Rare-earth-doped ceramic nanoparticles (RED-CNPs) have been studied for biological applications because they can convert biopermeable near-infrared light into visible and ultraviolet light. Both the host ceramic and the microenvironment around RED-CNPs may influence their luminescence characteristics; however, the effects of surrounding molecules have not been systematically clarified. In the present study, the effects of dispersion solvents and adsorbed polymers on the upconversion luminescence of Yb3+- and Er3+-doped NaYF4 nanoparticles (NaYF4:Yb3+,Er3+ NPs) were investigated. The effect of polymers was analyzed by encapsulating the luminescent NPs into various hydrophobic particle cores composed of poly(ethylene glycol) block copolymers. NaYF4:Yb3+,Er3+ NPs showed different upconversion spectra under different conditions when excited by 980 nm light. As the polarities of the surrounding organic molecules (solvents and hydrophobic core polymers) on the particle surface decreased, the thermal relaxation (4I11/2 → 4I13/2) was suppressed, whereas the excitation from 4I11/2 to 4F7/2 was enhanced in the upconversion excitation process, and thus, green upconversion luminescence (520–540 nm) increased. The solubility parameter was effective as an index of polarity affecting thermal relaxation. Our results showed the importance of material design considering the molecular polarity of the surrounding microenvironment for obtaining the desired upconversion emission of RED-CNPs.
- Hydrophobic polymer
- Organic solvent
- Rare-earth-doped ceramic nanoparticle
- Thermal relaxation
- Upconversion luminescence