Construction of LRET-Based Nanoprobe Using Upconversion Nanoparticles with Confined Emitters and Bared Surface as Luminophore

Upconversion nanoparticles (UCNPs) are promising energy donors for luminescence resonance energy transfer (LRET) and have widely been used to construct nanoprobes. To improve the LRET efficiency, which is currently a limiting factor for UCNPs-based bioassay, we herein propose a strategy to construct LRET-based nanoprobe using UCNPs with confined emitters and bared surface as the luminophore, with Ca2+ as the proof-of-concept target. The sandwich-structure upconversion nanoparticles (SWUCNPs) are designed with a core-inner shell-outer shell architecture, in which the emitting ions (Ln(3+)) are precisely located in the inner shell near the particle surface, which is close enough to external energy acceptors. The target receptor (Fluo-4) is directly tagged on bared surface of SWUCNPs, which further reduces the donor-to-acceptor distance. Our strategy contributes to significantly improved LRET efficiency and hence affords an ultrahigh sensitivity for Ca2+ detection. The as-constructed nanoprobe is structurally stable and exhibits good biocompatibility, which ensures uptake and reliable observation in living cells. The nanoprobe can be used for monitoring the different levels of cytosol [Ca2+] in living cells. Furthermore, it is applicable in Ca2+ imaging in mice liver tissues.