Label-Free Impedimetric Aptasensor with Antifouling Surface Chemistry: a Prostate Specific Antigen Case Study

We here report the development of an aptamer based biosensor for the investigation of prostate specific antigen (PSA), a biomarker found in blood for prostate cancer (PCa), using electrochemical impedance spectroscopy (EIS). The study concentrates on a clinical application where a specific and sensitive label-free detection could be performed in real blood samples using EIS. Two types of PSA aptasensor were fabricated by immobilizing (i) a self-assembled monolayer comprising of 6-mercaptohexanol (MCH) and thiolated-DNA aptamer and (ii) 11-mercaptoundecanoic acid for covalent immobilization of amine terminated DNA aptamers and sulfo-betaine terminated thiol as an antifouling agent on a polycrystalline gold surface. Upon incubation of PSA with the DNA aptamer-based biosensor a decrease of charge transfer resistance (Rct) was observed due to the partial screening of the DNA aptamer charge by PSA. Thiol terminated sulfo-betaine based sensors can detect PSA levels lower than 1 ng/ml. Validation of the aptasensor is provided by surface plasmon resonance measurements. Sulfo-betaine moiety also prevents any significant non-specific binding of the control protein human serum albumin (HSA) as compared to high non-specific binding with MCH-based sensors. A debate is presented on the effect of the measurement buffer on the values and the direction of Rct changes on the aptasensors and the need to cautiously select suitable measurement conditions in order to obtain reliable results. This is the first report to date where a DNA aptamer-based biosensor was developed using thiol-terminated sulfo-betaine, which is a simple surface chemistry for the development of promising, cost-effective, label-free and sensitive electrochemical detection platforms for a range of analytes.