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An ultrasensitive and selective chemiresistive sensor interface based on Ag nanocluster integrated polyaniline functionalized multi-walled carbon nanotubes (AgNC@PANI/MWCNTs) has been developed for trace-level detection of NH3 gas at room temperature. AgNC@PANI/MWCNTs was synthesized using surfactant-free, one-pot wet-chemical process, by controlled integration of active Ag sites onto MWCNTs. The structure and morphology of AgNC@PANI/MWCNTs nanocomposite have been extensively studied by various characterization techniques. The gas sensing properties of AgNC@PANI/MWCNTs nanocomposite towards trace-level concentrations NH3 (2-10 ppm), an important biomarker in exhaled human breath, was systematically evaluated. The sensor exhibited dramatic enhancement in the sensor response (26 %), fast response (∼5 s) and recovery (∼4 s) characteristics with good reproducibility and selectivity upon exposure to NH3 gas. The excellent performance of the sensor towards NH3 could be attributed to the rapid electronic sensitization of surface engineered active AgNC sites in the composite in which oxidized AgNC were found to play a critical role. Effect of humidity and the kinetics of the NH3 gas adsorption on nanocomposite were analyzed. The possible interactions between NH3 and AgNC@PANI/MWCNTs nanocomposite were discussed. This investigation can pave the way to novel strategies for designing and fabricating low-cost, high performance NH3 gas sensors for clinical breath analyzer application.