Hybrid assemblies of Eu-containing polyoxometalates and hydrophilic block copolymers with enhanced emission in aqueous solution.

Polyoxometalates (POMs), a type of giant transition metal oxide clusters on the nanoscale, are emerging as useful materials with unique catalytic, electrooptical, and biological properties. Lanthanide-containing POMs elements show excellent photoluminescent properties with narrow emission bands, large Stokes shift, long lifetime, and tunable emission, i.e., their long emission timescale (microto milliseconds) also allows autofluorescence from the biological samples to be removed by time-gating the emission decay. Therefore they are attractive for labeling and imaging of biological molecules. In spite of their intriguing functionality, practical POMbased materials are quite scarce because of their inherent limitations, that is, poor processability in the solid state and weakened functionality in solution. For example, the Eucontaining POM Na9EuW10O36·32H2O displays only weak photoluminescence in water in comparison to the solid state due to fluorescence quenching by water molecules. Therefore, the development of supramolecular methodologies to protect and utilize POMs is important for application of POMs. Polymers are ideal protective matrices for this purpose, into which POMs were incorporated to fabricate supramolecular assemblies for improved processability and enhanced functionality. Besides the widely used layer-bylayer assembly of POMs with polyelectrolytes for functional thin films, polymerizable surfactants and amphiphilic block copolymers have been used to organize POMs into stable supramolecular assemblies in organic solution. As far as we know, there are still no reports on the emission enhancement of lanthanide-containing POMs in aqueous solutions through supramolecular assembly strategies. Herein we report a straightforward procedure to fabricate hybrid assemblies of lanthanide-containing POMs with block copolymers in aqueous solution. Core–shell nanostructured organic–inorganic hybrid materials were produced by direct mixing of Na9EuW10O36·32H2O (Eu-POM) with doublehydrophilic neutral–cationic block copolymer poly(ethylene oxide-b-N,N-dimethylaminoethyl methacrylate) (PEO-bPDMAEMA) in aqueous solution (Scheme 1). Neither