Imaging techniques such as MRI help us see organs and help in the diagnosis of various diseases. A conventional microscope helps us see what is wrong at a cellular level. But to understand a disease at the molecular level one needs equipment that would elucidate molecular structures such as proteins.
Proteins play an important role in the development of disease. In fact, the most fundamental information about a disease is available at the proteinaceous level. For example, to understand how cancer develops, one has to study protein molecules. To visualize protein structures one needs some kind of enhancing technique, preferably one that can focus in the nanoscale range. In the context of medical imaging in particular, it’s becoming increasingly important to understand diseases at the molecular or protein level.
Now, researchers from the School of Electrical and Computer Engineering at Purdue University, West Lafayette, Indiana, are trying to address this issue by developing special structures called ‘nanoantennas’. Essentially nanoantennas are metal nanostructures comprised of two metallic subunits. Extremely high local fields can be created in the gap between these two metallic subunits. Acting like ‘nanolens’, nanoantennas can concentrate light in the nanoscale and allow one to visualize nanometer-sized structures such as proteins.
Elaborating on the concept behind nanoantennas, Vladimir Shalaev, a Professor at Purdue tells Technical Insights, "In the context of imaging and getting information on small structures, the idea behind using these nanoantennas is as follows: nanoantennas basically act as conventional antennas but conventional antennas work in the microwave range. They utilize larger wavelengths and relatively lower frequencies. Optical nanoantennas, on the other hand, are aimed to act in the optical range at very high frequencies, and couple strongly to the light in the visible spectral range with the wavelength being half a micron or so. Nanostructures are much smaller, say 10 nm, and because of this mismatch in size one needs a special coupler to interact efficiently with light. That’s where you need a nanoantenna that works as such a coupler."
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Currently, noble metals such as gold and silver are being used to prepare these nanoantennas. These metals exhibit a strong plasmon resonance; as a result, electrons in these metals oscillate up and down and create very high local fields that enable imaging on the nanoscale. In addition, such metal nanostructures are biocompatible, which is an important criterion for medical applications.
Besides molecular imaging, some of the other applications of Purdue’s nanoantenna technology are in drug discovery (to optically detect the binding between antibodies and antigens) and fundamental (protein) research (to detect and characterize conformational changes in proteins). Eli Lilly has been able to develop faster-acting insulin by modifying the position of two amino acids in the longer-acting insulin; and optical nanoantennas can clearly detect such tiny changes, which is important for drug characterization.
The researchers are actively engaged in adapting nanoantennas for specific applications. "It’s probably impossible to develop a structure, which would work universally for all possible applications in proteomics. We are learning more and more about specific biomedical applications. We are trying to optimize the design to fabricate structures that are suited to any particular application in the area of proteomics" explains Shalaev.
One of the major bottlenecks that the Purdue researchers are facing is in the fabrication of the nanostructures. Shalaev terms the new Birck Nanotechnology Center as ‘breakthrough’ (in terms of infrastructure availability). He adds, "Design is something that we can do, but to fabricate optimally and also in a reproducible and reliable way is still a challenge because, we have to fabricate on the nanoscale, with unprecedented precision. It comes down from physics to engineering. We also need to better understand bio-medical applications. We are working in close collaboration with our colleagues in the pharmaceutical department and companies such as Eli Lilly and InProteo LLC".
Nanoantennas for medical imaging applications are expected to be commercialized within the next 3 years. The Indiana Proteomics Consortium funded this research. And the technology developed has been licensed to InProteo LLC, which was cofounded by Eli Lilly, and Purdue and Indiana Universities.
Details: Vladimir M. Shalaev, Professor, School of Electrical & Computer Engineering, Purdue University, West Lafayette, IN 47907. Phone: 765-494-9855, E-mail: shalaev@purdue.edu