Ratiometric highly sensitive luminescent nanothermometers working in the room temperature range. Applications to heat propagation in nanofluids

Abstract

There is an increasing demand for accurate, non-invasive and self-reference temperature measurements as technology progresses into the nanoscale. This is particularly so in micro- and nanofluidics where the comprehension of heat transfer and thermal conductivity mechanisms can play a crucial role in areas as diverse as energy transfer and cell physiology. Here we present two luminescent ratiometric nanothermometers based on a magnetic core coated with an organosilica shell co-doped with Eu³⁺ and Tb³⁺ chelates. The design of the hybrid host and chelate ligands permits the working of the nanothermometers in a nanofluid at 293–320 K with an emission quantum yield of 0.38 ± 0.04, a maximum relative sensitivity of 1.5% K⁻¹ at 293 K and a spatio-temporal resolution (constrained by the experimental setup) of 64 × 10⁻⁶ m/150 × 10⁻³ s (to move out of 0.4 K – the temperature uncertainty). The heat propagation velocity in the nanofluid, (2.2 ± 0.1) × 10⁻³ m s⁻¹, was determined at 294 K using the nanothermometers' Eu³⁺/Tb³⁺ steady-state spectra. There is no precedent of such an experimental measurement in a thermographic nanofluid, where the propagation velocity is measured from the same nanoparticles used to measure the temperature.