Effect of pH on the formation mechanism of riboflavin mediated photocrosslinking hydrogel of soybean protein nanofibrils.

Riboflavin (RF) mediated photocrosslinking provides a green and controllable approach for soy protein nanofibril (SPF) hydrogel, but the pH-dependent reaction efficiency and structural stability remain key challenges to achieve reliable and sustainable applications. In this study, RF was used as a photosensitizer to induce the crosslinking of SPF under 365 nm UV irradiation, and the gelation mechanism of the photocrosslinked hydrogels was systematically investigated under different pH conditions (2, 4, 6, 8, and 10). The results demonstrated that pH not only altered the dispersibility and conformation of SPF but also modulated the photochemical behavior of RF. At pH 2, SPF formed hydrogels mainly through hydrogen bonding and amino acid oxidation, storage modulus increased from 20.20 to 186.98 Pa, but the photosensitivity of RF was limited. At pH 4-6, severe fibril aggregation hindered the exposure of reactive sites, preventing the formation of a continuous crosslinked network. At pH 8, SPF regained a well-dispersed conformation, while the enhanced photochemical properties of RF, possibly associated with a prolonged excited-state lifetime, facilitated amino acid oxidation and disulfide bond formation, resulting in the highest crosslinking efficiency and mechanical strength, with the storage modulus increasing from 5.19 Pa to 331.68 Pa. In contrast, at pH 10, fibril degradation and the limited light absorption efficiency of RF impeded gelation. Overall, pH governed the hydrogel structure and performance by synergistically regulating SPF conformation and RF photochemistry properties. These findings provide theoretical insights for the design of pH-responsive SPF-based photocrosslinked hydrogels.