Chitosan-encapsulated Keggin Anion [Ti2W10PO40]7−. Synthesis, Characterization and Cellular Uptake Studies

The Keggin type polyoxotungstate [Ti₂W₁₀PO₄₀]⁷⁻ forms stable associates with the biopolymer chitosan in the nanometer size range. The cluster compound crystallizes from aqueous solution as K₄H₃[Ti₂W₁₀PO₄₀] · 15H₂O having a tetragonal structure. Both, the cluster compound and the chitosan/[Ti₂W₁₀PO₄₀] associates show a high hydrolytic stability at pH 7.4. The associates formed between the cluster anion [Ti₂W₁₀PO₄₀]⁷⁻ with the polyaminosaccharide chitosan have been characterized by photon correlation spectroscopy, scanning electron microscopy, filtration, centrifugation and zeta potential measurements. The size of the associates formed is in the range of ca. 5×10¹ to 5×10² nm. These particles have a defined stoichiometry with 5–6 cluster anions bound per molecule chitosan. The isoelectric point determined by zeta potential measurements was found for a cluster anion to chitosan molar ratio of 5.5, indicating the charge neutralization between protonated chitosan and [Ti₂W₁₀PO₄₀]⁷⁻ anions. Cellular uptake studies with [Ti₂W₁₀PO₄₀]⁷⁻ using tumor cell lines FaDu (human squamous carcinoma) and HT-29 (human adenocarcinoma) showed that the tungsten amount inside the cells is remarkably enhanced in the presence of chitosan.