On-Chip Nucleic Acid Purification Followed by ddPCR for SARS-CoV-2 Detection

We developed a microfluidic chip integrated with nucleic acid purification and dropletbased digital polymerase chain reaction (ddPCR) modules to realize a `sample-in, result-out' infectious virus diagnosis. The whole process involved pulling magnetic beads through drops in an oilenclosed environment. The purified nucleic acids were dispensed into microdroplets by a concentricring, oil-water-mixing, flow-focusing droplets generator driven under negative pressure conditions. Microdroplets were generated with good uniformity (CV = 5.8%), adjustable diameters (50-200 mu m), and controllable flow rates (0-0.3 mu L/s). Further verification was provided by quantitative detection of plasmids. We observed a linear correlation of R-2 = 0.9998 in the concentration range from 10 to 10(5) copies/ mu L. Finally, this chip was applied to quantify the nucleic acid concentrations of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The measured nucleic acid recovery rate of 75 +/- 8.8% and detection limit of 10 copies/ mu L proved its on-chip purification and accurate detection abilities. This chip can potentially be a valuable tool in point-of-care testing.