Seed-mediated growth of Au@Ag core-shell nanorods for the detection of ellagic acid in whitening cosmetics.

Seed-mediated growth has been employed as a simple and powerful means to the shape-controlled synthesis of metal nanocrystals. In this work, we apply the principle of seed-mediated growth in analytical chemistry, and achieve improved sensitivity due to the low energy barrier in the target-induced formation of bimetallic nanoparticles with core-shell structure. As a result, a simple, reliable, highly sensitive and selective method for the detection of ellagic acid (EA), a naturally occurring polyphenolic antioxidant, has been developed. With the aid of EA in alkaline solution, Ag+ ions can be transformed to Ag atoms and deposit on the surfaces of Au nanorods (AuNRs, act as seeds here) to generate Au@Ag core-shell nanorods, accompanied by blue shift of the longitudinal localized surface plasmon resonance (LSPR) band of AuNRs from near-infrared region to shorter wavelengths. Based on the linear relationship between the wavelength change of longitudinal LSPR band and the concentration of EA, our method achieves a detectable range of 0.2-20 μM and a limit of detection as low as 40 nM toward EA. This approach is highlighted by its high sensitivity for EA assay, which benefits from the viewpoint of thermodynamics in the nucleation/growth mode of metal nanoparticles. Moreover, this method shows high selectivity for EA detection when potential species coexist, and thus has been successfully applied in the detection of EA in skin-whitening cosmetics. The proposed strategy of seed-mediated growth herein can also be extended to other systems for sensing.