What do dashes mean in chemical formulas?
This nomenclature refers to a core-shell arrangement of nanoparticles. This is where you have a nanoparticle core consisting of one material that has been coated/decorated/wrapped/overlaid with another material. There are a few accepted forms of nomenclature for describing these types of nanoparticles:


core@shell In your case, it would be Fe2O3@Au
core-shell In your case, it would be Fe2O3-Au
core/shell In your case, it would be Fe2O3/Au


There are plenty of examples of each syntax in the literature, and has been used for eons. 

Others may answers for the additional questions you have here.