Colorimetric recognition and sensing of nitrite with unmodified gold nanoparticles based on a specific diazo reaction with phenylenediamine

A colorimetric sensor for nitrite ion with high selectivity and sensitivity by unmodified citrate-capped gold nanoparticles (Au NPs) is presented. Recognition of nitrite is developed on the basis of a highly specific diazo reaction between nitrite and phenylenediamine (PDA). PDA caused the Au NPs to aggregate owing to the strong covalent NH-Au bond, with a clear color change of solution from red to blue being visualized. In the presence of phosphoric acid and nitrite, the amines of PDA would readily be converted to diazo bonds, and a red solution was observed after the subsequent addition of Au suspension due to the much less strength of electrostatic interaction between the positive diazo groups and the negative citrate-capped Au NPs. With this colorimetric light-up method, <1 ppm of nitrite can be easily detected within 5 min at room temperature without instrumentation. Since the diazo reaction and the colorimetric response are separate, this approach features the use of pristine Au NPs in an assay where acidic environment is a necessity, making it a more convenient and cost-effective method for the sensing of nitrite when compared with those utilizing chemically modified Au NPs.