Gold electrodes

Two millimeter diameter gold electrodes (CH Instruments) were mechanically polished using a polishing pad (Buehler) and sequential polishing using alumina powder of 0.3 μm then 0.05 μm followed by polishing with a bare polishing pad. In between each polishing stage, electrodes were cleaned by sonication in ultrapure (Milli-Q) water for 10 min. After the mechanical polishing was completed each electrode was further electrochemically cleaned by cycling in 0.5 M H₂SO₄ between −0.1 and 1.5 V vs. Saturated Calomel Electrode (SCE) for 60 cycles at 50 mV s⁻¹. Immediately after cleaning, electrodes were washed in ultrapure (Milli-Q) water, dried then soaked in probe solutions at a concentration of 1 μM in 10 mM Tris HCl buffer pH 7.5 containing 0.5 M NaCl for 16 h. The electrode surfaces were washed in buffer to remove excess DNA, then soaked in 1 mM mercaptohexanol in 10 mM Tris HCl buffer pH 7.5 containing 0.5 M NaCl for a further 2 h to ensure a passivated surface and a good presentation of the DNA probe in an upright position (Lee et al., 2006 (link)). Electrodes were washed in buffer (10 mM Tris HCl pH 7.5 containing 0.5 M NaCl) prior to use. For each experiment, three replicate electrodes were used for each target DNA to ensure that results were consistent for each target DNA product.

The aptasensors were prepared using gold electrodes of 2 mm diameter (CH Instruments, Austin, TX, USA). The electrodes were carefully cleaned according to the procedure described earlier [34 (link),35 (link)]. The gold surface was polished using an electrode-polishing kit consisting of 1.0 and 0.3 μm alumina powder (CH Instruments) and then sonicated for 15 min in ethanol followed by 20 successive cycles of electrochemical cleaning in 1 M H₂SO₄ in the potential range from +0.2 to +1.5 V vs. Ag/AgCl reference electrode with a scan rate of 100 mV/s. After that the electrodes were properly rinsed again in deionized water, ethanol and dried under nitrogen and the immobilization started immediately.