Self-assembled monolayers of supramolecular compounds for chemical sensors

We have studied the application of self-assembled resorcin[4]arene monolayers as chemically sensitive coating of quartz microalance sensors. Resorcin[4]arenes are typical supramolecular compounds with a molecular cavity formed by an aromatic ring system. The latter provides lipophilic binding sites and acts therefore as a molecular recognition structure for organic molecules. Reosrcinarenes are of particular interest for the selective detection of organic molecules in the gas and liquid phase. A high level of molecular order can be achieved by using modified cage compounds: dialkylsulfide-substituted resorcin[4]arene derivatives (with 4 aromatic rings) form well-ordered monolayers at Au surfaces by a self-assembling process which occurs spontaneously in ethanol/chloroform solutions of thes resorcin[4]arene derivatives. The driving force is the formation of very stable AuS bonds and the van der Waals interaction between the alkyl chains. We present results on the structure of resorcin[4]arene monolayers and their interactions with organic molecules. These results were obtained from angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and thermal desorption spectroscopy (TDS). The latter techniques makes it possible to determine binding energies of the molecule/resorcin[4]arene ‘key-lock’ interaction. The binding energies are then compared with mass changes Δm which are obtained from quartz microbalance oscillators with their Au electrodes coated with resorcin[4]arene monolayers. We tested different organic molecules in the gas and liquid phase such as tetrachloroethane (C2Cl4), Trichloroethylene (HClCCCl2), tetrachloromethane (CCl4), chloroform (CHCl3) and toluene (H3C-C6H5). By far the most pronounced sensor response and hence highest selectivity was observed to tetrachloroethene (C2Cl4).