Strategy design for ratiometric luminescence thermometry: circumventing the limitation of thermally coupled levels

The ratiometric fluorescence technique is believed to hold promise as the most important non-contact thermometry technique for future mass application due to the reliability and convenience originating from self-referencing. The discovery of thermally coupled levels in lanthanide ions initiated and boosted the fast development of the ratiometric fluorescence technique for temperature sensing in the past decades. However, the dilemma in the energy spacing between the two thermally coupled levels sets a limitation for further improvement of thermometric performance, which can be addressed by novel strategies other than thermally coupled level routes. The unique electronic structure of Ln(3+) ions offers great opportunities for conceiving such strategies. In this review, we have summarized recent progress in novel strategy design for ratiometric fluorescence temperature sensing, with the focus on the Ln(3+) luminescence involving dual emission construction. Various features of Ln(3+) luminescence dynamics have been described to play critical roles in judicious strategy design.