Towards ultra-high sensitive colorimetric nanothermometry: Constructing thermal coupling channel for electronically independent levels

Accurate intracellular thermal reading calls for luminescent nanothermometer with high temperature sensitivity in biological temperature region. Fluorescence intensity ratio method based on the conventional thermally coupled levels pair has been widely applied to thermal sensing. However, the narrow energy spacing between the thermally coupled levels, which is subject to the Boltzmann population distribution rule, leads to low relative temperature sensitivity in biological temperature region as well as inferior signal discriminability. By constructing thermal coupling channel for electronically independent levels, the restriction for narrow energy spacing could be broken by applying thermal coupling mechanisms unbound to Boltzmann population distribution rule. Herein, we show for the first time that the green H-2(11/2) and S-4(3/2) emitting states of Er3+ can be thermally coupled with the red F-4(9/2) emitting one via the T-4(1) state of Mn2+, through a distance-sensitive exchange interaction between Er3+ and Mn2+ in KMnF3 where lattice thermal expansion plays a vital role. As a consequence, a promising colorimetric up conversion luminescent nanothermometer working in the biological temperature region (303 K-343 K) with ultra-high sensitivity and excellent signal discriminability is achieved. This work provides a novel route for designing delicate luminescent nanothermometer applicable in intracellular thermal sensing and imaging. (C) 2017 Elsevier B.V. All rights reserved.