Highly sensitive and stable upconverted nanothermometer based on plasmonic near field assistant thermal coupling energy level population redistribution

• Upconversion fluorescence enhanced by over 1490% via plasmonic array. • Temperature sensitivity boosted by 91% with localized plasmonic heating. • Low-cost PS microsphere template enables large-area plasmonic fabrication. • Self-powered sensor exhibits high stability across varying power and angles. Non-contact and highly sensitive nanoscale temperature measurements are crucial for micro-nano devices, human health monitoring, and disease diagnosis applications. Although optical thermometry using upconversion nanoparticles (UCNPs) has advanced, the low efficiency and significant fluorescence intensity decay with temperature rise became a challenge. This study reported an optical thermometer based on a gold triangle nanoarray (AuTNA) and NaGdF 4 : Yb, Er@NaYF 4 Core–Shell UCNPs (C-S UCNPs) with simultaneously enhanced signal intensity and sensitivity. The near-infrared (NIR) pump of UCNPs can be localized and enhanced within the AuTNA, and local thermal effects induced by plasmon hot carriers amplified the near field temperature further increased the sensitivity. Such a local heat amplification can also result in a population redistribution of thermal coupling energy level which can accelerate the fluorescent intensity ratio (FIR) variation. Compared with bare UCNPs-based thermometer, the fluorescence signal intensity and sensitivity of the proposed thermometer was simultaneously enhanced by over 1450% and 91%. Furthermore, such temperature sensor possessed other merits such as easy fabrication, good stability and cyclicity. The excitation power independent signal enhancement factors and temperature sensitivity make this thermometer promising for future applications in biodetection, heat management in microscale and even low-power threshold environments