Programming Ratiometric Thermometric Windows in Polymeric Nanothermometers through Dual Thermoresponsiveness and Sequential Colloidal Evolution

Precise temperature mapping in microscopic biological environments requires fluorescent nanothermometers with a well-defined and application-matched sensing window. However, the operating range of conventional thermoresponsive polymer-based nanothermometers is typically governed by a single lower critical solution temperature (LCST) transition, limiting the rational programming of their functional window. Here, we report a dual-thermoresponsive diblock fluorescent copolymer (DTBFC) that enables programmable ratiometric thermometry through the intrinsic LCST tunability of thermoresponsive polymers. In this system, two independently adjustable LCTSs (LCST 1 and LCST 2 ) define the lower and upper boundaries of the thermometric window. The thermoresponsive fluorescent donor block (FDB) undergoes a coil-to-globule transition near LCST 1, initiating donor activation and micellization. The acceptor block (fluorescent acceptor block, FAB) then undergoes a gradual contraction between LCST 1 and LCST 2, continuously modulating donor–acceptor distance and regulating fluorescence resonance energy transfer (FRET) efficiency. This sequential structural evolution from hydrated chains to hierarchical aggregates establishes a direct relationship between polymer phase-transition behavior and optical output. By tuning the composition-dependent LCST 2 of the acceptor block, the thermometric window can be predictably shifted while maintaining a linear ratiometric response and environmental robustness. Overall, this work demonstrates that the intrinsic tunability of LCST-based polymers provides a general design principle for programming thermometric windows through polymer architecture and sequential phase transitions.