Direct electrochemical sensor for label-free DNA detection based on zero current potentiometry. 
A direct electrochemical DNA biosensor based on zero current potentiometry was fabricated by immobilization of ssDNA onto gold nanoparticles (AuNPs) coated pencil graphite electrode (PGE).  One ssDNA/AuNPs/PGE was connected in series between clips of working and counter electrodes of a potentiostat, and then immersed into the solution together with a reference electrode, establishing a novel DNA biosensor for specific DNA detection.  The variation of zero current potential difference (ΔE(zcp)) before and after hybridization of the self-assembled probe DNA with the target DNA was used as a signal to characterize and quantify the target DNA sequence.  The whole DNA biosensor fabrication process was characterized by cyclic voltammetry and electrochemical impedance spectroscopy with the use of ferricyanide as an electrochemical redox indicator.  Under the optimized conditions, ΔE(zcp) was linear with the concentrations of the complementary target DNA in the range from 10nM to 1μM, with a detection limit of 6.9nM. The DNA biosensor showed a good reproducibility and selectivity.  Prepared DNA biosensor is facile and sensitive, and it eliminates the need of using exogenous reagents to monitor the oligonucleotides hybridization.