Cost-effective α-MnO nanorods passivated with ethylenediamine utilizing charge transfer and oxidase-mimicking activity for ultrasensitive triple-mode colorimetric determination of TNT.

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-MnO@EDA-based colorimetric methods were developed for the rapid, selective, sensitive, and on-site determination of TNT. α-MnO@EDA, synthesized by passivation with ethylenediamine (EDA) of MnO 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-MnO@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-MnO@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 and 9.6 nmol L, 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-MnO@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-MnO@EDA display charge transfer-interaction with TNT, causing the α-MnO@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-MnO@EDA to be used as a novel, versatile triple-mode colorimetric sensor.