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Deep-Nulling Microscopy

An important focus of our research is the development of real label-free detection technologies with the goal of unravelling biomolecular interactions in microscopic dimensions. Labelling, e.g. with a fluorescent dye, is very often influencing the investigated biomolecular mechanisms. Furthermore the labelling procedure itself is in most cases complicated and subject to errors. Our group is thus exploring different approaches based on non-linear optics, deep-nulling interferometry and nanotechnology in order to achieve a real label-free detection of biomolecules or biomolecular interactions in aqueous environment. Below is shown a scheme of one of our current approaches: It comprises a standard Mach-Zehnder interferometer with a nulled dark exit (D) and a constructively interfering bright exit (C) with corresponding detectors. Without any disturbance no photon is detected at the dark exit in a perfect interferometer.

However, an object, e.g. a biomolecule, disturbing the coherence in one arm of the interferometer will be indicated directly by photons detected at the dark exit (D). To apply this scheme for a biomolecular analysis we combined a microscope set-up with a highly sensitive deep-nulling microscope. With this approach it is indeed possible to achieve a label-free single particle detection of larger nanospheres and a correlational analysis of proteins and smaller nanospheres

Single Nanoparticle-Transits:

Correlational analysis of the deep-nulling-signals:

Currently we are exploring the combination of this new approach with near-field apertures as coherent light-sources. The use of light-sources having dimension on the order of proteins will greatly enhance the methods sensitivity. To gain more specific information about the objects, the exploration of absorption effects (Kramers-Krönig relation) and the combination with non-linear optical techniques or the optical Kerr effect will be subject of future studies.

For details see:

A. Wehling, W. H. Pohl, B. Gerke, S. Kipp, P. J. Walla, "Generation of nanopores down to 10 nm for the use in deep-nulling interferometry", ChemPhysChem 9, Issue 2, 327-331 (2008).

M. Hilbert, H. Hippchen, A. Wehling and P. J. Walla, "Correlational analysis of proteins and non-metallic nano particles in a deep-nulling microscope”, Journal of Physical Chemistry B, 109, 18162-18170, (2005)

P. J. Walla, “New Frontiers in optical single molecule detection – Nanoparticle analysis based on deep-nulling interferometry?”, MPIbpc News 1-2, 1–5 (2005).

M. Hilbert, P. J. Walla “Correlational analysis of proteins and non-metallic nano particles in a deep-nulling microscope”, MPIbpc News 9, 18–19 (2005).

P. J. Walla, “Wellenfront-Analyse Mikroskop”, patent application (2005).

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