This communication demonstrates a facile method to detect CO32? by the naked eye through color change based on the pH dependence of inverse opal photonic crystal polymer films. These films are highly sensitive only to a certain range of pH values that make them less disturbed by other anions.

J. Mater. Chem. C, 2015

Schematic illustrations of the detection of CO₃²⁻ based on IOPP films sensitive to a certain range of pH values.