Water is the element of life. Without water, life on this planet could have neither developed nor would it have survived. That water plays such a crucial role in this context is partly due to its unique properties as a solvent. Water is polar, has a high boiling point considering the low molecular weight of water molecules, and dissolves many salts and polar organic molecules, while apolar molecules tend to phase‐separate. These properties are intimately linked with the propensity of water molecules to form an infinite and dynamic network in the condensed phase in which the individual components are held together by relatively strong cooperative hydrogen bonds.

This network can rearrange to incorporate ions or polar neutral molecules. The associated hydration of these solutes suppresses interactions between them, whereas interactions between apolar molecules, whose hydration is thermodynamically unfavorable, are reinforced. Consequently, apolar solvents do not mix with water, amphiphiles aggregate to form micelles or vesicles, and polymers (e.g. proteins) fold such that apolar regions are buried inside the resulting structure, whereas polar ones are positioned along the solvent‐accessible outside. The underlying recognition, aggregation, and folding processes encompass intra‐ or intermolecular molecular interactions under thermodynamic control, positioning them at the heart of supramolecular chemistry. Because aspects of water structure cause these recognition phenomena to be governed by effects that are absent in other solvents, supramolecular chemistry in water is special and often challenging.

Nevertheless, supramolecular chemistry has not shied away from water. For example, studies on the host–guest chemistry of cyclodextrins, cyclophanes, polyazacryptands or related receptors, micelles or liposomes, or transport phenomena were predominantly carried out in aqueous environments. The choice of solvent in these investigations was mostly based on practical considerations, however, such as the properties and solubilities of the binding partners. Only in recent years a clear trend emerged to deliberately develop supramolecular systems that work in water also with a view on potential practical uses. Examples are biomedical applications in which supramolecular receptors are used to mediate and control biochemical processes, analytical applications aiming at detecting harmful environmental contaminants, or the control of chemical transformations by suitable supramolecular catalysts. At the same time, the understanding of the peculiarities of water and the associated effects on recognition events have progressed to such a level that the development of supramolecular systems working in water has become increasingly more successful. This book should illustrate these developments.

Chapter 1 of the book shows how and to what state our knowledge about the structure of water in the condensed phase and the effects of water on recognition phenomena have developed in recent years. This chapter thus lays the foundation for understanding the behaviors of the systems presented in the rest of the book. Chapter 2 then introduces structures and properties of many important host systems that can be used in water, most of which will reappear in later chapters. The following four chapters focus on several types of substrates that can be bound with appropriate receptors in water, namely, peptides and proteins (Chapter 3), nucleotides (Chapter 4), carbohydrates (Chapter 5), and ions (Chapter 6). The focus will then shift to supramolecular systems that can be generated by self‐assembly in water. Chapter 7 describes coordination compounds, Chapter 8 polymers and gels, Chapter 9 foldamers, Chapter 10 vesicles and micelles, and Chapter 11 surface‐modified gold nanoparticles. All of these chapters elude to concepts, structural aspects of the respective systems, analytical techniques to characterize them, and also potential applications wherever appropriate. Applications are also the central topics of the final three chapters of the book, with Chapter 12 summarizing supramolecular strategies to sense analytes in water by means of optical methods, Chapter 13 presenting optical probes for medicinal imaging, and Chapter 14 focusing on supramolecular catalysts.

The broad range of subjects treated, from fundamental aspects to applied ones, should provide extensive insight into a timely and exciting research field at the same time illustrating some directions into which supramolecular chemistry is currently heading. It is thus hoped that the book will be a useful compendium for scientists working in the area and that it might also motivate interested readers to join the community and contribute with original and new ideas.

This book would not have been possible without the help from a group of highly competent and reliable authors. I am extraordinarily grateful to all of them for their efforts and excellent contributions. I also warmly thank Elke Maase and Shirly Samuel from Wiley for their help and support.

Kaiserslautern, Germany

October 2018

Stefan Kubik