Synthesis of Aqueous CdTe/CdS/ZnS Core/shell/shell Quantum Dots by a Chemical Aerosol Flow Method
© to the authors 2009
Received: 6 September 2009
Accepted: 5 October 2009
Published: 23 October 2009
This work described a continuous method to synthesize CdTe/CdS/ZnS core/shell/shell quantum dots. In an integrated system by flawlessly combining the chemical aerosol flow system working at high temperature (200–300°C) to generate CdTe/CdS intermediate products and an additional heat-up setup at relatively low temperature to overcoat the ZnS shells, the CdTe/CdS/ZnS multishell structures were realized. The as-synthesized CdTe/CdS/ZnS core/shell/shell quantum dots are characterized by photoluminescence spectra, X-ray diffraction (XRD), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Fluorescence and XRD results confirm that the obtained quantum dots have a core/shell/shell structure. It shows the highest quantum yield above 45% when compared to the rhodamine 6G. The core/shell/shell QDs were more stable via the oxidation experiment by H2O2.
In recent years, colloidal semiconductor nanocrystals or quantum dots (QDs) have attracted great scientific and technological interest due to their unique size-dependent properties [1–3]. By changing the size or composition of the QDs, their optical properties can be easily tuned to suit for various requirement [4, 5]. In the biological labeling especially [6, 7], numerous successful efforts have been made by using the QDs as the fluorescence agent. The organometallic synthesis of CdSe and CdTe QDs provides a good method to synthesize high quality QDs, which were synthesized using high boiling point solvents such as trioctylphosphine (TOP), trioctylphosphine oxide (TOPO) and so on. However, they were hydrophobic and need time consuming operation to convert into water soluble dispersions. The high toxicity of precursors and high price also limit their wide application. Aqueous colloid approaches become more attractive because they are much cheaper and suitable for biological studies.
Aqueous CdTe [CdTe (aq)] QDs with high fluorescence can be synthesized using a metal salt and NaHTe with mercaptan acid as the stabilize agent [8, 9]. It provides a simple way to generate aqueous QDs. But several hours to several days’ reaction time is needed. Chemical aerosol flow (CAF) synthesis of QDs  was first published by Suslick et al. and has been widely used to generate many kinds of nanoparticles, including mesoporous silica [11, 12], mesoporous carbon [13, 14], semiconductor nanocrystals [10, 15, 16], and other nanomaterials [17–19]. It provides a simple and fast way to continuously synthesize nanoparticles. In our previous work, we used the modified CAF method to generate CdTe/CdS core/shell quantum dots in several seconds . The QDs showed high stability, high quantum yield and have large scale. But for bioapplication, the cadmium-based QDs would release toxic Cd 2+ when used in the cell or tissues . Capping a shell of ZnS not only can decrease the toxicity of cadmium but also can increase the quantum yield , forming a core/shell or core/shell/shell structure with CdTe cores inside and ZnS shells outside is a good method to solve this problem.
With successive ionic layer adsorption and reaction (SILAR)  method, high quality of core/shell and multishell quantum dots can be synthesized [23–27] in organic media. Unfortunately, this method was complex, and it is hard to be carried out in aqueous solution. Synthesis of core/shell QDs especially core/shell/shell QDs in aqueous solution is a hard work as the surface stabilizers were fragile and sensitive to the environment. Many efforts have been made to obtain high quality core/shell in water ; however, only a few works pay attention to the synthesis of the CdTe/CdS/ZnS core/shell/shell quantum dots. Because of the large lattice mismatch between CdTe and ZnS (16.4%), it is hard to epitaxially grow ZnS shells on CdTe cores. A shell of CdS between CdTe and ZnS can work as a transition shell because the band gap and lattice contact of CdS is just between that of CdTe and ZnS. Using microwave irradiation method, high quantum yield CdTe/CdS/ZnS quantum dots can be obtained , but multistep was needed, the process was extremely troublesome, and the yield was low. So developing a low cost, simple, continuous way for preparing hydrophilic QDs with a shell of ZnS capped is an urgent need.
Here, we investigated a facial way to directly and continuously synthesize the CdTe/CdS/ZnS core/shell/shell QDs using a modified chemical aerosol flow method. In this integrated synthesis system, the Cd, Te precursors were first carried into the chemical aerosol flow system to result CdTe/CdS core/shell quantum dots. Then, they were brought out into a vessel containing Zn, S precursor solution for coating a shell of ZnS. The obtained core/shell/shell QDs were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometer (EDS), and PL spectra to confirm the core/shell/shell structure.
Cd(NO3)2·2H2O, Zn(NO3)2·2H2O, NaOH, NaBH4(99%), tellurium powder (99.8%), thiourea (97%), and 3-mercaptopropionic acid (MPA, 99%) were purchased from Beijing Chemical Reagent Co., Ltd. All chemicals were used without additional purification. Distilled water was used for preparation of all aqueous solutions.
Briefly, 0.5 mmol Te powder and 2 mmol NaBH4 were mixed in a tube, and then 2 mL water was added. The reaction mixture was heated at 80°C for 30 min to get a pink NaHTe solution. The NaHTe solution was stored at 4°C for further use. One millimole Cd(NO3)2·2H2O, 2.4 mmol MPA, and 100 mL water were mixed, and the pH of the solution was adjusted to 11.5. Subsequently, the NaHTe solution was injected into the mixture, and a clear deep red solution was obtained.
One millimole Zn(NO3)2·2H2O, 2.4 mmol MPA, and 100 mL water were mixed, and the pH was adjusted to 11.5. After stirring for 30 min, 2 mmol thiourea was added.
Synthesis of CdTe/CdS/ZnS
In a typical synthetic procedure, the stock solution containing Cd(MPA) complex and NaHTe was carefully transferred to the equipment and nebulized into microdroplets by a 1.7-MHz ultrasonic generator (Yuyue 402AI, Shanghai Yuyue Co. Ltd). The mist was carried to the furnace by a N2 flow at designed rate through a quartz tube located in a tube furnace kept at appointed temperature, and in the furnace, the solvent evaporated, then the reaction took place, and subsequently CdTe/CdS core/shell quantum dots were obtained; in the following collection and further reaction stage, the synthesized CdTe/CdS QDs were pumped into a three-neck flask containing the Zn and S precursor stock solution with continuous stirring and kept at about 80°C, capping a shell of ZnS was realized in this stage.
Fluorescence spectra were measured at room temperature using a FL-4600 spectrofluorimeter (HITACHI), Powder X-ray diffraction (XRD) measurements were performed on a D8 Focus XRD system (Bruker), and samples for XRD were prepared by dropping a colloidal solution of QDs in water on a glass sheet. EDS data were obtained on a scanning electron microscope S-4300 (HITACHI) system.
Chinese hamster ovary (CHO) cells were grown as a monolayer in a humidified incubator in a 95% air/5% CO2 atmosphere at 37°C in a dish containing DMEM supplemented with 10% (v/v) heat-inactivated fetal bovine serum, 100 IU/mL penicillin, and 100 IU/mL streptomycin. The CHO cells were detached mechanically and adjusted to the required concentration of viable cells as determined by counting in a hemocytometer. The CHO cells were plated 24 h before the start of the experiment in chamber slides at a density of 5 × 103 cell/cm2. One milliliter QDs was added and incubated with the CHO cells for 30 min. The slides were washed twice with PBS and then examined with a LEICA-Sp5 confocal microscope.
Results and Discussion
In our experiment, the intermediate CdTe/CdS QDs synthesized show great stability due to the good crystallization at high reaction temperature, and the in situ synthesized CdS shells on CdTe cores can play as a buffer layer to further epitaxially grow ZnS shells. So with this facile method, high quality of CdTe/CdS/ZnS core/shell/shell QDs can be synthesized.
As known, in the synthesis of QDs, especially that with core/shell/shell structure, large-scale production is difficult for many methods mainly due to its difficulty to ensure the same temperature and homogeneous mixing in the large volume of solution, which have a great influence on the monodispersity of the nanocrystals. Up to now, large-scale synthesis is still a challenge . Here, in our modified integrated apparatus, the continuous synthesis method makes it possible to realize large-scale synthesis of QDs with core/shell/shell structure. In our experiment, the rate of production can reach as high as 0.1 g/h. As the currently used quartz tube in the furnace only has a diameter of 30 mm, the flow rate is 1.5 L/min. It is easy to improve the production rate using a larger diameter tube or increasing the flow rate. Owing to its continuity, as much core/shell/shell QDs can be synthesized.
Control experiments to confirm the core/shell/shell structure of synthesized QDs
Fluorescence wavelength (nm)
In summary, CdTe/CdS/ZnS core/shell/shell quantum dots were synthesized by chemical aerosol flow method in a continuous system. This method can provide a simple, ultrafast, and continuous way to prepare core/shell/shell quantum dots. Importantly, compared with CdTe QDs prepared directly in aqueous solution and CdTe/CdS core/shell QDs synthesized by chemical aerosol flow method, the CdTe/CdS/ZnS core/shell/shell QDs have enhanced anti-oxide ability and stability. This is significant for further application of aqueous QDs. We also prove that the QDs can achieve multicolor label in living cells. Benefiting from their reduced toxicity, enhanced stability, and increased PLQY, this kind of core/shell/shell QDs has potential for future in vivo fluorescent imaging.
The current investigations were financially supported by the Hi-Tech Research and Development Program of China (863 program 2007AA021803 and 2009AA03Z302) and the National Natural Science Foundation of China (NSFC No. 60736001) and Natural Science Foundation of Beijing (2093044).
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