Is Graphene Toxic?
One critical issue to be resolved before further applications of graphene in nanomedicine and cancer therapy is the potential short- and long-term toxicity of this new nanomaterial. A number of groups have devoted efforts to explore the in vitro cytotoxic effects of graphene.[16,58–60] Zhang et al. reported that graphene prepared by the chemical vapor deposition technique increased the activation of caspase 3 (apoptosis marker), release of lactate dehydrogenase, and generation of reactive oxygen species (ROS), in neural pheochromocytoma-derived PC12 cells. Wang et al. demonstrated that GO would induce significant cytotoxicity of human fibroblast cells at a GO concentration above 50 mg/l. Hu et al. uncovered that as-prepared GO only lightly decreased A549 cells proliferation rates without inducing apoptosis or cell death at an exposure concentration up to 85 mg/l. By contrast, reduced GO (RGO) after treatment by hydrazine hydrate exhibited a remarkable cytotoxicity to the same cell line. On the other hand, GO after the biocompatible coating (e.g., PEGylation) exhibited neglectable in vitro toxicity to many cell lines, including Raji, HCT-116, OVCAR-3, U87MG, MDA-MB-435 and MCF-7, even at high concentrations up to 100 mg/l.[2,3]
Potential in vivo toxicity of graphene in animals has also been investigated by a few groups. In two recent studies, as-prepared GO showed dominate accumulation in lungs for long periods of time after being intravenously injected into rats or mice, inducing dose-dependent pulmonary toxicity (obvious toxicity at a dose of 10 mg/kg).[59,61] This is not surprising as GO without further surface functionalization is not stable in physiological environments, due to the screening of electrostatic charges and non-specific binding of proteins to the GO (Figure 2). After entering the bloodstream, the GO agglomerates would be trapped in the lung, the first organ that GO is carried into by the circulating blood after intravenous injection. GO after PEGylation shows significantly improved biocompatibility and appears to have a different story in terms of in vivo behaviors. In our photothermal treatment experiments, we found that NGO-PEG with small sizes in the range of 10 to 50 nm showed no obvious toxic side effects to the treated mice in 40 days. Our latest work indicated that radiolabeled NGO-PEG mainly localized in the reticuloendothelial system, including the liver and spleen, with neglectable accumulation in the lung after intravenous injection, and could be gradually excreted from mice without causing noticeable toxicity to the treated animals at a dose of 20 mg/kg over a course of 3 months.
PEGylation of graphene oxide. (A) PEGylated GO. (B) GO and (C) NGO-PEG in different solutions recorded after centrifugation at 10,000 × g for 5 min. GO agglomerated in phosphate-buffered saline (PBS), RPMI-1640 cell medium, and fetal bovine serum (top panel). In marked contrast, NGO-PEG was stable in all solutions.
GO: Graphene oxide; NGO: Nano-graphene oxide; PEG: Polyethylene glycol.
Similar to many other nanomaterials used in biomedicine, the toxicity of graphene is closely associated to its surface functionalization. As-prepared GO, although water soluble, is not stable in physiological solutions and causes dose-dependent toxicity both in vitro and in vivo.[59,61] In marked contrast, nano-graphene with biocompatible coatings (e.g., PEGylation) exhibits excellent stability in the presence of high-concentration salts and proteins, and appears to be less toxic in vitro and in vivo. The in vivo biodistribution of graphene is also highly dependent on its surface chemistry. However, how the size of nano-graphene, which is another important parameter, affects the in vivo behaviors of graphene requires further investigation. A lot more systematic explorations are demanded in order to fully understand the in vivo long-term fate and toxicology of graphene at different doses in various animal models before any clinical applications of this novel nanomaterial.
Nanomedicine. 2011;6(2):317-324. © 2011 Future Medicine Ltd.
Cite this: Graphene in Biomedicine: Opportunities and Challenges - Medscape - Mar 01, 2011.