Ömer Kaan Koç

There is a need for on-site applicable, simple, cost-effective, and rapid methods for the remediation of 2,4,6-trinitrotoluene (TNT)-contaminated soil and groundwater in military storage and reclamation sites, which necessitates extensive analyses to monitor remediation. Here, α-MnO₂@EDA-based colorimetric methods were developed for the rapid, selective, sensitive, and on-site determination of TNT. α-MnO₂@EDA, synthesized by passivation with ethylenediamine (EDA) of MnO₂ formed from permanganate with thiosulfate reduction, produces a colorimetric signal by entering into a charge-transfer interaction between the electron-rich amine groups in its structure and the electron-deficient nitro-groups of TNT. In addition, α-MnO₂@EDA oxidizes 3,3′,5,5′–tetramethylbenzidine (TMB) in pH 4 environment (acetate buffer) thanks to its oxidase-mimicking activity, causing the formation of a blue-colored charge-transfer complex (diamine-diimine complex) of TMB. However, in the presence of TNT, α-MnO₂@EDA interacts with TNT, inhibiting TMB oxidation and giving rise to a different TNT determination based on the lightening of the blue color. The TNT detection limits (LOD) of the charge transfer interaction and oxidase-mimicking activity methods are 1.2 nmol L⁻¹ and 9.6 nmol L⁻¹, respectively. Coefficients of variation (CV) were 1.32% (intra-assay) and 2.11% (inter-assay) for first mode and 2.01% (intra-assay) and 2.92% (inter-assay) for the second mode of colorimetric sensing. In both methods, TNT was determined selectively among different explosives. High recovery values (93.90–105.30%) ​​were obtained for TNT in the presence of possible interfering species (i.e., common metal cations, anions, camouflage materials, and explosive mixtures) which did not show a significant effect on the developed colorimetric methods. Additionally, an α-MnO₂@EDA-based swab sensor, which can be easily prepared and applied to on-site determination of TNT, was developed as a third mode. Swab cottons loaded with high stability α-MnO₂@EDA display charge transfer-interaction with TNT, causing the α-MnO₂@EDA swab to become colored and serve as a rapid test kit. The ability to use two different methods using the same nanoprobe for the same analyte in solution and having a swab-kit for practical applications allows α-MnO₂@EDA to be used as a novel, versatile triple-mode colorimetric sensor. Graphical Abstract

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.