Nitric oxide (NO•) is involved in various physiological and pathological processes in the cell and is implicated in vasodilation, neurotransmission, cytotoxicity, immune response and inflammation. Within cells, nitric oxide synthase (NOS) catalyses the conversion of arginine to citrulline and NO• in the presence of molecular oxygen, tetrahydrobiopterin, NADPH and flavin cofactors. Due to the importance of NO•, real time detection and quantification of NO• is of great interest. However, the extremely short half-life of NO• limits the study of its physiological effect in vivo. Therefore NO• -sensitive fluorescent probes, such as DAF-2 have been designed and used in real-time imaging of NO•.