Bile acids (BAs) are facial amphiphiles synthesized in the body of all vertebrates. They undergo the enterohepatic circulation: they are produced in the liver, stored in the gallbladder, released in the intestine, taken into the bloodstream and lastly re-absorbed in the liver. During this pathway, BAs are modified in their molecular structure by the action of enzymes and bacteria. Such transformations allow them to acquire the chemical–physical properties needed for fulling several activities including metabolic regulation, antimicrobial functions and solubilization of lipids in digestion. The versatility of BAs in the physiological functions has inspired their use in many bio-applications, making them important tools for active molecule delivery, metabolic disease treatments and emulsification processes in food and drug industries. Moreover, moving over the borders of the biological field, BAs have been largely investigated as building blocks for the construction of supramolecular aggregates having peculiar structural, mechanical, chemical and optical properties. The review starts with a biological analysis of the BAs functions before progressively switching to a general overview of BAs in pharmacology and medicine applications. Lastly the focus moves to the BAs use in material science.

Electrostatic energy plays a pivotal role in the minimization of the free energy of self-assembling oppositely charged ions. This is a fundamental principle that guides the formation of aggregates and the separation of phases characterized by a stoichiometric balance between the opposite charges. However, this study unveils the self-assembly of catanionic nanotubes from a mixture of oppositely charged surfactants in aqueous buffer, comprising an anionic bile salt derivative and the cationic cetyltrimethylammonium bromide (CTAB) at an exact strongly unbalanced charge ratio of 9 anions to 1 cation. This anomalous aggregation leads to the formation of highly stable and ordered supramolecular nanotubes with a uniform cross-section of about 20 nm. The key to this behavior lies in the unique, rigid, and facial amphiphilic structure of the bile salt derivative, allocating hydrogen bond donors/acceptors and an aromatic residue, which promotes geometrically constrained derivative–derivative interactions. This facilitates the assembly of the anionic derivative into parallel, helically wrapped, negatively charged ribbons with interspersed CTAB cations, acting to promote their adhesion. Supramolecular nanotubes are highly valued in nanotechnology for applications such as catalysis and tissue engineering due to their rigidity, intrinsic directionality, and capability for dual compartmentalization (inner cavity and outer surface). The catanionic structures reported here are ordered architectures that allow for a high charge and a minimal interference from free monomers, which are essential for optimizing functions such as material loading. A loading ability was demonstrated here by the nanotube interaction with positively charged carbon dots and gold nanorods.

Infrared-triggered photon upconversion in porous materials presents intriguing prospects for combined functionalities such as molecular sponge, energy harvesting and conversion functionalities. Metal-organic frameworks (MOFs) are one of the most versatile classes of porous crystals. So far only two-photon upconverting processes have been realized in MOFs both by ligand based triplet-triplet annihilation and directly in lanthanide ions. Here we report on Yb3+/Er3+-trimesate-based MOFs that exhibit photon avalanche (PA) characteristics. The PA process conventionally occurs through cross-relaxation within the lanthanide emitter manifold. In contrast, here PA proceeds in the organic molecule part and relies on a cooperative process, involving multiple emission centers. The IR photons are first absorbed and upconverted into high energy electronic population by the action of the lanthanide ions (Yb3+ and Er3+ are the sensitizer and the activator, respectively). Subsequently, the electrons are funneled into electronically coupled triplet states of the trimesate ligand, enabling accumulation in the organic matrix. This reservoir acts as source for a highly nonlinear spectrally broadband emission, arising mainly from ligand triplet states. The nonlinearity factor is comparable with the well-established PA inorganic nanoparticles. We prove that the PA is strongly related to the degree of crystallinity of the MOF: not well-formed frameworks support only the characteristic Er3+ emission with only linear increase as a function of the excitation power. Our work paves a path towards vastly expanding the range of materials exhibiting PA, well beyond a limited set of lanthanide ions. Moreover, it provides a path for much broader control of the PA emission characteristics.

Straight facets and sharp edges are among the most distinctive indicators of well-defined crystals, and often reflect the polyhedral geometry and symmetry of the underlying close-packed, molecular structure. Curved morphologies are sometimes observed in biogenic crystals where templating or non-classical crystallization processes (e.g., crystallization via amorphous states) occur. Here we report the formation and growth of copper-based metal-organic frameworks (MOFs) that crystallize in the hexagonal space group, P622. The crystals begin as hexagon-like structures, developing over time into complex flower-like structures having two decks joined in the center. Each layer has 6 well-defined petals with curved lateral surfaces. The growth mechanism shows initial straight petals that become progressively more curved with increased faceting. Remarkably, despite its multi-domain appearance, the entire entity is a single-crystal. The curved morphology is correlated with the crystallographic structure and the arrangement of nanosized channels within this structure. Crystal habits are typically considered inconsistent with curved morphologies. This work suggests that crystallographic explanations can support the development of such surfaces for low-density structures.