Spectroscopic characterization of laser-induced luminescence for remote environmental thermometry

Lanthanide-doped upconversion microparticles (UCMP) enable composites for luminescence thermometry with long luminescence lifetime and narrowband absorption and emission spectra. Being non-toxic, easily synthesizable, and having a bright, stable emission makes them an attractive candidate for in-vivo monitoring of key environmental parameters such as temperature. We use them to create soft, biodegradable, miniaturized seed-like robots endowed with fluorescence tags for the sustainable environmental monitoring of topsoil and air above soil environments. Our aim is an airborne platform with a sufficient signal-to-noise ratio to identify the concentration of targeted soil parameters. Here, we study the photoluminescence of Er, Yb: NaYF₄ UCMPs embedded in polylactic acid (PLA) polymeric matrix to assess their suitability for remote read-out. We assessed the signal-to-noise ratio in terms of excitation intensity, UCMP concentration, working distance, and sample orientation. We evaluated the signal stability over long exposure time as well as for amplitude-modulated excitation. Finally, we carried out ratiometric and lifetime measurements of luminescence emission in order to demonstrate the feasibility of such sensors in measuring the variation of temperature. Overall, the rare-earth doped UCMPs embedded in biodegradable polymer can be used for remote thermometry, displaying a significant signal-to-noise ratio for luminescence emission detection and subsequent derivation of temperature.