Colloidal Rare Earth Vanadate Single Crystalline Particles as Ratiometric Luminescent Thermometers

Thulium/ytterbium-doped yttrium vanadate particles provide a ratiometric thermal response as both colloids and powders via downshift or upconversion emissions. Here, we synthesized yttrium vanadates by controlled colloidal conversion of hydroxycarbonate precursors. A protected annealing process yielded single crystalline and readily dispersible particles that were manipulated individually by optical tweezers in water. Because individual particles displayed detectable emissions, this system has potential applications as a single-particle luminescent temperature sensor. Excitation on Yb³⁺ sensitizers (λ_exc = 980 nm) or at vanadate groups (λ_exc = 300 nm) resulted in Tm³⁺ emissions that effectively correlated with the temperature of the sample from 288 to 473 K with high relative thermal sensitivity (0.8-2.2% K^-1), one of the highest reported for vanadate nanocrystals so far. Different pairs of Tm³⁺ transitions afford a ratiometric thermal response, which fitted common sensing requirements such as large [³F_2,3 → ³H₆ (λ = 700 nm)/¹G₄ → ³H₆ (λ = 475 nm)] or small [³F_2,3 → ³H₆ (λ = 700 nm)/¹G₄ → ³F₄ (λ = 650 nm)] spectral gaps and emission wavelengths at the first near-infrared biological window [³F_2,3 → ³H₆ (λ = 700 nm)/³H₄ → ³H₆ (λ = 800 nm)]. Our findings open new perspectives for the use of luminescent nanothermometers with controllable spatial localization, which is a remarkably interesting prospect to investigate microscopically localized events related to changes in temperature.