Label-Free Selective Detection of Nitrobenzene Explosives at Picomolar Sensitivity in Single-Molecule Junctions

Developing single-molecule detection methods enables ultrasensitive identification of nitrobenzene explosives, offering groundbreaking significance in counterterrorism screening, environmental monitoring, and public safety. In this study, we demonstrate the selective and label-free detection of nitroaromatic explosives─2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrophenol (TNP), in single-molecule junctions using scanning tunneling microscopy break junction technique. Our findings reveal that the conductance peak areas of 4,4′-bipyridine-3-amine (Py-NH₂) exhibit pronounced concentration-dependent responses to nitroaromatic analytes. This behavior is attributed to the formation of Meisenheimer complexes between Py-NH₂ and the target molecules, which introduces steric hindrance to suppress molecular junction formations. Remarkably, this mechanism enables ultrasensitive detection with limits of detection as low as 0.95 × 10^–12 M for TNT, 0.71 × 10^–10 M for DNT, and 0.65 × 10^–10 M for TNP in a standard solution as well as a solution containing interfering compounds (toluene, xylene, and m-nitrobenzoic acid). Furthermore, the practicality of this single-molecule electrical sensing platform is validated through qualitative analysis of an environmental sample of soil. These findings demonstrate the substantial potential of single-molecule electrical measurement techniques in enabling highly sensitive, on-site detection of trace explosives for portable security screening and environmental surveillance systems.