Monitoring hydroquinone-quinone redox cycling by single molecule fluorescence spectroscopy.

Current research in the field of single-molecule chemistry is increasingly focused on the development of reliable experimental approaches for investigating chemical processes on a molecular level using single-molecule fluorescence spectroscopy (SMFS). Herein, we report on single-molecule observations of the copper(II)/air mediated oxidation of fluorescently labeled hydroquinone-based probe molecules followed by their reduction with cysteine. The redox cycle is signaled by quenching/recovery of fluorescence emission after addition of the oxidant/reductant, respectively. The experiments were realized by immobilizing the probe on a glass cover slide to allow single-molecule observation by means of total-internal-reflection fluorescence microscopy. Besides detection of successful oxidation and reduction events on single probe molecules, individual molecular intensity trajectories revealed dynamic processes and formation of intermediate states upon reaction. For the experimental design presented, we envision further reaction studies of catalytic redox-processes of single hydroquinone-moieties by means of SMFS.