a b s t r a c t Polymer hydrogels can exhibit large reversible volume changes in response to external stimuli, and thus are regarded as excellent materials for chemical sensors. In this report, we demonstrate a mechanically robust and fast response photonic crystal pH sensor fabricated by templated photo-polymerization of hydrogel monomers within the interstitial space of a self-assembled colloidal photonic crystal. Through-out a rigorous optimization of the photo-polymerization, pH sensors showing a response time of less than 10 s upon a pH change were fabricated. Repeated pH changes revealed that the sensor has a long lifetime (>6 months) without degradation of the response time or reproducibility in pH-driven color change.