Near-infrared luminescence of lanthanide calcein and lanthanide dipicolinate complexes doped into a silica-PEG hybrid material

The near-infrared luminescence of lanthanide complexes of 4',5'-bis[N,N-bis(carboxymethyl)aminomethyl] fluorescein (calcein) and pyridine-2,6-dicarboxylic acid (dipicolinic acid, dpa) doped in a hybrid sol-gel material was investigated. The silica-poly(ethylene glycol) (silica-PEG) inorganic-organic materials were prepared at a neutral pH. The lanthanide ions are well shielded from the environment by the calcein and dpa ligands, and the complexes are stable in the sol-gel matrix after preparation. The dysprosium and neodymium dipicolinate complexes showed near-infrared luminescence (NIR-luminescence) by direct excitation to the 4f-levels. The ytterbium dipicolinate complex doped in the sol-gel showed NIR-luminescence by excitation of the ligand in the UV region. All other tested lanthanide ions (Ln = Pr, Sm, Er, Ho) did not show luminescence. Neodymium and ytterbium complexes with calcein show intense NIR-luminescence when the ligand is excited by visible light. The corresponding erbium complex doped in the silica-PEG matrix also showed NIR-luminescence at 1525 nm. No NIR-luminescence could be detected for the other lanthanide complexes doped in the matrix (Ln = Pr, Sm, Dy, Ho).