Preparation, Imaging Of Quantum Dots And Their Applications In Chemical And Biological Analyses

Semiconductor quantum dots (QDs), as novel fluorescent nanomaterials, have attracted much interest due to their unique size-dependent optical properties. With the development of their synthetic techniques, surface processability and functional applications, QDs play important roles in the fields of chemical and biological analysis. Aiming at this important research field, based on a survey of a large number of documents, combining with materials preparation, optical imaging and nanometer analysis, by taking fluorescence technique as the main analytical approach, the preparation, their analytical detection and imaging of QDs as the line of this dissertation, following several works have been performed.1. Oleylammonium-selenide complex was exploited as a new precursor for synthesis of hydrophobic QDs. H2Se gas was firstly generated by reduction and subsequent acidation of Se powder. The gas was then introduced into oleylamine, which led to the formation of oleylammonium-selenide complex. The complex was used as precursor for preparation of hydrophobic CdSe QDs via hot-injection solvothermal synthetic procedures. The characterization results showed that the proposed method led to the formation of hexagonal wurtzite CdSe QDs with narrow fluorescence full-width at half-maximum (25 ~ 40 nm), high photoluminescence quantum yield (up to 23%), and a broad emission spectra tunable from 480 ~ 610 nm. Furthermore, core/shell CdSe/ZnS QDs were prepared. This work provides a low cost and environmentally benign technique for QDs preparation due to the absence of trialkylphosphine, and the obtained QDs with excellent properties could be further used in the fields of surface modification, nanomaterials assembly and analytical detections.2. A facile and mild chemical etching method was developed for resizing of CdTe QDs. Treated with BF4-, CdTe QDs gradually decreased in size and this was accompanied by corresponding blue-shifts of both the absorption and fluorescence spectra. The chemical etching process may involve both the removal of Cd dangling bonds by BF4- and the assistance of surface oxidation. The etching was performed at room temperature under neutral conditions, and different sizes of CdTe QDs could be obtained through termination of the etching reaction at different time intervals. This novel etching technology provides a means of downsizing and tailoring of the QDs and has great potential for the architecture of multifunctional nanostructures3. A strategy of combining physical embedding and covalent crosslinking was developed to encapsulate cysteamine-capped QDs into agarose hydrogel microbeads (AHM). Cysteamine-capped QDs were encapsulated into the pores of agarose hydrogel microbeads by virtue of hydrogen bonding between the amino groups of cysteamine and hydroxyl groups of agarose, resulting in more than 6.0×107 QDs per microbead. Polyethylenimine (PEI) and oxalaldehyde were then introduced to form a covalent cross-linked network to further stabilize the encapsulation. The resulting hybrid hydrogel microbeads exhibited high doping capacity and negligible QDs leaching. Furthermore, the microbeads possessed elevated fluorescence stability at wide pH ranges and two-color barcoded hydrogels were successfully obtained. We envision that further optimization of this method will allow high-through screen and array analysis of the hybrid hydrogel microbeads technology.4. A simple ions-mediated dispersing technique was developed to reduce the aggregation and non-specific adsorption of QDs. By the introduction of F- ions, the self-aggregation of cysteamine-capped QDs was disassembled, and the stability of QDs was greatly enhanced. We inferred the disaggregation was from the substitution of NHF hydrogen bonds for NHN hydrogen bonds of cysteamine-capped QDs. Meanwhile, we found in the presence of F- ions, the non-specific adsorption of QDs on the glass slides and cell was greatly decreased. Furthermore, the chemical cross-linking efficiency of QDs with biomolecules was greatly improved in the presence of F- ions. This work not only provides a new strategy for reducing the aggregation and non-specific adsorption of QDs, but also provides a guarantee for the following biomedical imaging and chemical sensing.5. A switchable fluorescent QD probe for F- ions was developed based on the aggregation/disaggregation mechanism. The cysteamine capped CdTe QDs undergo spontaneous self-aggregation via NHN hydrogen bonds, which results in efficient fluorescence self-quenching. In the presence of F- ions, the aggregates disassemble due to the substitution of NHF hydrogen bonds for NHN hydrogen bonds, which results in fluorescence recovery of the QDs. Thus, the fluorescence off/on process enables us to quantitate F- ions in aqueous media. This design provides a novel means for nanoscopic sensing, which has the advantage of rapid, simple, sensitive, and specific detection, with a limit of detection of 5.0μM for F- ions. This work provides a new signal transduction mechanism for QDs-based sensors and the methodology developed here may be expanded for design of other fluorescence nanoparticle sensors.6. A single nanoparticle imaging technique was set up to study the structure and photophysical properties of different phospholipid encapsulated QDs. Firstly, hydrophobic CdSe/ZnS QDs were phase transferred into water soluble by phospholipids derivatives, 1,2-distearoyl- sn-glycero-3- phosphor- ethanolamine (DSPE) and 1,2-distearoyl-sn- glycero-3- phosphoethanolamine- N-[methoxy(polyethylene glycol)-2000] (PEG2000- DSPE), respectively. Fluorescence fluctuation spectroscopy, fluorescence intensity statistic analysis, and single particle tracking were then executed to study the two types of phospholipid encapsulated QDs. The results showed that compared to DSPE-QDs micelles, PEG2000-DSPE-QDs micelles possessed higher monodispersity, narrower size- distribution, lower fluorescence intensity and lower diffusion coefficients. Hence, we inferred that statistically, many QDs were embedded in one DSPE-QDs micelle, while one QD was embedded in one PEG2000-DSPE-QDs micelle. The single nanoparticle imaging techniques can be further used for real time, in situ, dynamic biomedical research. In addition, the phospholipid encapsulation approach allows different ligands functionalized on the surface of QDs, to satisfy the particular requirements of the analytical applications.7. Fluorescence colocalization method was emplyed for sensitive protein detection. By taking the advantage of the robust fluorescence of QDs and high flexible total internal reflection fluorescence microscopy technique, fluorescence colocalization imaging of QDs at single nanoparticle level can be achieved. Thrombin employed as a protein model, we demonstrate a sandwich structure for protein detection using multicolor aptamer-functionalized QDs as nanoprobes. The presence of target protein can be determined based on colocalization measurements of the nanoassemblies. A low of detection with 0.8 pM was obtained. The fluorescence colocalization method broke the optical diffraction limit and reached nanometer resolution. Meanwhile, the method utilized the multicolor QDs, avoiding the separation and purifying process. Hence, this work provides a simple, fast procedure for sensitive protein detection at single nanoparticle level.