Current trends in the field of MXenes emphasize the importance of controlling their surface features for successful application in biotechnological areas. The ability to stabilize the surface properties of MXenes has been demonstrated here through surface charge engineering. It was thus determined how changing the surface charges of two-dimensional (2D) Ti₃C₂ MXene phase flakes using cationic polymeric poly-L-lysine (PLL) molecules affects the colloidal and biological properties of the resulting hybrid 2D nanomaterial. Electrostatic adsorption of PLL on the surface of delaminated 2D Ti₃C₂ flakes occurs efficiently, leads to changing an MXene’s negative surface charge toward a positive value, which can also be effectively managed through pH changes. Analysis of bioactive properties revealed additional antibacterial functionality of the developed 2D Ti₃C₂/PLL MXene flakes concerning Escherichia. coli Gram-negative bacteria cells. A reduction of two orders of magnitude of viable cells was achieved at a concentration of 200 mg L⁻¹. The in vitro analysis also showed lowered toxicity in the concentration range up to 375 mg L⁻¹. The presented study demonstrates a feasible approach to control surface properties of 2D Ti₃C₂ MXene flakes through surface charge engineering which was also verified in vitro for usage in biotechnology or nanomedicine applications.

Despite intensive research on the application of two-dimensional (2D) materials, including MXenes, in nanomedicine, the knowledge concerning the mechanisms responsible for their observed bio-effects is far from being understood. Here we present insight into the mechanism of toxicity in vitro of the 2D Ti₃C₂ MXene. The most important results of this work are that using simple, inexpensive, post-delamination treatments, such as ultrasonication or mild thermal oxidation it is possible to ‘tune’ the cytotoxicity of the Ti₃C₂T _z flakes. Sonication of Ti₃C₂T _z flakes, or sonication followed by mild oxidation in the water at 60 °C, renders them selectively toxic to cancer cells as compared to non-malignant ones. It relates to the appearance of superficial titanium (III) oxide (Ti₂O₃) layer corresponding to the type of post-treatment. The presence of surface-Ti₂O₃ results in a noticeably higher generation of oxidative stress compared to pristine 2D Ti₃C₂. Our findings give evidence that the sonication and thermal treatments were successful in changing the nature of the surface terminations on the Ti₃C₂T _z surfaces. This study makes a significant contribution to the future rationalized surface-management of 2D Ti₃C₂ MXene as well as encourages new rationalized applications in biotechnology and nanomedicine. Bullet points: 1. First study on 2D Ti₃C₂ MXene superficially oxidized to titanium (III) oxide i.e. Ti₂O₃. 2. By sonication Ti₃C₂Tz MXene flakes followed by mild thermal oxidation in the water at 60 °C for 24 h, it is possible to ‘tune’ the toxicity of the flakes to cancerous cell lines. 3. Decreases in cell viabilities were dose-dependent. 4. Highest cytotoxic effect was observed for thermally oxidized samples. 5. The thermally oxidized samples were also selectively toxic towards all cancerous cell lines up to 375 mg l⁻¹. 6. Reactive oxygen species generation was identified as a mechanism of toxicity.