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We report the first attempt to use a naturally occurring biopolymer, such as humic acid (HA), as modifiers to prepare a new organic-inorganic composite film with the 3-n-propylpyridiniumsilsesquioxane chloride Pt nanoparticles (Pt-SiPy+Cl−). By using layer-by-layer (LbL) technique, we have constructed multilayered films, (HA/Pt-SiPy+Cl−)n and (Pt-SiPy+Cl−/HA)n, with adjustable thickness due to negatively charged macromolecule HA, which assists the electrostatic interaction with the positively charged of the Pt-SiPy+Cl− nanohybrid. The combination of these partially charged polyelectrolytes in the LbL film was explored by spectroscopic and electrochemical techniques. The formation of the film (HA/Pt-SiPy+Cl−)n is more efficient, since the presence of the macromolecule in the inner layer provides a greater number of anchoring sites for the hybrid, ensuring a more linear and uniform growth of the LbL films. This film also presented a better electroanalytical response for oxidation of 17α-ethynylestradiol, EE2, indicating that the Pt nanoparticles strongly influences the electrochemical oxidation of estrogen. The analytical curve for EE2 was linear in the concentration range of 1.37 to 21.4 μmol L−1 (R = 0.998), with a detection limit of 1.10 μmol L−1 and quantification limit of 3.68 μmol L−1. The obtained results showed that the synergism between HA and Pt-SiPy+Cl− improves the electroactive properties of the sensor.

The detection of dopamine (DA) encapsulated in liposomes prepared by the hydrating of a thin film of dipalmitoyl phosphatidylcholine (DPPC) with DA solution, was investigated using LbL films containing sulphonate p-phenylenevinylene (PPV-SO3) and the hybrid formed by incorporation platinum nanoparticles (PtNPs) in the cavities of chloride-3-n-propylpyridiniumsisesquioxane polymer (SiPy+Cl-), designed as PPV/Pt-SiPy+Cl-. The LbL films of (PPV/Pt-SiPy+Cl-)n deposited on FTO was applied as sensors to detection of free and encapsulated DA into liposomes. The LbL films with 3 bilayers (PPV/Pt-SiPy+Cl-)3 present higher current values for DA oxidation (Ipa = 76 µA) and higher separation between the oxidation potential of the DA (0.97 V) and the oxidation potential of the ascorbic acid (AA) and uric acid (UA) interferences (1.60 V) (∆pa = 630 mV). The separation of the oxidation peak potentials for DA and interferences decreases to 320 mV when the DA is encapsulated in liposome. This behavior can be explained by approximation of the liposomes to electrode surface which can influences in the interaction between the Pt-SiPy+Cl- from (PPV/Pt-SiPy+Cl-)3 LbL film and the AA and AU interferences.

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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.