Study for optimizing the design of optical temperature sensor

The correlations between temperature sensitivity (relative sensitivity S_r and absolute sensitivity S_a) 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 (S_r, S_a) 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 S_r and ΔE are linearly positive correlation, which is invalid for S_a. 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.