Enhanced photodynamic selectivity of nano-silica-attached porphyrins against breast cancer cells

Abstract

The synthesis and characterization of bare silica (4 nm in diameter) nanoparticle-attached meso-tetra(N-methyl-4-pyridyl)porphine (SiO₂–TMPyP, 6 nm in diameter) are described for pH-controllable photosensitization. Distinguished from organosilanes, SiO₂ nanoparticles were functionalized as a potential quencher of triplet TMPyP and/or singlet oxygen (¹O₂) at alkaline pH, thereby turning off sensitizer photoactivity. In weak acidic solutions, TMPyP was released from the SiO₂ surface for efficient production of ¹O₂. By monitoring ¹O₂ luminescence at 1270 nm, quantum yields of ¹O₂ production were found to be pH-dependent, dropping from ∼0.45 in the pH range of 3–6 to 0.08 at pH 8–9, which is consistent with the pH-dependent adsorption behavior of TMPyP on the SiO₂ surface. These features make bare SiO₂-attached cationic porphyrin a promising candidate for use in PDT for cancer treatment in which efficient ¹O₂ production at acidic pH and sensitizer deactivation at physiological pH are desirable. The enhanced therapeutic selectivity was confirmed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) tests and trypan blue exclusion tests of cell viability in breast cancer cell lines. Bimolecular quenching rate constants of ¹O₂ by free TMPyP, SiO₂ and SiO₂–TMPyP nanoparticles were also determined.