Dual-Quenched, Dye-Sensitized Upconversion Nanoprobe Combines LRET and FRET to Break the Signal-to-Background Limit

Lanthanide-doped upconversion nanoparticles (UCNPs) are widely utilized in biosensing, owing to their exceptional photophysical properties. Conventional UCNP probes based on luminescence resonance energy transfer (LRET) show an "on-off-on" signal. However, this results in limited signal-to-background ratio (SBR) because only upconversion emitters near the surface of three-dimensional UCNPs can be fully quenched by LRET. To break through the bottleneck, we propose a novel "dye sensitization-dual quench-recovery" mode to construct a <i>de novo</i> UCNP probe. The organic dye IR783 is first modified onto the surface of UCNPs and functions as a sensitizer to significantly improve the luminescence of UCNPs. Next, CoOOH nanoflakes are modified onto UCNPs and serve to quench both UCNPs by LRET and surface-modified dyes by fluorescence resonance energy transfer (FRET). This simultaneous dual-quenching of both sensitizers and emitters significantly improves the overall quenching efficiency. The recovery of both IR783 and emitter, followed by total luminescence of UCNPs, was successfully achieved when CoOOH nanoflakes are reduced by ascorbic acid (AA). Total quenching efficiency is as high as ∼96.4% with the detection SBR of ∼20 for AA. Our UCNP-based probe was successfully applied for subsequent detection of AA in different complex samples. Therefore, we can conclude that this is a promising strategy with broad applications for the detection of different analytes.