RUI: High-Speed Imaging and Spectroscopy of Single Molecules

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With this award, the Chemical Measurement and Imaging Program is funding Dr. Nathan Lindquist and Dr. Nathan Lemke at Bethel University to develop single-molecule measurement techniques capable of high-speed (approximately MHz) imaging and spectroscopy with subwavelength (near 5 nm) spatial resolution using SERS(surface-enhanced Raman spectroscopy). The ability to image, track, and analyze single molecules with microscopes that use light opens up a vast array of new scientific possibilities. One promising area of research for probing single molecules is called surface-enhanced Raman spectroscopy (SERS). In these experiments, laser light is used to energize minuscule metal particles. These metal nanoparticles can then transfer energy to nearby molecules, resulting in their emission of light at specific energies. As a result, the molecules can be identified. However, many single-molecule experiments show large and random high-speed fluctuations in the amount of emitted light, which are not easily understood and also complicate analysis. This project sets out to develop a new microscope and detector system that can record super-high-resolution movies at a million frames per second, which is fast enough to capture, locate, track, and identify the fleeting signals from individual molecules. By using SERS, this new measurement technology has the potential to reveal more fully how light and matter interact on the nanoscale. This research is being performed at an undergraduate institution and its activities are deeply integrated into the curriculum through open-ended lab projects, summer research opportunities, and public outreach activities. The project aims to inspire, motivate, and train undergraduates for future careers in science and engineering.The concept of optical "hotspots" in laser-illuminated metallic nanoparticles is central to the interpretation of the SERS effect. However, while hotspots are generally portrayed as a static feature of a metal/molecule system, single-molecule SERS is punctuated with very high-speed (microsecond) fluctuations, both in signal intensity and spectral content. These random fluctuations originate from different areas of a single nanoparticle and can be uniquely activated with different laser wavelengths. Temperature and laser power also play a role in the fluctuation dynamics. High-speed SERS fluctuations are therefore interpreted to arise from single molecules interacting with a dynamic and restructuring metallic surface that generates short-lived, atomic-scale optical hotspots. Atomic-scale optical hotspots are a relatively uncharted area of basic research and new measurement technologies are therefore needed. This research uses image intensifiers integrated with multiple lasers, detectors, and fast single-photon counters to analyze atomic-scale optical hotspots and single-molecule SERS with unprecedented speed, sensitivity, and resolution.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.