Jorg C. Woehl

Physical Chemistry
Assistant Professor
PhD, University of California at Riverside
(414) 229-5223
e-mail: woehl@uwm.edu
Woehl Research Group Page
Selected Publications

The focus of our research is the imaging and, ultimately, the controlled manipulation of single nanoscale objects such as single fluorescent molecules. The Figure exemplifies one of several unique features of such a single quantum system, namely single-step photobleaching. The images, raster-scanned from top to bottom, show the diffraction-limited fluorescence spot from a single cyanine dye molecule (scale bar: 500 nm). During the second scan, a sudden disappearance of light emission is clearly visible where the molecule ceases to emit - this is a behavior that is not found when many molecules contribute to the fluorescence signal.

In order to spatially localize and select a given molecule, we utilize confocal scanning laser microscopy and near-field scanning optical microscopy (NSOM), in which case a tapered optical fiber serves as a nanoscale light source. We are actively developing new nanophotonic probes on the basis of photonic crystal fibers to push the resolution limit to the true molecular scale. Using single fluorescent nanobeads with a diameter of 20 nm as a nanoscale detector, we have also been able to map the three-dimensional point spread function of a confocal microscope at high resolution. These results have enabled us to develop an improved theoretical model that will serve as the basis for new confocal techniques and improvements for image reconstruction using deconvolution techniques.

One of the many advantages of single molecule measurements is a complete absence of ensemble averaging; samples can be studied one molecule at a time, and the whole distribution and fluctuations of molecular properties are obtained instead of just an average value, which yields more insight about underlying molecular mechanisms. We are utilizing this to better understand molecular diffusion in lipid films and biological membranes. Results from these studies will be directly relevant for an ambitious research goal: the trapping and manipulation of single molecules in molecular films, which would open up new possibilities for the fabrication of metal-molecule devices in molecular electronics, and could lead to the creation of artificial molecular arrangements and the assembly of biomimetic systems

Other single molecule studies in our group are closely related to biophysics and molecular electronics. We are interested in understanding the role of the electric field created by the hemoglobin subunits for the physiological function of ligand binding by heme groups; this is studied via Stark measurements at low temperatures using single molecule fluorescence of heme analogs. We are also investigating the photodynamics of electron transport molecules and study the effect of electric fields on their electronic structure, which will help answer important questions about the underlying conduction mechanism.

Students in my group gain broad experience and acquire specialized skills in a variety of fields such as nanophotonics, laser spectroscopy, thin-film deposition and cryogenic techniques, metal evaporation, and scanning probe microscopies. Whether you are a graduate or undergraduate student, please do not hesitate to contact me to discuss any short- or long-term research projects which you are interested in.