~~ MATERIALS SCIENCE & ENGINEERING ELSEVIER Materials Science and EngineeringC 5 (1998) 255-258 AC and DC measurements on Langmuir-Blodgett polypyrrole films for selective NH3 gas detection M. Penza *, E. Milella, F. Musio, M.B. Alba, G. Cassano, A. Quirini PASTIS-CNRSM, SS. 7, Appia, K~ 7 + 300 per Mesagne, 72100 Brindisi, haly Received 17 October 1996 Abstract The Langmuir-Blodgett method has been used to deposit onto quartz substrate polypyn'ole 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 10 I MHz). The AC and DC electrical resistance showed a highly sensitive response towards NH~ 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-I0 000 ppm has been performed at different frequencies (DC to 1 MHz) indicating a linear relationship between AC and/or DC resistance and NH~ concentration. Long-term stability tests have been cmTied out onto a polypyrrole detector exhibiting fairly reproducible responses towards NH3 during a six-month period. An efficient recognition of NH.~ as one component of a gas mixture containing CO, CH4, O_~in air has been successfully recorded. © 1998 Elsevier Science S.A. Keywords: Polypyrrole film; Langmuir-Blodgett technique: Ammonia gas sensor 1. Introduction In recent years, organic compounds have been investigated as vapour and gas sensing interfaces of solid-state devices [1-3]. The conducting polymers (polypyn'ole, polythio- phene, polyacetylene, etc.) are the most studied due to their reversible and highly sensitive response at room temperature and low Iimit of detection. These systems are being used in a number of application areas, including transportation, robotics, industriaI manufacturing, consumer electronics, health care, biomedicine, agro-industriaI sustainable improvement. It is well known that ammonia sensing is a task with a large demand for practicaI purposes. The Langmuir-Blodgett (LB) method perlnits the depo- sition of polypyrrole films with ultrathin thickness and well- ordered structure in order to selectively detect ammonia using a single sensing element [4]. While the DC electrical characteristics of the polypyrrole detector have been well studied [5], the AC parameters of resistance and capacitance of the sensing device are less well known. Therefore, the measurements at AC frequency of electrical parameters of the sensor may be useful to under- * Corresponding author. 0928-4931/98/S19.00 © 1998 Elsevier Science S.A. All rights reserved PILS0928-4931 (97) 00057-X stand the chemical interaction mechanisms between the adsorbed gas molecules and the sensitive coating and to dis- criminate a specific analyte from its interfering species [6]. This paper focuses on comparison between AC and DC electrical responses of the LB polypyrrole film sensor for selective detection of ammonia gas. 2. Experimental details 2.1. Synthesis of Langmuir-Blodgett polypyrrole jSIm The LB method was used for film deposition. A solution ( I mg mI- t) of stearic acid in chloroform was spread out onto a subphase of deionized water containing 4× 10 -s M ferric chloride. The subphase pH and temperature were kept constant at 5.6 and 15 °C (+0.02 °C), respectively. The pressure-area ('a-A) isotherms of the film deposition were performed by a KSV LB5000 apparatus ensuring high accu- racy of the surface pressure ( _+4 p~N m- ~ ) and the area per molecule ( > 99%). The film was compressed with a barrier rate of 5 mm min-~ up to a pressure of 35 mN m- 1, the dipping rate was 1 mm min- t and the drying time was about 60 min. A determined number of monolayers of feITiC stearate