Probing Individual Chemical Events

Fluorescence microscopy can be used to pinpoint individual chemical reactions in real time as they occur at the interface between a solution and the surface of a key industrial catalyst support, according to a study by researchers at the University of California, Irvine (J. Am Chem. Soc., DOI: 10.1021/ja105517d).

The technique provides molecular-resolution maps and live-action movies depicting the chemical evolution of discrete metal complexes as they bind to functionalized silica surfaces and thereby form model supported metal catalysts. That level of detail cannot be obtained from analytical methods that survey average properties of large numbers of molecules or from nanoscale techniques that probe static chemical systems. The advance could lead to new insights in inorganic and organometallic chemistry and surface catalysis.

To monitor the single-molecule events, UC Irvine chemists N. Melody Esfandiari, Yong Wang, Suzanne A. Blum, and coworkers treated glass (silica) microscope coverslips with a triethoxysilyl thiourea compound. The team carried out that preparation step in a manner that patterned the surface with alternating functionalized and unfunctionalized stripes measuring 25 μm in width. Then, the researchers exposed the surface to a boron dipyrromethene (BODIPY)-tagged diethylenetriamine platinum complex.

The reaction between the platinum species and sulfur in the thiourea groups selectively anchored the fluorophore-tagged metal complexes within the functionalized surface regions. By patterning the surface in that way and by conducting control experiments, the team demonstrated that points of light detected in the experiments were single-molecule fluorescence signals excited selectively from surface-bound metal complexes.

One key observation in the study is that the chemical reaction of one metal complex did not influence the location of a subsequent chemical reaction. That is, the team finds the reactions to be uncorrelated, suggesting that the molecules do not "communicate" via a charge-based or other mechanism.

Susannah L. Scott, a UC Santa Barbara chemical engineering professor, notes that "the ability to detect reactions at the solution-surface interface in real time and at the single-molecule level could be a real breakthrough in our understanding of how supported catalysts operate." Scott adds that a logical follow-up study would be to probe reactions of metal complexes already immobilized on a surface. Those experiments, she says, will begin to demonstrate the full potential of the new technique.