According to one embodiment of the present invention, an azo polymer forms a complex with a lithium ion and a sodium ion among alkali metal ions, but does not form a complex with a potassium ion. Therefore, the azo polymer is expected to be utilized as a material for a sensor capable of selectively detecting a specific alkali metal ion, or as a novel material capable of selectively trapping a specific alkali metal ion from a solution in which metal ions are mixed.