The Atomic Structure of Ti₂C and Ti₃C₂ MXenes is Responsible for Their Antibacterial Activity Toward E. coli Bacteria

The expanded Ti₂C and Ti₃C₂ MXene phases were synthesized from their parent Ti₂AlC and Ti₃AlC₂ MAX phases using the same conditions of the classical acidic aluminum extraction method. The assumption for the study was that the expanded Ti₂C and Ti₃C₂ MXenes are composed of the same atoms and if are synthesized from MAX phases using the same conditions of the classical acidic aluminum extraction method, the observed bio-effects can be related only to the changes in their structures. The scanning electron microscope investigations indicated that the expanded Ti₂C and Ti₃C₂ sheets formed the specific network of slit-shaped nano-pores. The x-ray photoelectron spectroscopy for chemical analysis (ESCA-XPS) showed almost no difference in surface chemistry of Ti₂C and Ti₃C₂ MXenes. The high-resolution transmission electron microscope investigations revealed, however, differences in atomic structure of the individual Ti₂C and Ti₃C₂ sheets. Measured distance between Ti-C atomic layers in Ti₂C was 9.76 Å and was larger by 0.53 Å in comparison with Ti₃C₂ (9.23 Å). Our investigations of bioactive properties toward model gram-negative Escherichia coli bacterial strain showed that the Ti₂C MXene did not influence the viability of bacteria. Contrarily, the Ti₃C₂ MXene showed antibacterial properties. The results of the study indicate that the structure at the atomic scale may play a key role in the bioactivity of MXenes of the same chemical composition, but different stoichiometry, just like in case of Ti₂C and Ti₃C₂.