Purely electronic diagnostic devices that are sensitive to biomolecules' intrinsic charge present a very attractive platform for point‐of‐care (PoC) applications, since they can operate under label‐free conditions. In this report, a graphene‐based electrolyte‐gated field‐effect transistor is developed as an immunosensor capable of sensitive analyte detection, even in the complex environment of physiological samples. The coimmobilization of an antibody fragment (F(ab′)2) and polyethylene glycol on the graphene surface allows for a highly sensitive and selective detection of a protein analyte, with a limit of detection in the low femtomolar range, both in high ionic strength buffer and undiluted serum. Multiparametric analysis of the device's analyte‐dependent electronic response shows that the mechanism behind this very sensitive detection cannot be explained by a commonly reported electrostatic gating effect. Rather, the observed combination of charge neutrality point shifts and asymmetric mobility changes are attributed to the modulation of scattering by charged impurities, which seem to dominate the device's transfer characteristics. Furthermore, the reproducibility of the normalized signal response obtained from several different devices shows that this graphene‐based immunosensor is capable of direct and quantitative measurements of protein analytes in untreated serum, imperative for diagnostic tools geared toward PoC applications.

Transistor-based biosensors fulfill many requirements posed upon transducers for future point-of-care diagnostic devices such as scalable fabrication and label-free and real-time quantification of chemical and biological species with high sensitivity. However, the short Debye screening length in physiological samples (<1 nm) has been a major drawback so far, preventing direct measurements in serum. In this work, we demonstrate how tailoring the sensing surface with short specific biological receptors and a polymer polyethylene glycol (PEG) can strongly enhance the sensor response. In addition, the sensor performance can be dramatically improved if the measurements are performed at elevated temperatures (37 °C instead of 21 °C). With this novel approach, highly sensitive and selective detection of a representative immunosensing parameter-human thyroid-stimulating hormone-is shown over a wide measuring range with subpicomolar detection limits in whole serum. To the best of our knowledge, this is the first demonstration of direct immunodetection in whole serum using transistor-based biosensors, without the need for sample pretreatment, labeling, or washing steps. The presented sensor is low-cost, can be easily integrated into portable diagnostics devices, and offers a competitive performance compared to state-of-the-art central laboratory analyzers.

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