| The quantum dots(QDs), with their characteristics such as adjusted size, wide excitation wavelength, narrow emission wavelength, good stability, and high quantum yield, have been attracting wide interest owing to their various potential applications in biological applications, solar cells, tracer, etc. The traditional organic preparation methods prepared QDs have high dispersion and high quantum yield. However, the surface of QDs, as-synthesized, is usually hydrophobic, making them unsuitable for direct use in biological environments. For many biomedical applications, QDs need to be monodisperse, biocompatible, small, and available functional groups for conjugation. So developing robust QD surface chemistries to ensure long-range colloidal stability for these challenging applications continues to be a primary research subject. Until now, a number of reports have described various methods to transfer QDs between organic and aqueous solvents, including ligand exchange reactions, amphiphilic polymer, and silica coating. Each method has the advantages. The amphiphilic polymers can provide bright and stable materials by “insulating” the QD from the aqueous environment. The ligand exchange reactions can prepare the hydrophilic quantum dots with small size, which is better to the further biological application. Previously, the method to prepare amine-functionlized hydrophobic QDs was based on the poly(ethyleneimine). But it may be very challenging owing to the poor on the transfer PL efficiency, colloidal stability, and poor ability to resist photobleaching. Meanwhile, the using of DHLA to prepare carboxyl-functionlized hydrophobic QDs faced the same problems. So it is necessary to make a research about improving the photoluminescence intensity and colloidal stability of hydrophobic QDs. In this paper, we use the n-alkylated branched poly(ethyleneimine) and zinc-metalated dihydrolipoic acid to make amine-functionlized and carboxyl-functionlized quantum dots, respectively. The main work and the results obtained are as follows:Chapter Two: We have synthesized N-alkylated branched poly(ethyleneimine) by means of the alkylation of poly(ethyleneimine) amino groups with the corresponding bromo-alkanes. The N-alkylated branched poly(ethyleneimine) could directly insulate the hydrophobic quantum dots to prepare amino-functionalized hydrophilic quantum dots. Herein, an exhaustive research on the transfer photoluminescence efficiency has been performed by analyzing three parameters(hydrophobic chain length, degree of alkylation, and molar ratio of N-alkylated branched poly(ethyleneimine) to quantum dots. In comparison with previous poly(ethyleneimine)-quantum dots prepared by direct ligand-exchange reactions, the N-alkylated branched poly(ethyleneimine)-quantum dots had enhanced 20% photoluminescence intensity and colloidal stability in various biologically relevant environmental conditions(e.g., over a wide pH range, temperature, salt concentrations, and PBS buffer solution). This procedure was versatile and has been successfully extended to other hydrophobic quantum dots, such as Ag and Fe3O4 nanocrystal.Chapter Three: we reducd lipoic acid to prpare the dihydrogen lipoic acid which was metalated with zinc to produce the metalated dihydrogen lipoic acid((DHLA)2Zn2-). Finally we used(DHLA)2Zn2- to prepare carboxyl-functionlized quantum dots. The obtained(DHLA)2Zn2--QDs were characterized by FTIR, TEM, and DLS which indicated that(DHLA)2Zn2- have effectively exchanged the ligand of the hydrophobic quantum dots. In comparison with previous DHLA-QDs,(DHLA)2Zn2--QDs had enhanced 15% photoluminescence intensity and colloidal stability in various biologically relevant environmental conditions(e.g., over a wide pH range, temperature, salt concentrations, and PBS buffer solution). The use of metalated ligands helps solve a lot of long-standing problems involving the application of small cap exchanged QDs for biological application. |
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