Preparation of a colorimetric hydrogel indicator reinforced with modified aramid nanofiber employing natural anthocyanin to monitor shrimp freshness.

The pH-responsive hydrogels have potential applications in food visualization detection, but their fragile mechanical properties limit their applicability. The excellent mechanical properties and thermal stability of aramid nanofibers (ANFs) can improve the structural stability of hydrogels. In this study, the surface properties of ANFs were enhanced through modification to improve their surface activity. The modified ANFs, designated as ANF-SN, were produced following treatment with a mixture of sulfuric acid (HSO) and nitric acid (HNO), which led to increased reactivity and dispersibility of the ANFs due to the proliferation of active groups on their nanofiber surface. The preferred anthocyanin extract from purple sweet potatoes (purple sweet potato extract [PSPE]) had significant color responses to pH (2-12) and ammonia vapor. A stable dual-network colorimetric hydrogel was fabricated by combining ANF-SN, polyvinyl alcohol/sodium alginate (PVA/SA), and PSPE through a two-step method (freeze-thawing and staining). Characterization analysis showed that the strong acid modification of ANFs effectively improved their chemical reactivity. ANF-SN was better than ANF in promoting the formation of hydrogen bond networks, enhancing hydrogel network structures, and improving the viscoelasticity of hydrogels. The optimal hydrogel indicator PVA/SA/ANF-SN/PSPE had good color responsiveness and sensitivity to ammonia. It can also be used to further determine shrimp freshness value using a smartphone and RGB color-picking software.