Utilizing Energy Transfer in Mn²⁺/Ho³⁺/Yb³⁺ Tri-doped ZnAl₂O₄ Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry

Lanthanide (Ln³⁺)-doped upconversion (UC) phosphors converting near-infrared (NIR) light to visible light hold very high promise toward biomedical applications. The scientific findings on luminescent thermometers revealed their superiority for noninvasive thermal sensing. However, only few reports showcase their potential for applications in extreme conditions (temperatures below -70 °C) restricted by low thermal sensitivity. Here, we demonstrate the tailoring of luminescence properties via introducing Ho³⁺-Mn²⁺ energy transfer (ET) routes with judicious codoping of Mn²⁺ ions in ZnAl₂O₄/Ho³⁺,Yb³⁺ phosphor. Preferentially, a singular red UC emission is required to improve the bioimaging sensitivity and minimize tissue damage. We could attain UC emission with 94% red component by a two-photon UC process. Higher temperature annealing brings the color coordinates to the green domain, highlighting the potential for color-tunable luminescence switch. Moreover, this work investigates the thermometric properties of ZnAl₂O₄/Yb³⁺, Ho³⁺ in the range of 80-300 K and influence of inducing extra ET pathways by Mn²⁺ codoping. Interestingly, the luminescence intensities for nonthermally coupled (⁵F₄,⁵S₂) and the ⁵F₅ radiative transitions of Ho³⁺ ions display opposite behavior at 80 and 300 K, which revealed competition between temperature-sensitive decay pathways. The codoping of Mn²⁺ ions is fruitful in causing a fourfold increase of absolute sensitivity. Notably, the color tunability from green through yellow to red is helpful in rough temperature estimation by naked eyes. The maximum relative (S_r) and absolute sensitivities (S_a) were estimated to be 1.89% K^-1 (140 K) and 0.0734 K^-1 (300 K), respectively. Even at 80 K, a S_a of 0.00447 K^-1 and S_r of 0.6025% K^-1 were achievable in our case, which are higher than most of the other Ln³⁺-based systems. The above-mentioned results demonstrate the potential of ZnAl₂O₄/Yb³⁺,Ho³⁺ for cryogenic optical thermometry and a strategy to design new Ln³⁺-based UC thermometers by taking advantage of ET routes.