Functionalized MXene Chemiresistors as Sensors for Humidity and NO in Breath Diagnostics

Introduction The development of reliable and low-cost gas sensors for breath analysis is gaining increasing attention as a non-invasive diagnostic tool for respiratory diseases [1-3]. Among gaseous biomarkers, nitric oxide (NO) is particularly relevant, as its concentration in exhaled breath correlates with airway inflammation [4,5]. Chemiresistive sensors based on 2D materials, such as MXenes, represent a promising alternative to conventional semiconductor metal oxides due to their high electrical conductivity, large surface area, and room-temperature operation [6,7]. However, their intrinsic hydrophilicity and limited selectivity remain critical challenges. Surface functionalization with organic molecules, such as 3-mercaptopropyltrimethoxysilane (MPTMS), offers a viable strategy to modulate both surface chemistry and electronic properties. In this work, Ti3C2Tx and Nb2CTx MXenes, both pristine and MPTMS- functionalized (20%), were investigated as chemiresistive sensors for NO detection under variable relative humidity (RH), with a focus on room-temperature operation. 2. Physicochemical characterization The successful synthesis and functionalization of MXenes were confirmed through XRPD, FT-IR, and TEM analyses. XRPD patterns show the characteristic (002) reflection of layered MXenes (Figures 1a,b). Upon functionalization, a shift toward lower diffraction angles is observed, indicating an increase in interlayer spacing due to the insertion of MPTMS molecules. This confirms effective intercalation and surface modification. FTIR spectra further support the functionalization process (Figures 1c,d). In both Ti3C2Tx and Nb2CTx, new vibrational bands appear in the 1000-1150 cm-1 region, assigned to Si–O–Si and Si–O–M (M = Ti, Nb) bonds, along with C–H stretching modes around 2900 cm-1. The decrease in –OH-related bands suggests partial replacement or shielding of surface terminations. TEM analysis reveals the typical layered morphology of MXenes (insets of Figure 1). Ti3C2Tx exhibits large, flexible flakes (micrometer scale), while Nb2CTx consists of smaller, more compact flakes (hundreds of nanometers). Functionalization does not significantly alter morphology but promotes slight aggregation and structural modification at the nanoscale. Gas sensing tests were performed at room temperature under different relative humidity conditions (10-85% RH) using 5 ppm NO as the reference concentration. The sensor response was defined as: S = Rair/RNO - 1 Distinct sensing behaviors were observed depending on the MXene composition and functionalization. In particular, pristine and modified-Ti3C2Tx exhibit a sensing response consistent with a p-type-like semiconducting behavior. In this case, exposure to NO (a reducing gas [8]) leads to a decrease in hole concentration, resulting in an increase in resistance. This trend is consistent with established models for p-type semiconductors. Conversely, both pristine and functionalized Nb2CTx exhibit positive responses (increase in conductance) upon NO exposure, indicating n-type behavior. According to the literature on n-type semiconductors such as WO3, reducing gases like NO donate electrons to the material, decreasing resistance. Functionalization with MPTMS significantly enhances the response amplitude, likely due to both reduced hydrophilicity (limiting interference from water vapor), and specific interactions between thiol groups (–SH) and NO molecules. All sensors demonstrate measurable responses down to 5 ppm NO at room temperature, with preliminary results indicating a limit of detection approaching 0.1 ppm, which is highly relevant for breath analysis applications. The ability to operate at RT represents a significant advantage compared to conventional metal oxide sensors, which typically require elevated temperatures (200- 400 °C) to achieve comparable performance. Lastly, concerning the humidity interference, it strongly influences sensor response. However, functionalized samples exhibit reduced humidity interference, particularly for Nb2CTx, confirming the role of MPTMS in modulating surface hydrophilicity. 4. Conclusions This study demonstrates that surface functionalization of MXenes with MPTMS is an effective strategy to tune both sensing performance and electronic behavior. Specifically, Ti3C2Tx exhibited a p-type behavior toward NO sensing, whereas Nb2CTx showed an n-type behavior, in both cases with enhanced sensitivity after modification. Moreover, MPTMS reduced humidity interference and improved NO interaction, likely via S–NO interactions, and detection at sub-ppm levels was achievable at room temperature, highlighting strong potential for breath diagnostics. These findings provide new insights into the design of MXene-based chemiresistors and open the way for portable, low-power gas sensing devices.