AuAg alloy film-based colorful SPR imaging sensor for highly sensitive immunodetection of benzo[a]pyrene in water

A colorful surface plasmon resonance imaging (SPRi) sensor with the hue-based enhanced sensitivity has been developed by using sputtered AuAg alloy thin film as the sensing layer. The condition for optimizing the hue-based sensitivity of the SPRi sensor was achieved, that is, the initial resonance wavelength is in the range from 595 to 610 nm. Under this condition, the hue-based refractive index sensitivity of the SPRi sensor was measured as high as Δhue∕Δn_c 29879∕RIU. This sensitivity is 8 times higher than that obtained with a gold-layer SPRi sensor (Δhue∕Δn_c 3658∕RIU) and 7.7 times as high in magnitude as the spectral sensitivity of the same sensor (Δλ_R∕Δn_c 3897.6 nm∕RIU). After functionalization of the AuAg alloy film with the monoclonal antibody, the sensor was used for quantitative immunodetection of highly carcinogenic benzo[a]pyrene (BaP) in water. According to the experimental results, the average hue of the SPR color image (300 pixels × 300 pixels) experiences an initial rapid increase and then stabilizes 15 min after exposure of the functionalized AuAg alloy film to an aqueous BaP solution sample. The variation of average hue obtained at the equilibrium of surface immunoreaction is a linear function of BaP concentration with the slope being Δhue∕Δc 132.2∕μg · L⁻¹. A cooled CCD camera is able to distinguish a change of Δhue 1, offering the colorful SPRi sensor the BaP detection limit of 0.01 μg · L⁻¹. The comparative measurements of the sensor’s responses indicate that the hue variation obtained with 0.1 μg · L⁻¹ of BaP is equal to that obtained either with 5 μg · L⁻¹ of benzanthracene or with 5 μg · L⁻¹ of benzofluoranthene, revealing the sensor’s excellent specificity to BaP. The work demonstrated that the AuAg alloy film-based colorful SPRi sensor can be used not only for visualized analysis of molecular interaction at the surface but also for quantitative trace detection of small-molecule analytes.