Optimization of a LaF-Coupled Au/BaTiO3/WS2 SPR Sensor for Multi-Ion Heavy Metal Monitoring in Water: A Numerical Study

Introduction: Heavy metal contamination in water represents a major environmental and public health challenge because toxic ions frequently occur as complex multi-species mixtures rather than isolated pollutants. This study presents a numerical design and optimization of a surface plasmon resonance (SPR) sensor based on a LaF/Au/BaTiO3/WS2 heterostructure for monitoring refractive-index changes associated with mixed heavy metal ions in aqueous media. Methodology: The optical response of the multilayer sensor was evaluated using the transfer matrix method under TM-polarized illumination at 633 nm. Systematic optimization was performed for the prism substrate, Au thickness, dielectric oxide layer, and 2D nanomaterial interface. The final configuration consisted of a LaF prism, 50 nm Au film, 2.0 nm BaTiO3 spacer, and 0.80 nm WS2 monolayer. Sensor performance was assessed using resonance-angle shift, sensitivity, detection accuracy, quality factor, figure of merit, FWHM, attenuation, and estimated limit of detection. Results and Discussion: The optimized LaF/Au/BaTiO3/WS2 configuration produced stable simulated SPR responses across single, binary, quaternary, and five-ion heavy metal matrices. The WS2 monolayer provided the highest angular displacement among the evaluated 2D materials, while BaTiO3 improved field confinement and limited optical damping in the numerical model. The configuration maintained attenuation near 1.6%, FWHM values around 7.9°, detection accuracy between 0.030 and 0.032 deg−1, and model-based refractometric LoD values down to 3.49 × 10−5 RIU under the assumed angular-resolution criterion. Conclusions: The proposed LaF/Au/BaTiO3/WS2 SPR configuration provides a numerical framework for label-free monitoring of refractive-index changes associated with complex heavy-metal-ion mixtures in contaminated water. Experimental fabrication and testing are required to validate the simulated performance.