Enhancement of near-infrared emission of neodymium-doped monoclinic gadolinium phosphate nanophosphors by surface coating with calcium phosphate

Core/shell-type nanoparticles composed of GdPO4: x mol% Nd3+/calcium phosphate (CaPO) (0 ≤ x ≤ 5) were synthesized by a precipitation method, as nontoxic bio imaging phosphors that can emit near-infrared (NIR) light under NIR excitation. The GdPO4: x mol% Nd3+ core, obtained in a single-phase form of the monoclinic rare-earth orthophosphate structure, exhibited characteristic emissions attributed to the f–f transitions of Nd3+. The strongest emission peak was observed at 1061 nm under excitation at 800 nm in a sample containing 3 mol% of Nd3+. The average particle size of GdPO4: 3 mol% Nd3+ was 42 nm, indicating that nano-sized particles were successfully obtained. Although the average particle size of the core/shell-type GdPO4: 3 mol% Nd3+/CaPO nanophosphor was slightly increased to 54 nm by a second calcination in the surface coating process, the CaPO shell was well formed with a thickness of 3 nm around the GdPO4: 3 mol% Nd3+ core. With surface coating with the CaPO4 shell, the NIR emission intensity increased to 4.2 times higher than that of GdPO4: 3 mol% Nd3+. This emission intensity was significantly higher than that of indocyanine green, moreover, which is used in practice as an organic bioimaging reagent.