Colorimetric sensors are convenient and can offer both simple and accurate readout. Many such detection techniques, however, rely on a change in optical density triggered by irradiation of a photo-sensitive chromophore and require external equipment for read-out in addition to stringent storage conditions to avoid device failure [2]. Some solution-based sensors which show clear colorimetric changes spanning a broader range of the visible spectrum provide an attractive means towards convenient, stand-alone sensors, but utilize inconvenient and toxic organic solvents [3–5]. Among the many existing approaches, film-based sensing techniques are appealing for general use because they avoid potentially toxic solvents, however may also require sealed containment or have time-sensitive restrictions on read-out due to a lack of chemical stability postirradiation