Synthesis And Characterization Of Organic/Inorganic Fluorescent Materials And Their Application For Optical Chemosensors

Fluorescent and colorimetric chemosensors have been developed to be useful tools for sensing heavy metal ions. Fluorescent chemosensors are attractive due to the simplicity and high detection limit. The advantage of colorimetric sensors is their capability to detect analyte by the naked eye. Among numerous indicators, rhodamine-based dyes are a kind of excellent candidate for the construction of an off/on-type fluorescent chemosensor due to their excellent spectroscopic properties of large molar extinction coefficients, high fluorescence quantum yields, and long absorption and emission wavelength elongated to visible region. Compared to traditional organic dyes and fluorescent proteins, semiconductor nanocrystals, often referred to quantum dots (QDs), have several intriguing advantages such as great water-solubility, large extinction coefficients, high photoluminescence efficiency, tunable size-dependent emission, wide absorption spectrum and sharp emission profile.In this thesis, a series of new optical chemosensors based on rhodamine and quantum dots were synthesized and investigated for their applications in the detection of heavy metal cations (Hg²⁺, Cu²⁺). In addition, rhodamine 6G and europium complexes are incorporated into polymer matrixes and electrospun into various composite nanofibers.The details are as follows:1. Novel rhodamine-based highly sensitive and selective colorimetric and off-on fluorescent chemosensors (RO1 and RS1) for heavy metal ions is designed and prepared. The photophysical characterization and Hg²⁺/Cu²⁺-binding properties of sensor RS1/RO1 are investigated and compared. The sensor RS1 show more excellent sensing ability for Hg²⁺ due to the incorporation of the S atom. The signal change of the chemosensor RS1 is based on a specific metal ion induced reversible ring-opening mechanism of the rhodamine spirolactam. The response of the chemise nsor RS1 for Hg²⁺ ions is instantaneous and reversible. Moreover, this sensor is applied for in vivo imaging in Rat Schwann cells to confirm that RS1 can be used as a fluorescent probe for monitoring Hg²⁺ in living cells with satisfying results, which further demonstrates its value of practical applications in environmental and biological systems.2. A rhodamine-based highly sensitive and selective chemodosimeter is designed and prepared. The system, which utilizes an irreversible Hg²⁺-promoted oxadiazole forming reaction of rhodamine derivative RO2, is monitored by colorimetric and fluorescence intensity changes that respond instantaneously at room temperature. In addition, the C=C is also incorporated into the rhodamine derivative. Next. the polymer through this C=C will be synthesized and electrospun into nanofibers with sensing ability.3. Novel multifunctional Hg²⁺ ions sensing nanocomposites were developed by applying SiO2 as the encapsulation agent to package Fe₂O₃ NRs and CdTe QDs. resulting in CdTe/Fe₂O₃@SiO2 core/shell nanostructures. The core/shell structural nanocomposites were confirmed by extensive characterizations. Photoluminescence (PL) spectroscopy and superconducting quantum interference device (SQUID) were used to investigate the optical and magnetic properties of the core/shell structural nanocomposites, respectively. The fluorescence of the obtained nanocomposites could be quenched effectively by Hg²⁺ions. The quenching mechanism was studied and the results show the existence of both static and dynamic quenching processes.4. A novel multifunctional microsphere with a fluorescent CdTe quantum dots (QDs) shell and a magnetic core (Fe3O4) has been successfully developed and prepared by a combination of the hydrothermal method and layer-by-layer (LBL) self-assembly technique. The resulting fluorescent Fe3O4@C@CdTe core/shell microspheres are utilized as a chemosensor for ultrasensitive Cu²⁺ions detection. The fluorescence of the obtained chemosensor could be quenched effectively by Cu²⁺ions. The quenching mechanism was studied and the results showed the existence of both static and dynamic quenching processes. The modified Stern-Volmer equation showed a good linear response (R2=0.9957) in the range of 1 to 10μM with a quenching constant (Ksv) of 4.9×104 M-1. Most importantly, magnetic measurements showed that the Fe3O4@C@CdTe core/shell microspheres were superparamagnetic and they could be separated and collected easily using a commercial magnet in 10s。5. Novel fluorescent composite nanofibrous films of rhodamine 6G/PAN and europium complexes/PAN are prepared by electrospinning. The optical properties of electruspun nanofibrous films are studied. For the rhodamine 6G/PAN fluorescent composite nanofibrous films, we have used casting films as reference material to compare the aggregation states of incorporation of Rh6G in electrospun nanofibrous films and casting films. The large specific surface area of the nanofibers and fast evaporation of the solvents in the electrospinning process reduced the aggregation of Rh6G. For the europium complexes/PAN fluorescent composite nanofibrous films, the thermal-stability, photo-stability and photoluminescence properties of these composite nanofibers are studied in comparison to that of the pure europium complexes in detail. The results indicate that, the PAN can provide a rigid environment for the europium complexes, so the thermal-stability and photo- stability of composite nanofibers are considerably improved. Most importantly, the luminescent quantum efficiency and luminescence lifetime of the composite nanofibers are of great improvement. The results demonstrate that the electrospun films are an ideal material for incorporation of fluorescent dyes and europium complexes.6. The Mannich reaction was successfully used in modification of the electrospun films surfaces. Further modification of electrospun films with fluorescent molecules presents the possibility of practical sensing applications of the electrospun films.