Optofluidic Systems Detect Ebola, Flu Viruses
SANTA CRUZ, Calif. — Two optofluidic devices hold promise as portable instruments for immediate diagnosis and monitoring of infectious diseases at the point of care.
One is a chip-scale hybrid device integrating a microfluidic chip for sample preparation and an optofluidic chip for optical detection of Ebola and other viral pathogenic RNA molecules. The other is a small, dedicated optofluidic chip for the multiplex fluorescence detection of different flu virus subtypes.
A hybrid device integrates a microfluidic chip for sample preparation and an optofluidic chip for optical detection of individual molecules of viral RNA. Courtesy of Joshua Parks.
The current standard for Ebola virus detection relies on polymerase chain reaction (PCR) amplification of the virus's genetic material for detection. Because PCR works on DNA molecules, and Ebola is an RNA virus, the reverse transcriptase enzyme is used to make DNA copies of the viral RNA prior to PCR amplification and detection.
Researchers from the University of California, Santa Cruz, and Brigham Young University in Provo, Utah, have reported on-chip sample preparation, amplification-free detection and quantification.
The silicon-based polymer microfluidic chip was designed and built in the lab of UC Santa Cruz optoelectronics professor Holger Schmidt, in collaboration with Richard Mathies at UC Berkeley, who pioneered the technology. The targeted Ebola virus RNA molecules were isolated by binding to a matching sequence of synthetic DNA attached to magnetic microbeads. The microbeads were collected with a magnet, nontarget biomolecules were washed off, and the bound targets were then released by heating, labeled with fluorescent markers, and transferred to the optofluidic chip for detection.
A schematic view shows the optical waveguide intersecting a fluidic microchannel containing target particles. Targets are optically excited as they flow past well-defined excitation spots created by multimode interference; fluorescence is collected by the liquid-core waveguide channel and routed into solid-core waveguides (red). Courtesy of Ozcelik et al./PNAS.
Optical detection was completed in under 10 minutes. The researchers reported high specificity, a detection limit of 0.2 plaque forming units per millimeter and a dynamic range of 13 orders of magnitude, outperforming other amplification-free methods.
The research was published in Nature Scientific Reports (doi: 10.1038/srep14494).
In a subsequent study, the researchers tested another optofluidic chip that incorporates principles of wavelength division multiplexing for detection of influenza.
By superimposing multiple wavelengths of light in an optical waveguide on the chip, the researchers created wavelength-dependent spot patterns in an intersecting fluidic channel. The fluorescent-labeled flu virus particles produced distinctive signals as they passed through the fluidic channel, depending on which wavelength of light the markers absorb.
While previous studies, such as the Ebola research, showed the sensitivity of Schmidt's optofluidic chips for detection of single molecules or particles, multiplexing could enable compact instruments for diagnostic assays that target a variety of biological particles and molecular markers.
"A standard flu test checks for about 10 different flu strains, so it's important to have an assay that can look at 10 to 15 things at once,” Schmidt said.
The team has not yet been able to test the system starting with raw blood samples, which requires additional sample preparation steps, and will have to be performed in a biosafety level 4 facility.
The flu study is published in Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1511921112).
Both studies were supported by the National Institutes of Health and National Science Foundation.
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