Beneficial role of gold nanoparticles as photoprotector of magnesium tetraphenylporphyrin

The photoprotection of porphyrins is of immense importance for their utilisation in several photo-related applications. In the present study, we have employed gold nanoparticles (AuNPs) to photoprotect a porphyrin, magnesium tetraphenylporphyrin (MgTPP). Absorption, fluorescence and X-ray photoemission spectroscopic (XPS) studies of MgTPP under illumination in the absence and in the presence of AuNPs have been carried out to assess their effect on the photostability of MgTPP. The results show that with an appropriate concentration of AuNPs, virtually 100% photoprotection can be provided to MgTPP. The protecting ability of AuNPs is the result of their binding with MgTPP at its nitrogen sites, inhibiting, thus, the binding of reactive oxygen species (ROS) at these sites, known to cause the photodegradation of MgTPP and several other porphyrins under aerobic conditions. Further, the decreased formation of excited singlet states of MgTPP, as a result of its binding with AuNPs, also suppresses the generation of ROS and improves the photostability of MgTPP. The binding between MgTPP and AuNPs may also be responsible for the photoprotection provided to MgTPP by AuNPs under anaerobic conditions too. The use of gold nanoparticles thus offers us a new strategy to photoprotect various porphyrins.