With the goal of enhancing the emission of ultra-sound (US) waves in photoacoustic imaging (PAI) for the detection of cancer we discovered an effective system for Förster Resonance Energy Transfer (FRET) composed of a near infrared fluorescent (NIRF) dye and a NIR quencher (NIRQ) dye separated by a polyproline peptide spacer. In PAI, a newly developed imaging technique, the signal is generated by excitation of cells using pulsed infrared lasers and measurement of resulting US waves using detectors. NIRF dyes can increase the sensitivity of PAI signal, particularly when used in targeted molecular imaging agents (TMIAs). However, most of the energy of absorption is likely lost during fluorescence. Our hypothesis was that if the fluorescence could be quenched by FRET, all of the energy from the laser could be converted to a substantially stronger US signal from the NIR dyes. In FRET, an optimized distance between the NIRF and NIRQ dyes is essential, but has not yet been determined for this dual dye system. To determine the appropriate distance, we designed and synthesized a polyproline spacer as a rigid “molecular ruler”. Multiple polyproline spacers (di, tetra, octa) were synthesized and coupled first to two NIRF dyes, Cy5.5 and IR770TI, using a modular synthetic approach developed by our group. When the emission resulting from FRET between NIRF dye pairs was evaluated, a strong FRET effect was observed from the octaproline dye pair, with twice the signal as the tetraproline. Furthermore, it was found that the FRET effect could be influenced by the solvents due to the dye aggregation. The polyproline approach was then applied to the synthesis and evaluation of NIRF-NIRQ dye pairs resulting in the discovery that tetra and octaproline NIRF-NIRQ systems yield nearly complete FRET quenching. The new FRET system is a valuable tool for future research in PAI to provide a higher, more sensitive signal. Samples have been submitted to collaborators for evaluation using PAI instrumentation at nearby cancer research centers.
Department, Program, or Center
School of Chemistry and Materials Science (COS)
Geng, Nisha, "Synthesis of Polyproline Spacers between NIR Dye Pairs for FRET to Enhance Photoacoustic Imaging" (2017). Thesis. Rochester Institute of Technology. Accessed from
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