Optimisation of an electrochemical impedance spectroscopy aptasensor by exploiting quartz crystal microbalance with dissipation signals

The response of an Electrochemical Impedance Spectroscopy (EIS) sensor using DNA aptamers is affected by many factors, such as DNA density, charge and conformational changes upon DNA-target binding, and buffer conditions. We report here for the first time on the optimisation of an EIS aptamer-based sensor by using Quartz Crystal Microbalance with Dissipation mode (QCM-D). As a case study, we employed a DNA aptamer against Prostate-Specific Antigen (PSA). PSA detection was achieved by functionalising the gold sensor surface via thiol chemistry with different ratios of thiolated-DNA aptamer and 6-mercapto-1-hexanol (MCH) used as spacer molecules. PSA binding efficiency can be monitored by measuring QCM-D signals which not only provide information about the mass of PSA bound on the sensor surface, but also crucial information about the aptamer conformation and layer hydration. Data generated through QCM-D analysis provided the optimal conditions in terms of aptamer/MCH ratio to maximise the PSA binding. The ratio of 1:200 for DNA aptamer/spacer molecule was found to be optimal for ensuring maximum PSA binding. However, this study showed how a maximum analyte binding does not necessarily correspond to a maximum EIS response, which revealed to be enhanced if a ratio of 1:100 for DNA aptamer/spacer molecule was used. Moreover, by monitoring the QCM-D signal, for the first time, a value of the dissociation constant (K_d), equal to 37 nM, was found for the PSA DNA aptamer towards its target. The combination of QCM-D with EIS techniques provides further insight into the effects of mass loading and charge effects that govern the response of an EIS aptasensor, serving as a valuable support for future EIS aptamer-based applications.