Highly sensitive voltammetric determination of arsenite by exploiting arsenite-induced conformational change of ssDNA and the electrochemical indicator Methylene Blue

The authors describe a voltammetric method for the determination of arsenite [As(III)] based on an As(III)-specific binding probe DNA (SBP DNA; a single-stranded DNA) and the electrochemical indicator Methylene Blue (MB). The SBP DNA was first hybridized with a capture probe (CP) DNA on the surface of a gold electrode. Then, MB was intercalated into the SBP/CP hybrid on the electrode. On addition of As(III), it specifically binds to the SBP DNA, and this results in a conformational change and the dissociation of the SBP DNA from the electrode into solution. Consequently, the amount of MB remaining on the modified electrode is reduced, and this decreases the peak current of MB (best measured at −0.28 V vs. SCE). The findings are exploited in an assay for As(III) that has a linear response in the 0.1 to 200 ppb concentration range and a detection limit as low as 75 ppt. Conceivably, this method can be extended by designing various specific ssDNA oligonucleotides for other heavy metal ions or for small molecules.