Synthesis Of Reversible And Ratiometric Fluorescent Molecular Probes For Biosensing Application

Recently, fluorescence probes have been applied to environmental protection, cell imaging and food safety field, and thus attract a broad attention of researchers on chemistry, biology and materials. Nowadays, there are pretty many fluorescence probes for the determination of chemical analytes such as metal ions, cations, anions and biological molecules in vitro and vivo systems. However, probes aim at sulfur dioxide and its derivatives are relatively few, and most of them can’t be used in biological system. There has been studies showing that bisulfites can attack the unsaturated C=C double bonds of cyanine dye through nucleophilic addition reaction, resulting in the quenching of fluorescence. Based on this mechanism, we designed two fluorescence probes, ratiometric fluorescent molecular probe HBT-Cy and composite fluorescent molecular probe Cy-CDs, which were synthesized by connecting cyanine dye with carbon dots and benzothiazole, respectively. The dissertation consists of three chapters summarized as follows:In chapter 1, we introduce the definition of fluorescent probes and three kinds of recognition mechanism including fluorescence resonance energy transfer (RET), photoinduced electron transfer (PET) and intramolecular charge transfer (ICT). We also summarize the meaning of studying sulfur dioxide and its derivatives, present the development of bisulfites fluorescence probes, and then propose the design idea of the dissertation.In chapter 2, we synthesized a novel fluorescent probe 2-(2’-hydroxyphenyl) benzothiazole cyanine (HBT-Cy), which shows dual emission bands at 450 nm and 590 nm upon a single excitation at 390 nm, derived from the benzothiazole and cyanine moieties, respectively. Upon addition of bisulfite, the emission band at 590 nm decreases gradually while the emission at 450 nm slightly increases. Accordingly, the visible-light color change from red to colorless and the fluorescence color change from rose red to blue. The fluorescence ratio (F450/F590) is also proportional to the amounts of bisulfite with a detection limit of 0.34 μM, which is indicative of the fact that probe HBT-Cy is potentially useful for the quantitative determination of bisulfite. The original HBT-Cy can be regenerated from the addition product HBT-CyO by peroxides such as H2O2 and TBHP. Such a reversible redox cycle could be repeated without much degradation of the reactivity and fluorescence properties. The cell imaging experiments suggested that HBT-Cy has the potential to sense exogenous and endogenous bisulfite as well as possesses the redox activity in biological system. We also successfully applied HBT-Cy for the visual detection of gaseous SO2 and the determination of bisulfite in real water samples and sugar samples. The results demonstrate the diverse application of the probe and suggest its potential for further imaging of oxidative stress in living cells.In chapter 3, we firstly synthesized amine-coated carbon dots (CDs) and cyanine dye with carboxyl group (Cy), and explored their energy relationship. We then prepared a novel composite fluorescence probe Cy-CDs by the fact that the cyanine dye containing carboxyl group can be easily covalently linked to the surface of the CDs through condensation reaction. The fluorescence intensity of the composite probe Cy-CDs is quite low because of the FRET process, since the absorption spectrum of cyanine dye is overlapped with the emission spectrum of CDs. The addition reaction between bisulfite and carbon-carbon double bond breaks the conjugated structure of the probe and interrupts the FRET process, leading to the enhancement of the intensity. As a result, the probe can be used to qualitatively and quantitatively detect bisulfite in aqueous solution. In addition, the composite probe is applied to the detection of SO2 gas in aqueous solution as well as the visually monitoring of SO2 gas in air. This nanomaterial-based probe is easy to prepare, low cost, and fast responding to bisulfite with high selectivity and sensitivity. These advantages expand the application potential of the probe Cy-CDs.