A novel metal–organic framework (MOF) was fabricated by spontaneous K+-induced supramolecular self-assembly with the embedded tripodal ligand units. When the 3D ligand was loaded onto Fe3O4@mSiO2 core-shell nanoparticles, it could effectively separate K+ ions from a mixture of Na+, K+, Mg2+, and Ca2+ ions through nanoparticle-assisted MOF crystallization into a Fe3O4@mSiO2@MOF hybrid material. Excess potassium ions could be extracted because of the specific cation–π interaction between K+ and the aromatic cavity of the MOF, leading to enhanced separation efficiency and suggesting a new application for MOFs.