COVID-19 can be diagnosed in 55 minutes or less with the help of programmed magnetic nanobeads and a diagnostic tool that plugs into an off-the-shelf cellphone. The stamp-sized micro-fluidic chip measures the concentration of SARS-CoV-2 nucleocapsid (N) protein in blood serum from a standard finger prick. The nanobeads bind to SARS-CoV-2 N protein — a biomarker for COVID-19 — in the chip and transport it to an electrochemical sensor that detects minute amounts of the biomarker.
The process simplifies sample handling compared to swab-based PCR tests that are widely used to diagnose COVID-19 and need to be analyzed in a laboratory. The entire new test can be performed and the results generated at the collection site, health clinic, or even a pharmacy. The entire system is easily transportable and easy to use.
To test the device, the researchers relied on donated serum samples from people who were healthy and others who were COVID-19-positive. A longer incubation yields more accurate results when using whole serum. The researchers found that 55 minutes was an optimum amount of time for the microchip to sense SARS-CoV-2 N protein at concentrations as low as 50 picograms (billionths of a gram) per milliliter in whole serum. The microchip could detect N protein in even lower concentrations, at 10 picograms per milliliter, in only 25 minutes by diluting the serum fivefold.
Paired with a Google Pixel 2 phone and a plug-in potentiostat, it was able to deliver a positive diagnosis with a concentration as low as 230 picograms for whole serum. There are standard procedures to modify the beads with an antibody that targets a particular biomarker. When combined with a sample containing the biomarker (in this case, SARS-CoV-2 N protein), they bond together.
A capillary tube is used to deliver the sample to the chip, which is then placed on a magnet that pulls the beads toward an electrochemical sensor coated with capture antibodies. The beads bind to the capture antibodies and generate a current proportional to the concentration of biomarker in the sample. The potentiostat reads that current and sends a signal to its phone app. If there are no COVID-19 biomarkers, the beads do not bind to the sensor and get washed away inside the chip.