LAUSR

We describe the synthesis and characterization of a thiol-functionalized porphyrin derivative 2 and its gold nanoparticle conjugates. The porphyrin 2 exhibited its characteristic intense Soret absorption at 420 nm with a molar extinction coefficient value of $$3.6\times 10^{5}\, \hbox {M}^{-1}\, \hbox {cm}^{-1}$$3.6×105M-1cm-1 and good fluorescence in the region of 650–660 nm. The porphyrin-capped gold nanoparticles (POPNPs) were synthesized from the citrate-capped gold nanoparticles by the ligand exchange method and characterized by spectroscopic and morphological analyses (UV–Vis, DLS and TEM). The broadening of the absorption spectrum and quenching of the fluorescence intensity for the porphyrin gold nanoconjugates suggest efficient incorporation of the porphyrin moiety onto the gold surface. The results of DLS and TEM analyses indicate that the nanoconjugate POPNPs are uniformly spherical in shape with a size of ca. $$25 \pm 5\, \hbox {nm}$$25±5nm and exhibits a negative zeta potential value of $$-\,16.0 \pm 2\, \hbox {mV}$$-16.0±2mV. The singlet oxygen generation efficiency of the porphyrin 2 and POPNPs was calculated and are found to be ca. $$0.53 \pm 0.02$$0.53±0.02 and $$0.43 \pm 0.03$$0.43±0.03, respectively. The in vitro photobiological studies revealed that POPNPs exhibited enhanced photodynamic activity compared to their parent porphyrin derivative 2 with an $$\hbox {IC}_{50}$$IC50 value of $$5\, \upmu \hbox {M}$$5μM in MDA MB 231 cell lines. The mechanism of the cell destruction was studied by Annexin-FITC and confirmed through TMRM assay. We observed the increase in the percentage of cell population corresponding to the late apoptotic stage ca. 37.7% and 51.2% for 5 and $$10\, \upmu \hbox {M}$$10μM of POPNPs, respectively, thereby demonstrating their apoptotic-mediated cell destruction and use in PDT applications.Graphical AbstractSynopsis We synthesized a thiol-functionalized porphyrin-based sensitizer 2 and its gold nanoparticles (POPNPs) and have investigated their photophysical properties, singlet oxygen generation and in vitro photobiological efficacy. Our results demonstrate that the incorporation of the sensitizer onto the surface of nanoparticles not only improved its solubility in the aqueous medium, photophysical and photobiological properties but also its potential as a novel sensitizer for photodynamic therapeutical applications.