Volume amplitude gratings made of mesoporous hydrogels are beneficial for sensing, but are difficult to fabricate because they involve creating high aspect ratio features in soft materials. A novel photofunctionalizable hydrogel is reported and its suitability for fabricating grating sensors is demonstrated comprising of features with an aspect ratio of ≈3.2. To make a photofunctionalizable hydrogel with high optical quality that can be patterned using widely available light sources, a water‐soluble photoactive monomer and sensitizer are synthesized. A transmission amplitude grating is subsequently fabricated in a ≈100 µm thick photofunctionalizable hydrogel film by reaction of the free amines generated in the photoexposed regions with pH‐responsive fluorescein isothiocyanate. The volume hydrogel grating described herein is shown to be suitable for real‐time sensing of pH as an exemplar analyte. This work will have a significant impact on the fabrication of diffractive optical structures in thick films of hydrogels that are highly promising for biomolecular sensing in disease diagnosis and healthcare monitoring.

Front Cover: In article number 1900228 by Ruchi Gupta and co‐workers, a water‐soluble chemical method for making a photo‐functionalizable hydrogel is reported for the fabrication of optical grating sensors with high (>3) aspect ratio. This grating sensor contains a dye that is sensitive to pH, resulting in changes in the diffraction pattern as the pH of the solution in which the grating is immersed changes.

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