Receptor‐Free Identification of Toxic Gases Enabled by Hygroscopic Aqueous Salt Films

ABSTRACT A large class of toxic compounds interacts strongly with water, yet gas sensors rarely exploit this chemistry because water evaporates rapidly under ambient conditions. Here, we demonstrate that hygroscopic aqueous salt films—formed from lithium chloride (LiCl), lithium bromide (LiBr), and phosphoric acid (H ₃ PO ₄ )—remain stable as liquid coatings on carbon nanotube (CNT) chemiresistors and enable receptor‐free identification of diverse toxic gases. Owing to their extremely low deliquescence relative humidity (DRH < 15%), these aqueous films persist in ambient air and create distinct water‐mediated interfacial microenvironments whose interactions with analytes produce highly salt‐dependent resistance responses. Using a simple four‐channel array consisting of one pristine CNT device and three devices coated with different hygroscopic films, we show that both aqueous‐phase transforming analytes (e.g., oxalyl chloride and oxalyl bromide) and primarily partitioning (non‐transforming) analytes (e.g., dimethyl methylphosphonate) generate unique response signatures that allow accurate identification without molecular receptors. These findings show that hygroscopic aqueous salt films on nanocarbon sensing layer provide a simple and broadly applicable platform for receptor‐free toxic gas sensing in ambient air, including industrial toxic chemicals and chemical warfare agents.