High sensitivity formalin detection in aqueous solutions using plasmonic multifunctional metal insulator metal nanoring based optical refractive index sensor platform

Formalin contamination in water poses major health and environmental risks, requiring rapid and sensitive detection. This study presents a plasmonic optical biosensor refractive index sensor based on a multifunctional metal-insulator-metal (MIM) nanoring structure optimized for formalin detection. The proposed design features nested nanorings with perpendicular arms and a reflective metal layer, enabling strong electromagnetic confinement and multi-resonance operation and multi-resonance operation. Finite-difference time-domain simulations demonstrate four resonance modes with sensitivities of 1289.30, 823.31, 662.71, and 557.62 nm/RIU, and corresponding FOM values up to 13.73 RIU⁻¹. The sensor exhibits a linear spectral shift versus refractive index variation of aqueous formalin (n = 1.3300-1.3600). Electric-field analysis confirms field localization at resonant sites, contributing to enhanced sensing performance. These results demonstrate the potential of the proposed MIM nanoring platform for highly sensitive and compact optical sensing applications, particularly in real-time environmental monitoring of hazardous chemicals.