- Fluorescence Microscopy Probes Surface-Dependent Catalysis
As the chemical industry develops techniques that use crystal surfaces to catalyze reactions to improve production efficiency, there is an interest in better understanding the process. Many analytical approaches involve single-crystal metal surfaces under high-vacuum conditions, however, and the industrial processes typically are at ambient or elevated pressure on nonuniform crystal surfaces. Also, although it is known that different surfaces of the same crystal perform different catalytic functions, the spatial variations have not been well characterized. Now a group of researchers at Katholieke Universiteit Leuven in Belgium have addressed both problems with a spatially resolved fluorescence technique.
By using wide-field fluorescence microscopy to image catalysis in action, the effects of catalyst crystal geometry can be investigated. These two images show the crystal structure and the reaction sites for a layered double hydroxide catalyst. Courtesy of Katholieke Universiteit Leuven.
In a demonstration of the method, Johan Hofkens and Dirk E. De Vos began with the nonfluorescent ester 5-carboxyfluorescein diacetate (CFDA), which becomes emissive only when undergoing catalyzed hydrolysis or catalyzed transesterification with a compound such as butanol. As a control experiment, they imaged bare glass slides and slides functionalized with amine molecules using an Olympus IX-71 wide-field fluorescence microscope and a Princeton Instruments electron-multiplying CCD. A Spectra- Physics argon-ion laser provided 488- nm excitation.
As expected, when C-FDA solution was introduced on the bare glass slides, no fluorescence was observed, while C-FDA on the functionalized slides showed individual spots of fluorescence confined to the surface. These results gave the investigators confidence that the observed fluorescence corresponded with individual C-FDA molecules activated at specific catalytic sites.
They next dispersed crystals of layered double hydroxide, a representative catalyst composed of well-formed octahedral layers, on glass slides. In separate experiments, they introduced C-FDA in water solution and in 1-butanol. The 24 × 24-μm image field of the reaction in butanol showed fluorescence distributed evenly about the surface of the crystals, and no fluorescence was observable elsewhere in the solution. In contrast, the C-FDA in water exhibited fluorescence concentrated on the crystal edges.
This technique offers the ability to image catalytic activity associated with different crystal faces. De Vos noted that, with layered double hydroxide used for industrial processes such as polymerization, methanol synthesis and aldol condensation, increasing industrial interest exists for catalytic transformations in the liquid phase.
The team is working to extend the technique by developing additional probe molecules, by applying the method to other catalysts and by enhancing the spatial resolution with stimulated emission-depletion techniques.
Nature, Feb. 2, 2006, pp. 572-575.
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