Performance enhancement of an Ag-Au bimetallic SPR sensor: A theoretical and experimental study

• Published in: IEEE sensors journal Vol. 23; no. 10; p. 1
• Main Authors: Babu, Roshni S., Colenso, Hamish R., Gouws, Gideon J., Auguie, Baptiste, Moore, Ciaran P.
• Format: Journal Article
• Language: English
• Published: New York IEEE 15-05-2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Bimetallic sensor; Bimetals; Chemical sensors; Glass; Gold; Gold coatings; Kretschmann configuration; Mathematical models; Matrix methods; Numerical models; Optical sensors; Parameter sensitivity; Performance enhancement; Reflectance; Refractivity; Sensitivity; Sensors; Silver; Stability; Stability criteria; Surface plasmon resonance; Thickness; Transfer matrices
• ISSN: 1530-437X; 1558-1748
• DOI: 10.1109/JSEN.2023.3265896

Ag-Au bimetallic surface plasmon resonance (SPR) sensors operating in the Kretschmann configuration were investigated by numerical modelling 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 modelling 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 characterisation of fabricated Ag-Au bimetallic sensors was carried out with a custom SPR testbed. Performance parameters, including minimum reflectivity, 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 ~1.5× that of the single-layer Au sensor.