Eduardo Ruiz-Hitzky

Advances in biomimetic and nanostructured biohybrid materials.

The rapid increase of interest in the field of biohybrid and biomimetic materials that exhibit improved structural and functional properties is attracting more and more researchers from life science, materials science, and nanoscience. Concomitant results offer valuable opportunities for applications that involve disciplines dealing with engineering, biotechnology, medicine and pharmacy, agriculture, nanotechnology, and others. In the current contribution we collect recent illustrative examples of assemblies between materials of biological origin and inorganic solids of different characteristics (texture, structure, and particle size). We introduce here a general overview on strategies for the preparation and conformation of biohybrids, the synergistic effects that determine the final properties of these materials, and their diverse applications, which cover areas as different as tissue engineering, drug delivery systems, biosensing devices, biocatalysis, green nanocomposites, etc.

Molecular access to intracrystalline tunnelsof sepiolite

The availability of the structural micropores in sepiolite for the access of molecular or ionic entities is an old and controversial subject, which is revised in this work in the light of the most recent experimental data obtained in our research group. In this way evidence of the accessibility of molecules, voluminous ions and polymeric species to the intracrystalline tunnels of the mineral is presented on the bases of the application of several techniques to the study of the mineral adsorption properties.

Hybrid materials based on clays for environmental and biomedical applications

Nanostructured hybrids derived from clays are materials of increasing interest based on both structural characteristics and functional applications, including environmental and biomedical uses. This review introduces some recent examples of nanostructured clay derivatives (organoclays) useful as adsorbents or photocatalysts for environmental applications such as the removal of pollutants or development of environmentally oriented pesticide formulations. The second group of nanostructured materials considered here are related to the so-called bio-nanohybrids, formed by combination of an inorganic solid (clay mineral) with organic entities from biological origin at the nanometric scale. Bionanocomposites are an emerging group of nanomaterials resulting from the assembly of different clay minerals and biopolymers. Among the proposed applications, the development of novel hybrid materials for scaffolds and regenerative medicine, as well as new substrates to immobilize biological species from enzymes to viruses, is notable. Hybrid materials based on layered double hydroxides are receiving special attention in view of the possible applications as drug delivery systems.

Functional biopolymer nanocomposites based on layered solids

Bio-nanocomposites are an emerging group of hybrid materials derived from natural polymers and inorganic solids interacting at the nanometric scale. These nanostructured organic–inorganic materials could be designed and prepared using a wide type of biopolymers and also inorganic solids with different compositions and topologies. Among these last solids, special attention is devoted to layered materials that show the ability to intercalate biopolymers giving hybrids with functional properties. This novel research topic envisages the future development of biomimetic materials to provide novel bio-nanocomposites as multicomponent and multifunctional materials.

Bionanocomposites based on alginate–zein/layered double hydroxide materials as drug delivery systems

The present work introduces new hybrid materials based on the combination of layered double hydroxides (LDH) and two biopolymers (a protein and a polysaccharide) to produce LDH–biopolymer nanocomposites, able to act as effective drug delivery systems (DDS) in comparison to the LDH or the biopolymers alone. Ibuprofen (IBU) has been chosen as a model drug, being intercalated in a Mg–Al LDH matrix. The resulting hybrid is used to prepare bionanocomposite materials by association with two biopolymers: (i) zein, a highly hydrophobic protein, and (ii) alginate, a polysaccharide widely applied for encapsulating drugs. Characterization of the IBU/Mg–Al LDH intercalation compound and the bionanocomposites resulting from its incorporation into alginate–zein matrices of different composition was carried out by means of different experimental techniques: X-ray diffraction, infrared spectroscopy, chemical and thermal analysis, as well as optical and scanning electron microscopies. Preliminary kinetic studies of IBU liberation from bionanocomposites processed as beads show a better protection against drug release at the stomach pH and a controlled liberation in the intestinal tract conditions. This effect can be attributed to the hydrophobic nature of zein, which limits the passage of water and swelling of biocomposite beads prepared with such systems, delaying the release of the drug.

Adsorption of monovalent organic cations on sepiolite; experimental results and model calculations

Adsorption of neutral organic molecules and the monovalent organic cations methylene blue (MB) and crystal violet (CV) to sepiolite was determined experimentally and investigated by an adsorption model. The largest amounts of MB and CV adsorbed were about 4-fold of the cation exchange capacity (CEC) of sepiolite. Consequently, it was proposed that most of the above described adsorption was to neutral sites of the clay. The adsorption model considered combines the Gouy-Chapman solution and specific binding in a closed system. The model was extended by allowing cation adsorption to neutral sites of the clay, in addition to adsorption to negatively charged sites and adsorption to neutral complexes formed from 1 cation adsorbed to a negative surface site. The amount of available neutral sites was determined from the adsorption of the neutral molecule Triton-X 100 (TX100). The model could adequately simulate the adsorption of the neutral molecules TX100 and crown ether 15-crown-5 (15C5) as well as the organic cations. Due to aggregation of MB molecules in solution, their adsorption was somewhat less than that of CV at the larger added concentrations. A consideration of the molecular dimensions of TX100, MB and CV suggested that their adsorption was mostly to external sites of the clay and that their entry to the sepiolite channels was largely excluded. This interpretation is supported by infrared spectroscopy (IR) measurements, which show large perturbations of the peak corresponding to vibrations of external Si-OH groups of the clay and confirm complete occupancy of external sites by MB and CV.

Poly(ethylene oxide)/NH4+-smectite nanocomposites

Abstract New materials based on the intercalation of poly(ethylene oxide), PEO, into homoionic NH 4 + -smectites (montmorillonite and hectorite) have been synthesised and characterised. IR spectroscopy of the resultant nanocomposites shows changes in the polymer helical conformation of PEO as well as in the T d symmetry of the ammonium ion. Intercalation of PEO into NH 4 + -hectorite produces a material with regular stacking of the clay layers, but in PEO/NH 4 + -montmorillonite, the layers appear to be irregularly stacked with PEO chains arranged in double layers. However, by exchanging NH 4 + with Na + ions, the layer stacking becomes ordered. Ionic conductivity of these materials is lower than that of analogous materials containing alkali metal cations. Typical conductivity values measured in a direction parallel to the silicate layers at 200°C are in the order of 10 −7 S/cm.

STRUCTURAL FLUORINE IN SEPIOLITE

Sepiolite from Vallecas-Vicálvaro, Spain, contains 1.3% fluorine. Laser microprobe mass spectrometry of this sepiolite suggests the presence of fragments of (SiO2)nOMgF and (SiO2)nOMgOH, which are typical of the sepiolite structure. During thermal dehydroxylation, the fluorine in this sepiolite is removed simultaneously with OH groups at about 750°C. Nuclear magnetic resonance spectroscopy (NMR) of 19F indicates that the fluorine is located in the interior of the sepiolite structure, probably substituting for OH groups, and is homogeneously distributed. In the Vallecas-Vicálvaro sepiolite, about one of every four OH groups bound to Mg2+ is substituted by fluorine. The kinetics of extraction of Mg2+ and F- ions by acid treatment (1 N HCl) shows a more rapid extraction of Mg2+, with a monotonous decrease of the Mg/F ratio as the extent of extraction increases. These results support the internal location of the fluorine, as suggested by the NMR data.

Pectin-coated chitosan-LDH bionanocomposite beads as potential systems for colon-targeted drug delivery.

This work introduces results on a new drug delivery system (DDS) based on the use of chitosan/layered double hydroxide (LDH) biohybrid beads coated with pectin for controlled release in the treatment of colon diseases. Thus, the 5-aminosalicylic acid (5ASA), the most used non-steroid-anti-inflammatory drug (NSAID) in the treatment of ulcerative colitis and Crohns disease, was chosen as model drug aiming to a controlled and selective delivery in the colon. The pure 5ASA drug and the hybrid material prepared by intercalation in a layered double hydroxide of Mg2Al using the co-precipitation method, were incorporated in a chitosan matrix in order to profit from its mucoadhesiveness. These compounds processed as beads were further treated with the polysaccharide pectin to create a protective coating that ensures the stability of both chitosan and layered double hydroxide at the acid pH of the gastric fluid. The resulting composite beads presenting the pectin coating are stable to water swelling and procure a controlled release of the drug along their passage through the simulated gastrointestinal tract in in vitro experiments, due to their resistance to pH changes. Based on these results, the pectin@chitosan/LDH-5ASA bionanocomposite beads could be proposed as promising candidates for the colon-targeted delivery of 5ASA, with the aim of acting only in the focus of the disease and minimizing side effects.

Fe-containing pillared clays as catalysts for phenol hydroxylation

An alternative method for the preparation of mixed Fe–Al-pillared clays (Fe–Al-PILCs) derived from two natural smectites, Wyoming SWy-1 and Tunisia-Gafsa VI (an iron-rich sample) based on the use of a mixture of FeCl3 and chlorhydrol is described. The effect of the pH of the pillaring solution and the Al/Fe ratio are studied in order to assess their influence in the characteristics of the resulting solids: specific surface area, porosity and thermal stability. The behaviour of these solids as catalysts has been checked in the hydroxylation of phenol, under conventional heating or microwave irradiation, this last one allowing better yields and shorter reaction times. The presence of redox centers in the layers or in galleries of the materials, together with a Bronsted acid environment in the galleries of the PILCs, (mixed Fe–Al-PILCs and Al-PILCs derived from the Tunisian clay) induces the hydroxylation of phenol reaching conversions close to 70% under microwave irradiation, even at low reaction time (5 min). These values are comparable or even greater than conversions obtained from other catalysts used for these reactions (i.e. modified MCM).