The measurements were made in a low ionic strength buffer (very diluted phosphate buffer with 10 mM NaCl) in order to slow down the current decay following the potential step. This allows an
easier sampling of the discharge current. In our experiments, a sampling frequency of 20 kHz was used (higher sampling frequencies permit measurements in higher ionic-strength solutions).
The subsequent data analysis is used to calculate the electrical capacitance by fitting the initial portion of the current decay with an RC model (in these experiments, linear regressions with r¼0.99 were obtained from fitting the first 500 ms of the decay). The capacitance at the electrode-solution interface was monitored before and after the hybridization events, therefore allowing the measurement of the capacitance variations due to the hybridization of target DNA and the HCR. Calibration curves were obtained working with series of different target concentrations (Fig. 5).