values of the target analytes from 0.017 μg/L to 0.050 μg/L. These results show that this method has high sensitivity and good repeatability.

Table 1
The validation data of MSPE-ECD procedure and analytical results for the eight OCPs in water samples (n = 3).

For practical application, the method was applied to analyze the analytes in water samples from river and well. A 0.45 mmm embrane filter was firstly applied to treat water samples. The recovery of the OCPs was studied by spiking OCPs standard solution containing eight 1 μg/L of OCPs into water samples. The recoveries for the OCPs in water samples from river and well ranged from 82% to 118% (Table 1). Fig. 2 showed the typical chromatograms of the extracted OCPs from river water sample before spiking (Fig. 2, curve a) and after spiking (Fig. 2, curve b) the above mentioned OCPs standard solution.

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Fig. 2. The GC chromatogram of river water sample before spiking (a) and after spiking (b) using the proposed method.

To make the performance advantages of this proposed method convictive, a comparison between this method and other reports has been made. As shown in Table 2, this proposed method exhibited the comparable and even superior performance to others in terms of LOD, precision and recovery and so on. All the results confirm that this proposed method is a promising tool for the extraction and determination of OCPs in water samples.

Table 2
Comparison of the proposed method with other methods for determination of OCPs in various samples.

In order to illustrate the role of MOFs, we synthesized polydopamine-coated magnetic Fe3O4 nanoparticles (Fe3O4@PDA) and used the magnetic nanocomposite as adsorbent to separate and determine OCPs from water sample under the same conditions. It can be seenfrom Table 2 that Fe3O4@PDA@Zr-SO3H has higher sensitivityas the adsorbent due to the large surface area, better hydrophilicity, more action points for enrichment of MOFs.

In this work, sulfonic acid-based metal organic framework functionalized magnetic nanocomposite was synthesized via a facile route. The nanocomposite possesses many advantages such as super-hydrophilicity and fast separation ability. By combining with GC-ECD, a rapid and sensitive method for extracting and determining OCPs was established. The method exhibited good adsorption capacity for mimic sample and was successfully employed for the pre-treatment of OCPs from water samples in environment. The Fe3O4@PDA@Zr-SO3H nanocomposite will have more effective applications in the separation field, and more extensive applications of Fe3O4@PDA@Zr-SO3H in the preconcentration and analytical methods are deserved to be expected.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary material related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.02.054.

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