Abstract
Self-assembled monolayers (SAMs) can be formed at the interface between solids and fluids, and are often used to modify the surface properties of the solid. One of the most widely employed SAM systems is exploiting thiol-gold chemistry, which, together with alkane-chain-based molecules, provides a reliable way of SAM formation to modify the surface properties of electrodes. Oligo ethylene-glycol (OEG) terminated alkanethiol monolayers have shown excellent antifouling properties and have been used extensively for the coating of biosensor electrodes to minimize nonspecific binding. Here, we report the investigation of the dielectric properties of COOH-capped OEG monolayers and demonstrate a strategy to improve the dielectric properties significantly by mixing the OEG SAM with small concentrations of 11-mercaptoundecanol (MUD). The monolayer properties and composition were characterized by means of impedance spectroscopy, water contact angle, ellipsometry and X-ray photoelectron spectroscopy. An equivalent circuit model is proposed to interpret the EIS data and to determine the conductivity of the monolayer. We find that for increasing MUD concentrations up to about 5% the resistivity of the SAM steadily increases, which together with a considerable decrease of the phase of the impedance, demonstrates significantly improved dielectric properties of the monolayer. Such monolayers will find widespread use in applications which depend critically on good dielectric properties such as capacitive biosensor.
Highlights
Oligo ethylene-glycol (OEG)-terminated alkyl thiols have been reported to form a SAM in which the alkane chains are tilted at a 30° angle with respect to the surface normal, with the terminal ethylene glycol groups forming either a liquid-like amorphous phase or a semicrystalline layer consisting of OEG molecules perpendicular to the surface.[11]
Xray photoelectron spectroscopy (XPS), ellipsometry and contact angle have been used to characterize the composition of the monolayers
This study shows that even a small amount of MUD in the mixed MUD:OEG SAMs leads to considerably improved dielectric properties, as seen by the considerable decrease of the phase of the impedance at low frequencies as well as a significant increase in the resistivity of the SAM
Summary
Self-assembled monolayers (SAMs) are a versatile means for modifying the physical and chemical properties of surfaces,[1,2] including their hydrophobicity, biocompatibility,[2] antifouling properties,[3−5] and ability to bind molecules covalently.[4,6,7] They have been employed to modify a variety of surfaces including silicon oxide,[8] gold,[1,9] and other metals.[10,11] The exploitation of thiol-gold binding chemistry, and in conjunction with molecules comprising alkane chains, provides an optimal combination of straightforward SAM preparation on gold surfaces, structural order, and versatility of modification of the functional group.[1,10]SAMs are commonly used in electronic biosensors both to provide the chemical functionality to attach the receptor molecules[4] to the sensor surface required for the biological detection,[12] as well as serving as an antifouling layer to reduce unwanted, nonspecific binding of material such as protein molecules to the device surface.[3,13] SAMs of oligo- and polyethylene glycol chains (OEG, PEG) have been used extensively to reduce nonspecific protein absorption[2,3] and improve biocompatibility[4] on solid surfaces,[11,13] with OEG-terminated alkyl thiols often the preferred choice of SAM molecules on gold electrodes.[4,6,11,13] OEG-terminated alkyl thiols have been reported to form a SAM in which the alkane chains are tilted at a 30° angle with respect to the surface normal, with the terminal ethylene glycol groups forming either a liquid-like amorphous phase or a semicrystalline layer consisting of OEG molecules perpendicular to the surface.[11].
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