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Posted: Nov 17th, 2010
Nanoplasmonic sensors detecting live viruses set to revolutionize infection diagnostics
(Nanowerk Spotlight) In physics, a plasmon is the quasiparticle resulting from the quantization of plasma oscillations just as photons and phonons are quantizations of light and sound waves, respectively. As the name indicates, surface plasmons are those plasmons that are confined to surfaces. Controlling these surface plasmons has become a 'hot' research area for optical signal processing, surface enhanced spectroscopy and sensor nanotechnology.
The latter is quickly becoming an increasingly attractive research field for developing label-free biosensors to be used as point-of-care diagnostic tools for cancer and infectious viral diseases. The recent emergence of H1N1 and H5N1 flu viruses and severe acute respiratory syndrome (SARS) has highlighted the importance of rapid detection and accurate diagnosis in health care and preventative medicine. The problem in these areas is that many virus detection platforms have limitations because they are not easily compatible with point-of-care use without the existence of significant infrastructure.
Cell culturing is a time consuming, highly specialized and labor intensive process. In some cases, viruses cannot be cultured at all. The ELISA technique requires multiple steps and agents with a potential to create quenching interactions among each other. Polymerase chain reaction (PCR), another powerful diagnostic tool based on detection of nucleic fragments in clinical samples, requires significant sample preparation, and can be confounded by inhibitors within a clinical sample. Therefore, highly sensitive/specific, compact, rapid and easy to use virus diagnostics are needed to prevent further spread at the onset of a viral epidemic.
A multi-disciplinary research team at Boston University, led by Hatice Altug and John H. Connor, has now introduced a novel label-free optofluidic-nanoplasmonic biosensor and demonstrated direct detection of live viruses from biological media at medically relevant concentrations with little to no sample preparation.
This novel platform can be easily adapted for point-of-care diagnostics to detect a broad range of viral pathogens in resource-limited clinical settings at the far corners of the world, in defense and homeland security applications as well as in civilian settings such as airports or other public spaces.
"Our work is the first demonstration of detection of intact viruses using extraordinary light transmission phenomena in plasmonic nanohole arrays," Altug tells Nanowerk. "So far, questions remain regarding the possible limitations of this technique for virus detection, as the penetration depth of the surface plasmon polaritons (SPP) is comparable to the dimensions of the pathogens."
The team, supported partly by Boston University Photonics Center and Army Research Laboratory, has been working in collaboration with the United States Army Medical Research Institute for Infectious Diseases (USAMRIID). Their work has successfully showed reliable detection of hemorrhagic fever virus surrogates (i.e. for Ebola virus) and poxviruses (like monkeypox or smallpox) in ordinary biological laboratory settings.
Dr. Connor at the Boston University School of Medicine explains their interest in focusing on detection and recognition of viruses. "Diseases caused by viruses are a constant and growing concern. Whether it is the most recent variant of influenza or the specter of an unexpected outbreak of Ebola, rapid and easy-to-use diagnostics are very important. Complicating the picture, patients presenting with virus infections often show symptoms that are not virus specific, but proper treatment is virus specific. Thus, there we are looking to develop easy-to-use, sensitive, and rapid viral diagnostics to help direct proper treatment as early as possible in the course of disease."
In their current work, the scientists have demonstrated detection of hemorrhagic fever virus surrogates by immobilizing group specific anti-bodies against these viruses on their sensor surfaces. Research associate Ahmet A. Yanik, who conducted the experiments, points out that this approach is powerful but also general, and can enable the recognition and identification of even previously unknown or highly divergent strains of rapidly evolving viruses.
"Our detection method is general and can be applied to detect other pathogens including bacteria, spores and toxins" Yanik adds. "By enabling high signal noise measurements without any mechanical or optical isolation, our platform opens up opportunities for fast and sensitive detection of a broad range of pathogens in ordinary clinical settings. Our platform is capable of quantifying virus concentrations, and can detect not only the presence of the intact viruses in the analyzed samples, but also the intensity of the infection process."
Altug explains that their sensors exploit plasmonic resonances supported by plasmonic nanohole arrays (PNA) for detection. PNAs are arrays of nanoscale apertures (holes, with diameters of ∼250-350 nm) that are defined periodically (with pitch ∼500-800 nm) on an optically thick noble metal films such as ∼100 nm thick gold. At certain wavelengths, these nanohole arrays can transmit light much more strongly than the classical aperture theory predicts. This phenomenon is called extraordinary optical transmission (EOT) effect.
EOT signals result from the involvement of surface plasmon-polariton resonances (SPR). The resonance wavelength of EOT signal is strongly correlated with the effective dielectric constant of the adjacent medium around the plasmonic sensor. Altug explains that, as pathogens bind to the sensor surface, the effective refractive index of the medium increases, and the red-shifting of the plasmonic resonance occurs.
"Unlike techniques based on external labeling, such resonance shifting operates as a reporter of the molecular binding phenomena in a label-free fashion and enables transduction of the capturing event directly to the far field optical signal. Specific detection of viruses in a label-free fashion requires an effective method to distinguish non-specific binding of the viruses to the plasmonic sensor surface. Selectivity is achieved by surface immobilized highly specific antiviral immunoglobulins showing strong affinity to the viral membrane proteins. Correspondingly, with the use of antibodies, we specifically capture viruses from a sample solution on the surface of our sensor."
This is illustrated in the figure below:
3-D renderings (not drawn to scale) and the experimental measurements illustrate the detection scheme using optofluidic-plasmonic biosensors based on resonance transmissions due to extraordinary light transmission effect. Detection of the VSV virus is experimentally observed with a strong red-shifting of the plasmonic resonances. (Image: Hatice Altug Research Group, Boston University)
The transition of this biosensing platform to the United States Army Medical Research Institute of Infectious Diseases (USAMRIID), is already underway to test the new approach for it ability to identify samples that contain Ebola, Marburg or Lassa hemorrhagic fever viruses. Additionally, the team is currently working on a highly portable version of this platform that integrates microfluidics and automation to allow a simple point-of-care diagnostic that can be useful for everything from identifying seasonal flu to identifying highly infectious diseases with potential pandemic and bioterrorism threats.
"After completing the testing of our system in U.S., it will be delivered to various sites in the developing world and in Africa as a means of quickly identifying potentially fatal infectious diseases. We hope that this technology might be helpful in preventing the spread of potential pandemic viruses by allowing early identification and treatment," " says Altug.