No abstract

In living organisms, enzyme catalysis takes place in aqueous media with extraordinary spatiotemporal control and precision. The mechanistic knowledge of enzyme catalysis and related approaches of creating a suitable microenvironment for efficient chemical transformations have been an important source of inspiration for the design of biomimetic artificial catalysts. However, in “nature-like” environments, it has proven difficult for artificial catalysts to promote effective chemical transformations. Besides, control over reaction rate and selectivity are important for smart application purposes. These can be achieved via incorporation of stimuli-responsive features into the structure of smart catalytic systems. Here, we summarize such catalytic systems whose activity can be switched ‘on’ or ‘off’ by the application of stimuli in aqueous environments. We describe the switchable catalytic systems capable of performing organic transformations with classification in accordance to the stimulating agent. Switchable catalytic activity in aqueous environments provides new possibilities for the development of smart materials for biomedicine and chemical biology. Moreover, engineering of aqueous catalytic systems can be expected to grow in the coming years with a further broadening of its application to diverse fields.

Time and again, nature utilizes the available building blocks for the preparation of molecules and ordered architectures for vital processes such as cell motility and intercellular transport. As biological catalyst, enzymes allow these chemical transformations at their active sites with high efficiency and selectivity. Importantly, these chemical transformations take place in aqueous media. To mimic the activity of natural enzymes, control over the reactivity of reactants (or intermediates) is necessary for artificial catalytic systems in aqueous media. Altering the selectivity of aqueous chemical reactions could be feasible with an organized and confined media in which reactants or intermediates would possess specific orientations, similar to active sites of an enzyme. Supramolecular hydrogels could be such organized media, where the fibrillar hydrogel network can provide active surface area with efficient diffusion properties. Herein, we summarize the use of supramolecular hydrogels in catalysis and as nanoreactors for efficient chemical transformations in aqueous media. The advantages of carrying out reactions in supramolecular hydrogels and key factors for hydrogel-phase catalysis are discussed. Overall, characteristics of supramolecular hydrogels and molecular engineering over the catalytic microenvironment within a hydrogel are key in the development of efficient chemical transformations with selectivity. However, this field is its infancy, as there is a limited set of chemical transformations resembling enzymatic activity. Based on recent developments in this field, we expect a promising future for cleverly designed chemical transformations in supramolecular hydrogels and encourage applications in biological systems and the pharmaceutical industry.