Figure 3. Single-molecule studies on the quenching of TMR-bipy-DNA by CuSO₄.

(A) Confocal fluorescence image of single ATTO620-bipy-DNA molecules immobilized on glass cover slides via biotin/streptavidin. Image (30×15 µm²) taken using pulsed excitation with a diode laser emitting at 635 nm with a repetition rate of 80 MHz at an average excitation power of 5.5 µW in presence of 0.1 µM CuSO₄. Time-resolved traces of single immobilized dye-bipy-DNA molecules labeled with TMR (B), ATTO 620 (C) and ATTO 633 (D) recorded under the same conditions show discrete dark states due to reversible complexation of Cu²⁺ and bipyridine which leads to intramolecular quenching of the fluorescence emission.