Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 1033∼1036
Synthesis and Characterization of a Quantum-dot doped Silica Network Prepared by Using a Microemulsion Jeong Ha Yoo and Jong Sung Kim∗ Department of Chemical and Biological Engineering, Kyungwon University, Seongnam 461-701 (Received 15 January 2010, in final form 15 July 2010) We have prepared a quantum-dot (QD)-doped silica network by using a microemulsion method and investigated the fluorescent properties of the hybrid particles for biological imaging and bioconjugation. The size of the particles was measured by using an electrophoretic light scattering system and transmission electronic micorscopy (TEM). The optical properties of the particles were characterized with an UV-Vis spectroscope, a photoluminescence (PL) spectrometer, and a fluorescence microscope. PACS numbers: 81.07.Ta, 81.20.Fw, 78.55.-m. Keywords: Quantum dots, QD-doped silica, Microemulsion, Hybrid DOI: 10.3938/jkps.57.1033
I. INTRODUCTION Recently, core-shell structures of CdSe/ZnS have been prepared and studied extensively due to their potential usage as biomarkers and to their applications in various photonic devices. Typically, the CdSe core is covered with a ZnS shell to enhance its quantum yield for radiative band gap recombination and to protect the core against photo-oxidation, as well as chemical and physical stress [1,2]. Meanwhile, the preparation of monodispersed quantum-dot nanoparticles with several techniques has been well established. For example, the synthesis of quantum dots via pyrolysis of organometallic reagents was reported by Murray et al. [3], and subsequently the method was improved by Hines and GuyotSionnest [4], which led to highly-monodisperse, passivated CdSe/ZnS core-shell nanocrystals. Nevertheless, CdSe/ZnS quantum dots are irregular in shape, and the quantum yield decreases with increasing shell thickness due to lattice imperfections [2]. Several hybrid quantum-dot nanostructures have been prepared, including nanoparticles conjugated to biomolecules [5], metalsemiconductor nanohybrid [6] and polymer-nanocrystal hybrids [7]. Among these core-shell hybrid-materials, silica doped with nanoparticles has attracted great attention in recent decades. Quantum-dot nanoparticles around a silica network are beneficial for applications in biological labelling as the surface of silica can be easily functionalized; also, silica is usually non-toxic and can be used with many biomaterials. Furthermore, the silica particle size can be controlled through a proper selecting ∗ E-mail:
of precursor materials [8], and silica can be assembled into bigger aggregates like photonic crystals [9]. In this study, CdSe and CdSe/ZnS QDs were synthesized. ZnS, which has a higher bandgap, was directly grown onto the surface of the CdSe core with little effect on the electronic state of the core. By using a simple microemulsion method, we prepared quantum-dot-doped silica networks without exchange of the surface-coordinating ligands (e.g., trioctylphosphineoxide (TOPO), hexadecylamine (HAD), or octadecylamine (ODA)) the synthesized QDs and the QD-silica were characterized by using an UV-Vis spectroscope, a photoluminescence (PL) spectrometer, a fluorescence microscope, and a transmission electronic micorscope.
II. MATERIALS AND METHODS 1. Materials and Reagents
Cadmium oxide (CdO, 99.99%), stearic acid (S.A. 95%), selenium powder (Se, 99.99%, 100 mesh), zinc acetate (Zn(Ac)2, 99.99%), sulfur powder (S, 99.98%), trioctylphosphine (TOP, 90%), TOPO (99%), HAD (90%), sodium dodecyl sulfate (SDS), octadecyltrimethoxysilane (C18 -TMS), triethoxyvinylsilane (TEVS), and (3-aminopropyl)triethoxy silane were purchased from Aldrich. All reagents were used without any further purification.
[email protected]; Fax: +82-31-750-5363
-1033-
-1034-
Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010
2. Synthesis of CdSe/ZnS@SiO2
The synthesis of CdSe/ZnS was performed as described in our previous report [10]. Then, the synthesis of CdSe/ZnS@SiO2 was done by using the methods described by Zhelev et al. [11]. Briefly, 800 µL of a CdSe/ZnS solution in chloroform was transferred to 10 mL of a detergent solution (15 mM SDS). The sample was mixed intensively with a magnetic stirrer and was subsequently heated to 50 ◦ C for complete evaporation of the chloroform. An additional ultrasonic treatment for 3 min was applied. During this process, the surfactant formed micelles over the QDs. The sample became slightly turbid in the process of micelle formation. For the formation of QD-silica, 20 µL of the first silica precursor (C18 -TMS) were added to 10 mL of the reaction mixture containing QD/detergent micelles. The mixture was stirred for 2 hours at 22 ◦ C. 110 µL of the second silica precursor (TEVS) were added to 10 mL of the reaction mixture. The mixture was stirred for 24 hours at 22 ◦ C. The silica network formed and grew inside the micelle during this time. Addition of TEVS resulted in a comparatively turbid solution due to the partial insolubility of the precursor in the water phase, but expansion of the polymerization process enhanced the transparency of the sample. The sample was subjected to dialysis for 48 hours at 22 ◦ C to remove the free ingredients. The aminofunctionalized silica spheres were prepared by using a third silica precursor, (3-aminopropyl)triethoxysilane. As the quantum-dot-nanoparticles-doped silica network was growing in the aqueous solution, 18 µL of (3amonopropyl)triethoxysilane was added to 10 mL of the reaction mixture. The mixture was stirred for 12 hours. Finally, the product was purified by using dialysis (48 r Dialysis Cassette hours at 22 ◦ C and a Slide-A-Lyzer - 10 000 MWCO (Pierce).
3. Instrument Analysis
The optical properties of the CdSe, CdSe/ZnS, and CdSe/ZnS@SiO2 nanoparticles were measured promptly, and the samples with a low optical density (extinction efficiency of first excitonic peak
