Upconverting mixed emitter nanocomposites as sensitive luminescent thermometers within plant-inspired artificial fliers

The applicability of current upconverting lanthanide-doped luminescent thermometers is limited by signal discriminability and thermal sensitivity. We overcome these limitations by creating fluorescent nanocomposites in biodegradable polyhydroxyalkanoates (PHAs). Nanocomposites that combine different lanthanide-based upconverting nanoparticles were designed. We created mixed emitter composites with bright red (Mn²⁺ doped with Er³⁺ and Yb³⁺ in NaYF₄), green (Er³⁺ and Yb³⁺ in BaYF₅) and blue (Tm³⁺ and Yb³⁺ in CaF₂) emitting particles to obtain clearly distinguishable and intense fluorescence signals. The resulting nanocomposites had maximum relative thermal sensitivities of 34% K^-1, outperforming existing thermometers. Importantly, their readout requires detection only in visible wavelength ranges, making them particularly suitable for drone-based environmental monitoring purposes. To demonstrate their applicability in this field, we integrated the nanocomposites into plant-inspired artificial fliers, creating self-deployable and biocompatible units for wireless monitoring of environmental temperature. The surface temperature of topsoil was reconstructed based on the fluorescence intensity ratio among the RGB (red-green-blue) wavelengths of the upconverting nanocomposites integrated into the fliers.