The G-quadruplex (GDNA) was immobilized on multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrode (CNT/GC). Prior to use, MWCNTs were treated with nitric acid and sulfuric acid to introduce the carboxylic acid groups. Then, the carboxylate-terminated MWCNTs covalently bound to the amino groups of GDNA (GDNA/CNT/GC), with the aid of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. Circular dichroism was used, to confirm the formation and stability of parallel form of GDNA in the solution using the named oligonucleotide and in the presence of the mentioned ligands. Morphologies of the modified electrodes in each step were studied by scanning electron microscopy. Characteristics of the modified electrodes and interaction of the immobilized GDNA with ethidium bromide/polyamines (spermine or spermidine) were investigated by different techniques. The cyclic voltammetry peak currents of [Ru(NH3)6]3+ decreased with increasing concentration of the employed ligands. The electrochemical impedance spectroscopy studies showed the increase of charge resistance after addition of each of these ligands, indicating their interaction with GDNA. The binding constant, the linear dynamic range and the limit of detection of this sensor was determined for each ligand. This modified electrode may be used as a label free nanobiosensor for detection of ligand–GDNA interaction, with respect to the binding mechanism.
