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).
IntroductionThe sol-gel process allows the preparation of silicabased or siloxane-based materials at ambient temperatures. Pure silica glasses doped with lanthanide complexes can be made by hydrolysis and polymerization of silicon alkoxides. 1,2 The resulting glasses are transparent and have good mechanical properties. However, these materials have some drawbacks, such as the low solubility of lanthanide complexes in the sol-gel matrix at the low pH needed for the hydrolysis reaction. 2,3 One way to overcome these solubility problems is to neutralize the solution after hydrolysis and to introduce organic components in the material. The properties of the inorganic-organic hybrid materials depend on the chemical nature of the different constituents. 4,5 Poly-(ethylene glycol) (PEG) can be introduced in a silica matrix by simply mixing PEG with the silica precursors, but no covalent bonds are formed between the PEG chains and the silica backbone. 6,7 The solubility of lanthanide complexes can be improved by performing the sol-gel synthesis in a buffered solution. 8