Stimuli-responsive hydrogels are materials with great potential for development of active functionalities in fluidics and micro-fluidics. Based on the current state of research on pH sensors, hydrogel sensors are described qualitatively and quantitatively for the first time. The review introduces the physical background of the special properties of stimuli-responsive hydrogels. Following, transducers are described which are able to convert the non-electrical changes of the physical properties of stimuli-responsive hydrogels into an electrical signal. Finally, the specific sensor properties, design rules and general conditions for sensor applications are discussed.

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Summary: The infrared absorption (IR) spectrum of alkyl phosphonic acid adsorbed on the α‐Al2O3 (0001) surface has been calculated by means of a density‐functional based tight‐binding method. Thereby mono‐dentate, bi‐dentate and tri‐dentate bonding of the acid to the surface have been considered. In addition, experimentally obtained Fourier Transform Infrared Spectra (FTIR) of octadecylphosphonic acid (ODPA) on the natural surface of aluminium have been included. The absence of the PO band in the experimental surface spectrum and in the calculated spectrum of the tridentate adsorption complexes showed that adsorption of (alkyl)phosphonic acids on aluminium favours tridentate bonding, where the acid is bound to the surface via three symmetric POAl bonds.

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