Engineering two-dimensional metal oxides via surface functionalization for biological applications

Two-dimensional (2D) metal oxides (MOs) have attracted a considerable amount of attention for various biological applications due to their unique physicochemcial properties such as high photothermal response, temperature superconductivity, photoluminescence, flexibility, unique catalytic capability, plasmonic tunability and relatively low toxicity. However, the sophisticated physiological environments in biosystems stimulate various explorations of surface functionalization to improve the dispersity, stability and biocompatibility of 2D MOs. Moreover, 2D MOs exhibit remarkably tuneable properties via creating oxygen vacancies or doping, which endow 2D MOs with additional capabilities in biological applications. The large surface to volume ratio inherent in these materials also allows easy functionalization and maximal interaction with the external environment. Much work has been done in tailoring 2D MOs through physical/chemical functionalization for use in a diverse range of biomedical applications such as biosensors, bioimaging, drug/gene delivery carriers or even as therapeutic agents. In this review, current progress on 2D MOs functionalized for various biological applications will be presented. Additional relevant issues concerning the research challenges, technology limitations, and future trends have also been discussed.