[Study of lanthanide complexes-doped silica gels and the Co-fluorescence effect by photoacoustic spectroscopy in situ].

Silica matrices, due to their good optical, thermal and chemical properties, are suitable candidates for the hosts of luminescent lanthanide complexes. However, lanthanide complexes would be unstable in the most common sol-gel precursor solution. It is important to study the coordination environment of lanthanide ions and the formation of lanthanide complexes in silica gels. In the present work, lanthanide complexes Ln(Sal)3 x H2O(Ln3+:La3+, Nd3+ and Tb3+; Sal:salicylic acid) were incorporated into silica gels via a sol-gel process. PA technique was firstly used to monitor the formation of lanthanide complexes in silica gels. Upon heat treatment at 110 degrees C, PA intensity of the ligand increased for Tb3+, La3+ and Nd3+ complexes in silica gels, respectively, while this difference could not be observed for the wet gels (samples without heat treatment). By comparison with fluorescence spectra, experimental data indicate that lanthanide complexes decompose in wet gels. The formation of lanthanide complexes in silica gels is discussed from two aspects: radiative and nonradiative processes. Co-luminescence effect was found for lanthanide complexes with aromatic carboxylic acid doped silica gels for the first time. For Tb0.8 Ln0.2 (Sal)3 x H2O (Ln3+:Gd3+ or Nd3+)-doped silica gel, the addition of Gd3+ increased the luminescence efficiency of Tb3+, while the luminescence of Tb3+ was quenched remarkably with the addition of Nd3+. Possible mechanism behind the co-luminescence phenomena of lanthanide complexes-doped silica gels is discussed.