The invention provides a general “signal-on” architecture for oligonucleotide-based detectors that leads to order of magnitude increases in signal gain and sensitivity as compared to prior art detectors. The detectors of the invention rely on base pairing between two oligonucleotide strands, the sensor strand and the blocker strand. In the ‘off’ position of the detector, i.e., in the absence of target, the blocker strand and sensor strand are base-paired. As shown in FIG. 1, the formation of comparatively rigid, duplex DNA prevents the redox moiety from approaching the electrode surface, thereby suppressing Faradaic currents. When target is added to the system, the target displaces the blocker strand, binds to the sensor strand, liberating the end of the redox-labeled oligonucleotide to produce a flexible element. This, in turn, allows the redox moiety to collide with the electrode surface, producing a readily detectable Faradaic current.