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    Estudo das interações entre ouro coloidal e óxidos de ítrio e titânio
    (Universidade Federal de São Paulo, 2016-03-15) Rosas, Victor Hildebrand [UNIFESP]; Giongo, Carolina Vautier Teixeira [UNIFESP]; Universidade Federal de São Paulo (UNIFESP)
    Many practical applications of gold nanoparticles (AuNP), such as detection, monitoring and pollution remediation of waters and soils, in addition to diagnosis and treatment of diseases, are based on the nature and magnitude of interactions between AuNP and different molecules, molecular aggregates or other colloidal particles, that occur in adsorption and mutual aggregation processes. Homoaggregation phenomena (aggregation between identical particles) and mutual aggregation (aggregation between different colloidal particles) have been studied for decades and, considering the fact that great advances have been achieved in understanding these phenomena, the presence of this new knowledge has been growing ever since in the fields of industrial processes and nanosciences. In this dissertation, homoaggregation of aqueous dispersions of AuNP has been studied as a function of media pH and ionic strength. In addition, studies of mutual aggregation between AuNP and Y2O3, on the one hand, and AuNP and TiO2, on the other hand, were undertaken. Localized surface plasmonic resonance (LSPR) spectroscopy was utilized in order to obtain kinetic and thermodynamic information concerning the aggregation experiments. Characterization of the aqueous dispersions of AuNP and inorganic oxides was achieved by LSPR spectroscopy in the visible region, dynamic and electrophoretic light scattering (DLS and ELS, respectively), and by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Aggregation experiments were performed with (i) approximately spherical AuNP having a mean diameter of 23 + 4 nm and a zeta potential around -35 mV in the pH range from 3 to 9; (ii) irregular Y2O3 particles presenting an apparent mean diameter of 600 + 180 nm and an isoelectric point (IEP) at pH = 8.2 + 0.1 and (III) spherical TiO2 particles having a mean diameter of 170 + 20 nm with an IEP at pH = 8.2 + 0.1. AuNP homoaggregation kinetic studies yielded information concerning the initial rate of the process as a function of pH and NaCl concentrations, enabling the obtainment of the stability factors of the AuNP dispersions and the NaCl critical concentration for the process of AuNP aggregation. Mutual aggregation kinetic studies between AuNP and Y2O3, and AuNP and TiO2, at pH 3.5, yielded apparent rate constants that showed very clearly the differences between the processes. Specific adsorption of Y+3 ions, originated from Y2O3 solubilization at pH 3.5, is probably the main factor that explains the differences observed for the experiments of mutual aggregation between AuNP with Y2O3 and with TiO2. Mutual aggregation isotherms between AuNP and Y2O3 at pH 3.5, obtained at different temperatures, indicate that the process is spontaneous, endothermic and that it proceeds with an increase of entropy. The isotherms profiles, which do not present typical plateaus of maximum coverage of Y2O3 particles by AuNP, suggests that either the nanoparticles deposit in the form of aggregates on the Y2O3 particles or that deposition of AuNP on the surface of Y2O3 favors additional aggregation of the nanoparticles onto the oxide?s surface. Our experimental results were rationalized in terms of generalizations of Mie?s theory of light extinction by metallic particles and the DLVO theory of colloidal stability. Great part of our observations can be justified by these theories.