Structural Effect of Thioureas on the Detection of Chemical Warfare Agent Simulants

The ability to rapidly detect, identify, and monitor chemical warfare agents (CWAs) is imperative for both military and civilian defense. Since most CWAs and their simulants have an organophosphonate group, which is a hydrogen (H)-bond acceptor, many H-bond donors have been developed to effectively bind to the organophosphonate group. Although thioureas have been actively studied as an organocatalyst, they are relatively less investigated in CWA detection. In addition, there is a lack of studies on the structure-property relationship for gas phase detection. In this study, we synthesized various thioureas of different chemical structures, and tested them for sensing dimethylmethylphosphonate (DMMP), a CWA simulant. Molecular interaction between DMMP and thiourea was measured by H-1 NMR titration and supported by density functional theory (DFT) calculations. Strong H-bond donor ability of thiourea may cause self-aggregation, and CH-pi interaction can play an important role in the DMMP detection. Gas-phase adsorption of DMMP was also measured using a quartz crystal microbalance (QCM) and analyzed using the simple Langmuir isotherm, showing the importance of structure-induced morphology of thioureas on the surface.