NIR to NIR luminescence thermometry in core/multishells-structured nanoparticles operating in the biological window

Optical thermometry via near-infrared (NIR) emitting luminescent nanoparticles has attracted a great deal of attention during the last few years. In particular, nanothermometers operating in the biological windows are being strongly demanded for biomedical applications. In this sense, Tm³⁺ emissions centered around 800 and 1800 nm were explored here for ratiometric thermometry. And a luminescent thermometer based on NaYbF₄/NaYF₄: Tm³⁺-Yb³⁺/NaYF₄ core/shell/shell-structured nanoparticles is designed. Excited by 980 nm laser, saturated emissions from Tm³⁺: ³H₄→³H₆ (~800 nm: first biological window NIR-I) and Tm³⁺: ³F₄→³H₆ (~1800 nm: third biological window NIR-III) transitions are readily obtained owing to the efficient upconversion processes in the core/multishells nanoparticles, finally resulting in the same dependence of NIR emissions on the pumping power. Different thermal response between 800 and 1800 nm emissions of Tm³⁺ ions enables optical thermometry via ratiometric method. In the physiological temperature range, the relative sensitivity S_r and absolute sensitivity S_a achieved here are about 0.33% K⁻¹ and 3.9% K⁻¹, respectively, with a temperature uncertainty of 0.75 K. These results illustrate that the nanothermometer developed in this work would be promising candidate for optical temperature measurement in bio-fields.