The chemistry behind the sol-gel synthesis of complex oxide nanoparticles for bio-imaging applications

The formation of nanoparticles in hydrolytic and non-hydrolytic reactions in organic media is treated as a borderline case of coordination chemistry equilibria. Metal cation complexes with O-donor ligands, such as metal alkoxides or beta-diketonates and carboxylates, experience normally no kinetic hinders in re-structuring and aggregation. The steps of hydrolysis and condensation constitute in this case parts of one and the same kinetic phenomenon. Introduction of oxo-ligands during the first steps of hydrolytic or thermal decomposition leads thus in homogeneous processes to well-defined oligonuclear oxo-species. The structure of the latter is originating from the densest possible packing of cations and ligands and is not in any mean related to the structure of the original precursor molecules. The oxo-alkoxide molecules serve apparently as nuclei for the formation of larger aggregates that become phase separated and can be referred to as Micelles Templated by Self-Assembly of Ligands. The core of the latter is practically always crystalline or at least short-order organized, while the shell is, in the hydrolytic approach, constructed of hydrated amorphous oxide and ligands that stabilize the colloid via interaction with the solvent. The thermal treatment either in solution or for a dry xerogel is thus indispensable for preparation of fully crystalline particles and offers also an opportunity to control the final size of the particles via further aggregation. To guarantee the formation of complex oxide nanoparticles, the conditions insuring phase separation for each component have to be achieved. This means that the use of stoichiometric or super-stoichiometric quantities of water is required together with a solvent guaranteeing minimal solubility of hydrolyzed species. The approaches to complex oxide phases and oxides doped with lanthanide cations are discussed.