Reproducible Single-Droplet multiplexed detection through Excitation-Encoded Tri-mode upconversion solid sensors

Upconversion multiplexed optical sensors are promising in clinical diagnostics, molecular mapping and environmental monitoring. However, traditional co-doped emitter sensors and mixture sensors show poor sensitivity and reproducibility due to the strong cross-relaxation quenching and instability. Herein, upconversion multi-shell nanoparticles are, for the first time, applied as excitation-controlled nanosensors for reproducible and channel-independent multiplexed detection. The multi-shell structure favors the excitation-encoded reproducible blue/green/red fluorescence signals from different upconversion shells in a single upconversion nanosensor, avoiding using instable upconversion nanoparticle mixtures for multiplexed detection. A reproducible and self-cleaning solid tri-mode sensor is constructed by the upconversion multi-shell nanoparticles coated with a thin superhydrophobic SiO₂ layer. The solid sensor shows superior single-droplet (≈10 µL) multiplexed detection capability with a low detection limit for dye molecules (∼0.1 μg/mL), high stability and excellent recyclability (more than 100 cycles). This work provides an intriguing strategy to achieve channel-independent, reproducible UC multiplexed sensors for simultaneous detection of multi-analytes.