For all bond-force measurements, it is absolutely necessary that there is no unspecific binding between the magnetic markers and the sample surface. Therefore, all markers were tested and carefully selected.
First, all 12 different magnetic markers we had at our disposal were
tested for unspecific binding to the SiO
surface of the wafers. To
test this, a drop of water containing many beads were pipette-spotted on
to the wafer. After waiting for a few minutes, the wafer is washed in
distilled water and blow dried with nitrogen. Then, the wafer is examined
for magnetic beads that still adhere to the wafer. Only the types of
beads that did not adhere to the wafer were used for a second test.
For the ultimate test, samples were prepared as described above with only one alteration. The pipette-spotting of the biotinylated oligonucleotides was omitted during the surface preparation. Because of the missing biotin, the beads cannot bind specifically to the surface. Therefore any binding event would be an unspecific bond, which is not allowed. It was tested several times with all beads used for the bond-force measurements that they never bind unspecific to the sample surface.
The three types of beads that are used, because they meet these requirements, were already presented in section 1.2.