A novel, selective and sensitive colorimetric detection method for Hg2+ is developed using Bismarck brown R (BBR) reductive products-immobilised silver nanoparticles (BBR products-AgNPs) as a sensing probe. In this research, the BBR reductive products were synthesised in situ and immediately immobilised on the surface of AgNPs. A surface plasmon resonance band of BBR products-AgNPs was observed at 396 nm. The absorbance at 396 nm gradually decreased, and a remarkable blue shift from 396 to 368 nm and a colour change from yellow to colourless by increasing the Hg2+ concentration were observed. This observation is due to the oxidation of Ag° to Ag+ and the reduction of Hg2+ to Hg° during the process, and thus Ag–Hg nanoalloy is formed. In addition, an exploratory study of the designed system is done by chemometrics approaches with the aim of making a clear perspective from the reaction system. The effective parameters for detection of Hg2+ based on the BBR products-AgNPs as optical probe were optimised. Under optimal conditions, the sensor exhibited a high selectivity and sensitivity with limit of detection of 6.1 nM (1.66 µg L−1) and the linear range from 0.01–19.0 µM (2.72–5160 µg L−1). The proposed method was simple, rapid and cost-effective without any complicated operation. Finally, it was successfully used for detection of Hg2+ in water samples with recoveries ranges of 86.7–103.6% and relative standard
