Multifunctional Titanium-Rich Gadolinium Titanate Nanophosphors: A Leap toward Advanced Optical Thermometry and Flexible NIR LED-Based Phototherapy

Upconversion (UC) materials are rapidly emerging as promising next-generation platforms for advanced optical technologies. Herein, multifunctional integration of TiO₂-Gd₂Ti₂O₇ (TGT):Er³⁺/Yb³⁺ UC nanophosphors via a facile wet-chemical technique is reported. As-prepared hollow brick-like particles, initially in a carbonate hydrate phase, transformed into nanoparticles with a cubic pyrochlore structure upon calcination. These nanophosphors display a dominant far-red emission alongside a moderate green emission under 980 nm laser excitation. Among different compositions, the TGT:1Er³⁺/10Yb³⁺ nanophosphors have shown excellent temperature sensing capabilities, achieving maximum absolute and relative sensitivities of 0.0048 K⁻¹ at 353 K and 0.78% K⁻¹ at 298 K for thermally coupled (TC) states, and 0.0017 K⁻¹ at 433 K and 1.35% K⁻¹ at 298 K for non-thermally coupled (N-TC) states. The study reveals that laser-induced heating has a negligible effect on N-TC-based sensing, whereas low pump powers are preferable for TC-based sensing. The multifunctionality of these nanophosphors is further established by fabricating a flexible phosphor film integrated into an NIR LED for horticultural lighting and phototherapy, and a far-red emitting UC ink for anti-counterfeiting. These findings underscore the potential of TiO₂-rich Gd₂Ti₂O₇:Er³⁺/Yb³⁺ nanophosphors as a versatile platform for advanced sensing, photonics, and security applications.