Ultra-Sensitive Techniques To Probe Structural Changes With Atomic Resolution

Self-assembled monolayers (SAMs) offer an approach for engineering interfacial structure at the molecular level with well-defined orientation and density on a solid substrate, for biopassivation and biosensing applications. As a result, the ability to monitor the effect of changes in molecular structure and composition of SAMs upon interfacial events is crucial. One such change is the variation in thickness upon molecular recognition at the interface. In this study, a series of alkyl monolayers [C10, C12, C14, C16 and C18] were covalently linked to the surface of Si(111) wafers. The structures of these surfaces were studied using X-ray reflectometry (XRR) and AC impedance spectroscopy. Both techniques are sensitive towards variation in thickness with the addition of two carbons and hence provide another useful means for monitoring molecular-scale events. The combinations of these techniques are able to probe the thickness and the interfacial roughness and capacitance of the individual layer at the immobilized surface with atomic resolution. A PAMAM G-5 dendrimer was also successful attached to a carboxylic acid functionalized Si(111) surface and was fully characterized using XRR. The utilizing of AC impedance spectroscopy as a biosensor was also demonstrated by its ability to differentiate single-stranded from double-stranded DNA through a vast increase in conductivity observed at the double-stranded DNA modified gold surface.