Impedimetric genosensors employing COOH-modified carbon nanotube screen-printed electrodes

Screen-printed electrodes modified with carboxyl functionalised multi-walled carbon nanotubes were used as platforms for impedimetric genosensing of oligonucleotide sequences specific for transgenic insect resistant Bt maize. After covalent immobilization of aminated DNA probe using carbodiimide chemistry, the impedance measurement was performed in a solution containing the redox marker ferrocyanide/ferricyanide. A complementary oligomer (target) was then added, its hybridization was promoted and the measurement performed as before. The change of interfacial charge transfer resistance between the solution and the electrode surface, experimented by the redox marker at the applied potential, was recorded to confirm the hybrid formation. Non-complementary DNA sequences containing a different number of base mismatches were also employed in the experiments in order to test specificity. A signal amplification protocol was then performed, using a biotinylated complementary target to capture streptavidin modified gold nanoparticles, thus increasing the final impedimetric signal (LOD improved from 72 to 22 fmol, maintaining a good reproducibility, in fact RSD<12.8% in all examined cases). In order to visualize the presence and distribution of gold nanoparticles, a silver enhancement treatment was applied to electrodes already modified with DNA-nanoparticles conjugate, allowing direct observation by scanning electron microscopy.