Structure-Induced Selectivity of Hydroxylated Covalent Organic Framework Nanofibers for Advanced Sensing Applications: An Experimental and Density Functional Theory Study

This study reports on the rational design of hydroxyl-functionalized covalent organic framework nanofibers (HO-COFs: PyTA-2,3-NA(OH)₂ and PyTA-2,6-NA(OH)₂) by a scalable solvothermal method. The resulting PyTA-2,3-NA(OH)₂ HO-COF is more hydrophilic than the PyTA-2,6-NA(OH)₂ HO-COF, which can effectively enhance the sensitivity of the sensor toward basic ethylenediamine (EDA). The fabricated HO-COF nanofiber-based quartz crystal microbalance sensor exhibits a rapid sensing response and a distinguished selectivity toward EDA vapor, arising from the strong hydrogen bonding interactions with the NH₂ groups of EDA, as investigated by a wide variety of chemical analysis techniques and density functional theory calculations. The presence of exposed neighboring hydroxyl groups that face the same direction in the PyTA-2,3-NA(OH)₂ HO-COF and the NH₂ groups present in EDA exhibited efficient interactions. The PyTA-2,3-NA(OH)₂ nanofiber with neighboring hydroxyl groups exhibits 1.6 times higher sensitivity to 100 ppm (ppm) EDA than PyTA-2,6-NA(OH)₂ with hydroxyl groups in opposite directions, with a low limit of detection of 2.9 ppm. The PyTA-2,3-NA(OH)₂ nanofiber structure has abundant active neighboring hydroxyl groups facing the same direction, making them favorable active sites for binding EDA molecules through strong hydrogen bond interactions. The color of the HO-COF changed after exposure to EDA vapor, as investigated by colorimetric assessment and naked-eye detection. These HO-COF nanofibers exhibit remarkable selectivity for EDA in the presence of other interfering chemical vapors and show high stability with only a 6.4% drop in sensitivity after 6 months. The adsorption of EDA on PyTA-2,3-NA(OH)₂ nanofibers follows a pseudo-first-order kinetic model, with an adsorption rate about 8.0 times faster than PyTA-2,6-NA(OH)₂ nanofibers. The findings of this study highlight the potential use of COFs, particularly those nanofibers with close neighboring hydroxyl groups, as effective sensing materials for the selective detection of harmful EDA.