Reduced photon quenching in Ce-doped NaYF4:Yb/Ho upconversion nanoparticles with core/shell structure

The use of red light or near-infrared radiation as a luminescent probe for in vivo bio imaging is crucial in order to restrict the strong absorption of short-wavelength light below 600 nm in tissue. It is demonstrated that the emission color of Yb/Ho codoped NaYF4 nanoparticles can be tuned from green to red by incorporating Ce3+ ions. However, compared with that of the NaYF4:Yb/Ho nanoparticles, the photoluminescence intensity of the Ce3+-tridoped NaYF4:Yb/Ho nanoparticles is drastically reduced. In this work, Ce3+-incorporated core/shell NaYF4: Yb(3+)50%@NaYF4:Ho(3+)0.5% nanoparticles are prepared. A strong red emission and a high-intensity ratio between the red emission and green emission are obtained in these upconversion nanoparticles. The emission intensity increases by a factor higher than 120 when compared with that of the NaYF4:Yb/Ho/Ce nanoparticles. This result indicates that the Ce3+ incorporation into the NaYF4:Yb/Ho nanoparticles promotes a strong quenching effect and reduces the emission intensity; however, the quenching effect can be significantly reduced by incorporating the Ce3+ ions into the core/shell NaYF4:Yb(3+)50%@NaYF4:Ho(3+)0.5% nanoparticles. A theoretical model is proposed to explain the presence of the quenching effect in the NaYF4:Yb/Ho/Ce nanoparticles, demonstrating that the quenching is mainly related to the interaction between the Yb3+ and Ce3+ ions.