DISSERTATION

_____________________________________________________________ Results and Discussion
Even though SAM formation has been an extensively investigated topic for decades until now,
the implementation of procedures for a fast and controlled SAM deposition is of great interest,
particularly when looking at future of point-of-care diagnostics devices, for which fast and
cheap techniques for sensor fabrication are becoming more and more important in order to
decrease production costs. The process of self-assembly supposedly consists of two
phases 22,90,91 , during which in the initial stage thiol molecules randomly cover the surface, and
in a second phase they slowly organize on the surface to form a monolayer. Although the twophase
process is widely accepted, there are conflicting reports on the kinetics of SAM
formation 92 .
Most approaches for SAM formation involve a self-assembly process at open circuit potential.
This is a simple method with, however, quite low reproducibility 24 and slow kinetics. It has
been reported in the literature that the application of constant positive potentials accelerates the
immobilization kinetics of long thiol chains providing compact SAMs in a shorter time 13,21,92 .
Nevertheless, further improvement of the immobilization process is still needed, especially
focusing on short-chain thiols commonly used for sensor fabrication 22 . Therefore, the
application of the potential pulse-assisted immobilization method for alkylthiol derivatives as
examples for uncharged molecules was investigated.
One of the most reliable methods for the investigation of the self-assembly process is the
measurement of the double-layer capacitance since it precisely describes the SAM adsorption
properties. The interfacial capacitance of a gold-SAM-electrolyte interface consists of the
capacitance of the SAM and the capacitance of the diffuse layer connected in series. The overall
capacitance is determined by the smaller one, which is the capacitance of the SAM. EIS is a
commonly used technique for the determination of the capacitance of a system. Real time
measurement of electrochemical impedance allows the continuous determination of the
capacitance with time and hence a detailed investigation of the SAM formation kinetics.
Subramanian and Lakshminarayanan 92 used real time impedance monitoring to study the selfassembly
mechanism of thiols by applying a constant DC potential superimposed with an AC
signal at a single frequency. This measuring principle was modified in that way, that a pulsetype
potential modulation employed for the above described potential-pulse assisted
immobilization method was applied as DC potential.
3.3 Importance of controlling the surface 68