AC and DC measurements on Langmuir-Blodgett polypyrrole films for selective NH3 gas detection

The Langmuir-Blodgett method has been used to deposit onto quartz substrate polypyrrole film as gas sensitive coating. The AC and DC electrical resistance and capacitance of the sensing device have been investigated under controlled ambient conditions, at room temperature, during gas exposures of NH3, CH4, CO in dry air. The AC capacitance of the polypyrrole sensor was not affected by any test gases throughout the examined frequency range (20 Hz to 1 MHz). The AC and DC electrical resistance showed a highly sensitive response towards NH3 and negligibly small response towards interfering CH4 and CO: the sensitivity and selectivity of the NH3 detector increased with the decreasing AC frequency obtaining the optimal working conditions in DC mode. The calibration curve of the sensor towards NH3 in the gas concentration range 100-10 000 ppm has been performed at different frequencies (DC to 1 MHz) indicating a linear relationship between AC and/or DC resistance and NH3 concentration. Long-term stability tests have been carried out onto a polypyrrole detector exhibiting fairly reproducible responses towards NH3 during a six-month period. An efficient recognition of NH3 as one component of a gas mixture containing CO, CH4, O-2 in air has been successfully recorded.