Luminescent thermometry in NaSrY(MoO4)3:Tm3+/Yb3+: achieving high thermal sensitivities across the visible and near-infrared-I/III windows

Abstract

Remote optical thermometry has garnered significant attention due to its non-contact nature, high sensitivity, and rapid response capabilities, which are essential for industrial and biomedical applications. Lanthanide-doped luminescent materials, particularly Tm³⁺-based systems, represent promising candidates for such thermometers owing to their tunable, temperature-dependent emissions and strong luminescence across biological transparency windows. Herein, Tm³⁺/Yb³⁺ co-doped NaSrY(MoO₄)₃ (NSYM) phosphors were synthesized via a sol–gel method. Under 975 nm excitation, the material exhibits upconversion emissions at 487 nm (¹G₄ → ³H₆) and 693 nm (¹G₄ → ³F₄), alongside near infrared (NIR) emissions at 797 nm (³H₄ → ³H₆; NIR-I) and 1625 nm (³F₄ → ³H₆; NIR-III). Additionally, it shows a band at 1450–1550 nm (³H₄ → ³F₄; NIR-III) enabling simultaneous operation across the visible spectrum as well as the first and third biological transparency windows. The luminescence intensity ratios (LIRs) of 693/487 nm (LIR3), 693/663 nm (LIR2), and 1625/1500 nm (LIR8) yield exceptional relative thermal sensitivities of 1.84% K⁻¹, 1.90% K⁻¹, and 0.82% K⁻¹, respectively. Notably, LIR3 operates within the third biological window (NIR-III), where tissue penetration is maximized, rendering it particularly valuable for deep-tissue applications. The system demonstrates temperature uncertainties as low as 0.4–0.5 K over the 297–356 K range, significantly outperforming most of the reported luminescent nanothermometers. Furthermore, strong NIR-III emission under low excitation power underscores the potential of NSYM:Tm³⁺/Yb³⁺ for deep-tissue imaging, optical signal amplification, and non-invasive biological thermometry. These results establish Tm³⁺-based phosphors as highly promising platforms for next-generation optical thermal sensors in biomedicine.