Abstract
Due to their unique physicochemical properties, graphene-family nanomaterials (GFNs) are widely used in many fields, especially in biomedical applications. Currently, many studies have investigated the biocompatibility and toxicity of GFNs in vivo and in intro. Generally, GFNs may exert different degrees of toxicity in animals or cell models by following with different administration routes and penetrating through physiological barriers, subsequently being distributed in tissues or located in cells, eventually being excreted out of the bodies. This review collects studies on the toxic effects of GFNs in several organs and cell models. We also point out that various factors determine the toxicity of GFNs including the lateral size, surface structure, functionalization, charge, impurities, aggregations, and corona effect ect. In addition, several typical mechanisms underlying GFN toxicity have been revealed, for instance, physical destruction, oxidative stress, DNA damage, inflammatory response, apoptosis, autophagy, and necrosis. In these mechanisms, (toll-like receptors-) TLR-, transforming growth factor β- (TGF-β-) and tumor necrosis factor-alpha (TNF-α) dependent-pathways are involved in the signalling pathway network, and oxidative stress plays a crucial role in these pathways. In this review, we summarize the available information on regulating factors and the mechanisms of GFNs toxicity, and propose some challenges and suggestions for further investigations of GFNs, with the aim of completing the toxicology mechanisms, and providing suggestions to improve the biological safety of GFNs and facilitate their wide application.
Background
Graphene, which is isolated from crystalline graphite, is a flat monolayer composed of single-atom-thick, two-dimensional sheets of a hexagonally arranged honeycomb lattice [1]. Because of its unique structural, specific surface area and mechanical characteristics, the functions and applications of graphene have gained considerable attention since the discovery of the material in 2004 [2, 3]. Graphene and its derivatives include monolayer graphene, few-layer graphene (FLG), graphene oxide (GO), reduced graphene oxide (rGO), graphene nanosheets (GNS), and graphene nanoribbons, etc. [4–7]. GO is one of the most vital chemical graphene derivatives of the graphene-family nanomaterials (GFNs), which attracts increasing attention for its potential biomedical applications. Graphene-based materials usually have sizes ranging from several to hundreds of nanometer and are 1-10 nm thick [8, 9], which is also the definition of ‘nanoparticles’ or ‘nanomaterials’. Due to their exceptional physical and chemical properties, graphene materials have been widely used in various fields, including energy storage; nanoelectronic devices; batteries [10–12]; and biomedical applications, such as antibacterials [13, 14], biosensors [15–18], cell imaging [19, 20], drug delivery [8, 21, 22], and tissue engineering [23–25].
Along with the application and production of GFNs increasing, the risk of unintentional occupational or environmental exposure to GFNs is increasing [26]. And recently, there are some investigation on GFNs exposure in occupational settings and published data showed that the occupational exposure of GFNs had potential toxicity to the workers and researchers [27–29]. GFNs can be delivered into bodies by intratracheal instillation [30], oral administration [31], intravenous injection [32], intraperitoneal injection [33] and subcutaneous injection [34]. GFNs can induce acute and chronic injuries in tissues by penetrating through the blood-air barrier, blood-testis barrier, blood-brain barrier, and blood-placenta barrier etc. and accumulating in the lung, liver, and spleen etc. For example, some graphene nanomaterials aerosols can be inhaled and substantial deposition in the respiratory tract, and they can easily penetrate through the tracheobronchial airways and then transit down to the lower lung airways, resulting in the subsequent formation of granulomas, lung fibrosis and adverse health effects to exposed persons [2, 29]. Several reviews have outlined the unique properties [35, 36] and summarized the latest potential biological applications of GFNs for drug delivery, gene delivery, biosensors, tissue engineering, and neurosurgery [37–39]; assessed the biocompatibility of GFNs in cells (bacterial, mammalian and plant) [7, 40, 41] and animals (mice and zebrafish) [42]; collected information on the influence of GFNs in the soil and water environments [43]. Although these reviews discussed the related safety profiles and nanotoxicology of GFNs, the specific conclusions and detailed mechanisms of toxicity were insufficient, and the mechanisms of toxicity were not summarized completely. The toxicological mechanisms of GFNs demonstrated in recent studies mainly contain inflammatory response, DNA damage, apoptosis, autophagy and necrosis etc., and those mechanisms can be collected to further explore the complex signalling pathways network regulating the toxicity of GFNs. It needs to point out that there are several factors which largely influence the toxicity of GFNs, such as the concentration, lateral dimension, surface structure and functionalization etc. Herein, this review presents a comprehensive summary of the available information on the mechanisms and regulating factors of GFNs toxicity in vitro and in vivo via different experimental methods, with the goals of providing suggestions for further studies of GFNs and completing the toxicology mechanisms to improve the biological safety of GFNs and facilitate their wide application.
