Thiol-mediated etching of gold nanorods as a neoteric strategy for room-temperature and multicolor detection of nitrite and nitrate

Abstract

The excessive presence of nitrite and nitrate in environmental matrixes has raised concerns among the scientific communities due to their negative impacts on human health and living organisms. Considering the necessity of regular monitoring and rapid evaluation of nitrite and nitrate, it is of great interest to develop methods capable of on-site detection of these compounds. This study presents a non-aggregation colorimetric method based on etching gold nanorods (AuNRs) for visual detection of nitrite and nitrate. Instead of temperature, we propose using thiourea as a sulfur-containing compound to accelerate the rate of AuNR etching. Thiourea forms stable cationic species with Au⁺ ions and consequently speeds up the etching process by reducing the redox potential of Au⁺/Au. In the presence of thiourea, the AuNRs are etched by nitrite resulting in wide obvious color changes from brown to light brown, green, blue, purple, pink, and colorless. In addition to nitrite, the developed method is capable of nitrate determination by reducing nitrate to nitrite through acid-washed zinc powder and is the first report of colorful detection of nitrate. Under optimized conditions, a good linear relationship was found between nitrite/nitrate concentration and the colorimetric response in the range of 8.0 to 100 μmol L⁻¹ and 0.5 to 3.0 mmol L⁻¹ with a limit of detection (LOD) as low as 1.3 μmol L⁻¹ and 173.3 μmol L⁻¹ for nitrite and nitrate, respectively. Furthermore, the practical application of our developed probe was confirmed by accurate determination of nitrite and nitrate in various complex media including water samples, soil extracts, and food products such as salami and sausage.