Thesis title: Self-assembly superstructures induced by synthetic derivative of steroidal amphiphiles
In last decades, the request of tiny devices for nanotechnological applications has driven the researchers towards the design of diverse functional nano-objects. A bottom up approach is used to reach specific organized structures, which can be rationalized based on the comprehension of self-assembly mechanism leading to the formation of nano and micro objects starting from atoms, molecules or nanoparticles. Surfactants play key roles in nanotechnology as building blocks of functional aggregates or as nanoparticle stabilizers and templates. These roles are strongly related to surfactant molecular structure and composition. Steroid surfactants coming from selective modification of the delicate molecular amphiphilic balance of bile salts have opened the scenario to a plethora of systems providing different superstructures by their self-assembly and co-assembly with other materials. This was confirmed here by reporting the supramolecular association and co-association with lecithin of two new bile salt derivatives. In addition, it was demonstrated that bile salt derivatives can confer specific features to surfactant capped nanoparticles and mesoporous silica surfactant templates that can be exploited to direct the assembly of nanorods and to control the shape of silica particles. Chirality was observed to emerge from the assemblies, which was imparted by the chiral molecular structure of the derivatives and revealed by circular dichroism (CD). It is remarkable the CD induced on the longitudinal localized surface plasmon resonance of assembled nanorods for fundamental interest and for implications in the determination of chiral analytes.
In details, concerning the supramolecular association, a very stable calcium induced hydrogel was obtained from a C3 L-phenilalanine substituted cholate, whereas organogels were given by a C12 naphtyl amido functionalized cholate mixed with lecithin in cyclohexane and limonene as solvents.
A selective tip to tip association of CTAB stabilized gold nanorods was induced by a C3 terz-butyl phenilamido functionalized cholate as a consequence of its ability to specifically destabilize the CTAB cap at its higher curvature. In the nanorod assembly the optical matching between the bile salt derivative placed in between two nanorods and the plasmon resonance of metal nanoparticles resulted in a CD revealed chirality transfer from the bile salt derivative to the plasmonic absorption of the metal nanoparticles.
Finally the ability of natural bile salts to control the shape of mesoporous silica particles when added to CTAB as templating agent was confirmed for a larger group of bile salts including analysing glycine and taurine conjugated BSs, cheno- and urso- derivatives and synthetically modified BSs beside the unconjugated ones. Based on these results, in a further step, a double templating process was carried out by employing a thiophene polymerizable functionalized CTAB and cholate in the templating mixture.