The new sensor is based on technology that could be quickly adapted to generate rapid and accurate diagnostics for future pandemics as well, the researchers said.
“A rapid test means that you can open up travel much earlier in a future pandemic. You can screen people getting off of an airplane and determine whether they should quarantine or not. You could similarly screen people entering their workplace and so forth,” said Michael Strano, senior author of the study.
“We do not yet have technology that can develop and deploy such sensors fast enough to prevent economic loss.”
The diagnostic is based on carbon nanotube sensor technology that Strano’s lab has previously developed.
Once the researchers began working on a Covid-19 sensor, it took them just 10 days to identify a modified carbon nanotube capable of selectively detecting the viral proteins they were looking for, and then test it and incorporate it into a working prototype. This approach also eliminates the need for antibodies or other reagents that are time-consuming to generate, purify, and make widely available.
Carbon nanotubes are hollow, nanometer-thick cylinders made of carbon that naturally fluoresce when exposed to laser light. The researchers have shown that by wrapping such tubes in different polymers, they can create sensors that respond to specific target molecules by chemically recognising them.
Their approach takes advantage of a phenomenon that occurs when certain types of polymers bind to a nanoparticle. Known as amphiphilic polymers, these molecules have hydrophobic regions that latch onto the tubes like anchors and hydrophilic regions that form a series of loops extending away from the tubes.
Those loops form a layer called a corona surrounding the nanotube. Depending on the arrangement of the loops, different types of target molecules can wedge into the spaces between the loops. This binding of the target alters the intensity or peak wavelength of fluorescence produced by the carbon nanotube.
Within 10 days of starting the project, the researchers had identified accurate sensors for both the nucleocapsid and the spike protein of the SARS-CoV-2 virus.
During that time, they also were able to incorporate the sensors into a prototype device with a fibre-optic tip that can detect fluorescence changes of the biofluid sample in real time, eliminating the need to send the sample to a lab, which is required for the gold-standard PCR diagnostic test for Covid-19.
The prototype device produces a result within about five minutes and can detect concentrations as low as 2.4 picograms of viral protein per millilitre of sample.
The researchers also showed that the device could detect the SARS-CoV-2 nucleocapsid protein (but not the spike protein) when it was dissolved in saliva. Detecting viral proteins in saliva is usually difficult because saliva contains sticky carbohydrate and digestive enzyme molecules that interfere with protein detection, which is why most Covid-19 diagnostics require nasal swabs.