Performance Enhancement of an Ag–Au Bimetallic SPR Sensor: A Theoretical and Experimental Study

Ag–Au bimetallic surface plasmon resonance (SPR) sensors operating in the Kretschmann configuration were investigated by numerical modeling and experiment. While Ag-based sensors have good sensitivity, they suffer from poor stability, and the addition of an Au capping layer offers improved chemical resistance and reliable analyte bonding via thiol–gold interactions. Sensors were evaluated with an Ag–Au combined thickness of 50 nm but different thickness ratios, and SPR reflectivity curves were obtained for 632.8 nm wavelength light incident over a range of angles. Numerical modeling using the transfer matrix method showed the SPR response improving as Au thickness decreased, giving the best results for 45 nm of Ag and 5 nm of Au. Experimental characterization of fabricated Ag–Au bimetallic sensors was carried out with a custom SPR testbed. Performance parameters, including minimum reflectivity, full-width at half-maximum (FWHM), stability, and sensitivity were measured and the results were compared to those of single-layer Ag and Au sensors. A 5 nm Au coating was unable to preserve stable bimetallic sensor performance; however, increasing the Au thickness to 10 nm was sufficient to protect the Ag sensing layer, allowing only a small variation in the minimum reflectivity and FWHM when exposed to analytes for multiple hours. The sensitivity of the single-layer Ag, bimetallic Ag–Au, and Au sensors was measured as 3041%/refractive index unit (RIU), 1817%/RIU, and 1229%/RIU, respectively. The sensitivity of the thickness-optimized bimetallic layer was $\sim 1.5\times $ that of the single-layer Au sensor.