Engineering excited state absorption based nanothermometry for temperature sensing and imaging.

Current luminescence nanothermometry exploits either temperature dependent quenching, temperature dependent energy transfer or thermal equilibrium between two metastable emitting levels, which are quantified to convert spectral features into absolute temperature. Although widely used and feasible, these methods are not always reliable enough in terms of flexibility, optimum temperature operating range and often require relatively complicated and expensive detection instrumentation, which may hinder wider adoption of luminescence based nanothermometry in technology and biomedical sciences. Therefore, not only more sensitive, brighter and robust phosphors are sought, but also novel temperature sensing schemes, which may potentially simplify remote quantification and imaging of temperature. In this work, we demonstrate the concept of contactless temperature readout and 2D temperature mapping by using excited state absorption (ESA) process instead of conventional approach based on ground state absorption (GSA) combined with multi-colour emission. The analysis of the excitation spectra of LiLaP4O12:Eu3+ nanocrystalline powders in a wide temperature range confirmed that the probability of populating higher levels of the ground 7FJ multiplet increases at increased temperatures. The Single Band Ratiometric Luminescent Thermometry (SBR-LT) opens new possibilities and offers luminescent thermometry at single emission band (5D0 → 7F1) under different excitation lines (7F2,3,4 → 5D0). In consequence, technically simple, temperature range adjustable, fast and affordable optical temperature imaging can be performed with high sensitivity reaching over 2.17% per °C in an unprecedentedly wide temperature range from -150 to 400 °C.