Controlled Annealing in Adaptive Multicomponent Gels.

We use a pH-driven annealing process to convert between co-assembled and self-sorted networks in multicomponent gels. The initially formed gels at low pH are co-assembled, with the two components coexisting within the same self-assembled structures. We use an enzymatic approach to increase the pH, resulting in a gel-to-sol transition, followed by a hydrolysis to lower the pH once again. As the pH decreases, a self-sorted network is formed by a two-stage gelation process determined by the pKa of each component. This approach can be expanded to layered systems to generate many varied systems by changing composition and rates of pH change, adapting their microstructure and so allowing access to a far greater range of morphologies and complexity than can be achieved in single component systems.

Spatiotemporal Control Over Base-Catalyzed Hydrogelation Using a Bilayer System.

Controlling the formation and directional growth of hydrogels is a challenge. In this paper, a new methodology to program the gel formation both over space and time is proposed, using the diffusion and subsequent hydrolysis of 1,1'-carbonyldiimidazole from an immiscible organic solution to the aqueous gel media.

Chemical crosslinking in 'reactive' multicomponent gels.

We show that the hydrolysis of EDC can be used to construct a reactive system to trigger permanent covalent crosslinking between the components in multicomponent gels comprising gelators with a carboxylic acid and amine group yielding an amide functionalized gel with enhanced mechanical properties.

Using Rheology to Understand Transient and Dynamic Gels.

Supramolecular gels can be designed such that pre-determined changes in state occur. For example, systems that go from a solution (sol) state to a gel state and then back to a sol state can be prepared using chemical processes to control the onset and duration of each change of state. Based on this, more complex systems such as gel-to-sol-to-gel and gel-to-gel-to-gel systems can be designed. Here, we show that we can provide additional insights into such systems by using rheological measurements at varying values of frequency or strain during the evolution of the systems. Since the different states are affected to different degrees by the frequency and/or strain applied, this allows us to better understand and follow the changes in state in such systems.

Controlling Syneresis of Hydrogels Using Organic Salts.

Supramolecular hydrogels can spontaneously undergo syneresis through fibre-fibre interactions and expel significant amounts of water upon aging. In this process, the hydrophobicity of fibres which regulates the 3D-rearrangement of the self-assembled structures during syneresis is important. Here, we show that we can control the hydrophobic microenvironment of gels by incorporating organic salts into the co-assembled gel fibres thereby enabling control of the macroscopic gel volume phase transition.

Controlling hydrogel properties by tuning non-covalent interactions in a charge complementary multicomponent system.

Mixing small molecule gelators is a promising route to prepare useful and exciting materials that cannot be accessed from any of the individual components. Here, we describe pH-triggered hydrogelation by mixing of two non-gelling amphiphiles. The intermolecular interactions among the molecules can be tuned either by controlling the degree of ionization of the components or by a preparative pathway, which enables us to control material properties such as gel strength, gel stiffness, thermal stability, and an unusual shrinking/swelling behaviour.

Varying the hydrophobic spacer to influence multicomponent gelation.

Mixing low molecular weight gelators (LMWGs) shows promise as a means of preparing innovative materials with exciting properties. Here, we investigate the effect of increasing hydrophobic chain length on the properties of the resulting multicomponent systems which are capable of showing ambidextrous phase behaviour on pH perturbation.

Urea-Urease Reaction in Controlling Properties of Supramolecular Hydrogels: Pros and Cons.

Supramolecular hydrogels are useful in many areas such as cell culturing, catalysis, sensing, tissue engineering, drug delivery, environmental remediation and optoelectronics. The gels need specific properties for each application. The properties arise from a fibrous network that forms the matrix. A common method to prepare hydrogels is to use a pH change. Most methods result in a sudden pH jump and often lead to gels that are hard to reproduce and control. The urease-urea reaction can be used to control hydrogel properties by a uniform and controlled pH increase as well as to set up pH cycles. The reaction involves hydrolysis of urea by urease and production of ammonia which increases the pH. The rate of ammonia production can be controlled which can be used to prepare gels with differing properties. Herein, we show how the urease-urea reaction can be used for the construction of next generation functional materials.

Stimuli responsive dynamic transformations in supramolecular gels.

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Programming Gels Over a Wide pH Range Using Multicomponent Systems.

Multicomponent hydrogels offer a tremendous opportunity for preparing useful and exciting materials that cannot be accessed using a single component. Here, we describe an unusual multi-component low-molecular weight gelling system that exhibits pH-responsive behavior involving cooperative hydrogen bonding between the components, allowing it to maintain a gel phase across a wide pH range. Unlike traditional acid-triggered gels, our system undergoes a change in the underlying molecular packing and maintains the ß-sheet structure both at acidic and basic pH. We further establish that autonomous programming between these two gel states is possible by an enzymatic reaction which allows us to prepare gels with improved mechanical properties.

Programming properties of transient hydrogels by an enzymatic reaction.

Supramolecular gels are usually stable in time as they are formed under thermodynamic equilibrium or at least in a deep well of a kinetically trapped state. However, artificial construction of kinetically controlled transient supramolecular gels is an interesting challenge. In these systems, usually a kinetically trapped transient aggregate is formed by active building blocks that leads to gelation; the gel then typically returns to the solution state. In this work, we show that such transient aggregation can occur by successive formation of two distinctly different kinetically controlled metastable states. Control over the first metastable state allows us to achieve significant control over the stability and properties of the second metastable state.

Annealing Supramolecular Gels by a Reaction Relay.

Supramolecular gels have potential in many areas. In many cases, a major drawback is that the gels are formed at a high rate. As a result, nonoptimal, kinetically trapped self-assembled structures are often formed, leading to gels that can be hard to reproduce and control. One method to get around kinetic trapping is annealing. Thermal annealing is one possibility, but it is not always desirable to heat the gels. Here, we describe a method to anneal pH-triggered gels after they are formed. We employ a reaction relay in a peptide-based hydrogel system to anneal the structures by a controlled and uniform pH change. Our method allows us to prepare gels with more controlled properties. We show that this can be used to enable homogeneous "molding and casting" of the hydrogels. This method of annealing is more effective in improving gel robustness than a conventional heat-cool cycle.

Effect of Substitution at Amine Functionality of 2,6-Diaminopyridine-Coupled Rhodamine on Metal-Ion Interaction and Self-Assembly.

2,6-Diaminopyridine-coupled rhodamines 1 and 2 have been synthesized, and the effect of substitution on amine functionality toward metal-ion interactions and self-assembly is thoroughly investigated. Both the compounds effectively recognize different metal ions of biological significance fluorimetrically and colorimetrically with a high degree of selectivity and sensitivities. While compound 1 is sensitive to Fe3+ ions, compound 2 is responsive to both Fe3+ and Al3+ ions in aqueous CH3CN (4/1, v/v; 10 mM tris HCl buffer, pH 6.8). The sensing mechanism involves the metal-ion chelation-induced spirolactam ring opening of the rhodamine scaffold that results in both color and fluorescence changes, while the extent of interactions with the metal ions is truly governed by the chemical structure of the compounds. Both 1 and 2 are proficient in detecting Fe3+ and Al3+ ions in human lung cancer cells (A549). As new findings, unlike 1, compound 2 formed a faint pink gel in the toluene-hexane mixture solvent (1:1, v/v), and the gel state of 2 selectively recognizes Ag+ ions by exhibiting a phase change from gel to purple sol. Experimental findings establish the role of the formamide moiety in forming the self-assembly.

Pathway Dependence in Redox-Driven Metal-Organic Gels.

Pathway dependence is common in self-assembly. Herein, the importance of pathway dependence for redox-driven gels is shown by constructing a FeII /FeIII redox-based metal-organic gel system is shown. In situ oxidation of the FeII ions at different rates results in conversion of a FeII gel into a FeIII organic gel, which controls the material properties, such as gel stiffness, gel strength, and an unusual swelling behaviour, is described. The rate of formation of FeIII ions determines the extent of intermolecular interactions and so whether gelation or precipitation occurs.

Gel to gel transitions by dynamic self-assembly.

Dynamic systems are of great interest from the perspective of mimicking biology through to preparing useful and exciting materials. Transient supramolecular gels are potentially useful, but there are limited applications that require a gel that only exists for a short length of time. Here, we show how a dynamic system can be designed to prepare materials with properties that cannot be directly accessed using the same gelator.

Temporally-Programmed Transient Supramolecular Gels.

In living systems, self-assembly processes are driven by the consumption of chemical fuels. Synthetic adaptation of living systems can be achieved by coupling of competing pathways that drive the assembly and disassembly, respectively, under the influence of chemical fuels. Here, a pH-responsive transient gel system is created by simultaneous incorporation of two triggers, of which one is responsible for the initiation of the self-assembly by increasing the pH and the second trigger drives the disassembly by reducing the pH. This method allows us to prepare transient gels with a high degree of control over the self-assembly lifetime as well as the mechanical properties of the transient gels.

Maintaining homogeneity during a sol-gel transition by an autocatalytic enzyme reaction.

Kinetic control over supramolecular gelation by increasing the pH can be achieved using an enzymatic reaction. This method allows us to produce homogeneous hydrogels with superior and improved mechanical properties as compared to gels obtained from simple addition of base.

Cholesterol-Appended Benzimidazolium Salts: Synthesis, Aggregation, Sensing, Dye Adsorption, and Semiconducting Properties.

A series of cholesterol-appended benzimidazolium salts 1-9 have been designed and synthesized. They have been explored in gel chemistry. The gelation of the benzimidazolium salts is dependent on the nature of the counteranions. In addition, the gelation behavior of the gelators is linked with the presence of both π-stacking and cholesteryl motifs. Whereas bisbenzimidazolium salt 2 forms a gel in dimethylsulfoxide/H2O (1:1, v/v) itself, under similar conditions, monobenzimidazolium salts 4 and 6 exhibit gelation in the presence of F- ions and validate the visual sensing of F-. As an application, the gel phase of 2 efficiently removes toxic dyes from waste water. Furthermore, all gels show thermally activated semiconducting property within a wide voltage window.