Detection analysis limit of nonlinear characteristics of DNA sensors with the surface modified by polypyrrole nanowires and gold nanoparticles

Surface modification of interdigitated DNA sensors by polypyrrole nanowires and gold nanoparticles has been analyzed systematically. Polypyrrole nanowires with diameter of 200 nm and length of 5 μm were electrochemically synthesized on the gold surface of interdigitated electrodes and subsequently decorated with 20 nm gold nanoparticles. Electrochemical impedance spectroscopy and differential voltage measurements were conducted to detect DNA concentrations. We have observed a logarithmic dependence of analytical signals on the DNA concentration. A formula for estimating the limit of detection has been derived. Instead of using a conventional method in which a blank measurement is performed to record the response of the sensor in a solution containing non-complementary DNA molecules, causing an errornous estimation of detection limit, we have proposed a novel approach for the calculation of the limit of detection. Limits of detection of 60.0 fM for the differential voltage method and of 84.5 fM for the electrochemical impedance spectroscopy method were calculated after taking into account all possible errors. These are the lowest values for the DNA sensors reported so far. The presence of the gold nanoparticles increases the effective electrode area, leading to an overall improvement of the detection limit. From the perspective of the detection limit, the differential voltage method is considered more advantageous as compared to the electrochemical impedance spectroscopy one.