Construction of 808 nm-activated bifunctional nanoparticles for effective photothermal conversion along with real-time thermal feedback

Photothermal therapy (PTT) integrated with the functionality of temperature self-monitoring has emerged as a prominent research focus in current scientific investigations. In this study, a bifunctional photothermal agent is developed based on the small-size β-NaLuF₄ nanoparticles (NPs) of about 40 nm, which can provide accurate optical thermometry through Nd³⁺-sensitized upconversion (UC) luminescence along with the ability of Cu₂S-mediated photothermal conversion. The small-size β-NaLuF₄ NPs are synthesized by a sequential layer-by-layer coating strategy, resulting in the generation of β-NaYF₄@β-NaLuF₄: 20 % Yb³⁺/2 % Er³⁺@β-NaLuF₄: 20 % Yb³⁺/2 % Nd³⁺ NPs. Then, the temperature sensing performance is realized via the thermally coupled levels of Er³⁺: ²H_11/2/⁴S_3/2 under the excitation of 808 nm wavelength. Whereafter, the above NPs suffer from a surface functionalization process with Cu₂S NPs, endowing the sample with efficient photothermal conversion property, which exhibits an obvious temperature increase from 293 K to 311 K under 808 nm near-infrared (NIR) light irradiation with a power density of 20 mW/mm² for 180 s. Meanwhile, an ex vivo experiment further demonstrates that the present NPs can implement efficient photothermal conversion and accurate optical temperature monitoring simultaneously within the biological tissues, while effectively preventing laser-induced overheating effect due to the weak absorption of water molecules for 808 nm NIR light. These findings highlight the promising potential of the prepared NPs for application in cancer treatment, offering a precise and safe approach to PTT with integrated temperature monitoring.