Figure 2: Three methodologies applied in the detection of thrombin using redox labeled aptamers. (a) An unfolded methylene blue labeled aptamer initially enables electron transfer between the probe and the electrode (signal on), a capability removed on target binding (signal off). (b) In an alternative approach, a long aptamer is utilized in which the redox reporter is initially too remote from the underlying electrode (signal off). After targeting binding, the ferrocene label approaches the electrode, increasing the electrochemical signal (signal on). (c) A DNA-duplex probe comprising an anti-thrombin aptamer and a complementary methylene blue labeled nucleic acid. In the presence of thrombin, the complementary sequence is liberated while the aptamer sequence forms a thrombin-binding complex. This event enables the methylene blue tag to access the electrode (signal on).