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In our experiments the design of individual sensor interfaces is common in all three approaches: scanning probe microscopy/spectroscopy, optical tweezers and cantilever array sensors. All techniques require the following issues to be fulfilled:

• Immobilization of the molecule under investigation in a native manner on a sensor interface using various chemical approaches (see figure)
• Determination of forces of a few pico-Newton (pN) and position deflection readout with nanometer accuracy
• Ability to detect conformational changes with nanometer accuracy

	Biomolecules are grafted in a native manner onto the underlying substrate by specific chemistry (e.g. self assembled monolayers) or direct chemical modification of a molecular site.

We explore and develop new schemes of direct chemical modification of single molecules or native immobilization of the biomolecule of interest via chemical cross-linkers onto the substrate a. Most of our experiments require covalent attachment of biomolecules on the interface or chemical modification of the molecules of interest. The main reason for this is that in some experiments we actively pull on a single molecule. Release of the biomolecule off the interface is not within the scope of the experiments. In other cases we regenerate receptor molecules bound to the interface by rinsing the interfaces with chemicals which release all bound ligand molecules interacting with their receptor sites. If physisorption were the ‘attachment’ mechanism for the receptor molecules, then some of the receptor molecules on the sensors surface would be stripped off during such processes and the functionality of the sensors would be hampered. Ref: a) M. Hegner, Single Mol. 1, 139 (2000)