A visual Hg2+ detection strategy based on distance as readout by G-quadruplex DNAzyme on microfluidic paper

In this work, we have developed a simple, fast and visual Hg2+ detection strategy based on distance as readout on paper chip by the Hg2+-mediated formation of G-quadruplex-hemin DNAzymes. In the presence of Hg2+, the two oligonucleotides hybridize to form G-quadruplex DNA by T-Hg2+-T base pair, which was able to bind hemin to form the catalytically active G-quadruplex-hemin DNAzymes. Once DNAzymes were added to react with the precipitated 3,3,5,5-tetramethyl benzidine (TMB) immobilized on the sample area, a visible color band was produced, and the formed length was positively correlated with the concentration of Hg2+. This biosensor is capable of selectively detecting mercuric ions with good reproducibility and satisfactory dynamic range. The limit of detection was low to 0.23 nM. Therefore, this strategy not only provides a visual and quick screen of Hg2+, but also shows a promising future in monitoring analysis of other metal ions in POC diagnostic field.