Gel-limited synthesis of dumbbell-like Fe3O4–Ag composite microspheres and their SERS applications

Abstract

A novel gel-limited strategy was developed to synthesize dumbbell-like Fe₃O₄–Ag composite microspheres through a simple one-pot solvothermal method. In such a reaction system, a special precursor solution containing oleic, water, ethanol and silver ions was used and transformed into a bulk gel under heating at the very beginning of the reaction, thus all the subsequent reactions proceeded in the interior of the gel. The gel-limited reactions had two advantages, on the one hand, the magnetic Fe₃O₄ microspheres were fixed in the gel which avoided them aggregating together, whereas on the other hand, the silver ions stored in the gel could be gradually released and tended to diffuse towards the nearest Fe₃O₄ microsphere, which favored the generation of a dumbbell-like Fe₃O₄–Ag structure. From the time-dependent experiments under optimal conditions, the typical growth process of dumbbell-like structures clearly demonstrated that a silver seed first appeared on the surface of a single Fe₃O₄ microsphere, which then grew bigger slowly and finally formed a dumbbell-like Fe₃O₄–Ag structure. Moreover, the formation of the gel was found to be strongly affected by the ratio of water and ethanol in the precursor solution, which further influenced the morphologies of the Fe₃O₄–Ag microspheres. Furthermore, the effect of lattice match between Fe₃O₄ and Ag on the final products was also proven from the control experiments by using a template with a different surface crystalline structure. When used as SERS substrates, the final dumbbell-like Fe₃O₄–Ag microspheres show fast magnetic separation and the selective detection of thiram for the surface capped oleic chain during the growth process.