Sol-gel synthesis of biocompatible Eu3+/Gd3+ co-doped calcium phosphate nanocrystals for cell bioimaging

Herein, a polyacrylic acid sol-gel method was presented to synthesize rare earth doped calcium phosphate nanocrystals (RE-CaP) as cell bioimaging agent. The effects of calcination temperature, calcination time and PAA concentration on phase composition , morphology, size and luminescence were discussed. In addition, the hemolysis, cytotoxicity, cell uptake and cell imaging of resulting RE-CaP nanocrystals were investigated. Results show that the increases in calcination temperature and time promote the transformation of product from impure crystalline phase (CaCO 3 , CaO) to pure CaP phase. The rise of calcination temperature results in significant enhancement of luminescence intensity. PAA concentration does not influence the crystalline phase composition and its increase leads to a little increase in luminescence intensity. In addition, the co-doping of Gd 3+ improves the luminescence intensity of Eu 3+ emissions. This method achieves the doping of Eu 3+ ions in the crystallographic Ca 2+ I and Ca 2+ II sites of HAP with a preferable occupation at Ca 2+ II site. The resulting RE-CaP nanocrystals (PAA: 0.6   mg/mL, Eu/Gd = 2:1.5, 600   °C, 3   h) show good biocompatibility and can be easily uptaken by cancer cells. The internalized nanocrystals show stable and strong red luminescence under visible wavelengths excitation. In summary, PAA sol-gel method can be used to synthesize biocompatible RE-CaP luminescent nanomaterials for cell bioimaging.