Facile synthesis of mono-disperse sub-20 nm NaY(WO 4 ) 2 :Er 3+ ,Yb 3+ upconversion nanoparticles: a new choice for nanothermometry

We report a novel optical nanothermometer based on ultra-small NaY(WO 4 ) 2 :Er 3+ ,Yb 3+ (NYW:Yb,Er) upconversion nanoparticles (UCNPs). The monodisperse, diamond-shaped sub-20 nm NYW:Er,Yb UCNPs with an average size of ∼8.5 × 12.5 nm were prepared via a facile one-step thermolysis protocol. Upon 980 nm laser excitation, the NYW:Er,Yb UCNPs yield bright green UC emission. The maximum relative sensitivity ( S rel ) and absolute sensitivity ( S abs ) were determined to be ∼1.2% K −1 at 293 K and ∼0.9% K −1 at 503 K, respectively. The excellent repeatability of fluorescence intensity ratio (FIR) and low temperature uncertainty Δ T min of ∼0.4 K at 293 K make them promising for optical nanothermometry covering a wide temperature range of 293–503 K.