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 (CaCO3, 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 Gd3+ improves the luminescence intensity of Eu3+ emissions. This method achieves the doping of Eu3+ ions in the crystallographic Ca2+ I and Ca2+ II sites of HAP with a preferable occupation at Ca2+ II site. The resulting RE-CaP nanocrystals (PAA: 0.6 mg/mL, Eu/Gd = 2:1.5, 600 degrees 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.