Similar to crown ethers and ionophores, these macrocyclic molecules show good selectivity for metal ions and can transport them across membranes. They are made by making the 2D, flat crown ether into a 3D cage for the metal ion. The simplest method is adding a bridge to make a bicyclic molecule, with nitrogens usually. This adds more ligands to the molecule and has, therefore, better coordination of the ion.

Tricyclic molecules have a more spherical shape, eg:


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