Monodisperse Bile‐Salt Nanotubes in Water: Kinetics of Formation

Abstract

Preparation of organic structures on the nanometer scale by the spontaneous self-assembly of molecular species is a very active field of research in materials science. Among the various morphologies reported, tubular architectures are of particular interest since they may find applications in catalysis, selective separation, drug release, sensors, containers, and conducting devices in nano-, opto-, or ionoelectronics. A brief review of the topic is given in our previous report in this journal.[1] It has been previously reported that organic nanotubes with strikingly monodisperse diameters can be very easily obtained from the alkaline salt of the naturally occurring lithocholic bile acid (LCA).[1] Cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS) techniques were used to characterize the structures, which appear as very uniform tubes with 52 nm outer diameters and internal cavities 49 nm in diameter. Nevertheless, some TEM images showed additional morphologies such as twisted and helical ribbons, and thin, long, and semi-rigid fibers (ªfibrilsº). At the same time, theoretical descriptions of the formation of tubes through such metastable intermediates have also been proposed. [2,3] For a complete description of the sodium lithocholate (SLC) nanotubes, it is now important to investigate the kinetics of their formation with different and complementary techniques. We report here such a study, taking advantage of the relations between structural information gained from both reciprocal-and real-space. For that purpose, SAXS experiments and cryo-TEM imaging were performed.For both SAXS and cryo-TEM techniques, there is a minimum time (ªdead timeº, t d ) required to disperse the LCA powder into NaOH aqueous solution before starting a measurement. The t d for the cryo-TEM samples was ca. 80 s. Figure 1A shows the evolution of the SAXS scattering profile versus time for a 1.01 wt.-% SLC suspension. The first curve is recorded at the SAXS t d of 160 s, and others are accumulated up to ca. 48 000 s, i.e., more than 13 h. For comparison, a scattering curve obtained from an aged suspension (Dt~270 000 s, over 3 days) is also shown as a reference scattering signature obtained with a stabilized suspension (curve R).As shown in Figure 1A, the scattering curve of the reference sample shows eight well-defined oscillations confirming ).

Supramolecular Structures Generated by a p‐tert‐Butylphenyl‐amide Derivative of Cholic Acid: From Vesicles to Molecular Tubes

“…That tubular morphology is lost at higher temperature, once lipid flip-flop is kinetically feasible, resulting in the formation of large liposomes. ii) Bean et al, [7] have studied the kinetics of the formation of sodium lithocholate nanotubes. After examining different possibilities, they concluded that the mechanism involved in the tube formation is complex, because coaxial cylinders, helical ribbons, fibrils, and singlewalled tubes were found to coexist in the first stage of the kinetics.…”

“…ii) They self-organize in different structures both in the solid state and in aqueous solution. For example, in aqueous solution they self-aggregate to form micelles, [3,4] gels, [5] fibers, [6] nanotubes, [7] or laminar structures. [8] In such processes, the number, location, and configuration of the hydroxyl groups play a leading role.…”

Direct-Imaging and Freeze-Fracture Cryo-Transmission Electron Microscopy of Molecular Gels

“…Recently, FFR, direct-imaging cryo-TEM, and SAXS studies were performed to elucidate the early structures leading to the formation of the uniform tubes (Figure 3). Direct-imaging cryo-TEM images showed that less than 2 min after initiating the process by mixing LCA powder in aqueous NaOH solution, single-and multi-walled tubules of different diameters were formed [4]. Other intermediates, including helical ribbons and thin long fibers, were also observed within the first 30 min.…”

“…In some cases, direct images provide the only way to prove a suggested or a theoretically predicted model. That was demonstrated for example in the case of the theoretically predicted branched micelles [1], the shape of the "end-caps" of thread-like micelles [2,3], and the exact nanostructure and mechanism of formation of lithocholate nanotubes [4,5].…”

“…In fact, one early success in imaging a network structure of molecular organogel was achieved by applying the FFR technique in the study of a steroid/cyclohexane physical gel by Wade et al [20]. While the technique is excellent to complement direct imaging cryo-TEM [4], it has lost popularity in the last two decades and, regrettably, is used in only a few research laboratories.…”