Clay-based hybrid nanoplatforms preparation, characterization and evaluation of properties for controlled release of bioactive species

Resumen

This Doctoral Thesis is centered in the field of the nanostructured hybrid materials, and more specifically addresses the study of the so-called biohybrid materials resulting from the assembly of molecular and polymeric species of biological origin with inorganic substrates at the nanometer scale. The Thesis deals with the preparation and characterization of hybrid and biohybrid materials by intercalation of bioactive species in silicates of the type of clays and other related inorganic layered solids, and their integration into more complex systems such as bionanocomposites and nanoarchitectures, for application as controlled release systems of drugs and herbicides. In this way, the first study focused on the development of controlled release systems of metformin (MF), the most commonly used oral drug for the treatment of type II diabetes and currently also explored in treatments of certain types of cancer. In a first stage, various hybrid materials were prepared by association of MF to clay minerals of the smectite group (montmorillonite and hectorite). Specifically, in this work a montmorillonite from Wyoming, marketed as Cloisite®Na (Mt), and a synthetic hectorite, commercialized under the tradename Laponite® XLG (Lap) were used. Laponite® XLG is usually used in cosmetics and pharmacy and was selected in view to have a well-regulated substrate for possible use in the production of pharmaceutical formulations. The combination of molecular modelling and experimental characterization techniques, such as FTIR, XRD, CHN chemical analysis and EDX, allowed to study the process of adsorption of the drug and the final molecular arrangement of MF species in the interlayered space of clays, confirming that the intercalated MF follows in both cases an ion-exchange mechanism. Furthermore, bionanocomposite systems in which the prepared intercalation compounds were incorporated into a biopolymer matrix of pectin or chitosan were also explored, processing them in the form of microspheres (beads). These beads were then coated with one or two layers of those polymers in order to take advantage of the resistance properties of pectin to acidic pH environments, which may help to protect the hybrid at pH 1.2, and the mucoadhesive properties of chitosan, which can favor residence time and assimilation of the drug in the intestinal tract, thus overcoming problems linked to its rapid elimination as occurs in the most typical formulations used for MF administration. The produced beads were tested for water stability and in vitro release simulating the pH changes occurring throughout the gastrointestinal tract with the aim of establishing the performance of each system as a possible formulation for controlled release of metformin. The results obtained show that it is possible to establish a control on the release by varying the characteristics of the bionanocomposite system and the coatings of the microspheres, which may be useful in view of the application of this type of systems in pharmacology. In a second study, the design of a controlled allantoin release system was addressed with the target of a possible topical application in skin disease treatments. Allantoin is a very particular molecule of great versatility for use in cosmetics and pharmacy, but it is very difficult to stabilize with other components for controlled release applications. In this sense, the preparation of hybrid materials by intercalation in layered metal hydroxides, typically layered double hydroxides (LDH), with different metals (MgAl or ZnAl) by different methods of synthesis (ion-exchange, reconstruction and co-precipitation) was explored. It was proven that hybrid materials based on LDH-MgAl incorporate up to a maximum of allantoin of approximately half of the LDH anionic exchange capacity, with the ion-exchange method being the least effective, and verifying that in all cases there is no intercalation of the associated allantoin. In the case of LDH-ZnAl, it is possible to incorporate a greater amount of allantoin, practically similar to the anion exchange capacity of the LDH, when the preparation of the hybrid is carried out by means of a co-precipitation process of the hydroxide in the presence of allantoin. The characterization of the resulting hybrid material indicates that actually the LDH was not formed because the Zn: Al ratio is much smaller than the expected 2:1, with practically no incorporation of Al in the structure of the precipitated solid. However, using only a Zn precursor, it was possible to successfully co-precipitate a compound in which allantoin is associated with a layered single hydroxide (LSH) of Zn. The obtained allantoin-Zn LSH hybrid was compared to the characteristics of a Zn-allantoin complex prepared according to a protocol previously reported in the literature, and for which the presence of Zn acts as an enhancer of the therapeutic effects of allantoin. A study of the release of allantoin from several of the prepared systems confirms that the allantoin-Zn LSH system is the most efficient, even better than the Zn-allantoin complex described in the literature. The incorporation of the systems developed in biopolymeric matrices such as agar, hydroxypropylmethylcellulose (HPMC) or nanocellulose, facilitates subsequent processing as stable films that can be used in applications, for example, as tissues for wound dressings, and in which the presence of Zn in the incorporated hybrid can play a bactericide action. The third group of materials prepared in the Doctoral Thesis focused on the development of hybrid heterostructures of LDH-sepiolite for herbicide release system applications. Specifically, the synthesis of a LDH of Mg-Al in the presence of sepiolite fibrous clay and the herbicide known as MCPA (2-methyl-4-chlorophenoxyacetic acid) was addressed through co-precipitation in a single stage to prepare the hybrid nanoarchitecture. It has been proven that the co-precipitation of the LDH in the presence of the herbicide and sepiolite results in systems that incorporate a large amount of herbicide, much greater than the amount adsorbed when only the LDH is co-precipitated in the presence of MCPA or when the herbicide is intercalated in the LDH-sepiolite nanoarchitecture by ion-exchange. MCPA-LDH/sepiolite nanoarchitectures were characterized by various physicochemical techniques (XRD, FTIR and 29Si NMR spectroscopy, CHN and FESEM analysis) revealing that LDH interacts and remains attached to the sepiolite fibers through silanol groups present on the external surface of the clay. It is also confirmed that MCPA is intercalated into the LDH as instead of an interlayer distance of 0.77 nm, characteristic of the LDH with chloride ions as compensating interlayer anions, the actual interlayer distance in materials with MCPA is 2.32 nm. The herbicide release tests in water at pH 5.5, which simulates the characteristics of rainwater, show that the release from the sepiolite-LDH hybrid nanoarchitectures is much faster and more complete compared to the MCPA-LDH hybrid prepared by co-precipitation of the LDH in the presence of the herbicide, which confirms its suitability for agricultural applications. In order to have release systems for application in soils in which the hybrid nanoarchitecture may act as a reservoir for the herbicide and where a longer action can be established, a system of microspheres composed of the polysaccharide alginate and the hydrophobic zein protein was also developed. In in vitro tests and in columns prepared with soils, the release results were very satisfactory and allow to confirm a more controlled release process of the herbicide from the bionanocomposite systems at levels closer to the outermost surface of the soil, avoiding leaching processes of the herbicide at greater depths thereof. Other additional advantages of the developed systems refer to the use of encapsulation as a means to allow better management and transport of the herbicide and to the use of sepiolite present in the hybrid nanoarchitecture to incorporate other species of interest that could be released simultaneously to the environment.