Conspectus: Recent advances in fluorescence microscopy allow us to track chem. reactions at the single-mol. level. Single-mol. measurements make it possible to minimize the amount of sample needed for anal. and diagnosis. Signal amplification is often applied to ultralow-level biomarker detection. Polymerase chain reaction (PCR) is used to detect DNA/RNA, and ELISA (ELISA) can sensitively probe antigen-antibody interactions. While these techniques are brilliant and will continue to be used in the future, single-mol.-level measurements would allow us to reduce the time and cost needed to amplify signals. The kinetics of chem. reactions have been studied mainly using ensemble-averaged methods. However, they can hardly distinguish time-dependent fluctuations and static heterogeneity of the kinetics. The information hidden in ensemble-averaged measurements would be extractable from a single-mol. experiment Thus, single-mol. measurement would provide unique opportunities to investigate unrevealed phenomena and to elucidate the questions in chem., physics, and life sciences. Redox reaction, which is triggered by electron transfer, is among the most fundamental and ubiquitous chem. reactions. The redox reaction of a fluorescent mol. results in the formation of radical ions, which are normally nonemissive. In single-mol.-level measurements, the redox reaction causes the fluctuation of fluorescence signals between the bright ON-state and the dark OFF-state, in a phenomenon called blinking. The duration of the OFF-state (ﾏОFF) corresponds to the lifetime of the radical ion state, and its reaction kinetics can be measured as 1/ﾏОFF. Thus, the kinetics of redox reactions of fluorescent mols. can be accessed at the single-mol. level by monitoring fluorescence blinking. One of the key aspects of single-mol. anal. based on blinking is its robustness. A blinking signal with a certain regular pattern enables single fluorescent mols. to be distinguished and resolved from the random background signal. In this Account, we summarize the recent studies on the single-mol. measurement of redox reaction kinetics, with a focus on our group窶ｲs recent progress. We first introduce the control of redox blinking to increase the photostability of fluorescent mols. We then demonstrate the control of redox blinking, which allows us to detect target DNA by monitoring the function of a mol. beacon-type probe, and we investigate antigen-antibody interactions at the single-mol. level. By tracing the time-dependent changes in blinking patterns, redox blinking is shown to be adaptable to tracking the structural switching dynamics of RNA, the preQ1 riboswitch. This Account ends with a discussion of our ongoing work on the control of fluorescent blinking. We also discuss the development of devices that allow single-mol.-level anal. in a high-throughput fashion.
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