Highly Sensitive Detection of Ammonia Using Solution Processable Polyaniline/Carbon Black Composite Sensors

A highly sensitive ammonia sensor has been fabricated by depositing solution processable polyaniline/carbon black conductive composites on to flexible plastic substrates with gold interdigitated electrodes. Polyaniline (PANI) is known to be an excellent sensing material for ammonia, since NH3 deprotonates the amine group in emeraldine salt (doped form of PANI), converting it to emeraldine base (undoped form) which may cause a significant decrease in conductivity. The very high affinity of PANI to ammonia has its drawbacks, mainly with regard to sensor recovery where long timescales are required to return the sensor to its baseline. Operation at elevated temperatures can improve desorption of NH3 molecules from the sensor film, hence improving the recovery time. Unfortunately, lack of solution processability, the inherent problem with conventional PANI especially in its doped stat, has limited implementation of this material in conventional sensor fabrication methods. Here, we have exploited sulphosuccinic acid (SSA), a multifunctional dopant with sulphonic and carboxylic acid groups, which not only renders the doped PANI soluble in aprotic solvents such as n-methyl pyrrolidone (NMP), but also makes it possible to prepare conductive composites of PANI/carbon black (CB) simply by mixing surface modified carbon black with PANI solution. Sensors were fabricated by depositing a thin layer of PANI/CB dispersion in NMP via dip coating on flexible polyimide and polyethylene naphthalate substrates incorporating a heater. Sensor response to very low concentrations of ammonia vapour at 80 ˚C was measured. Due to high affinity of polyaniline to ammonia and presence of conductive pathways of carbon black through out the composite layer, the sensor exhibits sensitive and fast response down to ppb concentrations. The high sensitivity, rapid response and good reversibility of the sensor make it promising for real-time ammonia sensing applications.