Label-free RNA aptamer-based capacitive biosensor for the detection of C-reactive protein

In this study, we report a novel aptamer-based capacitive label-free biosensor for monitoring transducing aptamer–protein recognition events, based on charge distribution under the applied frequency by non-Faradaic impedance spectroscopy (NFIS). This approach to capacitive biosensors is reported for the first time in this study, is reagent-less in processing and is developed using gold interdigitated (GID) capacitor arrays functionalized with synthetic RNA aptamers. The RNA atpamers served as biorecognition elements for C-reactive protein (CRP), a biomarker for cardiovascular disease risk (CVR). The signal is generated as a result of the change in relative capacitance occurring as a result of the formation of an RNA–CRP complex on GID capacitors with the applied AC electrical frequency (50–350 MHz). The dispersion peak of the capacitance curve was dependent on the CRP concentration and tends to shift toward lower frequencies, accompanied by the increase in relaxation time due to the increased size of the aptamer–CRP complex. The dissociation constant (Kd) calculated from the non-linear regression analysis of the relative capacitance change with the applied frequency showed that strong binding of CRP occurred at 208 MHz (Kd = 1.6 μM) followed by 150 MHz (Kd = 4.2 μM) and 306 MHz (Kd = 3.4 μM) frequencies. The dynamic detection range for CRP is determined to be within 100–500 pg ml−1. Our results demonstrates the behavior of an RNA–protein complex on GID capacitors under an applied electric field, which can be extended to other pairs of affinity biomolecules as well as for the development of electrical biosensor systems for different applications, including the early diagnosis of diseases.