RUI: Studying the photodynamics of FRET paired fluorescent molecules near gold nanogratings

Trinity University, San Antonio TX

Investigators

Abstract

Currently patterns on metal surfaces can be fabricated with features in the nanometer (one billionth of a meter) size range. These patterns can interact strongly with particular colors of light. Fluorescent molecules are designed to give off certain colors of light, and are used to mark specific molecules and proteins in biological applications. A nanopatterned metal surface can be made such that its optical properties match a particular fluorescent molecule and enhance the light signal from the molecule. Fluorescent molecules can also transfer light from one molecule to another, and this transfer is extremely sensitive to the distance between molecules. Biologists have used this property to measure distances at the sub-nanometer scale. The team will use the unique optical properties of arrays of gold wires hundreds of nanometers wide to enhance this transfer of energy. The result of this work will aid in the study of this energy transfer in photosynthesis and in applications such as improving light harvesting in organic solar cells. The team will be completely comprised of a diverse set of undergraduate students at Trinity University. The students will take a leading role in every aspect of the experiments, will present their finding at national scientific meetings, and will be co-authors on any publication resulting from this work. Equipment and experimental techniques from this project will be integrated into upper division lab courses, broadening the impact to additional students at Trinity University. This project characterizes the photophysics of Forster resonance energy transfer (FRET) paired donor and acceptor fluorescent molecules near a gold nanograting. The optical properties of structured metal surfaces can be engineered to influence the fluorescence of nearby quantum emitters. Applying this to FRET has many potential advantages, including the ability to enhance the energy transfer rate and increasing the Forster radius of FRET pairs. Metal enhanced fluorescence results from two effects, an enhanced excitation rate of the fluorophores and a decrease in their excited state lifetime of the fluorophore arising from altering the local density of optical states (LDOS). The geometry of gratings uniquely allows these two mechanisms to be measured separately, providing greater insight to the photophysics of the system. Surface plasmon modes on gratings follow a dispersion relationship, allowing for a wide wavelength range of relatively narrow surface plasmon resonances covering both fluorophores’ absorption and emission spectra using a single substrate. Time-correlated single photon counting will be used to directly measure both the FRET energy transfer rate and efficiency as a function of surface plasmon wavelength. DNA will be utilized to attach the donor and acceptor molecules to the nanogratings, allowing for precise spacing between donor and acceptor molecules with respect to each other as well as the nanograting surface. The results of these experiments will allow one to optimize the enhancement in FRET rate and efficiency, as well as directly measure any increase in the Forster radius. 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.