The probable underlying mechanism(s) of bacterial cell–TiO₂ nanoparticles (TiO₂ NPs) interaction in the absence of photo-irradiation has been less studied since most of the prior cytotoxicity studies focused on irradiated TiO₂. The present study draws attention to the possible role of cell surface–TiO₂ NP interactions under dark conditions, through an array of spectroscopic and microscopic investigations. A dominant freshwater bacterial isolate, Bacillus licheniformis, which interacted with environmentally relevant concentrations of TiO₂ NPs (1 𝜇g/mL), was analysed and compared under both light and dark conditions. Aggregation of cells upon NP interaction and adsorption of NPs onto the cell membrane was evident from the scanning electron micrographs under both light and dark conditions. The FT–IR and FT–Raman spectra suggested stress response of bacterial cells by elevated protein and polysaccharide content in the cell–NP interaction. The X-ray photoelectron spectroscopic data substantiated the reduction of titanium from Ti(IV) to Ti(III) species which might have contributed to the redox interactions on the cell surface under light as well as dark conditions. The internalization of NPs in the cytoplasm were obvious from the transmission electron micrographs. The consequent cell death/damage was confirmed through fluorescence spectroscopy and microscopy. To conclude, the current study established the substantial role of interfacial interactions in cytotoxicity of the TiO₂ NPs irrespective of the irradiation conditions.

To the best of our knowledge, we report for the first time the growth of brownmillerite Ca$_2$Fe$_2$O$_5$ (CFO) single crystals under Argon (Ar) only ambience by optical floating zone (OFZ) technique and compare its structural andspectroscopic properties with CFO single crystals grown by OFZ method under air and oxygen (O$_2$) atmospheres. Singlecrystal X-ray diffraction confirms growth of Ca$_2$Fe$_2$O$_5$ single crystals in all the atmospheres. Crystals grown in air andoxygen (O$_2$) atmospheres show both metallic Fe$^0$ and Fe$^{3+}$states, whereas in Ar-grown crystals only Fe$^{3+}$ oxidation state is observed. An enhancement in oxygen vacancies is also observed in Ar-grown crystals. Growth ambience is found tohave a strong influence on cationic environment and binding energy of constituent elements, as is evident from the X-ray photoelectron spectroscopy of the Ca$_2$Fe$_2$O$_5$ crystals.