Fluorescent probe for detection of enpp activity

A compound of formula (II) or a salt thereof:

• wherein
• R1a is one or two monovalent substituents present on a benzene ring, which are electron donating groups, and when a plurality of R1as are present, R1as are the same or different;
• R2a is one or two monovalent substituents present on a benzene ring, which have an anionic functional group at an end, and when a plurality of R2as are present, R2as are the same or different;
• R3a and R4a are each independently a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or a halogen atom;
• R5a and R6a are each independently an alkyl group having 1 to 6 carbon atoms, or an aryl group;
• R7a and R8a are each independently a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or a halogen atom;
• Za is an oxygen atom or N+R9aR10a,
• where R9a and R10a are each independently a hydrogen atom, or an alkyl group having 1 to 10 carbon atoms,
• R9a and R10a are optionally linked together to form a four- to seven-membered heterocyclyl containing a nitrogen atom bonded to R9a and R10a, and
• R9a or R10a, or each of both R9a and R10a is optionally linked with R3a or R7a to form a five- to seven-membered heterocyclyl or heteroaryl containing a nitrogen atom bonded to R9a or R10a, or optionally contains one to three additional hetero atoms selected from the group consisting of an oxygen atom, a nitrogen atom and a sulfur atom as ring forming members, and the heterocyclyl or heteroaryl is optionally substituted with an alkyl having 1 to 6 carbon atoms, an alkenyl having 2 to 6 carbon atoms, an alkynyl having 2 to 6 carbon atoms, an aralkyl group having 6 to 10 carbon atoms, or an alkyl-substituted alkenyl group having 6 to 10 carbon atoms;
• Ya is a single bond, —O—(CH2)n1—, —O—(CH2)n2—Ar1—, —NH—(CH2)n3— or —NH—(CH2)n4—Ar2—,
• where n1, n2, n3 and n4 are each independently an integer of 1 to 10, and
• Ar1 and Ar2 are each independently a substituted or unsubstituted arylene group;
• and
• (Nu) is represented by R—S—
• where R is an organic base, and
• S represents a sugar or a partial structure of a derivative thereof, or a single bond.