SenSARS: A Low-Cost Portable Electrochemical System for Ultra-Sensitive, Near Real-Time, Diagnostics of SARS-CoV-2 Infections

Abstract

A critical path to solving the SARS-CoV-2 pandemic, without further socioeconomic impact, is to stop its spread. For this to happen, pre- or asymptomatic individuals infected with the virus need to be detected and isolated opportunely. Unfortunately, there are no current ubiquitous (i.e., ultra-sensitive, cheap, and widely available) rapid testing tools capable of early detection of SARS-CoV-2 infections. In this article, we introduce an accurate, portable, and low-cost medical device and bio-nanosensing electrode dubbed SenSARS and its experimental validation. SenSARS' device measures the electrochemical impedance spectra of a disposable bio-modified screen-printed carbon-based working electrode (SPCE) to the changes in the concentration of SARS-CoV-2 antigen molecules ("S" spike proteins) contained within a sub-microliter fluid sample deposited on its surface. SenSARS offers real-time diagnostics and viral load tracking capabilities. Positive and negative control tests were performed in phosphate-buffered saline (PBS) at different concentrations (between 1 and 50 fg/mL) of SARS-CoV-2(S), Epstein-Barr virus (EBV) glycoprotein gp350, and Influenza H1N1 M1 recombinant viral proteins. We demonstrate that SenSARS is easy to use, with a portable and lightweight (< 200 g) instrument and disposable test electrodes (<U.S. [Formula: see text]5), capable of fast diagnosis (~10 min), with high analytical sensitivity (low limits of detection, LOD = 1.065 fg/mL, and quantitation, LOQ = 3.6 fg/mL) and selectivity to SARS-CoV-2(S) antigens, even in the presence of structural proteins from the other pathogens tested. SenSARS provides a potential path to pervasive rapid diagnostics of SARS-CoV-2 in clinical, point-of-care, and home-care settings, and to breaking the transmission chain of this virus. Medical device compliance testing of SenSARS to EIC-60601 technical standards is underway.

Keywords: Biosensors; SARS-CoV-2; electrochemical biosensors; electrochemical impedance spectroscopy (EIS).

Fig. 1.

Stepwise antigen-based diagnostics protocol in SenSARS. The lower right outlined inset figure shows the two main components in SenSARS, an SPCE (left) and a portable electrochemical impedance spectrometer (right). Human figure generated with iOS, Essential Anatomy 5 (https://3d4medical.com).

Fig. 2.

(a) Layout of screen-printed carbon electrodes (SPCEs) used in this work includes a WE and a CE made of carbon paste and the RE from Ag/AgCl, as well as the grey terminal contacts. The electrodes are partially protected with an insulating dielectric layer (ESL ElectroScience Europe, U.K., ESL 4917). Each SPCE measures . (b) SPCE photograph. SPCEs reported here were fabricated at the Universidad del Valle facilities.

Fig. 3.

Molecular model schematic of the WE’s modified surface (PABA, mAb(S), BSA) and an approaching “spike” glycoprotein onto the antibody binding domain.

Fig. 4.

Block diagram of SenSARS’ hardware architecture. The signal generation system produces the sinusoidal waveform that is applied to the SPCEs. The signal conditioning module is responsible for amplifying, filtering, and converting the current signal measured through the WE into an equivalent potential that is digitalized by the acquisition module and relayed to the data processing module where the EIS and diagnostics algorithms are executed.

Fig. 5.

(a) Schematic of the power supply circuit in SenSARS’ device. The operational amplifier OA1 provides a voltage follower configuration to isolate the 5- and 8-V sources. (b) Electrical diagram of the signal generation module using the XR2206 IC. The DS1809 potentiometers are driven by the Raspberry PI unit to enable user control of the excitation waveform frequency and amplitude. The capacitors are selected by analog switches from 74hc4066, controlled by the Raspberry Pi unit. (c) Schematic of the signal conditioning module. Amplifiers OA2 and OA6 are responsible for shifting the dc level of the generated signal. The first one takes the offset of the generated sinusoidal signal to 0 V to apply the full potential range on the electrode through amplifiers OA3 and OA4. The second is responsible for recovering the original DC of the input signal.

Fig. 6.

(a) Parameters associated with a typical EIS (adapted from [37]). (b) Equivalent Randles electronic model of our SPCE’s electrochemical cell. The measured opposition of the bio-functional layer to electric current flow under the alternating voltage signal, i.e., the impedance, is used to extract its charge transfer resistance ( ) across the biological layer. By fitting the curve obtained from the Nyquist plot to an equivalent Randles electrical circuit model, we extract as the difference between the maximum projected real impedance and the minimum starting real impedance, which corresponds to the ionic solvent’s resistance ( ).

Fig. 7.

(a) Exploded view of SenSARS portable device. (b) Photograph of the physical instrument and SPCE on a holder.

Fig. 8.

Example signals acquired and processed by SenSARS’ EIS instrument. (a) Voltage and current signals (amplified by the IxR value of the TIA) filtered with a median filter. (b) Estimated output phase (orange).

Fig. 9.

Flowchart for the calculation of the EIS in each SPCE characterization. The total processing time between generation, acquisition, and visualization of the result is approximately 10 min.

Fig. 10.

EIS test of SenSARS on a dummy-cell circuit. (a) Implemented dummy cell circuit with electrodes’ configuration. (b) EIS responses’ comparison between all the instruments and the Randles simulation.

Fig. 11.

SenSARS’ EIS validation. (a) EIS comparison between SenSARS, Palmsens4, and Autolab’s PGSTAT128N for a WE reference to BSA and the same electrode’s response with SARS-CoV2 spike protein (Sinobiological ref 40591-V08H Spike S1-His Recombinant Protein) at a concentration of 5 fg/mL. (b) Relative values’ comparison for each EIS obtained on different SPCE electrodes at a concentration of 5 fg/mL.

Fig. 12.

(a) Relative EIS between incremental spike protein concentrations in a fluid sample. (b) Linear response in concentration versus change between 1 and 20 fg/mL.

Fig. 13.

Relative values’ comparison for incremental spike protein concentrations and negative control tests.