[Recent advances in the use of graphene for sample preparation]

Sample preparation is playing an increasingly important role in sample analysis. The enrichment efficiency of the target and the removal effect of the sample matrix are strongly dependent on the extraction material. Therefore, the development of efficient extraction materials is an important research focus in the field of sample preparation. Various advanced materials such as nanomaterials, mesoporous materials, ionic liquids, aerogels, carbon materials, metal-organic frameworks, and covalent organic frameworks have been introduced to produce a diverse range of extraction materials for sample preparation. Owing to its unique physical and chemical properties, graphene, an excellent carbon nanomaterial, has attracted significant attention in different areas. Due to their unique advantages of large surface area, large π-electrons, excellent adsorption properties, abundant functional groups, and facile chemical modification, graphene-based materials have displayed excellent extraction performance for diverse analytes. Furthermore, graphene-based extraction materials have been applied to pretreat real samples from different fields. This paper provides an overview of the recent advances in graphene sample preparation from 2020 to date. The manuscript covers the use of graphene, graphene oxide, and the related functionalized materials as sorbents, as well as their specific applications in cartridge solid-phase extraction, dispersive solid-phase extraction, magnetic solid-phase extraction, stir bar sorptive extraction, fiber solid-phase microextraction, and in-tube solid-phase microextraction. To prevent the aggregation of graphene, three-dimensional graphene, porous graphene aerogels, graphene-modified silica, and stainless-steel mesh were developed for cartridge solid-phase extraction. Furthermore, some graphene-based extraction materials were used to develop online solid-phase extraction, which allowed for automatic and high-throughput tests. Graphene nanosheets and their hybrid materials with molybdenum disulfide or zinc oxide nanoparticles have been applied to dispersive solid-phase extraction, and several types of contaminants, including metal ions, bisphenol endocrine disruptors, paraben preservatives, and phthalates, could be captured. By combination with magnetic materials using the coprecipitation method or via chemical post-modification, many magnetic graphene extraction materials have been produced for magnetic solid-phase extraction. The introduction of magnetic graphene not only enhanced the extraction efficiency but also simplified the test process, making it highly suitable for complex samples such as food and biological samples. Similar to magnetic solid-phase extraction, stir bar sorptive extraction is a very simple and efficient extraction method that shows good extraction performance for metal ions and organic pollutants from environmental water, medicines in urine, and organic pollutants in cosmetics. In addition to its excellent applicability to solid-phase extraction, graphene delivered satisfactory performance for solid-phase microextraction. Graphene has been used as an extraction coating for the extraction of fibers or tubes by coupling solid-phase microextraction with chromatographic detection, and many kinds of organic pollutants, including polychlorinated biphenyls, phthalates, polycyclic aromatic hydrocarbons, toluene, xylenes, organophosphorus pesticides, phenoxy acid herbicides, and antibiotics, in environmental or biological samples have been successfully determined. The extraction mechanism, including π-π, electrostatic, hydrophobic, hydrophilic, and hydrogen-bonding interactions, is also discussed. Because of the mixed-mode interactions and rich functionalization, graphene-based extraction materials could effectively capture and selectively enrich different types of species. These extraction or microextraction techniques have been coupled with detection methods such as chromatography, mass spectrometry, and atomic absorption spectroscopy and widely used in environmental monitoring, food safety, and biochemical analysis. The future development of graphene in the field of sample pretreatment focuses on the following aspects: 1) functionalization of graphene with specific groups such as affinity groups, chelating groups, and molecularly imprinted sites to achieve unique extraction selectivity; 2) combination of graphene with the advanced materials, including covalent organic frameworks, metal organic frameworks, aerogels, and nanomaterials, thus realizing the complementary advantages between materials, so that the hybrid graphene materials find broad application prospects in sample preparation; 3) combination of electromagnetic materials with graphene to form electromagnetic composites, as well as the use of electromagnetic fields to improve extraction selectivity and efficiency; 4) exploiting the good performance of graphene-based materials to overcome the difficulty encountered in the pretreatment of complex samples; 5) development of more green methods to prepare graphene-based extraction materials or functionalize graphene, in line with the trends in green chemistry; 6) application of more graphene-based materials to online sample preparation for meeting the development trends in the field of analytical chemistry.