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Wednesday, June 15th, 2016, 7:30 pm
Room M-114, Stanford University Medical School
Scuba Diving at the Nanoscale: Novel Probes for Reliable Scanning Probe Microscopy in Liquids
Scuba Probe Technologies
Better understanding of biological processes at the molecular level requires the ability to image soft matter at high resolution. Using scanning probe force microscopy it remains challenging to visualize how proteins work together in native membranes. Mostly, this is because of the high viscous damping, which increases the interaction forces and results in deformation or even damaging of soft samples. The invention of encased cantilevers overcomes these limitations. Encased cantilevers trap a bubble around the cantilever and thus experience significantly less damping and gentle, non-invasive forces can be applied. As the encasement is optically transparent the devices are compatible with all existing commercial instruments using optical beam deflection detection. Superior performance is demonstrated on correct height measurements of soft matter such as lipid bilayers. Furthermore, self-assembly of collagen was observed in-situ and lattice resolution of mica could be achieved repeatedly. Using force spectroscopy we revealed multiple hydration layers demonstrating the low-noise characteristics of encased cantilevers. Beyond gentler imaging, encased cantilevers enable electrostatic excitation, interferometric detection, and quantitative mass sensing as entirely novel applications. Adding a conductive layer to the encasement the cantilever can be electrostatically driven without requiring any electrode alignment. In contrast to the commonly used piezo based excitation, such electrostatic actuation solely shakes the cantilever resulting in a clean resonance free from artifacts and spurious peaks. This enables quantitative studies of tip sample interaction. Adding a metal film enables to use the encasement for interferometric detection. The cantilever and the encasement form a Fabry-Pérot optical cavity. For specific gap size the light reflected from the encasement destructively interferes with light exiting the cavity. A first prototype achieved deflection noise densities that outperform commercially available instruments. Measuring intensity of the reflected light rather than its position also only requires crude alignment and enables high bandwidth position detection using a single photodiode. Another application beyond imaging is cantilever based mass sensing. Conventionally mass is measured indirectly over the induced stress resulting in a static bending of the cantilever. Unknown binding location and low Q-factors have limited use dynamic detection methods in liquid so far. Using encased cantilevers, however, the binding of a specific analyte only occurs at the exposed tip. Hence, the mass of the analyte can be quantitative extracted from the frequency shift, which can be easily tracked due to the high quality factor. We detected single binding events of nanoparticles with a detection limit of 80 attograms/√Hz. Improving cantilever geometries and/or using higher eigenmodes has the potential to extend this frontier of mass sensing in liquids down to few zeptograms. This corresponds to the mass of a single small protein.
Dominik Ziegler graduated from EPFL in Lausanne and received his Ph.D. from ETH Zurich. His research on MEMS includes implantable sensors, medical devices, and high precision instrumentation such as scanning probe microscopes. He has extensive experience with scanning probe techniques and ultra low-noise equipment in general. His various inventions related to Kelvin Probe Force Microscopy have improved the ability to measure charge distributions at the nanometer scale and helped studying charges in electret films, organic field effect transistors, and work function of few layers graphene devices. As a postdoc at the Lawrence Berkeley National Laboratory he focused on high-speed scanning probe techniques using non-raster scan patterns. Moreover, he combined his expertise in MEMS and scanning probe microscopy to develop encased cantilever. As co-founder of Scuba Probe Technologies LLC, he currently aims to market these innovative new sensors to provide a solution for more reliable and sensitive exploration of nanoscale science in liquids.
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