Supramolecular gels formed from multi-component low molecular weight species.

Low molecular weight supramolecular gels consist of small molecules (gelators) that in an appropriate solvent self-assemble into nano- or micro-scale network structures resulting in the formation of a gel. Most supramolecular gels consist of two parts, namely the solvent and the gelator. However, the concept of multi-component supramolecular gels, in which more than one compound is added to the solvent, offers a facile way (e.g. by changing the ratio of the different components) to tailor the properties of the gel. The simplest multi-component gels consist of two components added to the solvent and are the most widely studied to date. There are three general classes of such multi-component gels that have been investigated. The first class requires all the added components to access the gel; that is, no component forms a gel on its own. A second class uses two (or more) gelators which can either co-assemble or self-sort into distinct assemblies and the final class consists of one (or more) gelator and one (or more) non-gelling additive which can impact the assembly process of the gelator and therefore the gel's properties.

Structure and gelation mechanism of tunable guanosine-based supramolecular hydrogels.

The mechanism of gelation of 50/50 w/w mixtures of guanosine (G) and 2',3',5'-tri-O-acetylguanosine (TAcG) in aqueous 0.354 M KCl was investigated using a combination of static light scattering (SLS), polarized and depolarized dynamic light scattering (VV and VH DLS), small-angle neutron and X-ray scattering (SANS and SAXS), and viscometric experiments. SLS and viscometry show a dramatic increase in apparent molecular weight and hydrodynamic volume at 0.2 wt % and 0.3 wt %, respectively, indicating the critical concentration for self-association of G/TAcG quartets into columnar assemblies lies below 0.2 wt %. Above this concentration, SANS and SAXS generate complementary information on the structure of the individual columnar stacks. VV and VH DLS results indicate bimodal correlation functions, whose properties suggest, respectively, translational and rotational diffusion of a bimodal distribution of particles. The fast mode appears to originate from fibrillar agglomerates of G/TAcG columnar quartet assemblies, while the slow mode comes from microgel domains. Guinier plot analysis of the SLS data probes the internal structure of the microgel domains. Collectively, the results suggest that sol and microgel phases coexist below the macroscopic gel point, and that the sol phase contains individual columnar stacks of G/TAcG quartets and fibrillar aggregates formed via lateral aggregation of these columnar assemblies. With increasing concentration, the DLS data indicate a progressive increase in the volume fraction of microgel domains, which ultimately leads to macroscopic gelation. Prior observation of a transient network contribution to the gel rheology at low temperature is attributed to the presence of individual columnar stacks within the gel network.

Tailoring the properties of guanosine-based supramolecular hydrogels.

We demonstrate that very stable hydrogels can be formed in aqueous potassium chloride solution by mixing a well-known gelator (guanosine, G) with a nongelator of similar structure (2',3',5'-tri-O-acetylguanosine, TAcG), and through a variety of characterization methods including rheology, small-angle neutron scattering, differential scanning calorimetry, and atomic force microscopy, we report substantial progress toward elucidating the factors that control the structure and stability of this fibrous gel system. The results suggest that the tailorability, long lifetime stability, and thermomechanical behavior of these gels derives from a reduction in the driving force toward crystallization with increased hydrophobic (TAcG) content, accompanied by a simultaneous decrease in fiber length and an increase in fiber width.

Effect of monomer structure and solvent on the growth of supramolecular nanoassemblies on a graphite surface.

The self-assembly of high aspect ratio hierarchical surface assemblies, as observed by fluid tapping mode AFM, can be achieved through careful design of the supramolecular interactions between low-molecular-weight adsorbates. Needlelike assemblies of monotopic guanine end-capped alkanes grow on a graphite surface when deposited from a water/DMSO solution. The growth of these assemblies can be monitored by AFM in real time, and the growth rate along the two different axes can be understood (through molecular modeling) in terms of the specific adsorbate-adsorbate interactions along those axes. Additionally, through judicious solvent selection (e.g., use of non-H-bonding solvents such as o-dichlorobenzene), which allows the formation of hydrogen-bonding aggregates in solution and influences the surface-adsorbate interactions, dramatically different surface assemblies of these guanine derivatives are obtained.