Tuning the photodynamic efficiency of TiO₂ nanotubes against HeLa cancer cells by Fe-doping

In this study Fe-doped TiO₂ (0.35 to 3.50 wt% Fe) nanotubes (NTs) were prepared as the potential photosensitizer for near-visible light driven photodynamic therapy (PDT) against cervical cancer cells (HeLa). Characterization of the prepared nanotubes by X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of Fe³⁺ as a dopant into the TiO₂ matrix, which was mainly composed of an anatase phase, while elemental mapping using energy dispersive X-ray spectroscopy (EDX) showed homogenous distribution of the dopant ions in TiO₂ for both low and high doping levels. UV-Vis studies showed that Fe doping in TiO₂ increases the light absorption within the visible range, particularly in the case of 0.70 and 1.40 wt% Fe–TiO₂ and provides additional energy levels within the band gap, which promotes the photo-excited charge transport towards the conduction band. Photo-cytotoxic activity of the prepared Fe-doped TiO₂ NTs was investigated in vitro against cervical cancer cells (HeLa) and compared with human normal fibroblasts (GM07492). Fe-doped TiO₂ NTs exhibited no or lower dark cytotoxicity than un-doped TiO₂ NTs, which confirms their superior biocompatibility. Under the near-visible light irradiation (∼405 nm) Fe-doped TiO₂ NTs showed higher photo-cytotoxic efficiency than un-doped TiO₂ NTs, which was found to be dependent on the NTs concentration, but not on the incubation time of cells after near-visible light irradiation. The highest activity was observed for 0.70 and 1.40 wt% Fe–TiO₂ NTs. Fluorescent labeling of treated HeLa cells showed distinct morphological changes, particularly in the perimitochondrial area suggesting a mitochondria-involved apoptosis of cells, but also the nuclei and cytoskeleton were subject to Fe–TiO₂ NTs induced photo-damage. Apoptosis of PDT treated HeLa cells was also confirmed using ethidium homodimer (EthD-1).