Telling if a fruit is ripe is usually as simple as seeing if it has the right color. But that task is harder if a fruit does not change color as it ripens, like a kiwi, for example. A new sensor that turns from blue to red as a fruit ripens could make it easier to tell when a fruit is just right for eating (ACS Sens. 2020, DOI: 10.1021/acssensors.0c00117). Such sensors may also be useful in guiding fruit harvest, transport, and storage.
As fruits ripen, they release ethylene. Most color-changing sensors designed to detect this emitted gas rely on formation of metal complexes and require use of a scanner to read the change. Now, Fariba Dehghani and Sina Naficy of the University of Sydney and their colleagues have made a flexible ethylene sensor that makes that color change visible to the naked eye. Spherical vesicles made of polydiacetylenes are known to have a vivid blue-to-red color transition when exposed to different stimuli. So the researchers created vesicles using monomers of 10,12-pentacosadiynoic acid (PCDA) and an ethylene-reactive PCDA functionalized with a thiol group on the end. They then mixed a solution of the vesicles with chitosan, cast the mixture into a film, and used ultraviolet light to cross-link the monomers.
To test the resulting blue-colored films, the researchers sealed them with unripe kiwi fruit in a humid container and monitored them over 3 days. As the fruit ripened, the emitted ethylene reacted with the thiols, changing the structure of the polymer and gradually turning the film purple then red. The sensor was selective for ethylene and did not change color in response to other gases like air, carbon dioxide, or nitrogen.