Facile preparation of highly luminescent CdTe quantum dots within hyperbranched poly(amidoamine)s and their application in bio-imaging
© shi et al.; licensee Springer. 2014
Received: 13 November 2013
Accepted: 21 February 2014
Published: 13 March 2014
A new strategy for facile preparation of highly luminescent CdTe quantum dots (QDs) within amine-terminated hyperbranched poly(amidoamine)s (HPAMAM) was proposed in this paper. CdTe precursors were first prepared by adding NaHTe to aqueous Cd2+ chelated by 3-mercaptopropionic sodium (MPA-Na), and then HPAMAM was introduced to stabilize the CdTe precursors. After microwave irradiation, highly fluorescent and stable CdTe QDs stabilized by MPA-Na and HPAMAM were obtained. The CdTe QDs showed a high quantum yield (QY) up to 58%. By preparing CdTe QDs within HPAMAM, the biocompatibility properties of HPAMAM and the optical, electrical properties of CdTe QDs can be combined, endowing the CdTe QDs with biocompatibility. The resulting CdTe QDs can be directly used in biomedical fields, and their potential application in bio-imaging was investigated.
Quantum dots (QDs), also referred to as semiconductor nanocrystals, exhibit unique size and shape-dependent optical and electronic properties [1–7]. In recent years, significant progress has been made from the synthesis of QDs to the fabrication of nanodevices and nanostructured materials [8–14].
Until now, various ways have been developed to synthesize high-quality QDs, such as the nonaqueous trioctylphosphine oxide (TOPO)/trioctylphosphine (TOP) technique [15–18], the aqueous route with small thiols [2, 3, 19–23] or dendritic polymers [24–28] as stabilizers, and the biometric template method . The fluorescent QDs stabilized by small thiols or TOPO are inherently instable and should be stabilized by matrix materials in order to realize their successful applications, while the QDs prepared with dendritic polymers as stabilizers and nanoreactors can be directly applied to many fields. The dendritic polymers have three-dimensional globular architecture, numerous cavities, and plenty of peripheral functional groups, which offer dendritic polymers the capability of in situ preparing QDs with controlled size. The QDs prepared within dendritic polymer integrate the optical, electrical properties of QDs and the biocompatibility properties of polymers together, and they are easy to form films or to assemble on substrates. However, the low-quantum yield (QY) and the broad emission spectrum of QDs prepared within dendritic polymers still need to be improved further.
Now, preparation of CdS QDs within dendritic polymers has been reported [23–28]. However, the low QY and the broad emission spectrum of CdS QDs are still unresolved. There is also one work relating to preparing CdTe QDs within poly(amidoamine)s dendrimers (PAMAM) ; however, the highly fluorescence of CdTe QDs prepared within dendritic polymers has not been resolved. In our experiment, we found that if CdTe QDs were directly prepared within dendritic PAMAM without other stabilizers, they were easy to be oxidized and had very weak fluorescence even if microwave heating was used. There are also some works relating on modification of preformed CdTe QDs by dendritic PAMAM [31, 32]. By forming covalent bonds between CdTe QDs and PAMAM, CdTe/PAMAM nanocomposites were prepared. Nevertheless, this method is used for the functionalization of preformed CdTe QDs but not for in situ preparation of CdTe QDs, and the CdTe/PAMAM nanocomposites might form large aggregation or self-assembly. Compared with this route, in situ preparation of fluorescent CdTe QDs within dendritic polymers would be more convenient and effective.
In this paper, we propose a new method to synthesize highly fluorescent and stable CdTe QDs within hyperbranched poly(amidoamine)s (HPAMAM). HPAMAM was introduced to coat the CdTe precursors (not CdTe QDs) stabilized by mercaptopropionic sodium (MPA-Na). By this way, the growth of CdTe QDs can be further controlled, and the CdTe QDs can be endowed with biocompatibility by HPAMAM. After microwave irradiation, highly fluorescent and stable CdTe QDs stabilized by MPA-Na and HPAMAM were obtained. The resulting CdTe/HPAMAM nanocomposites combine the optical, electrical properties of CdTe QDs and the biocompatibility properties of HPAMAM together. They can be directly used in biomedical fields, and their potential application in bio-imaging was investigated.
HPAMAM with amine terminals was synthesized according to our previous work . After endcapping by palmitoyl chloride, the weight average molecular weight (Mw) of HPAMAM measured by gel permeation chromatography (GPC) was about 1.1 × 104 and the molecular weight polydispersity (PDI) was 2.7. CdCl2 · 2.5H2O (99%); NaBH4 (96%), tellurium powder (99.999%), and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. 3-Mercaptopropionic acid (MPA, >99%) was purchased from Fluka, St. Louis, MO, USA. The ultrapure water with 18.2 MΩ · cm was used in all experiments.
MPA (0.26 mL) was added to 100 mL CdCl2 (1.25 mmol) aqueous solution. After stirring for several hours, the aqueous solution was diluted to 950 mL, followed by adjusting the pH value to 8 with 1 M NaOH. After deaeration with N2 for 30 min, 50 mL oxygen-free NaHTe solution was injected at 5°C under vigorous stirring, thus CdTe precursor solution was obtained.
Proper amounts of HPAMAM (for example, 120 mg) was dissolved in 2 mL H2O in a one-neck flask, and then, 100 mL CdTe precursor solution was added. The mixture was deaerated with N2 for 15 min, followed by stirring for 24 h. Then, the mixture was irradiated at different times under ordinary pressure microwave (SINEO Shanghai Xinyi, Shanghai, China, 200 W, 100°C) to get a series of samples with various colors. The final CdTe/HPAMAM nanocomposites were abbreviated as CdTe/HPAMAM120. CdTe QDs prepared within 40, 80, and 200 mg HPAMAM were called CdTe/HPAMAM40, CdTe/HPAMAM80, and CdTe/HPAMAM200, respectively. The CdTe QDs stabilized by MPA-Na without HPAMAM were called CdTe/MPA-Na.
Cell imaging was characterized by confocal laser scanning microscopy (CLSM). HeLa cells (1 × 105 cells per well) were seeded on coverslips in 12-well tissue culture plates. After incubating the cells for 24 h, the CdTe/HPAMAM120 nanocomposites (obtained on heating for 80 min) in 200 μL Dulbecco's modified Eagle's medium (DMEM) were added into the wells and the cells were incubated at 37°C for 6 h. After washed with PBS, the cells were fixed with 4% formaldehyde for 30 min at room temperature. Then, the slides were rinsed with PBS for two times. The slides were mounted and observed by a LSM 510META.
No postpreparative treatment was performed on any as-prepared samples for optical characterization. pH values were measured by a Starter 3C digital pH meter, Ohaus, Parsippany, New Jersey, USA. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and elemental characterization were done on a JEOL 2010 microscope, Akishima-shi, Japan, with energy-dispersive X-ray spectrometer (EDS) at an accelerating voltage of 200 kV. X-ray powder diffraction (XRD) spectrum was taken on D/max-2550/PC X-ray diffractometer operated at 40 kV voltage and 40 mA current with Cu Ka radiation. For the XRD measurement, the CdTe QDs were rotary evaporated to remove water and then dried under vacuum. UV-visible (vis) spectra were recorded on a Varian Cary 50 UV/Vis spectrometer, Agilent Technologies, Inc., Santa Clara, CA, USA. Emission spectra were collected using a Varian Cary spectrometer. Dynamic light scattering (DLS) measurements were performed in aqueous solution at 25°C by using Zetasizer Nano S (Malvern Instruments Ltd., Malvern, Worcestershire, UK). The infrared measurements were performed on a Varian 800 Fourier transform infrared spectroscopy (FTIR) spectrometer. Thermogravimetric analysis (TGA) was performed under nitrogen on a STA 409 PC thermal analyzer, Netzsch, Germany.
where the subscripts s and x denote standard (rhodamine 6G) and test samples, respectively, φ is QY, F is the area of integrated fluorescence intensity, A is the absorbance at the absorption peak, and η is the refractive index of the solvent.
Results and discussion
HPAMAM have three-dimensional topological structures, many inner cavities, and a large amount of terminal functional groups. They have low cytotoxicity and have been widely used in biomedical science, such as gene transfections and drug delivery [34–36]. Based on this, we proposed new preparation strategies that combine the biomedical properties of dendritic polymers with the synthesis of CdTe QDs together in this paper. HPAMAM was introduced to stabilize the CdTe precursors (not CdTe QDs). After microwave irradiation, highly fluorescent and stable CdTe QDs stabilized by MPA-Na and HPAMAM were obtained. By preparing CdTe QDs within HPAMAM, the biocompatibility properties of HPAMAM and the optical, electrical properties of CdTe QDs can be combined, endowing the CdTe QDs with biocompatibility.
The QY of CdTe/MPA-Na in maximum is 33%, while the best aliquot with emission maximum at 554 nm of CdTe/HPAMAM120 show a high QY up to 58%, which is almost two times of that of CdTe/MPA-Na. Consequently, we can see that HPAMAM plays very important roles in enhancing the QY of CdTe QDs. The corresponding UV-vis spectrum and PL spectrum of CdTe/HPAMAM120 with emission maximum at 554 nm can be seen in the second set of curves from top to bottom in Figure 1c, and the corresponding fluorescent photograph under UV irradiation is located in the second place in the inserted photograph of Figure 1c.
In conclusion, a new strategy for preparing highly luminescent CdTe QDs within amine-terminated HPAMAM was proposed in this paper. The resulting CdTe/HPAMAM nanocomposites showed a high QY up to 58%. They combined the optical, electrical properties of CdTe QDs and the biocompatibility property of HPAMAM together. They could be directly used in biomedical fields, and their potential application in bioimaging was also investigated. Potential applications in gene transfection and drug delivery may be ideal because the fluorescent CdTe QDs can be used to monitor the entire process.
This work is supported by the Joint Fund for Fostering Talents of the National Natural Science Foundation of China and Henan Province (U1204213), the National Natural Science Foundation of China (21304001, 21201010, 21205003, 21273010), the social development projects of Anyang City (2012–218), and the Natural Science Foundation of Fujian Province (2012D124).
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