A method of detecting peroxynitrite in a sample is described comprising the steps of: (a) providing a complex of a saccharide with an aryl boronate compound of formula (I): Fp-L1-Z-L2-Ar—B(OH)2 (I) wherein: Fp comprises a fluorophore L1 and L2 are linker groups Z is a fluorescence switch and Ar is optionally substituted aryl (b) contacting said aryl boronate-saccharide complex with said sample, whereby peroxynitrite in said sample cleaves said aryl boronate-saccharide complex to produce a compound of formula (II): F-L1-Z-L2-Ar—OH (II) and (c) detecting a decrease in a fluorescence intensity of said fluorophore resulting from said cleavage reaction in step (b). Peroxynitrite reacts quantitatively, rapidly, and selectively in step (b) of the reaction, whereby medical conditions associated with elevated peroxynitrite can be diagnosed. Also provided are compounds of formula (I) for use in the methods.
