A naked-eye colorimetric assay for detection of Hg2+ ions in real water samples based on gold nanoparticles-catalyzed clock reaction

Designing a simple, inexpensive, and sensitive colorimetric sensor for the detection of toxic species in drinking water is of great importance. In this paper, we have demonstrated a simple and sensitive colorimetric assay for naked-eye detection of mercury ions (Hg²⁺) in water based on gold nanoparticles-catalyzed clock reaction. Citrate-capped Au nanoparticles (AuNPs) were used to catalyze the reduction of methylene blue (MB) to leucomethylene blue (LMB) by hydrazine. However, Hg²⁺ ions can reduce the catalytic activity of AuNPs in this clock reaction, resulting in the longer reaction time for color transformation of MB from blue to colorless LMB solution. Based on this principle, we have developed an ultrasensitive colorimetric assay that can selectively detect Hg²⁺ in the concentration range of 5 to 1000 nM with a limit of detection (LOD) of 4.3 nM, which is well below the maximum allowable concentration for Hg²⁺ in drinking water (10 nM) as defined by the United States Environmental Protection Agency. Owing to the rapidity and simplicity of the naked eye detection process, we expect that this AuNPs-catalyzed clock reaction could put forward an insight for the detection of toxic Hg²⁺ ions in real water samples, opening up new avenues for developing cost-effective and portable nanosensors for environmental applications.