Nanohole optical tweezers have been used by several groups to trap and analyze proteins. In this work, we demonstrate that it is possible to create high-performance double nanohole (DNH) substrates for trapping proteins without the need for any top-down approaches (such as electron microscopy or focused-ion beam milling). Using polarization analysis, we identify DNHs as well as determine their orientation and then use them for trapping. We are also able to identify other hole configurations, such as single, trimers and other clusters. We explore changing the substrate from glass to polyvinyl chloride to enhance trapping ability, showing 7 times lower minimum trapping power, which we believe is due to reduced surface repulsion. Finally, we present tape exfoliation as a means to expose DNHs without damaging sonication or chemical methods. Overall, these approaches make high quality optical trapping using DNH structures accessible to a broad scientific community.
Here we show that surface plasmon resonance sensors that typically use 760 nm wavelength Kretschmann-Raether coupling to a 50 nm thick gold film can have 3 times higher surface sensitivity by using local resonances from periodically arranged short-range modes in the same configuration. Considering shot noise, the resolution was found to improve four-fold. This was calculated by matching the design wavelength and minimum angle as calculated by rigorous coupled wave analysis, giving a period of 250 nm in a 10 nm thick gold film and a gap length of 40 nm. Finite difference time domain simulations were used to confirm that the short-range modes correspond to a localized surface plasmon resonances. The present short-range plasmon approach can be used to improve the sensitivity in monitoring biomolecule interactions.
Single molecule analysis of proteins in an aqueous environment without modification (e.g., labels or tethers) elucidates their biophysics and interactions relevant to drug discovery. By combining fringe-field dielectrophoresis with nanoaperture optical tweezers we demonstrate an order of magnitude faster time-to-trap for proteins when the counter electrode is outside of the solution. When the counter electrode is inside the solution (the more common configuration found in the literature), electrophoresis speeds up the trapping of polystyrene nanospheres, but this was not effective for proteins in general. Since time-to-trap is critical for high-thoughput analysis, these findings are a major advancement to the nanoaperture optical trapping technique for protein analysis.
There is a new class of technologies emerging for observing unmodified proteins in action and at the single molecule level. This presentation will give an introduction to the double-nanohole nanoaperture optical tweezer approach and overview the developments from ours and other groups working in the area. Particularly, I will focus on the analysis of the A sub-unit of protein phosphatase PP2A: PR65.
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