SYNTHESIS OF METAL-METAL OXIDE NANOCOMPOSITES FOR ANTIBACTERIAL APPLICATIONS

Abstract

Nanotechnology deals with understanding and utilizing materials in the nanoscale range (1-100 nm). Nanomaterials exhibit unique physicochemical properties that depend on their size, shape, composition, etc., with respect to their bulk counterparts. Among various nanomaterials, metal-metal oxide nanocomposites are of immense interest. The metal-metal oxide nanocomposites are multi-functional materials with at least one of the phases in the nanometer range. They possess unique design in which metal nanoparticles can be deposited on the surface of metal oxide nanoparticles. The metal-metal oxide nanocomposites possess different physicochemical properties compared to their individual constituents. The properties of metal-metal oxide nanocomposites can be easily tuned depending on the desired application by varying the size and shape of the metal or metal oxide nanoparticles or both. The flexibility in designing metal-metal oxide nanocomposites has led to their diverse applications in gene delivery, drug delivery, dental implants, photocatalysis, sensors, surface-enhanced Raman scattering, etc. Various chemical methods have been reported for the synthesis of metal-metal oxide nanocomposites, but their easy and economical synthesis is still challenging. New methods are often explored for the synthesis of metal-metal oxide nanocomposites. In the present thesis, the following nanoparticles and nanocomposites have been synthesized: (i) pure ZnO nanoparticles, (ii) pure Ag nanoparticles, and (iii) ZnO-Ag nanocomposites. The synthesis of metal-metal oxide nanocomposites was carried out in two steps. In the first step, pure metal oxide nanoparticles (ZnO) were synthesized using three different methods, namely, solid-state thermal decomposition, homogeneous precipitation, and wet chemical method. In the second step, deposition of Ag nanoparticles on the surface of ZnO nanoparticles was carried out using a thermal decomposition approach. The synthesized metal, metal oxide and metal-metal oxide nanocomposites were characterized using various analytical techniques that prove their formation. The optical properties of the nanoparticles and nanocomposites were studied using diffuse reflectance spectroscopy. After thorough characterization, the metal-metal oxide nanocomposites were explored for antibacterial applications. The present thesis consists of five chapters, and a brief description of each chapter is as follows. Chapter 1 starts with a brief introduction to nanotechnology, followed by an introduction to metal-metal oxide nanocomposites. Then, classification of metal-metal oxide nanocomposites and different chemical synthetic routes for their synthesis have been discussed. Further, this chapter deals with various interesting physicochemical properties of metal-metal oxide nanocomposites. In the end, some important applications of metal-metal oxide nanocomposites have been discussed. Chapter 2 deals with various analytical techniques, used in the present study, to characterize the synthesized nanoparticles and metal-metal oxide nanocomposites along with sample preparation methods for the measurements. The various analytical techniques used for the characterization of nanoparticles and nanocomposites include powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDXA), transmission electron microscopy (TEM), selected area electron diffraction (SAED), optical microscopy and atomic force microscopy (AFM). The optical properties of the metal-metal oxide nanocomposites were studied using diffuse reflectance spectroscopy (DRS). Chapter 2 also deals with the experimental details on the antibacterial activity studies against E. coli and S. aureus (by zone of inhibition and sprinkle methods) employed in the present study. Chapter 3 deals with the synthesis of three different types of pure ZnO nanoparticles (Z1, Z2, and Z3) by solid-state thermal decomposition, homogeneous precipitation, and wet chemical method, respectively. XRD analysis proves the formation of ZnO nanoparticles with wurtzite structure. The crystallite size of Z1, Z2, and Z3 nanoparticles was about 21.4 nm, 47.4 nm, and 25.4 nm, respectively. The formation of ZnO nanoparticles with different shapes were confirmed using FESEM and TEM studies. FESEM and TEM studies confirm rod-like morphology of Z1 and Z2 nanoparticles, while the morphology of Z3 nanoparticles was hierarchical assembly consisting of ZnO nanorods. EDX results confirmed the presence of Zn and O in all the three ZnO nanoparticles. SAED results confirm single crystalline nature of Z1 and Z2 nanoparticles, and polycrystalline nature of Z3 nanoparticles. The DRS studies of ZnO nanoparticles indicate that the bandgap values of Z1, Z2, and Z3 nanoparticles are 3.30 eV, 3.20 eV, and 3.24 eV, respectively. The antibacterial activity of the three types of ZnO nanoparticles (Z1, Z2 and Z3) was investigated using zone of inhibition and sprinkle methods against E. coli and S. aureus. According to zone of inhibition method, Z1 (ZnO nanoparticles synthesized by solid-state thermal decomposition), Z2 (ZnO nanoparticles synthesized by homogeneous precipitation), and Z3 (ZnO nanoparticles synthesized by wet chemical method) exhibit similar antibacterial activity against S. aureus. All the three ZnO nanoparticles (Z1, Z2 and Z3) did not exhibit antibacterial activity against E. coli according to ZOI method. According to sprinkle method, the antibacterial activity of Z3 (ZnO nanoparticles synthesized by wet chemical method) is better against E. coli compared to Z1 (ZnO nanoparticles synthesized by solid state thermal decomposition method) and Z2 (ZnO nanoparticles synthesized by homogeneous precipitation method). The antibacterial activity of Z2 and Z3 is higher against S. aureus compared to that of Z1. According to sprinkle method, size and morphology-dependent antibacterial activity is observed for ZnO nanoparticles against E. coli and S. aureus. From the FE-SEM and TEM studies, it was found that the Z3 sample possesses lesser diameter compared to Z1 and Z2 samples. Also, Z3 has higher surface area (48 m2/g) than Z1 (32 m2/g) and Z2 (36 m2/g) samples. The higher surface area of Z3 suggests more oxygen vacancies and more ROS generation compared to Z1 and Z2 and thus Z3 sample exhibits better antibacterial activity compared to Z1 and Z2. Chapter 4 deals with the synthesis of ZnO-Ag nanocomposites. The synthesis of two types of pure Ag nanoparticles (A-1 and A-2) and twelve types of ZnO-Ag nanocomposites (Z1-Ag-1 to Z1-Ag-5, Z2-Ag-1 to Z2-Ag-4, and Z3-Ag-1 to Z3-Ag-3, see Table 4.1) was carried out by thermal decomposition method. The ZnO-Ag nanocomposites were synthesized by thermal decomposition of silver acetate (Agac) in the presence of ZnO nanoparticles (Z1, Z2 and Z3) in diphenyl ether. The thermal decomposition time and the amount of Agac was varied during the synthesis. The prepared Ag nanoparticles and ZnO-Ag nanocomposites were then characterized using several analytical techniques. XRD results indicated the presence of ZnO and Ag in the ZnO-Ag nanocomposites. FT-IR studies proved the purity of ZnO-Ag nanocomposites. FE-SEM studies showed deposition of Ag nanoparticles on the surface of ZnO nanoparticles. EDX analysis confirmed the presence of zinc, oxygen, and silver in the ZnO-Ag nanocomposites. TEM studies showed the adherence of Ag nanoparticles on the surface of ZnO nanoparticles. SAED patterns of ZnO-Ag nanocomposites indicated polycrystalline nature of ZnO nanoparticles and cubic Ag. DRS results showed absorption bands due to ZnO (band gap absorption) and Ag (surface plasmon resonance) in the ZnO-Ag nanocomposites. After characterization, the antibacterial activity of ZnO-Ag nanocomposites was investigated by zone of inhibition and sprinkle methods against E. coli and S. aureus. The antibacterial activity of ZnO-Ag nanocomposites was found, in general, better than that of pure Ag and ZnO nanoparticles According to the zone of inhibition method, Z1-Ag-1 to Z1-Ag-4, Z2-Ag-1 and Z2-Ag-4, and Z3-Ag-1 to Z3-Ag-3 nanocomposites exhibit good antibacterial activity against S. aureus while only Z2-Ag-4 nanocomposite exhibits good antibacterial activity against E. coli. According to the sprinkle method, Z1-Ag-1 to Z1-Ag-3, Z2-Ag-4, and Z3-Ag-2 nanocomposites exhibit good antibacterial activity against E. coli, while Z1-Ag-1 to Z1-Ag-5, Z2-Ag-4, and Z3-Ag-3 nanocomposites exhibit good antibacterial activity against S. aureus. The difference in antibacterial activity among the ZnO-Ag nanocomposites has been explained on the basis of dimensions (e.g., average diameter) of Ag and ZnO nanoparticles, and the amount of Ag nanoparticles in the ZnO-Ag nanocomposites. Chapter 5 deals with an overall summary of the present thesis and discusses future prospects.