A colorimetric assay for the determination of heavy metal ions is presented that is based on the regulation of the oxidase-mimicking activity of Mn3O4 nanoparticles (NPs) by oligonucleotides. The chromogenic agent tetramethylbenzidine (TMB) is oxidized by the catalytic action of Mn3O4 NPs to generate products that have a yellow color in acidic solution, with a peak at 450 nm. It is found that random oligonucleotides are absorbed on the regular surface of the Mn3O4 NPs and temporarily inhibit the oxidation of TMB. This leads to a decrease in absorbance and a light-green coloration of the solution. The results show that the purine bases in oligonucleotides play a key role in their regulation of the activity of the NPs. The regulatory effect is assumed to be of the noncompetitive type. In the presence of heavy metal ions like Hg(II) or Cd(II), the inhibition is canceled due to the binding of heavy metal ions to thymine bases, and the color of the solution changes from light green to yellow. The increase in absorbance at 450 nm is related to the amount of heavy metal ions present. The method allows Hg(II) and Cd(II) to be determined visually in concentrations as low as 20 μg·L-1. The detection limit of the colorimetric assay is 3.8 and 2.4 μg·L-1 of Hg(II) and Cd(II), respectively. The assay displays good selectivity over other heavy metal ions. The method was successfully validated by analyzing several water samples. Graphical abstract Schematic representation of the colorimetric assay for Hg(II) and Cd(II) based on the intrinsic oxidase-mimicking activity of Mn3O4 nanoparticles that is regulated by oligonucleotides.
Keywords: Cadmium; Inhibition; Mercury; Nanozymes; Visual assay.