Sensitivity and performance enhancement of an SPR biosensor using a gold-silver alloy, zinc oxide and graphene

Over the past years, Surface Plasmon Resonance (SPR) has been used in various domains including medical diagnostics, chemical and biological sensing, and environmental monitoring. SPR-based biosensors have more advantages than other optical biosensors due to their rapid, label-free, and targeted detection capabilities. There have been challenges to designing SPR biosensors with enhanced performance and improved environmental adaptability, including considerations such as prism-metal bonding, biomolecule adhesion, and oxidation protection. This paper presents sensitivity and performance enhancement in an SPR biosensor based on Kretschmann configuration while using a low refractive index CaF₂ glass prism, a zinc oxide (ZnO) layer, a gold-silver alloy (Au₁₀Ag₉₀), and a single layer of 2D graphene. Theoretical analysis shows that the use of a CaF₂ prism and the alloy gives a good sensitivity of \(280^\circ /\text{RIU}\) ${280}^{\circ }/\text{RIU}$ . The sole addition of a ZnO layer improves this sensitivity to \(320^\circ /\text{RIU}\) ${320}^{\circ }/\text{RIU}$ . Further addition of a graphene layer to make the final proposed structure CaF₂/ZnO/Au₁₀Ag₉₀/graphene/SM, improves sensitivity to \(340^\circ /\text{RIU}\) ${340}^{\circ }/\text{RIU}$ , giving a figure of merit value of \(60.28/\text{RIU}\) $60.28/\text{RIU}$ and a quality factor value of \(102.48/{\text{RIU}}\) $102.48/\text{RIU}$ . Despite supporting the SPR mode and enhancing sensitivity, ZnO improves prism-alloy bonding while graphene promises better biomolecule adhesion and improved oxidation protection. Overall, our proposed structure has better sensing performance compared to other designs in the literature. The high sensitivity makes our proposed biosensor suitable for the detection of tiny cancer bio-markers in body fluids, enabling early medical interventions.