Syntheses And Properties Of Lanthanide-doped Fluoride Nanomaterials As Luminescent Biolabels

The use of luminescent labeling agents has greatly assisted the studies in the field of biology. Conventional luminescent labeling agents such as organic dyes and quantum dots (QDs) have several limitations caused by their intrinsic properties such as, for example, broad emission profiles, poor photochemical stability, intermittent on/off emission (blinking), and cytotoxicity etc. Recently, considering their attractive optical and chemical features such as low toxicity, large effective Stokes shifts, as well as high resistance to photobleaching, blinking, and photochemical degradation, lanthanide-doped luminescent nanoparticles were proposed to be a promising new class of luminescent labeling agents, which have the potential and ability to overcome a number of problems associated with the commonly used luminescent labels.This thesis gives an overview of the recent advances on luminescent materials for biological labeling, systemically introduces the luminescence theories of lanthanide ions and backgrounds of lanthanide-doped luminescent nanoparticles, summarizes the recent achievements on syntheses and surface modifications of lanthanide-doped luminescent nanoparticles, and puts forward the challenges they faced and the future directions of them in the area of biological labeling. On the above bases, we developed a series of mild wet chemical routes and in situ surface modification methods, and synthesized several kinds of lanthanide-doped luminescent fluoride nanoparticles accordingly. X-ray diffraction (XRD), transmission electronic microscope (TEM), photoluminescence spectroscopy (PL), fourier transform infrared spectroscopy (FT-IR), thermal analysis (DTA-TG), and MTT assay were used to study the crystal structure, size and morphology, surface property, and luminescence property of the nanoparticles. A series of important conclusions and innovative results with practical significance were obtained.In wet chemical routes to produce lanthanide-doped luminescent nanoparticles, organic ligands were usually required to control the particle growth in order to get products in the nanometer scale. In this thesis, we developed a very simplecoprecipitation method carried out in absolute ethanol to synthesize lanthanide-doped luminescent nanoparticles without the use of any ligands. A series of nanoparticles include CaF₂:Eu³⁺ (-15 nm), LaF₃:Eu³⁺ (-10 nm), and GdF₃:Eu³⁺ (-30 nm) were synthesized using this method. The products consist of well crystallized pure cubic, hexagonal, and orthorhombic phases, respectively. By varying the ratios of the start materials, nanoparticles doped with different concentrations of Eu³⁺ ions were synthesized. In the doping concentration range studied (the maximum doping concentrations of Eu³⁺ ions for CaF₂, LaF₃, and GdF₃ nanoparticles are 30 mol%, 60 mol%, and 40 mol% respectively), no second phase were found for all the three samples at high doping concentrations of Eu³⁺ ions and they all showed a concentration quenching with increasing the Eu³⁺ ions doping concentration (the quenching concentrations are -15 mol%, -30 mol%, and -20 mol%,for CaF₂, LaF₃, and GdF₃ respectively).Since most biological studies are situated in an aqueous environment, nanoparticles with hydrophilic surfaces are usually required. Conventional method for making hydrophilic nanoparticles is to use organic ligands with hydrophilic groups when synthesizing the nanoparticles. In this thesis, we developed a simple method to synthesize hydrophilic lanthanide-doped luminescent LaF₃ nanoparticles directly in aqueous solution without using any ligands. The nanoparticles have a nearly spherical shape with average size of below 30 nm and consist of well crystallized pure hexagonal phase. The nanopartcles are stable in aqueous solutions and show strong luminescence (quantum yield 16 %). LaF₃ nanoparticles doped with different lanthanide ions or ion pair (Eu³⁺, Ce³⁺-Tb³⁺, and Nd³⁺) were synthesized, which emit in the visible (VIS) and near-infrared (NIR) spectral regions, and can be used for different biological applications.For certain biological labeling, especially for in vivo cell and animal labeling, the nanoparticles should be biocompatible (free of cytotoxicity) and contain functional groups for further attachment of biomolecules. Conventional method for obtaining such nanoparticles is via post surface modification. In this thesis, we developed an in-situ surface modification method to prepare biocompatible lanthanide-doped luminescent nanoparticles with functional chemical groups in a one-pot synthesis process. Chitosan capped CTS/LaF₃:Eu³⁺ nanoparticles and polyethylenimine capped PEI/NaYF₄:Yb³⁺,Er³⁺ nanoparticles were synthesized using this method. The CTS/LaF₃:Eu³⁺ nanoparticles have an average size of about 25 nm and consist of well crystallized pure hexagonalphase. The PEI/NaYF₄:Yb³⁺,Er³⁺ nanoparticles have an average size of about 50 nm and consist of well crystallized cubic and hexagonal mixed phases. It is revealed that both CTS and PEI coatings are biocompatible to human colon HT-29 cells and have no impacts on the luminescence property and quantum yield of the nanoparticles. The as-prepared nanoparticles stay stable in aqueous solutions for more than 5 days and thus have good potentials for in vivo cell and animal labeling.Conventional luminescent labeling technologies are most based on the use of ultraviolet (UV) or short VIS irradiation. A major problem associated with these systems in biological applications is the autofluorescence from and photo-damage to the biological specimens. A possible way to circumvent this problem is labeling with lanthanide-doped up-conversion nanoparticles, which show VIS emissions under NIR irradiation. Using the above described coprecipitation method carried out in absolute ethanol, we prepared Yb³⁺ and Er³⁺ codoped LaF₃ nanoparticles and investigated the influence of synthesis temperatures on the size, morphology, and up-conversion luminescence intensity of the nanoparticles. It is found that elevating the synthesis temperatures will cause significant increase in the particle size and up-conversion luminescence intensity of the nanoparticles. Annealing at above 400 °C is necessary to obtain products with practical up-conversion luminescence. However, heat treatment at high temperatures will cause the nanoparticles to lose colloidal properties and functional chemical groups, and thus is not the optimal way to produce up-conversion luminescent biolabels.In order to prepare colloidal lanthanide-doped nanoparticles with intense up-conversion luminescence, we replace the host matrix with NaYF₄, which is the most efficient host material for performing up-conversion known to date. We synthesized the PEI/NaYF₄:Yb³⁺,Er³⁺ nanoparticles using the in situ surface modification method described above and investigated the influence of reaction time on the properties of the resulting products. It is revealed that prolonging the reaction time will cause phase transfer from cubic to hexagonal of the products and thus promote their up-conversion luminescence intensity. The size and morphology of the nanoparticles show no obvious changes with varying the reaction time. The optimum reaction time is found to be 24 h and the nanoparticles synthesized under this condition show strong up-conversion luminescence in aqueous solutions when excited with a 600 mW diode laser at 980 nm, and thus have good potentials for use as labels in up-conversion detection and in vivo animal imaging.For multiplexing labeling, which is of especial interest in biological field currently, nanoparticles of different emission colors should be readily available and the whole group of labels can be excited at a single wavelength. To cater for these applications, we prepared the PEI/NaGdF₄:Ce³⁺,Ln³⁺ (Ln = Tb, Eu, Sm, and Dy) nanoparticles using the in situ surface modification method described above. The nanoparticles have a rod shape (20-30 nm in diameter and 40-60 nm in length) and consist of well crystallized hexagonal phase. It is found that after excitation into the Ce³⁺ ions, the excitation energy can be transferred to the luminescent centers via the Gd³⁺ sublattice followed by emission from the these luminescent centers. As such, the PEI/NaGdF₄ nanoparticles activated with different lanthanide ions show bright luminescence of different colors under single wavelength excitation at 254 nm, and thus have good potentials for multiplexing labeling.