Summary of Chemistry doctoral thesis: Synthesis and properties of ferrite - metal (Ag, Au) hybrid nanostructures for biomedical application

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This work aims to fabricate ferrite - (Ag, Au) hybrid nanomaterials with a SPR peak located in the near-infrared region, high magnetic/optical-to-thermal conversion and ability to contrast MRI images for both T1- and T2- weighted modes, which are strongly bactericidal and able to be applied in biomedicine.

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MINISTRY OF VIETNAM ACADEMY OF EDUCATION AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY _________________________________________ NGUYEN THI NGOC LINH SYNTHESIS AND PROPERTIES OF FERRITE - METAL (Ag, Au) HYBRID NANOSTRUCTURES FOR BIOMEDICAL APPLICATION Major: Inorganic chemistry Code: 9.44.01.13 SUMMARY OF CHEMISTRY DOCTORAL THESIS Ha Noi - 2020
This thesis was done at: Laboratory of Electronic-Electrical Engineering, Institute for tropical technology, Vietnam Academy of Science and Technology. Laboratory of Biomedical Nanomaterials, Institute of Materials and Science, Vietnam Academy of Science and Technology. Supervisor 1: PhD. Le Trong Lu Supervisor 2: Assoc.Prof., PhD. Ngo Dai Quang Reviewer 1: Prof., PhD. Thai Hoang Reviewer 2: Assoc.Prof., PhD. Huynh Dang Chinh Reviewer 3: Assoc.Prof., PhD. Nguyen Thi Hien Lan The dissertation will be defended at Graduate University of Science and Technology, 18 Hoang Quoc Viet street, Hanoi. Time: .............,.............., 2020 This thesis could be found at National Library of Vietnam, Library of Graduate University of Science and Technology, Library of Vietnam Academy of Science and Technology.
INTRODUCTION 1. The necessary of the thesis In recent years, hybrid nanomaterials have attracted the attention of many researchers due to their integrated properties from individual components. The combination of magnetic and optical properties on a nanostructure has improved the application of single nanoparticles (NPs) and opened new directions in biomedical applications, especially in diagnosis and treatment. The advantage of magneto- plasmonic hybrid nanostructures in this area is that the desired target has merely been achieved after being activated by a physical stimulus, thus minimizing damaging effects on body. Furthermore, several functions can work synergically to enhance the efficiency in therapeutic methods. Currently, many magneto-plasmonic hybrid nanostructures have been studied in order to apply in the biomedical field in which Fe3O4/Au material has been one of the most typical materials. The studies on Fe3O4/Au hybrid NPs for magnetic resonance imaging (MRI) and magneto-photothermal therapy in cancer treatment have obtained some remarkable results. However, Fe3O4@Au core-shell hybrid NPs with Au layer coated on the surface of Fe 3O4 core significantly limit the connection of protons to the magnetic materials, leading to the reduction of the T2-weighted MRI contrast signal. In addition, Fe3O4-Au hybrid NPs exhibit the surface plasmon resonance (SPR) position in the range from 530 to 600 nm, limiting the deep penetration into thick tissue layers and reducing the efficiency of the photothermal therapy. In order to improve the effectiveness of the thermal therapy, the materials have to absorb radiation in near-infrared (NIR) region (650 ÷ 950 nm), also known as "biological windows” region, because the NIR radiation has the highest ability to penetrate the body. The previously fabricated hollow Fe3O4/Au hybrid NPs can meet this criterion, however, their large size (40 ÷ 100 nm) impact the blood circulation. Therefore the fabrication of the hollow Fe3O4/Au hybrid NPs with particle size 1
below 20 nm and the integration of both magnetic and plasmonic properties is still a big challenge. In Vietnam, to our best knowledge, the publication of the fabrication of the magnetic – noble metals hybrid nanomaterials (Ag, Au) integrated magnetic and plasmonic properties, which can be applied in the biomedical field, is still limited. Research that results on the application of Fe3O4-(Ag, Au) hybrid NPs as MRI contrast agents under both T1- and T2-weighted modes, and as a heat (optical/magnetic-thermal) substance in cancer treatment has not been reported. For these reasons, we conduct the thesis on topic “Synthesis and properties of ferrite - metal (Ag, Au) hybrid nanostructures for biomedical application”. 2. Research objectives of the thesis This work aims to fabricate ferrite - (Ag, Au) hybrid nanomaterials with a SPR peak located in the near-infrared region, high magnetic/optical-to-thermal conversion and ability to contrast MRI images for both T1- and T2- weighted modes, which are strongly bactericidal and able to be applied in biomedicine. 3. The main research contents of the thesis 1. Synthesis of magnetic ferrite nanoparticles MFe2O4 (M: Fe, Co, Mn) with uniform size and shape, monodisperse, and high saturation magnetization using thermal decomposition method in organic solvents. 2. Synthesis of small Fe3O4/Ag hybrid particles (below 20 nm) by seeded-growth method, and synthesis of Fe3O4/Au hollow hybrid nanoparticles with size below 20 nm and NIR light absorption by using Fe3O4/Ag nanoparticles as template via Galvanic replacement approach in organic solvents. 3. Phase transfer of the as-synthesized NPs from organic solvent to aqueous solvent, and evaluation of toxicity and durability of the hybrid particles in aqueous water. 4. Study on the applicability of the hybrid particle solution in biomedical: antibacterial activity, ability to convert optical/magnetic energy into heat, and the ability to contrast MRI images. 2
CHAPTER 1. OVERVIEW ABOUT MAGNETIC FERRITE - NOBLE METAL NANOMATERIALS 1.1. General introduction of magnetic ferrite - noble metal nanomaterials 1.1.1. Magnetic properties of magnetic ferrite materials The magnetic properties of materials vary depending on their electronic structure. The magnetic moments of an atom are generated by the magnetic moments of the electrons related to the intrinsic motion of the electron (spin motion) and the moment from the orbit caused by the motion of the electron around the atomic nucleus. 1.1.2. Plasmonic properties of noble metallic (Ag, Au) * The phenomenon of surface plasmon resonance: this phenomenon occurs when all the conducting electrons on the metal surface are stimulated simultaneously to form a collective oscillation. * Mie theory: Mie theory found that the extinction cross section (ext) includes the absorption (abs) and scattering (sct) cross sections of a particle The absorbance A of a sample of nanoparticles dispersed in a homogeneous medium is given by: where (ext) is the extinction coefficient of the sample at wavelength , N is the number of particles in one litter, and l is the thickness of the absorbing medium (cm). 1.1.3. Magnetic ferrite - noble metal materials When different materials are integrated in a nanostructure, in addition to inheriting the unique properties of each component, the hybrid structure also exhibits new properties generated from the interaction between the two material systems. 3
1.2. Properties of the magnetic ferrite - noble metal hybrid nanomaterials 1.2.1. Magnetic properties 1.2.2. Plasmonic properties 1.2.3. Biocompatibility and physiochemical stability 1.3. The magnetic ferrite - noble metal hybrid nanomaterials for biomedical application The combination of magnetic ferrite components and noble metals creates more diverse applications than individual nanoparticles. 1.3.1. Application of thermal therapy in cancer treatment 1.3.2. Bioimaging 1.3.3. Antibacterial application 1.3.4. Targeted drug delivery 1.4. Methods for synthesizing ferrite - noble metal hybrid nanomaterials There are many methods to synthesize magnetic ferrite - noble metal materials. Generally, the synthesis process consists of two steps: 1) Synthesize seeds which can be magnetic ferrite or noble metal (Ag, Au) nanoparticles. 2) Develop another component (noble metals or magnetic ferrite) on pre-synthesized seeds. This thesis focuses on the method of synthesizing magnetic ferrite NPs (seeds), then developing noble metal components (Ag, Au) on these seeds. 1.4.1. Synthesis of magnetic ferrite nanomaterials 1.4.2. Synthesis of magnetic ferrite - noble metal nanomaterials 1.4.2.1. Seeded-growth method 1.4.2.2. Synthesis of some magnetic ferrite - noble metal hybrid nanomaterials by the seeded-growth method 1.4.3. Surface modification of hybrid nanomaterials 1.4.3.1. Ligand-exchange method 1.4.3.2. Coating hybrid particles by bipolar polymers 4
CHAPTER 2. EXPERIMENTAL AND RESEARCH METHODS 2.1. Raw materials and chemicals 2.2. Synthesis of materials 2.2.1. Synthesis of magnetic ferrite NPs 2.2.1.1. Synthesis of Fe3O4 NPs with a low concentration of precursors 2.2.1.2. Synthesis of magnetic ferrite NPs with a high concentration of precursors 2.2.2. Synthesis of magnetic ferrite - noble metal (Ag, Au) hybrid NPs 2.2.2.1. Fe3O4/Ag hybrid NPs Fe3O4/Ag hybrid NPs were synthesized by seeded-growth method (Fe3O4 NPs were used as seeds) in the ODE solvent. The reactions were carried out with  = [Ag]/[Fe] in the range of 0.5 ÷ 13.6. The effect of temperature on the hybrid structure was investigated. 2.2.2.2. Fe3O4/Au hybrid NPs a) Fe3O4/Au hybrid NPs The synthesis of Fe3O4/Au NPs was similar to that of Fe3O4/Ag nanoparticles (section 2.2.2.1) but H[AuCl4].3H2O was used instead of AgNO3. b) Hollow Fe3O4/Au hybrid NPs The hollow Fe3O4/Au hybrid NPs were synthesized by the Galvanic replacement method and Fe3O4/Ag nanoparticles (section 2.2.2.1) were used as a template. The effect of the amount of the H[AuCl4] solution on the formation of the hollow Fe3O4/Au structure was investigated. 2.2.3. Phase transfer nanoparticles into water After synthesized, NPs were transferred phase into water using PMAO. 2.3. Material characterization 2.3.1. Transmission electron microscopy 5
2.3.2. X-ray diffraction 2.3.3. Vibrating sample magnetometer 2.3.4. Molecular absorption spectroscopy UV-Vis 2.3.5. Fourier Transform Infrared Spectroscopy 2.3.6. Energy Dispersive X-Ray Spectroscopy 2.3.7. Thermal gravimetric analysis 2.3.8. Dynamic light scattering 2.4. Methods of evaluating the toxicity of materials The toxicity of hybrid nanomaterials on AGS and MKN45 cells was evaluated by MTT method. 2.5. Methods of evaluating antibacterial activity of materials The evaluation of the antibacterial activity of materials was carried out by the agar well diffusion method. Bacterial species tested: Gram-positive bacteria: Bacillus subtilis (B. subtilis), Lactobacillus plantarum (L.plantarum), Sarcina lutea (S. lutea). + Gram-negative bacteria: Serratia marcescens (S. marcescens), Escherichia coli (E. coli). 2.6. Determination of the magneto-photothermal conversion efficiency The magnetic/photo induced heating efficiency of materials was carried out under three conditions: (i) Magnetic hyperthermia (MHT) at the magnetic field with an intensity of 100 - 300 Oe and frequency of 450 kHz, (ii) Photothermial therapy (PTT) at 808 nm laser, a power density of 0.2 – 0.65 W/cm2, and (iii) combined magnetic field and laser at the exact same conditions (MHT + PTT). 2.7. Magnetic resonance imaging MRI images of different material concentrations were taken on a Siemens magnetic resonance device (Model: MAGNETOM Avanto 1.5 T) with an alternating magnetic field (64 MHz and 1.5 tesla). 6
CHAPTER 3. RESULTS AND DISCUSSION 3.1. Magnetic ferrite nanoparticles 3.1.1. Morphology 3.1.1.1. Synthesis of Fe3O4 NPs at low concentration of precursors Normally, Fe3O4 NPs were synthesized by thermal decomposition in dibenzyl ether, a toxic organic solvent. In this thesis, 1-octadecene, a solvent with much lower toxicity level, was tested and used as a solvent. The effect of several factors such as time, reaction temperature, surfactant concentration, and inorganic precursor concentration on Fe3O4 nanoparticle size was determined by TEM and the results are presented in Table 3.1. Table 3.1. Effect of reaction conditions on Fe3O4 nanoparticle size. Surfactant Precursor concentration concentration Tempe (mM) Time (mM) rature Samples dTEM (nm) (min) FeSO4. FeCl2. (oC) Fe(acac)3 OA OLA 7H2O 4H2O 30 F1 3.6 ± 0.7 190 0 0 372 372 295 60 F2 4.5 ± 0.7 120 F3 7.2 ± 1.0 10 F4 3.2 ± 0.5 30 F5 4.1 ± 0.6 190 0 0 558 558 295 60 F6 6.3 ± 0.9 120 F7 10.7 ± 1.4 10 F8 3.4 ± 0.5 30 F9 6.7 ± 0.7 190 0 0 744 744 295 60 F10 8.1 ± 0.7 120 F11 13.9 ± 1.1 10 F12 5.8 ± 0.9 190 0 0 930 930 295 30 F13 11.3 ± 1.2 60 F14 14.7 ± 1.3 270 F15 4.8 ± 1.0 126.7 0 63.3 558 558 295 F16 8.4 ± 1.5 60 315 F17 10.2 ± 0.5 126.7 63.3 0 558 558 315 F18 10.8 ± 2.4 7
a) Effect of reaction time Figure 3.1. TEM images of the samples F8 (a), F9 (b), F10 (c), F11 (d) and corresponding particle size distribution histograms (e) All the Fe3O4 NPs are spherical, monodisperse, and uniform in size. When the reaction time increases from 10 ÷ 120 minutes, the particle size increases and is in the range of 3.2 ÷ 14.7 nm. b) Effect of surfactant concentration Figure 3.2. TEM images of the samples F2 (a), F6 (b), F10 (c) and F14 (d) and corresponding particle size distribution histograms (e) The obtained particles are spherical, uniform, and monodisperse, and the average particle size increases as the concentration of OA and OLA increases. c) Effect of inorganic precursors In order to reduce the cost and widen the applications, Fe(acac)3 was partly replaced by inorganic Fe(II) salts which are much cheaper than Fe(acac)3. 8
Figure 3.3. TEM images and corresponding particle size distribution histograms of samples F17 (a, b) and F18 (c, d) Of the two inorganic salts, FeCl2 gave uniform Fe3O4 NPs which were similar to those formed when only Fe(acac)3 was used. d) Effect of reaction temperature At 270 oC and 295 oC, the obtained Fe3O4 NPs are not uniform, and particle boundaries are not clear. At 315 oC (boiling point of the solvent), the average size of Fe3O4 particles is 10.2 ± 0.5 nm, the particles are uniformly distributed, and their boundaries are clearer. Figure 3.4. TEM images of samples F15 (a), F16 (b), F17 (c) and corresponding particle size distribution histograms (d). 3.1.1.2. Synthesis of magnetic ferrite NPs at high precursor concentration Hình 3.5. TEM images of Fe3O4 NPs (a1, a2), CoFe2O4 NPs (b1, b2) and MnFe2O4 NPs (c1, c2) synthesized at high precursor concentration. The magnetic ferrite particles synthesized at the high precursor concentration are still uniform and monodisperse. 3.1.2. Crystalline structure The as-synthesized materials have characteristic peaks corresponding to the (220), (311), (222), (400), (422), (511) và (440) planes in the ferrite spinel structure. 9
3.1.3. Magnetic properties When the particle size of Fe3O4 increases 3.2 ÷ 14.7 nm, the saturation magnetization (Ms) value rises 40.0 ÷ 66.5 emu/g and the coercivity (Hc) increases 0 ÷ 57 Oe. For the samples with particle size  10 nm, Hc is below 6 Oe. On the other hand, with the same particle size, the Ms value of magnetic ferrites decreases in the order of CoFe2O4, Fe3O4, and MnFe2O4. 3.1.4. Structure of the shell coating magnetic ferrite NPs The FT-IR spectra of magnetic ferrite samples have relatively similar absorption peaks, which characterize the vibrations of bonds in surfactants (OA/OLA). According to the TGA curve, the real mass of the magnetic core is about 90% Summary of results section 3.1: 1. Magnetic ferrite nanoparticles MFe2O4 (M: Fe, Co, Mn) were successfully fabricated by thermal decomposition in ODE solvent. The obtained NPs are highly uniform and monodisperse and have a spinel structure. By varying the synthesis conditions (time, reaction temperature, surfactant concentration, and precursor concentration), the particle size in the range of 3.2 ÷ 14.7 nm can be controlled. With the partial replacement of Fe(acac)3 by FeCl2 and at a high concentration of precursors, the magnetic ferrite particles are still highly uniform and monodisperse. 2. The value of the magnetic saturation of Fe3O4 NPs increases from 40.0 to 66.5 emu/g when the particle size increases from 3.2 to 14.7 nm. At room temperature, the obtained material has a very small coercivity and can be considered as superparamagnetic, that meets the requirements for biomedical applications. 3. Magnetic ferrite nanoparticles MFe2O4 are coated with a surfactant layer (OA/OLA) with a mass 10%. 3.2. Fe3O4-(Ag, Au) hybrid NPs According to the study of synthesis of the magnetic ferrite NPs, four Fe3O4 samples with different particle size: 6.7 ± 0,7 nm (F9), 8.1 ± 0.7 nm (F10), 10.2 ± 0.5 nm (F17) và 13.9 ± 1.1 nm (F11) were selected as the seeds for synthesizing Fe3O4-(Ag, Au) NPs. 3.2.1. Morphology 3.2.1.1. Fe3O4/Ag hybrid NPs Fe3O4/Ag hybrid NPs were synthesized by the seeded-growth method in ODE solvent. The parameters: [Ag]/[Fe] ratio and reaction time affecting the structure of Fe3O4/Ag are given in Table 3.5. 10
Table 3.5. Effect of synthetic conditions on Fe3O4/Ag hybrid structure  = [Ag]/[Fe] Reaction time Fe3O4/Ag hybrid structure 30 Core - shell 0.5 60 Core - shell 30 Core - shell 1.4 60 Core - shell 30 Core - shell 2.3 60 Core - shell 30 Core - shell 3.2 60 Core - shell 30 Core - shell 4.5 60 Core - shell 120 Core - shell 30 Core - shell 6.8 60 Core - shell 120 Core - shell 30 Dumbbell, Core - shell 9.0 60 Dumbbell, Core - shell 120 Dumbbell, Core - shell 30 Dumbbell 11.4 60 Dumbbell 120 Dumbbell 30 Dumbbell 13.6 60 Dumbbell 120 Dumbbell a) Effect of molar ratio between precursor and seed ([Ag]/[Fe]) Figure 3.10. TEM images (a - g) and corresponding particle size distribution histograms (h) of Fe3O4 (seeds) and Fe3O4/Ag hybrid NPs with the variation of  = [Ag]/[Fe] 11
When  is below 6.8, the obtained hybrid NPs have shell-core structure. When this value is increased to 9, the dumbbell structure is formed. Thus, when the value of  = [Ag]/[Fe] increases, the morphology of Fe3O4/Ag hybrid NPs changes from core-shell to the dumbbell. b) Effect of reaction time When the reaction time is 30 and 60 minutes, the particles are relatively uniform. As the reaction time is extended to 120 minutes the agglomeration of two or more nanoparticles occurs. This trend is similar to that in the case of using Fe3O4 seeds with different sizes. Figure 3.12. TEM images of Fe3O4/Ag hybrid structure synthesized with different reaction time: a1, b1) 30 min, a2, b2) 60 min, (a3, b3) 120 min. The size of the Ag shell in the core-shell structure and the Ag particles in the dumbbell structure increase as the reaction time increases. From the above analysis results, we can summarize the diagram describing the formation of Fe3O4/Ag hybrid structure as shown in Figure 3.13. Figure 3.13. Diagram describing the morphological development of Fe3O4/Ag hybrid structure 12
3.2.1.2. Fe3O4/Au hybrid NPs a) Fe3O4/Au NPs The Fe3O4/Au core-shell NPs were synthesized with [Au]/[Fe] molar ratio of 6.8. b) Hollow Fe3O4/Au hybrid NPs The hollow Fe3O4/Au hybrid NPs were synthesized by using Fe3O4@Ag (core-shell) NPs as the template via Galvanic replacement reaction between Ag and Au3+ in ODE solvent: 3Ag0 + Au3+  Au0 + 3Ag+ *) Effect of Fe3O4@Ag template on the formation of hollow Fe3O4/Au structure: When Fe3O4@Ag template is 12.8 nm, hollow Fe3O4/Au structure is not formed. As the template is 16.0 nm, the small holes in the middle of the hybrid NPs can be observed on TEM image, and the obtained solution has a characteristic blue color, demonstrating the formation of the hollow Fe3O4/Au hybrid NPs. Figure 3.15. TEM images and corresponding solutions of Fe3O4@Ag template (a1, a2) and Fe3O4/Au hybrid NPs (b1, b2) Thus, the formation of hollow Fe3O4/Au hybrid NPs depends on the Fe3O4@Ag template and the proposed mechanism is given in Figure 3.16. Figure 3.16. Diagram describing the formation mechanism of hollow Fe3O4/Au hybrid nanostructure 13
*) Effect of the amount of H[AuCl4] solution on the formation of hollow Fe3O4/Au hybrid structure When the volume of H[AuCl4] solution used in the reaction is 0.5 mL, the hollow structure is not formed. For larger amounts of H[AuCl4] (from 1.0 to 2.0 mL), the particles are converted into cage like structures, and the void size gradually increased. At 2.0 mL, the largest size of material reaches 17.0 nm. Further increasing the volume of H[AuCl4], the hollow spheres are gradually broken, and at 3.5 mL, the hollow structure is completely ruined. Figure 3.17. TEM images of Fe3O4@Ag NPs (a), Fe3O4/Au NPs (b-i) at different amounts of H[AuCl4] solution and particle size distribution histograms (k) of (a and e). The development of Fe3O4/Au hybrid structure morphology depends on the amount of H[AuCl4] solution used. This relationship can be summarized according to Figure 3.18. Figure 3.18. Effect of amounts of H[AuCl4] solution on the morphology of Fe3O4/Au hybrid structure. 14
3.2.2. Crystalline structure On the XRD pattern of Fe3O4@Ag core-shell structure, only characteristic peaks of Ag cubic structure can be observed while XRD data of Fe3O4-Ag dumbbell structure shows typical peaks of both Fe3O4 spinel (low intensity) and Ag (high intensity). Figure 3.19. XRD patterns of 3.2.3. Optical properties Fe3O4, Ag and Fe3O4/Ag NPs In the wavelength range from 300 - 900 nm, Fe3O4 NPs do not have absorption peaks. Ag NPs have an SPR peak at 405 nm, Fe3O4/Ag hybrid NPs have SPR peaks at 410 nm with the core-shell structure (sample F10@A60, 16.0 nm) and 420 nm with dumbbell structure (sample F10-A60, 8.1-16.3 nm). In general, the SPR peak of Fe3O4/Ag hybrid nanoparticles is below 450 nm. Figure 3.20. UV-Vis spectra of NPs in n-hexane solvent: a) Fe3O4, Ag and Fe3O4/Ag NPs, b) Fe3O4 and Fe3O4/Au NPs. The optical properties of Fe3O4/Au hybrid NPs depend on the morphology and structure of the materials. Fe3O4/Au hybrid NPs have an SPR peak at 530 nm, while hollow Fe3O4/Au hybrid NPs have an SPR peak at 707 nm (the sample with 2 mL of Au3+). The SPR position of the hollow Fe3O4/Au hybrid NPs depends on the amount of H[AuCl4] solution (Table 3.10). Table 3.10. Effect of the amounts of H[AuCl4] solution on the SPR position of Fe3O4/Au hybrid NPs. H[AuCl4] (mL) 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 The SPR position of 410 587 627 645 707 592 585 565 Fe3O4/Au (nm) 15
3.2.4. Magnetic property The values of saturation magnetization and coercivity of Fe3O4/Ag hybrid samples are lower than pure magnetic NPs, but the magnetic response of hybrid NPs is relatively good. Figure 3.21. a) The magnetization Figure 3.22. The magnetization curves of Fe3O4 and Fe3O4/Ag NPs; curves of Fe3O4@Ag template and b) Photographs of Fe3O4/Ag NPs in Fe3O4/Au NPs and image (inset) of n-hexane without and with magnet hollow Fe3O4/Au hybrid NPs in n-hexane without and with magnet. The formation of hollow Fe3O4/Au structure does not change the superparamagnetic properties when compared to that of Fe3O4@Ag template, but the value of saturation magnetization increases slightly (Figure 3.22). 3.2.5. Chemical composition The composition of Fe3O4/Ag hybrid NPs consists of the main elements Fe, Ag and O, and that of Fe3O4/Au, includes Fe, Ag, Au and O. According to SEM-EDS elemental mapping analysis, Fe3O4/Au hollow NPs (the sample with 2 mL of Au3+) show the good distribution Figure 3.25. SEM-EDS elemental of all elements in the hollow mapping of hollow Fe3O4/Au hybrid NPs. nanostructure (Figure 3.25). Summary of results of section 3.2: 1. Fe3O4/Ag hybrid NPs were successfully fabricated by the seeded-growth method in ODE solvent and their morphology can be controlled by varying  = [Ag]/[Fe] and reaction time. With   6.8, the particles have Fe3O4@Ag core-shell structure, while with  = 9 Fe3O4-Ag dumbbell structure is formed. The size of the Ag shell in the 16
core-shell structure and the Ag particles in the dumbbell structure increase as the reaction time increases. 2. The hollow Fe3O4/Au hybrid NPs were fabricated by using Fe3O4/Ag NPs as the template via Galvanic replacement reaction between Ag and Au3+. The obtained material has an average size of 17 nm and its SPR peak can be controlled to 707 nm. With [Au]/[Ag] = 0.83, Fe3O4/Au hybrid NPs have the largest void size, and all elements are well distributed. 3. The as-synthesized Fe3O4-(Ag, Au) NPs have superparamagnetic properties at room temperature with the value of the saturation magnetization in the range of 7 ÷ 27 emu/g, lower than that of the Fe3O4 seed samples (50 ÷ 64 emu/g). 3.3. Nanoparticles are coated by PMAO 3.3.1. Phase transfer of nanoparticles by PMAO Before being coated with PMAO, the hybrid NPs are dispersible in n-hexane and after coated, they are well-dispersed in water (Figure 3.26). Figure 3.26. Phase transfer of the NPs by PMAO (a), solutions of Fe3O4@Ag NPs (b), Fe3O4-Ag NPs (c) and hollow Fe3O4/Au NPs (d) before (1) and after phase transfer (2). 3.3.2. Optical properties of materials Figure 3.27. Solution (a) and UV-Vis spectra (b) of Fe3O4@Ag@PMAO and hollow Fe3O4/Au@PMAO NPs at different amounts of Au3+. 17
The change in color of the hybrid NPs solutions is also shown by the shift of the SPR peak of the materials (Figure 3.27). Among the solutions of Fe3O4/Au hybrid NPs coated by PMAO, the 2.0 mL Au3+ sample has the strongest SPR peak shifting to the near-infrared region. This sample was selected for further studies. 3.3.3. Structure of the coating shell FT - IR spectra confirm that the hybrid NPs were coated with PMAO. TGA curves indicate that PMAO shell accounts for 58% and Fe3O4/Au hybrid NPs make up 42% of the sample mass. Figure 3.28. FT-IR spectra of Hình 3.29. TGA curves of hollow hollow Fe3O4/Au hybrid NPs before Fe3O4/Au hybrid NPs before and after phase tranfer and after phase tranfer The hydrodynamic diameters determined by DLS of Fe3O4@Ag@PMAO and hollow Fe3O4/Au@PMAO are 25.85 and 28.84 nm, respectively. Zeta potential of Fe3O4@Ag@PMAO and hollow Fe3O4/Au@PMAO aqueous solutions is -42.5 mV and -40.0 mV, respectively. Moreover, hybrid NPs are stable in NaCl solution at concentration of 150 ÷ 250 mM and pH of 2 ÷ 11. These results demonstrate that the solution of the hybrid NPs is well dispersed and stable under the investigated conditions. 3.3.5. Toxicity evaluations of materials The toxicity of the solutions of Fe3O4@Ag@PMAO and hollow Fe3O4/Au@PMAO hybrid NPs was evaluated on two gastric cancer cell lines AGS and MKN45 by MTT method. *) Fe3O4@Ag hybrid NPs: At Fe3O4@Ag@PMAO concentration below 20 µg/mL, AGS and MKN45 cells grew normally, similar to those of the control sample. At higher sample concentrations (20 ÷ 100 µg/mL), the morphology and nuclei of the cells were altered. The IC50 values determined for AGS and MKN45 cell lines are 42 µg/mL and 58 µg/mL, respectively. 18