A smartphone-integrated ratiometric fluorescence probe using red/green dual-color carbon dots for on-site visual and quantitative determination of nitrite.

On-site sensitive detection of nitrite (NO2-) in food and environmental matrices is crucial to human health protection. Herein, a ratiometric fluorescence probe integrating red-emitting (R-CDs) and green-emitting (G-CDs) carbon dots was constructed for visual and quantitative determination of NO2-. The R-CDs acted as the nitrite-responsive unit via a diazotization reaction, while G-CDs served as a stable internal reference. The nitrite-triggered fluorescence quenching of R-CDs produced a concentration-dependent ratiometric response over 5-140 μM, with a detection limit (LOD) determined as 1.34 μM. The orange-red to green color change was visually discernible at 200 μM without the aid of any instrument. The R/G intensity ratio, analyzed via a smartphone, exhibited well linear correlation with log[NO2-] across 0.1-10 mM. This ratiometric sensor showed excellent specificity toward nitrite and achieved recoveries of 97.60%-104.6% in food (e.g. ham and bacon) and water samples, providing a promising approach for on-site food safety monitoring.