Methylene blue-based distinguishing DNA conformation for colorimetric detection of silver ions

A highly sensitive colorimetric sensing strategy based on the interaction between methylene blue (MB) and C-rich single-stranded DNA (ssDNA) (24C used here) has been developed for detection of Ag+. In the absence of target Ag+, the interaction between MB and 24C makes the color immediately change from blue to purple. However, specific C-Ag+-C pair forms when Ag+ ions are present in the aqueous solution containing 24C and MB, resulting in the desorption of MB from the surface of 24C into the solution. As a result, the color changes from purple to blue, and the absorbance values of MB at 660 and 605 nm gradually decrease. The detection limit can be as low as 1 nM, which satisfies the guideline concentration of Ag+ in drinking water set by the United States Environmental Protection Agency (EPA). The practical use of this method for Ag+ detection in river water samples is also demonstrated successfully.