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

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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.

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