Enabling Nonthermally Coupled Upconversion in a Core–Shell–Shell Nanoparticle for Ultrasensitive Nanothermometry and Anticounterfeiting

Luminescence intensity ratio (LIR)-based thermometry has the advantages of high relative sensitivity, fast temperature response, and high spatial resolution. However, the current LIR-based systems are mainly based on thermally coupled energy levels, which have low sensitivity due to the intrinsic limitation of the Boltzmann distribution theory. Here, we report a design of a core–shell–shell nanostructure to improve the thermal sensitivity by using the nonthermally coupled upconversion emissions. Ho³⁺ and Tm³⁺ were selected as emitters and spatially separated by an inert interlayer. The upconverted Tm³⁺ emissions show a dramatical thermal enhancement while the Ho³⁺ emissions show a decline with increasing temperature, resulting in a huge LIR (695 nm/645 nm) contrast and thereafter a high relative sensitivity (9.78% K^–1 at room temperature). In addition, this nanostructure design presents a color change from red to blue at different excitation powers and also from red to green by tuning the excitation laser pulse widths. These results hold great potential in the field of noncontact ultrasensitive temperature sensors and multimodel anticounterfeiting.