title: Sub-6 nm monodisperse hexagonal core/shell NaGdF4nanocrystals with enhanced upconversion photoluminescence.
abstract: The ability to fabricate lanthanide-doped upconversion nanocrystals (UCNCs) with tailored size and emission profile has fuelled their uses in a broad spectrum of biological applications. Yet, limited success has been met in the preparation of sub-6 nm UCNCs with efficient upconversion photoluminescence (UCPL), which enable high contrast optical bioimaging with minimized adverse biological effects entailed by size-induced rapid clearance from the body. Here, we present a simple and reproducible approach to synthesize a set of monodispersed hexagonal-phase core NaGdF4:Yb/Ln (Ln = Er, Ho, Tm) of ∼3-4 nm and core/shell NaGdF4:Yb/Ln@NaGdF4(Ln = Er, Ho, Tm) UCNCs of ∼5-6 nm. We show that the core/shell UCNCs can be up to ∼1000 times more efficient than the corresponding core UCNCs due to the effective suppression of surface-related quenching effects for the core. The observation of prolonged PL lifetime for the core/shell than that for the core UCNCs demonstrates the role of the inert shell layer for the protection of the core. The achievement of sub-6 nm NaGdF4UCNCs with significantly improved luminescence efficiency constitutes a solid step towards high contrast UCPL optical imaging with secured biological safety.
authors.affiliation: MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China. chenguanying@hit.edu.cn yangchh@hit.edu.cn.
authors.forename: Jing
authors.initials: J
authors.lastname: Liu
authors.forename: Guanying
authors.initials: G
authors.lastname: Chen
authors.forename: Shuwei
authors.initials: S
authors.lastname: Hao
authors.forename: Chunhui
authors.initials: C
authors.lastname: Yang