Two-dimensional (2D) Cu-CP (Cu-CP = [Cu(pca)₂∙Hpca]_n) (1) with an interpenetrating double-layer structure was prepared using a hydrothermal method based on 4-picolinic acid (Hpca) ligand. Because 3d–4f metal-incorporated polyoxometalate (POM) materials have received increasing attention, the extension of Cu-CP from a 2D to a three-dimensional (3D) structure was achieved by adding H₄SiW₁₂O₄₀ and rare earth metal. Three isostructural 3D 3d–4f metal-incorporated POMs were obtained: Cu-Ln-CPs (Cu-Ln-CPs = [Ln₂Cu (SiW₁₂O₄₀) (Hpca)₄(pca)₄(H₂O)₂]∙(Hpca)₂∙(I₂)_0.5, Ln = Sm (2), Gd (3), La (4)). The applications of these complexes in the fields of fluorescence and electrochemistry were explored. The results showed that the expanded structure of the complexes can realize the fluorescent sensing of Ni²⁺ and Cr³⁺ and the electrochemical sensing of nitrite.