Publications

Abstract Six dioxetanone molecules, ranging in complexity from simple dioxetanone to firefly dioxetanone, were studied by performing M06/6-311G(d,p) calculations. The quantum theory of atoms in molecules and the electron localization function was applied to analyze the peroxide and carbon-carbon bonds of the dioxetanone ring. Both approaches demonstrated that the peroxide bond is not covalent, but charge-shifted. This means that for this bond the covalent "electron sharing" is relatively unimportant, and it is the stabilizing resonance energy that causes the bonding. For the contrary, the carbon-carbon bond is covalent. These discoveries indicate that no biradical species should be formed in the dioxetanone decomposition, and that the most probable rate-determining step should be the carbon-carbon cleavage.

Abstract The main objective of this study was to simulate for the first time a complex sol-gel system aimed at preparing the (S)-naproxen-imprinted xerogel with an explicit representation of all the ionic species at pH 9. For this purpose, a series of molecular dynamics (MD) simulations of different mixtures, including species never studied before using the OPLS-AA force field, were prepared. A new parametrization for these species was developed and found to be acceptable. Three different systems were simulated, representing two types of pregelification models: the first one represented the initial mixture after complete hydrolysis and condensation to cyclic trimers (model A); the second one corresponded to the same mixture after the evaporation process (model B); and the last one was a simpler initial mixture without an explicit representation of all of the imprinting-mixture constituents (model C). The comparison of systems A and C mainly served the purpose of evaluating whether an explicit representation of all of the components (model A) was needed or if a less computationally demanding system in which the alkaline forms of the silicate species were ignored (model C) would be sufficient. The results confirmed our hypothesis that an explicit representation of all of the imprinting-mixture constituents is essential to study the molecular imprinting process because a poor representation of the ionic species present in the mixture may lead to erroneous conclusions or lost information. In general, the radial distribution function (RDF) analysis and interaction energies demonstrated a high affinity of the template molecule, 2-(6-methoxynaphthalen-2-yl)propanoate (NAP(-), the conjugate base of (S)-naproxen), for the gel backbone, especially targeting the units containing the dihydroimidazolium moiety used as a functional group. Model B, representing a nearly gelled sol where the density of silicates and solvent polarity were much higher relative to the other models, allowed for much faster simulations. That gave us the chance to observe the templating effect through a comparative analysis and observation of the trajectories from simulations with the template- versus non-template-containing mixtures. Overall, a strong coherence between the imprinting-relevant interactions, aggregation, or the silicate network texturing effects taken out of the simulations and the experimentally high imprinting performance and porosity features of the corresponding gels was achieved.

Abstract 4-Methylbenzylidene camphor (4MBC) is an organic UV filter commonly used in sunscreens and many personal care products due to its ability to protect human skin against UVB solar radiation. 4MBC can exist as a (E)- or (Z)-isomer due to an exocyclic carbon-carbon double bond. In sunscreen formulations the (E) isomer predominates but under light exposure isomerization occurs from (E) to (Z). In this study we have performed density functional theory calculations with the B3LYP hybrid functional and two basis sets: 6-31G(d) and 6-311G(d,p) to obtain the gas-phase molecular structure and energetics of the (E)- and (Z)-isomers of 4MBC. For comparison and validation purposes this study has been extended to the parent molecule: camphor. To obtain more accurate energy values we have also used the G3(MP2)//B3LYP method. The standard molar enthalpy of formation of 4MBC in the gas-phase, at T = 298.15 K, was derived from these calculations using appropriately chosen reactions.

Abstract In this study we have performed a density functional theory (DFT) study of the molecular structure and energetics of the enol and keto tautomers of UVA filter 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM) and for comparison and validation purposes a similar study has been carried out for the parent molecule: dibenzoylmethane (DBM). The molecular structure of the enol and keto tautomers were obtained at the B3LYP/6-311++G(d,p) level. The enol forms were found to have a strong intramolecular resonance assisted hydrogen bond that accounts for the greater energetic stability of the enol relative to the keto form. The aromatic character of the six-membered enol rings has been analyzed by calculation of Nucleus Independent Chemical Shifts (NICS). The standard molar enthalpy of formation in the gas phase of BMDBM was derived from appropriately chosen reactions.

Abstract This is the first theoretical study of the relationship between superoxide anion and firefly chemiluminescence, in DMSO. Electron transfer reactions between luciferin dianionic/carbanionic/radical species and superoxide were studied in order see if an alternative explanation existed for the consumption of the latter species, without correlating it with a role on luciferin chemiluminescence. Despite the finding that luciferin may indeed inhibit the formation of the superoxide anion, no theoretical evidence was found that showed that this molecule is consumed in a non-chemiluminescence reaction. Therefore, it is concluded that the superoxide anion is indeed related to the firefly luciferin chemiluminescence.

Abstract This a theoretical Letter based on density functional theory, on the role of superoxide anion in firefly chemiluminescence in DMSO. We have found that this anion can attack luciferin radical molecules, thus forming a luciferin-like trianion. This latter molecule transfers an oxygen atom, which results in the formation of oxyluciferyl radical dianion and carbon dioxide molecules. Oxyluciferin is finally formed after an electron transfer from oxyluciferyl radical dianion to tert-BuO center dot radical molecules. Thus, we have found evidence that firefly oxyluciferin can be formed in a energetically favorable superoxide anion-assisted reaction, without the need for the formation of firefly dioxetanone.

Abstract The preparation of polymers imprinted with common aromatic solvents such as benzene and toluene is an under-exploited subject of research. The present study was aimed at the understanding of whether true solvent memory effects can be achieved by molecular imprinting, as well as if they are stable at elevated temperature. A set of copolymers, comprising low and high cross-linking levels, was prepared from four different combinations of functional monomer and cross-linker, namely methacrylic acid (MAA)/ethylene glycol dimethacrylate (EGDMA), methyl methacrylate (MMA)/EGDMA, MAA/divinyl benzene (DVB) and MMA/DVB. Each possible combination was prepared separately in benzene, toluene and acetonitrile. The obtained materials were applied as coatings onto nickel-titanium (Ni-Ti) alloy wires which were incorporated into solid-phase microextraction devices and finally tested for their ability to competitively adsorb vapors from the headspace of an aqueous solution containing a few volatile organic compounds. Porosity analysis showed that, regardless of the solvent used, only a high cross-linking level permitted the preparation of mesoporous copolymers (BJH radius typically in the range 13-15 nm), a requirement for providing accessibility to the targeted nanoscale-imprinted cavities. A noticeable exception was, however, observed for the MMA/DVB copolymers which exhibited much diminished BJH radius. The porosity data correlated well with the extraction profiles found, which suggested the presence of benzene-imprinted sites in all the highly cross-linked copolymers prepared in benzene, except for the MMA/DVB copolymers. Concerning the effect of an elevated conditioning temperature on the memoryeffects created by the imprinting process, the results were clearly indicative that the tested copolymers, including the more robust highly cross-linked ones, are not suitable for high temperature applications such as solid-phase microextraction coupled to gas chromatography.

Abstract The behavior of a fluorescent nanocomposite, obtained by means of a thiol polypropylenimine dendrimer of third generation coated with CdSe quantum dots, and embedded in a hydrophilic cellulosic membrane as support is electrochemically studied in order to evaluate its applicability as a sensor in liquid media. The characterization of the nanocomposite by TEM and EDAX shows uniform nano morphology (size comprised in the range 60-90 nm) and composition, respectively. The analysis of the engineered hybrid cellulose-dendrimer quantum dots material by confocal fluorescence microscopy indicates almost mono-dispersion distribution of the nanocomposite when irradiated under UV light, while its presence on the film surface was determined by X-ray photoelectron spectroscopy. Impedance spectroscopy measurements performed with dry membrane samples show a decrease in the conductivity and dielectric constant of the modified membrane in comparison with the raw support. Electrical changes in the modified film associated to the presence of Cd(II) uptakes from a Cl2Cd solution were also obtained. These results support the possible application of this nanocomposite material as heavy metal sensor in liquid media.

Abstract In this work we present a fundamental study of the electrodeposition of tin from Deep Eutectic Solvents (DES) formed by a mixture of choline chloride and different hydrogen bond donors (HBD). Results shows that choline chloride based solvents can be successfully used for the electrodeposition of tin. Furthermore we demonstrate that the choice of hydrogen bond donor does not affect, significantly, the chemistry of tin in solution and we characterize the first stages of tin deposits at glassy-carbon (GC) electrode. The electrochemical characterization of tin deposits is carried out using cyclic voltammetry and chronoamperometry. The comparison of the theoretically and experimentally obtained current transients via dimensionless plots based on Bewick-Fleischman-Thirsk (BFT) theory, Scharifker and Hills (SH) and Scharifker and Mostany (SM) models and a non-linear fitting method showed that tin nucleation on GC surface occurs though a 3D instantaneous process with growth controlled by diffusion.

Abstract A water soluble complex of neodymium(III) with CdTe quantum dots nanoparticles was synthesized. The obtained homogeneous solutions were characterized by fluorescence, X-ray photoelectron and energy dispersive X-ray spectroscopies. The effect of the refluxing time of the reaction on the fluorescence intensity and emission wavelength has been studied. It was found that the emission wavelength of the solutions of neodymium(III) complex capped CdTe QDs nanoparticles shifted from about 540 to 735 nm. For an emission wavelength of 668 nm, the most reproducible nanoparticles obtained, the pH effect over the fluorescence emission and its intensity were studied. The purified and lyophilized solid obtained was morphologically characterized by transmission electron microscopy (TEM). The quantitative composition was determined by fluorescence X-ray spectroscopy (EDAX) and the X-ray photoelectron analysis (XPS) confirmed the presence of neodymium(III) at the surface of the CdTe nanoparticles forming a complex with the carboxylate groups from 3-mercaptopropanoic acid of the CdTe QDs. Due to the optical behavior of this complex, it could be of potential interest as a light source in optical devices.