Smartphone-Coupled Paper Ratiometric Fluorescence Strip for On-Site Biogenic Amines Detection and Shrimp Freshness Monitoring

Herein, a cost-effective, naked-eye-readable platform for on-site biogenic amines (BAs) detection and shrimp freshness monitoring was developed based on a ratiometric fluorescent CDs–RhB hybrid nanosystem that was constructed by integrating carbon dots (CDs) with rhodamine B (RhB). Biogenic amines induced a significant enhancement in the green fluorescence emission of CDs, while the red fluorescence emission of RhB remained unchanged, enabling an instant self-calibration during detection. Loaded on filter paper to form disposable test strips, the nanosystem enabled a visible color transition from orange to green with increasing BAs concentration. Readable via a smartphone RGB app, the paper-based assay showed a linear response of 5.0–250 μM with a limit of detection (LOD) of 3.56 μM using histamine as the model analyte. Validated through on-site shrimp quality evaluation, the paper-based sensor established a reliable “nanoprobe–paper–phone” strategy, facilitating on-site and quantitative freshness assessment in real-world demand scenarios.