In situ preparation of fluorescent CdTe quantum dots with small thiols and hyperbranched polymers as co-stabilizers
© Shi et al.; licensee Springer. 2014
Received: 6 December 2013
Accepted: 6 March 2014
Published: 17 March 2014
A new strategy for in situ preparation of highly fluorescent CdTe quantum dots (QDs) with 3-mercaptopropionic acid (MPA) and hyperbranched poly(amidoamine)s (HPAMAM) as co-stabilizers was proposed in this paper. MPA and HPAMAM were added in turn to coordinate Cd2+. After adding NaHTe and further microwave irradiation, fluorescent CdTe QDs stabilized by MPA and HPAMAM were obtained. Such a strategy avoids the aftertreatment of thiol-stabilized QDs in their bioapplication and provides an opportunity for direct biomedical use of QDs due to the existence of biocompatible HPAMAM. The resulting CdTe QDs combine the mechanical, biocompatibility properties of HPAMAM and the optical, electrical properties of CdTe QDs together.
Fluorescent quantum dots (QDs) exhibit unique size and shape-dependent optical and electronic properties [1–9]. They are of great interest to many applications such as optoelectronics, photovoltaic devices, and biological labels. Developing new method to prepare QDs with controlled size and shape is always an important research area. To be now, organometallic way [10–14], aqueous route with small thiols as stabilizers [15–19], dendritic polymers [20–22] as nanoreactors and biotemplate synthesis  are the common methods to prepare QDs. The QDs prepared by organometallic way or aqueous route with small thiols as stabilizers usually have high quantum yield, but they need to be modified in order to be suitable for their biological application. The QDs prepared by dendritic polymers or biotemplate always has low quantum yield and broad emission spectrum. So we would like to propose a new method by which highly fluorescent CdTe QDs which can be directly used for biomedical applications can be prepared.
In this study, we used 3-mercaptopropionic acid (MPA) and hyperbranched poly(amidoamine)s (HPAMAM) as co-stabilizers to prepare highly fluorescent CdTe QDs. MPA is always used to prepare luminescent CdTe QDs in aqueous phase. HPAMAM has low cytotoxicity and can be used to gene transfection and drug delivery . Consequently, by using MPA and HPAMAM as co-stabilizers, highly luminescent and biocompatible CdTe QDs can be synthesized. The resulting CdTe QDs can be directly applied to bioimaging, gene transfection, etc.
Amine-terminated HPAMAM was synthesized according to our previous work . After endcapping by palmityl 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 · 2.5 H2O (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.
Synthesis of CdTe QDs with MPA and HPAMAM as co-stabilizers
MPA (26 μL) was added to 100 mL CdCl2 (0.125 mmol) aqueous solution. After stirring for several hours, pH value of the aqueous solution was adjusted to 8.2 with 1 M NaOH. Then, 120 mg HPAMAM in 2 mL water was drop-added under N2 atmosphere and stirred for 24 h. After deaeration with N2 for 15 min, 10 mL oxygen-free NaHTe solution was injected at 5°C under vigorous stirring; thus, CdTe precursor solution stabilized by MPA and HPAMAM was obtained. Then, the mixture was irradiated at different times under microwave (PreeKem, Shanghai, China, 300 W, 100°C) to get a series of samples with various colors.
Characterization of the as-prepared CdTe QDs
pH values were measured by a Starter 3C digital pH meter, Ohaus, 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 Rigaku Ultima III X-ray diffractometer (Shibuya-ku, Japan) operated at 40 kV voltage and 30 mA current with Cu Ka radiation. 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. Thermogravimetric analysis (TGA) was done under nitrogen on a STA 409 PC thermal analyzer, Netzsch, Germany. The quantum yield (QY) of CdTe QDs was measured according to the methods described in  using rhodamine 6G as a reference standard (QY = 95%).
Results and discussion
In conclusion, a new strategy for in situ preparation of highly fluorescent CdTe QDs with MPA and HPAMAM as co-stabilizers was proposed in this paper. The resulting CdTe QDs combine the biocompatibility property of HPAMAM and the optical, electrical properties of CdTe QDs together. They also have a high QY up to 60.8%. They do not need to be post-treated and can be directly used in biomedical fields due to the existence of biocompatible HPAMAM.
This work is supported by the Joint Fund for Fostering Talents of National Natural Science Foundation of China and Henan province (U1204213), the National Natural Science Foundation of China (21304001, 21205003, 21273010), and the project of science and technology development of Henan province (122102310522).
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