Study On New System Of Ratiometric Fluorescent Analysis Based On Luminescent Carbon Quantum Dots

Carbon Quantum Dots(CQDs)are a new type of carbon nanomaterials.CQDs have been widely concerned since their properties of low cost,low toxicity,chemical inertness,great biocompatibility,great water-solubility and good electronic conductivity.The current sensors based on CQDs mostly use single emission fluorescent intensity as the detection signal,while their detection results are easy to affect by the interference of instrument parameters,microenvironment,probe concentration and photobleaching.Ratiometric fluorescent probes are generally composed of the emission peaks of two different fluorophores and have a self-calibration function,which can effectively eliminate the detection error caused by the interference factor of the single-emitting sensor,thereby improving the detection result.Until now,ratiometric fluorescent probes have been applied in ion detection,drug detection,biosensing and cell imaging and so on.Compared with ratiometric fluorescent probes based on organic materials,semiconductor quantum dots(QDs)and rare earth materials,ratiometric fluorescent probes based on CQDs are more advantageous on applications due to their low toxicity,high water-solubility and low-cost properties.Among its applications,CQDs can be designed as fluorescence response signals or as reference signals.However,there are little studies show direct synthesis of luminescent carbon quantum dots from a single carbon source and direct constitution of ratio probes based on the target responder.Therefore,it is significantly meaningful to develop dual emission new ratiometric fluorescent probes based on single CQDs and their analytical applications without using other fluorophores.In this thesis,we use o-phenylenediamine(o PD)or m-phenylenediamine(m PD)as single carbon precursor to directly synthesized new CQDs.Based on their responses to ions,p H,and small molecules,a new ratiometric fluorescent system was constructed.New methods of fluorescent analysis for the detection of the targets were developed.The main work of this paper is as follows:1.Using o-phenylenediamine as the sole carbon source,carbon quantum dots(oCQDs)with stable optical properties were synthesized by solvothermal method,and their morphology and optical properties were characterized.In BR buffer(p H 2.00),the oCQDs solution shows an independent fluorescent emission peak at 445 nm when use 388 nm as the excitation wavelength.With the addition of hexavalent chromium Cr(VI),the fluorescent emission signal of oCQDs solution at 445 nm decrease,while a new fluorescent emission peak appears at 565 nm.With the increase of Cr(VI),the fluorescent signal at565 nm sequentially increase while the fluorescent signal at 445 nm decrease.A dual emission fluorescent signal is forming with the fluorescent signals of these two peaks drop and rise.The dual emission mechanism was studied through the fluorescent response of Cr(VI)to oCQDs,the fluorescent lifetime and color change after adding different concentrations of Cr(VI),three-dimensional fluorescence spectrum,UV-visible spectrum signal and the effect of different solvents(including water,hydrochloric acid,sulfuric acid,acetic acid)for the synthesis of carbon quantum dots.Compared with o-phenylenediamine,CQDs synthesized with m-phenylenediamine and p-phenylenediamine as carbon source do not have this phenomenon.Studies have shown that the binding mode and site of amino groups on the benzene ring are important factors affecting the double emission characteristics,which indicate the o-phenylenediamine played a crucial role in the formation of this dual emission phenomenon.The ratio of the fluorescent signal at 565 nm and 445 nm of the probe have a good linear relationship with the concentration of Cr(VI)ranging from 0.12?M to 200?M.According to this,the concentration of Cr(VI)can be detected based on the ratio of the fluorescent probe.The detection limit(LOD)of this method was 37 n M,which the Cr(VI)content in river water and tap water was detected.2.Using m-phenylenediamine as the sole carbon source,new luminescent carbon quantum dots(mCQDs)with stable optical properties were synthesized by a one-step solvothermal method in phosphoric acid medium,and their morphology and optical properties were characterized.The experiment shows,with the excitation wavelength of300 nm,the mCQDs solution have three fluorescent emission peaks at 337 nm,430 nm and 501 nm,respectively.With the increase of p H value,the concentration of H⁺decrease.While the fluorescent signal of mCQDs at 337 nm decrease,the fluorescent signal at 430nm and 501 nm increase sequentially.Thus,the fluorescence signals show one drop and two rises,forming a multi-emission fluorescent signal.However,carbon quantum dots synthesized with o-phenylenediamine and p-phenylenediamine as carbon sources with different solvents(including water,hydrochloric acid,sulfuric acid,and acetic acid)do not show this phenomenon.The F_{(501+430)/337}and F_501/337 of this probe had a good linear relationship with p H changes,and a new type of ratiometric fluorescent probe can be established.The linear equations are F_{(501+430)/337}=0.1836p H-0.46 and F_501/337=0.1706p H-0.54 in the p H range of 5.00?9.00.The linear equations are F_{(501+430)/337}=1.3174p H-10.85and F_501/337=1.0161p H-8.30 in the p H range of 9.00?11.50.Accordingly,the p H can be detected based on this ratiometric fluorescent probe.Moreover,this ratiometric fluorescent probe is established with no requirement of multiple fluorophore complex construction and surface modication,the synthetic materials are simple and easy to acquire.It is a method with high sensitivity,fast response,and wide p H detection range.3.Using m-phenylenediamine as the carbon source,carbon quantum dots(mCQDs)with good luminescence properties were synthesized in one step.In Na₂HPO₄-citric acid buffer(p H 7.00),two separate fluorescent emission peaks at 340 nm and 500 nm of mCQDs are exhibited in the excitation wavelength of 310 nm.After adding Hg²⁺,the fluorescent emission signal of mCQDs at 340 nm decrease,while the fluorescent emission signal at 500 nm is basically unchanged.The fluorescent signal quenched at 340 nm is restored by the addition of DA,with no interference for the signal at 500 nm.Based on this phenomenon,a new ratiometric fluorescent probe for detection of DA sensitively and rapidly based on mCQDs can be established by using the fluorescent signal at 500 nm as a reference signal based on the decrease and recovery of the fluorescence signal at 340 nm.The fluorescent signal recovery ratio I_340/500 and the concentration of DA shows a good linear relationship within 0.5?M?10?M,so DA can be detected based on the ratiometric fluorescent probe.The detection limits of DA are 0.08?M,which can be used for the detection of DA in samples with satisfactory results.