An ultra-sensitive colorimetric Hg2+-sensing assay based on DNAzyme-modified Au NP aggregation, MNPs and an endonuclease

• A colorimetric assay for detection of trace Hg2+.
• Use of Au nanoparticle aggregates and endonuclease amplifies the colorimetric signal and decreases the background.
• The assay can be applied in drinking water and river water samples.
• A low Hg2+ detection limit (0.8 nM) was reported.

This paper reports the development of an ultra-sensitive colorimetric method for the detection of trace mercury ions involving DNAzymes, Au nanoparticle aggregation, magnetic nanoparticles and an endonuclease. DNAzyme-sensing elements are conjugated to the surface of Au nanoparticle-2, which can crosslink with the T-rich strands coated on Au nanoparticle-1 to form Au nanoparticle aggregation. Other T-rich stands are immobilized on the surface of MNPs. The specific hybridization of these two T-rich strands depends on the presence of Hg2+, resulting in the formation of a T-Hg2+-T structure. Added endonuclease then digests the hybridized strands, and DNAzyme-modified Au NP aggregation is released, catalysing the conversion of the colourless ABTS into a blue-green product by H2O2-mediated oxidation. The increase in the adsorption spectrum of ABTS+ at 421 nm is related to the concentration of Hg2+. This assay was validated by detecting mercury ion concentrations in river water. The colorimetric responses were not significantly altered in the presence of 100-fold excesses of other metal ions such as Zn2+, Pb2+, Cd2+, Mn2+, Ca2+ and Ni2+. The inclusion of both Au NP aggregation and an endonuclease enables the assay to eliminate interference from the magnetic nanoparticles with colorimetric detection, decrease the background and improve the detection sensitivity. The calibration curve of the assay was linear over the range of Hg2+ concentrations from 1 to 30 nM, and the detection limit was 0.8 nM, which is far lower than the 10 nM US EPA limit for drinking water.

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