Figure 5.

To overcome the limitations inherent to the surface attached depletants (which are easily saturated), we also show that the depletant probe can be simply added in solution at a fixed concentration. Here we use an unlabeled non-signalling probe (with the exact same sequence of the signalling redox-labelled probe) that sequesters the target DNA till a threshold level (fixed by the depletant concentration in solution) over which further increase in target concentration results in a steep dose-response curve. Because the depletant is free in solution, it rapidly reacts with the target (and with higher affinity) before this later can diffuse to the electrode surface and “activate” the signalling probe. (B) By using different concentrations of depletant in the reaction mix (0, 0.05, 0.4, 2, 6 μM) we can not only achieve steeper transitions than those observed with the depletant co-immobilized with the probe but we can also easily tune the threshold level at which we observe the sharp digital-like response of the sensor.