A CO2 sensor was fabricated by attaching CO2-sensitive spiropyran groups onto versatile photo-crosslinked poly(glycidyl methacrylate) (PGMA) precursor nanofibers via a nucleophilic ring-opening reaction. These nanofibers can quickly respond to dissolved and gaseous CO2. Hence, these spiropyran functional nanofibers are expected to be employed as CO2 sensors for the visual detection of CO2.