Nov 16, 2012
HD-DVDs provide low-cost starting mould for fabricating plasmonic gratings
Researchers at the University of Missouri in the US have shown how surface plasmon resonance (SPR)-based fluorescence amplification platforms can be produced at very low cost by using features found in commercially available HD-DVDs as a starting mould. The group reports that the discs, which come pre-fabricated with sub-micron-sized grating patterns, have the right dimensions to couple surface plasmons in the visible range.
Surface plasmons created at the interface of metals and dielectrics due to the optical coupling of light have applications in areas such as sub-wavelength optics, data storage, optoelectronic circuits, diffraction-limited microscopy and bio-photonics. In particular, these surface localized fields can be used to sense ultra-low concentrations of materials. For example, the enhanced electromagnetic field produced by plasmonic nanostructures can be used to couple light to fluorescent dye molecules in the immediate environment and thereby provide the extreme signal amplification necessary for detecting trace quantities of biomolecules tagged with fluorophores.
Nano-gaps are important
In the study, the scientists used a simple PDMS-based microcontact printing/replication process to reproduce the surface features of an HD-DVD-R disc (dissected into two parts to reveal the grating pattern on the inner side of the polycarbonate substrate) on conventional glass substrates. An important consequence of the fabrication process was the generation of defects in the form of nano-gaps that cut across the printed gratings.
The presence of nanogaps within the grating structures led to substantial field localization and amplification – propagating surface plasmon polaritons (SPPs) travel as surface waves with high field intensity towards the metallic nanogap where the sudden field discontinuity causes “extreme crowding” of the surface charges, leading to very high field intensities.
Gratings with embedded nanogaps, when studied with spin-coated rhodamine-590 dye, gave fluorescence enhancements as high as 36-fold on the gratings and 118-fold on nanogaps when compared with glass slides typically used in the lab.
Imaging single molecule fluorescence often requires sophisticated and expensive optical set-ups. A direct consequence of this research is the possibility of extending this work and fine-tuning the structures to enable visualization of single molecule fluorescence and Forster Resonance Energy Transfer (FRET) between single dye pairs. This will enable fundamental understanding of protein folding and conformational changes that were typically limited to laboratories having specialized equipment.
Full details can be found in the journal Nanotechnology.
About the author
The work discussed in the paper was performed at the Center for Nano/Micro Systems and Nanotechnology at the University of Missouri. Kunal Bhatnagar was a graduate student at the Department of Electrical and Computer Engineering where Avinash Pathak is currently pursuing his doctoral degree. Drew Menke is a doctoral student at the Department of Biochemistry and Dr Peter Cornish is his advisor. Dr Cornish's research interests include the study of ribosome and RNA biochemistry using single molecule and NMR techniques. Venumadhav Korampally, who was a research professor at the University of Missouri, is currently an assistant professor at the Northern Illinois University. Dr Keshab Gangopadhyay is a research professor of Electrical and Computer Engineering at the University of Missouri, and also holds an adjunct professorship at the university’s Nuclear Science and Engineering Institute. His areas of research are nanotechnology, nanoenergetics, neutron transport theory, particle simulation methods, and hydrodynamic and hydromagnetic stability. Dr Shubhra Gangopadhyay is a C W LaPierre Endowed Chair Professor in the Electrical and Computer Engineering Department as well as co-director of the MU Center for Nano/Micro Systems and Nanotechnology, which researches and develops miniaturized systems and nanotechnology.