Herein, we reported the study on the use of electrochemical techniques for the detection of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The objective was to determine optimal experimental conditions using a variety of substrates, electrochemical methods and immobilisation techniques. In our experiments, we tested the electrochemical detection of antibodies against SARS-CoV-2 (from the real serum sample) by affinity interaction with recombinant Spike protein (rSpike) immobilised on the working electrode surface.
In our earliest work [1], the rSpike was immobilised on the working electrode using a mixture of self-assembled monolayers (SAM): 1-mercaptoundecanoic acid (for protein binding) and 6-mercapto-1-hexanol (as a spacer). We utilised Au-sputtered microscopic glass as the working electrode. Hereafter, the sequential incubation in the antibodies (anti-rS) concentrations range was carried out. The electrochemical methods of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed for the detection of affinity interaction in phosphate-buffered saline (PBS) pH 7.4 in the presence of 2 mM of [Fe(CN)6]3-/4-. The limits of detection (LOD) for CV and EIS were 2.53 nM and 1.99 nM, correspondingly [1].
Another experiment [2] was based on the application of screen-printed carbon electrodes (SPCE) with Au nanostructures (AuNS) electrodeposited on the working electrode surface. In this case, L-cysteine was used as SAM for rSpike immobilisation and CV and differential pulse voltammetry (DPV) were applied for electrochemical observation in PBS/[Fe(CN)6]3-/4- medium. The resulting LOD values were 0.27 nM for CV and 0.14 nM for DPV [2].
In our recent study [3], we reported an EIS-based affinity sensor. The rSpike was immobilised on the phytic acid doped polyaniline films (PANI-PA) directly covering SPCE. Electrochemical measurements were performed in PBS without adding [Fe(CN)6]3-/4-. The calculated LOD value for this sensor was 8.00 nM [3].
We can assume that the system based on the AuNS-modified SPCE is more sensitive toward anti-rSpike detection by comparing LODs for the CV method in the two first tests. Moreover, it is notable that employing SPCE enhanced the reproducibility of the sensor. Among all the electrochemical methods used, it is possible to single out DPV as the fastest one and with the lowest LOD value. However, despite the high LOD value relative to other experiments, the PANI-based sensor had the advantage of label-free detection.