Sulfur-gold bond

The sulfur-gold bond has several advantages and was, therefore, the first bond tested in this thesis. One advantage is that it is a strong covalent bond with a high bond enthalpy of $418\pm 25$kJ/mol [135]. Because nearly all bonds between biomolecules are weaker, this would be a good proof of principle for the method. Furthermore, this bond is well known, e.g. GRANDBOIS et al.  presented AFM experiments in 1999 [54] where they measured a bond-force of $1.4\pm 0.3$nN at loading-rates of 10nN/sec for the sulfur-gold bond.

Figure 4.2: Sketch for the Sulfur-gold bond measurements

0.5

Another key advantage is the
simplified setup shown in figure 4.2. Magnetic markers functionalised
with a SH-group on the surface can directly bind to the patterned gold lines. The fabrication of the sample is much easier, because there is no surface preparation involved and even the SiO$_2$ protection layer is
omitted. Suitable magnetic markers are commercially available in a wide variety. The SICASTAR-SH particles from MICROMOD (with a diameter of 1.5$\mu$m) were used because these particles passed some important tests.

The most important property is that the markers only bind specifically to gold and not on the SiO$_2$ surface of the wafer. To ensure this, we tested the SICASTAR-SH markers on SiO$_2$ surface and MICROMOD SICASTAR markers without a SH-group on a gold surface. In both tests, the markers did not bind to the surface.

During the experiments, it became apparent that the binding only works good when the sample was freshly made. Even an ultrasonic bath in acetone didn't change this behaviour. This was very much unexpected, because the gold lines, buried in SiO$_2$, should not change over time, and the magnetic markers are always the same. So, we examined whether the gold surface changes over time. Auger measurements of a freshly made gold surface and of a 6 weeks old gold surface are compared in figure 4.3.

Figure 4.3: Auger measurements of sputtered gold surfaces

[Only 5 minutes on air] [6 weeks on air]

The Auger measurements clearly show that with time, carbon is deposited on top of the gold surface. So, the fact that the markers do not bind well to an older surface supports our main concern that we only examine the sulfur-gold bond and no unspecific binding. Therefore, we had to prepare fresh samples or expose the sample for 30sec to ion beam milling (see section 2.2).