| Rare earth elements perform strong fluorescence emission through 4f or 4f-5d transition and have incomparable spectral characteristics compared with common elements.Rare earth optical materials have also attracted much attention due to their unique luminescent properties.Rare earth upconversion nanoparticles excited by near-infrared light,have many advantages such as low radiation damage,strong penetrating power,weak self-fluorescence and narrow emission band,so they are considered as a new generation of luminescent probes.Rare earth luminescent materials prepared by traditional methods lack functional groups and show low upconversion luminescence efficiency,which limit their application in biomedical field.Graphene quantum dot（GQD）is another promising nano-optical material with the characteristics of good water-solubility and can be easily functionalizationed.However,the application of GQDs in biological detection is limited due to the large background fluorescence interference and the huge damage to organisms caused by UV excitation.Thus,it is of great practical significance to effectively compound graphene quantum dots and rare-earth nanoparticles to achieve their"complementary advantages"in terms of performance,and study the interaction mechanism between them.Histidine-functionalized graphene quantum dots were prepared by one-step pyrolysis,using citric acid as carbon source and histidine as functional reagent.The edge of the synthesized GQDs sheet contains abundant imidazole groups that can form stable complexes with rare-earth metal ions.This provides a basis for the preparation of future complexes.Histidine and hexamethylene diamine double functionalized graphene quantum dots were prepared by pyrolysis of citric acid,histidine and hexamethylene diamine.Studies have shown that the prepared GQDs have a size of about 2.9 nm and a thickness of 1-2 layers of graphene.Due to the introduction of hexamethylene diamine,the edge of the graphene sheet contains a large number of amino groups.Amino group has strong electron donating effect,which can improve the charge density in the graphene sheet.Thus the introduction of amino group is conducive to improving the luminous efficiency of GQDs.In addition,amino groups can be coupled with carboxylated DNA by amideation reaction,which extends the application of GQDs in the biological field.Histidine-functionalized graphene quantum dot was used as a stabilizer for hydrothermal synthesis of GQD@NaTbF4.The formed NaTbF4 is a hexagonal crystal,its surface was covalently coated by GQDs,and the crystal size is only in the range of 4-6 nm.The small size and rich of hydrophilic groups make the composite easy to disperse in water.The resulted dispersion is of excellent stability.As the absorption spectra of His-GQD and the fluorescence emission spectrum of NaTbF4 exist a great overlap as well a short distance between His-GQD and NaTbF4,their combination brings a remarkable fluorescence resonance energy transfer.The increase in the fluorescence intensity of GQD@NaTbF4 is more than 7 times that of sole GQD.The coordination of Cu2+with GQD on NaTbF4 surface results in an obvious fluorescence quenching process.However,the quenched fluorescence can be well recovered by adding histidine.Based on the above behaviors,one“On-Off”type fluorescence scheme was established for the detection of histidine.When the histidine concentration is in the range of 1.0×10-6-2.0×10-4 mol·L-1,the peak fluorescence intensity linearly increases with the increase of histidine.The analytical method gives a detection limit of 3.8×10-7 mol·L-1.It has been successfully applied in determination of histidine in human urine and cervical cancer cells（Hela cells）imaging.Histidine and hexamethylene diamine functionalized graphene quantum dot was used as a stabilizer for hydrothermal synthesis of GQD@NaYF4:Yb,Er hybrid.The resulting hybrid shows good water-solubility,small particle size of 11.5 nm.The graphene quantum dot has a strong absorption capacity for near-infrared excitation light,so it can be used as"antennas"to captured photon and then transfer the energy to up-conversion nanoparticles,resulting in significantly enhancement up-conversion emission.Further,the hybrid was designed as an upconversion biosensing nanoplatform for detection of carcinoembryonic antigen based on the exonuclease III-aided target recycling amplifcation.Firstly,the linker DNA（LDNA）is covalently attached to the hybrid surface.Then,the upconversion emission of LDNA-modifed hybrid is effectively quenched through the hybridization of gold nanoparticles with hairpin DNA（Hp）via the fluorescence resonance energy transfer.Finally,the Hp on the nanoassembly is specially opened by signal DNA（SDNA） and leads to rapid recovery of the quenched upconversion emission.In addition,the exonuclease Ⅲ-aided target cycling amplifcation strategy is introduced to generate the SDNA and release the carcinoembryonic antigen.The released carcinoembryonic antigen is circularly used to produce more SDNA,resulting in a signifcant signal amplifcation.The as-proposed biosensor displays good analytical performance in the detection of carcinoembryonic antigen with a wide linear range of 5.0×10-17-1.0×10-8 g·mL-1,giving a low detection limit of 2.4×10-17 g·mL-1（S/N=3） along with a high specifcity.Its practicality has been verifed by the analysis results of carcinoembryonic antigen in human serum. |
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