A colorimetric alginate-catechol hydrogel suitable as a spreadable pH indicator

• A spreadable pH-sensitive colorimetric hydrogel was prepared.
• The hydrogel showed physical and chemical stability with reversible pH indication.
• A pH indicator in the fluidic channel was demonstrated.

A pH-sensitive colorimetric hydrogel was prepared from a catechol-conjugated alginate hydrogel and a pyrocatechol violet dye. The alginate-catechol hydrogel functioned as a host matrix, holding the dye solution inside, while the entrapped pyrocatechol violet worked as a pH-indicating dye whose color changed reversibly by responding to external pH stimuli. Compared to the other commonly used hydrogels, the alginate-catechol hydrogel provided high mechanical strength and chemical stability without deformation over a wide range of pH values because of the internal cross-linking of the catechol groups. Furthermore, it conferred considerable adhesion strength to various substrates via the adhesive function of the catechol moiety. The usage of the hydrogel as a spreadable pH indicator was demonstrated on a variety of surfaces. Finally, the colorimetric hydrogel was implanted in a fluidic channel to function as a visible pH indicator.