Luminescence Properties Of Rare Earth Doped Upconversion Nanocrystalline @Au Multifunctional Composites

Upconversion materials have broad application prospects in fields such as displays,solar cells,biomedical imaging,biological diagnosis and treatment.Due to the low efficiency and weak intensity of upconversion fluorescence,the practical applications of these materials are severely restricted.In recent years,surface plasmon-enhanced fluorescence or Raman scattering using noble metal nanoparticles or nanorods have been extensively investigated.However,the preparation and the optical enhancement effect of ultrathin gold nanoshells with localized surface plasmon have been rarely reported.At present,there are still many problems such as large nanoparticle size,weakened fluorescence intensity,incomplete core-shell structure,weak penetrating capability,and unclear mechanism of surface plasmon enhancement/annihilation.Therefore,it is particularly important to further study the mechanism of surface plasmon interaction with the nanocrystal-targeting ligand system and provide an effective way for the upconversion luminescence performance improvement.In this paper,the in-situ reduction method was successfully used to prepare the uniform and ultra-thin gold shell-coated NaYF₄:Yb,Er@SiO₂@Au?NSA?nanostructures.The results of XRD,TEM,EDX,HRTEM-HAADF,mapping and absorption spectra indicated that the average thickness of SiO ₂ shell and nanogold shell were about 5 nm and 2 nm,respectively.Under a continuous laser excitation of 980 nm,the dependence of the upconversion fluorescence intensity of the core-shell structure on the concentration of chloroauric acid was investigated systematically.The steady-state spectral results showed that the red and green upconversion luminescence intensities of NS samples were 2.8 times enhanced simultaneously by coating the gold shell.The upconverting NSA nanostructures co-doped with Fe³⁺and Ho³⁺exhibited selective fluorescence enhancement at 540 nm,while co-doped with Fe³⁺and Tm³⁺at 808 nm,meeting the size and frequency(NIR_in-NIR_out)requirements in biomedical application.The mechanism of enhanced upconversion fluorescence by the surface plasmons was discussed based on the spectral analysis and FDTD simula tion method.The comparison of the electric field intensity distribution of the gold nanoshell and its solid counterpart was studied.After increasing the HAuC l₄ concentration,the maximum fluorescence intensity of upconversion luminescence were about 28 times and 5 times enhanced simu ltaneously stemming from the enhancement of electromagnetic field induced by external surface plasmon resonance of UCNPs and the synergistic effect of internal Fe³⁺codoping.By analyzing the up-conversion fluorescence dynamic s,the mechanism of enhanced upconversion fluorescence by surface plasmons was discussed.In addition,experimental results of cytotoxicity and in vitro near-infrared UC L imaging demonstrated that anti-EGFR-biofunctionalized NSA nanocomposites exhibited good biocompatibility for healthy cells and diagnostic potential for malignant cells.This provides a promising nanocomposite for fluorescent bioimaging,X-ray radiation therapy and photothermal therapy for deep cancers.In addition,the problem of environmental pollution and the energy crisis have become increasingly serious.People are very concerned about the development trend of new energy sources,especially the research of solar cells.We developed an integrated photoanodes with nanostructures of Mn²⁺,Yb³⁺,and Er³⁺triple doped NaYF₄/TiO₂?UCNPs/TiO₂?.The maximum conversion efficiency of 4.75%was achieved with the optimized composition of 8 wt%UCNPs/TiO₂.This study provides favorable conditions for further research on solar cell efficiency.