Surface-Engineered Ti3C2Tx and Nb2CTx MXenes for Nitric Oxide Detection

The development of compact and low-cost gas sensors for breath analysis is attracting increasing attention as a non-invasive strategy for diagnosing and monitoring respiratory diseases. Nitric oxide (NO) is a clinically validated biomarker of airway inflammation, while relative humidity (RH) strongly influences sensing performance in exhaled breath. Two-dimensional transition metal carbides (MXenes) are promising chemoresistive materials due to their high electrical conductivity, tunable surface terminations, and layered architecture enabling interfacial transport modulation. Here, Ti3C2Tx and Nb2CTx were synthesized via HF-based selective etching and functionalized with 3-mercaptopropyltrimethoxysilane (MPTMS). MPTMS was selected because sulfur-based terminations are theoretically predicted to enhance interaction with nitrogen-containing species such as NO, potentially promoting adsorption and interfacial charge redistribution. Structural and chemical characterizations (XRPD, FTIR, SEM/TEM, EDX) confirmed successful MXene formation and effective silane grafting without structural degradation. Chemoresistive devices were fabricated by spray-coating identical amounts of stable suspensions onto interdigitated electrodes, enabling nominally comparable film thicknesses, and tested at room temperature under controlled RH and NO concentrations (ppm to ppb level). Pristine Ti3C2Tx exhibited a decrease in current upon NO exposure. Considering NO as an oxidizing radical, electron withdrawal from the metallic MXene reduces carrier density, therefore causing a resistance increase. Conversely, Nb2CTx-based sensors displayed a resistance decrease under NO exposure. Despite identical deposition mass, the smaller flake size (around 200 nm by TEM) and distinct electronic structure of Nb2CTx likely promote stronger interfacial coupling among flakes, and more effective junction-barrier lowering upon NO adsorption. In this case, interfacial charge redistribution and barrier modulation dominate over carrier depletion, resulting in net conductivity enhancement. MPTMS functionalization further improves NO response for both MXenes thanks to the possible establishment of S---N interactions. These findings highlight that MXene sensing behaviour arises from the interplay between intrinsic electronic structure, morphology, and surface properties.