Propriedades fototérmicas de nanofluidos plasmônicos obtidos por síntese fotoquímica

Abstract

Nanofluids are stable colloids of nanoparticles that have applications in many fields, ranging from engineering, such as improving solar energy generation, to medicine, as theranostic agents or optical markers for evaluating the presence of antigens in biological samples. For such applications, metallic nanoparticles are the most desirable, due to an optical property called localized surface plasmon resonance (LSPR), which is the collective excitation of conduction band electrons due to interaction with incident electromagnetic radiation. For noble metals, this resonance occurs in the visible range of the electromagnetic spectrum and properties such as particle shape, size and aggregation affect the width and position of the LSPR absorption band of plasmonic nanofluids. Therefore, controlling the size and shape of nanoparticles during synthesis is necessary to obtain plasmonic nanoparticles. In the synthesis of nanoparticles, two approaches can be used: top down and bottom up. In the top down approach we can mention laser ablation as an example of obtaining nanoparticles, and in the bottom up approach we can mention chemical reduction. But in recent years, interest in syntheses using a light source to excite solutions with precursors has stood out due to their easy arrangement and possibility of controlling the size and shape of the particles through the energy that is supplied to the solution. In this thesis, the influence of the excitation wavelength of the light used in the production of nanoparticles is explored and the influence of the size and concentration of the nanoparticles on the nonlinear refractive index (n2) that characterizes the third-order optical property of the obtained samples. The technique used in this work to evaluate the nonlinear refractive index in nanofluids obtained by photochemical synthesis was spatial self-phase modulation (SSPM), a simple and effective method that measures n2 based on the formation of diffraction rings as a function of the power of the continuous laser used to excite nanofluids. The results obtained by ultraviolet-visible (UV-Vis) spectroscopy and transmission electron microscopy (TEM) indicate that the excitation wavelength has a direct influence on the shape and agglomeration of the nanoparticles obtained by the photochemical method. The solutions obtained by irradiation at wavelengths of 395 nm and 473 nm showed particles with better defined shapes and sizes between 10 and 25 nm and less agglomeration compared to those obtained at wavelengths of 530 nm and 650 nm. The nucleation kinetics of the AuNPs obtained by the photochemical method used in this work show a sigmoidal curve when observing the absorbances of the surface plasmon peaks of each solution obtained, which is in line with the theoretical Finke-Watzky model or two-step F-W model. In the photothermal characterization using the SSPM technique, the nanofluids obtained at the shorter wavelengths showed higher n2 values than the nanofluids obtained by exposure to longer wavelengths. This result indicates an improvement in the non-linear optical response when the solutions are obtained through irradiation with greater energy provided by the light source.