Smartphone-assisted ratiometric fluorescence sensing of hypoxanthine using N, B-co-doped carbon dots for on-site seafood freshness monitoring

Hypoxanthine (Hx) is a key biomarker for seafood freshness, accumulating during post-mortem storage. Conventional methods are cumbersome and not suited for on-site use. This work develops a low-cost, portable smartphone-assisted ratiometric fluorescence sensor using N, B-co-doped carbon dots (N, B-CDs) for Hx measurement. An enzymatic cascade yields DAP, quenching the 440 nm blue emission of N, B-CDs and triggering a new orange signal at 570 nm. Glutaraldehyde crosslinks N, B-CDs onto filter paper to fabricate a paper sensor. The liquid-phase assay achieves LODs of 0.106 μM (fluorescence) and 0.072 μM (RGB); the paper sensor enables visual detection from 0.1 to 225 μM with an LOD of 0.128 μM. The platform exhibits ideal selectivity and was validated in fish, shrimp and clam. Freshness classification into fresh, sub-fresh and spoiled batches is realized using R/B ratio thresholds. This work offers a simple, portable and cost-effective tool for on-site seafood quality monitoring.