Dear friend Marium Akram ,

You posed a good question.

As per my knowledge, The '@' symbol in Fe3O4@SiO2 is used to denote a core-shell structure. In this case, the Fe3O4 is the core and the SiO2 is the shell. The @ symbol is used because it resembles a core-shell structure, with the core at the center and the shell surrounding it. The symbol @ is also used in the title of the paper you mentioned, "Ligand complex of copper (II) supported on superparamagnetic Fe3O4@SiO2 nanoparticles: an efficient and magnetically separable catalyst for Narylation of nitrogen-containing heterocycles with aryl halides." In this case, the @ symbol is used to denote the support of the copper (II) ligand complex. The copper (II) ligand complex is supported on the Fe3O4@SiO2 nanoparticles, which means that it is attached to the surface of the nanoparticles.

Or,

The symbol "@" is often used in scientific literature to denote the encapsulation or attachment of one material or substance onto another. In the title you provided, "Fe3O4@SiO2" refers to superparamagnetic Fe3O4 nanoparticles that are coated or supported with SiO2 (silica) nanoparticles. The "@" symbol implies that the Fe3O4 nanoparticles are embedded or encapsulated within the SiO2 nanoparticles.

I hope this clears your doubts. Good luck with your research!

Best wishes & regards,

Vishwajit Chavda