Enhancement in room temperature ammonia sensing properties of naphthalene diimides through core expansion

Abstract

An amperometric type sensor whose active layer is derived from a tetra core-substituted organic semiconductor, naphthalene diimide (NDI-CN₄), has been evaluated for ammonia gas (3, 6, 25 and 50 ppm) sensing at room temperature against the parent NDI devoid of core-substitution effects. In empirical terms, the tetracyano derived sensor showed high sensitivity (178% at 50 ppm NH₃) and selectivity, quick response and recovery (12.5 s/39 s), low limit of detection (3 ppm), and importantly, excellent recyclability and stability. Density functional theory and cyclic voltammetry studies indicate that the incorporation of strong electron-withdrawing groups, i.e. cyano groups, causes a substantial decrease in the reduction potential of the NDI-CN₄ leading to a significant rise in the current of the sensor, mainly due to an efficient charge-transfer between NDI-CN₄ and ammonia. Furthermore, because of the low-lying LUMO energy level, the NDI-CN₄-based sensor shows high stability even with prolonged exposure to ambient air. A comparison with the corresponding core-unsubstituted NDI derivative confirms the positive cooperative effect of core-cyanation on gas sensing performance. It is anticipated that this work could give some useful information for preparing high performance organic sensing devices.