Plasmonic detection of microplastics and microorganisms using a circular-nanorod-coupled MIM sensor

What we believe to be a novel plasmonic refractive index (RI) sensor is proposed and numerically investigated for optical detection of microplastics and aquatic microorganisms. The sensor consists of a silver-air metal-insulator-metal (MIM) waveguide side-coupled to an octagonal resonant cavity, in which sixteen silver circular nanorods are strategically embedded to enhance localized surface plasmon resonance and strengthen light-matter interaction. The sensing region is assumed to be filled with an aqueous analyte, and variations in its effective refractive index are employed to emulate the presence and concentration of microplastic particles and aquatic microorganisms. The designed structure exhibits dual resonance modes with high RI sensitivities of 1320.86 nm/RIU and 2197.76 nm/RIU, respectively, for Peak 1 and Peak 2. Corresponding figures of merit of 54.604 1/RIU and 58.126 1/RIU are achieved, indicating a strong capability for resolving subtle refractive-index perturbations in water-based sensing scenarios. The pronounced dual-peak response, high sensitivity, and compact configuration demonstrate the potential of the proposed plasmonic sensor as an effective optical platform for both microplastic detection and aquatic microorganism analysis.