Synergistic enhanced photocatalytic and photothermal activity of Au@TiO2 nanopellets against human epithelial carcinoma cells

The photocatalytic and plasmonic photothermal cancer cell-killing activity of the metallic Au-capped TiO2 (Au@TiO2) composite colloidal nanopellets has been investigated on HeLa cells under UV-visible (350-600 nm) light irradiation. The Au@TiO2 composite nanopellets with the uniform Au-capped TiO2 structure were successfully synthesized by simple reduction of HAuCl4 on the surface of TiO2 nanoparticles. The morphological structure and surface properties of Au@TiO2 were characterized by using UV-visible absorption spectroscopy, TEM, SEM, XPS, EDX and XRD analyses. The formation of hydroxyl radicals ((OH)-O-center dot) was confirmed by photoluminescence (PL) spectra. The photocatalytic and photothermal cell-killing activity of the Au@TiO2 nanopellets was found to vary with the molar ratio of Au to TiO2. The direct involvement of the metal particles in mediating the electron transfer from the photoexcited TiO2 under the band gap excitation is considered to carry out the efficient photocatalytic reaction on the cells. The plasmonic absorption spectra of Au@TiO2 suspensions were also measured for the evaluation of photothermal cell killing. The charge separation, the interfacial charge-transfer and photothermal activity promoted the photocatalytic-photothermal cancer-cell killing more than TiO2 alone. The cytotoxic effect of Au@TiO2 nanopellets with low concentration of gold (TiO2 : Au molar ratio > 1 : 1) was found to be 100%, whereas that of the commercial TiO2 (P25) was ca. 50%. The comparative study of the cell viability using Au alone and TiO2 alone revealed that the synergistic effect of photocatalytic hydroxyl radical formation and Au-plasmonic photothermal heat generation plays a vital role in the cancer cell killing. A plausible mechanism was also proposed for photocatalytic cancer cell killing based on the obtained results.