Researchers have developed a paper-based device that changes colour, depending on whether a patient has Ebola, yellow fever or dengue, an advance that can help diagnose the diseases within minutes in remote, low-resource settings.
Standard approaches for diagnosing viral infections require technical expertise and expensive equipment, said researcher Kimberly Hamad-Schifferli.
"Typically people perform PCR and ELISA, which are highly accurate, but they need a controlled lab environment," she said.
Polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) are bioassays that detect pathogens directly or indirectly, respectively.
Colour-changing paper devices that work similarly to over-the-counter pregnancy tests offer a possible solution.
"These are not meant to replace PCR and ELISA because we can't match their accuracy. But this is a complementary technique for places with no running water or electricity," Hamad-Schifferli said.
Hamad-Schifferli and her team at the Massachusetts Institute of Technology, Harvard Medical School and the US Food and Drug Administration built silver nanoparticles in a rainbow of colours.
The sizes of the nanoparticles determine their colours. Therefore, the team uses different sizes of these chemical ingredients for various hues.
The researchers attached red, green or orange nanoparticles to antibodies that specifically bind to proteins from the organisms that cause Ebola, dengue or yellow fever, respectively.
They introduced the antibody-tagged nanoparticles onto the end of a small strip of paper. In the paper's middle, the researchers affixed "capture" antibodies to three test lines at different locations, one for each disease.
To test the device, the researchers spiked blood samples with the viral proteins and then dropped small volumes onto the end of the paper device.
If a sample contained dengue proteins, for example, then the dengue antibody, which was attached to a green nanoparticle, latched onto one of those proteins.
This complex then migrated through the paper, until reaching the dengue fever test line, where a second dengue-specific antibody captured it.
That stopped the complex from going farther down the strip, and the test line turned green.
When the researchers tested samples with proteins from Ebola or yellow fever, the antibody complexes migrated to different places on the strip and turned red or orange.
"Using other laboratory tests, we know the typical concentrations of yellow fever or dengue virus in patient blood. We know that the paper-based test is sensitive enough to detect concentrations well below that range," said Hamad-Schifferli.
The team will present their research at the 250th National Meeting & Exposition of the American Chemical Society (ACS) in Boston.