Halloysite nanotubes as nanoreactors for heterogeneous micellar catalysis

HYPOTHESIS

Electrostatic attractions between the anionic head group of sodium alkylsulphates and the positively charged inner surface of halloysite nanotubes (HNTs) drive to the formation of tubular inorganic micelles, which might be employed as nanoreactors for the confinement of non polar compounds in aqueous media. On this basis, sodium alkylsulphates/halloysite hybrids could be efficient nanocatalysts for organic reactions occurring in water.

EXPERIMENTS

Sodium decylsulphate (NaDeS) and sodium dodecylsulphate (NaDS) were selected for the functionalization of the halloysite cavity. The composition, the structure and the surface charge properties of the hybrid nanotubes were determined. The actual formation of inorganic micelles was explored by studying the microviscosity and polarity characteristics of the surfactant modified nanotubes through fluorescence spectroscopy experiments using DiPyme as probe. The performances of the sodium alkylsulphates/halloysite composites as micellar catalysts for the Belousov-Zhabotinsky (BZ) reaction were investigated.

FINDINGS

The halloysite functionalization with sodium alkylsulphates generated the formation of hydrophobic microdomains with an enhanced microviscosity. Compared to the surfactant conventional micelles, the functionalized nanotubes induced larger enhancements on the rate constant of the BZ reaction. This is the first report on the surfactant/halloysite hybrids showing their efficiencies as reusable nanocatalysts, which are dependent on their peculiar microviscosity and polarity properties.

Sepiolite-Hydrogels: Synthesis by Ultrasound Irradiation and Their Use for the Preparation of Functional Clay-Based Nanoarchitectured Materials

Sepiolite and palygorskite fibrous clay minerals are 1D silicates featuring unique textural and structural characteristics useful in diverse applications, and in particular as rheological additives. Here we report on the ability of grinded sepiolite to generate highly viscous and stable hydrogels by sonomechanical irradiation (ultrasounds). Adequate drying of such hydrogels leads to low-density xerogels that show extensive fiber disaggregation compared to the starting sepiolite—whose fibers are agglomerated as bundles. Upon re-dispersion in water under high-speed shear, these xerogels show comparable rheological properties to commercially available defibrillated sepiolite products, resulting in high viscosity hydrogels that minimize syneresis. These colloidal systems are thus very interesting as they can be used to stabilize many diverse compounds as well as nano-/micro-particles, leading to the production of a large variety of composites and nano/micro-architectured solids. In this context, we report here various examples showing how colloidal routes based on sepiolite hydrogels can be used to obtain new heterostructured functional materials, based on their assembly to solids of diverse topology and composition such as 2D and 1D kaolinite and halloysite aluminosilicates, as well as to the 2D synthetic Mg,Al-layered double hydroxides (LDH).

Pickering Emulsions Based on Wax and Halloysite Nanotubes: An Ecofriendly Protocol for the Treatment of Archeological Woods

A novel green protocol for the consolidation and protection of waterlogged archeological woods with wax microparticles has been designed. First, we focused on the development of halloysite nanotubes (HNTs) based Pickering emulsions using wax as the inner phase of the oil-in-water droplets. The optimization of the preparation strategy was supported by both optical microscopy and scanning electron microscopy, which allowed us to show the morphological features of the prepared hybrid systems and their structural properties, i.e., the distribution of the clay at the interface. Also, the dependence of the overall dimensions of the prepared systems on the halloysite content was demonstrated. Microdifferential scanning calorimetry (μ-DSC) was conducted in order to assess whether the thermal properties of the wax are affected after its interaction with HNTs. Then, the Pickering emulsions were employed for the treatment of waterlogged wooden samples. Compared to the archeological woods treated with pure wax, the addition of nanotubes induced a remarkable improvement in the mechanical performance in terms of stiffness and flexural strength. The proposed protocol is environmentally friendly since water is the only solvent used throughout the entire procedure, even if wax is vehiculated into the pores at room temperature. As a consequence, the design of wax/halloysite Pickering emulsions represents a promising strategy for the preservation of wooden artworks, and it has a great potential to be scaled up, thus becoming also exploitable for the treatments of shipwrecks of large size.

Halloysite Nanotubes Coated by Chitosan for the Controlled Release of Khellin

In this work, we have developed a novel strategy to prepare hybrid nanostructures with controlled release properties towards khellin by exploiting the electrostatic interactions between chitosan and halloysite nanotubes (HNT). Firstly, khellin was loaded into the HNT lumen by the vacuum-assisted procedure. The drug confinement within the halloysite cavity has been proved by water contact angle experiments on the HNT/khellin tablets. Therefore, the loaded nanotubes were coated with chitosan as a consequence of the attractions between the cationic biopolymer and the halloysite outer surface, which is negatively charged in a wide pH range. The effect of the ionic strength of the aqueous medium on the coating efficiency of the clay nanotubes was investigated. The surface charge properties of HNT/khellin and chitosan/HNT/khellin nanomaterials were determined by ζ potential experiments, while their morphology was explored through Scanning Electron Microscopy (SEM). Water contact angle experiments were conducted to explore the influence of the chitosan coating on the hydrophilic/hydrophobic character of halloysite external surface. Thermogravimetry (TG) experiments were conducted to study the thermal behavior of the composite nanomaterials. The amounts of loaded khellin and coated chitosan in the hybrid nanostructures were estimated by a quantitative analysis of the TG curves. The release kinetics of khellin were studied in aqueous solvents at different pH conditions (acidic, neutral and basic) and the obtained data were analyzed by the Korsmeyer-Peppas model. The release properties were interpreted on the basis of the TG and ζ potential results. In conclusion, this study demonstrates that halloysite nanotubes wrapped by chitosan layers can be effective as drug delivery systems.

Functional biohybrid materials based on halloysite, sepiolite and cellulose nanofibers for health applications

Biohybrid materials were prepared by co-assembling the three following components: nanotubular halloysite, microfibrous sepiolite, and cellulose nanofibers dispersed in water, in order to exploit the most salient features of each individual component and to render homogeneous, flexible, yet strong films. Indeed, the incorporation of halloysite improves the mechanical performance of the resulting hybrid nanopapers and the assembly of the three components modifies the surface features concerning wetting properties compared to pristine materials, so that the main characteristics of the resulting materials become tunable with regard to certain properties. Owing to their hierarchical porosity together with their diverse surface characteristics, these hybrids can be used in diverse biomedical/pharmaceutical applications. Herein, for instance, loading with two model drugs, salicylic acid and ibuprofen, allows controlled and sustained release as deduced from antimicrobial assays, opening a versatile path for developing other related organic-inorganic materials of potential interest in diverse application fields.

Multicomponent bionanocomposites based on clay nanoarchitectures for electrochemical devices

Based on the unique ability of defibrillated sepiolite (SEP) to form stable and homogeneous colloidal dispersions of diverse types of nanoparticles in aqueous media under ultrasonication, multicomponent conductive nanoarchitectured materials integrating halloysite nanotubes (HNTs), graphene nanoplatelets (GNPs) and chitosan (CHI) have been developed. The resulting nanohybrid suspensions could be easily formed into films or foams, where each individual component plays a critical role in the biocomposite: HNTs act as nanocontainers for bioactive species, GNPs provide electrical conductivity (enhanced by doping with MWCNTs) and, the CHI polymer matrix introduces mechanical and membrane properties that are of key significance for the development of electrochemical devices. The resulting characteristics allow for a possible application of these active elements as integrated multicomponent materials for advanced electrochemical devices such as biosensors and enzymatic biofuel cells. This strategy can be regarded as an "a la carte" menu, where the selection of the nanocomponents exhibiting different properties will determine a functional set of predetermined utility with SEP maintaining stable colloidal dispersions of different nanoparticles and polymers in water.

Why does vacuum drive to the loading of halloysite nanotubes? The key role of water confinement

The filling of halloysite nanotubes with active compounds solubilized in aqueous solvent was investigated theoretically and experimentally. Based on Knudsen thermogravimetric data, we demonstrated the water confinement within the cavity of halloysite. This process is crucial to properly describe the driving mechanism of halloysite loading. In addition, Knudsen thermogravimetric experiments were conducted on kaolinite nanoplates as well as on halloysite nanotubes modified with an anionic surfactant (sodium dodecanoate) in order to explore the influence of both the nanoparticle morphology and the hydrophobic/hydrophilic character of the lumen on the confinement phenomenon. The analysis of the desorption isotherms allowed us to determine the water adsorption properties of the investigated nanoclays. The pore sizes of the nanotubes' lumen was determined by combining the vapor pressure of the confined water with the nanoparticles wettability, which was studied through contact angle measurements. The thermodynamic description of the water confinement inside the lumen was correlated to the influence of the vacuum pumping in the experimental loading of halloysite. Metoprolol tartrate, salicylic acid and malonic acid were selected as anionic guest molecules for the experimental filling of the positively charged halloysite lumen. According to the filling mechanism induced by the water confinement, the vacuum operation and the reduced pressure enhanced the loading of halloysite nanotubes for all the investigated bioactive compounds. This work represents a further and crucial step for the development of halloysite based nanocarriers being that the filling mechanism of the nanotube's cavity from aqueous dispersions was described according to the water confinement process.

Layered composite based on halloysite and natural polymers: a carrier for the pH controlled release of drugs

We have prepared new biohybrid materials based on halloysite nanotubes and natural polymers (alginate and chitosan) for the controlled and sustained release of bioactive species.

Filling of Mater-Bi with Nanoclays to Enhance the Biofilm Rigidity

We investigated the efficacy of several nanoclays (halloysite, sepiolite and laponite) as nanofillers for Mater-Bi, which is a commercial bioplastic extensively used within food packaging applications. The preparation of Mater-Bi/nanoclay nanocomposite films was easily achieved by means of the solvent casting method from dichloroethane. The prepared bio-nanocomposites were characterized by dynamic mechanical analysis (DMA) in order to explore the effect of the addition of the nanoclays on the mechanical behavior of the Mater-Bi-based films. Tensile tests found that filling Mater-Bi with halloysite induced the most significant improvement of the mechanical performances under traction force, while DMA measurements under the oscillatory regime showed that the polymer glass transition was not affected by the addition of the nanoclay. The tensile properties of the Mater-Bi/halloysite nanotube (HNT) films were competitive compared to those of traditional petroleum plastics in terms of the elastic modulus and stress at the breaking point. Both the mechanical response to the temperature and the tensile properties make the bio-nanocomposites appropriate for food packaging and smart coating purposes. Here, we report a preliminary study of the development of sustainable hybrid materials that could be employed in numerous industrial and technological applications within materials science and pharmaceutics.

Selective adsorption of oppositely charged PNIPAAM on halloysite surfaces: a route to thermo-responsive nanocarriers

Halloysite nanotubes were functionalized with stimuli-responsive macromolecules to generate smart nanohybrids. Poly(N-isopropylacrylamide)-co-methacrylic acid (PNIPAAM-co-MA) was selectively adsorbed into halloysite lumen by exploiting electrostatic interactions. Amine-terminated PNIPAAM polymer was also investigated that selectively interacts with the outer surface of the nanotubes. The adsorption site has a profound effect on the thermodynamic behavior and therefore temperature responsive features of the hybrid material. The drug release kinetics was investigated by using diclofenac as a non-steroidal anti-inflammatory drug model. The release kinetics depends on the nanoarchitecture of the PNIPAAM/halloysite based material. In particular, diclofenac release was slowed down above the LCST for PNIPAAM-co-MA/halloysite. Opposite trends occurred for halloysite functionalized with PNIPAAM at the outer surface. This work represents a further step toward the opportunity to extend and control the delivery conditions of active species, which represent a key point in technological applications.