Smartphone-Assisted Dual-Mode Ratiometric Film Sensor-Based GS-CDs@AgNPs for the Sensitive Detection of Rhodamine B in Food via FRET and IFE

Rational design of dual-emissive optical materials offers opportunities for real-time visual analyte detection. In this work, we report a Green Shifted Carbon Dots decorated with Silver Nanoparticles (GS-CDs@AgNPs) nanocomposite embedded in a sodium alginate film as a ratiometric optical material for the dual-mode detection of Rhodamine B (RhB). The composite exhibits a distinct green photoluminescence (∼534 nm), which undergoes a ratiometric transition upon RhB exposure, characterized by a decrease in donor emission and the emergence of an acceptor peak at ∼568 nm. The spectral evolution arises from a synergistic interplay of Förster Resonance Energy Transfer (FRET) and the Inner Filter Effect (IFE) mechanisms enabled by spectral overlap and proximity, where the quenching mechanism was confirmed via fluorescence lifetime analysis. The biopolymer film provides environmental stability and optical clarity, supporting smartphone-assisted quantifications. The sensor exhibits excellent analytical performance (LOD: 0.88 μM colorimetric, 0.2146 μM fluorimetric) under instrumental analysis and 1.88 μM (R/G vs concentration) and 1.1 μM (Hue vs concentration) under smartphone-based detection. The system offers rapid, visual monitoring across a wide concentration range with high linearity, low detection limits, and validated performance in real samples, highlighting its potential for food safety and environmental monitoring applications.