Demonstration of an intelligent hydrogel based diffraction grating

We report the fabrication of a diffraction grating cast into a responsive hydrogel using a silicone rubber intermediate cast from an engraved glass master grating. The aim to investigate if changes in the swelling of this gel in response to changes in the concn. of a specific analyte led to changes in the line spacing, and hence diffraction pattern, of the grating. The protocol for casting gratings was initially developed using a composite carboxymethyl dextran/bovine serum albumin gel produced using carbodiimide chem. to assess the optimum gel properties required. Examn. under a light microscope showed that, formed under appropriate synthesis conditions, CM-dextran-BSA hydrogels retained the grating structure and appeared to have similar optical properties to the silicone rubber sub-master used for casting. For a facile initial evaluation of the detection principle a no. of similar gels were produced using cross-linked alginic acid. In this case excess carboxylic groups remaining after crosslinking were able to form addnl. ionic cross-links in the presence of divalent cations (Ca2+). Test expts. with these gels showed that both the size and position of the reflected and refracted spots obtained from illumination with a Helium-Neon laser changed as gel swelling changed with calcium ion concn. i.e. the size of both diffraction and reflection spots increased as the alginate hydrogel shrank in response to changes in environmental Ca2+. The utility of the alginate based gels for the detection of cations, together with evidence that dextran protein gels can retain grating structures, suggest that this assay procedure should applicable to any hydrogel where the response is based on protein-ligand interactions. The key requirement is that the crosslinking interactions constraining gel swelling can be quant. displaced by the analyte acting as a specific competitor.