Synthesis of CdSe/CdS AND CdSe/CdS/SiO2 nanoparticles VIA wet chemical method

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-0035 Mathematical and Physical Sci., 2015, Vol. 60, No. 7, pp. 75-80 This paper is available online at SYNTHESIS OF CdSe/CdS AND CdSe/CdS/SiO2 NANOPARTICLES VIA WET CHEMICALMETHOD Chu Viet Ha1, Hoang Thi Hang2, Nguyen Thi Bich Ngoc3, Ngo Thi Huong1, Vu Thi Kim Lien1 and Tran Hong Nhung3 1Faculty of Physics, Thai Nguyen University of Education 2Faculty of Physics, Hanoi National University of Education 3Institute of Physics, Vietnam Academy of Science and Technology Abstract. This work presents the synthesis of CdSe/CdS and CdSe/CdS/SiO2 nanoparticles via wet chemical method for the purpose of preparing fluorescence SiO2 nanoparticles. The CdSe/CdS nanoparticles have been synthesized directly in an aqueous solution using citrate as the surfactant agent. The CdSe/CdS nanoparticles are then coated by a silica shell using tetraethylorthosilicate (TEOS) and aminopropyltriethoxysilane (APTEOS) as precursors, and ammonium hydroxide as the catalyst. The size of the nanoparticles can be controlled by synthesis conditions. The CdSe/CdS nanoparticles in citrate aqueous solution have strong intensity of emission, high photostability and quantum yield that is suitable for biological applications. The emission intensity of SiO2 coated quantum dots is remarkable. The CdSe/CdS quantum dots-based fluorescence silica nanoparticles exhibit a photostability for a long time during storage. The results show that SiO2 coated quantum dots (CdSe/CdS/SiO2 nanoparticles) can be used as biomarkers. Keywords: CdSe/CdS quantum dots, citrate, CdSe/CdS/SiO2 nanoparticles, Stober method, optical properties. 1. Introduction Semiconductor colloidal nanoparticles (quantum dots) have emerged as a new class of fluorescent probe for in vivo biomolecular and cellular imaging because they are highly photo-stable with broad absorption spectra, narrow size-tunable emission spectra, remarkably resistant photobleaching can span the light spectrum from the ultraviolet to the infrared region, and they have long fluorescence lifetimes [1-6]. Semiconductor nanoparticles with CdSe as the workhorse are increasingly being used as photoluminescence markers because of the spanning visible light of their spectrum. For biological applications, the semiconductor colloidal nanoparticles should be dispersed in an aqueous solution because most biological environments are aqueous. Most methods to prepare colloidal semiconductor nanoparticles make use of toxic precursors at high temperature in organic solvents and then the nanoparticles are dispersed in water by exchanging ligands to replace the hydrophobic layer with bifunctional molecules containing Received November 9, 2015. Accepted November 30, 2015. Contact Chu Viet Ha, e-mail address: chuvietha@tnu.edu.vn 75 C. V. Ha, H. T. Hang, N. T. B. Ngoc, N. T. Huong, V. T. K. Lien and T. H. Nhung thiol and hydrophilic moieties separated by a molecular spacer [7-10]. One simple route known to fabricate water soluble CdSe and CdSe/CdS nanoparticles is synthesis in an aqueous solution with sodium citrate used as surfactant agent [11]. This is a green method with non-toxic chemicals which gives a number of good quality water soluble nanoparticles. Due to the toxicity of quantum dots, reducing the toxicity is still being studied for in vivo applications. One route known to reduce the toxicity and also avoid the blinking of quantum dots is coating the quantum dots by silica layers. The silica matrix is inert in many environments, biocompatible, prevents agglomeration, functional, and it serves as the substrate for easy bioconjugation [12-15]. For the synthesis of silica nanoparticles, the most common approach is the Sto¨ber method which involves grafting organic groups by chemical reaction of pre-synthesized silica particles with certain coupling agents [16, 17]. This simple method can be carried out with non-toxic solvents such as water or ethanol, and it has been modified to incorporate quantum dots inside silica nanoparticles and reform high uniform beads. In this work, we report on the synthesis process of water soluble CdSe/CdS quantum dots in a citrate aqueous solution. In order to fabricate CdSe/CdS/SiO2 nanoparticles, the CdSe/CdS quantum dots are coated with a silica layer in an ethanol solvent via the Sto¨ber method using ammonium hydroxide (NH4OH) as the catalyst. We used 3-aminopropyl triethoxysilane (APTEOS) to balance the electrostatic repulsion between the CdSe/CdS quantum dots and the silica intermediates. The CdSe/CdS nanoparticles in citrate aqueous solution have strong intensity of emission, high photostability and a quantum yield that is suitable for labeling applications. The CdSe/CdS quantum dots-based fluorescence silica nanoparticles exhibit a photostability that will withstand storage over a long period of time. The results show an ability to use the SiO2 coated quantum dots (CdSe/CdS/SiO2 nanoparticles) as biomarkers. 2. Content 2.1. Experiment * Synthesis of CdSe/CdS quantum dots CdSe/CdS quantum dots were synthesized using redistilled water and the following chemicals: selenium powder (Se), sodium borohydride (NaBH4, 99%), absolute ethanol, Na2S.9H2O (98%), CdCl2.2.5H2O (99%), trisodium citrate dihydrate (99%), tris (hydroxymethyl) aminomethane (Tris) (99%), hydrochloric acid, sulfuric acid, and sodium hydroxide (96%). First, in absolute ethanol under magnetic stirring and in an N2 atmosphere, Se powder reacted with sodium borohydride to form a NaHSe solution. The time required for this synthesis step was 30 minutes. On the other side, trisodium citrate dihydrate was added to a tris-HCl buffer solution with a initial pH value in a three neck bottle, then a cadmium chloride solution was added dropwise with magnetic stirring, forming the solution containing Cd2+ ions protected by citrate molecules. The volume of the original solution was 50 ml. Second, H2Se gas was created in the reaction of above NaHSe solution with diluted H2SO4. The reaction was passed through the oxygen-free original solution at an appointed synthesis temperature in a water-bath together with a slow flow of nitrogen. The H2Se gas reacted with the above ion Cd2+ solution forming CdSe quantum dots under vigorously stirring conditions. CdSe/CdS quantum dots solutions were synthesized due to blowing H2S gas generated by the reaction of Na2S solution with diluted H2SO4 into the CdSe core solutions synthesized as described above with a slow nitrogen flow. 76 Synthesis of CdSe/Cds and CdSe/Cds/SiO2 nanoparticles via wet chemical method * Synthesis of CdSe/CdS/SiO2 nanoparticles Fluorescent SiO2 nanoparticles with CdSe/CdS quantum dots have been synthesized using the Sto¨ber method with tetraethylorthosilicate (TEOS, Sigma Aldrich) and aminopropyltriethoxysilane (APTEOS, Merck) as the precursors, NH4OH (Sigma Aldrich) as the catalyst, ethanol (Merck) as the solvent and non-ionic water from Millipore. First, the mixture of CdSe quantum dots and APTEOS was vibrated in ethanol. This was then added to the ethanol solution containing the TEOS that was stirred previously. After that, an ammonium hydroxide catalyst was added to the solution to initiate a reaction which formed silica particles containing quantum dots inside. The solution was magnetically stirred for 24 hours. The silica-coated quantum dot (CdSe/CdSe/SiO2) nanoparticle samples were then cleaned by centrifugation in ethanol. The absorption spectra of the nanoparticles were measured using a JASCO-V570-UV-Vis-NIR spectrometer. The fluorescence spectra were recorded on a Cary Eclipse spectrofluorometer (Varian). Transmission electron microscopes (TEM, JEM 1011) were used to determine the shape and size of the nanoparticles. 2.2. Results and discussion The prepared CdSe/CdS nanoparticles samples are a transparent aqueous solution under visible light with a brown color which is the color of the cadmium selenide semiconductor. They have a strong luminescent emission intensity due to excitation under an ultra violet lamp with the emission color dependent on the size of the CdSe particles (Figure 1). Figure 2 presents the TEM images of CdSe/CdS nanoparticles in an aqueous citrate solution. The TEM images show that the nanoparticles are quite mono-dispersed in water. The experimental results show that as the citrate concentration increases, the size of the CdSe decreases. We prepared the synthesis conditions in order to obtain the emission colors red, orange, yellow, and green, the color dependent on the size of the CdSe core. The size of CdSe nanoparticles prepared is 2 - 6 nm and the size of CdSe/CdS nanoparticles is estimated to be 3.5 - 10 nm. The size of the nanoparticles was determined by TEM and controlled by changing the synthesis conditions. Figure 1. Photo image of CdSe/CdS nanoparticle samples under ultra violet light Figure 2. TEM image of CdSe/CdS nanoparticles with emission peak at 586 nm (orange color) Figure 3 presents the photoluminescence spectra of CdSe/CdS nanoparticles prepared with different emission colors corresponding to different ratios of citrate concentration under excitation of 480 nm at room temperature. The photoluminescent maxima of the nanoparticles are 602, 586, 77 C. V. Ha, H. T. Hang, N. T. B. Ngoc, N. T. Huong, V. T. K. Lien and T. H. Nhung 568, and 556 nm corresponding to the emission colors red, orange, yellow and green. The quantum yield of CdSe/CdS nanoparticles was estimated by comparing the quantum yield of Rhodamine 6G (Rh 6G) dye with the same optical density (absorbance) at 480 nm because quantum yield of Rh 6G dissolved in water with an excitation of 480 nm is known to be 0.95 [18]. The quantum yield of these nanoparticles is estimated at 20 - 50% (shown in Table 1). This quantum yield is remarkably high compared with the yield of water soluble quantum dots in many other reports. The sample with the highest quantum yield is the orange emission nanoparticles sample (w = 2). This quantum dots sample was used to prepare fluorescence silica CdSe/CdS/SiO2 nanoparticles. Figure 3. Photoluminescence spectra of CdSe/CdS nanoparticles with different w ratios of citrate concentration Table 1. Photoluminescence emission and quantum yield of CdSe/CdS with different w ratios of citrate concentration w Emission peak (nm) Emission color Full width at half maximum (FWHM - nm) Quantum yield 1.5 602 Red 45 0.46 2 586 Orange 55 0.64 2.5 568 Yellow 53 0.38 3 556 Green 58 0.25 Figure 4 presents the TEM image of CdSe/CdS/SiO2 nanoparticles. The size of silica nanoparticles can be controlled depending on the concentration of reactants and the catalyst of the synthesis. We can see small quantum dots inside each silica nanoparticle clearly in Figure 4b. The results show the success of the synthesis of SiO2 nanoparticles containing quantum dots. Measurement of the absorption spectra in the UV - VIS region of the CdSe/CdS quantum dots and CdSe/CdS/SiO2 nanoparticles was performed at room temperature. Figure 5 presents the absorption spectra of the CdSe/CdS quantum dots and CdSe/CdS/SiO2 nanoparticles solutions with the same concentration of quantum dots. We can see that the absorption edges of the CdSe/CdS quantum dots and CdSe/CdS/SiO2 nanoparticles are the same. The absorbance of CdSe/CdS/SiO2 nanoparticles is higher than that of CdSe/CdS quantum dots due to an absorption of silica matrix. 78 Synthesis of CdSe/Cds and CdSe/Cds/SiO2 nanoparticles via wet chemical method Figure 4. TEM images of CdSe/CdS/SiO2 nanoparticles The fluorescence spectra of the nanoparticles were recorded under excitation of 350 nm of 450 W Xe light source. Figure 6 presents the fluorescence spectra of CdSe/CdS quantum dots and CdSe/CdS/SiO2 nanoparticle solutions with the same concentration of quantum dots. The shape of the fluorescence spectra of CdSe/CdS/SiO2 nanoparticles is similar to that of uncoated CdSe/CdS quantum dots with the same emission peak at 574 nm and full width at half maximum (FWHM) of 48 nm. It is worth noting that the prepared CdSe/CdS/SiO2 nanoparticles exhibit a fluorescence intensity that is higher than the fluorescence intensity of CdSe/CdS quantum dots. This intensity increases up to 30 % compared to uncoated silica CdSe/CdS quantum dots. This is an expected result because silica nanoparticles containing quantum dots prepared via the Sto¨ber method as noted in previous publications (such as refs. 19 and 20) have a fluorescence intensity that is significantly less than that of quantum dots. Figure 5. Absorption spectra of CdSe/CdS quantum dots and CdSe/CdS/SiO2 nanoparticles with the same concentration of quantum dots Figure 6. Fluorescence spectra of CdSe/CdS quantum dots and CdSe/CdS/SiO2 nanoparticles with the same concentration of quantum dots 3. 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