Halogen-controlled engineering of fluorescent carbon dots with adjustable color for food freshness detection

Abstract

Fluorescent carbon quantum dot-based visualization methods for food freshness analysis have gained increasing attention. However, traditional carbon quantum dots (CDs) exhibit single-color fluorescence and are prone to background light interference. Herein, a novel series of multi-color fluorescent CDs is successfully synthesized through a one-pot method, using o-phenylenediamine (OPD) ligands of the same halogen family as the precursor and citric acid as the carbon source. The CDs doped with different halogens exhibit a distinct color change from blue to red, attributed to differences in the key fluorophore molecules of corresponding pyridone derivatives. The halogen-doped CDs display pH-responsive characteristics based on color change sensitivity, enabling their use as putrescine-sensing probes. Excitingly, the chlorine-doped ratiometric fluorescent CDs (ClOPD-CDs) show the most significant color change (ΔE = 44.66), delivering a stark color shift from orange-red to yellow based on the deprotonation of edge amino groups in the presence of biogenic amine molecules. Furthermore, the relationship between total volatile basic nitrogen (TVB-N) and RGB outputs of a homemade fluorescent biosensor for meat spoilage (R² = 0.9386) confirms that ClOPD-CDs function effectively as an on-line fluorochromic probe for visual and quantitative food freshness monitoring via smartphone. The ratiometric fluorescence signals have facilitated the development of a smartphone-compatible sensing application, anticipated to provide a convenient tool for real-time food quality detection.