Browsing by Author "Homem-de-Mello, Paula"
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- Chlorhexidine digluconate on chitosan-magnetic iron oxide nanoparticles modified electrode: Electroanalysis and mechanistic insights by computational simulationsPublication . Sousa, Camila P.; Oliveira, Raissa C. de; Freire, Tiago. M.; Fechine, Pierre B.A.; Salvador, Michele A.; Homem-de-Mello, Paula; Morais, Simone; Lima-Neto, Pedro de; Correia, Adriana N.In this work, an electrochemical sensor based on modification of a glassy carbon electrode (GCE) with magnetic iron oxide nanoparticles (Fe3O4) and chitosan (CS) was developed to quantify chlorhexidine digluconate (CHD), a worldwide used antiseptic. Cyclic voltammetric and electrochemical impedance spectroscopy assays showed that the permeable Fe3O4/CS film on the GCE surface improved the conduc-tivity and facilitated electron transfer. CHD response at GCE/Fe3O4/CS corresponds to an irreversible anodic diffusion-controlled process (at about 1.13 V in 0.04 mol L−1 Britton-Robinson buffer, pH 4.0) involving the transfer of two electrons and an equal number of proton. Quantum mechanics and Monte Carlo simulations were performed to give insights on the CHD oxidation process. The modification with Fe3O4/CS induced alterations in CHD geometry that led to degeneration of the highest occupied molecular orbitals (HOMO and HOMO−1), facilitating the oxidation process because both sides of the molecule contribute to these orbitals. It facilitated also, at least thermodynamically, the first electron lost. The oxidation of CHD probably conducts to the formation of two p-chloroaniline and, possibly, two biguanidine molecules. GCE/Fe3O4/CS exhibited suitable electroanalytical characteristics in terms of sensitivity (8.78 ± 0.58 A mol−1 L), linearity range (2.10 × 10−8–2.09 × 10−7 mol L−1), detection limit (5.7 × 10−9 mol L−1; 0.005 mg kg−1), intra-day repeatability (2.8% RSD), and reproducibility (4.6% RSD). To evaluate the accuracy and applicability of the proposed electroanalytical methodology, two CHD com-mercial formulations were analysed and the attained results were in agreement with those attained by the chromatographic reference method. Overall, the developed GCE/Fe3O4/CS exhibits appropriate performance and relevant advantages for CHD electroanalysis in commercial products.
- Evaluation of degradation mechanism of chlorhexidine by means of Density Functional Theory calculationsPublication . Salvador, Michele Aparecida; Sousa, Camila Pinheiro; Morais, Simone; Lima-Neto, Pedro de; Correia, Adriana Nunes; Homem-de-Mello, PaulaChlorhexidine (CHD), a germicidal drug, has degradation products that can be hemotoxic and carcinogenic. However, there is no consensus in literature about the degradation pathway. In order to shed light on that mechanism, we have employed Density Functional Theory to study reactants, in different protonation states, products and intermediates involved in the different pathways. Based on free energy values comparison and frontier molecular orbital analysis, we have obtained the most stable structures in each protonation state. CHD in saturated form has HOMO localized in one p-chloroaniline, and, due to molecule's symmetry, HOMO-1 has contributions from the other side of the molecule, but mainly from the biguanide portion of the molecule, instead of from the p-chloroaniline. For the saturated form, we have studied two possible degradation pathways, starting from the monoprotonated structure, and three pathways starting from the neutral structure. We found out that the mechanisms proposed in literature, whose pathways lead to p-chloroaniline (PCA) formation in a smaller number of steps, are more likely than the mechanisms with more intermediate steps or pathways that do not predict PCA formation. Also, based on free energy results, we have found that the formation of another sub-product (PBG-AU) is favorable as well.
- Experimental and computational studies of the interactions between carbon nanotubes and ionic liquids used for detection of acetaminophenPublication . Salvador, Michele A.; Sousa, Camila P.; Maciel, Cleiton D.; Gomes, Rayane N.; Morais, Simone; Lima-Neto, Pedro de; Coutinho-Neto, Maurício D.; Correia, Adriana N.; Homem-de-Mello, PaulaThe interactions between multi-walled carbon nanotubes and different amounts of an ionic liquid (IL), as well asthe interactions between this system (used as electrochemical sensor) and acetaminophen (ACOP), were in-vestigated through both experimental and theoretical methodologies. Experiments indicated that there is anoptimal concentration of ionic liquid for ACOP detection. A host of techniques and model systems were em-ployed to investigate the adsorption and oxidation processes. To investigate the source of the increased elec-trochemical current in the presence of an IL, we computed the adsorption energy values of ACOP in the nanotube–IL system via Monte Carlo simulations and Density Functional Theory (DFT). DFT allowed us to explore thechanges in adsorption energy due to oxidation. Our theoretical results support the experimentalfindings thatmoderate amounts of IL modulates ACOP/ACOP+adsorption, pointing to a cooperative effect that tends to wanewith increasing amounts of IL pairs. We observed that the IL favors desorption of the oxidized species andfacilitates charge transfer from the ACOP to the nanotube. Therefore, our studies point towards multifactorialeffects with clear physical basis that modulates binding leading to an optimal ratio to promote ACOP detection