RB-OPD (NO-red) is a o-phenylenediamine (OPD)-locked rhodamine nitric oxide (NO) fluorescent probe with great sensitivity and selectivity (λex=550 nm, λem=590 nm).In Vitro: RB-OPD (compound 1) exhibits “turn-on” type fluorogenic and chromogenic behavior toward NO in aqueous solution with great sensitivity and selectivity, and it can be used for the sensing of NO under normal physiological conditions. NO-red is composed of two moieties: rhodamine B spirolactam as the potential strong fluorophore and chromophore, and o-phenylenediamine as an NO-reactive group in a lactam form as a “masked” NO-sensitive modulator.