The usage of reconstructed zigzag graphene nanoribbon (RZGNR) as a sensor for nitrogen based chemicals is being analyzed. The principle of sensing is based on reaction of nitrogen atom with the edge of RZGNR, which causes a measurable change in electrical properties of the nanoribbon. In order to characterize the RZGNR based sensor, reaction mechanisms of a graphene nanoribbon with an ammonia molecule (NH3) are analyzed in detail by using ab initio and DFT calculations. Ammonia molecule reacts with RZGNR via a two stage reaction. The first stage represents the rate limiting step of the entire reaction and produces a reaction intermediate. The reaction intermediate undergoes proton transfer as a second stage reaction in order to form energetically stable product. The negative differential resistance (NDR) behavior was noticed in our model of RZGNR. Upon reaction with the NH3 molecule, the electronic properties of RZGNR are significantly modified and the observed NDR behavior disappears. Present findings show that adsorption of guest NH3 molecule can be sensed by the change in current passing through the ribbons. (C) 2015 Elsevier B.V. All rights reserved.