Anion recognition by synthetic receptors in water is not a new field, indeed the first receptors that were shown to interact with anionic species exhibited high affinity in aqueous solutions. Anion recognition in aqueous solution was, however, for a long time the domain of receptors containing multiple positive charges and/or metal ions while interactions of neutral receptors with anions were believed to be too weak to be efficient in water. Independent work in several groups has recently shown that this assumption is not necessarily correct. As a consequence, a much wider range of receptors is now available with which anion recognition in competitive aqueous media can be achieved. This tutorial review presents selected examples of synthetic anion receptors active in aqueous solutions and guidelines to achieve anion recognition in water.

Nature has devised highly efficient and selective ways of recognizing anionic substrates by using surprisingly few building blocks the most important of which are the amino acids serine, tryptophane, and arginine in addition to NH groups along the protein backbone. Deliberate use of amino acids for the construction of abiotic anion receptors could lead to systems that mimic the anion coordinating properties of anion binding proteins. With this aim in mind several groups have focused their attention on the development of anion receptors containing amino acid building blocks and came up with remarkably efficient systems. This critical review summarizes the different approaches (174 references).