Magnetic nitrogen-doped reduced graphene oxide as a novel magnetic solid-phase extraction adsorbent for the separation of bisphenol endocrine disruptors in carbonated beverages
Graphical abstract
In this work, a novel magnetic nitrogen-doped reduced graphene oxide (Fe3O4@N-RGO) was fabricated for the first time based on a simple solvothermal method and successfully applied to determine four bisphenol endocrine disruptors (bisphenol A, bisphenol B, bisphenol F and bisphenol AP) in carbonated beverages.
Introduction
Bisphenols, a group of high-production volume industrial chemicals with two hydroxyphenyl functionalities, have a wide range of applications in the production of polycarbonate plastics and epoxy resins [1]. As is well known, polycarbonate plastics are a principle member of plastic food packaging materials and epoxy resins are widely applied as inner surface coating of food and beverage cans [2,3]. Hitherto, decades of research have illustrated bisphenols can be released from packaging materials and migrate into beverages and foods, especially when they contact with either acid or basic contents [4]. Nevertheless, bisphenols belong to the family of endocrine disruptors. They can mimic natural hormones and have adverse effects on reproduction and development, neural network, cardiovascular, metabolic and immune systems in humans [5]. Many countries and organizations have formulated legal provisions to control their maximum migration levels in food and beverage and restrict their use. Therefore, it is urgently required to exploit rapid analytical methods for the detection of bisphenol endocrine disruptors in food and beverage.
However, the detection of bisphenol endocrine disruptors in food and beverage is often accompanied with complicated matrix effect and their content is always at trace level. In consequence, various sample preparation methods, including liquid-liquid extraction (LLE) [6], solid phase extraction (SPE) [7], dispersive liquid-liquid microextraction (DLLME) [8], solid phase microextraction (SPME) [9], selective pressurized liquid extraction (SPLE) [10], supramolecular solvents (SUPRAS)-based microextraction [11] and magnetic solid phase extraction (MSPE) [12], have been developed sequentially.
Among aforesaid extraction methods, MSPE has proven to be an interesting technique and a breakthrough development in sample preparation on account of its simple and speedy phase separation process, during which magnetic adsorbents can be retrieved from complex matrices with ease under an external magnetic field [13]. In the MSPE technique, magnetic adsorbents are directly dispersed into sample solutions, and this dispersive extraction mode can enhance the contact area between adsorbents and analytes [14]. As a result, the extraction efficiency of MSPE is superior to that of typical SPE. Meanwhile, typical SPE problems related with adsorbent packing, such as high pressure and packed bed clogging, can be avoided. Magnetic adsorbents, commonly consisting of magnetic carriers and functionalities, play a vital role in the MSPE technique since they not only influence extraction efficiency but also settle on the sensitivity and selectivity of the method [15]. Therefore, it is a wise choice to develop new magnetic adsorbents with high adsorption capacities and selectivity.
Magnetic graphene composite, which fuses the excellent adsorption capacity of graphene with the convenient separation of magnetic materials, has become a famous star on the horizon of MSPE and extensively applied for the analysis of deleterious compounds and biomacromolecule in complex samples (such as environmental, biological, pharmaceutical, or food samples). However, the application of the magnetic graphene composite is still at its infant phase with many challenges remaining. Under such circumstances, a series of functionalized magnetic graphene composites come into being. Different surfactants [16], ionic liquids [17] and deep eutectic solvents [18] were introduced into magnetic graphene composites to ameliorate their adsorption capacity. Similarly, boronate affinity technique [19] and molecular imprinting technology [20] were put to use to improve the selectivity of magnetic graphene composites. Although these modification hit the mark to some extent, it was very regrettable that the majority of researches did not take the irreversible aggregation of graphene sheets into account, which was basic and ineluctable in the preparation process of magnetic graphene composites [14]. Undoubtedly, the aggregation of graphene sheets would result in the decrease of their adsorption capacity. Supposing this question can be fundamentally solved, the application of the magnetic graphene composite or functionalized magnetic graphene composites would further boost and enter a new era.
In recent years, nitrogen-doped graphene (NG) or nitrogen-doped reduced graphene oxide (N-RGO), as a new class of graphene material, has become a hot topic by means of a unique set of properties. The introduction of nitrogen can create many defects, which may result in a more crinkled surface in the NG or N-RGO structure [21], and simultaneously prevent the irreversible aggregation of graphene sheets [22]. There is no doubt that both the increase of wrinkle level and the decrease of aggregation degree would make the specific surface area of NG or N-RGO increased and expose more adsorption sites, which was conducive to the improvement of adsorption capacity. Meanwhile, the hydrophilicity of NG or N-RGO can be enhanced due to N-doping [23]. In principle, Lewis-basic nitrogen atoms can also endow NG or N-RGO with good adsorption capacity toward Lewis acids. Moreover, nitrogen atoms doped in the NG or N-RGO structure can change its energy band structure and bestow itself a high active surface area. To sum up, NG or N-RGO is expected to become a new talent in sample preparation.
The main purpose of this work is to prepare a novel magnetic nitrogen-doped reduced graphene oxide (Fe3O4@N-RGO) and then apply it for the separation of four bisphenol endocrine disruptors (bisphenol A, bisphenol B, bisphenol F and bisphenol AP) in carbonated beverages. In order to obtain high extraction efficiency, a series of analytical parameters influencing the extraction efficiency were optimized. Finally, a novel and eco-friendly method of MSPE combined with HPLC-DAD was developed and successfully utilized to determine trace bisphenol endocrine disruptors in carbonated beverages for food quality control.
Section snippets
Reagents and materials
All reagents used in the experiment were of analytical grade unless otherwise mentioned. Ferric chloride hexahydrate (FeCl3·6H2O) was purchased from Beijing Chemicals Corporation (Beijing, China). Ethylene glycol, polyethylene glycol (M.W. 6000) and anhydrous sodium acetate were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium chloride (NaCl), hydrochloric acid (HCl), sodium hydroxide (NaOH) and ammonia were obtained from Sichuan Xilong Chemical Industry Co., Ltd.
TEM analysis
TEM was employed to address the morphological aspects of RGO, N-RGO and Fe3O4@N-RGO. As shown in Fig. 2a and b, both RGO and N-RGO were randomly compact and stacked together, showing uniform laminar morphology like crumpled silk veil waves, indicating N-doping did not destruct the intrinsic structure of graphene. Moreover, it was worth noting that N-RGO was more wrinkled and less aggregated in comparison with RGO. This manifestation may originate from the defective structures formed during the
Conclusion
In this work, a novel Fe3O4@N-RGO composite was fabricated via a simple solvothermal method and then applied as adsorbents for the separation of four bisphenol endocrine disruptors (BPA, BPB, BPF and BPAP) in carbonated beverages. The introduction of nitrogen atoms not only made the wrinkle level of N-RGO increased, but also retarded the irreversible aggregation of graphene sheets. As a result, Fe3O4@N-RGO owned larger specific surface area and more adsorption sites for bisphenol endocrine
Acknowledgements
The work was supported by the National Natural Science Foundation of China (Nos. 21575150, 21775153 and 21804080).
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