Taggable fluorescent probe for calcium ion detection

A compound represented by the following general formula (I) or a salt thereof:

• (wherein,
• R1 at each occurrence represents a hydrogen atom or a monovalent substituent on the benzene ring, and when more than one monovalent substituent are present, they may be the same or different, and said monovalent substituent is a halogen or an alkyl group which may be substituted;
• R2 and R3, each independently, represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms or a halogen atom;
• R4 and R5, each independently, represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms or a halogen atom;
• R6 represents an acetoxy group, an acetoxymethoxy group, or a hydroxyl group;
• X is selected from a carbonyl group, an amide group or an alkylcarbonyl group;
• U represents a group capable of capturing calcium ions after hydrolysis, wherein said group capable of capturing calcium ions after hydrolysis is selected from the group consisting of an ester compound of an aminophenol triacetic acid derivative and an ester compound of 1,2-bis(o-aminophenoxide)ethane-N,N,N′,N′-tetraacetic acid derivative;
• Y is azetidine;
• T, if present, represents a cross-linking group, and said cross-linking group is a carbonyl group;
• S represents a label site, and said label site is 2-(2-((6-chlorohexyl)oxy)ethoxy)ethaneamino group;
• m is an integer of 1 to 3, and
• n is an integer of 1 to 3, with the proviso that m+n=4).