Halloysite nanotubes as a vector for hydrophobic perfluorinated porphyrin-based photosensitizers for singlet oxygen generation

The development of effective drug delivery systems represents a significant advancement in cancer treatment. Anisotropic, natural, and cost-effective nano-vectors, such as halloysite nanotubes (HNT), can be utilized for this purpose. In this study, we loaded the HNT with the apolar photosensitizers (PS) 5,10,15,20-tetrakis(perfluorophenyl)porphyrin (H₂TPPF₂₀) and its Zn(II) complex (ZnTPPF₂₀) to produce singlet oxygen for photodynamic therapy (PDT). The loading was achieved through repeated vacuum/N₂ cycles using both pristine HNT and HNT modified with tetradecylphosphonic acid (HNT-TDP) to promote the uptake of the lipophilic PS in the inner lumen. To slow down the release of PS from HNT, the nanotubes were treated with dextrin. The ability of the free base and Zn perfluorinated porphyrins to produce singlet oxygen (¹O₂) was confirmed by irradiating the samples with a low-power visible LED emitter (23 mW cm^-2), showing a ¹O₂ quantum yield of 22% and 34%, respectively, in ethanol. The characterization of the nanocomposite is not trivial, so we employed a wide range of analytical techniques to investigate the material thoroughly, particularly the location of PS within the HNT. All nano-hybrids were analyzed by attenuated total reflectance infrared (ATR-FTIR), diffuse reflectance (DRS) and solid-state emission spectroscopy. Thermogravimetric analysis (TGA) was used to determine the loading capacity of HNT. To better understand the interactions between the PS and the nanoclay, we compared all the loaded HNT samples with mechanically mixed HNT and solid H₂TPPF₂₀ or ZnTPPF₂₀ samples, where the interaction with the HNT inner lumen is assumed to be absent. We measured the release kinetics using UV-vis spectroscopy, observing a delayed release of the PS. Finally, we studied the cellular uptake of pristine HNT and a loaded sample (HNT-TDP-H₂TPPF₂₀-dextrin) by confocal microscopy using three distinct tumor cell lines. The cytotoxicity on PC3, 5637 and UMUC3 cells was then assessed as reduction of cell viability both on cells left in the dark and those irradiated with a visible light emitting LED (1.3 mW cm^-2), ascertaining the ability to induce cell death especially after light administration.