Crescimento de nanopartículas de ouro em substrato de sílica para futura aplicação em sensores ópticos

Abstract

Nanoparticles (NPs) are considered relatively new materials, as they are produced from already known compounds but, due to their small proportions, have differentiated proprerties from those of solid bulk. There are several methods for the synthesis of NPs, which can be physical, chemical or biological. Among the metallic NPs, the gold ones are the most studied and applied, been included in these applications its use in optic fiber sensors. Controlling some characteristics of the NPs, such as average size and shape, we believe that it is possible to enhance a sensor’s sensibility and sensitivity. The absorption band of a particular set of metallic NPs depends on size, shape, metal that composes the nanoparticle and other factors, such as the surrounding environment. This absorption band occurs in the visible region of the spectrum for gold, silver and copper, and it is called plasmon band, occuring due to the surface electrons ressonance upon the exposition to a specific wavelenght of electromagnetic radiation. The occurance of the plasmon band makes that the sensors with deposited NPs on their surface differenciate small changes in the refractive index close to the sensor’s surface. By applying a layer of gold NPs on the optic fiber’s surface, it’s possible to create a highly efficient surface plasmon ressonance sensor. The objective of this work was focused on the synthesis and characterization of gold NPs deposited in silica slides for future application of these NPs on the surface of optic fiber sensors. For this, it was used microscope slides, which were funcionalized with different concentrations of 3- aminopropyltriethoxysilane and in sequence coated with solutions with different concentrations of chloroauric acid and then reduced with different concentrations of sodium borohydrade. The characterization of these slides was made with the aid of techniques such as X-ray diffraction, Raman and UV-Vis spectroscopy, transmission electron microscopy and atomic force microscopy. Only one between the analyzed samples presented a film thick enough to get information on XRD, providing an average cristallite’s size of 11,10 nm. Most of the samples had a plasmon band easily observed by UV-Vis, but the Raman spectra showed na –OH signal due to the water adsorbed on the material’s surface, and the samples did not present a relevant SERS effect. The scannings in AFM were conclusive about the homogeneity of NPs distribution over the substrate, as well as the film’s thickness. The TEM images showed the presence of nanospheres and, in some samples, nanorods also. The synthetic rout must be thoroughly studied, even applying other variables, such as the film’s exposure time to an analyte in order to evaluate the plasmon band intensification.