Fast Imaging of Single Molecules and Nanoparticles by Wide-Field Microscopy and Spectrally Resolved Confocal Microscopy

The dynamic photophysical behaviour of single Cy5- maleimide molecules and core-shell (CdSe)ZnS quantum dots is compared. To this end a home-built fast and sensitive wide-field microscopy system and a scanning confocal microscope coupled to a spectrograph for spectral imaging are employed. The ability to measure time-resolved spectra of quantum dots is shown to be particularly useful in characterizing single quantum dots. The difference in blinking behaviour of single Cy5-maleimide molecules and (CdSe)ZnS quantum dots can be understood by their physical origin, i.e. the presence of a single ‘dark’ state in case of Cy5 versus a distribution of ‘dark’ states in case of (CdSe)ZnS. This intrinsic difference results in higher probabilities to find a quantum dot in a short-lived (<0.02 s) or long-lived (>0.2 s) state than a Cy5-mal molecule. Moreover, from a comparison to data in the literature, it was found that the lifetime of the ‘off’-state of the Cy5-mal molecule is longer than for the non derivatized Cy5 molecule.