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14 February 2007 Single-molecule studies on individual metal complexes
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Some fluorescent dyes, like Atto635 or Cy5, exhibit alterations in fluorescence emission in the presence of various metal ions and metal ion complexes. We used this effect to design dye-ligand conjugates that can be immobilized on glass surfaces and allow studying metal-ion binding using time-resolved single-molecule fluorescence spectroscopy (SMFS). Double-stranded DNA served as a rigid scaffold carrying 2,2'-bipyridene-4,4'-dicarboxylic acid as chelating ligand and a fluorescent dye as reporter, placed in close vicinity to the ligand. In the absence of metal ions, the probes showed high fluorescence quantum yield, whereas strong fluorescence quenching upon binding of Cu2+-ions was observed. Time-resolved single-molecule measurements revealed stochastic switching between a highly fluorescent ("on") and a low fluorescent ("off") state. The coordination of the metal ion to the ligand is thus indicated by intramolecular fluorescence quenching of the dye. We screened various fluorescent dyes for their sensitivity to Cu2+-coordination, and found that both Atto620 and MR121 are well-suited for this application. Ensemble studies of the fluorescence lifetimes of metalsensors with Atto620 showed only small dependence on the metal-ion concentration, while single-molecule studies reported strong changes in the fluorescence lifetimes which were correlated with the observed on- and off-states. Our results further indicate that the fluorescence of Atto620 is not completely quenched upon association of the metal-ion complex; either because a less fluorescent complex is formed or because of intramolecular collisional quenching due to conformational changes of the C6-linker used for covalent coupling of the fluorescent dye.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Alexander Kiel, Anne Järve, Janos Kovacs, Andriy Mokhir, Roland Krämer, and Dirk-Peter Herten "Single-molecule studies on individual metal complexes", Proc. SPIE 6444, Ultrasensitive and Single-Molecule Detection Technologies II, 64440C (14 February 2007);

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