Phenylenediamine-derived carbon dots coupled with rhodamine B: Ultrafast ratiometric fluorescence and smartphone-enabled imaging detection of nitrite

Nitrite (NO 2 − ) is common in food and water and poses health risks, requiring rapid detection. Here, a ratiometric fluorescent probe combining phenylenediamine-derived carbon dots (Ph-CDs) with Rhodamine B (RhB) was developed for quick, accurate NO 2 − sensing. Under acidic conditions, NO 2 − reacts with amino groups on Ph-CDs via diazotization, causing a clear fluorescence change within 200 s. RhB provides a stable reference, allowing self-calibrating dual-emission output that reduces matrix and environmental interferences. Coupled with smartphone imaging, the probe enables real-time visual detection and quantification without complex instruments. The system showed high sensitivity (LOD: 0.10 μM) and good accuracy (recoveries: 96.8 %–101.8 %) when tested in food and tap water, demonstrating strong reliability in complex samples like cured meats. This portable, user-friendly platform offers an effective tool for on-site NO 2 − screening, supporting food safety and environmental monitoring, especially in resource-limited settings.