Irreversible adsorption behavior of CuS nanoplate-based QCM sensors toward aqueous Ammonia: Adsorption kinetics and isotherm insights

In this study, the ammonia (NH₃/NH₄⁺) sensing performance of two-dimensional covellite phase copper sulfide (CuS) nanoplates in aqueous media was investigated for the first time using QCM-based sensors. Hydrothermally synthesized CuS nanoplates were characterized by X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscope (FE-SEM) analyses. The CuS-coated quartz crystal microbalance (QCM) electrode exhibited clear and concentration-dependent frequency shifts ranging from approximately 1.4 to 2 MHz for NH₃ concentrations between 18.25 and 91.25 ppm. It was observed that the frequency shifts were related to NH₃ adsorption, and at higher concentrations, irreversible adsorption became dominant on the surface. UV–vis measurements showed these findings, and chemical changes in the solution pointed to the adsorption mechanism. The BET analysis also confirmed the mesoporous nature of the CuS nanoplates, providing a suitable surface for NH₃ adsorption and correlating well with the concentration-dependent QCM frequency shifts. The Elovich model showed the best fit in kinetic analyses, and the Langmuir model in isotherm analyses. These results demonstrate that both monolayer and heterogeneous adsorption behaviors are effective on the CuS surface. The findings provide a basis for the development of next-generation, low-cost, and portable CuS-based sensors that can be used in areas such as environmental monitoring, water quality control, and nuclear waste management.