MXenes, two-dimensional (2D) transition metal carbides and nitrides, have been arousing interest lately in the field of gas sensing thanks to their remarkable features such as graphene-like morphology, metal-comparable conductivity, large surface-to-volume ratio, mechanical flexibility, and great hydrophilic surface functionalities. With tunable etching and synthesis methods, the morphology of the MXenes, the interlayer structures, and functional group ratios on their surfaces were effectively harnessed, enhancing the efficiency of MXene-based gas-sensing devices. MXenes also efficiently form nanohybrids with other nanomaterials, as a practical approach to revamp the sensing performance of the MXene sensors. This Mini-Review summarizes the recent experimental and theoretical reports on the gas-sensing applications of MXenes and their hybrids. It also discusses the challenges and provides probable solutions that can accentuate the future perspective of MXenes in gas sensors.

In the present work, we report highly sensitive and selective nanosensors constructed with metal-decorated graphene-like BC₆N employing nonequilibrium Green’s function (NEGF) formalism combined by density functional theory (DFT) toward multiple inorganic and sulfur-containing gas molecules (NO, NO₂, NH₃, CO, CO₂, H₂S, and SO₂) as disease biomarkers from human breath. Monolayer sheets of pristine BC₆N and Pd-decorated BC₆N were evaluated for their gas adsorption properties, electronic property changes, sensitivity, and selectivity toward disease biomarkers. The pristine BC₆N nanosheets exhibited sharp drops in the bandgap when interacted with gases such as NO₂ while barely affected by other gases. However, the nanosecond recovery time and low adsorption energies limit the gas sensing applications of the pristine BC₆N sheet. On the other hand, the Pd-decorated BC₆N-based sensor underwent a semiconductor to metal transition upon the adsorption of NO_x gas molecules. The conductance change of the sensor’s material in terms of I–V characteristics revealed that the Pd-decorated BC₆N sensor is highly sensitive (98.6–134%) and selective (12.3–74.4 times) toward NO_x gas molecules with a recovery time of 270 s under UV radiation at 498 K while weakly interacting with interfering gases in exhaled breath such as CO₂ and H₂O. The gas adsorption behavior suggests that metal-decorated BC₆N sensors are excellent candidates for analyzing pulmonary disease and cardiovascular biomarkers, among other ailments of the stomach, kidney, and intestine.

Inspired by prior advancements and the growing utilization of two-dimensional (2D)-based gas sensors, this work presents the potential of black phosphorene for sensing volatile organic compound (VOC) gas molecules. First-principles density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) calculations was employed to evaluate the VOC sensing performance of suspended pristine and Pt-decorated phosphorene materials. Several VOCs, such as acetone, ethanol, formaldehyde, methanol, and toluene, along with CO₂ and H₂O molecules, were considered as the target gases. The results indicated that although the electronic properties of pristine phosphorene were slightly affected by the VOC adsorption, decorating the phosphorene sheet with Pt atoms drastically increased its sensitivity to alcohols. Sensors based on Pt-decorated phosphorene were found to be more sensitive and selective toward methanol among the other considered gas molecules with a gas response of 41.10%. A short recovery time of 0.17 s at room temperature and under UV light was obtained theoretically, and it confirmed the potential application of Pt–phosphorene-based sensors for alcohol detection at room temperature.