Dually cross-linked supramolecular hydrogels for sensor applications

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The submitted project deals with the investigation of sensors based on (hydro-)gels having supramolecular host/guest structures incorporated. As interesting and relevant analytes special biomarker (spermine, spermidine), doping substances (N-methyl hexane amine) and fungicides (difenoconazole) were identified. (Hydro-)gels are established as active materials since some years. With the aid of a transducer the volume change as reaction to a stimulus can be utilized to generate a sensor signal. The incorporation of suitable host/guest (HG) structures offer to achieve a high selectivity regarding the examined analyte. In the presented project idea the cross-linking yields from the formation of physical interactions by inclusion of the guest molecule into the host molecule. Hereby, the host molecule forms a topological cavity. Molecules that penetrate this cavity and interact with the host are summarized as guest molecules. The kind of interaction being formed between the two species is purely physical and not covalent and hence, reversible. In “intelligent” hydrogels with HG cross-linking the band width of possible bindable guest molecules will be used to initiate a substance specific reaction of the gel. To achieve this a foreign molecule that is also able to interact with the host will be delivered to the HG system (HG1). The very particular interaction with the cavity of the host molecule lead to a competing formation of another HG complex (HG2) resulting in a repulsion of the original guest from the host cavity. Due to this exchange the cross-link breaks and yields in a swelling of the hydrogel in an (aqueous) environment. In addition to the primary swelling due to inclusion of solvent in the gel dually cross-linked hydrogels swells by additional decrease of the degree of cross-linking. While several molecules can act as guest species if they fit into the host cavity the choice of possible host molecules is more limited. Of particular interest within this project are cyclodextrins (a-CD and b-CD) as well as the pillar arenes (P[5]a and P[6]a). For a proper detection of the analyte the host/guest pair shall have a lower binding constant than the host/analyte (HA) pair. On the other side the binding constant must not be too small since then no effect with analyte addition will be observed. 100 is expected to be a proper ratio of the binding constants (HA / HG). In a micro sensor the swelling change of an “intelligent” hydrogel (chemo-mechanical transducer) will finally result by a piezo-resistive Si pressure sensor (mechano-electric transducer) in a sensor signal.