Vertical nanogap nanoelectrodes: an alternative to large-scale fabrication of field-deployable electrochemical biosensors

Abstract

Electrochemical biosensors are promising platforms toward point-of-care (POC) and mass testing. However, these devices have not yet successfully reached the commercial market beyond the personal blood glucose assays. Analyses in complex biofluids and large-scale production are key challenges facing the routine application of the electrochemical biosensors. The complexity of biological samples leads to a low clinic accuracy due to cross-reactivity and electrode surface fouling issues. To address these challenges, this work aims to develop a scalable approach for fabricating label-free impedimetric biosensors based on vertical nanogap nanoelectrodes (VN²Es) capable of delivering high-accuracy analyses in biofluids. The advantages derived from the use of nanoelectrodes (two-electrode cell and enhanced signal-to-noise ratio) in a vertically structured nanogap (shorter functionalization times by applying AC voltages and antifouling effect) will be essential to achieve a scalable setting and accurate analyses in biological samples. The fabrication will involve one step of conventional photolithography, one technique for thin film deposition (sputtering), and one step of direct writing lithography (focused ion beam, FIB) for opening the nanocavities. Thin films of Au acting as working and quasi-reference electrodes will be separated by an insulating layer of Al‚Oƒ with nanometer thickness (50 to 300 nm). The areas opened via FIB will present a square geometry of about 500 nm in length. The proposed fabrication method is expected to be able to produce tens to hundreds of VN²Es on a single glass substrate (75 mm x 35 mm) for analyses with low sample consumption (0.5 µL). The VN²E fabrication is already under development. Microscopy and spectroscopy techniques will be employed for the surface characterization of the nanoelectrodes, whereas electrochemical analyses of a single and arrays of electrodes (4 to 8) will be performed to assess the properties that are supposed to be achieved by the nanoelectrodes and the vertical nanogap design. Lastly, label-free impedimetric biosensors will be constructed for evaluation of the immobilization time and analytical performance. The Spike protein will be used as recognition element to detect standard IgG SARS-CoV-2 antibodies after electrostatic immobilization on the nanoelectrodes. The scale-up compatibility of the VN²Es combined with their capability in providing accurate analyses in complex biofluids may help to pave the way for translating platforms into the clinical practice for POC and mass testing.