Preparation Of Rare-earth Upconversion Materials And Its Application In Detecting Hg²⁺ And Pb²⁺ Ions

With the development of modern industry,heavy metal pollution has become a continuous concerning topic globally.Heavy metal ions,such as Hg²⁺,Pb²⁺,Cu2+ and Ba²⁺ etc.,can be accumulated in human and other vivo organisms through the food cycle,even the air.Therfore,it will be of great significance to detect the content of heavy ions accurately.Fluorescence resonance energy transfer（FRET）has been widely applied in the detection of metal ions owing to its high sensitivity and good selectivity.The typical energy donors of FRET,such as organic fluorescent dyes and carbon quantum dots,result of lower sensitivity when detecting biological and complex system due to their strong background fluorescence intensity and short Stokes shift.Rare-earth doped up-conversion nanoparticles（RE-UCNPs）,a new generation of energy donors,exhibit unique properties,such as low toxicity,weak background fluorescence,high sensitivity,deep light penetration depth in tissue,large Stokes shifts,high chemical and good optical stability et al.Rhodamine B and its derivatives（RBD）can be the energy donors of FRET system,owing to their good optical stability and photophsical properties,such as high molar absorptivity and good fluorescence quantum yield and so on.Therefore,the fluorescence probes are constructed by RE-UCNPs combined with RBD.The detection of metal ions Hg²⁺ and Pb²⁺ is realized by investigating the change of the upconversion spectra and UV-visible spectra.Besides,the probes,which can weaken autofluorescence and scattering under the excitation of 980 nm laser,is expected to be utilized for detecting Hg²⁺ and Pb²⁺ ions in biological and complex systems.The major works are summarized as follows:（1）NaYF₄:Yb³⁺,Er³⁺ upconversion nanoparticles（UCNPs）were prepared by thermal decomposition method,the structure,morphology and optical properties of samples were characterized by X-Ray Diffraction（XRD）or Fourier Transform-Infrared（FT-IR）,Field Emission-Scanning Electron Microscope（FE-SEM）or Transmission Electron Microscope（TEM）and Upconversion Luminsence（UPL）,respectively.XRD,TEM and SEM showed that hexagonal and high crystallinity upconversion nanoparticles were obtained,the particles had good dispersibility and uniform size was about 50 nm.FT-IR revealed that UCNPs bear a hydrophobic surface because of a certain amount of oleic acid capped on their surface.The green light was excitated by 980 nm laser irradiated in the cyclohexane solution of UCNPs.（2）The surface of upconversion nanoparticles were modified by SiO₂ coating（abbr.UCNPs@mSiO₂）,UCNPs@mSiO₂-RHO composites were obtained by RHO attached to UCNPs@mSiO₂.Under the excitation of 980 nm laser,the upconversion spectra of UCNPs,UCNPs@mSiO₂ and UCNPs@mSiO₂-RHO showed that there existed the characteristic emission peaks of Er³⁺,they were green light at 545 nm,red light at 660 nm and blue light at 490 nm.After being coated by silica and attached by RHO,the fluorescence intensity of UCNPs gradually decreased,but had no effect on the internal structure of UCNPs.（3）Under the excitation of 980 nm laser,the coordination complex RHO-Hg²⁺ emitted a weak fluorescence at 593 nm.When the concentration of Hg²⁺ ions was 0¹00 μM,the relative fluorescence intensity of green and red upconversion showed a good linear relationship with its concentration.When the concentration of Hg²⁺ ions was increased（0⁸0 μM）,the absorbance intensity of the probe at 550 nm was linearly proportional to it,and the detection limit was 8.7328 μM.The competing ions had little effect on the upconversion luminescence intensity of the probe,which showed that the probe had a good selectivity for Hg²⁺ ions.（4）The UCNPs@mSiO₂-RBH nanoparticles were successfully synthesized and applied in Pb²⁺ ions detection.A weak fluorescence at 593 nm was excitated by 980 nm laser irradiated in the coordination complex RBH-Pb²⁺.While the the fluorescence intensity of green upconversion was decreased,the intensity of green upconversion also showed a good linear relationship with concentration of Pb²⁺ ions（0¹00 μM）.When the Pb²⁺ concentration was increased,the absorbance intensity of the probe at 560 nm was linearly proportional to it,and the detection limit was 4.0951 μM.Hg²⁺ and Cu²⁺ ions had a great effect on the upconversion luminescence intensity of the probe,indicating that the probe had a general selectivity for Pb²⁺ detecting,altering the energy receptor can further improve the probe selectivity for Pb²⁺ ions.