Sensing of Fish Freshness Using Smart Pectin Films Incorporated with <i>Spirulina</i> Extract and Carbon Dots

Multifunctional and biodegradable smart packaging films were developed here using pectin, phycocyanin rich Spirulina (PCS) extract and citric acid derived carbon dots (CDs). The physicochemical, active and intelligent properties of the films were systematically examined as a function of CD concentration. Incorporation of CDs enhanced tensile strength from 10.88 to 17.70 MPa, increased crystallinity and thermal stability and maintained high biodegradability (above 80% mass loss after 28 days in soil). The addition of CD enhanced antioxidant activity (from 12.8% to 39.5% ABTS scavenging) and imparted concentration dependent antimicrobial activity against E. coli and S. aureus . The PCSP/CD12.5 film displayed a distinct colorimetric response to ammonia vapor, displaying a linear Δ E –NH 3 correlation ( R 2 = 0.93) within the range of 0–50 mg N/100 g and a detection limit of 9.04 mg N/100 g, and exhibited colorimetric stability over two months of storage. In real food trials, the PCSP/CD12.5 film enabled effective visual tracking of fish freshness at 23 °C with color changes closely correlating to increases in TVB-N (from 13.3 to 36.2 mg N/100 g) and TVC (from 4.5 to 7.4 log cfu/g) during storage. These results demonstrate that PCSP/CD films effectively integrate active and intelligent functionalities thereby extending the application of pigment and protein complexes.