g-C₃N₄/CoN₄ Heterojunction as a Sensor for Detecting Volatile Organic Compounds in Exhaled Human Breath: A Density Functional Study

Utilizing density functional theory (DFT) calculations, we investigated the adsorption behavior of key exhaled breath biomarkers - benzene (bz), toluene (bz-CH₃), aniline (bz-NH₂), and o-toluidine (NH₂-bz-CH₃) - on both pristine g-C₃N₄ and g-C₃N₄/CoN₄ composite surfaces. By analyzing the key parameters such as adsorption energy, electronic density of states (DOS), band structures, charge density difference, conductivity and work function we gained the critical insights into the interaction mechanisms between the gas molecules and the sensor surfaces. Our results reveal that the incorporation of CoN₄ significantly enhances the chemical reactivity and stability of the g-C₃N₄ substrate, further improving upon gas adsorption. Notably, benzene, toluene, and aniline exhibit reversible adsorption behavior on g-C₃N₄/CoN₄, highlighting their suitability for reusable gas sensor applications. In contrast, o-toluidine shows irreversible binding, potentially limiting its reusability. Aniline exhibits lowest band gap and highest conductivity, highest sensitivity and strongest orbital hybridization, confirming g-C₃N₄/CoN₄ substrate acting as a best sensor towards aniline gas molecule, among the investigated molecules. Band structure analysis of aniline adsorbed on g-C₃N₄/CoN₄ heterostructure further confirms that the composite exhibits improved electrical conductivity, even at room temperature, reinforcing its potential for real-time biomarker sensing in human breath analysis. The emergence of g-C₃N₄/CoN₄ heterostructure as a highly promising material for the selective and sensitive detection of aniline vapors is still illusive.