Magnetically-controllable hollow multi-shell magnetite spheres as stable and reusable SERS substrates

Abstract

This study successfully synthesized magnetically-manipulatable and reusable SERS active buoyant substrates, i.e. Ag@SiO2@Fe3O4 composite hollow spheres. The composites comprised mesoscopic hollow spheres with a Fe3O4 inner shell of 15–30 nm in thickness and strong ferromagnetism, which were synthesized using spray pyrolysis and the subsequent reduction, and a SiO2 outer shell to maintain the stability of Fe3O4, as well as Ag nanoparticles on the sphere surface providing localized surface plasmon resonance. The coercive field (Hc) of bare Fe3O4 hollow sphere reached 200 Oe, and the saturation magnetization (Ms) was 130 emu/g at 300 K, which was at least 30% higher than the reported values no matter in bulk or nanoscales. Compared to composite spheres with Fe3O4 solid cores, the SERS signal intensity of hollow structures was 2.6 times greater, which could be attributed to the buoyancy of hollow structure which contribute to more interaction with the target analytes. Under external magnetic field, these ferromagnetic hollow composites can be concentrated and separated easily, resulting in a more augmented SERS effect (about 1.5 times higher than those collected without applying magnetic field). This study also demonstrated the possibility to reuse those composites for SERS measurements.