Two in One: Poly(ethyleneimine)-Modified MnO2 Nanosheets for Ultrasensitive Detection and Catalytic Reduction of 2,4,6-Trinitrotoluene and Other Nitro Aromatics

The development of a portable sensor for on-site sensing of 2, 4, 6-trinitrotoluene (TNT) and other nitro aromatics (NAs) is still needed to strengthen environmental protection and public security. Herein, we report the fabrication of fluorescent few-layered MnO2 nanosheets (FL-MnO2 NSs) via N,N-dimethyl formamide-mediated ultrasonication and hydrothermal treatment. Electron and spectroscopy studies demonstrate that the modification of the FL-MnO2 NSs (named as PEI-FL-MnO2 NSs) with amine-rich polyethyleneimine (PEI) produced planar, β-phase, well-crystalline, and positively charged nanosheets. This PEI-related surface modification not only improves the quantum yield of the FL-MnO2 NSs from 7.6 to 13.2% but also provides multiple binding sites with nitro groups of NAs. It was found that the presence of TNT, as an example of NAs, efficiently quenched the fluorescence of the PEI-FL-MnO2 NSs due to the formation of Meisenheimer complexes through the efficient adsorption process. In comparison to PEI-modified MnO2 quantum dots, the PEI-FL-MnO2 NSs exhibited relatively high sensitivity to TNT with a limit of detection of 20 pM. By depositing PEI-FL-MnO2 NSs on a filter paper, the as-made paper sensor was capable of visualizing 10 nM TNT in drinking water and 10 μM TNT on a postal envelope through the appearance of a quenching spot under a UV lamp. Besides, the adsorbed PEI polymer enables the PEI-FL-MnO2 NSs to electrostatically interact with citrate-capped metal nanoparticles, producing the composites for catalyzing NaBH4-mediated reduction of NAs. Since the PEI-FL-MnO2 NSs possess a large surface area to capture NAs and serve as an electron acceptor for BH4–, the formed composites efficiently catalyze the reaction of NaBH4 and NAs as compared to metal nanoparticles and PEI-FL-MnO2 NSs alone.