Point-of-Care Detection of Antioxidant in Agarose-Based Test Strip through Antietching of Au@Ag Nanostars.

Antioxidants are crucial for human health, and the detection of antioxidants can provide valuable information for disease diagnosis and health management. In this work, we report a plasmonic sensing approach for the determination of antioxidants based on their antietching capacity toward plasmonic nanoparticles. The Ag shell of core-shell Au@Ag nanostars can be etched by chloroauric acid (HAuCl), whereas antioxidants can interact with HAuCl, which prevents the surface etching of Au@Ag nanostars. We modulate the thickness of the Ag shell and morphology of the nanostructures, showing that the core-shell nanostars with the smallest thickness of Ag shell have the best etching sensitivity. Owing to the extraordinary surface plasmon resonance (SPR) property of Au@Ag nanostars, the antietching effect of antioxidants can induce a significant change in both the SPR spectrum and the color of solution, facilitating both the quantitative detection and naked-eye readout. This antietching strategy enables the determination of antioxidants such as cystine and gallic acid with a linear range of 0.1-10 μM. The core-shell Au@Ag nanostars are further immobilized in agarose gels to fabricate test strips, which can display different color changes in the presence of HAuCl from 0 to 1000 μM. The agarose-based test strip is also capable of detecting antioxidants in real samples, which allows naked-eye readout and quantitative detection by a smartphone.