Etchant and etching method

Fluorescent probes represented by the general formula (I) wherein R and R are each hydrogen or a substituent for capturing a proton, a metal ion, or an active oxygen species; R is a monovalent substituent except hydrogen, carboxyl, and sulfo; R and R are each hydrogen, halogeno, or alkyl; R to R are each alkyl; R and R are each hydrogen, halogeno, or alkyl; M is a counter ion; and before the capture of a proton, a metal ion, or an active oxygen species, the combination of R , R , and R gives the benzene ring to which they are bonded such a substantially high electron density as to make the compound substantially nonfluorescent, while after the capture of a proton, a metal ion, or an active oxygen species, the combination brings about such a substantial lowering in the electron density of the benzene ring to which they are bonded as to make the compound substantially highly fluorescent.