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.