| Graphene quantum dots and rare-earth upconversion nanoparticles are two kinds of extremely distinctive luminescent materials.Graphene quantum dots are small pieces of graphene with a lateral dimension of 1-10 nm with excellent biocompatibility,low cytotoxicity and stable optical properties,which have been widely used in optical sensing and biological imaging.However,the single function defect of graphene quantum dots limits their application in bioanalysis.Different from graphene quantum dots,rare-earth upconversion nanoparticles can generate high-energy ultraviolet,visible and near-infrared light under low-energy near-infrared excitation,which have the characteristics of low background fluorescence,strong penetration ability and little damage to biological tissue.However,the disadvantages of rare-earth upconversion nanoparticles,such as low quantum yield and poor water dispersivity,bring inconvenience to its application in biomedicine.Therefore,it is extremely necessary to compound graphene quantum dots with rare-earth upconversion nanoparticles to realize the complementary advantages properties between them.To solve the problem of functional simplicity of graphene quantum dots,histidine and D-penicillamine co-functional graphene quantum dot was designed and synthesized.The introduction of histidine not only improved the catalytic activity of peroxidase,but also laid a foundation for the subsequent composite with rare-earth metal ions.The presence of D-penicillamine enhances the fluorescence emission capacity of graphene quantum dot.The average size of the synthesized graphene quantum dot was about 3.6 nm,which was composed of 1-2 layers of graphene nanosheets.The graphene quantum dot was used as optical probe to construct a fluorescence sensing platform for the determination of acetamiprid.Firstly,G-rich DNA probe 2?P2?hybridizes with aptamer DNA probe 1?P1?to form a triple-helix DNA structure.The free probe P2 was released by the specific binding between acetamiprid and the aptamer fragment in the triple-helix structure.The released probe 2,in the presence of of K+,undergoes structural change into stem-loop structure that bears G-quadruplex structure.Then,G-quadruplex combines with hemin to form G-quadruplex/hemin DNAzyme?G4/hemin DNAzyme?,which can accelerate the oxidation of o-phenylenediamine by H2O2 and generate a large number of yellow oxidation product with excitation/emission maxima at 420/560 nm.The fluorescence of graphene quantum dot gradually decreased with the increase of oxidation product fluorescence.The quantitative determination of acetamiprid was realized by the change of fluorescence signals.Meanwhile,the presence of graphene quantum dot not only improves the signal-to-noise ratio of the analytical method,but also enhances the catalytic activity of G4/hemin DNAzyme and enhances the fluorescence response of the sensing system to acetamiprid.The linear range of the method was 1.0×10-15-1.0×10-9 mol·L-1,and the corresponding linear regression equation was?F420+560=145.27×LOG[Cacetamiprid,mol·L-1]+2183,correlation coefficient 0.992.The detection limit was 3.8×10-16 mol·L-1?S/N=3?,which has been successfully used for the detection of acetamiprid in tea samples.In order to facilitate the connection of biomolecules,the histidine and pentaethylenehexamine co-functional graphene quantum dot was prepared by introducing pentaethylenehexamine molecules with abundant amino groups at the edge of graphene quantum dot.The quantum yield of this graphene quantum dot was 90.2%.The introduction of pentaethylenehexamine not only increases the fluorescence emission intensity of graphene quantum dot,but also provides a large number of sites for the connection of biomolecules.Histidine significantly enhanced the catalytic activity of G4/hemin DNAzyme to H2O2.The prepared graphene quantum dot was composed of 1-3 layers of graphene nanosheets with average size of 2.7 nm and thickness of 0.9 nm.Meanwhile,it was used as an optical probe to construct fluorescence sensing system for the quantitative determination of miRNA.The specific combination of target miRNA and molecular beacon triggers the cycle amplification of target-molecular beacon,which achieves efficient DNA nanoassembly on the surface of graphene quantum dot and generates a large number of G-quadruplex structures.The resulted G-quadruplexes properly fold and bind hemin to produce stable G-quadruplex/hemin complexes.Then,the fluorescence of graphene quantum dot was effectively quenched via the photo-induced electron transfer by the electron acceptor hemin and O2,which was in situ generated by the decomposition of H2O2 under the catalysis of G-quadruplex/hemin DNAzymes.As a result,the proposed biosensing nanoplatform exhibits an ultrasensitive fluorescence response towards miRNA.In the concentration range of 1.0×10-18-1.0×10-12mol·L-1,the fluorescence signal decreases linearly with the increase of the logarithm of miRNA concentration.The correlation linear equation was FP=-25217×LOG[Cmi RNA-141,mol·L-1]+145686,the correlation coefficient was 0.9998,and the detection limit was4.3×10-19 mol·L-1?S/N=3?.The analytical method has been successfully applied to fluorescence detection of miRNA in human serum.For the problem that the interference of biological tissue on the fluorescence signal of graphene quantum dots,and the defects of rare-earth upconversion nanoparticles,such as low luminescence efficiency and lack of functional groups,graphene quantum dot/NaYF4:Yb,Er nanocomposite was designed and synthesized.Graphene quantum dot,as stabilizer,not only provides rich functional groups for the surface of the composite,but also significantly enhance the upconversion fluorescence of rare-earth materials.The average particle size of the composite was about 13.2 nm,with good water dispersibility.Meanwhile,it was used as an optical probe for the quantitative determination of miRNA.Firstly,Mg2+-dependent DNAzyme was formed by catalytic hairpin assembly induced by the target miRNA.Secondly,the hairpin H3 on the surface of gold nanoparticles?AuNP?was cut into DNA fragment S1 by Mg2+-dependent DNAzyme.Then,the DNA probe on the surface of the composite was combined with S1 to make AuNP close to the surface of the composite,resulting in the energy resonance transfer between the complex and AuNP,which effectively quenching the upconversion fluorescence intensity of the complex.Meanwhile,mi RNA released by the target driven cycle amplification strategy will continue to participate in the cycle process to achieve significant signal amplification.The linear range of the constructed system was2.5×10-15-5.0×10-9 mol·L-1,the correlation linear equation was F=-1078.6×LOG[CmiRNA-200b,mol·L-1]-5631.5,R2=0.995,the detection limit was 6.9×10-16 mol·L-1?S/N=3?,which has good selectivity.In this paper,we aim to prepare graphene quantum dot-rare earth upconversion nanocomposite based on the design and synthesis of different functionalized graphene quantum dots.The prepared nanomaterials as optical probes to carry out corresponding biological detection research,provides new research directions for biolabel and the early diagnosis of diseases. |
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