Modulating the Luminescence of Upconversion Nanoparticles with Heavy Metal Ions: A New Strategy for Probe Design.

Upconversion nanoparticles (UCNPs) are attracting increasing attention in biosensing and imaging. The design of UCNP-based probes currently relies exclusively on the luminescence resonance energy transfer (LRET) principle. The prerequisite spectral overlap in LRET leads to limited flexibility in probe design, thus hindering the construction and application of upconversion (UC) probes. To change this situation, we herein present a new approach to construct UC probes by use of heavy metal ion-induced quenching. We reveal that heavy metal ions can quench the upconversion luminescence (UCL) to >95% without the occurrence of spectral overlap. A proof-of-concept UC probe for biothiols by manipulating Cu2+ as the switch of luminescence exhibits satisfying performance both in vitro and in bioimaging. This is the first report of a UC probe utilizing heavy metal ions to govern the read-out signal. The strategy is much simpler than the LRET principle and highly efficient, which provides a new way to design and apply UCNP-based probes.