Gold-silver alloy film based spectral surface plasmon resonance imaging sensor with high sensitivity

This paper reports, for the first time, a gold-silver alloy film based broadband spectral surface plasmon resonance imaging (SPRI) sensor that enables in situ quantitative detection of chemical and biological molecules adsorbed on the partial or entire surface of the alloy film. The use of the gold-silver alloy film as the sensing layer makes the SPRI sensor lower in detection cost and higher in detection sensitivity as compared with the conventional sensor with a pure gold film. The gold-silver alloy films of ~50 nm thicknesses were deposited on glass substrates using a sputtering target made of gold (50%)-silver (50%, w, mass fraction) alloy. Both the SPR spectra and SPR color images for the gold-silver alloy films covered with pure water were measured at different incident angles using the laboratory-made Krestchmann-type multifunctional platform. The two-dimensional (2D) hue profile and the average hue for each SPR color image were obtained by calculation with the hue algorithm. Using the average hue as the sensitivity parameter, the spectral SPRI sensor enables quantitative detection. The spectral range in which the average hue is most sensitive to refractive index (RI) changes of bulk solution and to molecular adsorption was determined to be between 595 and 610 nm. In this narrow spectral range the average hue is linearly dependent on the resonant wavelength and its slope (representing the hue variation induced by per unit change in resonant wavelength) is ∆hue/∆λ_R = 7.52 nm⁻¹, implying that the hue-based RI sensitivity is 7.52 times as high as the wavelength-based RI sensitivity. This implication was experimentally demonstrated in this work. After setting the initial resonant wavelength of the sensor in the hue-sensitive spectral range, the hue-based RI sensitivity of the SPRI sensor was measured to be S = 29879 RIU⁻¹, which is 8 times higher than that obtained with the gold-film SPR chip under the same conditions (S = 3658 RIU⁻¹ for the gold-film SPR chip). Nonspecific adsorption of bovine serum albumin (BSA) molecules on the gold-silver alloy film was monitored in real time by the time-resolved spectral SPRI method, and the temporal change in the average hue was obtained. The time required for BSA adsorption to reach equilibrium is determined to be about 15 min. This study illustrates that the gold-silver alloy film based SPRI sensor has the powerful capability of quantitative detection of sub-monomolecular adsorption of proteins.