Responsive Soft Materials: Functional Elements in Biosensing and Beyond

Development of hydrogels that respond to changes in environment with high specificity and sensitivity requires highly sensitive detection methods. The presentation will highlight application of an fiber-optic interferometric platform with 2 nanometer sensitivity for detection of changes in the optical length of the hydrogels. The 2 nanometer resolution realized for characterization of the hemispherical ~60 mm radius hydrogels yields high sensitivity potentially supporting detection of minute concentrations of analyte. The generic swelling properties of ionic hydrogels are transformed to biospecific ones by including specific elements that binds, catalyze or induce some reaction, and where these changes affect the overall swelling properties.

In the presentation, we include examples of glucose responsive hydrogels, oligonucleotide based recognition as well as examples where hydrophobic and electrostatic interactions are dominating. The glucose sensitive hydrogel combined with the fiber-optic interferometer supported continuous monitoring of glucose within physiological relevant range and temperatures. The olinucletide based recognition hydrogel comprises hybridized dioligonucleotides grafted to the polymer network as network junctions in addition to the covalent crosslinks. This supports detection of complementary oligonucleotides or other biological molecules based on their aptamer sequences. Insight into the coupled processes of transport, binding, competitive displacement and swelling in this hybrid hydrogels was obtained using time-lapse confocal imaging.

Furthermore, changes in swelling properties of anionic hydrogels following impregnation of polycations that either penetrate the network or preferentially deposit at the surface of the hydrogel show that the distribution of the polycationic component strongly affect the swelling behaviour. In particular, the case of polycation that distribute nearly evenly within the anionic hydrogel do show swelling decrease with reduced ionic strength, e.g. a polyampholyte character. For the quantitative interpretation of the swelling behaviour of the hydrogels, initial results applying finite element modelling indicate that the covalent linkage at the end of the fiber-optic waveguide reduces the swelling capacity compared to that predicted by the theory for unconstrained hydrogels.