Ratiometric highly sensitive luminescent nanothermometers working in the room temperature range. Applications to heat propagation in nanofluidsElectronic supplementary information (ESI) available. See DOI: 10.1039/c3nr02335d

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 Eu 3+ and Tb 3+ chelates. The design of the hybrid host and chelate ligands permits the working of the nanothermometers in a nanofluid at 293320 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' Eu 3+ /Tb 3+ 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. Highly luminescent Eu 3+ /Tb 3+ -based nanothermometers operating in fluids, with temperature uncertainty of 0.4 K and spatio-temporal resolution of 64 m/150 ms.