In this study, we developed a kind of gold-silver alloy film based SPR (AuAg-SPR) sensors with wavelength interrogation to detect Cancer antigen 125 (CA125) by sandwich immunoassay. The experiment shows that the changes in resonance wavelength (Δλ<sub>𝑅</sub>) of AuAg-SPR sensors are higher than that of the gold film based SPR (Au-SPR) sensors when the final sandwich structures were formed. We have applied the Fresnel reflection equations and Bruggeman approximation formula to calculate the volume fractions of adsorbates in different adlayers, and the dispersion curves of the effective refractive index (RI) of adsorbates with different volume fractions of adlayers were obtained. Moreover, we simulated the sensitivity of RI of two kinds of sensors by using the waveguide equations, when the initial resonance wavelength (λ<sub>𝑅</sub>) was fixed at 628nm, changing RI from 1.333 to 1.335, the Δλ<sub>𝑅</sub> of AuAg-SPR sensor is 2.25-fold compared with that of Au-SPR sensor. Finally, the effect of the size of antigen on the detection result and effective RI were analyzed. In conclusion, the AuAgSPR sensors with a higher sensitivity than the conventional Au-SPR sensors in detecting CA125, owing to the advantages of low-cost, stability, highly efficient for the clinical determination of CA125 levels
Layer-by-layer self-assembly ultrathin films continue to be of significant interest among researchers for its broad application in diverse areas. Based on electrostatic attraction between materials with opposite charges, multilayer films of (PSS/Cyt c) <sub>6</sub>, (GNPs/Cyt c) <sub>5</sub> and (PDDA/DNA) <sub>6</sub> had been successfully fabricated on the surface of gold chip, respectively. The adsorption process had been followed by in suit spectral surface plasmon resonance (SPR) technique in real time. The experimental results demonstrate that adsorption of materials onto the gold chip can induce a redshift of resonance wavelength and the amount of adsorption can be determined by the change of resonance wavelength. Kinetics studies suggest that adsorption of cytochrome c (Cyt c) or deoxy ribonucleic acid (DNA) obeys Langmuir-isotherm theory, while adsorption of gold nanoparticles (GNPs) obey diffusion-controlled model. These results also indicate that GNPs need more time to reach adsorption equilibrium than Cyt c and DNA, due to a small value of diffusion coefficient.0 increasing with the layer, however, (GNPs/Cyt c) <sub>5</sub> and (PDDA/DNA) <sub>6</sub> show a nonlinearly increase. Moreover, through controlling the number of assembled layer, the thickness of multilayer films can be precise designed.