Experimental Research

Physical characterization of model systems for the study of curvature generation

I am collaborating with Kathleen Howard’s research group to develop good model systems for the study of curvature generation by influenza M2 protein, a membrane protein involved in the generation of curvature that facilitates virus budding. With several research students, we have used light scattering and a colorimetric assay to optimize the challenging process of reliably reconstituting the full-length protein into liposomes. The long term goal of the work is to carry out structural studies that can elucidate the process by which M2 is involved in curvature generation, and thus how drugs could target its role in influenza infection.

We have submitted this work to PLOS One (preprint available on request):

Catherine H. Crouch, Margaret H. Bost,* Tae H. Kim,* Bryan M. Green,* D. Stuart Arbuckle, Carl H. Grossman, and Kathleen P. Howard, “Optimization of Detergent-Mediated Reconstitution of an Integral Membrane Protein” (August 2016).

Prior to launching this project with the Howard lab, I spent a sabbatical with the research group of Tobias Baumgart at the University of Pennsylvania developing optical techniques for the study of curvature sensing in peripheral membrane proteins.

Single-nanocrystal fluorescence (2003-2011)

An intriguing property of single III-V semiconductor nanocrystals (NCs) is that they exhibit intermittent fluorescence — under steady illumination, individual NCs switch between a dark (or “off”) state, in which nonradiative recombination of excitons occurs much more rapidly than radiative recombination, so no light is emitted, and a bright (“on”) state, in which the reverse is true. These on or off states last for milliseconds to many minutes at a time. [1,2] A power-law distribution of off-time durations and a truncated power-law distribution of on-times has been repeatedly observed. Although this blinking behavior is not fully understood, the dark state is thought to occur when the electron escapes from the NC core and gets trapped either at the surface or in the environment, leaving the core charged; the bright state is recovered when the electron tunnels back to the core.

Here are the projects my group and collaborators carried out:

Nanorod blinking

My coworkers in  Prof. Marija Drndic’s group at the University of Pennsylvania, a Swarthmore undergraduate, and I showed that rod-shaped semiconductor nanocrystals, or nanorods (NRs), also blink, with very similar statistics as NQDs. NRs show truncated power law on-time statistics with a truncation time that increases with decreasing aspect ratio. [3] Reprint

We used correlated transmission electron microscopy and fluorescence to show that small clusters with 2-3 NRs have indistinguishable blinking statistics from single NRs, while clusters with 5 – 15 NRs have increased truncation times and altered on- and off-time exponents.[4]  DOI We also found evidence for collective enhancement of blinking in clusters. [5] DOI

Binning dependence of blinking statistics

In collaboration with Matthew Pelton and Xiaohua Wu of Argonne National Laboratory’s Center for Nanoscale Materials, and Prof. Marija Drndic’s group at the University of Pennsylvania, we have shown that the time binning involved in typical blinking data analysis can lead to substantial artifacts in on probability distributions. [6]  DOI

Excitation energy dependence of nanocrystal blinking

We measured excitation energy dependence of the blinking statistics of different sizes of spherical core/shell nanocrystals (NCs) using excitation energies corresponding to 532 nm, 488 nm, and 405 nm laser light. We found that the energy dependence may reflect a change in dynamics at energies corresponding to the 1P state, or may simply be attributable to changes in absorption cross-section. [7] [Reprint]

References

  1. F. Cichos, C. von Borczyskowski, and M. Orrit, “Power-law intermittency of single emitters,” Current Opinion in Colloid and Interface Science 12, 272 (2007) and references therein.
  2. P. Frantsuzov, M. Kuno, B. Janko, and R. A. Marcus, “Universial emission intermittency in quantum dots, nanorods, and nanowires,” Nature Physics 4, 519 (2008) and references therein.
  3. Siying Wang, Claudia Querner, Thomas Emmons*, Marija Drndic, and Catherine H. Crouch, “Fluorescence blinking statistics from CdSe core and core-shell nanorods,” Journal of Physical Chemistry B 110. 23221 (2006).
  4. Siying Wang, Claudia Querner, Michael Fischbein, Lauren Willis, Dmitry Novikov, Catherine H. Crouch, and Marija Drndic “Blinking statistics correlated with nanoparticle number,” Nano Letters 8, 4020 (2008).
  5. Siying Wang, Claudia Querner, Tali Dadosh, Catherine H. Crouch, Dmitry Novikov, and Marija Drndic, “Collective fluorescence enhancement in nanoparticle clusters,” Nature Communications 2, 364 (2011).
  6. Catherine H. Crouch, Orion Sauter*, Xiaohua Wu, Robert Purcell*, Claudia Querner, Marija Drndic, and Matthew Pelton, “Facts and artifacts in the blinking statistics of semiconductor nanocrystals,”  Nano Letters 10, 1692 (2010).
  7. Catherine H. Crouch, Robert Mohr*, Thomas Emmons*, Siying Wang, and Marija Drndic, “Excitation energy dependence of fluorescence intermittency in CdSe/ZnS core-shell nanocrystals,” Journal of Physical Chemistry C 113, 12059 (2009).

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