S-CQD-enhanced cellulose-based multifunctional ratiometric fluorescent films: Improved fluorescent performance of FITC for freshwater shrimp freshness monitoring

TPO-S-CQD was fabricated from waste tires via a top-down approach. • TPO-S-CQD endows film with enhanced FRET-based fluorescence response. • The C3 film achieves a strong ΔE–TVB-N linear correlation (R 2  = 0.9945). • The C3 film possesses good antibacterial and physicochemical properties. Sulfur-doped carbon quantum dots (S-CQD) were synthesized via hydrothermal (bottom-up approach) or pyrolysis (top-down approach) methods, denoted as CA-S-CQD and TPO-S-CQD respectively. These were incorporated into cellulose substrate to construct fluorescent composite sensing films. Structural and spectral characterization revealed that, compared with CA-S-CQD, TPO-S-CQD possessed smaller particle size and superior dispersibility, enabling the formation of composite films with denser and more uniform microstructure. Based on the fluorescence resonance energy transfer (FRET) mechanism, the composite films exhibited more sensitive fluorescence responses to volatile biogenic amines. Specifically, the C3 film incorporating TPO-S-CQD demonstrated optimal performance in sensing sensitivity, recyclability, and long-term stability. Physicochemical properties tests indicated that the incorporation of S-CQD significantly enhanced the hydrophilicity, antioxidant activity, UV-blocking efficiency, and mechanical properties of the films, while maintaining excellent biodegradability. Moreover, the C3 film exhibited broad-spectrum antibacterial activity, with notably stronger inhibitory effects on E. coli than S. aureus . Using Litopenaeus vannamei as the detection object, the fluorescent color of the film changed gradually with spoilage degree at different storage temperatures, which was highly consistent with the national standard for total volatile basic nitrogen (TVB-N) in freshwater shrimp. The color difference (ΔE) showed a strong linear relationship with TVB-N content, with a coefficient of determination (R 2 ) of 0.9945 for the C3 film. This study realizes the high-value utilization of waste tires and provides a novel high-performance functional material and research strategy for intelligent indicator packaging and rapid visual freshness monitoring for freshwater shrimp. Download: Download high-res image (149KB) Download: Download full-size image