Preparation Of Graphene Quantum Dots With Orange Emission And Its Application In The Construction Of Fluorescent Nanoprobes

Fluorescent nanoprobes have been widely used in chemical sensing,biological detection and recognition,owing to the benefits of excellent sensitivity,favorable selectivity,operational simplicity and real-time detection.However,the design of analyte recognition probe is complicated,because the probe must be specifically sensitive to the analyte and with good fluorescence properties at the meantime.The important component of fluorescent nanoprobes is the nanomaterials with photoluminescence properties.As a new class of fluorescent nanomaterial,GQDs inherit the perfect optical properties of traditional semiconductor quantum dots and also have its unique properties such as good biocompatibility,very low cytotoxicity and tunable band gap.GQDs is rich in functional groups and is easy to surface modification to meet the needs for the construction of different types of fluorescent nanoprobes.So far,many synthesis methods for GQDs have been developed,but most of the obtained materials are blue or green emission.Therefore,it is necessary to prepare GQDs with long wavelength emission to meet different needs and expand the application fields.Therefore,the preparation,structure and properties of yellow fluorescent GQDs were studied in this paper.On this basis,a series of fluorescent nanoprobes were constructed using GQDs after further ion regulation or surface functionalization as luminescent units.And its application in the analysis of small molecules,environmental pollutants and enzyme activity were also studied.The details are as follows:In chapter I,we first briefly introduce graphene based nanomaterials and their development process.Then,we make a systematic introduction about the basic properties,preparation methods,luminescence mechanism,quenching mechanism and applications in the field of analysis of GQDs.Finally,the main contents and research significance of this thesis are given.In chapter II,the authors describe a method for synthesizing GQDs possessing orange fluorescence with emission wavelength that can be tuned over the range from537 to 593 nm by variation of the excitation wavelength.The GQDs display peroxidase-mimicking catalytic activity.Specifically,they catalyze the oxidation of dopamine to produce 4-（2-aminoethyl）benzene-1,2-quinone（AQ）which is colored and can quench the fluorescence of GQDs.However,quenching is reversed by addition of NADP⁺,but not by its reduced form（NADPH）.Based on these findings,an assay was worked out to monitor enzymatic reactions involving NADP⁺.The method allows NADPH to be detected in the 2-175μmol L^-1 concentration range,with a 0.6μmol L^-1 detection limit.In chapter III,we describe a detection method for acid phosphatase（ACP）based on the aggregation-caused quenching of GQDs.The fluorescence of GQDs could be quenched by poly-（dimethyl diallyl ammonium chloride）（PDDA）,the high efficiency of the quenching was caused by the non-covalent binding of positively charged PDDA to negatively charged GQDs through electrostatic interactions,aggregating to form a GQDs-PDDA complex.Addition of（NaPO3）6 could effectively turn on the quenched fluorescence due to the stronger electrostatic interactions between positively charged PDDA and negatively charged（NaPO₃）₆.The introduction of ACP would lead to the breakdown of（NaPO3）6 into small fragments and disassemble the complex of PDDA-（NaPO₃）₆.As a result,the recovered fluorescence could be quenched again by the addition of ACP.Quantitative evaluation of ACP activity in a broad range from 30nU mL^-1 to 420 nU mL^-1 with a detection limit of 12 nU mL^-1 can be achieved in this way.In chapter IV,we develop a dual-readout mode amifostine detection method based on the inner filter effect（IFE）from gold nanoparticles（AuNPs）to sulfanilic acid functional graphene quantum dots（SGQDs）.The fluorescence of SGQDs can be adequately suppressed by AuNPs via IFE.Besides,alkaline phosphatase（ALP）can catalyze the hydrolysis of WR2721 to generate WR1065,which combined with AuNPs by S-Au conjunction and induced the aggregation of AuNPs with the color change from red to blue.Thus,the IFE from AuNPs to SGQDs was weakened and the switched off fluorescence of SGQDs was switched on accordingly.Therefore,amifostine can be detected through the changes of the color of solution and the fluorescence recovery of SGQDs.This sensing platform showed a good leaner relationship from 1 to 175 nmol L^-1,with a detection limit of 0.4 nmol L^-1.In chapter V,a rapid fluorescence“turn off-on”assay was developed to detect ALP and ascorbic acid（AA）based on SGQDs and MnO₂ nanosheets.ALP catalyzed the hydrolysis of amifostine to WR1065,which in the presence KMnO₄ yielded MnO₂nanosheets and WR33278.Then,the energy transfer platform was established by attaching SGQDs to MnO₂ nanosheets via WR33278 as the linker.As a result,the fluorescence of SGQDs quenched efficiently by MnO₂ nanosheets.Moreover,AA could reduce Mn O₂ into Mn²⁺and trigger the disintegration of the MnO₂ nanosheets.As a result,the SGQDs were released and the quenched fluorescence was recovered efficiently.Besides,both the addition of ALP and AA can cause the change of solution color owing to the formation or breakdown process of MnO₂ nanosheets.Thus,MnO₂nanosheets could also be used as colorimetric probes for the detection of ALP and AA through directly visualizing the color variation by bare eyes.In chapter VI,a functionalized GQDs-based Cu²⁺modulated dual-functional fluorescent nanoprobe was developed.The attachment of sulfanilic acid and glutathione onto GQDs through amide linkage to obtain the functionalized graphene quantum dots（termed as SSGQDs）.The SSGQDs exhibit dramatically enhanced green fluorescence compared with GQDs.And the fluorescence of SSGQDs was found to be effectively quenched by Cu²⁺via aggregation kinetics with the formation of SSGQDs-Cu（II）complex.S^2-exhibits a good capacity to remove Cu²⁺from SSGQDs and dissociated SSGQDs-Cu（II）complex,as a result the fluorescence of SSGQDs was turned on again.Moreover,the introduction of AA could also induce the fluorescence recovery,because it could reduce Cu²⁺into Cu⁺and result in the destruction of the SSGQDs-Cu（II）complex.This method can be used for the sensitive analysis of S^2-and AA down to 12.6 nmol L^-1 and 20.3 nmol L^-1,respectively.