Upconversion luminescence and photothermal properties of the NaGdF4 :Er3+ /Yb3+/Al3+ nanoparticles in the hybrid films

<title>Abstract</title> <p> Upconversion nanoparticles (NPs) play an important role in photothermal therapies, imaging techniques and photothermal sensing. However, simultaneous improvement of the fluorescence intensity, thermometric sensitivity is still a challenge. The coupling of Ag NPs and gold nanorods (GNR), photonic crystals (PCs) and single-walled carbon nanotubes (SWCNT) may be an effective way to improve their photothermal performance, the upconversion luminescence (UCL) and thermometry sensitivity. In this work, based on NaGdF ₄ :Er ³⁺ /Yb ³⁺ /Al ³⁺ NPs (UC), the hybrid films of PCs/Ag/UC, PCs/Ag/SWCNT/UC and PCs/Ag/GNR/UC were fabricated, the effect of the GNR and SWCNT on the UCL and the temperature sensing behavior has been investigated based on the fluorescence intensity ratio (FIR) technique. The results indicate that the PCs/Ag/UC and PCs/Ag/SWCNT/UC samples exhibit the strongest and weakest UCL intensities, respectively. The local electromagnetic field caused by the PCs effect and plasmonics not only can enhance the UCL intensity, but dramatically create the thermal self-amplification effect. Overlapping of the strong absorption band for the GNR and SWCNT with the excitation band of the upconversion NaGdF ₄ :Er ³⁺ /Yb ³⁺ /Al ³⁺ NPs is responsible for the decrease of the UCL intensity compared with the PCs/Ag/UC sample, and the mechanisms of fluorescence quenching is clarified. However, the photothermal conversion performance and absolute sensitivity (S _A ) are improved after adding the SWCNT and GNR in the PCs/Ag/UC, respectively. The PCs/Ag/GNR/UC sample exhibits the optimal comprehensive performance of upconversion luminescence and photothermal properties. This work presents a route for enhancing the comprehensive performance of upconversion thermometers through multiphysics <bold/> coupling. </p>