Theoretical Investigation of Early Cancer Biomarker Sensing Using a PMMA–Gold Hybrid Quasi-D-Shaped Photonic-Crystal-Fiber-Based Surface Plasmon Resonance Biosensor

In this work, a quasi-D-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) biosensor is proposed and numerically investigated using the finite element method (FEM) implemented in COMSOL Multiphysics version 6.2 for the detection of cancer cells with different refractive indices. The biosensor has a coating of plasmonic material gold (Au) and a polymer coat of polymethyl methacrylate (PMMA). The effects of plasmonic material thickness and air hole dimensions on key sensing parameters, including confinement loss (CL), wavelength sensitivity (WS), and amplitude sensitivity (AS), are systematically analyzed. The results revealed that increasing plasmonic thickness beyond its optimum value significantly raises CL while reducing sensitivity due to reduced penetration depth of the evanescent field. Similarly, variations in the geometrical dimensions of the air holes (±10%) also affect the sensor response, emphasizing the importance of precise structural optimization. For the optimized design the proposed biosensor exhibits high performance with a maximum WS of 31,000 nm/RIU for MDA-MB-231 cells under x-polarization and 29,500 nm/RIU under y-polarization. The corresponding resolutions achieved are as low as 3.22 × 10^-6 RIU and 3.38 × 10^-6 RIU, respectively, with AS exceeding 9000 RIU^-1. The WS, AS, and other sensing parameters obtained from our sensor are relatively higher than some of the PCF-SPR sensors reported recently. These numerical results demonstrate that the optimized quasi-D-shaped PCF-SPR biosensor exhibits enhanced sensitivity to refractive index (RI) variations associated with cancerous cells, suggesting its suitability for early detection.