Results and Discussion

In all our experiments with magnetic gradient fields, the sulfur-gold bond could not be broken. The markers either did not bind at all, or could not be ripped off the gold line, even with currents of up to 150mA. The maximal force with a current of 150mA, a magnetic moment of the beads of 0.4fAm$^2$ and at a distance of 9.63$\mu$m, can be calculated as 129fN. This is 4 magnitudes lower than observed in AFM measurements [54] and obviously too weak. But it may not be that far away, when thinking about the loading-rate dependency found for other bonds (confer section 1.1.4).

Figure 4.4: The sulfur bond cannot be broken with the maximum magnetic field (a), but with an electric field between the outer electrodes, (b) and (c). See the CD for the complete Video.

[Applying a current of 80mA through the wide line in the middle]     [Directly before the electric field is turned on]     [Directly after the electric field is turned on]

Although the sulfur-gold bond could not be broken with an applied magnetic gradient field, it can be broken with an electric field. Using the setup, already presented in section 3.3.3, the rupture of the sulfur-gold bond works in principle. Figure 4.4 shows three images from the video of the rupture events. In the first image the two single beads on the thin line and the bead agglutination below still bind to the gold surface (a), so the magnetic force at the maximal possible current of 80mA is still too weak. But when the electric field is turned on, all beads are ruptured. See before (b) and after (c) the electric field is turned on. However, the uncertainty of the charge carried by each bead (confer section 3.3.3) prohibit a calculation of the bond-force.

Because this very strong covalent bond could not be broken with the magnetic gradient field, non-covalent but still strong ligand-receptor bonds were used for the next experiments. The following sections present measurements of the streptavidin-biotin and the avidin-biotin bonds.