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.
Enhanced degradation of mesoporous silica particles in neutral and acidic aqueous solutions was achieved by embedding diimine moieties in the silica network.
Over
the last years, advancements in the use of nanoparticles for
biomedical applications have clearly showcased their potential for
the preparation of improved imaging and drug-delivery systems. However,
compared to the vast number of currently studied nanoparticles for
such applications, only a few successfully translate into clinical
practice. A common “barrier” that prevents nanoparticles
from efficiently delivering their payload to the target site after
administration is related to liver filtering, mainly due to nanoparticle
uptake by macrophages. This work reports the physicochemical and biological
investigation of disulfide-bridged organosilica nanoparticles with
cage-like morphology, OSCs, assessing in detail their bioaccumulation in vivo. The fate of intravenously injected 20 nm OSCs was
investigated in both healthy and tumor-bearing mice. Interestingly,
OSCs exclusively colocalize with hepatic sinusoidal endothelial cells
(LSECs) while avoiding Kupffer-cell uptake (less than 6%) under both
physiological and pathological conditions. Our findings suggest that
organosilica nanocages hold the potential to be used as nanotools
for LSECs modulation, potentially impacting key biological processes
such as tumor cell extravasation and hepatic immunity to invading
metastatic cells or a tolerogenic state in intrahepatic immune cells
in autoimmune diseases.
Supramolecular rearrangements are crucial in determining the response of stimuli sensitive soft matter systems such as those formed by mixtures of oppositely charged amphiphiles. Here mixtures of this kind were prepared by mixing the cationic block copolymer pAMPTMA-b-pNIPAAM and an anionic surfactant obtained by the modification of the bile salt sodium cholate. As pure components, the two compounds presented a thermoresponsive self-assembly at around 30-35 °C; a micelle formation in the case of the copolymer and a transition from fibers to tubes in the case of the bile salt derivative. When both were present in the same solution they associated into mixed aggregates that showed complex thermoresponsive features. At room temperature, the core of the aggregate was comprised of a supramolecular twisted ribbon of the bile salt derivative. The block copolymers were anchored on the surface of this ribbon through electrostatic interactions between their charged blocks and the oppositely charged heads of the bile salt molecules. The whole structure was stabilized by a corona of the uncharged blocks that protruded into the surrounding solvent. By increasing the temperature to 30-34 °C the mixed aggregates transformed into rods with smooth edges that associated into bundles and clusters, which in turn induced clouding of the solution. Circular dichroism allowed us to follow progressive rearrangements of the supramolecular organization within the complex, occurring in the range of temperature of 20-70 °C.
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