PUB - Publikationen an der Universität Bielefeld

This thesis presents an on-chip manipulation and positioning system for single magnetic markers. The setup is simple and easily customisable. Conducting lines are patterned with optical lithography on a Si-wafer chip. An applied current through the conducting lines creates a magnetic gradient field that interacts with the magnetic markers, so the magnetic markers follow the gradient of the field to a local maximum. It is shown that this manipulation and positioning technique works in principle and, moreover, several applications are introduced. One application is a special design that allows the transportation of several markers and the positioning in predefined places. Additionally, the trapping of markers inside a ring structure is studied and the effects of an applied electric field on the magnetic markers are investigated. Single magnetic markers were positioned directly on top of small TMR sensors using a specifically designed structure. The magnetic stray field of the magnetoresistive sensors helped to position the magnetic markers on top of the sensors, although this is not necessary. The accuracy of the positioning system only depends on the accuracy of the used lithography. So for e-beam lithography the accuracy is below 100nm. The manipulation system is eminently suited for a small handheld biosensor device because it fits together with the sensors directly on the Si-wafer chip. As an application of the manipulation technique, bond-force measurements between two biomolecules were done. One biomolecule is attached to the sample surface and the other is connected to a magnetic marker. After the biomolecules bound, a magnetic field is turned on using the conducting lines on the chip. The magnetic gradient field is slowly increased until the bonds between the biomolecules break. This event is monitored with a CCD-camera and evaluated to calculate the corresponding bond-force. Compared to other bond-force measurements, this method has an extremely low loading-rate (about 1 fN/sec) and, therefore, the measured bond-forces are extremely low as well. The two well known bonds streptavidin-biotin and avidin-biotin were investigated, and very low bond-forces were measured (full bond of streptavidin-biotin: about 245 fN, avidin-biotin: about 58 fN). The measured bond-forces are about 1000 times lower than produced by earlier measurements, which fits well to the loading-rate, which is also about 1000 times lower. This method also provides evidence for the cooperativity of these ligand-receptor bonds.