Remote detection of gaseous ammonia using the near infrared transmission properties of polyaniline

Ammonia is a key component of many industrial processes where it is used in very high concentrations. The applications range from high quality steel production and fertiliser manufacture, to the refrigeration of food products and ice ring leisure facilities. Ammonia escapes have been identified as a large and serious problem by both government and industry. Simple and robust ammonia sensors for remote monitoring applications remain an area of continuous interest.

Polyaniline is a conducting polymer used in ‘electronic nose’ instrumentation and has been shown to be electronically sensitive to ammonia. In this paper, we report on the application of electrochemically-prepared polyaniline films for the measurement of gaseous ammonia at 1300 nm. This wavelength corresponds to a region of high optical transmission for optical fibres and is also compatible with telecom devices and technology. A simple and robust measurement system based on a standard telecom 1300 nm LED is described and remote sensing using 100 m of duplex multimode fibre is demonstrated.

Typically, the transmission of the polyaniline films at 1300 nm increases by approximately 1% in response to gaseous ammonia levels of 6 ppm in 50% RH. This represents the lower limit of detection in our study. Although initially the sensors react to the presence of ammonia very fast it takes over several hours for the output to reach the equilibrium. Clearly this is impractical however, it is possible to differentiate between different concentrations of ammonia by taking two readings at fixed intervals. A calibration curve for the sensors was obtained using two readings 15 s apart. The response time of the polyanaline films was found to be insensitive to the humidity variations in the range of 30–70% RH, however, over the 10–90% variation the changes were of the same order of magnitude as those induced by 6 ppm of ammonia.