Nanocompósitos formados por nanopartículas de ouro e polianilina ancorados em substrato de silica: síntese, caracterização e uso em sensor ácido/base

Abstract

The chemical synthesis of Polyaniline (PANI) can be carried out via oxidation of the monomers and results in different forms, depending on its degree of oxidation. The Emeraldine Salt (ES) form is the most commonly used for applications, due to its high conductivity and stability values. Nanocomposites formed by PANI and gold nanoparticles (AuNps) can, among other applications, be used as a pH-sensitive material, as the ES form and the Emeraldine Base (EB) form are interconvertible depending on the pH of the medium. Thus, the present work focused on the synthesis and characterization of nanocomposites containing PANI/ESAuNPs anchored on silica substrate, obtained in a single-step synthesis (one-pot), where the oxidizing agent, responsible for PANI polymerization, is the Au3+ cation, which, in the process, is reduced to Au0 forming gold Nps. The different samples were obtained in the form of a film deposited on a substrate, which facilitates their application. In the proposed methodologies, silica slides used as substrates were cleaned and previously prepared in piranha solution and subsequently functionalized with 3-aminopropyltriethoxysilane (APTES) in toluene under heating (60°C) for 1 hour, with deposition of metal cations via aqueous solution of HAuCl4 and subsequent reduction by solution composed of aniline in Hcl for 24 hours, this being the final step of the synthesis where the polymer and the Nps are formed concomitantly, resulting in a PANI-AuNPs film anchored on the substrate. Variations in the concentrations of the solutions were made to study the effect on the characteristics of the obtained films. UV-Vis spectroscopy showed the typical PANI spectrum in the ES form. Through Raman spectroscopy, it was possible to assign the characteristic bands of the polyaniline polymer. X-ray Diffraction (XRD) confirmed the presence of gold nanoparticles by presenting characteristic peaks of the AuNPs in the fcc phase. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) images showed a film with characteristic polymeric organization of PANI. Transmission Electron Microscopy (TEM) images demonstrated a polymeric environment with dispersed gold Nps, and the crystallinity of the Nps could be confirmed by dark-field images. The proposed routes for obtaining such systems were successful, as seen by the results of the different characterization techniques. The samples were also tested as an acid/base sensor both in vapor and in liquid, showing a rapid change in the film coloration, being a system with potential use for this purpose.