DISSERTATION

______________________________________________________________________ Introduction
specialized ink-jet printer ran by a robot (Figure 1.11). By this, spots with a diameter of 100-
150 µm are created on the surface. The number of spots is limited to prevent crosscontamination.
Therefore, the density of these microarrays is moderate with 10,000 to 30,000
spots per array. Furthermore, this technique requires strict monitoring of the production
reproducibility and quality control. Difficulties with efficiency and accuracy are another
drawback of this technique 47,51 . Nevertheless, an advantage is that the content of the
microarrays is flexible.
Figure 1.11. Schematic representation of DNA solution spotting on the electrode surface.
Electronic microarrays use an electric field to control the immobilization of DNA. The
Company Nanogen developed a Nanochip with 12 connectors controlling 400 individual sites
on a chip. The principle of immobilization consists of the transport of negatively charged DNA,
modified with biotin, through an agarose permeation layer towards specific sites, modified with
streptavidin, on the chip where a positive current is applied 52 . Even though the density of these
chips is limited to 400 spots, this is sufficient for the majority of diagnostic applications 51 .
Furthermore, the content of the microarray can be specified by the user, which decreases
microarray manufacturing costs. Biotin-streptavidin chemistry offers a very strong bonding but
comes with some limitations. The synthesis of streptavidin-modified surfaces consists of
several steps, such as the activation of the surface, immobilization of streptavidin and blocking,
which increases the production costs and time 47 . Furthermore, one of the drawbacks of using
streptavidin is the problem with unspecific interactions.
1.3 DNA immobilization 20