Optical microscopes, such as those found in almost every high school to look at cells, have an inherent limit to their spatial resolution. They cannot resolve two objects that are closer to each other than the wavelength of light, which for visible colors is about 500 nm, or about 1/100 the diameter of a human hair. Recently super-resolution microscopes have been developed that can resolve individual molecules that are only 10 nm apart. Super-resolution microscopy has had tremendous success when imaging fluorescent dyes near other molecules, such as proteins, but it has trouble when the molecule is located near a metal nanostructure. The problem arises when the molecule interacts with the loosely held electrons in the metal, which blurs its location. With support from the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Willets at Temple University is studying the interaction of fluorescent molecules with metal nanoparticles. Professor Willets and her students are developing protocols that can better pinpoint the location of the fluorescent molecule. The project could pave the way to producing higher fidelity super-resolution images, which may help to advance nanotechnologies for biosensing, nanomedicine, and solar energy conversion. The work also provides training opportunities for graduate and undergraduate students, furthering the development of the Nation's scientific workforce. In addition, the Willets lab is actively engaged with the 9th grade science classes at Freire Charter School in downtown Philadelphia, helping the students to foster an appreciation for scientific exploration and discovery.
Working alongside her students, Professor Willets is functionalizing gold nanorods and nanospheres at specific sites with fluorescently-labeled DNA. A combination of super-resolution imaging and polarization-resolved microscopy is then used to track how the position and orientation of the fluorescent molecules impacts the localization precision of the fluorescent molecule. In super-resolution imaging, single fluorescent molecules are localized by fitting their diffraction-limited emission to a model function, such as a two-dimensional Gaussian, to localize their spatial position with better than 10 nm precision. However, coupling between the fluorescent emitter and the plasmon modes of the nanostructure impacts the localization accuracy, resulting in the calculated position of the fluorescent molecule being shifted from its true position. Theoretical modeling provides insight into how the position and orientation of a single molecule relative to the plasmonic nanostructure affects the localization error. By creating well-defined hybrid organic-plasmonic nanoparticles, the team is developing new analysis tools to extract information hidden in the fluorescence images to improve agreement between the super-resolved images and the actual nanostructure properties.
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.
