Plasmonic Resonance Optical Microscope Has Capability of an Electron Microscope

A novel, relatively low cost microscopy technique enables an optical fluorescent microscope to display images with the resolution of an electron microscope.

Super-resolution imaging has advanced the study of biological and chemical systems, but the required equipment and platforms are expensive and unable to observe single-molecules at the high fluorescent dye concentrations required to study protein interactions and enzymatic activity.

In a notable advance in this area, investigators at the University of Missouri (Columbia, USA) designed a plasmonic platform that utilized an inexpensively fabricated plasmonic grating in combination with a scalable glancing angle deposition (GLAD) technique using physical vapor deposition.

Plasmonic resonance is a phenomenon that occurs when light is reflected off thin metal films, which may be used to measure interaction of biomolecules on the surface. An electron charge density wave arises at the surface of the film when light is reflected at the film under specific conditions. A fraction of the light energy incident at a defined angle can interact with the delocalized electrons in the metal film (plasmon) thus reducing the reflected light intensity. The angle of incidence at which this occurs is influenced by the refractive index close to the backside of the metal film, to which target molecules are immobilized. If ligands in a mobile phase running along a flow cell bind to the surface molecules, the local refractive index changes in proportion to the mass being immobilized. This can be monitored in real time by detecting changes in the intensity of the reflected light.

The investigators described the imaging system in the June 28, 2016, issue of the journal Nanoscale. They reported that the GLAD technique created an abundance of plasmonic nano-protrusion probes that combined the surface plasmon resonance (SPR) from the periodic gratings with the localized SPR of these nano-protrusions. The resulting platform enabled simultaneous imaging of a large area without point-by-point scanning or bulk averaging for the detection of single Cyanine-5 dye molecules using epifluorescence microscopy. The new system was able to resolve grain sizes down to 65 nanometers, a resolution normally obtained only by electron microscopes.

“Usually, scientists have to use very expensive microscopes to image at the sub-microscopic level,” said senior author Dr. Shubra Gangopadhyay, professor of electrical and computer engineering at the University of Missouri. “The techniques we have established help to produce enhanced imaging results with ordinary microscopes. The relatively low production cost for the platform also means it could be used to detect a wide variety of diseases, particularly in developing countries.”

“In previous studies, we have used plasmonic gratings to detect cortisol and even tuberculosis,” said Dr. Gangopadhyay. “Additionally, the relatively low production cost for the platform also means it could be used to further detect a wide variety of diseases, particularly in developing countries. Eventually, we might even be able to use smartphones to detect disease in the field.”

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