The role of surface chemistry in the cytotoxicity profile of graphene.

Graphene and its derivative, because of their unique physical, electrical and chemical properties, are an important class of nanomaterials being proposed as foundational materials in nanomedicine as well as for a variety of industrial applications. A major limitation for graphene, when used in biomedical applications, is its poor solubility due to its rather hydrophobic nature. Therefore, chemical functionalities are commonly introduced to alter both its surface chemistry and biochemical activity. Here, we show that surface chemistry plays a major role in the toxicological profile of the graphene structures. To demonstrate this, we chemically increased the oxidation level of the pristine graphene and compared the corresponding toxicological effects along with those for the graphene oxide. X-ray photoelectron spectroscopy revealed that pristine graphene had the lowest amount of surface oxygen, while graphene oxide had the highest at 2.5% and 31%, respectively. Low and high oxygen functionalized graphene samples were found to have 6.6% and 24% surface oxygen, respectively. Our results showed a dose-dependent trend in the cytotoxicity profile, where pristine graphene was the most cytotoxic, with decreasing toxicity observed with increasing oxygen content. Increased surface oxygen also played a role in nanomaterial dispersion in water or cell culture medium over longer periods. It is likely that higher dispersity might result in graphene entering into cells as individual flakes ~1 nm thick rather than as more cytotoxic aggregates. In conclusion, changes in graphene's surface chemistry resulted in altered solubility and toxicity, suggesting that a generalized toxicity profile would be rather misleading.