Synthesis and characterization of ZnS@Fe3O4 ...

79 downloads 0 Views 2MB Size Report
Aug 24, 2017 - (QDs) and SiO2 coated Fe3O4 nanoparticles were synthesized separately by ... tetraethyl orthosilicate (TEOS) by the typical sol-gel method [2e7]. ... and ZnS QDs were encapsulated by using TEOS and MPS, respectively.
Superlattices and Microstructures 110 (2017) 198e204

Contents lists available at ScienceDirect

Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices

Synthesis and characterization of ZnS@Fe3O4 fluorescentmagnetic bifunctional nanospheres Kenan Koc a, Baris Karakus b, Kausar Rajar b, c, Esra Alveroglu b, * a

Yildiz Technical University, Faculty of Science and Letters, Department of Physics, 34210, Esenler, Istanbul, Turkey Istanbul Technical University, Faculty of Science and Letters, Department of Physics Engineering, 34469, Maslak, Istanbul, Turkey c National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, 76080, Pakistan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 May 2017 Received in revised form 14 August 2017 Accepted 23 August 2017 Available online 24 August 2017

Herein, we synthesized and characterized fluorescent and super paramagnetic ZnS@Fe3O4 nanospheres. First, (3-mercaptopropyl) trimethoxysilane (MPS) capped ZnS quantum dots (QDs) and SiO2 coated Fe3O4 nanoparticles were synthesized separately by using solution growth and co-precipitation techniques. After synthesis and characterization of these two nanoparticles, they were conglutinated together in a nano sized sphere. The QDs were attached to the surface of the Fe3O4 nanoparticles by SieOeSi bonds and so SieOeSi bonds created a SiO2 network around the nanoparticles during the formation of the ZnS@Fe3O4 nanospheres. The synthesized MPS capped ZnS fluorescent QDs, SiO2 coated magnetite super paramagnetic nanoparticles and ZnS@Fe3O4 fluorescent-magnetic bifunctional nanospheres were characterized by using UVeVis Absorption Spectroscopy, Fluorescence Spectroscopy, X-ray analysis, Vibrating Sample Magnetometer analysis, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy, Scanning Electron Microscope and Energy-dispersive X-ray spectroscopy. ZnS@Fe3O4 bifunctional nanospheres were shown to retain the magnetic properties of magnetite, while exhibiting the luminescent optical properties of ZnS nanoparticles. The combination of fluorescent and magnetic behaviors of nano composites make them useful for potential applications in the field of bio-medical and environmental. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Magnetic nanoparticle Semiconducting nanoparticle Nanostructure

1. Introduction During the last decades, magnetic iron oxide nanoparticles and II-VI group semiconducting quantum dots (QDs) have been extensively investigated due to their size dependent magnetic optical and electronic properties and their potential applications in different areas. Especially, unique properties of fluorescent-magnetic nanocomposites make them useful for many different areas such as monitoring of various biological processes, drug delivery, biosensor and magnetic separation [1e6]. Despite of their widely usage area, there are some challenges in their preparation procedures because of their lack of chemical stability. Therefore, generally they have been used by introducing them into a matrix material or coated with a shell material. Within various coating materials, silica is almost the best choice can be used for liquid solution and bio applications. It is because silica can prevent the core from unwanted chemical reactions with liquid. SiO2 has high chemical stability,

* Corresponding author. E-mail address: [email protected] (E. Alveroglu). http://dx.doi.org/10.1016/j.spmi.2017.08.044 0749-6036/© 2017 Elsevier Ltd. All rights reserved.

K. Koc et al. / Superlattices and Microstructures 110 (2017) 198e204

199

amorphous structure, biocompatibility and a wide bandgap (9.1 eV) [7,8]. Hence, SiO2 is an ideal matrix or shell material for most of nanoparticles and the toxicity of nanoparticles can be reduced by encapsulating them by using SiO2 shell. Therefore, in several study, magnetic nanoparticles were produced by coating with SiO2. Mostly in these studies, SiO2 is produced using tetraethyl orthosilicate (TEOS) by the typical sol-gel method [2e7]. In our present study, the fluorescent magnetic composite nanospheres were formed with a SiO2 network by linking Fe3O4 and ZnS nanoparticles. Ferrite nanocomposites have been investigated in wide areas of science because of their unique magnetic and electronic properties. Particularly, when the particle sizes below roughly 30 nm, Fe3O4 nanoparticles show super-paramagnetic behaviour [9]. These unique properties are let Fe3O4 nanoparticles to use in many different area especially in biomedical applications [10e14]. However, their surfaces do not let strong bounds with other molecules, so surface coating for these particles have been used to overcome this problem, it will also help to adjust composites properties like thickness and magnetization. ZnS nanoparticles are very attractive semiconductor material among II-VI group elements due to its high photosensitivity and relatively less toxicity. These properties make it a promising candidate especially for biomedical applications. The present study, colloidal ZnS QDs were prepared by using 3-mercaptopropyltrimethoxysilane (MPS) in pure water. MPS used as a capping agent to obtain nanoscale particle size. The thiol groups of MPS molecules has the ability of making covalent bonds on the surface of ZnS nanoparticles and thus MPS molecules can use as a capping agent [15e17]. Thus MPS molecules also has also trimethoxysilane groups, so this groups from MPS molecules creates a SiO2 network around the nanoparticles due to SieOeSi bonds by hydrolysis and condensation reactions in sol-gel methods [17e20]. In the present study Fe3O4 nanoparticles and ZnS QDs were encapsulated by using TEOS and MPS, respectively. Then these two nanoparticles were combined by using the silica network. 2. Experimental part All chemicals were purchased from Sigma-Aldrich and used as received without any purification. 2.1. Synthesis of MPS caped ZnS QDs ZnS QDs were synthesized by solution growth technique at room temperature. The 0.04 mol/L zinc nitrate (Zn(N03)2), 0.02 mol/L sodium sulphide (Na2S) and 0.04 mol/L MPS (HS(CH2)3Si(OCH3)3) solutions were prepared in deionized water. All solutions were stirred during 5 min. Then 2 ml of zinc nitrate (0.04 mol/L) was added in 41 ml of MPS (0.04 mol/L) solution by keeping on stirring at room temperature and maintains the pH 12 by using tetrapropylammonium hydroxide (CH3CH2CH2)4N(OH). After then 4 ml of sodium sulphide (0.02 mol/L) rapidly transferred in the same sample. Finally, 2 ml of zinc nitrate (0.04 mol/L) was added by dropwise under continuously stirring [21,22]. 2.2. Synthesis of SiO2 caped Fe3O4 nano particles Fe3O4 nanoparticles were prepared by co-precipitation method. Firstly, iron (II) chloride tetrahydrate (FeCl2$4H2O) and iron (III) chloride hexahydrate (FeCl3$6H2O) (molar ratio 1:2) were dissolved in 30 ml pure water. After mixing this solution up to half hour on stirring, 50 ml of sodium hydroxide (NaOH) (2 M) was added by drop wise and the stirring was continued for 30 min after the addition of sodium hydroxide. After that the synthesized particles were collected, washed and dried in oven at 40  C. €ber method via the hydrolysis of tetraethylorthosilicate To capped the Fe3O4 nano particles with SiO2, the modified Sto (TEOS) (Si(OC2H5)4) was performed [23e26]. For this purpose, 0.045 g of Fe3O4 nanoparticles was dispersed in 16 ml of pure water by using ultrasonic treatment for 10 min. After that 2 ml of ammonia solution (25 wt %) and 80 ml ethanol were added and solution was kept to stirring. Finally, 0.8 ml of TEOS was added and stirring was continued for 18 h. After that, the sample was separated by using a magnet and washed with pure water. 2.3. Preparation of ZnS@Fe3O4 nanospheres 1 ml of synthesized MPS capped ZnS QDs and 1 ml of Fe3O4@SiO2 nanoparticles were dispersed in 18 ml pure water and kept on stirring for 18 h to bind each other structures. The MPS capped ZnS QDs were attached to the magnetite nanoparticles via SieOeSi bonds, which was created between the SieO groups of Fe3O4 nanoparticle surface and the trimethoxysilane groups of MPS during the reaction process. After mixing, the solution was centrifuged, washed and vacuum dried. 2.4. Characterization The optical absorption spectra of the sample were recorded with a UVevisible spectrometer (Agilent 8453). X-ray diffraction (XRD) measurements of the samples were taken by an MMA (GBC Scientific Equipment) model X-ray diffractometer. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) measurements of the samples were taken by a Spectrum One FT-IR spectrometer (Perkin Elmer). The magnetic characterization of the samples was performed at room temperature using a Vibrating Sample Magnetometer (VSM). Fluorescence measurements were studied by

200

K. Koc et al. / Superlattices and Microstructures 110 (2017) 198e204

Varian Cary Eclipse model spectrometer equipped with a quartz vessel. Scanning Electron Microscopy (SEM) images and Energy Dispersive Analytical X-ray spectroscopy (EDAX) spectra of the ZnS@Fe3O4 nanospheres were recorded using a SEM microscope (FEI Nova NanoSEM 450). 3. Results and discussion UVeVis absorption of ZnS QDs is given in Fig. 1. The first exciton peak energy (E1s1s) values of the sample was calculated from the first minimum value of the second derivatives of the absorption spectrum which was found as 4.68 eV. The blue-shift in energy (E1s1s) above the bulk band gap of ZnS (3.6 eV [27]), owing to the quantum confinement effect, are indicative of ZnS QDs with very smaller average size [28]. In the particle in a spherical box model, the first exciton peak is given as [29,30]:

E1s1s ¼ Eg þ

Z2 p2 2r 2

1 1 þ m*e m*h

! 

1:786e2 e4  0:248 εr 2ε2 Z2

1 1 þ m*e m*h

!1 (1)

where Eg is the band gap energy of the bulk ZnS, m*e and m*h are the effective mass values of electrons and holes respectively, (m*e ¼ 0:34m0 and m*h ¼ 0:23m0 for ZnS), r is the radius of the nanoparticle, and ε is the dielectric constant (8.76 for ZnS) [27]. To calculate the average radius of the nanoparticles, the E1s1s value was inserted into the Eq. (1) and average radius was found to be 1.46 nm. The result showed that the average radius of the ZnS QDs is below the exciton Bohr radius of bulk ZnS (2.5 nm) [27]. According to this result, there are a strong confinement in the ZnS QDs for electrons and holes. XRD results of SiO2 capped Fe3O4 nanoparticles and ZnS@Fe3O4 are given in Fig. 2. The difractions comes from (220), (311), (400), (422), (511) and (440) planes represents that Fe3O4 nanoparticles in cubic inverse spinel structure given by JCPDS card no 19e629 [31]. The noisy background (seen in between the 20 30 ) shows SiO2 covering occurs on the shell. The average size of crystals were found to be 5.8 nm by calculation of Scherrer's formula. In Fig. 2 the XRD pattern of ZnS@Fe3O4 composite, both diffraction peaks come from ZnS and Fe3O4. Diffractions come from (111), (200), (220) and (311) planes represents that ZnS QDs in face centered cubic structure given by ICDD PDF 65e1691 with lattice parameter of 5.41 Å [21,32]. The result shows that the both structures keep their form constant after combining to each other. The ATR-FTIR spectra of Fe3O4 nano particles and Fe3O4eSiO2 are shown in Fig. 3 in the range 650e4000 cm1. As known from the literature there is a strong absorption in the range between 500 and 600 cm1 for Fe3O4 nanoparticles due to characteristic vibration of FeeO in the Fe3O4 lattice [13]. Therefore, ATR- FTIR spectrum of naked Fe3O4 nanoparticles exhibit strong absorption tendency below 700 cm1. On the other hand, ATR- FTIR spectrum of SiO2 coated Fe3O4 nanoparticles exhibit prominent peaks at 792, 969 and 1051 cm1 these peaks can be assigned to the SieO, SieOeH and SieOeSi bands, respectively [33,34]. The other peak of Fe3O4: SiO2 nanoparticles in ATR-FTIR curve at 1341 cm1 belong to CeOH bond [13]. The bands seen at around 1630 cm1 and 3400 cm1 resulted from HeOeH streching modes and bending vibration of the free or adsorbed water, respectively [34]. The ATR-FTIR results show that the Fe3O4 nanoparticles were successfully coated by silica. M-H curves of the samples at room temperature are given in Fig. 4. The saturation magnetization of naked Fe3O4 particles is measured as 56 emu/g, that is less than the bulk Fe3O4 (92 emu/g). When the particles covered with SiO2, the saturation value decreased to 41 emu/g, due to the non-magnetic SiO2 on the surface. Additionally, saturation magnetization value of Fe3O4:ZnS is measured as 30 emu/g, this value is much less than the Fe3O4:SiO2 as expected, since more SiO2 layer was covered to keep Fe3O4 and ZnS particles together in a sphere. It should also be noted that the composite shows superparamagnetic behaviour (unsaturated magnetization and immeasurable coercivity), because of the Fe3O4 nanoparticles size

Fig. 1. UV absorption spectrum of MPS caped ZnS QDs.

K. Koc et al. / Superlattices and Microstructures 110 (2017) 198e204

Fig. 2. XRD difraction pattern of Fe3O4:SiO2 and ZnS@Fe3O4.

Fig. 3. ATR-FTIR spectra of Fe3O4 and Fe3O4eSiO2 samples.

Fig. 4. M~H curve of naked Fe3O4, Fe3O4eSiO2, ZnS@Fe3O4 samples and Langevin fitting curve.

201

202

K. Koc et al. / Superlattices and Microstructures 110 (2017) 198e204

Fig. 5. Fluorescence spectrum of ZnS QDs and ZnS@Fe3O4 nanospheres during the preparation of ZnS@Fe3O4 nanospheres. (t ¼ 0, 6, 12,18 h).

in single domain limit. The Langevin Function fit curve is also given in Fig. 4 for naked Fe3O4. The grain size of nanoparticles was calculated as 6.9 nm by using this fitting curve. The fluorescence emission spectrum of the ZnS QDs under 300 nm wavelength excitation is shown in Fig. 5. As shown from the figure, at the initial stage of ZnS@Fe3O4 combination process, the intensity of emission spectrum quenched and shifted to the lower wavelength. After that, while the maximum emission wavelength located at the same place with increasing the time duration, the intensity started to increase. This quenching come from chemical instability of the solution at the initial stages of the reaction, after 18 h the intensity of the emission kept constant which confirm the ending of the conglutinating process. As seen from the figure the maximum of the spectrum placed at 395 nm for MPS capped ZnS QDs. This broad emission peak (3.14 eV) is below the band gap value of bulk ZnS (3.6 eV) so it is primarily associated with point defects in the nanocrystal lattice, such as interstitial ions and vacancies, which are known to produce deep trap states [35]. In this study, zinc nitrate and sodium sulphide were used as the source of Zn2þ and S2 respectively and the Zn2þ:S2 stoichiometric molar ratio was fixed as 2:1 during the synthesis of ZnS QDs. In addition, vacancy defect states lie deeper than interstitial ions defect states in the band gap and interstitial ions defect states occurs at a level closer to the bulk band gap value [36]. Hence, most probably this peak arising from excess of zinc ions and can be assigned to the interstitial zinc ions in the crystal lattice. On the other hand, luminescence properties of the ZnS QDs were significantly influenced by the interaction of the ZnS QDs with the Fe3O4 super paramagnetic particles. The maximum intensity wavelength of the ZnS QDs shifts to 330 nm, when Fe3O4 and ZnS nanoparticles combined in the same matrix. This peak value (3.76 eV) is above the band gap value of bulk ZnS and below E1s1s value (4.68 eV) of the ZnS QDs so it can be associated with surface defects states for the ZnS QDs [37]. Therefore, we can say that, when the quantum dots combined with the magnetic nanoparticles, defects occurred on the surface of the quantum dots. The disappearance of the peak at 395 nm for the emission spectrum of ZnS@Fe3O4 nano structures can be explained that interstitial ions go out from the crystal lattice while the quantum dot and the magnetic nanoparticles combination process and this prevent the formation of deep trap states. Although magnetite nano particles

Fig. 6. SEM images of ZnS@Fe3O4 nanospheres in various magnification.

K. Koc et al. / Superlattices and Microstructures 110 (2017) 198e204

203

Fig. 7. EDAX analysis of ZnS@Fe3O4 nanospheres and elemental maps of S, Zn, O and Fe.

causes a shifting in the fluorescence spectrum of the ZnS QDs, in the composite ZnS@Fe3O4 nano sphere spectra do not result in quenching at the end of the process. SEM images of the ZnS@Fe3O4 nanospheres are given in Fig. 6. The SEM images show that the ZnS@Fe3O4 nanosphere are agglomerate in spherical shape and size distributions are homogeneous. As seen in the SEM images the average size of ZnS@Fe3O4 nanospheres is around 100 nm (range between 50 nm and 150 nm). Hence, these spheres on the SEM images indicate that the ZnS@Fe3O4 nanospheres were formed by assembled from various numbers of ZnS and Fe3O4 nanoparticles. To provide evidence for the existence of the both Fe3O4 and ZnS nanostructures in the SEM images, EDAX analysis (seen in Fig. 7) had been performed. In the EDAX pattern, the presence of Zn, S, Fe, O and Si is clear and there are no other elements existing in addition to these elements in the samples. The corresponding EDAX elemental mappings of the SEM image are shown in below images of Fig. 7 which illustrates the actual distribution of SeZn and OeFe. The homogeneous distribution of S, Zn, O and Fe can be easily observed from the EDAX elemental mappings. 4. Conclusion Based on the above discussions, ZnS@Fe3O4 nanospheres were synthesized and characterized successfully. The method of preparation of ZnS@Fe3O4 nano structure can easily reproduced. The morphology of the ZnS@Fe3O4 showed spherical like structure with an average size of around 100 nm. This final nanocomposite shows both superparamagnetic and fluorescent behavior. The saturation magnetization value of the ZnS@Fe3O4 nanospheres 30 emu/g and emission wavelength is 330 nm. While magnetic properties of the nanospheres comes from magnetite, fluorescent properties are provided by ZnS QDs. ZnS@Fe3O4 nanospheres possesses both properties of fluorescence and magnetism, and may serve as targeted and tracing drug delivery carrier in tumor treatment. Acknowledgments Kausar Rajar acknowledges the scholarship support from the Scientific and Technological Research Council of Turkey-2216 Research Fellowship Program for International Researchers and Department of Physics Engineering, Istanbul Technical University Istanbul for of financial assistance and facilities during this research.

204

K. Koc et al. / Superlattices and Microstructures 110 (2017) 198e204

References [1] D. Thakur, et al., pH sensitive CdS-iron oxide fluorescent-magnetic nanocomposites, Nanotechnology 20 (48) (2009). [2] L. Sun, et al., Synthesis of magnetic and fluorescent multifunctional hollow silica nanocomposites for live cell imaging, J. Colloid Interface Sci. 350 (1) (2010) 90e98. [3] Y. Xu, et al., Multifunctional Fe3O4 cored magnetic-quantum dot fluorescent nanocomposites for RF nanohyperthermia of cancer cells, J. Phys. Chem. C 114 (11) (2010) 5020e5026. [4] C. Jiang, et al., Synthesis and characterisation of magnetic-fluorescent Fe3O4@ SiO2@ ZnS nanocomposites, Micro & Nano Lett. 9 (3) (2014) 171e174. [5] D.K. Yi, et al., Silica-coated nanocomposites of magnetic nanoparticles and quantum dots, J. Am. Chem. Soc. 127 (14) (2005) 4990e4991. [6] P. Sun, et al., Preparation and characterization of Fe3O4/CdTe magnetic/fluorescent nanocomposites and their applications in immuno-labeling and fluorescent imaging of cancer cells, Langmuir 26 (2) (2010) 1278e1284. [7] X. Zhao, et al., Preparation of silica-magnetite nanoparticle mixed hemimicelle sorbents for extraction of several typical phenolic compounds from environmental water samples, J. Chromatogr. A 1188 (2) (2008) 140e147. [8] R. Thielsch, T. Bohme, H. Bottcher, Optical and structural properties of nanocrystalline ZnS-SiO2 composite films, Phys. Status Solidi a-Applied Res. 155 (1) (1996) 157e170. [9] S.M. Obrien, O.R.T. Thomas, P. Dunnill, Non-porous magnetic chelator supports for protein recovery by immobilised metal affinity adsorption, J. Biotechnol. 50 (1) (1996) 13e25. [10] L. Babes, et al., Synthesis of iron oxide nanoparticles used as MRI contrast agents: a parametric study, J. Colloid Interface Sci. 212 (2) (1999) 474e482. [11] S. Kalia, et al., Magnetic polymer nanocomposites for environmental and biomedical applications, Colloid Polym. Sci. 292 (9) (2014) 2025e2052. [12] E. Alveroglu, et al., Fluorescence and magnetic properties of hydrogels containing Fe3O4 nanoparticles, J. Mol. Struct. 1037 (2013) 361e366. [13] K. Koc, E. Alveroglu, Adsorption and desorption studies of lysozyme by Fe3O4-polymer nanocomposite via fluorescence spectroscopy, J. Mol. Struct. 1089 (2015) 66e72. [14] M. Mahmoudi, et al., Superparamagnetic iron oxide nanoparticles (SPIONs): development, surface modification and applications in chemotherapy, Adv. Drug Deliv. Rev. 63 (1e2) (2011) 24e46. [15] R. Tremont, et al., 3-Mercaptopropyltrimethoxysilane as a Cu corrosion inhibitor in KCl solution, J. Appl. Electrochem. 30 (6) (2000) 737e743. [16] K. Koc, F.Z. Tepehan, G.G. Tepehan, Preparation and characterization of self-assembled thin film of MPS-capped ZnS quantum dots for optical applications, J. Nanomater. 2012 (2012). [17] K. Koc, Effect of MPS concentration on ripening kinetics and structural properties of CdS quantum dots in self-assembled thin films, J. Mol. Struct. 1102 (2015) 127e134. [18] W.R. Thompson, et al., Hydrolysis and condensation of self-assembled monolayers of (3-mercaptopropyl)trimethoxysilane on Ag and Au surfaces, Langmuir 13 (8) (1997) 2291e2302. [19] J. Singh, J.E. Whitten, Adsorption of 3-mercaptopropyltrimethoxysilane on silicon oxide surfaces and adsorbate interaction with thermally deposited gold, J. Phys. Chem. C 112 (48) (2008) 19088e19096. [20] A. Scott, J.E. Gray-Munro, The surface chemistry of 3-mercaptopropyltrimethoxysilane films deposited on magnesium alloy AZ91, Thin Solid Films 517 (24) (2009) 6809e6816. [21] M. Cakici, E. Alveroglu, Stabilisation of (3-mercaptopropyl)-trimethoxysilanecapped ZnS nanocrystals in polyacrylamide hydrogel, Int. J. Nanotechnol. 12 (3e4) (2015) 142e153. [22] H. Li, W.Y. Shih, W.H. Shih, Stable aqueous ZnS quantum dots obtained using (3-mercaptopropyl) trimethoxysilane as a capping molecule, Nanotechnology 18 (49) (2007). € ber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1) (1968) 62e69. [23] W. Sto [24] C. Hui, et al., Large-scale Fe3O4 nanoparticles soluble in water synthesized by a facile method, J. Phys. Chem. C 112 (30) (2008) 11336e11339. [25] G. Liu, X. Yang, Y. Wang, Synthesis of ellipsoidal hematite/silica/polymer hybrid materials and the corresponding hollow polymer ellipsoids, Langmuir 24 (10) (2008) 5485e5491. [26] C. Hui, et al., Core-shell Fe3O4@SiO2 nanoparticles synthesized with well-dispersed hydrophilic Fe3O4 seeds, Nanoscale 3 (2) (2011) 701e705. [27] B. Bhattacharjee, et al., Synthesis and characterization of sol-gel derived ZnS : Mn2þ nanocrystallites embedded in a silica matrix, Bull. Mater. Sci. 25 (3) (2002) 175e180. [28] M.H. Yukselici, Growth kinetics of CdSe nanoparticles in glass, Journal of Physics-Condensed Matter 14 (6) (2002) 1153e1162. [29] L.E. Brus, Electron electron and electron-hole interactions in small semiconductor crystallites - the size dependence of the lowest excited electronic state, J. Chem. Phys. 80 (9) (1984) 4403e4409. [30] Y. Kayanuma, Quantum-size effects of interacting electrons and holes in semiconductor microcrystals with spherical shape, Phys. Rev. B 38 (14) (1988) 9797e9805. [31] S. Khashan, et al., Novel method for synthesis of Fe3O4@TiO2 core/shell nanoparticles, Surf. Coatings Technol. 322 (2017) 92e98. [32] N. Soltani, et al., Visible light-induced degradation of methylene blue in the presence of photocatalytic ZnS and CdS nanoparticles, Int. J. Mol. Sci. 13 (10) (2012) 12242e12258. [33] L.L. Dong, et al., Thermally and magnetically dual- responsive mesoporous silica nanospheres: preparation, characterization, and properties for the controlled release of sophoridine, J. Appl. Polym. Sci. 131 (13) (2014). [34] J.S. Lee, E.J. Lee, H.J. Hwang, Synthesis of Fe3O4-coated silica aerogel nanocomposites, Trans. Nonferrous Metals Soc. China 22 (2012) S702eS706. [35] T.T. Jiang, et al., Aqueous synthesis of color tunable Cu doped Zn-In-S/ZnS nanoparticles in the whole visible region for cellular imaging, J. Mater. Chem. B 3 (11) (2015) 2402e2410. [36] F.P. Ramanery, A.A.P. Mansur, H.S. Mansur, One-step colloidal synthesis of biocompatible water-soluble ZnS quantum dot/chitosan nanoconjugates, Nanoscale Res. Lett. (2013) 8. [37] F. Hache, et al., Photoluminescence study of schott commercial and experimental CDSSE-doped glasses - observation of surface-states, J. Opt. Soc. Am. B-Optical Phys. 8 (9) (1991) 1802e1806.