Tracking chemical reactions with high-resolution microscopy

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Non-contact atomic force microscope images of reactants and products (center) are remarkably recognizable compared to structural diagrams (bottom) and show greater clarity compared to scanning tunneling microscope images (top)

While working towards new syntheses of functional nanomaterials for use in molecular electronic devices, UC Berkeley chemist Felix Fischer and physics collaborator Michael Crommie came upon an astounding discovery. Using a technique called non-contact atomic force microscopy (nc-AFM), they were able to directly image Fischer’s reactions on a single molecule level in incredible detail. This ground-breaking work marks the first time single reactants and their consequential products have been visualized. Not only do the images afford sub-nanometer clarity, but regions of increased electron density (e.g. multiple bonds) are starkly apparent. Maybe as a chemist I am biased, but I find these images quite breathtaking.

Though these findings are certainly fundamentally exciting, the science behind them is also pretty cool. Operating without electromagnetic radiation, AFM is a bit of a black sheep amongst its optical and electron microscope cousins. A type of scanning probe microscopy, AFM images surfaces by interacting directly with the force fields (e.g. van der Waals forces) associated with the specimen of interest. In this way, the microscope is said to “feel” the surface of the sample independent of electromagnetic radiation. Consequently, unlike other varieties of scanning probing microscopy, AFM imaging is not limited to that of semiconducting or conducting materials, making it an ideal candidate for Fischer’s work. Its basic construction consists of a cantilever arm with a tip attached at the terminus that raster scans across the sample. A laser is aimed at the backside of the arm and reflection of this beam off the arm terminates in a photodiode. The changing position of this beam on the diode detects the displacement of the cantilever as the tip bounces up and down the sample. Operating distances are anywhere from a couple to a few hundred angstroms away, as the magnitude of the force fields felt by the tip are distant-dependent. Put it in Fisher’s words, AFM on molecules is a bit like reading Braille.

Atomic Force Microscope.

Atomic Force Microscope.

One other, perhaps more subtle, motivation for the use of AFM is the capacity for single molecule resolution. Currently a burgeoning field of interest, single molecule studies investigate topics ranging from molecular motors to nanodevices. Historically, reactions are studied by identifying bulk samples of reactants, intermediates, and products with spectroscopy. These characterization methods rely on compiling spectroscopic features to piece together molecular structures. Data collected in this fashion is thus both indirect and subject to ensemble averaging effects. In contrast, the study of individual molecules allows for the elucidation of properties obfuscated in the ensemble by unambiguously mapping physical properties to their structures. In Fischer’s case, nc-AFM is employed to directly resolve structural changes and observe minor products that may be washed out in ensemble characterization approaches.

Beautiful both in experimental design and ground-breaking results, Fischer and Crommie’s  work is an excellent example of the power of collaboration and elegant approach. For those interested in the UC Berkeley news releases, head on over here and here. What are some other exciting modern findings that inspire you? Hit us up in the comments below.

 

 

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