Elicia L. S. Wong

Electrical characterizations of biomimetic molecular layers on gold and silicon substrates

Electrical impedance technology was used to characterize DNA recognition in a monolayer containing single-stranded DNA probes immobilized on a gold substrate using thiol self-assembly chemistry. Recognition of targeted complementary DNA was principally correlated with an eight-fold increase in the conductance of the monolayer and attributed to electron conduction through double helices formed upon the binding of the DNA targets to the probes. The high recognitive sensitivity was possible without the use of the redox labels or large bias voltages required for recognition using cyclic and Osteryoung square wave voltammetry. The impedance technology also provided atomic resolution of a hybrid bimolecular lipid membrane formed by deposition of a phospholipid:cholesterol monolayer onto a hydrophobic alkyl monolayer covalently attached to a silicon substrate via silicon-carbon bonds. Atomic resolution was achieved through preparation of membranes on surfaces approaching atomic flatness and the performance of impedance measurements over precisely defined areas of the surface in contact with solutions. Principally capacitive properties distinguished between the immobilized (octadecyl) and more fluidic (lipid:cholesterol) leaflets of the hybrid membrane. The lipid:cholesterol leaflets were structurally similar to those leaflets in free-standing bimolecular lipid membranes. The hybrid membrane therefore provides a highly stable and physiologically relevant surface for studying biomolecular interactions with membrane surfaces.

ELECTRICAL IMPEDANCE SPECTROSCOPY CHARACTERIZATIONS OF ALKYL-FUNCTIONALIZED SILICON(111)

This organic thin-film systems that are based on silicon-carbon covalent bonds have been shown to lead to densely packed alkyl monolayers that have potential bio-passivation or bio-sensing applications. Presented are electrical impedance spectroscopy (EIS) characterisations of a series of alkyl monolayers [CH3(CH2)mCH=CH2; m = 7, 9, 11, 13, 15] that were covalently linked to Si(111) wafers. The characterizations reveal capacitance, conductance and geometrical properties of the monolayers. The capacitance properties yield estimates of thicknesses for the monolayers that increase proportionally with each additional CH2 unit and are consistent with the known physical properties of these films such as dielectric constants and chain canting angles. This study illustrates that EIS charcterizations are able to probe immobilized surfaces on silicon with sub-atomic resolution which is so important in the development of practical bio-passivation or bio-sensing applications.

Electrochemical transduction of DNA hybridization by long-range electron transfer

For the detection of DNA hybridization, there are two main challenges that current research aims to overcome: lower detection limits and higher selectivity. We describe here the development of an electrochemical biosensor that used redox-active intercalators to transduce DNA hybridization by long-range electron transfer through DNA duplexes. This study outlines how the sensitivity and selectivity of the biosensor was tuned by careful control of the surface chemistry of the DNA-modified interface. The DNA-modified interface is composed of thiolated DNA and a diluent component, both of which are self-assembled onto a gold electrode. The resultant DNA biosensor has excellent selectivity towards single-base mismatch detection, whilst both the detection limit and sensitivity can easily be adjusted by varying the length of the diluent molecule relative to the length of the thiol linker at the 3´ end of the DNA. The one limitation of such a detection scheme is the slow assay time, which is a consequence of the slow kinetics of intercalation of the redox molecule into the duplexes. Approaches to reducing the assay time to a more commercially viable timescale are outlined.

Probing Biomimetic Molecular Structures on Gold and Silicon(111) with Electrical Impedance Spectroscopy

Self-assembled monolayers (SAMs) offer an ap- proach for engineering molecular interfaces that mimic bio- logical structures and their processes on solid substrates. Here we demonstrate the ability of electrical impedance spectros- copy to monitor the biological process of DNA recognition in a monolayer comprised of single stranded DNA assembled on a gold substrate and characterize the structure of a hybrid bi- molecular lipid membrane (BLM) on an atomically flat silicon surface. These biomimetic examples demonstrate the crucial importance of the impedance phase in being able to distinguish between electrical conductive and capacitive properties of the molecular interfaces.

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.