Competitive Impedance Spectroscopy in a Schottky-Contacted ZnO Nanorod Structure for Ultrasensitive and Specific Biosensing in a Physiological Analyte.

To enable detection and discovery of biomarkers, development of label-free, ultrasensitive, and specific sensors is the need of the hour. For addressing this requirement, here, a Schottky-contacted ZnO nanorod biosensor has been demonstrated, which explores the interplay between Schottky junction capacitance and solution resistance, resulting in an interesting sensing principle of competitive impedance spectroscopy. When the transition of dominating impedance occurs from solution resistance to junction capacitance, a notch or a peak appears in the impedance response at a particular frequency (referred to as the corner frequency) depending on the charge of the target molecule. The appearance of the peak or notch acts like an electronic label for selectivity since it is visible only for target molecules even at ultralow concentrations in the physiological analyte, where the magnitude of impedance change overlaps with that for nonspecific molecules. This phenomenon has been successfully applied for the positively charged vascular endothelial growth factor (VEGF) and the negatively charged hepatitis B surface antigen (HBsAg), where the shifts in the higher corner frequencies for 1 aM concentration of the target molecules have been observed to be more than 3 times the changes in the impedance magnitude. Further, the area of the ZnO nanorods was segmented into two zones corresponding to the lower and higher concentration regimes, thereby expanding the dynamic range. To summarize, an ultralow detection limit of 1 aM with a dynamic range up to 1 pM was achieved for VEGF and HBsAg, which is 4 orders of magnitude and 20 times lower than their most sensitive label-free reports, respectively.

PSA detection using label free graphene FET with coplanar electrodes based microfluidic point of care diagnostic device.

Affordable point-of-care (PoC) diagnostic devices enable detection of prostate specific antigen (PSA) in resource limited settings. Despite the advancements in PoC systems, most of the reported methods for PSA detection have unsatisfactory detection limits and are based on labelled assays, requiring multiple reagent flow steps which increases both expenses and inconvenience. Circumventing these constraints, we report here the development and validation of a label free, affordable dielectrophoresis (DEP) based graphene field effect transistor (FET) sensor implemented using coplanar electrodes and integrated uniquely with a compact disc based microfluidic platform along with electronics readout for the estimation of PSA at the point of care. Design of coplanar gate electrode which has not been explored earlier is not a straightforward approach. In fact, it has been observed that there is a non-monotonic dependence of the capture of PSA molecules in the channel region of the FET with varying widths and spacings of the gate electrode. The graphene FET based PoC device with optimized coplanar gate electrode is the only label free analytical system for PSA detection requiring simple operation and achieving a detection limit of 1 pg/ml in serum with a wide dynamic range upto 4 ng/ml and appreciable selectivity against potential interferents like bovine serum albumin (BSA) and human immunoglobulin G (IgG). Further, it has been validated satisfactorily with commercially available existing systems using human serum samples. Moreover, the proposed sensing system lowers the detection limit by three orders of magnitude compared to a recent study on label free PoC device on other cancer biomarkers.