An optical sensor (1) for detecting a chemical species includes a substrate (3), a mesoporous matrix (5) disposed on the substrate, and a microporous matrix (7) disposed within the mesoporous matrix. The microporous matrix (7) includes an indicator dye (9) dispersed therein, the indicator dye (9) exhibiting changes in its optical properties in response to the presence of the chemical species. This arrangement significantly increases the amount of dye present, while allowing the medium to be analyzed greater access through the porous structure, thereby increasing the performance of the sensor.