2.2.2. Electrochemical Characterization and Measurement

The immobilization of the aptamer was confirmed by scanning electron microscopy (SEM) using a FlexSEM 1000 instrument (Hitachi, Düsseldorf, Germany) at an acceleration voltage of 20 kV and characterized by cyclic voltammetry using Gamry Reference 600+ potentiostat (Gamry instruments, Warminster, PA, USA) with scanning ranges from −0.7 to +0.5 V, scan rate 50 mV s⁻¹. The SWV measurements were performed with a scanning range from −0.5 to +0.5 V at a frequency of 60 Hz, step size of 4 mV, and an amplitude of 40 mV. All measurements were performed at laboratory RT (23 ± 1 °C). The decrease of the peak currents (ΔI) was defined as the different currents in the absence and presence of the target bacterium measured by SWV and was calculated by:

The signal loss is the percentage of decrease of peak current compared to the current measured in the absence and presence of the target bacterium and was calculated as:

The modified SPEs were stored at 4 °C in 10 mM Tris buffer, pH 7.0, for several weeks. Every week, three modified electrodes underwent the SWV measurement for a period of 5 weeks. The signal loss in percentage every week was recorded compared to the initial SWV measurement as in Equation (2). The stability of the SWV signal was tested for 2 h in PBS 1X buffer, pH 7.4. The signal loss in percentage was recorded every 15 min and was calculated as in Equation (2) above.