Two-dimensional transition metal carbides/nitrides, known as MXenes, have been recently receiving attention for gas sensing. However, studies on hybridization of MXenes and 2D transition metal dichalcogenides as gas-sensing materials are relatively rare at this time. Herein, Ti 3 C 2 T x and WSe 2 are selected as model materials for hybridization and implemented toward detection of various volatile organic compounds. The Ti 3 C 2 T x /WSe 2 hybrid sensor exhibits low noise level, ultrafast response/recovery times, and good flexibility for various volatile organic compounds. The sensitivity of the hybrid sensor to ethanol is improved by over 12-fold in comparison with pristine Ti 3 C 2 T x. Moreover, the hybridization process provides an effective strategy against MXene oxidation by restricting the interaction of water molecules from the edges of Ti 3 C 2 T x. An enhancement mechanism for Ti 3 C 2 T x /WSe 2 heterostructured materials is proposed for highly sensitive and selective detection of oxygencontaining volatile organic compounds. The scientific findings of this work could guide future exploration of next-generation field-deployable sensors.

Two-dimensional (2D) transition-metal carbides (Ti3C2T x MXene) have received a great deal of attention for potential use in gas sensing showing the highest sensitivity among 2D materials and good gas selectivity. However, one of the long-standing challenges of the MXenes is their poor stability against hydration and oxidation in a humid environment, limiting their long-term storage and applications. Integration of an effective protection layer with MXenes shows promise for overcoming this major drawback. Herein, we demonstrate a surface functionalization strategy for Ti3C2T x with fluoroalkylsilane (FOTS) molecules through surface treatment, providing not only a superhydrophobic surface, mechanical/environmental stability but also enhanced sensing performance. The experimental results show that high sensitivity, good repeatability, long-term stability, and selectivity and faster response/recovery property were achieved by the FOTS-functionalized when Ti3C2T x was integrated into chemoresistive sensors sensitive to oxygen-containing volatile organic compounds (ethanol, acetone). FOTS functionalization provided protection to sensing response when the dynamic response of the Ti3C2T x -F sensor to 30 ppm of ethanol was measured over in the 5 to 80% relative humidity range. Density functional theory simulations suggested that the strong adsorption energy of ethanol on Ti3C2T x -F and the local structure deformation induced by ethanol adsorption, contributing to the gas-sensing enhancement. This study offers a facile and practical solution for developing highly reliable MXene based gas-sensing devices with response that is stable in air and in the presence of water.

Two-dimensional titanium carbide MXenes, Ti 3 C 2 T x , possess high surface area coupled with metallic conductivity and potential for functionalization. These properties make them especially attractive for the highly sensitive roomtemperature electrochemical detection of gas analytes. However, these extraordinary materials have not been thoroughly investigated for the detection of volatile organic compounds (VOCs), many of which hold high relevance for disease diagnostics and environmental protection. Furthermore, the insufficient interlayer spacing between MXene nanoflakes could limit their applicability and the use of heteroatoms as dopants could help overcome this challenge. Here, we report that S-doping of Ti 3 C 2 T x MXene leads to a greater gas-sensing performance to VOCs compared to their undoped counterparts, with unique selectivity to toluene. After S-doped and pristine materials were synthesized, characterized, and used as electrode materials, the as-fabricated sensors were subjected to room-temperature dynamic impedimetric testing in the presence of VOCs with different functional groups (ethanol, hexane, toluene, and hexyl-acetate). Unique selectivity to toluene was obtained by both undoped and doped Ti 3 C 2 T x MXenes, but an enhancement of response in the range of ∼214% at 1 ppm to ∼312% at 50 ppm (3−4 folds increase) was obtained for the sulfur-doped sensing material. A clear notable response to 500 ppb toluene was also obtained with sulfur-doped Ti 3 C 2 T x MXene sensors along with excellent long-term stability. Our experimental measurements and density functional theory analysis offer insight into the mechanisms through which S-doping influences VOC analyte sensing capabilities of Ti 3 C 2 T x MXenes, thus opening up future investigations on the development of high-performance room-temperature gas sensors.

Semiconducting two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are considered promising sensing materials due to the high surface-to-volume ratio and active sensing sites. However, the reported strategies for 2D TMDCs toward sensing of volatile organic compounds (VOCs) present with some drawbacks. These include high operation temperatures, low gas response, and complex fabrication, limiting the development of room-temperature gas sensors. In this study, 2D MoS2 nanoflakes were prepared by liquid-phase exfoliation, and their surface was functionalized with Au nanoparticles (NPs) through a facile solution mixing method. MoS2 decorated with Au NPs with an average size of 10 nm was used as a material platform for an electrochemical sensor to detect a wide variety of VOCs at room temperature. Through dynamic sensing tests, the enhancement of gas-sensing performance in terms of response and selectivity, especially in detecting oxygen-based VOCs (acetone, ethanol, and 2-propanol), was demonstrated. After Au functionalization, the response of the gas sensor to acetone improved by 131% (changing from 13.7% for pristine MoS2 to 31.6% for MoS2-Au­(0.5)). Sensing tests under various relative humidity values (10–80%), bending or long-term conditions, indicated the sound robustness and flexibility of the sensor. Density functional theory simulations suggested that the adsorption energy of VOC molecules on MoS2-Au is significantly higher than that on pristine MoS2, contributing to the gas-sensing enhancement; a VOC-sensing mechanism for Au-decorated MoS2 nanoflakes was proposed for the first time for the highly sensitive and selective detection of oxygen-based VOCs.

Coronavirus disease 2019 (COVID-19) is an emerging human infectious disease caused by severe acute respiratory syndrome 2 (SARS-CoV-2, initially called novel coronavirus 2019-nCoV) virus. Thus, an accurate and specific diagnosis of COVID-19 is urgently needed for effective point-of-care detection and disease management. The reported promise of two-dimensional (2D) transition-metal carbides (Ti 3 C 2 T x MXene) for biosensing owing to a very high surface area, high electrical conductivity, and hydrophilicity informed their selection for inclusion in functional electrodes for SARS-CoV-2 detection. Here, we demonstrate a new and facile functionalization strategy for Ti 3 C 2 T x with probe DNA molecules through noncovalent adsorption, which eliminates expensive labeling steps and achieves sequence-specific recognition. The 2D Ti 3 C 2 T x functionalized with complementary DNA probes shows a sensitive and selective detection of nucleocapsid (N) gene from SARS-CoV-2 through nucleic acid hybridization and chemoresistive transduction. The fabricated sensors are able to detect the SARS-CoV-2 N gene with sensitive and rapid response, a detection limit below 10 5 copies/mL in saliva, and high specificity when tested against SARS-CoV-1 and MERS. We hypothesize that the MXenes’ interlayer spacing can serve as molecular sieving channels for hosting organic molecules and ions, which is a key advantage to their use in biomolecular sensing.