Highly sensitive biosensors with resonant coupling of plasmon-waveguide resonance to localized surface plasmons†

Sensors based on plasmon-waveguide resonance (PWR) offer narrow linewidths, but their low surface electric field intensity hinders the detection of low concentrations of biomolecules. In this study, we design and fabricate a novel biosensor composed of Au and TiO₂ layers, and gold nanoparticles (Au–TiO₂–AuNPs), and achieve high-performance sensing based on refractive index changes by integrating the biosensor into microfluidics. The evanescent field of PWR supported by the Au–TiO₂ structure can effectively stimulate localized surface plasmon resonance (LSPR) supported by AuNPs by the prism coupling mechanism, and then the resonant coupling mode between PWR and LSPs (PWR–LSP) is generated in the Au–TiO₂–AuNPs structure. Numerical analyses based on the finite element method show that PWR–LSP has an improvement in the surface electric field intensity by 2.9 times in comparison with PWR and is, therefore, more sensitive to small refractive index changes. Compared with the Au film sensor, the PWR–LSP sensor shows increases in the bulk sensitivity (5000 nm RIU⁻¹) and figure of merit (114 RIU⁻¹) by 3 and 4.4 times, respectively. Furthermore, the specific detection of carcinoembryonic antigen (CEA) within a linear range of 5–100 ng mL⁻¹ is achieved with a limit of detection of 3 ng mL⁻¹ (15 pM) using a double-antibody sandwich method, and the dynamic range is clinically applicable to human CEA levels. The PWR–LSP biosensor enables highly sensitive immunoassays and offers an innovative approach to biochemical sensing.