Fabrication, Characterization, and Optimization of Robust Sensing Materials for MEMS LC Resonators in Harsh Aqueous Environments

Sensing in harsh aqueous environments remains a critical challenge due to the limited lifetime and reliability of conventional electrode-based sensors. While laboratory-based techniques such as mass spectrometry, infrared spectroscopy, Raman spectroscopy, chromatography, filtration, multiple fermentation tube techniques, DNA amplification, capillary electrophoresis, field-flow fractionation, and fluorescence in situ hybridization (FISH) technique offer high accuracy for heavy metal ion detection, their complexity, cost, and lack of portability restrict their use for on-site monitoring. Resonant sensing, such as MEMS-based LC resonators, offers a promising alternative; however, their long-term performance is often constrained by chemical degradation of electrode materials when exposed to harsh aqueous media. Thus, in the presented work, a hard and chemically inert multilayer electrode stack (Ti/TiN/Al/Ti/TiN) is developed, with material selection optimized using the TOPSIS approach, and deposition is optimized within a single sputtering system. Titanium nitride is employed as the top protective layer due to its excellent chemical and mechanical stability under both acidic and alkaline conditions. The robustness of the proposed materials stack is experimentally validated across a wide pH range representative of industrial runoff, river water, and seawater, demonstrating negligible degradation in resonator performance. As a case study, lead nitrate is introduced to illustrate the sensor response, where changes in the dielectric and conductive properties of the aqueous medium result in measurable shifts in the resonator’s S21 parameter. Both simulated and measured results show good agreement, confirming the viability of the proposed platform. The presented materials stack and resonator architecture provide a robust sensing platform, which can be readily extended to resonant sensing of various analytes in harsh aqueous environments.