Polylactic acid/graphene oxide/nickel oxide electrode for ammonia sensing

Despite advancements in ammonia (NH₃) sensing, existing metal oxide-based sensors often require high operating temperatures, lack biodegradability, and atomic-scale understanding of their adsorption mechanisms. Polylactic acid (PLA) modified with graphene oxide (GO), forming a PLA/GO composite is proposed to interact with nickel oxide (NiO) through two different schemes. Density functional theory (DFT) calculations at the B3LYP/LANL2DZ level were performed for geometry optimization, frequency calculations, and the determination of total dipole moment (TDM), HOMO–LUMO orbitals, energy gap (∆E), molecular electrostatic potential (MESP), density of states (DOS) and projected density of states (PDOS). Adsorption energy for interacting between the composite and the NH₃ was calculated, along with global reactivity descriptors. Results indicated that only the Ni-mediated PLA/GO/NiO interaction with NH₃ converged at this level of theory. A high TDM of 6.952 Debye, indicating polar material, and the change of ∆E from 2.093 eV to 1.374 eV, combined with frontier HOMO/LUMO orbitals and Mulliken analysis, confirm enhanced charge transfer upon interacting with NH₃. The negative value of adsorption energy (E_a = −0.448 eV) indicate selectivity for NH₃ interaction. The positive frequency values confirm a true optimized structure, with characteristic Infra-red (IR) bands aligning well with the experimental IR data. Additionally, quantum theory of atoms in molecules (QTAIM) analysis was performed to confirm the structural stability, revealing critical points, bond paths and the type of bond formed upon adsorption. Non-covalent interactions (NCI) analysis with reduced density gradient (RDG) was performed to visualize the physical interactions within the composite and with NH₃. By elucidating these atomic-scale mechanisms, these findings highlight the potential of PLA/GO/NiO as selective, efficient and sustainable PLA-based sensing applications for NH₃.