Preparation Of Carbon-dot-based Fluorescent Detection Systems And Their Application In Biosensing

Neutral molecules(ROS, RNS), anions (I-, F-), Cations(K+, Ca2+, Fe3+, Zn2+，H+) play avery important role in regulating the balance of living organisms. Currently, the detection ofbiomolecules in living systems has attracted considerable attention.However, due to thecomplexity of physiological system and the diversity of biologicalmolecules, the applicationof many detection methods faces daunting challenges. Amongvarious detection techniques,fluorescence method is considered to be sensitive, simple, rapid, real-time detection; hencethis method has been widely used in biological detection.Carbon dots (CDs) is a new fluorescence carbon nanomaterial， which possessuniqueoptical properties and smallsize effect. With good biocompatibility and withoutcytotoxicity it may be a nice substitute for QDs. In this dissertation, we prepared three kindsof nanoparticle sensors through the modification of carbon dots. These sensors can sensitivelyand quickly detect iodide, pH and hydrogen peroxide.First, we report on a simple method for the determination of iodide in aqueous solutionby exploiting the fluorescence enhancement that is observed if the complex formed betweencarbon dots and mercury ion is exposed to iodide. The sensor was characterized by XPS,AFM, UV-vis, fluorescence spectrophotometer and FT-IR.Fluorescent carbon dots (C-dots)were treated with Hg(II) ion which causes quenching of the emission of the C-dots. Onaddition of iodide, the Hg(II) ions are removed from the complex due to the strong interactionbetween Hg(II) and iodide. This causes the fluorescence to be restored and the detection limitis~436nM. The test is highly selective for iodide and was used for the determination ofiodide in buffer solution and urine.Then，we demonstrate a fluorescence resonance energy transfer(FRET)-based ratiometricpH nanosensor with carbon-dot (CD) as the carrier. The sensor was prepared by covalentlylinking a pH-sensitive fluorescent dye (fluorescein isothiocyanate, FITC) onto carbon-dot.The CD-based system exhibits a significant change in fluorescence intensity ratio between pH4and8with a pKa value of5.69. It also displays excellent water dispersibility, good spectralreversibility, satisfactory cell permeability and low cytotoxicity. Following the living celluptake, this nanoplatform with dual-chromatic emissions can facilitate real-time visualization of the pH evolution involved in the endocytic pathway of the nanosensor.Finally, we demonstrate a multifunctional fluorescent nanoprobe for detectingmitochondrialH2O2. The nanoprobe was prepared by covalently linking amitochondria-targeting ligand (triphenylphosphonium, TPP) and a H2O2recognition element(PFl) onto carbon dots (CDs). The content for the targeting ligand (TPP) and the H2O2recognition element (PFl) were determined to be4.21mg/g and31.47mg/g respectively. Thenanoprobe displays excellent water dispersibility, high sensitivity and selectivity, satisfactorycell permeability, very low cytotoxicity with detection limit of0.75μM. Following the livingcell uptake, this nanoprobe can specifically target and stain the mitochondria; and it can detectthe exogenous H2O2in L929cells, as well as the endogenously produced mitochondrial H2O2in Raw264.7cells upon stimulation by PMA.This study shows that CDs can serve as promising nano-carriers for fabricating practicalmultifunctional fluorescent nanosensors.