Enhancement effect of branch built-in gap on SERS performance of AuAg nanostars for improved detection of mercury ion

A type of nanostar with brand-new morphology called AuAg@gap@AuAg nanostar (NS) was synthesized. Specifically, we made it by adopting the AuAg nanostar as the core and the AuAg@Ag core-shell nanostar with a fully covered Ag layer as a template. Through tuning the thickness of the Ag layer on the etched template, the gap width in the branches of the AuAg@gap@AuAg NS was regulated artificially with a range of 3–20 nm. The results revealed that the SERS efficiency could be greatly enhanced due to the local field coupling in the branch gaps and the lightning rod effect of the branch tips, and the strongest SERS performance occurring when the built-in gap width was 14.5 nm. Established on the excellent SERS performance and stability of the AuAg@gap@AuAg NS, a capillary detection substrate loaded with abundant 3D “hot spots” was prepared, which realized a SERS enhancement factor of 2.0 × 10⁸ and maintained stable over a month. Via modifying 4-mercaptopyridine (4-Mpy) on the substrate surface, rapid and solid detection of mercury ions was achieved through the complexation reaction, with a detection limit of 0.33 pM. The developed capillary platform is expected to become an ideal candidate for on-site analysis and monitoring of various pollutants.