RB-OPD, also known as NO-red, is a fluorescent probe that combines o-phenylenediamine (OPD) and rhodamine nitric oxide (NO). With a λex of 550 nm and a λem of 590 nm, it exhibits exceptional sensitivity and selectivity.

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[1].