Rapid Dual‐Mode Detection of Congo Red Using Silver Nanoparticles‐Decorated Red Carbon Dot Films for Food Safety Applications

The illegal adulteration of foods with Congo Red (CR) poses significant health risks, necessitating rapid and reliable detection strategies. Herein, we report a dual‐mode optical sensing film based on ethylenediamine‐mediated silver nanoparticles‐decorated red carbon dots embedded in carboxymethyl cellulose (EDA–AgNPs@RCDs/CMC) for selective CR detection. The nanocomposite displayed bright yellow emission at 556 nm ( λ ex = 420 nm), which was efficiently quenched and redshifted via the inner filter effect, π–π stacking, and surface interaction with CR. The probe achieved limits of detection of 0.416 µM (fluorimetry) and 0.439 µM (colorimetry), with rapid response within 2 min under ambient conditions and high stability toward ionic strength and storage. Smartphone‐assisted hue–saturation–value (HSV)/hexadecimal (HEX) analysis enabled quantitative on‐site evaluation. Excellent selectivity and recoveries of 94.8%–109% (RSD ≤ 5.2%) were obtained in juices, sauces, powders, and candies, demonstrating strong practical utility. The film offers a stable, low‐cost platform for field‐level monitoring of azo dye adulteration.