Rare-earth-doped GdPO₄ nanoparticles have recently attracted much scientific interest due to the simultaneous optical and magnetic properties of these materials and their possible application in bio-imaging. Herein, we report the hydrothermal synthesis of GdPO₄:Eu³⁺ nanoparticles by varying different synthesis parameters: pH, : molar ratio, and Eu³⁺ concentration. It turned out that the Eu³⁺ content in the synthesized nanoparticles had little effect on particle shape and morphology. The synthesis media pH, however, has showed a pronounced impact on particle size and distribution, i.e., the nanoparticle length can be adjusted from hundreds to tens of nanometers by changing the pH from 2 to 11, respectively. Increasing the : molar ratio resulted in a decrease in nanoparticle length and an increase in its width. The temperature-dependent measurements in the 77–500 K range revealed that the GdPO₄:50%Eu³⁺ sample maintains half of its emission intensity, even at room temperature (TQ_1/2 = 291 ± 19 K).

Current efforts to improve photodynamic therapy focus on nanomaterials that integrate deep tissue imaging with efficient reactive oxygen species generation. Gold nanoclusters (Au NCs) are promising alternatives to conventional photosensitizers due to their effective ROS production and enhanced biocompatibility when stabilized by a protein corona. However, both photosensitizers and Au NCs are typically activated by ultraviolet or visible light, which cannot penetrate deeper into tissues and is limited to superficial applications. Here, we report a near-infrared (NIR)-activated photodynamic nanoplatform based on core–shell upconverting nanoparticles (UCNPs; NaGdF₄:Yb³⁺,Er³⁺@NaGdF₄:Yb³⁺,Nd³⁺), functionalized with a protein corona containing bovine serum albumin-stabilized Au NCs (BSA–Au NCs) and photosensitizer chlorin e6 (Ce6). Spectroscopic data confirmed the formation of the UCNP-BSA–Au-Ce6 nanoplatform and demonstrated 32% energy transfer efficiency from UCNPs to Ce6, resulting in efficient reactive oxygen species generation under 808 nm irradiation. Cellular experiments confirmed the effective internalization and optimal biocompatibility of the nanoplatform in human breast cancer and healthy cells. Upon irradiation at 808 nm, the nanoplatform significantly reduced the viability of MDA-MB-231 cancer cells. These findings indicate that the UCNP-BSA–Au-Ce6 nanoplatform couples NIR activation with enhanced singlet oxygen production, providing a multifunctional platform for deep tissue imaging and NIR-activated photodynamic therapy.