Photon upconverting nanoparticles for luminescent sensing of temperature

Photon upconverting nanoparticles convert near-infrared into visible light (anti-Stokes emission), which strongly reduces the background of autofluorescence and light scattering in biological materials. Hexagonal NaYF4 nanocrystals doped with Yb3+ as the sensitizer and Er3+/Ho3+/Tm3+ as the activator display at least two emission lines that respond differently to temperature changes. The ratio of the main emission line intensities enables a self-referenced optical readout of the temperature in the physiologically relevant range from 20 to 45 degrees C. Upconverting nanoparticles of the type NaYF4:Yb, Er covered by an inactive shell of NaYF4 are bright and allow for resolving temperature differences of less than 0.5 degrees C in the physiological range. The optical readout of this nanoparticle-based thermometer offers many options for imaging the two-dimensional distribution of temperature.