Reports: AC5
46878-AC5 Imaging Single Molecule Catalysis at the Solid-Liquid Interface
These studies are being carried out to explore the relationship between the catalytic activity of the surface and its nanometer scale structure and topography. To accomplish this research, we are developing advanced experimental techniques that allow precise spatial registry of single molecule fluorescence and AFM images. The fluorescence images allow us to determine the precise position of the catalytic sites with nanometer scale precision, as well as the kinetics and the catalytic activity of the site. The corresponding AFM images reveal the nanometer scale structure of the surface surrounding the catalytic site. Using this information, we can resolve surface defects or other topographical features and see if such features enhance or suppress the catalytic activity.
In the past year, we have expanded our single molecule fluorescence and AFM capabilities to include fluorescence lifetime ananlysis of individual molecules and nanocrystals. This enables kinetics studies of chemical and photophysical reactions with nanosecond time resolution that can be combined with the AFM imaging. To demonstrate these capabilities, we have examined the fluorescence dynamics of individual isolated semiconductor nanocrystals and semiconductor nanocrystal clusters. The individual isolated nanocrystals exhibit single exponential behavior with lifetimes in the tens of nanoseconds range. The nanocrystal clusters exhibit multiple lifetimes ranging from nanoseconds to tens of nanoseconds. This heterogeneity of lifetimes is characteristic of energy transfer between nanocrystals in the clusters. We have developed a physical model for this energy transfer in which excitation energy of a nanocrystal can be transferred to a “dark” quantum dot in the cluster, thereby quenching the fluorescence. This model is consistent with the blinking behavior of individual nanocrystals and nanocrystal clusters. Individual quantum dots exhibit slow blinking with characteristic correlation times ranging from milliseconds to seconds. However, nanocrystal clusters exhibit much faster blinking, with correlation time in the microseconds to milliseconds. Furthermore, single nanocrystal blinking exhibits discreet on and off times; whereas, multiple intensity levels are observed for the nanocrystal clusters. In the case of nanocrystal clusters, lower intensity blinking correlates with shorter lifetimes, and higher intensity blinking correlates with longer lifetimes.
In the remaining year of this project, we will apply these same techniques to the study of single molecule catalysis on individual isolated gold nanoparticles and gold nanoparticle clusters. In so doing, we will investigate the effects of crystal size and number of interacting particles on the catalytic activity.
