Operando monitoring of gelation kinetics of polyacrylamide hydrogel using in-fiber dual-MZI

• A novel in-fiber dual-MZI superposition sensing technology is first proposed. • Operando monitoring of gelation kinetics of polyacrylamide hydrogel is realized. • Kinetic curves of temperature and RI information during gelation is established. • Two gelation laws regarding the monomer, crosslinker and catalyst is confirmed. • This technology has the potential to monitor other biochemical reaction processes. Operando monitoring of gelation kinetics and exploring the gelation law are crucial for optimizing the performance of hydrogels. In this paper, we propose a novel superposition sensing structure based on an integrated dual-Mach–Zehnder interferometer (MZI), which can simultaneously sense temperature and refractive index (RI), with an RI sensitivity of −10,173 nm/RIU and a temperature sensitivity of 8.14 nm/ °C. For the first time, its application for the operando monitoring of gelation kinetics of polyacrylamide hydrogel (PAH) is demonstrated. By establishing the kinetic curves of temperature and RI information in the gelation process, it is confirmed that the gelation process can be divided into three stages: initial lag, rapid gelation, and final saturation. In addition, two gelation laws are verified through experiments: (1) As the concentration of monomers and crosslinkers increases, RI changes, and the amount of heat released gradually increases. (2) As the concentration of catalysts increases, the gelation time decreases significantly. This superposition sensing technique based on dual-MZI can be further applied to monitor other complex dynamic biochemical processes, thereby providing insights into the laws and mechanisms of reaction processes.