Thin Amphiphilic Polymer-Capped Upconversion Nanoparticles: Enhanced Emission and Thermoresponsive Properties

Upconversion nanoparticles (beta-NaYF4:Er3+, Yb3+, UCNPs) were capped with a thin polymer shell by replacing the oleate ligand of hydrophobic UCNPs by multidentate thiolate-grafting of P(MEO(2)MA-co-SEMA) copolymers. The presence of the 2-(2-methoxyethoxy)ethyl side chains of MEO(2)MA extending out of the nanohybrid made them water-dispersible. The UCNP@P(MEO(2)MA-co-SEMA) nanohybrids exhibited an enhanced emission by up to a factor of 10, as compared with that of their hydrophobic precursor in dichloromethane and even in water (a factor of 2). Moreover, their thermoresponsiveness was modulated by the pH; this is consistent with the presence of some thiol groups at the nanohybrid periphery. Remarkably, the nanohybrid emission, as well as its stability, was almost independent of the aggregation state (in the basic-acid and temperature range studied here). The formation of stable water-dispersible UCNPs with enhanced emission, together with their amphiphilic and temperature-responsive polymer coating, is promising for building multifunctional nanostructures for intracellular imaging, therapy, and drug delivery.