Colloidal LaPO4:Gd3+ nanocrystals: X-ray induced single line UV emission

Colloidal solutions of nearly monodisperse 5 nm LaPO4:Gd3+ nanocrystals are shown to strongly emit UV radiation upon excitation with tungsten K radiation (59.3 keV) or vacuum UV radiation (160 nm). The UV emission of the particles consists mainly of a single line at 311 nm corresponding to the P-6(7/2)-S-8(7/2) transition of Gd3+. The highest emission intensity is observed for LaPO4 nanocrystals with a Gd3+ concentration of 20%. Since the absorption cross section of biomaterials is low for X-rays but high for 311 nm radiation, the UV emission of particles embedded in the biological tissue can only affect the direct vicinity of the particles. Nanocrystals of LaPO4:Gd3+ could, therefore, be interesting for biomedical applications such as strongly localized drug release by X-ray triggered UV uncaging reactions.