Unraveling Ammonia and Trimethylamine Uptake on Conductive Doped Polyaniline

Polyaniline (PAni)-based sensors are a promising solution for ammonia (NH₃) detection at the ppb level. However, the nature of the NH₃-PAni interaction and underlying drivers remain unclear. This paper proposes to characterize the interaction between doped PAni (dPAni) sensing material and NH₃ by using a Knudsen cell. First, to characterize the dPAni interface, the probe-gas method, i.e., titration of surface sites with a gas of specific properties, is deployed. The dPAni interface is found to be homogeneous with more than 96% of surface sites of acid nature or with hydroxyl functional groups. This result highlights that basic gases such as amines might act as interfering gases for NH₃ detection by polyaniline-based sensors. Second, the adsorption isotherms of NH₃ and trimethylamine (TMA) on dPAni are reported at ambient temperature conditions, 293 K. The uptake of NH₃ and TMA on dPAni follows a Langmuir-type behavior. This approach allows for the first time to quantify the uptake of NH₃ and TMA on gas-sensor materials and determine typical Langmuir adsorption parameters, i.e., the partitioning coefficient, K_Lang, and the maximum surface coverage, N_max. The corresponding values obtained for NH₃ and TMA are K_lang (NH₃) = 19.7 × 10^-15 cm³ molecules^-1 N_max (NH₃) = 11.6 × 10¹⁴ molecules cm^-2, K_Lang (TMA) = 7.0 × 10^-15 cm³ molecules^-1 N_max (TMA) = 5.0 × 10¹⁴ molecules cm^-2. K_Lang and N_max values of NH₃ are higher than those of TMA, suggesting that NH₃ is more efficiently taken up than TMA on dPAni. The results of this work suggest that strong hydrogen bonding drives the performance of a polyaniline-based gas sensor for NH₃ and amines. In conclusion, the Knudsen cell approach allows reconsidering the fundamentals of NH₃ interactions with dPAni and provides new insights on drivers to enhance sensing properties.