New Analytical Methods Based On The Fluorescence Energy Transfer Of Rare Earth Luminescence Nanoparticles

Fluorescence analysis, due to its some merits such as good selectivity, highsensitivity, multiple measurement parameters and etc., is widely used in many fields.The properties of the fluorescence reagents and the selected method play an importantrole in the sensitivity and selectivity of the fluorescence analysis method. So, todevelop new method for preparation of luminescence reagents （or probes） withexcellent luminescence properties such as good stability, high fluorescence quantumyields, good water-solubility, good biocompatibility and etc., has become the frontierand hot issue in the fluorescence analysis. In early research, organic fluorescence dyeswere widely studied. So far, organic fluorescence dyes were still widely applied forbiological labeling, chemical and biological sensing, bioimaging, due to its lowermolecular weight, facile to be conjugated with biomolecules （the bioactivity of thebiomolecules was still kept after conjugated）. However, organic dyes have certainlimitations such as their relatively short fluorescence lifetime, narrow excitationranges, broad emission spectra, and photobleaching. The development of luminescentcolloidal nanoparticles referred to quantum dots （QDs） have led to an explosivegrowth in research on these materials for two decades. Compared to organic dyes,luminescent nanomaterials show some unique properties, such as composition-andsize-dependent absorption and emission, narrow emission peak, wide excitationranges. As an alternative to dye molecules and QDs, the luminescence of rare earthdoped nanoparticles shows certain special chemical and optical properties, includingexcellent chemical stability, large effective Stokes shifts, low toxicity, as well as highresistance to photobleaching and non-blinking，sharp emission bandwidths, longfluorescence lifetime and etc. In addition, some rare earth doped nanoparticles shows upconversion luminescence property. So, preparation of rare earth dopednanomaterials with excellent luminescence properties and combination of thesenanomaterials with the fluorescence spectrometry technique to develop newfluorescence analysis based on fluorescence energy transfer principle with highselectivity and sensitivity is meaningful.In this dissertation, we prepared down-conversion green luminescence CePO₄:Tb³⁺nanoparticles by hydrothermal method. Based on the fluorescence energy transferprinciple of rare earth luminescence nanoparticles and fluorescence inner filter effect（IFE） principle, we developed new methods for detection of Fe²⁺, Al3+by thesynchronous fluorescence technique. We prepared upconversion-visible luminescenceNaYF₄:Yb³⁺,Er³⁺nanoparticles by solvothermal method. Based on fluorescence innerfilter effect principle and fluorescence resonance energy transfer, we developed newmethods for identify and detection of Fe²⁺, Al3+, Cr（VI） and biothiols （cysteine andhomocysteine） by upconversion luminescence technique. These new methods wereapplied in the determination for real samples. The main contributions are as follows:1) Water-soluble and well-stable CePO₄:Tb³⁺nanoparticles were synthesized inaqueous solutions by a facile solvothermal method, and then characterized by TEMand EDS. The luminescence mechanism of CePO₄:Tb³⁺nanoparticles is based on astrong absorption of Ce³⁺within the UV range followed by an efficient energytransfer from Ce³⁺to Tb³⁺. The emission peak corresponded to the⁵D₄–⁷F₆and⁵D₄–⁷F₅transitions of Tb³⁺. When Fe²⁺was added to the CePO₄:Tb³⁺nanoparticles-H2O2system, hydroxyl radicals were generated by the Fenton reactionbetween Fe²⁺and H₂O₂. In this case, the reaction of hydroxyl radicals with Ce³⁺generated Ce⁴⁺. Therefore, the efficient energy transfer from Ce³⁺to Tb³⁺wassuppressed, which led to the decrease in the fluorescence intensity. At the same time,Fe³⁺ cannot react with H₂O₂. Based on the principle above, combined with thesynchronous fluorescence scan technique, a new method was developed forquenching kinetic discrimination of Fe²⁺and Fe³⁺, and ultra-sensitive, selectivedetection of trace amount of Fe²⁺. The effects of certain experimental conditions (suchas the Δλ, the concentration of CePO₄:Tb³⁺nanoparticles and H₂O₂, pH, the reaction temperature and time) were investigated, and the possible mechanism was discussed.Under the optimal condition, the linear range is3.0nM-2.0μM, the correlationcoefficient is0.999, and the limit of detection （3σ） is2.0nM. The influence of someinterference substance was tested. K⁺, Na⁺, Mg²⁺, and Ca²⁺have small interference.Some transition metal ions, such as Ni²⁺, Mn²⁺, Cu²⁺have certain interference. But inthe same concentration, the change of synchronous fluorescence intensity was lessthan5%. At the last, and the method was applied in the determination of Fe²⁺in watersamples.2) In acid solution, chrome Azurol S （CAS） reacts with Al3+quantitatively andgenerates a pink product of CAS-Al³⁺. The degree of the overlap was effective forinner filter effect between the emission spectrum of CePO₄:Tb³⁺nanoparticles and theabsorption spectrum of CAS-Al³⁺. Furthermore, the two materials did not chemicallyinteract with each other or show interference effects on the fluorescence properties ofCePO₄:Tb³⁺nanoparticles. Based on the results above, combined with thesynchronous fluorescence scan technique, a new inner filter effect optical sensor wasdeveloped for sensitive and selective detection of trace amount of Al3+. The effects ofcertain experimental conditions (such as the Δλ, the concentration of CePO₄:Tb³⁺nanoparticles and chrome Azurol S, buffer solution, the reaction temperature and time)were investigated, and the possible mechanism was discussed. Under the optimalcondition, the linear range is8.0ng mL^-1-900.0ng mL^-1, the correlation coefficient is0.993, and the limit of detection （3σ） is5.3ng mL^-1. The method described here issensitive than the method of CAS-Al spectrophotometer (the linear range is0.02⁰.14μg mL^-1). The influence of some interference substance was tested. K⁺, Na⁺, Zn²⁺,Mg²⁺, Ca²⁺, Ba²⁺and some coexisting ions have small interference. At the last, themethod was applied in the determination of Al3+in water samples.3) Stable and water-soluble NaYF₄:Yb³⁺,Er³⁺luminescent nanoparticles with highupconversion efficiency were synthesized by hydrothermal method, and characterizedby transmission electron microscopy （TEM） XRD and upconversion luminescencespectroscopy. Under acid condition, diphenylcarbazide （DPC） and Cr （VI） can reactquantitatively and at the same time generate a pink chelate complex （Cr（Ⅲ）-diphenylcarbazone）（To be noted, the direct reaction of Cr（Ⅲ） withdiphenylcarbazone does not occur to any appreciable extent on account of the wellknown inertness of the Cr（Ⅲ） aquo-complex）. The green emission band ofupconversion NaYF₄:Yb³⁺,Er³⁺luminescent nanoparticles possesses a complementaryoverlap with the absorption spectrum of the pink chelate complex. Based on thisprinciple, the inner filter effect method was well established for the determination oftrace chromium（VI）. The effects of certain experimental conditions were investigated(such as the concentration of NaYF₄:Yb³⁺,Er³⁺nanoparticles and diphenylcarbazide,acidity), and the possible mechanism was discussed. Under the optimal condition, thedecrease in the upconversion luminescent nanoparticles was proportional to theconcentration of chromium （VI） due to IFE. The linear range is7.00×10^-8-1.00×10^-5mol L-1, the correlation coefficient is0.995and the limit of detection （3σ） is2.40×10^-8mol L^-1Cr （VI）. In comparison to the method for the determination of Cr（VI） inclean waters by spectrophotometry with diphenylcarbazide, in which the limit ofdetection is approximate3.80×10^-8mol L^-1, the inner filter effect method describedhere is more sensitive. The influence of some interference substance was tested. K+,Na+, Ca²⁺，Mg²⁺, Cu²⁺, Ni²⁺, Pb²⁺and some coexisting ions have small interference. Atthe last, this assay was used in the determination of Cr （VI） in water samples.4) The emission spectrum of upconversion luminescent NaYF₄:Yb³⁺,Er³⁺nanoparticles was effectively overlapped with the absorption spectrum of goldnanopartilcles （AuNPs）. Furthermore, NaYF₄:Yb³⁺,Er³⁺nanoparticles synthesizedshow positive charge, while AuNPs have negative charge due to the negative cappingagent’s （citrate ions）. Thus, there is electrostatic interaction between the negativelycharged AuNPs and the positively charged NaYF₄:Yb³⁺,Er³⁺nanoparticles, and thedistance between NaYF₄:Yb³⁺,Er³⁺nanoparticles and AuNPs is shortened. Based onthis result, an efficient fluorescence resonance energy transfer system betweenNaYF₄:Yb³⁺,Er³⁺nanoparticles and AuNPs was created. In the presence of biothiols（such as cysteine and homocysteine）, AuNPs will interact with biothiols, which led tothe aggregation of AuNPs. In this case, the absorption of AuNPs was significantlydecreased, and the luminescence intensity of NaYF₄:Yb³⁺,Er³⁺was restored （“turn on”）. Based on the principle above, a simple and sensitive “turn on” luminescentsensor for biothiols based on fluorescence resonance energy transfer between greenupconversion NaYF₄:Yb³⁺,Er³⁺nanoparticles and gold nanoparticles was developed.The effects of certain experimental conditions (such as the concentration ofNaYF₄:Yb³⁺,Er³⁺nanoparticles and gold nanoparticles, buffer solution, the reactiontemperature and time) were investigated, and the possible mechanism was discussed.Under the optimal condition, the linear range is0.04-1.00μg mL^-1, the correlationcoefficient is0.996, the limit of detection （3σ） is1.24×10^-2μg mL^-1for cysteine; andthe linear range is0.06-1.80μg mL^-1, the correlation coefficient is0.992, and thelimit of detection （3σ） is1.51×10^-2μg mL^-1for homocysteine, respectively. Theinfluence of some interference substance was tested. Na⁺, K⁺, Ca²⁺, Zn²⁺, Mg²⁺andsome amino acid have small interference. At the last, the method was successfullyapplied for the detection of biothiols in samples.