Direct capture and smartphone quantification of airborne SARS-CoV-2 on a paper microfluidic chip

Abstract

SARS, a new type of respiratory disease caused by SARS-CoV, was identified in 2003 with significant levels of morbidity and mortality. The recent pandemic of COVID-19, caused by SARS-CoV-2, has generated even greater extents of morbidity and mortality across the entire world. Both SARS-CoV and SARS-CoV-2 spreads through the air in the form of droplets and potentially smaller droplets (aerosols) via exhaling, coughing, and sneezing. Direct detection from such airborne droplets would be ideal for protecting general public from potential exposure before they infect individuals. However, the number of viruses in such droplets and aerosols is too low to be detected directly. A separate air sampler and enough collection time (several hours) are necessary to capture a sufficient number of viruses. In this work, we have demonstrated the direct capture of the airborne droplets on the paper microfluidic chip without the need for any other equipment. 10% human saliva samples were spiked with the known concentration of SARS-CoV-2 and sprayed to generate liquid droplets and aerosols into the air. Antibody-conjugated submicron particle suspension is then added to the paper channel, and a smartphone-based fluorescence microscope isolated and counted the immunoagglutinated particles on the paper chip. The total capture-to-assay time was <30 min, compared to several hours with the other methods. In this manner, SARS-CoV-2 could be detected directly from the air in a handheld and low-cost manner, contributing to slowing the spread of SARS-CoV-2. We can presumably adapt this technology to a wide range of other respiratory viruses.

Keywords: Airborne pathogens; Bioaerosol; COVID-19; Paper microfluidics; Respiratory virus; Smartphone microscope.

Fig. 1

Assay procedure. (a) Chamber design with dimensions and the placements of a paper microfluidic chip and a sprayer. (b) Fluorescence images of a paper microfluidic channel for 0 ng/mL (control) and 600 pg/mL (sample) UV-inactivated SARS-CoV-2 in 10% saliva. Three different images from a single channel were taken with a benchtop fluorescence microscope. (c) The assay procedure, starting from the air collection, antibody-particles, smartphone-based fluorescence microscopic imaging, and image processing with the MATLAB script.

Fig. 2

Assay results with a benchtop fluorescence microscope. Pixel counts from three different images of a single channel were summed and used as a single data point. Experiments were repeated four times (each time with three images), each using a different paper microfluidic chip. Average pixel counts (n = 4) are plotted with the error bars representing standard errors, each time with different spraying/capture and with different paper microfluidic chips. (a) With two-times spraying, and (b) with five-times spraying. * denotes p ≤ 0.05, **p ≤ 0.01, and n.s. = not significant.

Fig. 3

Assay results with a smartphone-based fluorescence microscope. Pixel counts from five different images of a single channel were summed and used as a single data point. Experiments were repeated four times (each time with five images), each using a different paper microfluidic chip. Average pixel counts (n = 4) are plotted with the error bars representing standard errors. (a) With two-times spraying, (b) with five-times spraying, and (c) two-times spraying and fans installed. n = 4, each time with different spraying/capture and with different paper microfluidic chips; * denotes p ≤ 0.05; ** denotes p ≤ 0.01; n.s. denotes not significant.