Mario E. Flores

Immobilization of hydrophilic low molecular-weight molecules in nanoparticles of chitosan/poly(sodium 4-styrenesulfonate) assisted by aromatic-aromatic interactions.

The immobilization of the hydrophilic low molecular-weight cationic molecules rhodamine 6G, methylene blue, and citidine in nanoparticles composed of two opposite charged polyelectrolytes, poly(sodium 4-styrenesulfonate) and chitosan, is studied, and the results correlated with their physicochemical properties. Nanoparticles containing both polyelectrolytes have been synthesized showing hydrodynamic diameters of around 200 nm and tunable zeta potential. It was found that the strength of binding of the cationic molecules to the polyanion bearing charged aromatic groups poly(sodium 4-styrenesulfonate) by means of short-range aromatic-aromatic interactions increases with their hydrophobicity and polarizability, as seen by (1)H NMR and UV-vis spectroscopies, and diafiltration. Consequently, association efficiencies of 45, 21, and 12% have been found for the three molecules, respectively, revealing the different ability of the molecules to be immobilized in the nanoparticles. These results provide a proof of concept on a new strategy of immobilization of hydrophilic low molecular-weight molecules based on aromatic-aromatic interactions between polyelectrolytes and their aromatic counterions.

Effect of Polymer Micelles on Antifungal Activity of Geranylorcinol Compounds against Botrytis cinerea.

Herein, we explore the potential use of two micelle-forming block copolymers, i.e., Pluronic F-127 and poly(ethylene oxide)-b-poly(caprolactone), for application of fungicide agents. The polymer effect on the in vitro fungicide activity of a series of geranyl orcinol derivatives against Botrytis cinerea has been assessed. The results show that, for all test compounds, the incorporation into micelles, formed by Pluronic F-127, produces a great enhancement of the inhibitory effect on the growth of B. cinerea. For some compounds, at the lowest tested concentration (50 ppm), the percentage of inhibition increases significantly (from 0-10 to 80-90%) when the application is made using a polymer solution instead of an ethanol/water mixture. The synthesis and structural determination of a series of eight geranylphenols/diacetates, which were used as fungicide agents, are also discussed. These results suggest that polymer micelles are promising systems for application of crop-protecting agents.

Polyaromatic-Anion Behavior of Different Polyelectrolytes Containing Benzenecarboxylate Units

The polyaromatic-ion behavior of poly(sodium N-maleoyl-2-aminobenzoate-co-sodium acrylate) in a comonomer composition 1:2, poly(sodium N-maleoyl-4-aminobenzoate-co-sodium acrylate) in a comonomer composition 1:2, and poly(sodium N-maleoyl-4-aminobenzoate-co-N,N-dimethylacrylamide) in a comonomer composition 1:1 is studied. The copolymers undergo short-range aromatic-aromatic interactions with methylene blue and rhodamine B, as a consequence of which their UV-vis monomer band is shifted to lower energies. As a result of their polyaromatic-anion behavior, methylene blue is easily dispersed on the polymeric domains. Moreover, the pK(a) of rhodamine B is shifted from 3.2 to 4.5-5 in the presence of these copolymers. This behavior is also observed in the presence poly(sodium 4-styrenesulfonate), but not in the presence of poly(sodium vinyl sulfonate), which are taken as reference polymers. Compared with poly(sodium 4-styrenesulfonate), a lower resistance to the cleaving effect of added NaCl on the interaction with methylene blue is found. A different influence on the reduction of 2,3,5-triphenyl-2H-tetrazolium chloride with ascorbic acid was found for the different polyelectrolytes.

Different Models on Binding of Aromatic Counterions to Polyelectrolytes

The standard theory regarding the interactions between polyelectrolytes and their counterions is based on long-range electrostatic interactions. However, aromatic counterions may undergo short-range aromatic-aromatic interactions with polyelectrolytes containing aromatic rings so that ion pairs may be formed. The charge of the polymeric aromatic groups and the linear aromatic density of the polyelectrolytes play an important role on the behavior of the systems. Self-aggregation of counterions on the polymer environment can be controlled.

An easy synthetic way to exfoliate and stabilize MWCNTs in a thermoplastic pyrrole-containing matrix assisted by hydrogen bonds

This work focuses on the design of an engineered thermoplastic polymer containing pyrrole units in the main chain and hydroxyl pendant groups (A-PPy-OH), which help in achieving nanocomposites containing well-distributed, exfoliated and undamaged MWCNTs. The thermal annealing at 100 °C of the pristine nanocomposite promotes the redistribution of the nanotubes in terms of a percolative network, thus converting the insulating material in a conducting soft matrix (60 μΩ m). This network remains unaltered after cooling to r.t. and successive heating cycles up to 100 °C thanks to the effective stabilization of MWCNTs provided by the functional polymer matrix. Notably, the resistivity–temperature profile is very reproducible and with a negative temperature coefficient of −0.002 K−1, which suggests the potential application of the composite as a temperature sensor. Overall, the industrial scale by which A-PPy-OH can be produced offers a straightforward alternative for the scale-up production of suitable polymers to generate multifunctional nanocomposites.

Water-Induced Phase Transition in Cyclohexane/n-Hexanol/Triton X-100 Mixtures at a Molar Composition of 1/16/74 Studied by NMR

Molecular aggregation in a mixture of Triton X-100/n-hexanol/cyclohexane at a molar ratio of 1/16/74 is studied upon addition of small amounts of water. The composition of organic components has been chosen at a ratio n-hexanol/cyclohexane where a well-formed hydrogen bond network has been described. The ratio Triton X-100/n-hexanol has been chosen to afford a stoichiometry of ethylene oxide (EO) residues/n-hexanol of 1/2. At these conditions the addition of water consecutively produces the appearance of three defined phases: a clear solution, a lamellar phase, and a microemulsion. The two corresponding transitions occur at water/EO/n-hexanol molar ratios of 2/1/2 (clear to lamella), and 3/1/2 (lamella to microemulsion), while phase separation occurs at a molar ratio of 4/1/2, highlighting the important role of stoichiometry. Molecular dynamics measured by 1H NMR techniques, such as DOSY, and calculations of T1 and T2 relaxation times allow distinguishing the transition between the different phases and justifying their structure. Molecular assembly in the three phases is organized around hydrogen bond networks in which the hydroxyl groups of both TX-100 and n-hexanol, ethylene oxide groups of TX-100, and water participate. 1D 1H NMR spectral changes correlate with the different characteristics of the different phases. As the main characteristics of the lamellar phase we find a very restricted mobility of the molecules involved, and water chemical shifts in 1D 1H NMR spectra of around 5.0 ppm, higher than that of bulk water appearing at 4.7 ppm.

Facile Formation of Redox-Active Totally Organic Nanoparticles in Water by In Situ Reduction of Organic Precursors Stabilized through Aromatic–Aromatic Interactions by Aromatic Polyelectrolytes

The formation of redox-active, totally organic nanoparticles in water is achieved following a strategy similar to that used to form metal nanoparticles. It is based on two fundamental concepts: i) complexation through aromatic-aromatic interactions of a water-soluble precursor aromatic molecule with polyelectrolytes bearing complementary charged aromatic rings, and ii) reduction of the precursor molecule to achieve stabilized nanoparticles. Thus, formazan nanoparticles are synthesized by reduction of a tetrazolium salt with ascorbic acid using polyelectrolytes bearing benzene sulfonate residues of high linear aromatic density, but cannot be formed in the presence of nonaromatic polyelectrolytes. The red colored nanoparticles are efficiently encapsulated in calcium alginate beads, showing macroscopic homogeneity. Bleaching kinetics with chlorine show linear rates on the order of tenths of milli-meters per minute. A linear behavior of the dependence of the rate of bleaching on the chlorine concentration is found, showing the potential of the nanoparticles for chlorine sensing.

Stability of Water/Poly(ethylene oxide)43-b-poly(ε-caprolactone)14/Cyclohexanone Emulsions Involves Water Exchange between the Core and the Bulk

The formation of emulsions upon reverse self-association of the monodisperse amphiphilic block copolymer poly(ethylene oxide)43-b-poly(ε-caprolactone)14 in cyclohexanone is reported. Such emulsions are not formed in toluene, chloroform, or dichloromethane. We demonstrate by magnetic resonance spectroscopy the active role of the solvent on the stabilization of the emulsions. Cyclohexanone shows high affinity for both blocks, as predicted by the Hansen solubility parameters, so that the copolymer chains are fully dissolved as monomeric chains. In addition, the solvent is able to produce hydrogen bonding with water molecules. Water undergoes molecular exchange between water molecules associated with the polymer and water molecules associated with the solvent, dynamics of major importance for the stabilization of the emulsions. Association of polymeric chains forming reverse aggregates is induced by water over a concentration threshold of 5 wt %. Reverse copolymer aggregates show submicron average hydrodynamic diameters, as seen by dynamic light scattering, depending on the polymer and water concentration.

A mechanistic approach for the optimization of loperamide loaded nanocarriers characterization: Diafiltration and mathematical modeling advantages

&NA; Oral bioavailability of loperamide is restricted by its limited absorption in the gastrointestinal tract due to its poor aqueous solubility and its P‐glycoprotein (Pgp) substrate characteristic. In addition, ammonium methacrylate copolymers have shown to have mucoadhesive properties, whereas poloxamer 188, has been suggested as a Pgp inhibitor. Thus, in this work, we evaluate conditions that affect physicochemical parameters of ammonium methacrylate/poloxamer 188‐based nanocarriers loaded with loperamide hydrochloride. Nanocarriers were synthesized by nanoprecipitation, enhancing loperamide encapsulation efficiency by modifying the aqueous phase to basic pH. The isolation of the non‐encapsulated drug fraction from the nanocarriers‐incorporated fraction was conducted by centrifugation, ultrafiltration, vacuum filtration and diafiltration. The last method was effective in providing a deeper understanding of drug‐nanocarrier loading and interactions by means of modeling the data obtained by it. Through diafiltration, it was determined an encapsulation efficiency of about 93%, from which a 38% ±6 was shown to be reversibly (thermodynamic interaction) and a 62% ±6 irreversibly (kinetic interaction) bound. Finally, release profiles were assessed through empirical and semi‐empirical modeling, showing a biphasic release behavior (burst effect 11.34% and total release at 6 h = 33% ±1). Thus, encapsulation efficiency and release profile were shown to have a strong mathematical modeling‐based correlation, providing the mechanistic approach presented in this article a solid support for future translational investigations.

Aggregation Number in Water/n-Hexanol Molecular Clusters Formed in Cyclohexane at Different Water/n-Hexanol/Cyclohexane Compositions Calculated by Titration 1H NMR

Upon titration of n-hexanol/cyclohexane mixtures of different molar compositions with water, water/n-hexanol clusters are formed in cyclohexane. Here, we develop a new method to estimate the water and n-hexanol aggregation numbers in the clusters that combines integration analysis in one-dimensional 1H NMR spectra, diffusion coefficients calculated by diffusion-ordered NMR spectroscopy, and further application of the Stokes-Einstein equation to calculate the hydrodynamic volume of the clusters. Aggregation numbers of 5-15 molecules of n-hexanol per cluster in the absence of water were observed in the whole range of n-hexanol/cyclohexane molar fractions studied. After saturation with water, aggregation numbers of 6-13 n-hexanol and 0.5-5 water molecules per cluster were found. O-H and O-O atom distances related to hydrogen bonds between donor/acceptor molecules were theoretically calculated using density functional theory. The results show that at low n-hexanol molar fractions, where a robust hydrogen-bond network is held between n-hexanol molecules, addition of water makes the intermolecular O-O atom distance shorter, reinforcing molecular association in the clusters, whereas at high n-hexanol molar fractions, where dipole-dipole interactions dominate, addition of water makes the intermolecular O-O atom distance longer, weakening the cluster structure. This correlates with experimental NMR results, which show an increase in the size and aggregation number in the clusters upon addition of water at low n-hexanol molar fractions, and a decrease of these magnitudes at high n-hexanol molar fractions. In addition, water produces an increase in the proton exchange rate between donor/acceptor molecules at all n-hexanol molar fractions.