TiO2/ZIF-67 nanocomposites synthesized by the microwave-assisted solvothermal method: a correlation between the synthesis conditions and antimicrobial properties

Abstract

Zeolitic imidazole frameworks (ZIFs) and TiO₂ based composites have recently attracted interest to control pathogenic microorganism growth. In this context, two novel TiO₂/ZIF-67 nanocomposites were synthesized by the microwave-assisted solvothermal method using two different routes: one pot composite (OPC) and two step composite (TSC) syntheses. X-ray powder diffraction and vibrational spectroscopy data revealed the formation of crystalline ZIF-67 and disordered TiO₂ in the OPC composite, while both phases were crystalline in the TSC sample. Sphere-like morphologies were obtained for both materials, as indicated by SEM and TEM images. XPS measurements showed mixed-valence Ti (Ti³⁺/Ti⁴⁺) and Co (Co²⁺/Co³⁺) on the surface of both materials, while EPR analysis confirmed the presence of Co²⁺ ions and oxygen-related defects. The results showed that TiO₂ has no antimicrobial activity against Staphylococcus aureus, while the TSC has higher and the OPC has lower activity than pure ZIF-67. Highly efficient biocidal activity at a concentration of 2.5 mg mL⁻¹ was observed for the TSC. The difference in the antimicrobial performance of the nanocomposites can be correlated with the balance between structural order/disorder, which depends on the synthesis route used, and the material solubility. The results also indicate the release of Co ions and the ligand as a possible mechanism together with redox reactions to inactivate bacteria. Thus, this work provides simple and promising synthesis routes to obtain TiO₂/ZIF-67 nanocomposites for potential use as new antimicrobial agents.