Water-Dispersible Nanoemulsions Containing Pheophorbide A-Functionalized Upconversion NaYF4:Yb,Er@NaYF4 Nanoparticles for Photonic Bioapplications

Upconversion nanoparticles (UCNPs) such as NaYF₄:Yb,Er are a promising platform for targeted theranostics due to the ease of surface biofunctionalization and their intrinsic upconversion luminescent properties. These properties include the conversion of NIR photons, which are within the optical biological window and therefore can be used to excite the nanomaterials in vivo, into visible photons.

In this work, NaYF₄:Yb,Er@NaYF₄ (YYE@Y) was synthesized using oleic acid (OA) as a capping ligand via an adapted high-temperature coprecipitation method. The obtained nanoparticles were then conjugated with pheophorbide A (PPA), a well-established porphyrin photosensitizer for photodynamic therapy. The composite exhibited a decreased red Er³⁺ emission when compared to fresh YYE@Y-OA nanoparticles  (Figure 1a), arising from PPA absorption in the same region.This decrease is a consequence of the energy transfer from the nanoparticles to PPA. Finally, a water-dispersible nanoemulsion containing the conjugated system was prepared, with potential applications in NIR-activated photodynamic therapy.