Fluorescent probe

A fluorescent probe comprising a compound represented by the following general formula (I):

• (Fluorophore A)-S-(Fluorophore B) (I)
• wherein Fluorophore A and Fluorophore B are fluorophores which emit fluorescence when they are irradiated with an excitation light of a wavelength of 600 to 950 nm, Fluorophore A has a property that it shows change of fluorescence characteristic before and after a specific reaction with a substance to be measured, and S represents a spacer which connects Fluorophore A and Fluorophore B, which compound shows substantial change in efficiency of fluorescence resonance energy transfer between Fluorophore A and Fluorophore B before and after the specific reaction with the substance to be measured,
• wherein Fluorophore A is represented by the following formula (AI):
• wherein R1 and R2 independently represent a C1-18 alkyl group which may have a substituent;
• R3 and R4 independently represent hydrogen atom, or a substituent for detecting the substance to be measured, provided that R3 and R4 do not simultaneously represent hydrogen atom, and R3 and R4 may bind to each other to form a ring structure for detecting the substance to be measured;
• Y1 and Y2 independently represent —O—, —S—, —Se—, —CH═CH—, —C(R5)(R6)—, or —N(R7)—(wherein R5, R6 and R7 independently represent hydrogen atom, or a C1-6 alkyl group which may have a substituent);
• n and n′ independently represent 0, 1 or 2;
• Z1 and Z2 independently represent a nonmetallic atom group required to form a benzo-condensed ring which may have a substituent or a naphtho-condensed ring which may have a substituent;
• L1, L2, L3, L5, L6 and L7 independently represent a substituted or unsubstituted methine group, L4 represents a substituted methine group, provided that when n or n′ is 2, each two of L1 and L2, or each two of L6 and L7 may be the same or different;
• M−represents a counter ion in a number required for neutralizing electrical charge, and the combination of R3 and R4 is a combination that provides change in the maximum excitation wavelength of Fluorophore A represented by the formula (AI) before and after detection of the substance to be measured;
• Fluorophore B is a tetramethylindodicarbocyanine derivative;
• and
• S is a spacer of a length so that FRET is induced between Fluorophore A and Fluorophore B.