The correlations between temperature sensitivity (relative sensitivity Sr and absolute sensitivity Sa) and thermally coupled level gaps (ΔE) are vital but less-studied for potential applications in scientific research, industrial production, clinical medicine, and so on. We take YbPO4:Ln3+ (Ln = Tm3+, Ho3+, and Er3+) up-conversion phosphors as a case to study the relationships between temperature sensitivity (Sr, Sa) and ΔE. The results of various discussions, including the experimental data of temperature sensitivity based on YbPO4:Ln3+ (Ln = Tm3+, Ho3+, and Er3+) and theoretical derivation from original formulas, show that Sr and ΔE are linearly positive correlation, which is invalid for Sa. Noticeably, YbPO4:Tm3+ nanoparticles display intense near infrared red emission within the biological window, leading to great potential application in biological sensing and biological imaging. All the research studies would benefit the design of optical temperature sensing.

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