Synthesis and evaluation of a nano silver/Congo red-based colorimetric sensor for sensing mercury ions in water

Colorimetric detection of Hg²⁺ ions is one of the most promising and highly needed areas of environmental research. Silver nanoparticles (AgNPs) provide an efficient platform for visual detection of Hg²⁺ ions because these particles are easy to synthesize. However, the application of AgNPs suffers from certain shortcomings such as AgNPs aggregation, low detection limit, and poor selectivity. In the present study, AgNPs have been synthesized using a conventional method. After synthesis, AgNPs were treated with Congo red forming complex (Ag–CR). The synthesis procedure adopted was to prevent AgNPs from aggregation and to achieve higher sensitivity and selectivity. Ag–CR exhibited its function as a colorimetric sensor and facilitated clear visual detection of Hg²⁺ ions in aqueous media. After treatment, the solutions turned from yellow to colorless in just 30 s. The sensor can sense Hg ion up to the minimum concentration of 11 nM limit of detection (LOD). Similarly, limit of quantification was found to be 35 nM. Ag–CR efficiently sensed Hg²⁺ in real samples also. In comparison to AgNPs in the bare form, that is, without CR, Ag–CR imparts stability by blocking AgNPs from aggregation, removing toxicity of CR, exhibiting higher selectivity, and detecting only Hg²⁺ ions in water.