Construction And Application Of Carbon Dot-based Fluorescent Nanoprobes

As a carbon-based nanomaterial,carbon dots（CDs）are widely used in environmental monitoring,biological imaging,anti-bacterial and other fields because of their simple synthesis methods.Moreover,carbon dots have excellent physical and chemical properties（such as low toxicity,good water solubility,high biocompatibility and rich surface functional groups）.Therefore,it is of great significance to design and synthesize a new type of CD-based fluorescent probe for the target,which can improve the sensitivity and selectivity of detection,and expand its application in the field of analysis.Because of the good stability and luminous efficiency of CDs,in this paper,CDs with excellent optical properties were firstly prepared with one-pot solvothermal synthesis method and column chromatography separation technology using aniline as carbon source.Then,CD-based fluorescence nanoprobes were constructed,which can selectively detect intracellular p H and hydrogen sulfite（HSO₃^-）in the environment.The specific works and results are as follows:1.In this work,label-free carbon dots（p-CDs）,which were fabricated using a straightforward one-pot solvothermal treatment of p-phenylenediamine and urea,were employed to create a new ratiometric p H nanosensor.Under singlewavelength excitation(λ_ex=500 nm),the p-CDs gave dual emission bands at 525 and 623 nm.The fluorescent intensity ratio(I_{525 nm}/I_{623 nm})was linearly related to p H over the range 4.0 to 8.8 in buffer solutions,indicating that the ratiometric fluorescence nanoprobe may be useful for p H sensing.In p H measurements,the p-CDs also demonstrated outstanding selectivity,reversibility,and photostability.Owing to the advantages outlined above,the nanoprobe was used to monitor the p H of He La cells effectively,suggesting a new method for detection of p H-related diseases.2.In this work,we constructed a ratiometric fluorescent nanoprobe,Coum-CDs,by covalently coupling the coumarin dye（Coum）onto the surface of yellow carbon dots（Y-CDs）via amide bonds.The Coum-CDs showed peculiar dual fluorescence emission at 480 and 565nm under single-wavelength excitation.However,upon treatment with HSO₃^-,theπ-conjugation structure of Coum was destroyed due to the selectively nucleophilic reaction between the C-4 atom of Coum and HSO₃^-,which shut off the pathway of fluorescence resonance energy transfer from Coum to Y-CDs.As a result,the fluorescence emission of Coum-CDs at 565 nm decreased significantly,accompanied by a slight change in fluorescence intensity at 480 nm,allowing the ratiometric detection of HSO₃^-.In addition,the detection of HSO₃^-by Coum-CDs has the advantages of low detection limit（15 n M）and excellent selectivity without interference from other relevant analytes,especially reactive sulfur-containing species.Importantly,preliminary fluorescence confocal experiments showed that Coum-CDs have been successfully applied to ratiometric detecting of exogenous and endogenous HSO₃^-in living cells.In addition,the probe can also be used for quantitative measurement of HSO₃^-in sugar and wine.3.In this work,a new merocyanine-based fluorescent turn-on probe MC-Hg was developed,which used merocyanine Schiff base dye as fluorophore and thiocarbonate as recognition group for the determination of Hg²⁺in aqueous solution.MC-Hg with thiocarbonate has a low p K_a value,and it mainly exists in the form of Schiff base（SB）without fluorescence under weak acid and neutral conditions.After reacting with Hg²⁺,its thiocarbonate group was removed to afford a merocyanine dye as the final product,whose p K_avalue increased,and was present mainly as the fluorescent protonated Schiff base（PSB）form under weak acid and neutral conditions,which can be utilized for the detection of Hg²⁺.The results showed that the probe MC-Hg had good stability and water solubility,and show fast response and high selectivity toward Hg²⁺.In addition,the probe MC-Hg has potential application and has been successfully applied to the determination of Hg²⁺in real water samples.