Gold-silver alloy film based surface plasmon resonance sensor for biomarker detection

In this study, we developed a gold‑silver alloy film based surface plasmon resonance (AuAg-SPR) sensor with wavelength interrogation to detect cancer antigen 125 (CA125) using a sandwich immunoassay. We first theoretically simulated the sensitivity of conventional gold film based SPR (Au-SPR) sensor and AuAg-SPR sensor, and conducted a series of experiments to investigate the sensitive characteristics of AuAg-SPR sensor, including the angle and refractive index (RI) sensitivity. We then conducted CA125 detection experiments on these two types of sensors. The results demonstrated that the limit of detection (LOD) of CA125 on the AuAg-SPR sensor was 0.1 U/mL (0.8 ng/mL) based on its direct reaction with an immobilised antibody, which was two orders of magnitude lower than that of the Au-SPR sensor (10 U/mL). The total changes in the resonance wavelength (∆λ_R) of the former were 1.7-fold those of the latter. The volume fractions of the adsorbates (f_ad) and effective RIs (n_adlayer) in each adlayer were then calculated and the effect of the antibody size on the detection results was analysed. The AuAg-SPR sensors had a higher sensitivity than the conventional Au-SPR sensors for detecting CA125 due to their electric field characteristics. Therefore, these will have better application prospects.