Colorimetric sensing of sinapine based on competitive interaction of papain-coated gold nanoparticles and sinapine toward Pb2+†
Direct selective and sensitive sensing of sinapine (SA) in diets and plants is of great importance due to its crucial roles in the functions of life. In this study, we demonstrate a sensitive and selective colorimetric method for SA sensing in rapeseed with papain-coated gold nanoparticles (P-AuNPs) as signal readouts based on the competitive interaction of papain and SA toward Pb2+. Initially, P-AuNPs are wine red in color and exhibit absorption at 524 nm (A524) in the UV-vis spectra. The addition of an aqueous solution of Pb2+ to the P-AuNPs dispersion triggers the aggregation of P-AuNPs, resulting in the wine-red-to-blue color change and the appearance of a new absorption peak at 626 nm (A626) in the UV-vis absorption spectra. The addition of SA into P-AuNPs and Pb2+ (AuNPs/Pb2+) solution induces changes in both the color and the UV-vis spectra of the P-AuNPs aggregation because of the strong binding between Pb2+ and SA. Thus, the colorimetric sensing of SA can be realized through the blue-to-wine-red color change of the P-AuNPs solution and measuring the ratio of absorbance at a wavelength of 524 nm to that at 626 nm in the UV-vis spectra. The study demonstrated here offers a simple yet effective method for SA assay and expands the applications of AuNPs in bioassays.