The new sensors also may be able to detect viruses such as influenza and HIV, as well as bacteria and proteins, paving the way for further development of a lightweight, portable, real-time diagnostic tool that can be used in homes, clinics and in the field.
Los Alamos researchers Duncan McBranch, David Whitten, Hsing-Lin Wang and Liaohai Chen, along with Fred Wudl and Roger Helgeson from the University of California, Los Angeles, discovered they could use polymers as luminescent sensors while studying electron transfer in conducting polymers for possible applications in photovoltaics and nonlinear optics.
Using a laptop computer, hand-sized spectrometer, fiber-optic cabling and a sample holder, the researchers demonstrated the principle behind this technique by "turning on" a polymer's luminescence by adding the sample protein avidin, derived from egg whites.
Certain polymers transfer their electrons over to electron-accepting molecules when excited by laser light. However, the polymer's luminescence is "quenched" once the polymer transfers its electrons to the acceptor molecule, which is attached to the polymer.
The researchers discovered they could use this effect for sensing by attaching a specific ligand to the acceptor molecule; the new molecular package then gets pulled away from the polymer by a biological or chemical agent's receptor site, restoring the polymer's luminescence.
All pathogens, proteins, viruses and bacteria contain receptor sites, which allow them to latch onto specific ligands and provide a route for cells to be infected. By matching the right ligand with a receptor, researchers can positively identify the specific proteins, viruses or bacteria.
The process is analogous to a lock and key. The ligand part of the molecular package, called a quencher-tether-ligand (QTL), is the "key," which fits into a receptor site, or "lock," on the biological and chemical species to be detected, explained McBranch.
"This recognition event literally pulls away the QTL from the polymer, thereby restoring the polymer's luminescence," he said.
To operate the device, McBranch and his colleagues place a water-soluble, quenched polymer inside a sample holder attached to one end of a fiber-optic cable. The other end of the cable connects to the spectrometer which, in turn, connects to the laptop computer.
The water-soluble polymer is essential because it contains the negative charge that attracts the positively-charged QTL. In the demonstration, the QTL contained the ligand biotin. Next, the researchers added a solution containing the egg white protein avidin inside the sample holder. Biotin and avidin are a well-known ligand/receptor combination often used for biochemical binding studies, said McBranch.
As expected, the receptors within the protein latched onto the QTL and pulled it away from the polymer. The spectrometer picked up the restored luminescence signal from the polymer and displayed it on the computer screen, confirming the presence of the protein. The analysis and identification process took about a second to complete.
McBranch says a researcher can identify unidentified bacteria or viruses simply by placing a sample into an array of sample holders containing different, receptor-specific QTLs. The spectrometer would analyze each sample holder simultaneously and identify any bacteria or virus, again within seconds.
The researchers currently are seeking funds to develop a library of QTLs and to reduce the size of their biosensor device.
Initial funding for their work came from Los Alamos' Laboratory-Directed Research and Development Program. DOE's Office of Biological and Environmental Research is providing $435,000 over three years to further develop the biosensor. The project is in its second year.
The researchers' paper detailing their luminescent sensor finding was published in the October issue of the Proceedings of the National Academy of Sciences.
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and the Washington Division of URS for the Department of Energy's National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.