Halloysite for smart nano-structured materials in sustainable applications
Biohybrid nanostructured materials were prepared by the co-assembling of inorganic particles, i.e. nanoclays, with organic moieties such as biopolymers. Among clays, Halloysite Nanotubes (HNTs) were deeply investigated due to their peculiar properties: morphology, different chemistry and charges on the internal and external surfaces, high aspect ratio, no toxicity, eco-friendliness and low cost. Hence, this thesis begins with a fundamental study of the thermodynamics of water confinement within the cavity of halloysite, which is at the basis for the loading mechanism of guest molecules inside its inner volume. By using Knudsen Thermogravimetry, it was found that the confined solvent exhibits a vapor pressure larger than the bulk solvent and, consequently, a faster evaporation rate. In particular, it was found that the variation of the pressure conditions during the loading procedure is responsible for the optimization of the encapsulation efficiency within halloysite nanotubes and it also allows to target the accumulation site of the guest molecules inside/outside the nanoclay, as observed by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). More importantly, these insights are crucial in order to tune and control the kinetics of release of active species from the clay. After this fundamental physico-chemical studies, we worked on the selective functionalization of HNTs with oppositely charged polymers and biopolymers by exploiting the electrostatic interactions arising between the organic/inorganic counterparts, as investigated by Differential Scanning Calorimetry (DSC), Dynamic Light Scattering (DLS) and contact angle measurements. The precise tailored adsorption site allowed to generate smart stimuli-responsive nanocarriers, as resulted from UV-Vis spectroscopy. Then, gel beads based on chitosan with uniformly dispersed halloysite nanotubes were obtained by a dropping method. Alginate was used to generate a coating layer over the hybrid gel beads, as observed by Laser Scanning Confocal Microscopy. These systems were used for applications in controlled drug release. Furthermore, we used nanoclays with the aim to improve the properties of Mater-Bi based bioplastics. The evaluation of the mechanical performance was carried out by Dynamic Mechanical Analysis (DMA), which allowed to recognize halloysite as the most efficient among the investigated clays. Hence, the effect of HNTs content was also focused and TGA results showed its effectiveness in improving the thermal properties of the bioplastics especially for low concentrations of nanotubes, which are homogeneously distributed within the biopolymeric matrix. The attained knowledge was exploited for the development of functional biohybrid nano-engineered materials for health applications. Indeed, we prepared a tablet-like material composed of a chitosan based nanocomposite film with drug loaded embedded halloysite and alginate, as external layers. The structure of the sandwich-like architecture was highlighted by morphological and wettability analysis. Moreover, we reported the preparation of a new type of multicomponent hybrid nanopaper constituted be the co-assembling in water of cellulose nanofibers, sepiolite and halloysite nanotubes. The physico-chemical characterization of the resulting materials was carried out by SEM, X Ray Diffraction (XRD) and DMA analysis. Both the two nano-engineered hybrids were evaluated as functional delivery systems by investigating the release profiles of model drugs through UV-Vis spectroscopy and, also, by in vitro antibacterial assays. Finally, we designed a new protocol for the preparation of Pickering emulsions based on halloysite and paraffin wax. Optical and Scanning Electron Microscopy allowed to study the morphological features of the prepared particles and to find a dependence between their dimensions and the clay content. DSC experiments highlighted that the presence of halloysite affects the thermal properties of the paraffin. Then, the Wax/HNTs Pickering emulsions were employed as consolidants for waterlogged archaeological woods, with an enhancement of the treatment efficiency compared to the use of pure wax, as showed by the analysis of the mechanical performances of the wooden samples. Overall, this thesis provides a significant contribution to some critical and societal challenges, which are addressed with the aim to enable a more environmentally respectful development model. In particular, the design of green and sustainable materials with different architectures and tailored properties at the nanometric scale is crucial to promote the green and sustainable transition.
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