Construction Of Supermolecular Combined Fluorescence Chemical Sensor And Its Detection Of Amino Acids In Water

With increasing attention paid to healthy problem, disease diagnosis and treatment, and realizing the important roles of amino acids playing in both animal and human bodies, the detection and analysis of amino acid has draw extensive attention. Amino acids are a kind of substances that are indispensable for vital movements and other specific physiological processes. Moreover, they are also one of nutrients required for living bodies. The lacking or reducing one of the kinds will disturb the normal life metabolism, more seriously, even lead to diseases or the termination of life activities. Hence, the detection of amino acids takes an important position which is highly related to disease diagnosis, nutrition analysis, drug monitoring, food processing, as well as life science research. However, amino acids have features such as structural similarity, spectral inertness, weak chromophores and none electrochemical activity, and all these features challenge the detection of amino acids. At present, the methods for separation and determination of amino acids are continuously developed and improved, and plenty of methods have been achieved for detection of amino acids. Nevertheless, new methods for rapid, simple and precise sensing amino acids are still of great significance. Fluorescence sensors for molecular identification have drawn extensive attention because of high sensitivity and selectivity, and have been used for detection of metal ions, anions and organic bioactive molecules. In addition, amphiphilic surfactant molecules can spontaneously form supramolecular self-assemblies such as premicelle, micelle and vesicle. Fluorescent probes are encapsulated in the hydrophobic microdomain of supramolecular assemblies through electrostatic and hydrophobic interactions with these assemblies. As a result, the fluorescence stability and water-solublity of the fluorescence molecules in aqeous solution are significantly increased, and the photophysical behaviors of fluorescent molecules can be adjusted. Moreover, the sensing behaviors of fluorescent probes can be modulated by the concentration, surface charge and hydrophobic chain of the surfactants.Based on reviewing the progress in fluorescent sensors for amino acids, comparison of the advantages and disadvantages of various methods, and evaluating the fluorescence sensors developed in our laboratory, my dissertation proposes to design and construct fluorescent supramolecular sensors for amino acids since they are barely reported so far. In the present dissertation, the following three projects were mainly accomplished.In the first project, dansyl was chosen as the signal output group since it has high fluorescence quantum yield and polarity sensitivity to microenvironment. A cationic dansyl-based fluorophore, DIlSD, was designed and synthesized. Its combination with anionic surfactant SDS assemblies shows enhanced fluorescence intensity and blue-shifted maximum wavelength. Its fluorescence can be slightly quenched by Cu2+, however, the fluorescence quenching efficiency by Cu2+ is highly increased upon titration of arginine (Arg). As a result, the ternary system containing the cationic fluorophore, anionic surfactant, and Cu2+ function as a highly sensitive and selective sensor to Arg. The optimized sensor system displays a detection limit of 170 nM, representing the highest sensitivity to Arg in total aqueous solution by a fluorescent sensor. Control experiments reveal that the imidazolium groups in the fluorophore, the anionic surfactant, and Cu2+ all play important roles in the process of sensing Arg. The electrostatic interaction between the cationic fluorophore and anionic surfactants facilitates the binding of imidazolium rings with Cu2+, the surfactants surface-anchored Cu2+ is responsible for further binding of Arg, and the electrostatic interaction between anionic surfactants and positively charged amino acids accounts for the selective responses to Arg.In the second project, pyrene was used to substitute dansyl group in DIlSD since pyrene not only has features such as high fluorescence quantum yield, long fluorescent lifetime, and sensitivity to microenvironment, but also provide rich fluorescent signals such as monomer and excimer emission. Therefore, a similar cationic fluorophore based on pyrene derivative, DIlSPy, was designed and synthesized. Firstly, the photophysical behaviors of DIlSPy in various surfactants and neat water reveal that DIlSPy emits both monomer and excimer emission in aqueous solutions. Moreover, the fluorescence emission of DIlSPy can be easily modulated by controlling the concentration of SDS. Compared to DIlSD/SDS/Cu2+, the ternary sensor system based on DIlSPy/SDS/Cu2+ exhibits more interesting and different sensing performance to amino acids. The ternary sensor system shows on-off responses (excimer quenching) to low concentrated Arg and Lys and off-on responses (monomer enhancement) to high concentrated Arg and Lys. The detection limits of Arg and Lys turn out to be 5.2 nM and 14.6 nM, respectively, which represent the lowest detection limits for these two amino acids. The sensing mechanism was explored by conducting systematical control experiments. A possible assumption for the extraordinary sensing behavior is proposed as follows:the low concentration of Arg (0.2-10 μM) binding with SDS assemblies enhances the electron transfer from Cu2+ to the fluorescent probe and gives rise to fluorescence quenching. On the contrary, the high concentration of Arg (over 10 μM) leads to dissociation of SDS aggregates and leak of the fluorophore back into aqueous environment, and as a result gives rise to monomer emission.In the third project, DI1SD, the fluorescent probe from the first project, shows favorable sensing behaviors to Asp and Glu in non-buffered aqueous solution containing SDS. The highest sensitivity to these two amino acids was found when the concentration of SDS is 10 mM, and the detection limit for Asp and Glu are 1.6 μM and 6.0 μM, respectively. At the same time, the binary sensor system, DI1SD/SDS, shows a fine selectivity to these two amino acids, and a linear response over a certain concentration range. The preliminary study of sensing mechanism by ultraviolet absorption spectrum indicates that there may be a complex formed among SDS, DI1SD and Asp. Since Asp and Glu are acidic and negatively charged amino acids among the 13 kinds of amino acids, the effect by hydrochloric acid on the fluorescence emission of the system was also examined. It turns out that H+ could also quench the DIlSD/SDS assembly fluorescence, which suggests that the sensing to Asp and Glu may be due to the acidic nature of these two amino acids. Further studies are still needed for exploring the sensing mechanism of the binary sensor system to Asp and Glu.