Design and optimization of a high-sensitivity symmetric BP/WS₂-based surface plasmon resonance biosensor: A theoretical study for carcinoembryonic antigen (CEA) detection

Surface plasmon resonance (SPR) biosensors are widely recognized for their high sensitivity and real-time detection capabilities, particularly in biomedical diagnostics. This study presents a theoretical investigation of an SPR biosensor designed in a symmetric Kretschmann configuration with a BK7 prism, a silver (Ag) layer, black phosphorus (BP), and tungsten disulfide (WS₂). The optical response of the structure was analyzed using the finite-difference time-domain (FDTD) method. Optimization results indicate that the highest sensitivity and FoM are obtained with a symmetric configuration that includes a 45 nm layer of Ag, a three-layer BP, and a monolayer of WS₂ placed on both sides of the sensing medium. This configuration produces two distinct resonance modes. The proposed configuration for the first resonance mode achieves a maximum sensitivity of 263.14 deg/RIU, with an FWHM of 2.04 degrees and an FoM of 128.99 RIU^-1. The second mode achieved a sensitivity of 326.3 deg/RIU, with an FWHM of 1.95 degrees and an FoM of 167.33 RIU^-1. These measurements were obtained with a refractive index change of Δn = 0.005, all at an operating wavelength of 632.8 nm. The sensor's optical performance is further evaluated using carcinoembryonic antigen (CEA) as a model analyte to demonstrate the sensing capabilities. Incorporating layered 2D materials into a symmetric SPR sensor greatly improves detection performance and provides a promising framework for future applications in biomarker detection.