Ratiometric thermometry is among the emerging applications in phosphor materials. Particularly, the technique of ultrafine-space thermometry has garnered significant attention in bio-imaging. Near-infrared (NIR) light, with its high tissue permeability, serves not only as an excitation source for up-conversion photoluminescence (UCPL) but also induces down-shifting photoluminescence (DSPL) at longer wavelengths. These luminescence mechanisms offer promising avenues for bio-available thermometry. In this study, we focused on highly bio-adaptable yttria-stabilized zirconia (YSZ), Zr0.85Y0.15O1.925, and prepared phosphor materials with yttrium partially substituted by ytterbium (Yb), erbium (Er), and thulium (Tm) using a hydrothermal reaction method. The synthesized YSZ:Yb–Er/Tm phosphors with different Yb3+ contents showed multi-line UCPL in the visible to NIR region due to Er3+ and Tm3+ ions under the laser irradiation at 980 nm; in particular, it showed strong UCPL at 800 nm originating from Tm3+. Furthermore, under the same excitation conditions, the phosphor exhibited not only UCPL but also DSPL due to 4I13/2 → 4I15/2 transition of Er3+ in the deeper NIR region of 1400–1700 nm. Interestingly, these DSPLs exhibit significant PL enhancement (anti-thermal quenching; anti-TQ) with increasing temperature. The thermometric properties based on the luminescence intensity ratio (LIR) of UCPL, which shows normal thermal quenching, and DSPL, which shows anti-thermal quenching, demonstrated excellent temperature sensitivity (Sr > 3% K−1 @ 283 K) and temperature resolution (δT < 0.1 K @ 283 K). This study suggests that the LIR thermometry technique using UCPL/DSPL, specifically anti-TQ/normal-TQ, can contribute to further advancements in luminescence-based temperature measurement.
