Nanocrystalline TiO2 anatase doped with lantanide ions: a novel luminescent material for optical imaging

In recent years, considerable interest has been devoted to the use of trivalent lanthanide ions (Ln3+) in the field of optical imaging based on fluorescence [1, 2]. This technique is characterised by the clear advantage that no hazardous ionising radiation or magnetic field is required, and by the fact that it is possible to study drugs kinetics and biodistribution in vitro and in vivo. Possible breakthroughs in this area critically depend on the development of suitable fluorescent non-toxic compounds which can be used in vivo, without being harmful to living organisms. Our approach to this problem consists in the development of a biocompatible nanocrystalline material that can be suitably doped with Ln3+, that give rise to efficient visible fluorescence and that could be dispersed in biological fluids. Our choice has been nanosized titania, in particular in the anatase form. TiO2 is considered to be non-toxic, and is widely employed as a harmless white pigment in the food, cosmetic, and pharmaceutical industries. Some of us have recently shown that "chimie douce" procedures lead to the facile synthesis of nanocrystalline titania powders doped with Ln3+ ions, and having a particle size in the range 5-20 nm [3, 4]. In this perspective, we have prepared, using the sol-gel technique, and characterized several nanocrystalline anatase samples activated with the Eu3+, Sm3+, Nd3+ and Pr3+ luminescent ions The potentialities of the samples have been investigated using a commercial equipment for optical imaging with different filter setups. Almost all the samples have shown better fluorescence efficiency using an excitation radiation in the 445-490 nm wavelength range and emission in the 575-650 nm range with respect to the other wavelength ranges investigated. The nanopowders activated with Eu3+ have shown the best fluorescence efficiency reaching values comparable to Eu-activated commercial powders.