110th Anniversary: Synthesis of Plasmonic Silica-Coated TiN Particles

Titanium nitride is emerging as an alternative plasmonic nanomaterial to costly (e.g., Au) or potentially toxic (e.g., Ag) noble metals. So TiN is most promising as replacement for Au-based nanostructures in photothermal cancer therapy due to its localized surface plasmon resonance (LSPR) in the desired near-infrared region (750-900 nm). In such biological applications, a SiO2 surface is most desirable as it enables easy functionalization with specific moieties. Here, TiN nanoparticles coated with SiO2 films of precisely controlled thickness are prepared by nitridation of flame-made monocrystalline SiO2-coated TiO2 particles (similar to 40 nm in crystal size). The average SiO2 thickness and polydispersity are obtained by microscopy and are in good agreement with those from fluid-particle dynamic simulations. The coating quality is assessed by photocatalysis of methylene blue by SiO2-coated and -mixed (co-oxidized) TiO2 particles. Nitridation results in polycrystalline TiN (similar to 20 nm) and proceeds from the particle surface inward even through the most hermetically coated (similar to 7 nm in SiO2 thickness) particles as determined by microscopy and elemental mapping. A photothermally optimal SiO2 shell thickness of a couple of nanometers, however, is found and attributed to plasmonic decoupling and stoichiometry of the constituent TiN particles. The biocompatibility of these particles was excellent as assessed with THP-1 monocytes.