No abstract

Bile salts (BSs) are naturally occurring rigid surfactants with a steroidal skeleton and specific self-assembly and interface behaviors. Using bile salts as precursors, derivatives can be synthesized to obtain molecules with specific functionalities and amphiphilic structure. Modifications on single molecules are normally performed by substituting the least-hindered hydroxyl group on carbon C-3 of the steroidal A ring or at the end of the lateral chain. This leads to monosteroidal rigid building blocks that are often able to self-organize into 1D structures such as tubules, twisted ribbons, and fibrils with helical supramolecular packing. Tubular aggregates are of particular interest, and they are characterized by cross-section inner diameters spanning a wide range of values (3−500 nm). They can form through appealing pH-or temperature-responsive aggregation and in mixtures of bile salt derivatives to provide mixed tubules with tunable charge and size. Other derivatives can be prepared by covalently linking two or more bile salt molecules to provide complex systems such as oligomers, dendrimers, and polymeric materials. The unconventional amphiphilic molecular structure imparts specific features to BSs and derivatives that can be exploited in the formulation of capsules, drug carriers, dispersants, and templates for the synthesis of nanomaterials.

Biocompatible molecules that undergo self-assembly are of high importance in biological and medical applications of nanoscience. Peptides and bile acids are among the most investigated due to their ability to self-organize into many different, often stimuli-sensitive, supramolecular structures. With the aim of preparing molecules mixing the aggregation properties of bile acid and amino acid-based molecules, we report on the synthesis and self-association behavior of two diastereomers obtained by substituting a hydroxyl group of cholic acid with a l-phenylalanine residue. The obtained molecules are amphoteric, and we demonstrate that they show a pH-dependent self-assembly. Both molecules aggregate in globular micelles at high pH, whereas they form tubular superstructures under acid conditions. Unusual narrow nanotubes with outer and inner cross-section diameters of about 6 and 3 nm are formed by the derivatives. The diasteroisomer with α orientation of the substituent forms in addition a wider tubule (17 nm cross-section diameter). The ability to pack in supramolecular tubules is explained in terms of a wedge-shaped bola-form structure of the derivatives. Parallel or antiparallel face-to-face dimers are hypothesized as fundamental building blocks for the formation of the narrow and wide nanotubes, respectively.

An amino acid-substituted bile acid forms tubular aggregates with inner and outer diameters of about 3 and 6 nm. The diameters are unusually small for surfactant self-assembled tubes. The results enhance the spectrum of applications of supramolecular tubules and open up possibilities for investigating a novel class of biological amphiphiles.

Morphology control and tuning of nanomaterials are crucial to determine their properties and applications. Solutions based on different synthetic methodologies have been proposed, and in general they required variation of several parameters. Here, a new facile and cost-effective bottom-up strategy to control the morphology of mesoporous silica particles is presented. Specifically, catanionic templating systems composed of bile acids and CTAB enable the production of submicrometer MCM-41 particles of various shapes, high porosity, and remarkable features. The variation of a single component, the bile acid, leads to the preparation of particles with different morphologies. For instance, small (<500 nm) well-separated hexagonal platelets and twisted rods, with tunable aspect ratio and chiral pore channels, were prepared. Experiments aimed at elucidating the role of the bile acids showed that the control of shape is due to the specific interactions between bile acids and CTAB.