1 Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States.
2 The Institute for Lasers, Photonics and Biophotonics, State University of New York at Buffalo, Buffalo, New York 14260, United States.
3 Department of Chemistry and Biochemistry and Center for Nanoscience Research, Concordia University, 7141 Sherbrooke Street W., Montreal, QC H4B 1R6, Canada.

Emission bands from thermally coupled states in lanthanide-doped nanoparticles have been studied for ratiometric nanothermometry in biological applications. Unfortunately certain factors such as water absorption distort the intensity, limiting the accuracy of ratiometric nanothermometry. However, the decay time of such states does not suffer from such distortions. We introduce the decay time of the ³H₄ state in Yb³⁺, Tm³⁺-doped nanoparticles for improved nanothermometry. The strong 800 nm upconversion emission exists in the first biological transparency window. This is the first use of a single upconversion band for lifetime nanothermometry.