Photopolymerization Induced by Upconversion Emission for Poly(pentaerythritol acrylate) Shell Coating on NaYF<sub>4</sub> Nanoparticle Surface Toward Ratiometric Optical Thermometry

Rare-earth doped ceramics nanoparticles are promising near-infrared fluorescent emitters for ratiometric nanothermometry in biological system. This study investigated the use of polypentaerythritol tetraacrylate as a coating of β-NaYF4:Yb3+,Ho3+,Er3+ nanoparticles to suppress luminescence quenching in water. It was found that upconversion emission from the particles facilitated photopolymerization of pentaerythritol tetraacrylate, forming roughly 10-nm thick shells on the particle surfaces. Temperature dependence of luminescence intensity ratio was evaluated and shows an increase of Ho3+ emission at 1200 nm by the polymer coating.