Method for Detecting Enzyme Activity and Fluorescent Probe Used in Said Method

A fluorescent probe comprising a compound represented by General Formula (I) or a salt thereof,

• wherein
• T is a reactive site that reacts with the thiol group (R—SH), the selenol group (R—SeH), or the poly sulfur atom (R—(S)n-H);
• S is a hydrophilic group selected from the group consisting of a sulfonic acid group, a carboxyl group, and a phosphonic acid group;
• n1 represents an integer of 1 to 10;
• represents a fluorophore, in which molecular design is carried out so that optical properties of the entire molecule of the compound change by the reaction of the compound represented by General Formula (I) or the salt thereof with the thiol group, the selenol group, or the poly sulfur atom;
• and
• S and T may be bonded to the fluorophore through a linker.

A compound represented by General Formula (IIb) below or a salt thereof,

• wherein
• R1, when present, represents the same or different monovalent substituent present on a benzene ring, and the substituent may have a hydrophilic group selected from the group consisting of a sulfonic acid group, a carboxyl group, and a phosphonic acid group;
• m is an integer of 0 to 4, and when m is 2 or more, each R1 may be the same or different;
• R2 to R7 each independently represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, the hydrophilic group, or an alkyl group having the hydrophilic group and having 1 to 6 carbon atoms;
• R8 represents a hydrogen atom or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms,
• provided that at least one of R3 and R6 is a sulfonic acid group, a carboxyl group, or a phosphonic acid group;
• and
• X1 and X2 are each independently selected from a fluorine atom, an alkyl group, an alkoxy group, an aryl group, the hydrophilic group, or an alkyl group having the hydrophilic group and having 1 to 6 carbon atoms.