Using of Humic Acid and Platinum Nanoparticle As Surface Modifier for the Construction of a Electrochemical Sensor Using in Detection of Ethinylestradiol

This work presents a study for preparation of thin films obtained with a natural and synthetic polyelectrolyte, such as humic acid (HA) and a hybrid composite prepared by encapsulation of platinum nanoparticles in the matrix of chloride 3-n-propylpyridinium silsesquioxane (Pt-SiPy⁺Cl^-). Even when dealing with molecules of complex nature, the film deposition occurred regardless of the type of substrate. It was found that there was growth of films by alternate deposition of polyelectrolytes HA and Pt-SiPy⁺Cl^-, governed largely by electrostatic interactions between functional groups COOH, OH-phenolic of HA with pyridinium ring of SiPy⁺Cl^-. The presence of polyelectrolytes in LbL films built on molecular architectures (HA/Pt-Pt-SiPy⁺Cl^-)_n and (Pt-SiPy⁺Cl^-/HA)_n, was obtained by spectroscopic measurements. From the shifts of the bands in the absorption spectra in the UV-Vis region of LbL films in relation to the precursor solutions, it was inferred on the interactions between these molecules. Measurements of absorption spectroscopy FTIR and Raman scattering of the LbL film were observed by displacement or disappearance of absorption bands due to the presence of precursors of Pt-NPs cavities of SiPy⁺Cl^-. It was found that bands were more intense and defined in the spectra of the film (HA/Pt-SiPy⁺Cl^-)₃₀ in relation to the (Pt-SiPy⁺Cl^-/HA)₃₀ film. This observation corroborates the results of UV-Vis showing that from 12 bilayers no linear growth of absorbance as a function of number of bilayers (Pt-SiPy⁺Cl^-/HA)₁₂. The AFM images showed a linear increase when the film (HA/Pt-SiPy⁺Cl^-)₃₀ was obtained with a thickness of 2.6 nm per bilayer, confirming the nanostructured nature provided by the molecular organization of polyelectrolytes. Morphological characterization showed that the homogeneity of the surface in function of the amount of deposited materials influence of surface phenomena, such as the number of active sites, as evidenced by electrochemical action of the hormone 17α-ethinylestradiol (EE2). From the differential pulse voltammetric response of the investigated in the presence of EE2 62.5 mmol L^-1, it was found that the film (HA/Pt-SiPy⁺Cl^-)₃ showed high peak current (Ipa = 17.26 µA) and minor oxidation potential (Epa = 0.74 V vs Ag/AgCl) in relation to no modified electrode (FTO) (Ipa = 8.97 µA e Epa = 1.1 V). Under these conditions, the electrode (HA/Pt-SiPy⁺Cl^-)₃ obtained a linear response in the concentration range of 1.37 to 21.4 mmol L^-1, yielding a sensitivity 0.68 mmol L^-1 and limits of detection (LOD) and quantification (LOQ ) equal to 1.06 and 3.52 mmol L^-1, respectively. These values are close to those reported in the literature for electrodes modified with metals nanoparticles. Finally, the technique of electrochemical impedance spectroscopy confirmed that the sequence of deposition, the number of bilayers, as well as the nature of the nanostructured films LbL affect the charge transfer resistance. Therefore, the presence of HA in the architecture of the film decreases the charge transfer resistance of the fact that this macromolecule consist of large number of functional groups, which provide a greater number of sites occur for interactions with the pyridinium group SiPy⁺Cl^- and consequently higher amounts of Pt-Np on the electrode surface.