A paper-based optical sensor for the screening of viruses through the cysteine residues of their surface proteins: A proof of concept on the detection of coronavirus infection

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a serious threat to human health. Current methods such as reverse transcription polymerase chain reaction (qRT-PCR) are complex, expensive, and time-consuming. Rapid, and simple screening methods for the detection of SARS-CoV-2 are critically required to fight the current pandemic. In this work we present a proof of concept for, a simple optical sensing method for the screening of SARS-CoV-2 through its spike protein subunit S1. The method utilizes a target-specific extractor chip to bind the protein from the biological specimens. The disulfide bonds of the protein are then reduced into a biothiol with sulfhydryl (SH) groups that react with a blue-colored benzothiazole azo dye-Hg complex (BAN-Hg) and causes the spontaneous change of its blue color to pink which is observable by the naked eye. A linear relationship between the intensity of the pink color and the logarithm of reduced S1 protein concentration was found within the working range 130 ng.mL^-1-1.3 pg mL^-1. The lowest limit of detection (LOD) of the assay was 130 fg mL^-1. A paper based optical sensor was fabricated by loading the BAN-Hg sensor onto filter paper and used to screen the S1 protein in spiked saliva and patients' nasopharyngeal swabs. The results obtained by the paper sensor corroborated with those obtained by qRT-PCR. The new paper-based sensing method can be extended to the screening of many viruses (e.g. the human immunodeficiency virus, the human polyomavirus, the human papilloma virus, the adeno associated viruses, the enteroviruses) through the cysteine residues of their capsid proteins. The new method has strong potential for screening viruses at pathology labs and in remote areas that lacks advanced scientific infrastructure. Further clinical studies are warranted to validate the new sensing method.

Keywords: Biothiol; Colorimetric assay; Paper based sensor; SARS-CoV-2.

Graphical abstract

Scheme 1

Schematic illustration of selective binding and paper based colorimetric sensor for SARS-CoV-2 detection through its S1 protein.

Fig. 1

(A) UV–Visible spectra of the BAN-Hg sensor, (B) UV–Visible spectra of the BAN-Hg sensor after mixing with unreduced S1 protein (390 ng mL⁻¹), (C) UV–Visible spectra of the BAN-Hg sensor after mixing with reduced S1 protein (390 ng mL⁻¹).

Fig. 2

(A) UV–Visible spectra of the BAN-Hg sensor upon addition of various concentrations of the reduced S1 protein (130 ng.mL⁻¹- 130 fg mL⁻¹), (B) Plot of the intensity of the absorption band at 530 nm versus log concentration of reduced S1 protein (130 ng mL⁻¹ to 130 fg mL⁻¹).

Fig. 3

The color change of the paper-based optical sensor in the presence of trace concentrations of the reduced S1 protein (390 ng.mL⁻¹- 390 fg mL⁻¹). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Change in UV–Visible spectra and the color of the BAN-Hg solution and paper sensor after reaction with (A) a negative control sample and, (B) a positive control sample (spiked with S1 protein), (C) blank saliva, (D) saliva spiked with S1 protein. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

Color change of the paper-based optical sensor after interaction with nasal swab samples from healthy and infected persons. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)