Harnessing LRET in a rationally designed "sandwich" fluorescent probe for selective ClO sensing.

Upconversion nanoparticles (UCNPs), as a type of light-emitting nanomaterials excited by near-infrared (NIR) light, can circumvent interference from spontaneous fluorescence and scattered light emitted by biological molecules in sensing applications. Traditional homogeneous core-shell UCNPs struggle to locate the position of luminescent doped ions (such as Tm) within the NaYF matrix, only the luminescent ions close to the surface of the particles can be efficiently quenched, with a low bursting efficiency that produces a considerable background. The full effectiveness energy transfer exists only by sufficiently close proximity between donor and acceptor. Here, a distance dependence "sandwich" structured NIR fluorescent probe UCNPs-IR820 was developed based on luminescence resonance energy transfer (LRET), where the luminescent ions (Tm) were secured in a intermediate layer of a core-middle-shell UCNPs structure, which could be quenched by the specific distance external adjacent receptor molecule (cyanine NIR dye IR820). In the presence of the analyte ClO, the double bonds of IR820 were oxidized, unable to absorb the luminescence from the core-middle-shell "sandwich" UCNPs, thus an "off to on" luminescence was restored. This core-middle-shell design could effectively enhance the effect of LRET-based detection strategies through physically meaningful distance constraints, providing new ideas for the design of future UCNP-based NIR fluorescent probes.