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.

Oxygen- and nitrogen-heteroatom-doped, water-dispersible, and bright blue-fluorescent carbon dots (ON-CDs) were prepared for the selective and sensitive determination of 2,4,6-trinitrophenol (picric acid, PA). ON-CDs with 49.7% quantum yield were one-pot manufactured by the reflux method using citric acid, d-glucose, and ethylenediamine precursors. The surface morphology of ON-CDs was determined by scanning transmission electron microscopy, high-resolution transmission electron microscopy, dynamic light scattering, Raman, infrared, and X-ray photoelectron spectroscopy techniques, and their photophysical properties were estimated by fluorescence spectroscopy, UV-vis spectroscopy, fluorescence lifetime measurement, and 3D-fluorescence excitation-emission matrix analysis. ON-CDs at an average particle size of 3.0 nm had excitation/emission wavelengths of 355 and 455 nm, respectively. With the dominant inner-filter effect- and hydrogen-bonding interaction-based static fluorescence quenching phenomena supported by ground-state charge-transfer complexation (CTC), the fluorescence of ON-CDs was selectively quenched with PA in the presence of various types of explosives (i.e., 2,4,6-trinitrotoluene, tetryl, 1,3,5-trinitroperhydro-1,3,5-triazine, 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, pentaerythritol tetranitrate, 3-nitro-1,2,4-triazole-5-one, and TATP-hydrolyzed H₂O₂). The analytical results showed that the emission intensity varied linearly with a correlation coefficient of 0.9987 over a PA concentration range from 1.0 × 10^-9 to 11.0 × 10^-9 M. As a result of ground-state interaction (H-bonding and CTC) of ON-CDs with PA, an orange-colored complex was formed different from the characteristic yellow color of PA in an aqueous medium, allowing naked-eye detection of PA. The detection limits for PA with ON-CDs were 12.5 × 10^-12 M (12.5 pM) by emission measurement and 9.0 × 10^-10 M (0.9 nM) by absorption measurement. In the presence of synthetic explosive mixtures, common soil cations/anions, and camouflage materials, PA was recovered in the range of 95.2 and 102.5%. The developed method was statistically validated against a reference liquid chromatography coupled to tandem mass spectrometry method applied to PA-contaminated soil. In addition, a poly(vinyl alcohol)-based polymer composite film {PF(ON-CDs)} was prepared by incorporating ON-CDs, enabling the smartphone-assisted fluorometric detection of PA.