2.4. DNA sensor development Prior to electrode modification, silicon chip was cleaned by sonicating in ethanol and DI water, for ten minutes each. Once clean, the chip was dried in a stream of nitrogen, placed in a chip holder and connected to the potentiostat using either an SD connector or probe pins. Each sensor IDE was modified with gold nanoparticles, electrodeposited by applying -0.2 V for 60 s in 500 µL of 400 ppm HAuCl4 dissolved in 10 mM sodium acetate, pH 3. Subsequently, Cht-Au nano-complex was electrodeposited on top of the pre-AuNPs modified IDE. The solution was prepared by diluting chitosan from a stock solution (0.2%) to 0.04% (v/v) with DI water containing 1% acetic acid. Afterwards, 1 M NaOH was added to the solution to increase its final pH to ~pH 5. Gold ions from a (DI water) stock solution were added to chitosan just before the deposition to achieve a final concentration of 50 ppm HAuCl4. The Cht-Au solution was mixed using a vortex and 500 µL was added to the sample well. A voltage of -1.5 V was applied for 15 seconds to each electrode. Subsequently, the modified chip was immersed in 0.01 M PBS buffer, pH 7, for 10 minutes to increase the stability of the layer, washed thoroughly with DI water and dried in a stream of nitrogen. The electrodes were then immersed in 0.2% (v/v) glutaraldehyde for 2 hours. Afterwards, a 50 µL droplet of 0.5 µM aminemodified probe ssDNA was deposited on top of an electrode, incubated for 2 hours, then the unattached probe was removed with DI water.