A CO2-responsive hydrogel film for optical sensing of dissolved CO2.

CO2-monitoring plays an important role in medicine, environmental sciences, and food industries. Here, a new CO2-responsive hydrogel film was fabricated from branched polyethyleneimine (BPEI) and partially oxidized dextran (PO-Dex) via layer-by-layer (LBL) assembly, based on the in situ Schiff base reaction between the two polymers. The swelling behaviours of the films were studied using Fabry-Perot fringes on their reflection spectra. Like ordinary hydrogels, the BPEI/PO-Dex films swell in water. In addition, they swell to a larger degree when CO2 is introduced, due to the reaction between CO2 and the amino groups in BPEI. The CO2-induced swelling can be reported by the shift of the Fabry-Perot fringes on their reflection spectra; therefore, the BPEI/PO-Dex film can be used as an optical sensor for dissolved CO2. In the new sensor, the BPEI/PO-Dex film acts as a CO2-sensing material and Fabry-Perot cavity simultaneously. The introduction of ordered structure is no longer required. The response of the sensor to CO2 is linear and reversible. Unlike other hydrogel-based sensors that suffer from a slow response, the new sensor can respond to CO2 quickly, making it applicable for real-time, continuous monitoring of CO2 levels in solution.