Intelligent pH- and ammonia-sensitive indicator films using neutral red immobilized onto cellulose nanofibrils

In this study, colorimetric indicator films (CIFs) were developed by integrating neutral red covalently immobilized onto TEMPO-oxidized cellulose nanofibrils (NR@TOCNFs) and poly(acrylic acid) (PAA) inside a poly(vinyl alcohol) (PVA) matrix. The successful covalent immobilization of NR onto the TOCNFs was confirmed using attenuated total reflection-Fourier transform infrared, X-ray photoelectron spectroscopy, and thermogravimetric analyses. The CIFs had a visible color change from red to yellow as the pH changed from 2.0 to 10.0. The colorimetric response of CIFs improved as the NR@TOCNF content increased, while it decreased as the PAA level increased. The critical pH ranges for the color change of CIFs were 6–7, 7–8, and 8–9 for 3 %, 5 %, and 7 % PAA, respectively, at 0.3 % NR@TOCNF. The best ammonia sensitivity was found in the indicator films containing 3 % PAA and 0.3 % NR@TOCNF. These results showed that the CIFs could be applied for freshness detection in food packaging.