Responsive interfaces are interfaces that show a defined and reversible change in physical properties in response to external stimuli. Typically, responsive interfaces result from the immobilization of responsive molecular components at the interface that translate a nanoscale signal into a macroscopic effect. Responsive interfaces can also be obtained if the topology of the interface can be reversibly changed using an external stimulus. As the surface of any material is its connection to the environment, responsive interfaces provide opportunities for interactive materials which are not only able to change properties upon demand, but also sense their environment and act autonomously. The application of responsive molecular components at interfaces, however, requires chemical and physical compatibility with the material surface of interest, posing a challenge not least in the retention of the responsive functionality. The state of the art in "active" interfaces which display responsive wettability, permeability, or adhesion is discussed, with a particular emphasis on microscale and nanoscale patterning since patterned interfaces can give rise to unique material properties. Finally, perspectives in the development of responsive interfaces, as well as promising approaches for bypassing the most prominent challenges are discussed.

BiographiesSebastian Lamping received his MSc degree in 2015 and is currently a PhD candidate in chemistry at the WWU Munster. His research focuses on the functionalization of glass and silicon surfaces with various polymer brushes for the development of responsive adhesive surfaces.Christoph Buten received his MSc degree from WWU Munster in 2016. He is currently working on his PhD thesis in the field of surface functionalization with biomolecules and the development of functional surfaces using polymer brush functionalized glass, silicon, and silica substrates.Bart Jan Ravoo received his MSc and PhD degree from the University of Groningen. He was a postdoctoral fellow at University College Dublin and assistant professor at the University of Twente. Currently, he is full professor at the WWU Munster. The "leitmotiv" of his research is self-assembly, and together with his group, he develops responsive soft materials and functional surfaces by bottomup self-assembly of molecular and nanoscale building blocks. ■ ACKNOWLEDGMENTSThis work was funded by the Deutsche Forschungsgemeinschaft (Ra 1732/9).

We developed a simple method to pattern self-assembled monolayers of tetrazole triethoxylsilane with a variety of different molecules by photochemical microcontact printing. Under irradiation, tetrazoles form highly reactive nitrile imines, which react with alkenes, alkynes, and thiols. The covalent linkage to the surface could be unambiguously demonstrated by fluorescence microscopy, because the reaction product is fluorescent in contrast to tetrazole. The modified surfaces were further analyzed by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), and contact angle goniometry. Protein-repellent micropatterns, a biotin-streptavidin array, and structured polymer brushes could be fabricated with this straightforward method for surface functionalization.

In this paper, we show that carboxylic acid-functionalized molecules can be patterned by photochemical microcontact printing on tetrazole-terminated self-assembled monolayers. Upon irradiation, tetrazoles eliminate nitrogen to form highly reactive nitrile imines, which can be ligated with several different nucleophiles, carboxylic acids being the most reactive. As a proof of concept, we immobilized trifluoroacetic acid to monitor the reaction with X-ray photoelectron spectroscopy. Moreover, we also immobilized peptides and fabricated carbohydrate-lectin as well as biotin-streptavidin microarrays using this method. Surface-patterning was demonstrated by fluorescence microscopy and time-of-flight secondary ion mass spectrometry.