Chitosan vs chitin: Comparative study of functional pH bioindicators synthesized from natural red dyes and biopolymers as potential packaging additives

Abstract

In this study, we compare pH-sensitive bioindicators fabricated via the facile one-pot reaction of biopolymers (chitosan or chitin) and natural red dyes (carminic acid and purpurin). The relationship between the biocomponents and the structure, stability, and pH-sensing activity of the resulting materials is also investigated. XPS, FT-IR, SEM and pH assessments confirmed the presence of interactions between the biopolymers and the natural dyes, leading to the formation new functional smart colorants. The type of biopolymer determined the stability of the materials, and the incorporation of natural dyes significantly increased their chemical and thermal stability. All the colorimetric indicators were highly sensitive to HCl and NH3 vapors, changing color after just a few minutes. Their application as new biocolorants for ethylene-norbornene copolymer (EN) resulted in composites with attractive color performance and good photostability. This was evidenced by the lower color changes and carbonyl index increment of the samples containing the new color systems after 50 h, 100 h, and 200 h of UV irradiation compared to pure natural dyes. For the real-time analysis of the application of colorimetric pH indicators, the colored EN composites were applied to colorimetric monitor the freshness of shrimps at room temperature. The findings indicated that the designed biopolymer-dye materials could be used simultaneously as colorants, pH indicators, and additives for polymers, providing protection against unfavorable aging conditions. Importantly, the new colorants also exhibited lower tendency to leach from the polymer matrix compared to natural dyes. This could elegantly overcome the strong migration problems associated with natural dyes and enable widespread application of the new biocolorants in packaging systems.