Figure 2.

Change in electron transfer rate upon target binding provides a mechanism for tuning the signal gain of the E-DNA clamp-switch probe. Following target binding, the clamp-switch probe folds back to form a triplex structure, and the methylene blue reporter is held in close proximity to the electrode surface, providing faster electron transfer than the unbound probe, which has more freedom to occupy positions distant from the electrode surface. (a) The ratio of the measured peak current to SWV frequency (i_p/f) as a function of frequency provides a way to measure the apparent electron transfer rate of the methylene blue reporter.²² The bound E-DNA triplex (black) has a critical frequency around 100 Hz, for an apparent electron transfer rate of ∼85 s^–1.The unbound free probe (blue) has a critical frequency ≤10 Hz, showing much slower electron transfer. (b) By varying the SWV frequency used to measure the probe, the ratio of signal between bound and unbound states is variable, providing highly tunable signaling characteristics. For most measurement frequencies, the signal current increases upon target binding with signal gain of up to 400% for measured frequencies. Only when the frequency falls below 25 Hz, a time scale in which the rapid electron transfer of the bound state rapidly exhausts the signaling current, the observed signal of the unbound probe is higher than that in the presence of the target (signal-off behavior). For a matter of clarity in these binding curves and in those in the following figures, error bars have been depicted for only one point on each curve and represent the average and standard deviations of measurements performed on at least three independent sensors.