A team of researchers recently published a paper in the journal Sensors and Actuators: A. Physical that reviewed the applicability of two-dimensional (2D) nanomaterials as breath analyzers for disease diagnosis.
Study: Emergence of two-dimensional nanomaterials-based breath sensors for non-invasive detection of diseases. Image Credit: TY Lim/Shutterstock.com
2D Nanomaterials in Sensor Design
Human metabolism leads to the emission of minuscule quantities of inorganic species and several volatile organic compounds (VOCs) such as isoprene, ethanol, and methanol, which are exhaled through pulmonary ventilation. The combination of VOCs in exhaled gases can be considered as breath signatures.
Breath signature comparison between a healthy person and a sick person can potentially act as biomarkers for specific diseases as such comparison can depict the current metabolic state of an individual, which can help in accurate analyses and early diagnoses of diseases.
Exhaled breath analysis is one of the safest, most convenient, and non-invasive techniques to assess patient health conditions. However, the significant presence of water vapor and carbon dioxide in the exhaled breath often hinders the detection of certain trace compounds, which necessitates the formulation of methods or fabrication of devices that can effectively detect these trace compounds, specifically disease biomarkers, with high accuracy and sensitivity. For instance, robust, flexible, and wearable gas sensing devices with high stability and sensitivity can facilitate real-time monitoring and early detection of disease.
2D nanomaterials have gained considerable attention in sensing-related applications owing to their exceptional properties such as extremely high carrier mobility and surplus adsorption sites. The evolution of 2D nanomaterials into a single layer form has further increased their feasibility in sensing applications, specifically for gas sensing.
In this study, researchers reviewed the characteristics and sensing mechanism of functionalized 2D nanomaterials, 2D material-based nanocomposites, and pristine 2D materials, as well as the opportunities and challenges of using 2D nanomaterial-based gas sensors for disease diagnosis.
These semiconductors are primarily composed of conductive polymers with a high degree of conjugation and exhibit p-type characteristics. The gas sensing mechanism in organic semiconductors depends on the change in their conductivity when the doping state and carrier density of polymers are controlled after the material comes into contact with the target gases. Organic semiconductors such as doped polypyrrole can detect gases such as ammonia and hydrogen sulfide.
Traditionally, bulk metal oxides were used extensively for gas sensing applications. However, advancements in nanotechnology have led to the development of nanostructures with intrinsic defects and enormous surface area, which facilitated the detection of gas at extremely low concentrations.
Wide bandgap semiconductors are the most commonly used metal oxides for gas sensing applications. In these semiconductors, the majority of carriers are determined by the non-stoichiometry and the dopant type during their fabrication.
N-type metal oxides are often preferred over p-type metal oxides in most sensor-related applications due to their higher charge carrier mobility, which facilitates higher sensitivity and quick response and recovery.
Pristine graphene possesses a single layer atomic structure composed of carbon atoms organized in a hexagonal honeycomb lattice form. These materials demonstrate good gas sensing performance owing to their extremely high electron mobility and large surface area.
Transition Metal Dichalcogenides (TMDs)
Type MX2 semiconductors are referred to as TMDs, where M represents a transition metal atom and X represents a chalcogen atom. Among the TMDs, molybdenum disulfide (MoS2) and tungsten disulfide (WS2) demonstrated decent gas sensing characteristics at room temperature owing to their atomically thin structures that increase the number of sites for adsorption of target gases.
Black Phosphorous (BP)
BP comprises a skeleton of orthorhombic crystal structure and puckered hexagonal domains. BP displays good gas sensing performance due to its high specific surface area and carrier mobility.
A novel type of 2D material, MXene consists of transition metal carbonitrides, carbides, and nitrides. MXene-based gas sensors display extremely high sensitivity and low electrical noise while detecting gas molecules.
Metal-organic Frameworks (MOFs)
Metal-organic frameworks constitute conjugated organic ligands arranged around the metal centers to take the shape of crystals. MOFs such as copper 2-hexahydroxytriphenylene (Cu3(HHTP)2) are used for gas sensing applications as the Cu metal centers improve the gas sensing abilities.
Metal Oxide Nanosheets (MONs)
MONs such as nanoparticles and nanotubes are typically used to build semiconductor gas sensors, with MONs with 2D geometry displaying significant potential in low-level gas detection.
Additional 2D Materials
Recent studies demonstrated that 2D layered materials such as gallium selenide (GaSe) and tin sulfide (SnS2) have considerable potential in gas sensing applications due to their high surface-to-volume ratio and temperature-dependent electronic band structure, respectively.
2D Material-based Composites
Recently, 2D material-based composites fabricated with 2D structures, such as metal nanoparticles and organic polymers, gained significant attention owing to their better gas sensing performance compared to their pristine forms.
Synthesis Techniques Used to Fabricate 2D Materials as Breath Analyzers
Exfoliation techniques, chemical vapor deposition, atomic layer deposition, and wet chemical approach are primarily used to fabricate 2D materials as breath analyzers for disease diagnosis.
Existing Gas Sensors as Breath Analyzers
Commonly used gas sensors that can be developed to detect disease-related biomarkers in exhaled breath include conductometric sensors, field-effect transistors, impedance-based sensors, and surface acoustic wave-based sensors.
Detection of VOCs as Disease Biomarkers
The presence of certain VOCs in exhaled air can act as disease biomarkers. For instance, the detection of acetone indicates diabetes mellitus, detection of ammonia indicates renal failure, hydrogen sulfide indicates hepatic disease, and large quantity VOCs indicate lung cancer.
Opportunities and Challenges
Among all 2D materials, graphene exhibits the highest sensitivity and selectivity at room temperature. Thus, ternary and binary graphene-based hybrids can be developed to detect disease biomarkers in exhaled breath in the future. Additionally, TMDs can also be synthesized in a way that improves the disease biomarkers in exhaled breath.
However, slow recovery and response, the presence of contaminants in the sensing layer, and issues with selectivity and stability are some of the major challenges associated with 2D materials. Moreover, more research is required to find optimal ways for the practical implementation of these 2D material-based sensors for disease biomarker detection.
Sett, A., Rana, T., Sha, R. et al. (2022) Emergence of two-dimensional nanomaterials-based breath sensors for non-invasive detection of diseases. Sensors and Actuators: A. Physical. https://www.sciencedirect.com/science/article/pii/S0924424722001455?via%3Dihub