Bare eye detection of Hg(II) ions based on enzyme inhibition and using mercaptoethanol as a reagent to improve selectivity

The authors describe a colorimetric method for the determination of Hg²⁺ ions based on the inhibition of the activity of the enzyme urease. The pH value of solution increases when urease hydrolyzes urea, which can be visualized by adding a pH indicator such as Phenol Red (PhR). Mercaptoethanol as a typical thiol is added to the system to improve selectivity because it binds metal ions and then - unlike the Hg²⁺ mercaptoethanol complex - does not inhibit urease. Hence, the color of the pH indicator PhR turns from yellow to pink as the solution becomes alkaline. The Hg²⁺ mercaptoethanol complex, in contrast, strongly inhibits urease and the color of the solution remains yellow. The findings were used to design a photometric assay based on the measurement of the ratio of absorptions of PhR at 558 nm and 430 nm. It has a linear response over the 25 to 40 nM Hg²⁺ concentration range and a 5 nM detection limit. This is well below the guideline values of Hg²⁺ specified by the US Environmental Protection Agency and the World Health Organization for drinking water (10 nM and 30 nM, respectively). The method was employed to the determination of Hg²⁺ in water samples spiked with 10 nM levels of Hg²⁺ where color changes still can be observed visually. Graphical Abstract Schematic presentation of a colorimetric method for the ultrasensitive detection of Hg²⁺ based on the inhibition of urease activity. Mercaptoethanol is used to improve the selectivity. Even at Hg²⁺ concentrations as low as 5 nM, the color change still can be easily observed by bare eyes.