| Abstract
| - We demonstrate that commercially available unmodified carbocyanine dyes such as Cy5 (usuallyexcited at 633 nm) can be used as efficient reversible single-molecule optical switch, whose fluorescentstate after apparent photobleaching can be restored at room temperature upon irradiation at shorterwavelengths. Ensemble photobleaching and recovery experiments of Cy5 in aqueous solution irradiatingfirst at 633 nm, then at 337, 488, or 532 nm, demonstrate that restoration of absorption and fluorescencestrongly depends on efficient oxygen removal and the addition of the triplet quencher β-mercaptoethylamine.Single-molecule fluorescence experiments show that individual immobilized Cy5 molecules can be switchedoptically in milliseconds by applying alternating excitation at 633 and 488 nm between a fluorescent andnonfluorescent state up to 100 times with a reliability of >90% at room temperature. Because of theirintriguing performance, carbocyanine dyes volunteer as a simple alternative for ultrahigh-density opticaldata storage. Measurements on single donor/acceptor (tetramethylrhodamine/Cy5) labeled oligonucleotidespoint out that the described light-driven switching behavior imposes fundamental limitations on the use ofcarbocyanine dyes as energy transfer acceptors for the study of biological processes.
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