Ultrasensitive Thermochromic Upconversion in Core-Shell-Shell Nanoparticles for Nanothermometry and Anticounterfeiting.

Upconversion nanoparticle based ratiometric nanothermometry has shown many advantages including high relative sensitivity, fast temperature response, and high spatial resolution. However, most of the existing designs are on the basis of thermally coupled upconversion emissions, and it remains a challenge to improve the thermo-sensitivity. Here, we report a new nanoplatform of NaYF:Yb/Er/Ce@NaYF@NaYF:Yb/Tm core-shell-shell nanostructure to improve the thermal sensitivity through the nonthermally coupled upconversion emissions. With the increase of temperature, the green upconversion of Er shows a decline while the blue upconversion of Tm exhibits a rapid increase, leading to a huge contrast in both intensity ratio and emission colors. The maximum relative sensitivity can reach up to 9.86% K at 303 K. It is further found that introducing Ce is able to improve the sensitivity and expand the thermochromic green-to-blue gamut greatly. These results show great potential in ultrasensitive lanthanide-based nanothermometry and anticounterfeiting.