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 recent successes in the development of two-dimensional based gas sensors capable of single gas molecule detection, we investigate the adsorption of gas molecules (N₂, NO, NO₂, NH₃, CO, CO₂, CH₄, SO₂, and H₂S) on silicene nanoribbons (SiNRs) using density functional theory (DFT) and nonequilibrium Green's function (NEGF) methods. The most stable adsorption configurations, adsorption sites, adsorption energies, charge transfer, quantum conductance modulation, and electronic properties of gas molecules on SiNRs are studied. Our results indicate that NO, NO₂, and SO₂ are chemisorbed on SiNRs via strong covalent bonds, suggesting its potential application for disposable gas sensors. In addition, CO, NH₃, and H₂S are chemisorbed on SiNRs with moderate adsorption energy, alluding to its suitability as a highly sensitive gas sensor. The quantum conductance is detectably modulated by chemisorption of gas molecules which can be attributed to the charge transfer from the gas molecule to the SiNR. Other studied gases are physisorbed on SiNRs via van der Waals interactions. It is also found that the adsorption energies are enhanced by doping SiNRs with either B or N atoms. Our results suggest that SiNRs show promise in gas molecule sensing applications.

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.

In this paper, we report the development of the nanotube-CTC-chip for isolation of tumor-derived epithelial cells (circulating tumor cells, CTCs) from peripheral blood, with high purity, by exploiting the physical mechanisms of preferential adherence of CTCs on a nanotube surface. The nanotube-CTC-chip is a new 76-element microarray technology that combines carbon nanotube surfaces with microarray batch manufacturing techniques for the capture and isolation of tumor-derived epithelial cells. Using a combination of red blood cell (RBC) lysis and preferential adherence, we demonstrate the capture and enrichment of CTCs with a 5-log reduction of contaminating WBCs. EpCAM negative MDA-MB-231/luciferase-2A-green fluorescent protein (GFP) cells were spiked in the blood of wild mice and enriched using an RBC lysis protocol. The enriched samples were then processed using the nanotube-CTC-chip for preferential CTC adherence on the nanosurface and counting the GFP cells yielded anywhere from 89% to 100% capture from the droplets. Electron microscopy (EM) studies showed focal adhesion with filaments from the cell body to the nanotube surface. We compared the nanotube preferential adherence to collagen adhesion matrix (CAM) scaffolding, reported as a viable strategy for CTC capture in patients. The CAM scaffolding on the device surface yielded 50% adherence with 100% tracking of cancer cells (adhered vs. non-adhered) versus carbon nanotubes with >90% adherence and 100% tracking for the same protocol. The nanotube-CTC-chip successfully captured CTCs in the peripheral blood of breast cancer patients (stage 1–4) with a range of 4–238 CTCs per 8.5 ml blood or 0.5–28 CTCs per ml. CTCs (based on CK8/18, Her2, EGFR) were successfully identified in 7/7 breast cancer patients, and no CTCs were captured in healthy controls (n = 2). CTC enumeration based on multiple markers using the nanotube-CTC-chip enables dynamic views of metastatic progression and could potentially have predictive capabilities for diagnosis and treatment response.

The popularity of graphene owing to its unique and exotic properties has triggered a great deal of interest in other two-dimensional nanomaterials. Among them silicene shows considerable promise for electronic devices with a carrier mobility comparable to graphene, flexible buckled structure, and expected compatibility with silicon electronics. Using first-principle calculations based on density functional theory, the electronic properties of armchair silicene nanoribbons perforated with periodic nanoholes (ASiNRPNHs) are investigated. Two different configurations of mono-hydrogenated (:H) and di-hydrogenated (:2H) silicene edges are considered. Pristine armchair silicene nanoribbons (ASiNRs) can be categorized into three branches with width W = 3P − 1, 3P, and 3P + 1, P is an integer. The order of their energy gaps change from “E_G (3P − 1) < E_G (3P) < E_G (3P + 1)” for W-ASiNRs:H to “E_G (3P + 1) < E_G (3P − 1) < E_G (3P)” for W-ASiNRs:2H. We found the band gaps of W-ASiNRs:H and (W + 2)-ASiNRs:2H are slightly different, giving larger band gaps for wider ASiNRs:2H. ASiNRPNHs' band gaps changed based on the nanoribbon's width, nanohole's repeat periodicity and position relative to the nanoribbon's edge compared to pristine ASiNRs because of changes in quantum confinement strength. ASiNRPNHs:2H are more stable than ASiNRPNHs:H and their band gaps are noticeably greater than ASiNRPNHs:H. We found that the value of energy band gap for 12-ASiNRPNHs:2H with repeat periodicity of 2 is 0.923 eV. This value is about 2.2 times greater than pristine ASiNR:2H and double that of the 12-ASiNRPNHs:H with repeat periodicity of 2.