Tuning the thermal sensitivity of β-NaYF4: Yb3+, Ho3+, Er3+ nanothermometers for optimal temperature sensing in OTN-NIR (NIR II/III) biological window

Our group reported the synthesis of a β-NaYF4: Yb3+, Ho3+, Er3+ nanothermometer operating in the NIR-II/III region by the luminescence intensity ratio (LIR) approach. It was shown that the sensing mechanism is defined by the surrounding medium, though luminescent thermometry properties were not quantitatively assessed, yet. In the current framework, we optimized the luminescent thermometry properties by systematically studying the impact of nanoparticle (NP) composition and size on the temperature-dependent spectroscopic properties. The ratio between the Ho3+ (NIR-II) and Er3+ (NIR-III) luminescent bands was adjusted by the Ho3+ dopant concentration. Nanothermometers with larger areas responded stronger to temperature changes and exceptionally high S were found for all NP compositions and sizes in water, indicating a higher sensitivity of the Er3+ emission towards temperature changes. Our current work illustrates how NP characteristics affect their luminescent thermometry properties, building on this, thermal sensitivities comparable to UC nanothermometers were reached for nanothermometers operating in the NIR-II/III region.