Preparation Of Multifunctional Fluorescent Nanomaterials Based On CdTe Quantum Dots And Their Applications In Biological Recognition

Recently, quantum dots (QDs) have been successfully applied in the field of tumor cellslabeling and biochemical analysis because of their novel physicochemical properties. Nevertheless,the potential cytotoxicity of cadmium ions released from the degradation of CdTe QDs exposed tothe physiological environment is an issue of concern in the bio-application field such as celllabeling. It is essential to develop proper strategies on surface modification and functionalizationto improve the biocompatibility and fluorescence properties of the QDs. The major challenge ofthe current QDs research is the selectivity when they are applied in biochemical analysis.Molecular imprinting is an attractive strategy to design a matrix that can be tailor-made materialswith high selectivity for a target molecule. The resulting molecularly imprinted polymers (MIPs)exhibit a higher affinity for the template molecule than for other structurally related compounds.Combining the high selectivity of molecular imprinting and the fluorescent properties of QDscould develop a novel fluorescent sensor for target analyte recognition. The fluorescent sensorsprepared recently commonly employ QDs for recognition signal amplification and optical readout,which have some drawbacks such as poor fluorescence signal stability, bad specific recognitionand long response time. Such existing problems are mainly led by the organic molecules orpolymers enclosing the QDs which have effects on the surface status, spatial distribution andinteraction. It is found that the combination of QDs with carbon nanotubes (CNTs) or graphenecan effectively improve the fluorescent properties of QDs. While the composites of CNT-QDs orgraphene-QDs have exhibited enhanced photocurrent and photocatalysis in electrochemical sensing, very little attention was paid to the preparation of molecularly imprinted materials basedon CNT-QDs or graphene-QDs up to date. According to the discussion above, we have carried outthe following research in this thesis.(1) High-quality CdTe QDs were rapidly synthesized in aqueous phase using both L-Cysteine(L-Cys) and mercaptopropionic acid (MPA) as mixed-ligand and were employed as fluorescentbiomarkers for labeling tumor cells. In consideration of the different properties of L-Cys and MPA,CdTe QDs with superior fluorescent properties were prepared using L-Cys and MPA as mixedligands. The ratio of L-Cys and MPA was found to have influence on the growth kinetics andoptical properties of CdTe QDs. Compared to single ligand of L-Cys or MPA, L-Cys/MPA asmixed-ligand could accelerate the red shifted rate of fluorescence emission peak wavelengtheffectively. CdTe QDs capped with mixed-ligand exhibited a high uniformity in diameter and anexcellent photostability.50L-CdTe (L-Cys/MPA molar ratio was50%) held a high quantum yieldof66.4%and75L-CdTe (L-Cys/MPA molar ratio was75%) exhibited a luminescence decaylifetime of46.8ns. MTT assays showed that75L-CdTe with emission of615nm maintained75-95%SiHa cell viability. Further study of labeling SiHa cell with the as-prepared75L-CdTeindicated that the QDs are excellent biomarkers for SiHa cell. L-Cys and MPA as mixed-ligandwith proper design can be employed to fabricate CdTe QDs with superior fluorescence propertiesand biocompatibility, which have potential in biological and medical fields as biomarker-candidates.(2) Three kinds of protein imprinted optosensing material based on CdTe/CdS QDs,multi-walled carbon nanotubes-QDs (MWCNTs-QDs) and graphene oxide-QDs (GO-QDs) havebeen designed and synthesized separately for the recognition of a target protein bovine serumalbumin (BSA). CdS shell was capped on CdTe core to obtain CdTe/CdS core/shell QDs.Polyethylenimine (PEI) was chosen to modify MWCNTs and GO separately to obtain PEIfunctionalized MWCNTs (PEI-MWCNTs) and PEI functionalized GO (PEI-GO) which possessexcellent dispersible in aqueous solutions. MWCNTs-QDs and GO-QDs fluorescentnano-composites were got through the formation of covalent bonds between MWCNTs and QDsor GO and QDs using EDC·HCl/NHS as coupling agents. Then three kinds of protein imprintedmaterial BMIP1, BMIP2and BMIP3were prepared using BSA as the template protein via sol-gelprocess based on CdTe/CdS QDs, MWCNTs-QDs and GO-QDs respectively. Among the threefluorescent sensors, BMIP2which was prepared using MWCNTs-QDs composite for recognitionsignal amplification and optical readout exhibited a much better sensibility in fluorescence–0.9mg/ml of the point of adsorption equilibrium for BSA,25min of the response time,4.08of theimprinting factor. Compared to non-imprinted materials, the template protein could further quench the fluorescence intensity of the imprinted materials, indicating an excellent recognition capabilityof the imprinted materials for the template protein through a series of adsorption experiments.