The photodynamic anti-cancer and anti-bacterial behaviour of meso-substituted trans-A2B2 porphyrin conjugated silica‑gold nanoparticles

Abstract

The synthesis and characterization of 5,15-bis(4-(methylthio) phenyl)-10,20-di(pyridine-4-yl) porphyrin (1) and its zinc(II) derivative (2) are reported along with their gold-coated silica nanoparticle (AuSiO₂) conjugates. The heavy atom effect of the zinc(II) ion resulted in complex 2 having high singlet oxygen quantum yields. Upon conjugation of complex 1 and 2 to AuSiO₂ NPs the triplet lifetimes decreased, while the singlet oxygen quantum yields increased due to an external heavy atom effect. The photodynamic therapy (PDT) activity of all the complexes was determined against MDA-MB-231 cells, and all the porphyrin complexes exhibit a relatively low in vitro dark cytotoxicity percentage cell viability of >77 %. Both 1-AuSiO₂ and 2-AuSiO₂ conjugates display more favourable PDT activity properties upon illumination with a Thorlabs 625 nm light-emitting diode (288 J cm^-2) for 30 min against MDA-MB-321 cancer cells. In a similar manner, the conjugates were found to also have significantly higher photodynamic antimicrobial activity (aPDT) against planktonic Gram-(+) cells of methicillin-sensitive Staphylococcus aureus (MSSA) as compared to methicillin-resistant Staphylococcus aureus (MRSA). The observed enhancements in the PDT and aPDT activity of 1-AuSiO₂ and 2-AuSiO₂ could be attributed to the high singlet oxygen generated by the nanoconjugates compared to the porphyrin complexes alone.