The ability to detect specific oligonucleotides in complex, contaminant-ridden samples, without the use of exogenous reagents and using a reusable, fully electronic platform could revolutionize the detection of pathogens in the clinic and in the field. Here, we characterize a label-free, electronic sensor, termed E-DNA, for its ability to simultaneously meet these challenging demands. We find that because signal generation is coupled to a hybridization-linked conformational change, rather than to only adsorption to the sensor surface, E-DNA is selective enough to detect oligonucleotides in complex, multicomponent samples, such as blood serum and soil. Moreover, E-DNA signaling is monotonically related to target complementarity, allowing the sensor to discriminate between mismatched targets: we readily detect the complementary 17-base target against a 50 000-fold excess of genomic DNA, can distinguish a three-base mismatch from perfect target directly in blood serum, and under ideal conditions, observe statistically significant differences between single-base mismatches. Finally, because the sensing components are linked to the electrode surface, E-DNA is reusable: a 30-s room temperature wash recovers >99% of the sensor signal. This work further supports the utility of E-DNA as a rapid, specific, and convenient method for the detection of DNA and RNA sequences.