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 Tm³⁺) within the NaYF₄ 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 (Tm³⁺) 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.