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A two-step approach including the assembly of QDs and subsequent Stöber synthesis has been developed to encapsulate hydrophobic CdSe/Cd(x)Zn(1-x)S core/shell QDs into SiO2 beads. TEM observation shows that the core/shell QDs have high concentration in the SiO2 bead. The fabrication mechanism was proposed for these SiO2 bead. First, partially hydrolyzed tetraethyl orthosilicate (TEOS) replaces the initial ligands of the QDs and assembles the QDs into seeds by condensation reaction. 3-mercaptopropyltrimethoxysilane (MPS) was applied to control the assembly process and inhibit a heavy aggregation. Second, the mixed silane TEOS/carboxyethylsilanetriol sodium (CES) was used for Stöber process. The carboxyl group of CES can react with the sulfhydryl of the MPS facilitating the deposition of the SiC2 on the QDs seeds. Sonication was required for the formation of well-dispersed and well-coated SiO2 beads. The combined application of MPS and CES, the decent silane concentration, and the sonication during the Stöber process were crucial for the synthesis of SiO2 beads with high concentrated QDs.

CdTe quantum dots (QDs) were prepared in an aqueous solution using various mercaptocarboxylic acids, such as 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA), as stabilizing agents. The experimental result indicated that these stabilizing agents played an important role for the properties of the QDs. Although both TGA and MPA-capped CdTe QDs exhibited the tunable photoluminescence (PL) from green to red color, the TGA-capped QDs revealed a higher PL quantum yield (QY) up to 60% than that of MPA-capped QDs (up to 50%) by using the optimum preparation conditions, such as a pH value of ~11.2 and a TGA/Cd molar ratio of 1.5. PL lifetime measurements indicate that the TGA-capped QDs exhibited a short average lifetime while the MPA-capped QDs revealed a long one. Furthermore, the average lifetime of the TGA-capped QDs increased with the increase of the QDs size, while a decreased lifetime for the MPA-capped QDs was obtained. This means that the PL lifetime depended strongly on the surface state of the CdTe QDs. These results should be utilized for the preparation and applications of QDs.

A sol-gel approach including partially removing capping agents, depositing SiO(2) monomers, and growing a SiO(2) shell was developed to generate a bio-compatible functionalization on CdTe quantum dots (QDs). The QDs retained their high photoluminescence (PL) efficiency after coating with a SiO(2) shell (22.5%) by controlling the surface state of the QDs to decrease PL degradation during a sol-gel preparation. Furthermore, the CdTe QDs with a SiO(2) shell were conjugated with IgG using 3-sulfo-N-hydroxysuccinimide or streptavidin-maleimide as linkers. The biotin-streptavidin linker resulted in a high PL efficiency retained (only ~23% lower than the initial value of the QDs), which is crucial for bio-applications., (Copyright © 2010 Elsevier Inc. All rights reserved.)

CdTe0.5Se0.5/Cd0.5Zn0.5S core/shell quantum dots (QDs) with a tunable photoluminescence (PL) range from yellow to dark red (up to a PL peak wavelength of 683 nm) were fabricated using various reaction systems. The core/shell QDs created in the reaction solution of trioctylamine (TOA) and oleic acid (OA) at 300 °C exhibited narrow PL spectra and a related low PL efficiency (38%). In contrast, the core/shell QDs prepared in the solution of 1-octadecene (ODE) and hexadecylamine (HDA) at 200 °C revealed a high PL efficiency (70%) and broad PL spectra. This phenomenon is ascribed that the precursor of Cd, reaction temperature, solvents, and ligands affected the formation process of the shell. The slow growth rate of the shell in the solution of ODE and HDA made QDs with a high PL efficiency. Metal acetate salts without reaction with HDA led to the core/shell QDs with a broad size distribution. [ABSTRACT FROM AUTHOR]

The synthesis and photoluminescence (PL) properties of aqueous CdTe/ZnO core/shell quantum dots (QDs) have been investigated by using thiolglycolic acid as a capping reagent. The highlighted contribution of the present study was CdTe QDs coated with a ZnO shell by controlling the hydrolysis process of Zn(OAc). The QDs benefitted from overcoming the high lattice mismatch between CdTe and ZnO. The PL peak wavelength of the CdTe/ZnO QDs with high PL quantum yields up to 88% was located in a range between 547 and 596 nm by adjusting the size of CdTe cores and the thickness of ZnO shells. The results of X-ray diffraction analysis and transmission electron microscopy observation indicate that the dot-shaped CdTe/ZnO QDs (566 nm) with an average size of 2.2 nm in diameter belong to the cubic CdTe crystal structure. Due to the passivation of surface defects, it is found that the luminescence decay curves accord with a biexponential decay model of exciton and trap radiation behavior. The average PL lifetimes of CdTe (571 nm) and CdTe/ZnO (596 nm) QDs at room temperature are 27.3 and 35.1 ns, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Hydrophobic oleic acid-capped CdSe/Cd x Zn1−x S core/shell quantum dots (QDs) with high photoluminescence (PL) efficiency of 62% and a PL peak wavelength of 602nm were assembled in SiO2 particles through a novel silanization technique. Ligand exchange (3-mercaptopropyltrimethoxysilane (MPS) instead of oleic acid) occurred via the connection of mercapto groups and QDs when MPS was added in the toluene solution of the QDs. After being silanized using MPS, partially hydrolyzed tetraethyl orthosilicate (TEOS) was attached to the surface of the silanized QDs. The hydrophobic CdSe/Cd x Zn1−x S QDs were then transferred from toluene phase to water phase by the hydrolysis of TEOS through the addition of ethanol, ammonia, and water. Further hydrolysis and condensation of the TEOS and MPS causes them to form assemblies in the water phase. MPS is crucial for proper assembly of the QDs in SiO2 particles and for controlling the size of resulting SiO2 particles. The resulting SiO2 particles exhibited tunable diameters from several tens to 100nm. After being encapsulated in SiO2 particles, the QDs revealed a PL efficiency of 25%. Because of the resulting SiO2 particles encapsulating multiple hydrophobic CdSe/Cd x Zn1−x S QDs and having a functional surface (mercapto group), they are applicable to bio-probes. [Copyright &y& Elsevier]

CdSe/Cd x Zn 1-x S core-shell quantum dots capped with oleic acid have been encapsulated into SiO 2 nanospheres by a modified reverse microemulsion method. As an effective approach to convert quantum dots with nonpolar capping agent into hydrophilic and biocompatible ones, silica coating technology is limited by the inevitable luminescence degradation during the silication process. Cd x Zn 1-x S shell involved to protect CdSe core quantum dots from luminescence quenching was found to be quite helpful to restrain the luminescence degradation during the silica coating process. The luminescent intensity has been highly enhanced by controlling the thickness of Cd x Zn 1-x S shells. Luminescence quenching has also been observed when the as-prepared silica coated quantum dots were transferred from the reaction solution into different solvent. According to the time-resolved luminescence and stability examination, the micro channels in the SiO 2 shells should be responsible for the degradation. Polyethylene glycol has thus been introduced to block the channel, resulting in a product with much higher emission efficiency, which performed well as a fluorescent labeling probe for MCF-7 cells. • The Cd x Zn 1-x S shell plays a significant role in keeping PLQY during SiO 2 coating process. • The micro channels in the SiO 2 shells are responsible for the solvent-related PL quenching. • PEG modified CdSe/Cd x Zn 1-x S@SiO 2 nanospheres applied as a fluorescent labelling probe for MCF-7 cells. [ABSTRACT FROM AUTHOR]