CdTeZnS 量子点的合成及表征

Synthesis and Characterization of the Core-shell CdTe/ZnS QuantumDotsXU Shichao 1, YAO Cuicui 1, ZHANG Jimei 1, a , DAI Zhao 1, ZHENG Guo 1, SUN Bo 2, b , SUN Shuqing 3, HAN Qing 1, HU Fei 1, ZHOU Hongming 11School of Material Science and Chemical Engineering, Key Laboratory of Hollow Fiber Membrane Materials and Membrane Processes (Ministry of Education), Tianjin Polytechnic University, Tianjin300160, China;2 School of Chemistry, Nankai University, Tianjin, 300071, China3 Chemistry Department, Tianjin University, 30071, Chinaa Zhangjimei@,b Sunb@Keywords: Quantum dots (QDs), core-shell structure, synthesis and characterization, optical propertiesAbstract: Core-shell quantum dots are colloidal particles consisting of a semiconductor core and a shell material as an outer coating layer. It can be utilized to develop sensitive methods for the detection of specific biological entities, such as microbial species, their transcription products, and single genes etc. The goal of current research is to synthesize CdTe and core-shell CdTe/ZnS quantum dots (QDs) with an improved process, and to investigate their properties. Well-dispersed CdTe core was prepared in aqueous phase with using 3-mercaptopropionic acid (MPA) as stabilizer under conditions of pH 9.1, temperature of 100 °C, refluxing for 6h, and mol ratio of Cd 2+/Te 2-/MPA is 1:0.5:2.4. Average size of 8 nm CdTe core was conformed via transmission electron microscopy (TEM). Core-shell CdTe/ZnS QDs were then synthesized to improve the optical properties and biocompatibility of CdTe core. Various conditions were researched to obtain the core-shell QDs with the best optical properties, such as quantum yields, fluorescence intensity etc. The results indicated that the core-shell qualified CdTe/ZnS was prepared under conditions of pH 9.0, temperature of 45 °C, refluxing for 1h, and mol ratio of CdTe/S 2-/Zn 2+ is 4/1/1. CdTe/ZnS with average size of 10 nm were achieved and conformed via TEM. Moreover, red shift of a maximum emission wavelength from 547 nm of CdTe to 587 of CdTe/ZnS was observed via fluorescence spectrum (FS), which inferred the growth of QDs and formation of ZnS shells. The achieved ZnS shell make CdTe core less toxic and more biocompatible, it will be useful in biological labeling, diagnostic process and biosensing system based on fluorescence resonance energy transition (FRET).IntroductionThe luminescent semiconductor nanocrystal or quantum dots (QDs) has gained increasing concerns during the past two decades, which showed high quantum yields, broad absorption spectra with narrow size tunable photoluminescent emissions, and exceptional resistance to photobleaching and chemical degradation [1-3], these excellent properties make QDs have great potentialbiosensing base on fluorescence resonance energy transfer (FRET) [4- 6]. QDs with core-shell structures were developed to enhance some important properties, such as quantum yields (QYs), electronic accessibility, and photo-stability [7, 8]. Some core-shell QDs were prepared, investigated, and reported [9-11]. Tsay et al. recently reported the synthesis of CdTe/ZnS core-shell QDs in a basic aqueous phase, the morphology and some optical properties of CdTe/ZnS core-shell QDs were investigated and illustrated [12]. However, the QYs and some optimum details were not investigated or well illuminated yet.We currently demonstrated a simple process to prepare CdTe/ZnS core-shell QDs in aqueous phase. In which CdTe QDs was used as core, and ZnS were selected as the shell materials, because of their relative small lattice mismatch (11.5 and 16.0%) [13, 14]. Some optimum conditions to synthesize core-shell QDs were investigated, and the obtained QDs were characterized by spectroscopy techniques. We believe exploiting core-shell QDs can provide a potential candidate for FERT-based biosensing applications.ExperimentalPreparation of CdTe and CdTe/ZnS QDs. All chemicals were of analytical grade or better, and were used without further purification. Double-distilled water was applied throughout, and the thorough reaction system were deaerated and protected under nitrogen environment.The preparation of CdTe and core-shell CdTe/ZnS QDs was performed according to previous methods with some modifications [12, 15, 16]. CdTe core was prepared in aqueous phase with MPA as stabilizer at pH 9.1, temperature of 100 °C, refluxing for 6h, and mol ratio of Cd 2+/Te 2-/MPA is 1:0.5:2.4. During preparation of core-shell CdTe/ZnS, Zn(CH3COO)2 was used instead of ZnSO4, various pH , mol ratio of CdTe/S 2-/Zn 2+, and refluxing temperature were utilized to determine the maximum conditions.Characterization of the nanocrystals . The optical properties of the colloidal suspensions of CdTe and CdTe/ZnS QDs were investigated via ultraviolet spectrum (UV; Helios-γ, Thermo Spectronic Corporation, USA) and fluorescence spectrum (FS; WGY-10, Tianjin East port of Science and Technology Development Co., Ltd., PRC). And the morphology of the CdTe and CdTe/ZnS QDs were characterized via transmission electron microscope (TEM; Tecnai G2 20, FEI Company, USA). Results and discussionCharacterization of CdTe core and core-shell CdTe/ZnS QDs. TEM images of CdTe and core-shell CdTe/ZnS were showed in Fig 1. The particle size of prepared CdTe and CdTe/ZnS QDs were estimated to be 8 nm (A) and 10 nm (B), respectivelyFig. 1 TEM image of CdTe core (A) and core-shell CdTe/ZnS QDs (B)(A) (B)0.00.20.40.60.81.0dc b aA bso r ba nc eW avelength(nm )/Zn 2+ was 4:1:1, and refluxing time was 2h)A b s o r ba n ce W avelength(nm )Fig. 3 UV spectrum of CdTe/ZnS QDs at various ratio of CdTe/S 2-/Zn 2+ (a) control, i.e. CdTe core (b) 4:1:1 (c) 2:1:1 (d) 1:1:1 (e) 1:2:1; mol ratio. (pH is set at 9.0; refluxing temperature was 45°C, and refluxing time was 2h)In te ns ity (a .u )W avelength(nm )Fig. 4 FS of CdTe/ZnS QDs reacted at various pH (a) control, i.e. CdTe (b) pH 8.0 (c) pH 9.0. (mol ratio of CdTe/S 2-/Zn 2+ was 4:1:1; refluxing temperature was 45°C, and refluxing time was 2h)Investigations on optimum conditions. A suitable condition has great influence on the properties of prepared core-shell CdTe/ZnS QDs, such as QDs’ stability, quantum yield, and probe stability etc., and optimum conditions were consequently investigated. The data in Fig 2. showed the red-shift of maximum absorption wavelength (MAW) kept increasing when the refluxingsize-increasing of QDs, however, the typical absorption of CdTe QDs was disappeared gradually when the temperature increased, and we inferred that the decreased fluorescence intensity may be observed at high temperature, consequently, relative low temperature (45 °C) is preferred for the current case.CdTe/ZnS solution showed obvious red-shift of MAW when mol ratio of CdTe/S 2-/Zn 2+ was 4:1:1compared with CdTe core (Fig 3.), this result indicated the size of CdTe core increased, and which suggested the formation of ZnS shell. The other mol ratio of CdTe/S 2-/Zn 2+, in contrast, caused blue-shift. According to the current results, higher mol ratio of CdTe/S 2-/Zn 2+=4:1:1 is the best candidate for the synthesis of CdTe/ZnS.pH dependence of CdTe/ZnS on fluorescence intensity was investigated and results were showed in Fig 4. CdTe/ZnS core-shell QDs exhibited the higher fluorescence intensity than the CdTe core when pH was 9.0, but lower than that of the resulting CdTe core when pH was 8.0. Furthermore, a shoulder peak was observed in Fig 4 (b), which indicated the QDs’ size was inhomogeneous, this result simultaneously revealed that size distribution of CdTe/ZnS core-shell QDs were sensitive to pH, and pH 9.0 is suitable to obtain the qualified CdTe/ZnS core-shell QDs in the present research.I n t e ns it y (a .u)Wavelength(nm)Fig. 5 FS of CdTe and core-shell CdTe/ZnS QDs (refluxed for 1h at optimum conditions; a, CdTe; b, CdTe/ZnS)FS of CdTe/ZnS core-shell QDs were showed in Fig 5. The emission wavelength showsa significant red-shit from ca 550 nm (CdTe) to mean 580 nm (after 1h of refluxing) was observed, this result revealed formation of ZnS-shell. Moreover, fluorescence intensity increased compared with CdTe core, which likely due to the formation of ZnS on the CdTe-core.Conclusion sWe have developed a simple and improved method to obtain CdTe/ZnS core-shell nanoparticles in the aqueous phase. Future researches about characterization of the QDs, such as crystallinity, properties of surface absorptions, and biological sensing applications are in progress, and results will be reported elsewhere.AcknowledgementThe current research was supported by the Natural Science Foundation of Tianjin (No.07JCYBJC15900), the National Natural Science Foundation of China (No. 20802051), andReferences[1] D.R. Marc, A.H. Andrew, H.De P.L. Silvia and L.B. Matthew: Chem. Commun. (2008), p.2106[2] W.J. Parak, D. Gerion, T. Pellegrino, D. Zanchet, C. Micheel, S.C. Williams, R. 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