ELSEVIER Sensors and Actuators B 40 (1997) 205-209 s S ORS ACTUATORS B CHEMICAL Selective NH 3 gas sensor based on Langmuir-Blodgett polypyrrole film M. Penza a,*, E. MiMla a, M.B. Alba a, A. Quirini a, L. Vasanelli b ~ PASTIS-CNRSM, SS. 7, Appia, tOn 7+300per Mesagne, 72100 Brindisi, Italy u University of Lecce and IME-CNR, Lecce, Via per Arnesano, 73100 Lecce, Italy Received 11 March 1996; received in revised form i4 January 1997; accepted 16 January 1997 Abstract Polypyrrole thin films have been deposited onto a glass substrate by the Langmuir-Blodgett technique to fabricate a selective ammonia ('NI'{3) gas sensor. The d.c. electrical resistance of the sensing elements is found to exhibit a specific increase upon exposure to different gases such as NH> CO, CI-{4, H2 in N2 and pure Oz. The polypyrrole thin-film detector shows a considerable increase of resistance when exposed to NH3 in N2, and negligible response when exposed to comparable concentrations of interfering gases such as CO, CH4, H a in N2 and pure O> The calibration curve for NH3 in Na at room temperature is measured in the concentration range from 0.01 to 1%. The relative change of the electrical resistance is about 10% for the lower detectable limit of 100 ppm of NH3 in N2. The sensitivity of the Lan~dnuir-Blodgett polypyrrole towards ammonia is considerably higher than that of the electrochemical polypyrrole. The fast rise time and the high sensitivity of the detector are reported as a function of number of the polypyrrole layers. Long-term aging tests of the seiective NH3 gas sensor are performed. © 1997 Elsevier Science S.A. Keywords: Ammonia gas sensor; Polypyrrole thin film; Langmuir-Blodgetttechnique 1. Introduction The detection of the NH3 gas is an important task in many technological fields such as industrial processes, clinical diagnosis, environmental monitoring. Many gas sensors based on metal-oxide thin films [1,2] and/or thick films [3,4] with dopants and catalyzers [5,6] have been developed, but they need elevated working tem- perature in the range from 250 to 500°C in order to activate the adsorption and desorption phenomena of the test gases. On the contrary, the organic films and in particular the conductive polymers such as the polypyrrole have great advantages in comparison to metal-oxide devices for their higher sensitivity towards toxic gases, for the lower detectable limit in the range of few tens of ppm, and for their possibility to operate at or near room temperature [7-10]. In the oxidised form, the polypyrrole is a p-type semiconductor. When it is exposed to electron-donating * Corresponding author. Tel.: +39 83I 507374; fax: +39 83i 507379; e-mail: Penza@ENRSM.IT 0925-4005/97/$i7.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-4005(97)00066-X gases, such as ammonia, a redox reaction occurs and its electronic density varies. Generally, the direct electro- chemical synthesis of the conducting polypyrrole is the most used technique [11,12] to prepare gas sensing elements, because it is convenient to study materials in thin solid film form. Considering that the Langmuir-Blodgett technique allows to prepare chemically sensitive ultrathin films with an excellent molecular order and controlled thick- ness. We have prepared thin film polypyrrole by solid state reactions, modifying a preformed Langmuir-Blod- gett film by exposure to gaseous reactants. The selectivity of a single sensing element towards a gaseous analyte remains an open problem due to the cross-sensitivity of the chemical interface towards the interfering analytes. Usually, a simple sensitive layer cannot discriminate a test gas from the counterpart gases, except for some exclusive cases. An ongoing approach to solve the problem of the selectivity is the use of an array of non-selective sensing elements and the exploit of the pattern recognition techniques to identify an unknown test gas. If realized, the solution is effective but sophisticated.