Located at the crossroads of biophysical chemistry, optics, nanoengineering, and ultrasensitive biomedical analysis, research in the Woehl Lab is interdiscplinary by nature. It focuses on the optical spectroscopy and imaging of single molecules and their controlled manipulation using a novel device pioneered in our research group: the corral trap. The particular aim of our research is to better understand
1. In many cases, biological molecules carry out their tasks by making use of molecular electric fields (such as in photosynthesis, enzymatic activity, electrostatic steering of ligands to active sites, and protein folding and assembly). In a collaborative effort with the Geissinger group, we employ single molecules as nanoscale reporters of molecular electric fields at the active sites of the oxygen carriers myoglobin (see image to the right) and hemoglobin, where molecular fields may play an important role for the proteins' biological function. The spectroscopic measurements are carried out at cryogenic temperatures by applying an external electric field, which shifts the single molecule absorption or emission line shifts (Stark shift). We have developed two novel methods of analysis based on quantum chemical calculations that allow for the determination of internal electric fields (magnitude and orientation) from the experimental data.
2. All known physical and chemical properties of a molecular species are the direct result of quasi-electrostatic interactions, and it appears sensible to focus on electrostatic forces as the most efficient means of exercising control over single molecules. Based on this idea, we have conceived and developed a new approach for the trapping and manipulation of single nanoparticles (including single molecules): the corral trap. Corral trapping of single molecules opens up new possibilities for the planned (as opposed to purely heuristic) assembly of molecular-scale devices, detection of single base substitutions (single nucleotide polymorphisms, SNPs) in the human genome at the single molecule level, and may lead to new approaches for water purification.
A common component of all our experiments is the ability to image single molecules with high spatial resolution, and the development and improvement of instrumentation and theoretical tools for optical microscopy is therefore at the core of our efforts. In order to spatially localize and select a given molecule, we utilize widefield microscopy, confocal scanning laser microscopy, or near-field scanning optical microscopy (NSOM). In aperture-type NSOM, a tapered optical fiber serves as a nanoscale light source, and we have developed new NSOM tips on the basis of photonic crystal fibers (see SEM image to the right) with improved imaging capabilities. We have also mapped the three-dimensional point spread function (PSF) of a confocal microscope at high spatial resolution and developed an rigorous theoretical model based on a vectorial approach that has resulted in PSF Lab, a software package for calculating PSFs that is currently used by research groups from more than 50 countries worldwide.
Students in my group gain a broad experience in physical chemistry and acquire specialized skills in a variety of fields such as single molecule imaging, nanophotonics, laser spectroscopy, thin-film deposition, cryogenic techniques, metal evaporation, and scanning probe microscopies. Please do not hesitate to contact me if you wish to participate in any of these research projects.