Enhanced heavy metal detection in water using silver-based plasmonic sensors with nickel and gold interlayers

Abstract Developing highly sensitive and reliable sensors for heavy metal detection in water is crucial for environmental monitoring and public health. This work proposes and optimizes a plasmonic sensor based on surface plasmon resonance (SPR) with a heterostructure design consisting of CaF 2 /Ag/Ni/Au in a Kretschmann configuration. Using the transfer matrix method (TMM), we systematically investigated the influence of Ag, Ni, and Au layer thicknesses on the reflectance spectra and detection performance. The optimized configuration, CaF 2 /Ag (50 nm)/Ni (4 nm)/Au (1 nm), achieved a sensitivity of 471 deg/RIU and a figure of merit (FOM) of 109/RIU, outperforming conventional Ag- or Au-based SPR sensors. The inclusion of Ni as an interlayer not only improved the adhesion between Ag and Au but also enhanced resonance sharpness. At the same time, a thin Au coating contributed to improved stability and overall performance. Further analysis demonstrated that the optimized sensor can have dual detection modes: resonance angle shifts dominate at high heavy metal concentrations, whereas linear changes in reflectance intensity enable sensitive detection at low concentrations (0–100 ppm). These findings underline the potential of the proposed SPR design for accurate, stable, and wide-range detection of toxic heavy metals such as Hg, Cu, and Pb in aqueous environments, providing a strong foundation for future development of compact and field-deployable sensing platforms.