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
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 . They also can be used to prepare nanocrystals such as CdS nanocrystals, but they cannot cap the nanocrystals very compactly compared to small thiols. If nanocrystals are not capped closely, they might be unstable and tend to be oxidized. Based on this, we proposed a new strategy for preparing CdTe QDs with MPA and HPAMAM as co-stabilizers, so the resulting CdTe QDs can be coated closely and high QY can be reached. 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, as illustrated in Figure 1. By preparing CdTe QDs by MPA and HPAMAM, the mechanical, biocompatibility properties of HPAMAM and the optical, electrical properties of CdTe QDs can be combined, endowing the CdTe QDs with biocompatibility.
Figure 2 shows the photograph of different-sized CdTe QDs (stabilized by both MPA and HPAMAM) made under an UV lamp (top) and the corresponding absorption (bottom) and photoluminescence (PL) spectra (bottom). The fluorescent color of CdTe QDs under UV light changed from green to yellow orange, and red with prolonging heating time. All the absorption shoulders in the UV-vis spectra shifted to a longer wavelength during the heating treatment, indicating the growth of CdTe QDs. The maximum peak of PL emission also shows red shift, and this can also be seen in Figure 3a. While increasing the heating time, the QY of CdTe QDs increased significantly. The QY increased markedly from 11.2% at 15 min to a maximum value of 60.8% at 70 min. Further heating resulted in a slight decrease of QY, as shown in Figure 3b. The sizes of CdTe QDs can be estimated from the absorption peaks using Peng's empirical formula . From the absorption peaks, the Peng's empirical formula predicts that the diameter of CdTe QDs is from 2.8 to 3.6 nm.
The stability of CdTe QDs is important for their application, so we kept some samples taken at different irradiation times to investigate their stability. Figure 4 shows the absorption and emission spectra of CdTe aqueous solution before and after being aged for 2 months. After being kept for 2 months, the absorption and photoluminescence spectra of CdTe QDs (the excitonic absorption peak at 515 nm) had only slight changes, indicating the high stability of CdTe QDs.
The morphology of CdTe QDs (the excitonic absorption peak at 589 nm) was characterized by TEM, as shown in Figure 5. From the TEM image, we can see the size of CdTe QDs is about 3.5 nm, and the size is quite uniform. The SAED pattern inside Figure 4a shows that the synthesized fluorescent nanoparticles are polycrystalline. The corresponding EDS spectrum (Figure 5b) gives the signals of Cd and Te elements, confirming the existence of CdTe QDs.
Figure 6 shows XRD pattern of the resulting CdTe QDs (the excitonic absorption peak at 589 nm). The CdTe QDs exhibit X-ray diffraction pattern consistent with cubic (zinc blende) CdTe, as represented by the broad diffraction peaks at 23.8° (111), 41.2° (220), and 48.1° (311).
Figure 7 shows a comparison of FT-IR spectra between 4,000 and 500 cm−1 of pure HPAMAM and CdTe QDs stabilized both by MPA and HPAMAM. The broad band at 3,298 cm−1 in Figure 7a is characteristic for the N-H stretching bond frequency of primary and secondary amine groups, and it has shifted to 3,281 cm−1 in Figure 7b. The characteristic bands assigned to amides I and II for HPAMAM are at 1,654 and 1,552 cm−1, while the band positions of amides I and II slightly shift to 1,649 and 1,559 cm−1 for the CdTe QDs stabilized both by MPA and HPAMAM. The band at 1,559 cm−1 in Figure 7b can also be attributed to the asymmetric carboxylate peak, which is from the MPA stabilizer.
The composition of CdTe QDs stabilized both by HPAMAM and MPA was characterized by TGA. From the TGA thermogram in Figure 8a, we can see a long temperature range from 200°C to 450°C, which is the decomposition temperature for HPAMAM. For the CdTe QDs stabilized both by HPAMAM and MPA, the weight fraction is 45.6% at 794°C, as shown in Figure 8b. This means that the content of CdTe QDs in the nanocomposites is 45.6%.
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).
School of Chemistry and Chemical Engineering, Anyang Normal University
Instrumental Analysis Center, Shanghai Jiao Tong University
Gaponik N, Talapin DV, Rogach AL, Hoppe K, Shevchenko EV, Kornowski A, Eychmüller A, Weller H: Thiol-capping of CdTe nanocrystals: an alternative to organometallic synthetic routes. J Phys Chem B 2002, 106: 7177–7185.View Article
Zhou D, Lin M, Chen ZL, Sun HZ, Zhang H, Sun HC, Yang B: Simple synthesis of highly luminescent water-soluble CdTe quantum dots with controllable surface functionality. Chem Mater 2011, 23: 4857–4862. 10.1021/cm202368wView Article
Gu YP, Cui R, Zhang ZL, Xie ZX, Pang DW: Ultrasmall near-infrared Ag2Se quantum dots with tunable fluorescence for in vitro imaging. J Am Chem Soc 2012, 134: 79–82. 10.1021/ja2089553View Article
Fang T, Ma KG, Ma LL, Bai JY, Li X, Song HH, Guo HQ: Mercaptobutyric acid as an effective capping agent for highly luminescent CdTe quantum dots: new insight into the selection of mercapto acids. J Phys Chem C 2012, 116: 12346–12352. 10.1021/jp302820uView Article
Cushing BL, Kolesnichenko VL, O’Connor CJ: Recent advances in the liquid-phase syntheses of inorganic nanoparticles. Chem Rev 2004, 104: 3893–3946. 10.1021/cr030027bView Article
Burda C, Chen X, Narayanan R, El-Sayed MA: Chemistry and properties of nanocrystals of different shapes. Chem Rev 2005, 105: 1025–1102. 10.1021/cr030063aView Article
Lin Y, Skaff H, Emrick T, Dinsmore AD, Russell TP: Nanoparticle assembly and transport at liquid-liquid interfaces. Science 2003, 299: 226–229. 10.1126/science.1078616View Article
Balazs AC, Emrick T, Russell TP: Nanoparticle polymer composites: where two small worlds meet. Science 2006, 314: 1107–1110. 10.1126/science.1130557View Article
Lim J, Park M, Bae WK, Lee D, Lee S, Lee C, Char K: Highly efficient cadmium-free quantum dot light-emitting diodes enabled by the direct formation of excitons within InP@ZnSeS quantum dots. ACS Nano 2013, 7: 9019–9026. 10.1021/nn403594jView Article
Peng XG, Manna L, Yang WD, Wickham J, Scher E, Kadavanich A, Alivisatos AP: Shape control of CdSe nanocrystals. Nature 2000, 404: 59–61. 10.1038/35003535View Article
Shi YF, He P, Zhu XY: Materials research bulletin photoluminescence-enhanced biocompatible quantum dots by phospholipid functionalization. Mater Res Bull 2008, 43: 2626–2635. 10.1016/j.materresbull.2007.10.034View Article
Murray CB, Norris DJ, Bawendi MG: Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites. J Am Chem Soc 1993, 115: 8706–8715. 10.1021/ja00072a025View Article
Talapin DV, Rogach AL, Shevchenko EV, Kornowski A, Haase M, Weller H: Dynamic distribution of growth rates within the ensembles of colloidal II − VI and III − V semiconductor nanocrystals as a factor governing their photoluminescence efficiency. J Am Chem Soc 2002, 124: 5782–5790. 10.1021/ja0123599View Article
Jing LH, Ding K, Kalytchuk S, Wang Y, Qiao R, Kershaw SV, Rogach AL, Gao MY: Aqueous manganese-doped core/shell CdTe/ZnS quantum dots with strong fluorescence and high relaxivity. J Phys Chem C 2013, 117: 18752–18761. 10.1021/jp406342mView Article
Qian HF, Dong CQ, Weng JF, Ren JC: Facile one-pot synthesis of luminescent, water-soluble, and biocompatible glutathione-coated CdTe nanocrystals. Small 2006, 2: 747–751. 10.1002/smll.200500530View Article
Shi YF, Wang JJ, Li SJ, Wang ZY, Zang XX, Zu XM: Photoluminescence-enhanced CdTe quantum dots by hyperbranched poly(amidoamine)s functionalization. J Mater Res 2013, 28: 1940–1946. 10.1557/jmr.2013.166View Article
Zhang H, Wang LP, Xiong HM, Hu LH, Yang B, Li W: Hydrothermal synthesis for high-quality CdTe nanocrystals. Adv Mater 2003, 15: 1712–1715. 10.1002/adma.200305653View Article
Gao MY, Kirstein S, Möhwald H, Rogach AL, Kornowski A, Eychmuller A, Weller H: Strongly photoluminescent CdTe nanocrystals by proper surface modification. J Phys Chem B 1998, 102: 8360–8363. 10.1021/jp9823603View Article
Lemon B, Crooks RM: Preparation and characterization of dendrimer-encapsulated CdS semiconductor quantum dots. J Am Chem Soc 2000, 122: 12886–12887. 10.1021/ja0031321View Article
Shi YF, Tu CL, Wang RB, Wu JL, Zhu XY, Yan DY: Preparation of CdS nanocrystals within supramolecular self-assembled nanoreactors and their phase transfer behavior. Langmuir 2008, 24: 11955–11958. 10.1021/la801952vView Article
Zhou L, Gao C, Hu XZ, Xu WJ: General avenue to multifunctional aqueous nanocrystals stabilized by hyperbranched polyglycerol. Chem Mater 2011, 23: 1461–1470. 10.1021/cm1030359View Article
Bao HF, Hao N, Yang YX, Zhao DY: Biosynthesis of biocompatible cadmium telluride quantum dots using yeast cells. Nano Res 2010, 3: 481–489. 10.1007/s12274-010-0008-6View Article
Zhou YF, Huang W, Liu JY, Zhu XY, Yan DY: Self-assembly of hyperbranched polymers and its biomedical applications. Adv Mater 2010, 22: 4567–4590. 10.1002/adma.201000369View Article
Shi YF, Tu CL, Zhu Q, Qian HF, Ren JC, Liu C, Zhu XY: Self-assembly of CdTe nanocrystals at the water–oil interface by amphiphilic hyperbranched polymers. Nanotechnology 2008, 19: 445609. 10.1088/0957-4484/19/44/445609View Article
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