Correlating electrochemical impedance spectroscopy and quartz crystal microbalance with dissipation signals for optimisation of aptamer-based biosensors

DNA aptamers provide new avenues towards the development of a wide range of biosensors with high specificity and controlled surface chemistry for signal optimisation. Electrochemical Impedance Spectroscopy (EIS) is an extremely promising technique towards the development of DNA-based biosensors since it can be used to accurately monitor changes in the charge density of the DNA layer. However, EIS optimisation for DNA aptamers is somewhat complex due to the different characteristics that induce a signal change: namely DNA density, change in charge density close to the electrode upon DNA conformational changes, size and charge of the analyte, screening of DNA charges upon analyte binding. The use of techniques such as Quartz Crystal Microbalance with Dissipation mode (QCM-D) can provide valuable information about conditions for maximum analyte binding as well as the hydration, folding and behaviour of the aptamer distribution on the electrode.

We here report on the correlation of EIS and QCM-D signals for optimisation of aptamer-based biosensors using a DNA aptamer against Prostate Specific Antigen (PSA) as a case study. Given the unreliability of current prostate cancer diagnostic tools, it is critical to develop prostate cancer biosensors with high selectivity and sensitivity. Although PSA by itself is not an ideal prostate cancer biomarker, it is believed that any reliable diagnosis will involve a panel of biomarkers comprising PSA.

The sensor setup comprises a gold surface modified with a mixed Self Assembled Monolayer (SAM) made of 6-mercapto-hexanol (MCH) and thiolated-DNA aptamer. QCM-D results provided the optimal MCH:aptamer ratios, pH and buffer solutions for the binding of PSA to the aptamers. The efficiencies of the two techniques under similar conditions were compared and correlated in order to develop a reliable, label-free and cost effective biosensor for prostate cancer as well as to serve as support for further EIS aptamer-based sensors.