High-loading red carbon dots@NH2-MIL-53(Al) composite for ultrasensitive and selective picric acid detection via synergistic FRET and competitive absorption

Rapid and sensitive detection of picric acid (PA), a highly toxic and water-mobile nitroaromatic pollutant, is essential for environmental safety and public health protection. Herein, we fabricate a dual-emission ratiometric fluorescent probe by immobilizing high-load red-emitting carbon dots (R-CDs) onto NH₂-functionalized MIL-53(Al), forming an R-CDs/NH 2 -MIL-53(Al) composite for selective aqueous-phase PA sensing. R-CDs were hydrothermally synthesized with citric acid and neutral red as co-precursors. The loading efficiency was   maximized by systematically optimizing precursor molar ratios and dilution conditions. The composite exhibits distinct selective fluorescence quenching in the presence of PA. Mechanistic characterizations, including fluorescence lifetime decay measurements and spectral overlap analysis, verify that the fluorescence quenching originates from two synergistic quenching pathways, namely competitive light absorption and fluorescence resonance energy transfer (FRET), rather than nonspecific physical interactions. This ratiometric sensing platform achieves a wide linear detection range of 0.1–70 μM and an ultralow limit of detection (LOD) of 32.9 nM. Spike-and-recovery assays using real environmental water samples (tap water, commercial bottled mineral water, and Jialing River surface water) produced recoveries of 99.7%–105.1%, with relative standard deviations (RSD) below 5.86% ( n  = 3). Furthermore, a smartphone-enabled visual quantitative method based on RGB color channel analysis was constructed, enabling portable on-site detection without large-scale laboratory instrumentation.