Here is how the system works (Figure 1). At the beginning, the two strands of DNA are restricted in closed loop structures, and they are known as ‘hairpins’. The 2 redox tags (MB and Fc) positioned on them can transfer 2 respective electrons to a proximal electrode (colored in yellow in the figure). If for some reason, the hairpin is released to become a straight strand, the redox tag is too far away to transmit the electron to the electrode, and a decrease in intensity of the I/V plot will be observed. This ‘proximity effect’ is indeed very common in molecular biology, and I believe my biological readers are definitely aware of something called ‘FRET’.
This is where the ‘logic’ comes into play: when the 2 corresponding single-stranded DNA comes to bind with each of this hairpin, the effect will the release of the hairpin and a decrease of I/V will be the output. For course, the output will be ‘1’ if both single-stranded DNAs (inputs) are present to release the two hairpins (outputs). The observed I/V characteristics on Figure 2 clearly illustrates the action of the genosensor. Impressive!