Colorimetric and fluorometric sensing of polar E120 in juice and environmental water samples using mannitol-functionalized magnetic nanoparticles and nitrogen-doped carbon dots

In this study, mannitol-functionalized magnetic nanoparticles (MMNPs) as a unique nanosorbent and N-doped fluorescent carbon dots (N-CDs) as a cost-effective nanosensor were created and utilized, for the first time, for dispersive micro-solid-phase extraction (Dµ-SPE) to determine carmine (E120) dye in water samples and juices. The modification of the magnetic nanoparticles with mannitol was designed to enhance the responsive potential for adsorption of the polar E120 dye from complex sample matrices through electrostatic interaction. The as-fabricated N-CDs fluorescent probe exhibited a high fluorescence quantum yield (Φs) of 43.1 %, allowing for accurate fluorometric detection of E120 dye. The as-synthesized MMNPs nanosorbent and fluorescent N-CDs nanoprobe were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), thermogravimetric analysis (TGA), and vibrating-sample magnetometer (VSM). Density functional theory (DFT) studied the E120 dye structure using Gaussian 09 to explore the interactions between E 120 dye molecules and MMNPs/N-CDs. The impact of the critical adsorption and detection experimental factors was investigated and adjusted. A minimal amount of MMNPs nanosorbent (150 mg) is sufficient for E120 extraction in an acceptable time of 15 min. Furthermore, with a high determination coefficient, the adsorption characteristics fit with the models of Langmuir isotherm and first-order kinetics. The maximum adsorption capacity (qm) of the as-fabricated MMNPs was 87.7 mg.g-1. After adsorption, E120 dye was fluorometrically analyzed using nitrogen-doped carbon dots as a fluorescent nanosensor via the inner filter effect (IFE) mechanism. Under the optimized conditions, the proposed fluorometric procedures showed a linear increase in the fluorescence ratio with increasing the E120 concentration in the range of 1.0 - 160.0 μg.mL-1 with detection (LOD) and quantitation (LOQ) limits of 0.27 and 0.83 μg.mL-1, respectively. The relative standard deviation (%RSD) did not exceed 2.34 %. The proposed methodology was successfully applied to determine E120 dye in juice and environmental water samples with % recovery ranged from 89.2-106.1 % and 92.9-107.2 %, respectively offering a reliable and environmentally friendly alternative to traditional detection methods with potential applications across various industries.