The achievement of high hybridization efficiency in DNA microarrays is largely affected by the surface density of immobilized functional DNA probes. We investigated the chemical conditions for the formation of mixed self-assembled monolayers of thiol-modified oligonucleotides and mercaptohexanol deposited onto gold surfaces. The surface density of DNA was studied by means of high-resolution X-ray photoemission spectroscopy. The measurements revealed that the spatial density of DNA strands can be controlled within a wide range by the concentration of MgCl2 in the immobilization buffer. Moreover, improved preparation conditions for high-quality self-assembled hybrid monolayers are presented. Compared to our previous work, a reduction in unbound thiol was achieved by changing from ethanolic to aqueous solvent and lowering the MCH concentration.

We report investigations on the use of film bulk acoustic resonators for the label-free, multiplexed biosensing of DNA and proteins. The used acoustic resonators were operated in shear mode at about 800 MHz. From the measured changes of frequency and in dissipation, the mass and the viscoelasticity of biomolecular films formed at the top electrode of the device could be derived, respectively. A mass sensitivity of ∼2 kHz cm2/ng and a minimum detectable mass of ∼1 ng/cm2 were achieved. To demonstrate the highly sensitive detection of the time evolution of protein adsorption, the adsorption kinetics and recrystallisation of bacterial surface layer proteins on gold surfaces were investigated.

Mercaptohexanol (MCH) monolayers assembled on gold surfaces were investigated by means of high-resolution X-ray photoemission spectroscopy (XPS). The presence of three chemical states of sulfur was detected which exhibit different strength of interaction with the gold surface. Comparing the relative intensities of the corresponding subspectra allowed to estimate which fraction of the deposited MCH molecules is not bound to the metal surface via the thiol group, and thus, the characterization of the quality of the self-assembled monolayer (SAM). The number of sulfur atoms bound to the gold surface decreased when part of the the hydroxyl endgroup of the assembled MCH molecules is substituted by DNA duplexes. Long-term irradiation experiments on MCH layers revealed that soft X-ray radiation induces damage of the SAMs, accompanied with a preferential cleavage of both hydroxyl groups and hydrogen atoms from the alkane chains, leading in turn to a rearrangement of those MCH molecules which were not bound to the gold surface during the SAM preparation. The fraction of molecules which are bound to the metal via the thiol group, however, is practically not affected by the irradiation with soft X-rays.

We report investigations on the use of film bulk acoustic resonators for the label-free, multiplexed biosensing of DNA and proteins. The used acoustic resonators were operated in shear mode at about 800 MHz. From the measured changes of frequency and in dissipation, the mass and the viscoelasticity of biomolecular films formed at the top electrode of the device could be derived, respectively. A mass sensitivity of ∼2 kHz cm2/ng and a minimum detectable mass of ∼1 ng/cm2 were achieved. To demonstrate the highly sensitive detection of the time evolution of protein adsorption, the adsorption kinetics and recrystallisation of bacterial surface layer proteins on gold surfaces were investigated.