Synthesis of Europium(III) Complex-Based Hydroxyapatite Nanocrystals for Biolabeling Applications

We synthesized the inorganic/organic hybrid nanocrystal (EHA) by hydroxyapatite (HA) nanocrystal growth under the existence of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium(III) (EuTH) complex. Then, folic acid derivative (FA-NHS: folate N-hydroxysuccinimidyl ester) as the targeting ligand for the HeLa cancer cells was immobilized on the EHA by the mediation of 3-aminopropyltriethoxysilane (APTES) and methyltriethoxysilane (MTES). When the FA-NHS molecular occupancy ratio on the EHA surface is around 3 to 5 %, the strong luminescence from the f-f transition of the Eu³⁺ ion and luminescence associated with energy transfer between the EuTH-FA monomer near 518 nm were observed. Moreover, the dispersibility in phosphate buffer saline was confirmed with immobilizing the positively-charged FA-NHS. The affinity and non-cytotoxicity between the nanocrystals and HeLa cancer cells were confirmed for 3 days. The red luminescence from the cells could be observed by fluorescence microscopy and the luminescence spectra.