Highly conjugated water soluble CdSe quantum dots to multiwalled carbon nanotubes

Available online at www. sciencedirect.comCHINES EScienceDirectCHEMICALLETTERSEL SEVIERChinese Chemical Letters 20 (2009) 356- -357www.elsevier.com/locate/ccletHighly conjugated water soluble CdSe quantumdots to multiwalled carbon nanotubesGui Zheng ZouSchool of Chemistry and Chemical Engineering, Shandong Universiy, Jinan 25010, ChinaReceived 17 July 2008AbstractHighly conjugated multiwalled carbon nanotube-quantum dot heterojunctions were synthesized by ethylene carbodiimidecoupling procedure. The functional multiwalled carbon nanotube with carboxylic groups on sidewall could react with the aminogroup of L-cysteine capped CdSe quantum dots and then resulted in nanotube-quantum dot heterojunctions. Scanning electronmicroscopy was used to characterize the heterojunctions.◎2008 Gui Zheng Zou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.Keywords: Multiwalled carbon nanotubes; Quantum dots; HeterojunctionsCarbon nanotube-based optical and biological devices were in dire need to image and manipulate them in solutionstate [1]. As quantum dots (QDs) possessed size-tunable optical properties and have been extensively utilized inbiochemical sensing domain [2], the synthesis of carbon nanotube-quantum dot heterojunctions has received muchattention with the achievements obtained in carbon nanotube modification [3,4]. In this work, we developed a simpleway to extensively conjugate multiwalled carbon nanotube (MWCNT) (Shenzhen Nanoport Co.) with water solubleQDs, as shown in Fig. 1. First, L-cysteine capped CdSe QDs was chosen as they posses both carboxylic groups andamino groups. The L-cysteine capped CdSe QDs was synthesized by adding Na2SSeO3 (prepared as reportedpreviously [5]) into N-saturated 2 mM cadmium chloride solution in presence of L-cysteine. The molar ratio ofCd+:Se2- :L-cysteine was fixed at 1:0.5:2. The QD solution was then purified by being dialyzed against doublydistilled water for 24 h under dark condition. Second, highly functional MWCNT with carboxylic groups on sidewallwas obtained by refuxing MWCNT in 3:1(V:V) H2SO4/HNO3 solution for 12 h. The MWCNT-COOH was purified bybeing filtered through a 0.22 μm Millipore polycarbonate membrane and washed with distilled water until pH of thefiltrate was ca. 7. The purified MWCNT-COOH was then dispersed in doubly distilled water. Finally, heterojunctionsof CNT-QDs were synthesized by the reaction between amino groups of L-cysteine capped CdSe QDs and carboxylicgroups of the functional MWCNT in the presence of 3 mg/mL ethylene carbodimide (EDC) and 5 mg/mL N-hydroxysuccinimide (NHS). Excess L-cysteine capped CdSe QDs was used to form highly conjugated heterojunctions.The heterostructures was purified by filtrating through a 0.22 μm Millipore polycarbonate membrane and thenultrasonically dispersed in water.E-mail address: zouguizheng @ sdu.edu.cn.中国煤化工1001-8417/$ - see front matter◎2008 Gui Zheng Zou. Published by Elsevier B.V. onMYH，CNMHG..All rights reserved.doi:10. 101 6/.cclet.2008.11.014G.Z Zou/ Chinese Chemical Letters 20 (2009) 356 -357357HOOC-IINHOOC.HSO/HNO3-cooH HN-C=0> HOOEDCNHSpNHO--c-CoHooc'Fig. 1. The schematic for MWCNT-C0OH and the heterojunctions.A)453400 20KV x70.0k0.43um453400 20KV x70.aK".43unFig. 2. SEM image of MWCNT-COOH (A) and the heterojunctions (B).Fig. 2 was scanning electron microscopy (SEM) image of the MWCNT-COOH and the heterojunctions. L-cysteinecapped CdSe QDs were spotted one by one onto the sidewall of MWCNT-COOH (Fig. 2B), manifesting the highlyconjugated MWCNT-QDs heterojunctions. Although all carboxylic groups from MWCNT-COOH has been consumedby the L-cysteine capped CdSe QDs, there were carboxylic groups remaining on the surface of QDs, as demonstrated inFig. 1. Thus, this synthesizing procedure not only made it possible for the fluorescence visualization of nanotubes, butalso for further labeling the heterojunctions with bio- molecules.AcknowledgmentsThis project was supported by the National Natural Science Foundation of China (No. 20705016), State KeyLaboratory of Electroanalytical Chemistry (No. 2008009) and China Postdoctoral Science Foundation (No.20080431213).References[1] s. Chaudhary, JH. Kim, K.V. Singh, M. Ozkan, Nano Lett. 4 (2004) 2415.[2] J.M. Costa. Fermandez, R. Pereiro, A. Sanz- Medel, Trends Anal. Chem. 25 (2006) 207.[3] S. Ravindran, S. Chaudhary, B. Colbum, M. Ozkan, C.S. Ozkan, Nano Lett. 3 (2003) 447.[4] B. Pan, D. Cui, C.S. Ozkan, M. Ozkan, P. Xu, T. Huang, F. Liu, H. Chen, Q. Li, R. He. E. Gao.J. Phvs. Chem. C 112 (2008) 939.[5] G.Z. Zou, H.X. Ju, Anal. Chem. 76 (2004) 6871.中国煤化工YHCNMHG