The severe toxicity and easy availability of nerve agents have caused an increase in the need to develop effective methods for identifying these lethal substances. Among the traditional platforms employed for this kind of purpose, chromophore-based chemosensors have garnered a significant amount of attention. In this study, we have designed, prepared, and characterized (N1E,N4E)-N1,N4-bis((Z)-3-(4-(dimethylamino)phenyl) allylidene)benzene-1,4-diamine, BDPA, a simple chromogenic probe of symmetrical diimine, having the extreme capability to sensing diethylchlorophosphate (DCP), the mimic of chemical warfare agent sarin not only in solution but also in solid and vapor phases with highly selective and sensitive manner. In the presence of DCP, a remarkable color change from light yellow to blue is observed in pure acetonitrile solvent and also shows pink color in 10 % acetonitrile–water by the naked eye, comes back to its former state after the introduction of triethylamine (TEA) which is found to be five times recyclability by alternately adding DCP and TEA. The detection limit of BDPA with DCP has been estimated in the nanomolar range, and also, response time towards sarin surrogate DCP is withina few minutes (∼5–6 min.).To determine its practical usability, we have developed a portable test kit that uses paper strips to assess the capability of BDPA for in-situ recognition and measurement of DCP. A dipstick method has also been executed to recognize gaseous DCP within the stores of analogous hazardous analytes to prove the effectiveness of the sensor, BDPA. A smartphone-based readout novel technique, i.e., RGB analysis, has provided an innovative platform for the instant, on-site, visual identification and measurement of DCP in remote areas. The current work presented an incredible probe and method for conveniently and instantly detecting and quantifying sarin gas mimics in actual dangerous circumstances.