UV−Curable Choline Chloride and Bromophenol Red Covalent Functionalized Chitosan Antibacterial and pH−Sensitive Hydrogels

Food safety and food freshness issues have become important concerns for consumers and hydrogels with high antibacterial efficiency, low toxicity and pH−sensitive colorimetric intelligence are one of candidates to meet the requirement of food package and food spoilage monitoring. Nevertheless, hydrogels prepared only by assembling the ingredients with non−covalent interactions through time−consuming molding process usually encounter drawbacks such as relatively poor mechanical properties, stability and thermal stability, which always restrict their further applications. To overwhelm these shortcomings, UV−curable choline chloride (ChCl) and bromophenol red (BR) covalent functionalized antibacterial and pH−sensitive cross−linked hydrogels (UV−hydrogels) were developed in this paper. The UV−hydrogels possessed good thermal stability because the initiated thermal decomposition temperature (Td5) is in the range of 135.8−184.7 oC. The DMA analysis revealed that the UV−hydrogels are with good mechanical properties because the elongation at break is in the range of 117−525% and the tensile strength is in the range of 0.8−2.7 MPa. The UV−hydrogels with BR groups exhibited naked eye visible color variation from yellow to dark blue-green with the variation of the pH values from 2 to 14. The antibacterial efficiency of the UV−hydrogels against S. aureus, E. coli and M. albican increased with the increment of covalent−grafted ChCl amount. Cytotoxicity assay demonstrated that the UV−hydrogels are low toxicity against L929 cells. The experiment of monitoring freshness of beef denoted the UV−hydrogels have potential application in animal–derived high protein food packaging and freshness monitoring.