A 2-phenyl-6-azolylpyridine-based ligand is represented by the following formula (I): wherein, X represents nitrogen or C—R10; R1, R2, R3, R4, R5, R6, R7, R8 and R10 independently represent hydrogen, halogen, a cyano group, an amino group, an optionally substituted aryl group, an optionally substituted heteroaryl group, sulfate, —COOH, an alkoxy group, a carbonyl group, a C1-C10 haloalkyl group, a C1-C12 alkyl group, a C2-C10 alkenyl group, a C2-C10 alkynyl group, a C3-C20 cycloalkyl group, a C3-C20 cycloalkenyl group, a C1-C20 heterocycloalkyl group, or a C1-C20 heterocycloalkenyl group; and R9 represents hydrogen or alkali metal.