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Collaborative effort leads to high-resolution imaging breakthrough

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Diagram of gratings.

This diagram shows the difference between regular and plasmonic gratings in terms of fluorescent intensity. Photo courtesy of Shubhra Gangopadhyay/Nanoscale.

A collaborative effort between the University of Missouri departments of Electrical and Computer Engineering, Bioengineering and Biochemistry could improve the ability of imaging and sensing equipment and lower costs for super-resolution imaging.

ECE doctoral student Biyan Chen and bioengineering doctoral student Aaron Wood were co-lead authors on a paper called “Plasmonic gratings with nano-protrusions made by glancing angle deposition (GLAD) for single-molecule super-resolution imaging” recently accepted by the journal Nanoscale, published by the Royal Society of Chemistry in Great Britain.

ECE Professor Shubhra Gangopadhyay and Biochemistry Assistant Professor Peter Cornish were the co-corresponding authors. Bioengineering Professor Sheila Grant and Keshab Gangopadhyay were co-authors.

The research illustrated how fabricating a relatively inexpensive plasmonic grating can create a platform that allows for higher resolution imaging down to 65 nanometers. The grating coupled with the use of particular wavelengths and angles allows for the enhancement of the image, allowing simple and cost effective microscopes to capture visuals down to a subdiffraction limit typically reserved for expensive optical microscopes and other imaging equipment, such as Scanning Electron Microscopes and Transmission Electron Microscopes.

“Imaging biological molecules at higher concentrations has been challenging due to overlapping signals present in conventional imaging technologies,” Cornish said. “Recent techniques that overcome these barriers come with a high price tag. Plasmonic gratings generated through GLAD provides a less expensive technology that produces similar measurements while allowing for the use of more readily available microscopes. This has the potential to allow more researchers to perform fluorescence imaging technologies opening up new avenues of research in the process.

A plasmonic surface is a metal surface in which free electrons can be excited by the electric field in light waves to produce oscillations. These oscillations allow plasmonic materials to achieve optical properties not possible naturally.

“You usually have to use very expensive microscopes to image to that level,” Gangopadhyay said. “Plasmonics couple the light very efficiently. And with this coupling, we can enhance the signal a lot, and we are able to image single molecules using a simple microscope without the need for an expensive microscope.”

The gratings use HD-DVD and Blu-Ray discs as starting materials. The pattern is replicated, then fabricated onto the devices where the specimen will be placed. The gratings are covered in silver and a particular kind of dye for biocompatibility purposes and are then ready for use.

“This is a basic platform Shubhra and I have used to detect cortisol, to detect tuberculosis, essentially,” Grant said, alluding to previous collaborations with Gangopadhyay on sensor projects. “You just have to change your sensing elements on there. But the whole purpose of these plasmonic gratings is to enhance your fluorescent signal.”

The potential not only for microscopy but also a wide variety of sensing applications makes this new plasmonic platform a potentially versatile tool in various disciplines across the sciences and in the medical field. As a sensor, the plasmonic grating serves to amplify the signal as well as increase sensitivity, making it more accurate in many ways — for example, testing for the presence of particular molecules in urine or sweat.

There are two patents pending for the platform — one for the process of replicating the pattern onto different devices and another for the process of combining the silver and other dielectric layers to stop corrosion of silver and for biocompatibility — and there are plans in place to take the technology to market in the future, with the goal of making microscopy and sensing much more cost efficient. Since the patterns come from previously existing technology, scalability is not a big concern.

“Think about if this platform is available. The microscopes are there in high schools and everywhere,” Gangopadhyay said. “We can really learn the nanoscale [biological] processes without going into very expensive microscopes.”

And low cost means potentially inexpensive sensors, which could be used to detect a wide variety of diseases, particularly in developing countries.

“Eventually, we want to take these plasmonic platforms that are super sensitive and, and we can just use our iPhone,” Grant said. “We can have one of these platforms, I can put it on my skin and see if there are cortisol levels, and I can just use the iPhone to excite [the material] and capture the fluorescence of it. We’re on the road to wearable sensor systems, potentially.”

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