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 (λexc = 980 nm) or at vanadate groups (λexc = 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 [3F2,3 → 3H6 (λ = 700
nm)/1G4 → 3H6 (λ
= 475 nm)] or small [3F2,3 → 3H6 (λ = 700 nm)/1G4 → 3F4 (λ = 650 nm)] spectral gaps and emission
wavelengths at the first near-infrared biological window [3F2,3 → 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.