Photothermal therapy (PTT) using plasmonic nanoparticles for cancer treatment is on the verge of clinical application. Titanium nitride (TiN) nanoparticles offer a promising alternative to commonly used gold-based systems at a fraction of the costs. Little is known, however, about the relationship between TiN particle characteristics and their optical properties in colloidal systems. Here, TiN nanoparticles with closely controlled characteristics are prepared by nitridation of TiO2, and their use as PTT agents is explored. Emphasis is placed on the particle surface and core oxygen content, which dominate the TiN optical properties. Colloidal suspensions were studied under UV-vis and near-infrared (NIR) laser irradiation and correlated to particle characteristics. High nitridation temperatures and long residence times lead to increased NIR light absorption. Too high nitridation temperatures, however, lead to particle aggregation that deteriorated their optical properties. This was overcome with SiO2 coating of TiO2 nanoparticles prior to nitridation: the resulting SiO2-coated TiN particles exhibited increased plasmonic performance compared to bare TiN, which is attributed to reduced plasmonic coupling effects. The optimized SiO2-coated TiN had a photothermal efficiency of 58.5% and mass extinction coefficient of 31.6 L g-1 cm-1, outperforming commercial gold nanoshells that are used in clinical trials. The potential of SiO2-coated TiN for photothermal therapy was demonstrated by controllably killing HeLa cells in vitro.
Keywords: SiO coating; coupling; flame spray pyrolysis; photothermal therapy; plasmonic; titanium nitride.