3, 3'-Diaminobenzidine with dual o-phenylenediamine groups: two in one enables visual colorimetric detection of nitric oxide.

Nitric oxide (NO) plays an important role in the generation of smog and ozone. Although great efforts have been made to determine NO by using o-phenylenediamine (OPD)-based fluorescent probes, more simple and reliable colorimetric assays for detection of NO are extremely scarce because a single OPD structure cannot produce enough optical absorption for chromogenesis. In this study, we report an innovative two-in-one visual colorimetric methodology. Commercially available 3,3'-diaminobenzidine (DAB) with two OPD structures in a single molecule is selected as the colorimetric probe, and it reacts with NO via diazo-coupling reaction to generate 1H,3'H-[5,5']bibenzotriazolyl because of the increase of conjugated double bonds, accompanying a distinct color change from colorless to brownish yellow. This two-in-one colorimetric assay can determine NO at a concentration as low as 3 ppm by the naked eye and 40 ppb by UV-vis spectrometry, which is the lowest limit of detection (LOD) among reported colorimetric assays for NO. Moreover, the present two-in-one visual colorimetric assay also has good selectivity toward NO over other common potential gas interferents such as CO2, NO2, NH3, N2, O2, and SO2. This present study provides a new insight for the design and development of assays for NO. Graphical abstract.