Using correlated microscopy to understand nanoscale assemblies
Combining microscopy techniques to cancel out each other’s disadvantages.
Microscopy has been of enormous importance in many fields of science. Though each new microscopy technology provides new information, they also have disadvantages. When the sample contains molecule-to-molecule or particle-to-particle variation, combining and correlating different microscopy techniques can eliminate these disadvantages and prove vital towards the correct interpretation of measurements. PhD candidate Michael Beuwer of the TU/e department of Applied Physics researched correlated microscopy to study gold nanoparticle dimers optically and structurally, and relate structure to function.
With optical microscopy, it is easy to study certain aspects of nanoparticles, for instance their location, conformation, orientation, and clustering. However, since optical microscopy does not allow us to observe details smaller than 250 nm, we cannot see the actual structure of the sample. With atomic force microscopy (AFM), a very sharp tip moves over the sample, generating an image of sample height. By combining the structural information from AFM with the location and optical information obtained by optical imaging, we can improve our understanding of the nanostructure.
Gold nanoparticles
In his PhD research, Beuwer studied different self-assembled nanostructures to show the strengths of correlated microscopy. One of the nanostructures he studied were gold nanoparticle dimers. Gold nanoparticles strongly absorb and scatter light; they do so at a resonance wavelength, or color, specific to their size and shape. These particles are sensitive to changes in the local environment, which means they can be used as sensors. For instance, if they bind even a single molecule, this can be detected by a color change in the gold nanoparticle. A gold nanorod is most sensitive at its tips. Therefore, binding location matters. The shift in color depends on multiple factors, so it is not possible to only use optical measurements to determine the binding location. However, by combining the optical response with AFM data, we can relate the binding position to the optical output. This information can then be used to learn more about other molecules of interest, such as proteins involved in diseases, and to improve plasmonic sensing.
Sensitivity
Binding two nanorods together increases the sensitivity of a gold nanoparticle. Two particles that are joined have a region in between them with high sensitivity. Beuwer was able to study this sensitivity by combining simulations with AFM images and monitoring the color change optically. He also showed that the theoretical increase in sensitivity may not be practically useful: the accessibility of the high-sensitivity region may be obstructed by the way the nanorods are linked.
Beuwer’s research shows the enormous power of correlated microscopy for nanoscale assemblies. Information about both structural and optical properties is necessary to understand systems where variation between particles and molecules plays an important role. Beuwer’s research results pave the way to study structure-function relationships in other nanostructures.
Title of PhD-thesis: Correlative Microscopic Characterization of Nanoscale Assemblies at Interfaces. Promotor: Peter Zijlstra, Eindhoven University of Technology. Co-promotor: Menno W. J. Prins, Eindhoven University of Technology Other main parties involved: ICMS.