Utilização de quitosana no controle sobre a formação de hot spots e protetor superficial de nanoesferas de ouro para estudos de adsorção por espalhamento Raman intensificado por superfície

Abstract

The surface protection of gold nanospheres (AuNEs) can provide greater colloidal stability, important when work with biological system, because the present diverse situations that can lead to the aggregation of AuNEs. The chitosan biopolymer (Quit) is a non-toxic material, presents low cost, and has been widely in biological systems for the diagnosis of diseases. Sensitive systems, such as biosensors used in detection of adsorbed molecules in AuNEs, can be monitored by changing in Localized Surface Plasmon Resonance´s band (LSPR) and by use of Surface-Enhanced Raman Spectroscopy technique (SERS). When the molecules are adsorbed on the AuNEs surfaces is possible to observe the induction of aggregation of the colloidal suspension by the change in the LSPR band. Due to the analytical sensitivity, the surfaces of the AuNES are protected to avoid aggregation and colloidal precipitation, and to obtain false positive results. This thesis aimed to perform several studies using the AuNEs surface protected by Quit to offer greater stability and to control the formation of hot spots in the presence of the analyte. For this, experiments were always done to compare the AuNEs with and without surface protection by Quit. The initial studies observed the resistance to AuNEs aggregation in relation to exposure time at room temperature. Following the reasoning, the AuNEs were also exposed to different concentrations of the aggregating agents ethanol (EtOH) and KCl. The analytes that were used as a test molecule for the SERS and also to observe possible changes in the LSPR band were the dyes: crystal violet (CV), rhodamine 6G (R6G) and IR-820, which have positive or negative surface charges; in order to identify some preference in surface adsorption of the analyte. For the SERS spectroscopy studies, measurements were performed varying the concentration of Quit in the presence of the dyes together or separated, besides observing the interference in the order of dye addition in the SERS signal. The Quit proved promising to increase the stability of AuNEs and, therefore, modification experiments, aiming at modulate and control the formation of hot spots, were performed. The SERS effect was used as an analytical tool in which the direct relationship of the SERS signal obtained from the dyes with their concentration in solution was studied. From these results, it was possible to construct adsorption isotherms of the studied dyes, which were adjusted to the Langmuir model.