This paper reports a novel method to detect ammonia by using the ninhydrin - PDMS composite. The polymer composite film is prepared by integrating ninhydrin into the PDMS polymer matrix. Further, an optical lab-on-a-chip device is developed by integrating the ninhydrin-polymer composite into a microfluidic device for the detection of ammonia. The chemisorption of ammonia onto the composite resulted in the change in its optical absorption property. The proposed device has an integrated light emitting diode and photoresistor in order to measure the change in absorption and hence the detection and quantification of ammonia are performed. The response time of the sensor was found to be linear for a wide range of ammonia concentrations and it is shortest for the thin (100 mu m) composite film. The limit of detection of the proposed device is found to be as low as 2ppm. The proposed sensor platform is also demonstrated for the detection of amino acids. (C) The Author(s) 2018. Published by ECS.