Thiol self-assembled monolayers (SAMs) are popular due to their utility for various applications. In biosensing platforms SAMs are often used as linkers to tether receptors to biochip surfaces . To get a full understanding of SAM-properties, it is important to study also their formation kinetics and thermal-transport properties. This latter point was not addressed before. However, it is already known that DNA and adsorbed cells at solid-liquid interfaces show a heat-blocking effect, which is utilized in the biosensing technique HTM (heat transfer method) .
In this work we employed HTM to study the formation kinetics of thiol SAMs on gold surfaces using ethanol as a solvent. The thiols were 1-dodecanethiol and 11-mercaptoundecanoic acid (11-MUA) differing in their specific head groups, being -CH3 for 1-dodecanethiol and -COOH in case of 11-MUA. Complementarily, the layer formation was monitored with quartz crystal microbalance, Fourier-transform infrared spectroscopy, and atomic force microscopy. The results indicate that presence of a SAM on the gold surface leads to a surprisingly strong increase of the interfacial heat-transfer resistance Rth, given the fact that thiol molecules are only 1.5 nm in length. Results show a concentration-dependent jump in Rth for concentrations higher than 0.5 mM. The thermal resistance displays a two-step evolution for concentrations below 0.5 mM: Initially, the thermal resistance decreases in comparison to a blank gold substrate. In a second phase, Rth increases gradually, eventually reaching a stable plateau value. This behavior can be attributed to a transition from a lying-down to a standing-up conformation of the thiols. The results demonstrate that a nanometer thin ‘thiol carpet’ causes an unexpectedly strong heat-blocking effect at gold/ethanol interfaces. Furthermore, the absolute increase of the thermal-boundary resistance depends on the used head group. This observation points to an interface effect, which can possibly be explained by the mismatch between the phonon frequencies of gold and the vibration frequencies of ethanol and thiol molecules in the THz regime.
Acknoledgments: FWO project G.0B62.13N “Exploration of heat-conducting effects for applications in bio- and chemosensors” is gratefully appreciated.
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