We designed a biodegradable hybrid nanostructure for near-infrared (NIR)-induced photodynamic therapy (PDT) using an ultrasmall upconversion (UC) phosphor (β-NaYF4:Yb3+, Er3+ nanoparticle: NPs) and a hydrocarbonized rose bengal (C18RB) dye, a hydrophobized rose bengal (RB) derivative. The UC-NPs were encapsulated along with C18RB in the hydrophobic core of the micelle composed of poly(ethylene glycol) (PEG)-block-poly(ε-caprolactone) (PCL). The UC-NPs were well shielded from the aqueous environment, owing to the encapsulation in the hydrophobic PCL core, to efficiently emit green UC luminescence by avoiding the quenching by the hydroxyl groups. The hydrophobic part of C18 of C18RB worked well to be involved in the PCL core and located RB on the surface of the PCL core, making the efficient absorption of green light and the emission of singlet oxygen to surrounding water possible. Moreover, as the location is covered by PEG, the direct contact of RB to cells is prohibited to avoid their irradiation-free toxic effect on the cells. The hybrid nanostructure proved to be degradable by the hydrolysis of PEG-b-PCL. This degradation potentially results in renal excretion by the decomposition of the nanostructure into sub-10 nm size particles and makes them viable for clinical uses. These nanostructures can potentially be used for PDT of cancer in deep tissues.
Keywords: biodegradable polymer; hydrocabonized rose bengal; rare-earth-doped ceramics; theranostics; ultrasmall nanoparticles; upconversion.