Label-Free, Sub-Picomolar Detection of Neurofilament Light Chain with Electrolyte-Gated Organic Field-Effect Transistor-Based Biosensors

In the last years, organic electronic-based immunosensors, such as electrolyte-gated organic transistors (EGOTs), have emerged as a promising alternative strategy for the ultra-sensitive and label-free detection of biological analytes. Their intrinsic characteristics, such as high amplification, low cost, flexibility, and biocompatibility, make them the ideal candidates for point-of-care testing.<br/>Among EGOTs, two different architectures can be distinguished: organic electrochemical transistors (OECTs) and electrolyte-gated organic field-effect transistors (EGOFETs). Both, OECTs and EGOFETs, are three-terminal devices in which the source and drain electrodes are connected by an organic semiconductor (OSC) layer, that acts as a conducting channel, and is separated by an electrolyte from the gate electrode. The application of a potential difference between gate and source (V_GS) leads to the formation of two electrical double layers, at the gate/electrolyte and electrolyte/OSC interfaces, promoting the accumulation of charge carriers in the OSC, which results in a current flow between the source and drain (I_DS) following the application of a second potential difference (V_DS).<br/>In the present work, we report the first EGOFET-based immunosensor for the detection of neurofilament light chain (NF-L), a candidate biomarker for several neurological disorders. NF-L is a major component of the axonal cytoskeleton of the neurons and, following injuries of the central nervous system, NF-L is released into the body fluids, providing a direct indication of neural damage.<br/>In order to endow the EGOFET device with the ability to selectively recognize NF-L, the gate electrode was used as the sensing element, and specific anti-NF-L antibodies were immobilized on the gate surface, with a potentially controlled and uniform orientation. A decrease in the output current was observed after the incubation of the gate electrode in buffered solutions containing increasing concentrations of NF-L. In addition, following the binding events occurring on the gate surface, a concomitant shift of the threshold voltage (V_th) towards more negative values, and a decrease in transconductance (g_m) were observed.<br/>The biosensor exhibited the maximum response, defined as the normalized relative current change, in the subthreshold regime. The observed trend suggested the presence of two different regimes, which were interpretated as the simultaneous formation of two layers on the gate surface: a first antigen-antibody layer, and second layer corresponding to weak protein-protein interactions, observed at high NF-L concentrations. The successful fit using the Guggenheim-Anderson-de Boer (GAB) adsorption model and further morphological characterization of the gate electrode supported this hypothesis. In addition, a large set of control experiments was performed in order to evaluate the selectivity and specificity of the biosensor.<br/>In summary, our EGOFET biosensor demonstrated to selectively detect NF-L in a wide dynamic range of concentrations (100 fM-10 nM), providing a label-free, rapid, and reproducible response, indicating its potential as an alternative sensing platform for the detection of NF-L in neurological disorders such as Parkinson’s disease or multiple sclerosis.<br/>The authors acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 813863.