Praperation Of Rare Earth Upconversion Luminescence Nano-materials And The Construction Of FRET-based Sensor

As a main category of pollutants,heavy metal ions have greatly affected the environment and the health of human beings.Therefore,it is of great significance to realize the effective detection of metal ions.At present,among the numerous novel sensing detection methods,fluorescence sensing analysis method because of its high stability,small cell toxicity,high sensitivity and other advantages,has good application potential in the detection of heavy metal ions.The emergence of some new nanometer fluorescence probes in recent years further promoted the application and research of fluorescent probes in all aspects.Fluorescence resonance energy transfer?FRET?is one of the most popular strategies,which has high sensitivity and selectivity for the detection of targets.To construct a FRET detection platform,the key procedure is to design a suitable pair of energy donor and energy receptor.Rare earth upconversion nanoparticles?UCNPs?as a new type of nano materials,has high chemical stability,low toxicity,adjustable multicolor luminescence,large optical penetration depth in tissue and low autofluorescence in organisms et al,which make it become the ideal fluorescence in FRET experiment donor selection.In addition,graphene quantum dots?GQDs?as a new kind of carbon materials was widely studied in various fields because of its high surface area,low cytotoxicity and chemical inertness.In previous studies,GQDs are often used as a kind of luminescent material,which is rarely reported as a quencher.Herein,we design a new fluorescent sensor platform for highly sensitive and selective detection of Ag⁺ in this article,using single-stranded DNA?ssDNA?functionalized UCNPs as the energy donor and GQDs as the energy acceptor based on FRET mechanism.The concentration of Ag⁺ was quantified by detecting the change of the fluorescence intensity of the donor.The major works are summarized as follows:?1?Upconversion nanoparticles NaYF₄:Yb,Er were synthesized by thermal decomposition method,and as the core structure,NaYF₄ was used as passivation layer to prepare two layer upconversion nanoparticles Na YF₄: Yb,Er@ NaYF₄?CS-UCNPs?with core-shell structure,and then characterized by XRD,TEM,DLS and FT-IR.The results indicated that the CS-UCNPs are pure hexagonal crystals and have high degree of crystallinity;the as-prepared OA-UCNPs and CS-UCNPs display uniform,monodisperse and high qualities with average sizes of about 16 nm and 20 nm,respectively;FT-IR showed that the surface of the material is covered with oleic acid ligands,which has hydrophobic properties.Then,the hydrophobic nanoparticles of CS-UCNPs were successfully converted into hydrophilic nanoparticles of Cit-UCNPs through ligand exchange method,which could be used for the following experiments.?2?Graphene oxide was synthesized firstly,and then the graphene quantum dots?GQDs?were synthesized by microwave cutting method,then both characterized by TEM,HRTEM,DLS,SEM,FT-IR and Raman.The results indicated that the GQDs display uniform and monodisperse with average diameter of 4.5 nm,and have a very clear lattice structure and a high degree of crystallinity;FT-IR confirmed that the existence of a lot of oxygen-containing functional groups on the GQDs surface,which has good hydrophilic properties;Raman showed that the GQDs have a large defect.The successful synthesis and characterization of the GQDs were well prepared for the construction of subsequent nanoprobe.?3?A novel fluorescence nanoplatform for the sensitive and selective sensing of Ag⁺ was fabricated based on fluorescence resonance energy transfer?FRET?technique between sodium citrate functionalized upconversion nanoparticles?Cit-UCNPs,energy donor?and graphene quantum dots?GQDs,energy acceptor?.Amino-labeled single-stranded DNA?NH2-ssDNA?containing a number of cytosine?C?was conjugated on the surface of the Cit-UCNPs to capture Ag⁺ ions.The upconversion fluorescence intensity of fluorescence probes at 543 nm enhanced gradually with the increase of the concentration of Ag⁺,and a good linear relationship was obtained between the relative fluorescence intensity versus the logarithm of Ag⁺ concentration in the range from 2×10-4 to 1 ?M.The detection limit for Ag⁺ was estimated to be 60 p M,meanwhile the high selectivity of this detection platform toward Ag⁺ over the other metal ions.Therefore,this paper proposes a novel monitor nanoplatform is expected to detect Ag⁺ with sensitive in complex biological or environmental samples.