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 similar to 1.5x that of the single-layer Au sensor.