The adhesion of some marine organisms to almost any kind of surface in wet conditions has aroused increasing interest in recent decades. Numerous fundamental studies have been performed to understand the scientific basis of this behaviour, with catechols having been found to play a key role. Several novel bio-inspired adhesives and coatings with value-added performances have been developed by taking advantage of the knowledge gained from these studies. To date there has been no detailed overview focusing exclusively on the complex mode of action of these materials. The aim of this Review is to present recent investigations that elucidate the origin of the strong and versatile adsorption capacities of the catechol moiety and the effects of extrinsic factors that play important roles in the overall adhesion process, such as pH value, solvent, and the presence of metal ions. The aim is to detail the chemistry behind the astonishing properties of natural and synthetic catechol-based adhesive materials.

Abstract.Catechols are found in nature taking part in a remarkably broad scope of biochemical processes and functions. Though not exclusively, such versatility may be traced back to several properties uniquely found together in the o -dihydroxyaryl chemical function; namely, its ability to establish reversible equilibria at moderate redox potentials and pHs and to irreversibly cross-link through complex oxidation mechanisms; its excellent chelating properties, greatly exemplifi ed by, but by no means exclusive, to the binding of Fe 3 + ; and the diverse modes of interaction of the vicinal hydroxyl groups with all kinds of surfaces of remarkably different chemical and physical nature. Thanks to this diversity, catechols can be found either as simple molecular systems, forming part of supramolacular structures, coordinated to different metal ions or as macromolecules mostly arising from polymerization mechanisms through covalent bonds. Such versatility has allowed catechols to participate in several natural processes and functions that range from the adhesive properties of marine organisms to the storage of some transition metal ions. As a result of such an astonishing range of functionalities, catechol-based systems have in recent years been subject to intense research, aimed at mimicking these natural systems in order to develop new functional materials and coatings. A comprehensive review of these studies is discussed in this paper.

A facile one-step polymerization strategy is explored to achieve novel catechol-based materials. Depending on the functionality of the catechol, the as-prepared product can be used to modify at will the surface tension of nano and bulk structures, from oleo-/hydrophobic to highly hydrophilic. A hydrophobic catechol prepared thus polymerized shows the ability to self-assemble as solid nanoparticles with sticky properties in polar solvent media. Such a versatile concept is ideal for the development of catechol-based multifunctional materials.

A series of catechol derivatives with a different number of linear alkyl chain substituents, and different length, have been shown to polymerize in the presence of aqueous ammonia and air, yielding hydrophobic coatings that present the ability to provide robust and efficient water repellency on weaved textiles, including hydrophilic cotton. The polymerization strategy presented exemplifies an alternative route to established melanin- and polydopamine-like functional coatings, affording designs in which all catechol (adhesive) moieties support specific functional side chains for maximization of the desired (hydrophobic) functionality. The coatings obtained proved effective in the transformation of polyester and cotton weaves, as well as filter paper, into reusable water-repellent, oil-absorbent materials capable of retaining roughly double their weight in model compounds (n-tetradecane and olive oil), as well as of separating water/oil mixtures by simple filtration.

The large tendency of catechol rings to adsorb on surfaces has been studied by STM experiments with molecular resolution combined with molecular-dynamics simulations. The strong adhesion is due to interactions with the surface and solvent effects. Moreover, the thermodynamic control over the differential adsorption of 1 and the nonanoic solvent molecules has been used to induce a new temperature-induced switchable interconversion. Two different phases that differ in their crystal packing and the presence of solvent molecules coexist upon an increase or decrease in the temperature. These results open new insight into the behavior of catechol molecules on surfaces and 2D molecular suprastructures.