Exploiting the Yb³⁺ and Er³⁺ Codoped β-NaYF₄ Nanoparticles as Luminescent Thermometers for White-LED-Free Thermal Sensing at the Nanoscale

Luminescent ratiometric technology has been regarded as one of the most promising methods for temperature measurement, as it enables noncontact thermal sensing with minimal disturbance to the object of interest. Particularly, it, as expected, is free from the influences of many surrounding factors. However, we demonstrate here that, in some cases, this technology is under the influence of white LED that are commonly used in our daily life and industrial processes, considering the fact that these involved emitting lines used for thermal sensing are overlapped severely with the emitting spectrum of white LED. It is found that using the traditional green upconversion (UC) luminescence emanating from Er³⁺, namely, the ²H_11/2/⁴S_3/2-⁴I_15/2 transitions, for thermal sensing leads to a very large temperature error, up to 17 at 303 K in the case where there is the influence of white LED. While the two violet UC luminescence bands, separately originating from the ⁴G_11/2/²H_9/2-⁴I_15/2 transitions of Er³⁺ embedded in the β-NaYF₄:40% Yb³⁺, 2% Er³⁺ nanoparticles, are capable of enabling white-LED-free thermal sensing in that these two emission bands are absent from the emitting spectrum of white LED. Our work is likely to provide a new perspective for the study of luminescent ratiometric technology for thermal sensing. Most importantly, it presents a strategy for white-LED-free thermal sensing when the influence of white LED cannot be ignored in practical applications.