The precise modification of a nanoscaled Keplerate-type polyoxometalate with NH2-groups: reactive sites, mechanisms and dye conjugation

Abstract

The interaction of alkoxysilanes with nanoscaled giant polyoxoclusters is a challenging route for efficiently building blocks for supramolecular smart design. 3-Aminopropyltrimethoxysilane (APTMS) treatment grafts amino groups onto the surface of the Keplerate {Mo₁₃₂} in a controlled manner. By means of XPS and elemental analysis, we studied the influence of the component ratio, the solvent nature, acidity and mixture preparation on the stoichiometry of POM–APTMS conjugates. The reaction mechanism shows the role of {Mo₂} bridging units as reactive sites for APTMS binding. Comprehensive use of IR and Raman spectroscopy shows ring-like siloxane formation above the {Mo₉O₉} pores in {Mo₁₃₂}. This approach to the covalent modification of giant polyoxometalates (POM) was successfully applied to the functionalization of another Keplerate {Mo₇₀Fe₃₀}. Grafting NH₂ groups opened a way to covalently bind POMs with fluorescein-NHS (FL-NHS), where the strong charge transfer near 526 nm (UV–vis) appears to promote potential photocatalytic activities. Studying the conjugates POM–APTMS with SEM, we discovered a novel nano-object consisting of nanostrips: these are unknown for structures based on nanoclusters from the giant POM family. Thus, these pioneering approaches to giant POM covalent functionalization can be applied to designing templates for supramolecular structures and functional materials.