Fibrillar Network Dynamics during Oscillatory Rheology of Supramolecular Gels

Supramolecular gels are three-dimensional network structures formed by the hierarchical self-assembly of small molecules through weak noncovalent interactions. On the basis of the various interactions contributed by specific functional groups/moieties, gels with different architectures can be constructed that are smart to the external stimuli such as pH, type of solvent, stress, temperature, etc. In the present work, we explore the oscillatory shear response of supramolecular self-assembled systems formed by the low-molecular-weight (LMW) gelator based on difunctionalized amino acid, florenylmethoxycarbonyl (Fmoc)-lysine(Fmoc), Fm-K(Fm) in aqueous buffer solution, at two different pH (6 and 7.4). Small amplitude oscillatory shear (SAOS) reported weak frequency dependence of moduli indicating a gel-like network structure. Large amplitude oscillatory shear (LAOS) indicated flow regimes dictated by yielding and subsequent network dynamics analogous to cagelike soft glassy events reported for colloidal systems. The three interval thixotropy test (3iTT) indicated recovery of moduli due to regelation contributed by the reversible interactions. A generalized network model framework is utilized to investigate the transient network characteristics of the Fm-K(Fm) gels. The "network creation" and "network loss" rates were chosen as exponential functions of scaled shear stress (= |τ12(t)G|) to effectively describe the complex response. On the basis of the insights, possible mechanisms to explain the differences/similarities in the response at different pH are speculated. It is further illustrated that the modeling strategy can be extended to supramolecular gels of different classes because of the commonality/universality of their response features under oscillatory shear.

Toxic Metal Sequestration Exploiting a Unprecedented Low-Molecular-Weight Hydrogel-to-Metallogel Transformation

We report herein the development of a unique low-molecular-weight gelator-induced technique for environmental remediation. The motive of this work is wastewater purification using a gel-based toxic heavy metal sequestration. The essence of this technique was to bring two different functionalities, one capable of multiple coordination and another with gel-forming ability, arranged in tandem within a single ligand molecule. Naturally, the success of the approach depends on whether the two tandem-arrayed functionalities are indeed working in tandem. Our results show that the ligand molecule is an excellent example of concomitant hydrogelator and metallogelator. The most interesting aspects of this study involve the toxic metal sequestration of Pb, Cd, and Hg which was further studied in detail with spectroscopic, microscopic, and diffraction techniques. We also report here a rare property of pure organic hydrogel-to-metallogel transformation which could open up a new avenue on wastewater purification. In essence, the hydrogels can be envisaged as a unique class of metal-free zeolite analogue for environmental remediation not by just absorbance but through absorbance cum coordination, which are further corroborated by the inductively coupled plasma-optical emission spectroscopy results.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.