High Quantum Yield Nitrogen-Doped Carbon Quantum Dot-Based Fluorescent Probes for Selective Sensing of 2,4,6-Trinitrotoluene

Abstract

Fluorescent carbon quantum dots hold great promise for the determination of nitro-explosive 2,4,6-trinitrotoluene (TNT), a potential environmental pollutant. Here, a nitrogen-doped carbon quantum dot (N-CQD)-based fluorometric probe with high quantum yield (73.10%) was developed for the selective determination of TNT. N-CQDs were synthesized by the reflux method using citric acid (CA) as a carbon precursor and ethylenediamine (EDA) with diaminocyclohexane (DACH) as nitrogen precursors. N-CQDs, with excitation and emission wavelengths of 360 and 460 nm, respectively, were highly selective toward TNT among different types of explosives [i.e., tetryl, 2,4,6-trinitrophenol (TNP), 2,4-dinitrotoluene (DNT), 4-amino-2,6-dinitrotoluene (ADNT), HMX, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN)]. As a result of the dominant FRET mechanism secondarily assisted by ground-state donor–acceptor complexation, the fluorescence of N-CQDs showed excellent quenching in the presence of TNT. The structure of N-CQDs was elucidated using X-ray photoelectron spectroscopy, X-ray diffraction, IR, Raman, high-resolution transmission electron microscopy, and dynamic light scattering techniques, and their interaction mechanism with TNT was resolved using the Gaussian (i.e., highest occupied molecular orbital–lowest unoccupied molecular orbital energy levels) program. The limit of detection and limit of quantification in aqueous media were 30.0 and 90.0 nM, respectively, while the Benesi–Hildebrand binding constant of N-CQDs was 2.22 × 103. In the presence of common soil ions, explosive mixtures, and camouflage materials, the N-CQDs could determine TNT with recoveries between 94.9 and 104.9%. In addition, sandy soil specimens were artificially contaminated with TNT as real samples, and the TNT extracted from these samples was determined by both N-CQDs and the reference liquid chromatography with tandem mass spectrometry method to validate the proposed fluorometric method. To design field detection techniques, two different support materials based on the thin layer chromatography-paper and hydrogel (N-CQD-gel) were developed, and TNT was determined visually at a μM level with both materials. The developed support materials make N-CQDs potentially useful in environmental sensor applications with their selective and sensitive sensing ability.