Nanocrystalline NIR-to-NIR luminescent thermometer based on Cr3+,Yb3+ emission

A new type of nanocrystalline luminescent thermometer based on luminescent intensity ratio (LIR) between Cr3+ and Yb3+ luminescence intensities is investigated in a wide range of temperatures. The presented temperature sensor under single excitation wavelength at 650 nm is matching d-d Cr3+ absorption, exhibits emission readout from d-d Cr3+ and f-f Yb3+ bands in optical window of biological tissues and its sensitivity to temperature changes reaches around 0.32%/K at physiological temperature range. The mechanism of population of excited states of Yb3+ was discussed in terms of reabsorption of Cr3+ emission by the co-doping Yb3+ ions. Therefore, thermal quenching of Cr3+ emission intensity concurrently leads to reduced Yb3+ excited state population, and thus diminish sensitivity of such non-contact thermometer as compared to another Cr3+ thermometer, which related Cr3+ emission intensity to barely temperature sensitive Nd3+ emission. For this reason, if only the Cr3+ and Yb3+ were photoexcited individually at 650 and 920 nm respectively, and emission intensity ratio of such excited Cr3+ and Yb3+ was used as LIR, the sensitivity improved 3-fold and reached 1.2%/K. Moreover concentration studies revealed that, beside absolute Cr3+-to-Yb3+ LIR value, the content of Cr3+ ions does not affect neither usable temperature range nor sensitivity of these nanothermometers.