Upconverting Nanoparticles in Aqueous Media: Not a Dead-End Road. Avoiding Degradation by Using Hydrophobic Polymer Shells

The stunning optical properties of upconverting nanoparticles (UCNPs) have inspired promising and unique biomedical technologies. However, as a necessary step for their use in bio-applications, their transfer to aqueous media is accompanied by highly detrimental effects. Namely, intense luminescence quenching and partial dissolution by water, as well as complete degradation by other molecules such as phosphates. Whereas the translation of UCNPs to the medical market would open new and interesting avenues for ultrasensitive diagnostics and other medical technologies, their concentration- and time-dependent degradation currently challenges the realistic bio-application of these nanomaterials. In this work, we developed a strategy to protect UCNPs by creating an isolating hydrophobic polymer shell (HPS) through miniemulsion polymerization. Stability studies revealed that these HPS served as a very effective barrier, impeding polar molecules to affect UCNP’s optical properties. Even more, it allowed UCNPs to withstand even extremely aggressive conditions such as very high dilutions, high phosphate concentrations, and high temperatures. In-depth optical, morphological, and atomic-level chemical characterization proved the potential of HPS to overcome the current limitations of UCNPs. This strategy, which can be applicable to other nanomaterials with similar limitations, pave the way towards more stable and reliable UCNPs with expanded applications in life sciences.