 signals in PBS (curve a) and very small currents in BSA (curve b), 1000 fM ncDNA(curve c) and 10 fM ctDNA (curve d). However, a well-defined voltammetric peak from 1000 fM ctDNA loading on SPE is distinguished (curve e), indicating the strong affinity among PNA, ctDNA and anti-5-mC antibodies to form sandwich-like PNA-AuNPs/ctDNA/LPA-anti-5- mC complex. These observations suggest that the effects of non-specific adsorption could be neglected.

Optimization of detection conditions for ctDNA
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Since the PNA-AuNPs conjugates were used here as capturing platform, the amount of PNA on the surface of AuNPs influenced the amount of captured ctDNA, and this accounted for the amount of LPA-anti-5-mC. One can see from Fig. [3](#F3){ref-type="fig"}A that the voltammetric currents increased upon raising the concentrations of the PNA on the surface of AuNPs and achieved the highest signal at 6 mM PNA. Thus, 6 mM PNA was used in the experiments.

The current responses also depended on the incubation time of both PNA capturing and LPA-anti-5-mC recognition of ctDNA. As shown in Fig. [3](#F3){ref-type="fig"}B that both the responses increased with increasing incubation time and tended to steady values after 30 min (black and red curve), which was used for the construction of DNA sensor. These results were consistent with the EQCM results above.

Electrochemical measurements on the DNA biosensor
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For the detection of ctDNA, high sensitivity and selectivity play important roles in improving the treatment. In view of this, the proposed DNA biosensor was challenged with different concentrations of ctDNA, as shown in Fig. [4](#F4){ref-type="fig"}. The SWV currents (from lead ions) increased with the increasing concentration of ctDNA from 20 fM to 10000 fM. The resulting calibration plot of current versus known concentration of ctDNA (Fig. [4](#F4){ref-type="fig"} inset) is linear over 50 fM to 10000 fM range, with the detection limit of 1