Tetrakis(2,4,6‐Trihydroxyphenyl)porphyrin‐Conjugated Gold Nanoparticles as Efficient Photosensitizers With Plasmon‐Enhanced Fluorescence and Singlet Oxygen Generation

Till date, porphyrin molecules are best available photosensitizers (PS) for cancer therapy with good singlet oxygen generation (SOG) capability; however, with minimal fluorescence. Suitable coupling of PS with the plasmonic nanoparticle may result into enhanced fluorescence and SOG, which is attractive for image-guided photodynamic therapy (PDT). Here, Tetrakis(2,4,6-trihydroxyphenyl)porphyrin (THPP) molecules and gold nanoparticles (AuNPs) of two different sizes are synthesized and subsequently conjugated via L-cysteine and introduced as novel plasmon-enhanced nano-PS. Upon excitation at 418 nm, simultaneous enhancement in fluorescence and SOG was achieved for hybrid nanostructure, when compared with fluorescence intensity (FI) and SOG of only THPP molecule in absence of AuNPs. Among two different sizes of AuNPs, more enhancement is observed with average diameter of 25 nm, whereas 45 nm showed less enhancement. The possible mechanism is ascribed to local electric field enhancement by the AuNPs, where overlap of Q-band of THPP, localized surface plasmon resonance peak of AuNPs, and the excitation wavelength is the important factor. We witness that enhancement in fluorescence is accompanied with no reduction in faster lifetime component of THPP. Overall, the work shows a rational approach to design PS-metal configurations with desired emissive properties with higher SOG for useful PDT approach.