Colorimetric determination of the microcystin leucine-arginine based on the use of a hairpin aptamer, graphene oxide, and Methylene Blue acting as an optical probe

The authors describe an aptamer-based colorimetric assay for the specific detection of the toxic microcystin leucine-arginine (MC-LR). The method is based on the use of graphene oxide (GO) in a hydrogel matrix, Methylene Blue (MB) acting as an optical probe, streptavidin-modified magnetic particles, and an aptamer (Apt) as a the recognition probe. A complementary strand of the aptamer and adenosine act as promoters in the formation of the GO hydrogel, and the hairpin structure of the aptamer warrants selectivity. In the absence of MC-LR, the gelation of GO sheets is promoted by complementary strand and adenosine, and MB is trapped between GO sheets. Hence, no blue color can be observed in the sample solution. In the presence of MC-LR, GO does not cause the formation of a strong hydrogel structure and a distinct blue color can be observed and measured (at 660 nm) because MB is present in solution. The assay has high selectivity for MC-LR, with a limit of detection (LOD) as low as 219 pM. It was successfully applied to the determination of MC-LR in spiked tap water and serum samples where it gave LODs of 221 and 412 pM, respectively.