Polymeric nano-thermometer exploiting reverse intersystem crossing: a potential solution to common interferences

Thermometry with spatial resolution from tens to hundreds of nanometers is critically important for applications such as intracellular temperature sensing, microcircuit thermal mapping, and microfluidics. Most fluorescence-based nanothermometry approaches rely on temperature-induced fluorescence quenching, leveraging their non-invasive nature, low cost, and high throughput. However, fluorescence enhancement with increasing temperature offers an attractive and potentially more robust alternative, as it is inherently less sensitive to background fluctuations and external perturbations. In this paper, we present a proof-of-concept nano-thermometry platform based on reverse intersystem crossing (RISC) in a dye-doped polymer nanoparticle as a model system (eosin and pheophorbide A co-doped polystyrene nanoparticle). The photophysical behavior of eosin in different environments was investigated experimentally and theoretically to elucidate the mechanisms governing the temperature response. In particular, the contribution of anionic eosin species in aqueous media was found to be essential for the observed fluorescence enhancement at elevated temperatures. The temperature-dependent emission is attributed to thermally activated RISC from higher-lying triplet states to the first excited singlet state, consistent with either E-type delayed fluorescence or a hybridized local and charge-transfer (HLCT) excited-state character. As a model thermometry probe, the system operates over a biologically relevant temperature range of 10-80 °C, exhibiting a sensitivity of 1.9 ± 0.2% °C^-1, and remains functional at the single-particle level. While eosin serves here as a convenient model dye, this study establishes a general design principle for fluorescence-enhancement-based nanothermometry. The performance of the probe is expected to be further improved through the incorporation of more efficient thermally activated delayed fluorescence (TADF) dyes with optimized RISC rate, triplet yield and photostability, paving the way toward highly sensitive and robust nanoscale temperature sensors.