Performance enhancement of a prism-based label-free SPR biosensor using a Cu/Si₃N₄/Al₂O₃ multilayer structure for cancer detection

Surface plasmon resonance (SPR) biosensors are becoming more and more popular in label-free and highly sensitive biomedical sensing. However, they generally suffer from limited sensitivity and poor resonance stability, which restrict their practical biomedical sensing performance. Considering these, a prism-coupled multilayer SPR biosensor with the configuration BK7/Cu/Si₃N₄/Al₂O₃/analyte is proposed in this study for cancer cell detection. The sensor design is modeled and analyzed using the transfer matrix method (TMM) and further validated through finite element method (FEM) simulations. The optimized configuration consisting of Cu (50 nm), Si₃N₄ (10 nm), and Al₂O₃ (2 nm) achieves a maximum sensitivity of 446.07 deg per RIU with a resonance angle shift of 6.245° for breast cancer detection. The corresponding detection accuracy and quality factor are found to be 0.293 deg^-1 and 130.62 RIU^-1, respectively. Compared with the conventional BK7/Cu/analyte configuration, the proposed multilayer structure exhibits nearly 2.7 times higher sensitivity. The incorporation of the Si₃N₄ layer improves electromagnetic field confinement at the sensing interface, which contributes to improved sensor response. Additionally, the Al₂O₃ layer together with Si₃N₄ helps suppress Cu oxidation and supports stable resonance characteristics. A fabrication tolerance analysis with ±2% thickness variation shows that the maximum sensitivity deviation remains within 6.97%, indicating good structural robustness. The proposed sensor exhibits good linearity within the refractive index range of 1.360-1.399. The electric field distribution indicates evanescent field penetration into the analyte region with an estimated penetration depth of approximately 174 nm, allowing effective interaction between the sensing medium and the plasmonic field. In addition to breast cancer cells, the proposed sensor also shows promising response for adrenal gland, blood, cervical, and skin cancer cells with sensitivities of 347.14, 281.43, 267.71, and 210.00 deg per RIU, respectively. These results demonstrate the potential of the suggested SPR biosensor for biomedical sensing applications including cancer diagnosis.