Study Of Fluorescence Characteristics And Enhancement Of Upconversion In Water-soluble Er³⁺/Yb³⁺ Co-doped NaLuF₄

With the rapid development of nanotechnology fluorescent probes, drug deliveryand photodynamic therapy, NaLuF4is a new upconversion matrix material that hasseveral of merits such as good light stability, non-toxicity, weak photobleaching, andstrong tissues penetration ability. Therefore, it has the potential applications in thebiomedical field compared with other materials. However, the upconversionfluorescence intensity of the rare earth doped NaLuF4is not strong enough and thedispersion in an aqueous solution is low. These shortcomings limit its application inclinical medicine. The main aim in this paper is to improve the water solubility andupconversion luminescence intensity of the sample by considering the Yb3+/Er3+co-doped NaLuF4as research object.One-pot method is used to prepare NaLuF4:Yb3+/Er3+nanocrystalline materialswith two kinds of crystal phase structures, as well as the upconversion luminescenceproperties of the sample are explored. By changing the NaOH dosage, the amount ofoleic acid, the ratio of reactants F-/Ln3+and the reaction temperature, the control ofnanocrystalline structure can be realized. Subsequently, the upconversion luminescenceproperties of different ratio of reactants F-/Ln3+and reaction temperature is studiedusing the976nm near-infrared light to excite the sample. The results indicate that theupconversion fluorescence intensity reaches a maximum at the ratio of reactantsF-/Ln3+of5or reaction temperature of200℃. Meanwhile, the reasons of fluorescenceenhancement are analysised. We further measure the fluorescence lifetime curve for twocrystal phase structures, and the dependence of the fluorescence lifetime on the crystalphase structure is analyzed.As such, NaLuF4:Yb3+/Er3+/SiO2core-shell composite materials are prepared, andthe on upconversion fluorescence properties are researched. First of all,NaLuF4:Yb3+/Er3+micron bar is prepared by oil heating method, and then SiO2is coatedon the micron rod surface by reverse microemulsion method. The characteristic of thestructure and morphology is tested by XRD and TEM, as well as the water-solubleperformance of the samples is dissected by virtue of Fourier infrared spectroscopy. Byusing near-infrared light with976nm to excitated the test samples, the fluorescenceintensity ratio of the red and green light beams changes with the pump power increases.This phenomenon can be explained by combining double logarithmic curve with thefluorescence lifetime.Finally, Sc3+ion enhanced the upconversion fluorescence intensity ofNaLuF4:Yb3+/Er3+nanocrystals and microrods are studied. Different concentrations of Sc3+are doped into NaLuF4:Yb3+/Er3+nanocrystals and microrods by one-pot methodand oil heating method. The upconversion luminescence characteristics of nanocrystalsand microrods with various Sc3+doping concentration are explored by employingnear-infrared light with976nm to excitate the two proposed samples. On the one hand,the fluorescence intensity ratio of the red and green light beams improves with thedoped concentration of Sc3+enhances. This mechanism is further analyzed by measuringXRD spectra and upconversion fluorescence spectroscopy. On the other hand, theupconversion luminescence intensity of the micron bar reaches to two times when thedoped concentration of Sc3+is20%. This is because the doped Sc3+ion decreases thesymmetry of the crystal field around Er3+, and makes the non-radiative transitionprobability reduction possible. By measuring XRD spectra and fluorescence lifetimecurve, the enhanced mechanisms of fluorescence upconversion are discussed.