Rare-earth-based fluorescence intensity ratio (FIR) thermometry has attracted considerable attention due to its non-contact operation, rapid response, and high sensitivity. However, enhancing thermal sensitivity, particularly in the visible emission region, remains a key challenge. Herein, we demonstrate a significant enhancement in optical thermometric performance through Bi3+ doping in ultrasmall SrF2: Yb3+/Tm3+ (22% Yb3+, 0.5% Tm3+) upconverting nanoparticles synthesized via a hydrothermal route. The obtained nanoparticles are uniform and spherical with a narrow size distribution. Systematic temperature-dependent studies (283–333 K) reveal that Bi3+ incorporation strongly modulates the upconversion emission dynamics, leading to markedly improved FIR thermometry performance. Among the investigated transitions, the visible blue-emitting levels exhibit substantially higher thermal responsiveness compared to near-infrared counterparts. In particular, the FIR based on Tm3+ transitions shows optimal performance at 1.5% Bi3+ doping, achieving a maximum relative sensitivity of 1.171% K−1 at 283.15 K. Notably, the blue emission-based thermometric response is more than four times higher than times the corresponding near-infrared value of 0.293% K−1. This blue ratio is treated as an empirically calibrated, non-thermally coupled-level thermometer. These findings establish Bi3+-doped SrF2: Yb3+/Tm3+ nanoparticles as a highly promising platform for high-sensitivity optical nanothermometry and highlight the previously underexplored potential of blue upconversion emission for precision temperature sensing.
