The main goal of this work is to apply a emergent technique, based on nonlinear optics, to the detection of the molecular recognition process in biosensing devices. For this purpose, we implement the sum-frequency generation (SFG) spectroscopy, which is able to extract a vibrational fingerprint from the studied interface. Our experimental route begins with a study of molecular adsorption mechanisms that constitute an essential prerequisite to the biosensor technology. The adsorption properties of two molecules (decanethiol and decyl thiocyanate) that differ only through their anchoring groups are investigated. Complementary techniques such as XPS or STM complete the SFG spectroscopic information. We show that the use of thiocyanate groups allows the formation of self-assembled monolayers on platinum substrate, even if the persistence of cyanate ions disturbs the organization of the layers. Our study continues with the analysis of a model biosensor based on the biotin/avidin system. The SFG confirms the formation of a organized monolayer of probe molecules (biotin) on Au, Ag, Pt and CaF2 substrates. The molecular recognition with the target molecule (avidin) is observed on CaF2 substrates in the spectral range of the CH and NH stretching vibrations. The use of alternative target molecules (BSA, saturated avidin) demonstrates that the SFG detection occurs in a specific way. In a next step, we identify a preparation method leading to the formation of DNA based sensors that are suitable for the SFG investigations. The quality of the hybridization process of four different methods is investigated by surface plasmon resonance (SPR). That one shows that the best chemical architecture consists to adsorb mercaptohexanol (MCH) molecules in between the DNA strands, which are previously assembled on the substrate. In a final step, the SFG is applied to DNA based sensors formed with the method selected by SPR. The obtained spectra exhibit a poor organization of the probe molecules, which prevents us to detect the hybridization process by SFG. Besides, the spectroscopic information reveals the presence at the interface of residues from the buffer solution.
|Date of Award||24 Aug 2009|
|Supervisor||Paul THIRY (Supervisor), Laurent HOUSSIAU (President), Laurent Dreesen (Jury), Gérard Tourillon (Jury) & Francesca Cecchet (Jury)|