Film with induced halochromic properties engineered by grafting neutral red on silk backbone

In this work, a silk-based device with halochromic properties is presented. After the removal of sericin from natural fibers, silk fibroin was grafted with neutral red through Mannich reaction and casted into pH-sensing films with a thickness of 140 ± 31µm. The colored silk fibers retained their hipsochromic behavior after being washed with DMF, suggesting a covalent bonding between the silk and the neutral red. The nature of the linkage was confirmed through evolved gas analysis, which indicates that neutral red is attached to the tyrosine residues of silk. Once the substrate was casted into colored films, its ability to determine the pH within a wide range, 1 to 12, was proved in the liquid phase. Its reversibility and reusability was verified by subsequent immersions. The color evolution was tracked in terms of CIELAB parameters with a spectrophotometer and by image analysis using a smartphone. Both sets of experiments show consistent results. Finally, the silk sensor device was used to monitor shrimp spoilage. It proved to be color-responsive when capturing the volatile nitrogen of the ammonia released along the shrimp degradation.