Prepartion Of Quantum Dots Based Fluorescence Probes And Their Application In Detection

Due to quantum confinement effect, the inherent physical or chemical properties of nanoscaled materials could be tuned by changing their particle sizes. One of the intuitionistic performances of quantum confinement effect in semiconductor nanocrystal (also called quantum dots, QDs) is that the energy band gap of QDs increases upon the reduction of particle size. Therefore, different emission wavelengths can be achieved with use of the same kind of nanomaterial with different particle sizes. Compared to traditional organic dyes, QDs exhibit better photostability and continuous absorbance spectra. These properties make QDs more suitable in long-time investigation and bring forward a wider range of excitation wavelengths for QDs. Thus QDs with different emission colors can be excited by a single excitation wavelength, allowing multiplexed detection. Furthermore, QDs have narrow and symmetrical emission spectra, high emission intensity (the emission intensity of an individual particle is about100times stronger than a single dye molecular) and long fluorescence lifetime. These advantages make QDs alternative emitter for super-sensitive, multicolored, and long-time detection, tracing and imaging in biomedicine. In recent years, QDs have attracted more and more interests in the area of analysis and biological imaging. In this dissertation, QDs are used as the luminescence materials to build fluorescence probes for the selective and sensitive detection of Hg2+, dopamine and biothiols. The main research contents are as follows:(1) QDs-based ratiometric fluorescence probe for the detection of Hg2+The analysis of toxic heavy metal ions is always an important issue in environment monitoring. Ratiometric fluorescence probes that can significantly eliminate the external effects by self-calibration of two different emission bands are preferable for the detection of real samples. Here, we designed a dual-emission QDs nanocomposite as a ratiometric probe to visually detect Hg2+in aqueous solutions. The dual-emission QDs nanocomposite consists of two differently sized CdTe/CdS QDs. The red-emitting larger sized CdTe/CdS QDs were embedded in silica nanoparticles, while the green-emitting smaller sized ones were covalently conjugated onto the silica nanoparticles surface. The addition of Hg2+can only quench green fluorescence in the dual-emission QDs nanocomposites because of the electron transfer between green-emitting QDs and Hg2+. However, the red-emitting QDs were insensitive to Hg2+due to the protection of SiO2shell, and their fluorescence intensities were accordingly barely changed. The quenching of green fluorescence triggered the change of fluorescence intensity ratio of two different emission wavelengths and hence induced the evolution of fluorescence color of the probe solution with the variation of Hg2+concentration. Based on this feature, the dual-emission QDs nanocomposites can be used to develop a ratiometric fluorescence probe for visual detection of Hg2+. Under UV-light irridation, with the increasing of the Hg2+concentration, the emission color of the probe solution changed from yellow-green to red gradually, which could be directly observed by naked eye for visual detection of Hg2+. In our experimental conditions, the detection limit can reach up to3.1nM. The probe also displayed good selectivity to Hg2+under the existence of other common metal ions. In addition, it had been successfully used in the determination of Hg2+in biological samples.(2) Adenosine capped QDs based fluorescence probe for the detection of dopamine.Dopamine (DA) is a kind of neurotransmitter, its deficiency in human body may lead to some neurological diseases. DA is very susceptible to be oxidized in aqueous solution, generating dopamine quinone (DAQ), which is a good electron acceptor. According to this property of DA, a QDs based fluorescent probe was synthesized for highly sensitive and selective detection of DA. In this assay, adenosine served as a capping ligand or stabilizer for QDs to render initial oil-soluble QDs with high quality dispersed in water; and as a receptor for DA to attach DA onto the surface of adenosine capped QDs. DA molecules can bind to adenosine capped QDs via non-covalent bonding, resulting in the fluorescence quenching of QDs due to the electron transfer between QDs and DAQ, thus enabling the detection of DA. The A-QDs based fluorescence probe showed a limit of detection of ca.29.3nM for DA detection. This facile method exhibited high selectivity and anti-interference in the presence of amino acid, ascorbic acid (AA), uric acid (UA) and glucide with100-fold higher concentration. Moreover, it was successfully used in the detection of DA in the human urine samples with quantitative recoveries.(3) CdTe/CdS QDs based fluorescence probe for the detection of biothiolsBiothiols (glutathione (GSH), cysteine (Cys), homocysteine (Hcy)) involved in a great number of important vital activities in human body, and unnormal alternations in the level of cellular thiols will cause many serious diseases. Thus, achieving rapid and instant detection of biothiols shows great significance for the diagnosis and treatment of illness. In this dissertation, we directly used water phase synthesized mercaptopropionic acid (MPA) capped CdTe/CdS QDs to design a fluorescent probe for the detection of biothiols in water environment. The quenched fluorescence of QDs attached to TiO2nanoparticles (TiO2NPs) was selectively switched on by biothiols through ligand replacement, which makes it feasible for facilely sensing biothiols based on the fluorescence turn on mechanism. The detailed principle is as follows:when TiO2NPs were added into the QDs solution, QDs were absorbed on the surface of TiO2NPs through the covalent bond between the terminal carboxyl group of MPA ligands and the Ti atoms on the surface of TiO2NPs, leading to the fluorescence quenching of QDs due to the electron injection from QDs to TiO2NPs. When biothiols were introduced into the QD-TiO2system, the MPA ligands on the surface of QDs may be replaced by biothiols due to the stronger coordination capacity of biothiols with metal ions on the QDs surface, and the distance between QDs and TiO2NPs is consequently enlarged. As a result, the initially quenched fluorescence of QDs is effectively recovered by the interruption of the electron transfer pathway. Based on the above features, a facil probe with excellent selectivity and high sensitivity was construced for simply sensing biothiols including GSH, Cys and Hcy. Under the optimal conditions, the detection limits of GSH, Cys and Hcy are0.17μM,0.28μM and0.15μM, respectively. The probe exhibited excellent selectivity and anti-interference ability, the presence of other19kinds of essential amino acids will not interfere with the detection of biothiols. The determination of GSH in biological samples also received satisfactory results. In addition, a novel fluorescent indicating paper was constructed by immobilizing the probe on a piece of filter paper to visually detect GSH in which only a UV lamp was used. The indicating paper provided a simple platform for facial and visual detection of biothiols.