Colloidal LaPO ₄ :Gd ³⁺ 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 Kα 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 ⁶P_7/2-⁸S_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.