The Influence of Several Compounds and Liposome Encapsulation on the Optical Properties of Quantum Dots
|Keywords||Blue shift of quantum dots Photobleaching of quantum dots Blinking in quantum dots Triplet state quenching Liposome encapsulation of quantum dots|
Compared with conventional fluorescence dyes, quantum dots (QDs) have many excellent optical properties, such as broad excitation spectrum, narrow emission spectra, high quantum yields, good photostability and so on. They have gotten extensive use in molecular biology, cell biology, biochemistry, proteomics, genomics, drug screening, medical diagnosis, and so on. This thesis studied the influence of several compounds and liposome encapsulation on the optical properties of quantum dots. The main contents were as follows:1. The influence of several compounds on the optical properties of quantum dots. The effect of several compounds such as mercaptoacetic acid, cysteine, dithiothreitol, ethyl gallate, thiourea, glutathione, surfactants and sodium sulfite’s depleting oxygen to fluorescence blinking, photobleaching, blue shift of single quantum dots in solution was investigated in detail. The results of experiment proved that sulfhydryl compounds such as mercaptoacetic acid, cysteine, dithiothreitol, thiourea, glutathione, and antioxidant ethyl gallate could suppress blinking in quantum dots by passivating surface trap states of quantum dots. And the power-law distribution exponentialαon decreased which meant long“on”time after addition of these compounds. The addition of an appropriate dose of triplet quencher such as dithiothreitol, glutathione, and antioxidant ethyl gallate into the sample and depleting the oxygen from solution by use of sodium sulfite could inhibit photobleaching of quantum dots. We also found QDs’photobleaching was speeded up by cationic surfactants (hexadecyl trimethyl ammonium bromide, CTAB, hexadecyltrimethylammonium chloride, CTAC) because they could protect QDs’triplet excited state from quenching at a concentration above the critical micellar concentration (CMC) of them. Thus we speculate that photooxidation and accumulation of the triplet excited state together induce photobleaching of quantum dots. The oxidation of quantum dots cores was blocked by addition of an appropriate dose of strong reducing agents (dithiothreitol, ethyl gallate) and by efficient oxygen removal using sodium sulfite, and as a result, blue shift of quantum dots was suppressed. In a word, the optical properties of quantum dots were improved by several compounds which could effectively suppress the photobleaching, blue shift and blinking in quantum dots and the possible mechanism behind these optical phenomena was elucidated. Consequently, the application scope of quantum dots would expand further.2. The influence of liposome encapsulation on the optical properties of quantum dots. The water-souble carboxyl quantum dots were encapsulated in liposomes by the method of reverse-phase evaporation. It was characterized by the combination of fluorescent imaging and scattering imaging and we also investigated the impact of liposome encapsulation on the optical properties of quantum dots. The result indicated that most of the quantum dots were encapsulated in liposomes and the quantum dots had lower photobleaching resistance after liposome encapsulation. This method raised the biological compatibility of quantum dots, but meanwhile, the QDs’photostability was lowered.